library/ssl_tls12_server.c

/*
 *  TLS server-side functions
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0
 *
 *  Licensed under the Apache License, Version 2.0 (the "License"); you may
 *  not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

#include "common.h"

#if defined(MBEDTLS_SSL_SRV_C) && defined(MBEDTLS_SSL_PROTO_TLS1_2)

#include "mbedtls/platform.h"

#include "mbedtls/ssl.h"
#include "ssl_misc.h"
#include "mbedtls/debug.h"
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"
#include "constant_time_internal.h"
#include "mbedtls/constant_time.h"
#include "hash_info.h"

#include <string.h>

#if defined(MBEDTLS_USE_PSA_CRYPTO)
#define PSA_TO_MBEDTLS_ERR(status) PSA_TO_MBEDTLS_ERR_LIST(status,   \
                                                           psa_to_ssl_errors,             \
                                                           psa_generic_status_to_mbedtls)
#endif

#if defined(MBEDTLS_ECP_C)
#include "mbedtls/ecp.h"
#endif

#if defined(MBEDTLS_HAVE_TIME)
#include "mbedtls/platform_time.h"
#endif

#if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY)
int mbedtls_ssl_set_client_transport_id(mbedtls_ssl_context *ssl,
                                        const unsigned char *info,
                                        size_t ilen)
{
    if (ssl->conf->endpoint != MBEDTLS_SSL_IS_SERVER) {
        return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
    }

    mbedtls_free(ssl->cli_id);

    if ((ssl->cli_id = mbedtls_calloc(1, ilen)) == NULL) {
        return MBEDTLS_ERR_SSL_ALLOC_FAILED;
    }

    memcpy(ssl->cli_id, info, ilen);
    ssl->cli_id_len = ilen;

    return 0;
}

void mbedtls_ssl_conf_dtls_cookies(mbedtls_ssl_config *conf,
                                   mbedtls_ssl_cookie_write_t *f_cookie_write,
                                   mbedtls_ssl_cookie_check_t *f_cookie_check,
                                   void *p_cookie)
{
    conf->f_cookie_write = f_cookie_write;
    conf->f_cookie_check = f_cookie_check;
    conf->p_cookie       = p_cookie;
}
#endif /* MBEDTLS_SSL_DTLS_HELLO_VERIFY */

#if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_conf_has_psk_or_cb(mbedtls_ssl_config const *conf)
{
    if (conf->f_psk != NULL) {
        return 1;
    }

    if (conf->psk_identity_len == 0 || conf->psk_identity == NULL) {
        return 0;
    }


#if defined(MBEDTLS_USE_PSA_CRYPTO)
    if (!mbedtls_svc_key_id_is_null(conf->psk_opaque)) {
        return 1;
    }
#endif /* MBEDTLS_USE_PSA_CRYPTO */

    if (conf->psk != NULL && conf->psk_len != 0) {
        return 1;
    }

    return 0;
}
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_renegotiation_info(mbedtls_ssl_context *ssl,
                                        const unsigned char *buf,
                                        size_t len)
{
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    if (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE) {
        /* Check verify-data in constant-time. The length OTOH is no secret */
        if (len    != 1 + ssl->verify_data_len ||
            buf[0] !=     ssl->verify_data_len ||
            mbedtls_ct_memcmp(buf + 1, ssl->peer_verify_data,
                              ssl->verify_data_len) != 0) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("non-matching renegotiation info"));
            mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                           MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE);
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }
    } else
#endif /* MBEDTLS_SSL_RENEGOTIATION */
    {
        if (len != 1 || buf[0] != 0x0) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("non-zero length renegotiation info"));
            mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                           MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE);
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }

        ssl->secure_renegotiation = MBEDTLS_SSL_SECURE_RENEGOTIATION;
    }

    return 0;
}

#if defined(MBEDTLS_PK_CAN_ECDH) || defined(MBEDTLS_PK_CAN_ECDSA_SOME) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
/*
 * Function for parsing a supported groups (TLS 1.3) or supported elliptic
 * curves (TLS 1.2) extension.
 *
 * The "extension_data" field of a supported groups extension contains a
 * "NamedGroupList" value (TLS 1.3 RFC8446):
 *      enum {
 *          secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
 *          x25519(0x001D), x448(0x001E),
 *          ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096(0x0102),
 *          ffdhe6144(0x0103), ffdhe8192(0x0104),
 *          ffdhe_private_use(0x01FC..0x01FF),
 *          ecdhe_private_use(0xFE00..0xFEFF),
 *          (0xFFFF)
 *      } NamedGroup;
 *      struct {
 *          NamedGroup named_group_list<2..2^16-1>;
 *      } NamedGroupList;
 *
 * The "extension_data" field of a supported elliptic curves extension contains
 * a "NamedCurveList" value (TLS 1.2 RFC 8422):
 * enum {
 *      deprecated(1..22),
 *      secp256r1 (23), secp384r1 (24), secp521r1 (25),
 *      x25519(29), x448(30),
 *      reserved (0xFE00..0xFEFF),
 *      deprecated(0xFF01..0xFF02),
 *      (0xFFFF)
 *  } NamedCurve;
 * struct {
 *      NamedCurve named_curve_list<2..2^16-1>
 *  } NamedCurveList;
 *
 * The TLS 1.3 supported groups extension was defined to be a compatible
 * generalization of the TLS 1.2 supported elliptic curves extension. They both
 * share the same extension identifier.
 *
 * DHE groups are not supported yet.
 */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_supported_groups_ext(mbedtls_ssl_context *ssl,
                                          const unsigned char *buf,
                                          size_t len)
{
    size_t list_size, our_size;
    const unsigned char *p;
    uint16_t *curves_tls_id;

    if (len < 2) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }
    list_size = ((buf[0] << 8) | (buf[1]));
    if (list_size + 2 != len ||
        list_size % 2 != 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    /* Should never happen unless client duplicates the extension */
    if (ssl->handshake->curves_tls_id != NULL) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER);
        return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
    }

    /* Don't allow our peer to make us allocate too much memory,
     * and leave room for a final 0 */
    our_size = list_size / 2 + 1;
    if (our_size > MBEDTLS_ECP_DP_MAX) {
        our_size = MBEDTLS_ECP_DP_MAX;
    }

    if ((curves_tls_id = mbedtls_calloc(our_size,
                                        sizeof(*curves_tls_id))) == NULL) {
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR);
        return MBEDTLS_ERR_SSL_ALLOC_FAILED;
    }

    ssl->handshake->curves_tls_id = curves_tls_id;

    p = buf + 2;
    while (list_size > 0 && our_size > 1) {
        uint16_t curr_tls_id = MBEDTLS_GET_UINT16_BE(p, 0);

        if (mbedtls_ssl_get_ecp_group_id_from_tls_id(curr_tls_id) !=
            MBEDTLS_ECP_DP_NONE) {
            *curves_tls_id++ = curr_tls_id;
            our_size--;
        }

        list_size -= 2;
        p += 2;
    }

    return 0;
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_supported_point_formats(mbedtls_ssl_context *ssl,
                                             const unsigned char *buf,
                                             size_t len)
{
    size_t list_size;
    const unsigned char *p;

    if (len == 0 || (size_t) (buf[0] + 1) != len) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }
    list_size = buf[0];

    p = buf + 1;
    while (list_size > 0) {
        if (p[0] == MBEDTLS_ECP_PF_UNCOMPRESSED ||
            p[0] == MBEDTLS_ECP_PF_COMPRESSED) {
#if !defined(MBEDTLS_USE_PSA_CRYPTO) &&                             \
            (defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C))
            ssl->handshake->ecdh_ctx.point_format = p[0];
#endif /* !MBEDTLS_USE_PSA_CRYPTO &&
          ( MBEDTLS_ECDH_C || MBEDTLS_ECDSA_C ) */
#if !defined(MBEDTLS_USE_PSA_CRYPTO) &&                             \
            defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
            mbedtls_ecjpake_set_point_format(&ssl->handshake->ecjpake_ctx,
                                             p[0]);
#endif /* !MBEDTLS_USE_PSA_CRYPTO && MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
            MBEDTLS_SSL_DEBUG_MSG(4, ("point format selected: %d", p[0]));
            return 0;
        }

        list_size--;
        p++;
    }

    return 0;
}
#endif /* MBEDTLS_PK_CAN_ECDH || MBEDTLS_PK_CAN_ECDSA_SOME ||
          MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_ecjpake_kkpp(mbedtls_ssl_context *ssl,
                                  const unsigned char *buf,
                                  size_t len)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

#if defined(MBEDTLS_USE_PSA_CRYPTO)
    if (ssl->handshake->psa_pake_ctx_is_ok != 1)
#else
    if (mbedtls_ecjpake_check(&ssl->handshake->ecjpake_ctx) != 0)
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    {
        MBEDTLS_SSL_DEBUG_MSG(3, ("skip ecjpake kkpp extension"));
        return 0;
    }

#if defined(MBEDTLS_USE_PSA_CRYPTO)
    if ((ret = mbedtls_psa_ecjpake_read_round(
             &ssl->handshake->psa_pake_ctx, buf, len,
             MBEDTLS_ECJPAKE_ROUND_ONE)) != 0) {
        psa_destroy_key(ssl->handshake->psa_pake_password);
        psa_pake_abort(&ssl->handshake->psa_pake_ctx);

        MBEDTLS_SSL_DEBUG_RET(1, "psa_pake_input round one", ret);
        mbedtls_ssl_send_alert_message(
            ssl,
            MBEDTLS_SSL_ALERT_LEVEL_FATAL,
            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE);

        return ret;
    }
#else
    if ((ret = mbedtls_ecjpake_read_round_one(&ssl->handshake->ecjpake_ctx,
                                              buf, len)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecjpake_read_round_one", ret);
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER);
        return ret;
    }
#endif /* MBEDTLS_USE_PSA_CRYPTO */

    /* Only mark the extension as OK when we're sure it is */
    ssl->handshake->cli_exts |= MBEDTLS_TLS_EXT_ECJPAKE_KKPP_OK;

    return 0;
}
#endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_max_fragment_length_ext(mbedtls_ssl_context *ssl,
                                             const unsigned char *buf,
                                             size_t len)
{
    if (len != 1 || buf[0] >= MBEDTLS_SSL_MAX_FRAG_LEN_INVALID) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER);
        return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
    }

    ssl->session_negotiate->mfl_code = buf[0];

    return 0;
}
#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */

#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_cid_ext(mbedtls_ssl_context *ssl,
                             const unsigned char *buf,
                             size_t len)
{
    size_t peer_cid_len;

    /* CID extension only makes sense in DTLS */
    if (ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER);
        return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
    }

    /*
     *   struct {
     *      opaque cid<0..2^8-1>;
     *   } ConnectionId;
     */

    if (len < 1) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    peer_cid_len = *buf++;
    len--;

    if (len != peer_cid_len) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    /* Ignore CID if the user has disabled its use. */
    if (ssl->negotiate_cid == MBEDTLS_SSL_CID_DISABLED) {
        /* Leave ssl->handshake->cid_in_use in its default
         * value of MBEDTLS_SSL_CID_DISABLED. */
        MBEDTLS_SSL_DEBUG_MSG(3, ("Client sent CID extension, but CID disabled"));
        return 0;
    }

    if (peer_cid_len > MBEDTLS_SSL_CID_OUT_LEN_MAX) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER);
        return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
    }

    ssl->handshake->cid_in_use = MBEDTLS_SSL_CID_ENABLED;
    ssl->handshake->peer_cid_len = (uint8_t) peer_cid_len;
    memcpy(ssl->handshake->peer_cid, buf, peer_cid_len);

    MBEDTLS_SSL_DEBUG_MSG(3, ("Use of CID extension negotiated"));
    MBEDTLS_SSL_DEBUG_BUF(3, "Client CID", buf, peer_cid_len);

    return 0;
}
#endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */

#if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_encrypt_then_mac_ext(mbedtls_ssl_context *ssl,
                                          const unsigned char *buf,
                                          size_t len)
{
    if (len != 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    ((void) buf);

    if (ssl->conf->encrypt_then_mac == MBEDTLS_SSL_ETM_ENABLED) {
        ssl->session_negotiate->encrypt_then_mac = MBEDTLS_SSL_ETM_ENABLED;
    }

    return 0;
}
#endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */

#if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_extended_ms_ext(mbedtls_ssl_context *ssl,
                                     const unsigned char *buf,
                                     size_t len)
{
    if (len != 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    ((void) buf);

    if (ssl->conf->extended_ms == MBEDTLS_SSL_EXTENDED_MS_ENABLED) {
        ssl->handshake->extended_ms = MBEDTLS_SSL_EXTENDED_MS_ENABLED;
    }

    return 0;
}
#endif /* MBEDTLS_SSL_EXTENDED_MASTER_SECRET */

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_session_ticket_ext(mbedtls_ssl_context *ssl,
                                        unsigned char *buf,
                                        size_t len)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    mbedtls_ssl_session session;

    mbedtls_ssl_session_init(&session);

    if (ssl->conf->f_ticket_parse == NULL ||
        ssl->conf->f_ticket_write == NULL) {
        return 0;
    }

    /* Remember the client asked us to send a new ticket */
    ssl->handshake->new_session_ticket = 1;

    MBEDTLS_SSL_DEBUG_MSG(3, ("ticket length: %" MBEDTLS_PRINTF_SIZET, len));

    if (len == 0) {
        return 0;
    }

#if defined(MBEDTLS_SSL_RENEGOTIATION)
    if (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("ticket rejected: renegotiating"));
        return 0;
    }
#endif /* MBEDTLS_SSL_RENEGOTIATION */

    /*
     * Failures are ok: just ignore the ticket and proceed.
     */
    if ((ret = ssl->conf->f_ticket_parse(ssl->conf->p_ticket, &session,
                                         buf, len)) != 0) {
        mbedtls_ssl_session_free(&session);

        if (ret == MBEDTLS_ERR_SSL_INVALID_MAC) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("ticket is not authentic"));
        } else if (ret == MBEDTLS_ERR_SSL_SESSION_TICKET_EXPIRED) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("ticket is expired"));
        } else {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_ticket_parse", ret);
        }

        return 0;
    }

    /*
     * Keep the session ID sent by the client, since we MUST send it back to
     * inform them we're accepting the ticket  (RFC 5077 section 3.4)
     */
    session.id_len = ssl->session_negotiate->id_len;
    memcpy(&session.id, ssl->session_negotiate->id, session.id_len);

    mbedtls_ssl_session_free(ssl->session_negotiate);
    memcpy(ssl->session_negotiate, &session, sizeof(mbedtls_ssl_session));

    /* Zeroize instead of free as we copied the content */
    mbedtls_platform_zeroize(&session, sizeof(mbedtls_ssl_session));

    MBEDTLS_SSL_DEBUG_MSG(3, ("session successfully restored from ticket"));

    ssl->handshake->resume = 1;

    /* Don't send a new ticket after all, this one is OK */
    ssl->handshake->new_session_ticket = 0;

    return 0;
}
#endif /* MBEDTLS_SSL_SESSION_TICKETS */

#if defined(MBEDTLS_SSL_DTLS_SRTP)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_use_srtp_ext(mbedtls_ssl_context *ssl,
                                  const unsigned char *buf,
                                  size_t len)
{
    mbedtls_ssl_srtp_profile client_protection = MBEDTLS_TLS_SRTP_UNSET;
    size_t i, j;
    size_t profile_length;
    uint16_t mki_length;
    /*! 2 bytes for profile length and 1 byte for mki len */
    const size_t size_of_lengths = 3;

    /* If use_srtp is not configured, just ignore the extension */
    if ((ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM) ||
        (ssl->conf->dtls_srtp_profile_list == NULL) ||
        (ssl->conf->dtls_srtp_profile_list_len == 0)) {
        return 0;
    }

    /* RFC5764 section 4.1.1
     * uint8 SRTPProtectionProfile[2];
     *
     * struct {
     *   SRTPProtectionProfiles SRTPProtectionProfiles;
     *   opaque srtp_mki<0..255>;
     * } UseSRTPData;

     * SRTPProtectionProfile SRTPProtectionProfiles<2..2^16-1>;
     */

    /*
     * Min length is 5: at least one protection profile(2 bytes)
     *                  and length(2 bytes) + srtp_mki length(1 byte)
     * Check here that we have at least 2 bytes of protection profiles length
     * and one of srtp_mki length
     */
    if (len < size_of_lengths) {
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    ssl->dtls_srtp_info.chosen_dtls_srtp_profile = MBEDTLS_TLS_SRTP_UNSET;

    /* first 2 bytes are protection profile length(in bytes) */
    profile_length = (buf[0] << 8) | buf[1];
    buf += 2;

    /* The profile length cannot be bigger than input buffer size - lengths fields */
    if (profile_length > len - size_of_lengths ||
        profile_length % 2 != 0) { /* profiles are 2 bytes long, so the length must be even */
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }
    /*
     * parse the extension list values are defined in
     * http://www.iana.org/assignments/srtp-protection/srtp-protection.xhtml
     */
    for (j = 0; j < profile_length; j += 2) {
        uint16_t protection_profile_value = buf[j] << 8 | buf[j + 1];
        client_protection = mbedtls_ssl_check_srtp_profile_value(protection_profile_value);

        if (client_protection != MBEDTLS_TLS_SRTP_UNSET) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("found srtp profile: %s",
                                      mbedtls_ssl_get_srtp_profile_as_string(
                                          client_protection)));
        } else {
            continue;
        }
        /* check if suggested profile is in our list */
        for (i = 0; i < ssl->conf->dtls_srtp_profile_list_len; i++) {
            if (client_protection == ssl->conf->dtls_srtp_profile_list[i]) {
                ssl->dtls_srtp_info.chosen_dtls_srtp_profile = ssl->conf->dtls_srtp_profile_list[i];
                MBEDTLS_SSL_DEBUG_MSG(3, ("selected srtp profile: %s",
                                          mbedtls_ssl_get_srtp_profile_as_string(
                                              client_protection)));
                break;
            }
        }
        if (ssl->dtls_srtp_info.chosen_dtls_srtp_profile != MBEDTLS_TLS_SRTP_UNSET) {
            break;
        }
    }
    buf += profile_length; /* buf points to the mki length */
    mki_length = *buf;
    buf++;

    if (mki_length > MBEDTLS_TLS_SRTP_MAX_MKI_LENGTH ||
        mki_length + profile_length + size_of_lengths != len) {
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    /* Parse the mki only if present and mki is supported locally */
    if (ssl->conf->dtls_srtp_mki_support == MBEDTLS_SSL_DTLS_SRTP_MKI_SUPPORTED &&
        mki_length > 0) {
        ssl->dtls_srtp_info.mki_len = mki_length;

        memcpy(ssl->dtls_srtp_info.mki_value, buf, mki_length);

        MBEDTLS_SSL_DEBUG_BUF(3, "using mki",  ssl->dtls_srtp_info.mki_value,
                              ssl->dtls_srtp_info.mki_len);
    }

    return 0;
}
#endif /* MBEDTLS_SSL_DTLS_SRTP */

/*
 * Auxiliary functions for ServerHello parsing and related actions
 */

#if defined(MBEDTLS_X509_CRT_PARSE_C)
/*
 * Return 0 if the given key uses one of the acceptable curves, -1 otherwise
 */
#if defined(MBEDTLS_PK_CAN_ECDSA_SOME)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_check_key_curve(mbedtls_pk_context *pk,
                               uint16_t *curves_tls_id)
{
    uint16_t *curr_tls_id = curves_tls_id;
    mbedtls_ecp_group_id grp_id = mbedtls_pk_ec(*pk)->grp.id;
    mbedtls_ecp_group_id curr_grp_id;

    while (*curr_tls_id != 0) {
        curr_grp_id = mbedtls_ssl_get_ecp_group_id_from_tls_id(*curr_tls_id);
        if (curr_grp_id == grp_id) {
            return 0;
        }
        curr_tls_id++;
    }

    return -1;
}
#endif /* MBEDTLS_PK_CAN_ECDSA_SOME */

/*
 * Try picking a certificate for this ciphersuite,
 * return 0 on success and -1 on failure.
 */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_pick_cert(mbedtls_ssl_context *ssl,
                         const mbedtls_ssl_ciphersuite_t *ciphersuite_info)
{
    mbedtls_ssl_key_cert *cur, *list;
#if defined(MBEDTLS_USE_PSA_CRYPTO)
    psa_algorithm_t pk_alg =
        mbedtls_ssl_get_ciphersuite_sig_pk_psa_alg(ciphersuite_info);
    psa_key_usage_t pk_usage =
        mbedtls_ssl_get_ciphersuite_sig_pk_psa_usage(ciphersuite_info);
#else
    mbedtls_pk_type_t pk_alg =
        mbedtls_ssl_get_ciphersuite_sig_pk_alg(ciphersuite_info);
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    uint32_t flags;

#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
    if (ssl->handshake->sni_key_cert != NULL) {
        list = ssl->handshake->sni_key_cert;
    } else
#endif
    list = ssl->conf->key_cert;

    int pk_alg_is_none = 0;
#if defined(MBEDTLS_USE_PSA_CRYPTO)
    pk_alg_is_none = (pk_alg == PSA_ALG_NONE);
#else
    pk_alg_is_none = (pk_alg == MBEDTLS_PK_NONE);
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    if (pk_alg_is_none) {
        return 0;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("ciphersuite requires certificate"));

    if (list == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("server has no certificate"));
        return -1;
    }

    for (cur = list; cur != NULL; cur = cur->next) {
        flags = 0;
        MBEDTLS_SSL_DEBUG_CRT(3, "candidate certificate chain, certificate",
                              cur->cert);

        int key_type_matches = 0;
#if defined(MBEDTLS_USE_PSA_CRYPTO)
#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
        key_type_matches = ((ssl->conf->f_async_sign_start != NULL ||
                             ssl->conf->f_async_decrypt_start != NULL ||
                             mbedtls_pk_can_do_ext(cur->key, pk_alg, pk_usage)) &&
                            mbedtls_pk_can_do_ext(&cur->cert->pk, pk_alg, pk_usage));
#else
        key_type_matches = (
            mbedtls_pk_can_do_ext(cur->key, pk_alg, pk_usage));
#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
#else
        key_type_matches = mbedtls_pk_can_do(&cur->cert->pk, pk_alg);
#endif /* MBEDTLS_USE_PSA_CRYPTO */
        if (!key_type_matches) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("certificate mismatch: key type"));
            continue;
        }

        /*
         * This avoids sending the client a cert it'll reject based on
         * keyUsage or other extensions.
         *
         * It also allows the user to provision different certificates for
         * different uses based on keyUsage, eg if they want to avoid signing
         * and decrypting with the same RSA key.
         */
        if (mbedtls_ssl_check_cert_usage(cur->cert, ciphersuite_info,
                                         MBEDTLS_SSL_IS_SERVER, &flags) != 0) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("certificate mismatch: "
                                      "(extended) key usage extension"));
            continue;
        }

#if defined(MBEDTLS_PK_CAN_ECDSA_SOME)
        if (pk_alg == MBEDTLS_PK_ECDSA &&
            ssl_check_key_curve(&cur->cert->pk,
                                ssl->handshake->curves_tls_id) != 0) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("certificate mismatch: elliptic curve"));
            continue;
        }
#endif

        /* If we get there, we got a winner */
        break;
    }

    /* Do not update ssl->handshake->key_cert unless there is a match */
    if (cur != NULL) {
        ssl->handshake->key_cert = cur;
        MBEDTLS_SSL_DEBUG_CRT(3, "selected certificate chain, certificate",
                              ssl->handshake->key_cert->cert);
        return 0;
    }

    return -1;
}
#endif /* MBEDTLS_X509_CRT_PARSE_C */

/*
 * Check if a given ciphersuite is suitable for use with our config/keys/etc
 * Sets ciphersuite_info only if the suite matches.
 */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_ciphersuite_match(mbedtls_ssl_context *ssl, int suite_id,
                                 const mbedtls_ssl_ciphersuite_t **ciphersuite_info)
{
    const mbedtls_ssl_ciphersuite_t *suite_info;

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
    mbedtls_pk_type_t sig_type;
#endif

    suite_info = mbedtls_ssl_ciphersuite_from_id(suite_id);
    if (suite_info == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("trying ciphersuite: %#04x (%s)",
                              (unsigned int) suite_id, suite_info->name));

    if (suite_info->min_tls_version > ssl->tls_version ||
        suite_info->max_tls_version < ssl->tls_version) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("ciphersuite mismatch: version"));
        return 0;
    }

#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
    if (suite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECJPAKE &&
        (ssl->handshake->cli_exts & MBEDTLS_TLS_EXT_ECJPAKE_KKPP_OK) == 0) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("ciphersuite mismatch: ecjpake "
                                  "not configured or ext missing"));
        return 0;
    }
#endif


#if defined(MBEDTLS_PK_CAN_ECDH) || defined(MBEDTLS_PK_CAN_ECDSA_SOME)
    if (mbedtls_ssl_ciphersuite_uses_ec(suite_info) &&
        (ssl->handshake->curves_tls_id == NULL ||
         ssl->handshake->curves_tls_id[0] == 0)) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("ciphersuite mismatch: "
                                  "no common elliptic curve"));
        return 0;
    }
#endif

#if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
    /* If the ciphersuite requires a pre-shared key and we don't
     * have one, skip it now rather than failing later */
    if (mbedtls_ssl_ciphersuite_uses_psk(suite_info) &&
        ssl_conf_has_psk_or_cb(ssl->conf) == 0) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("ciphersuite mismatch: no pre-shared key"));
        return 0;
    }
#endif

#if defined(MBEDTLS_X509_CRT_PARSE_C)
    /*
     * Final check: if ciphersuite requires us to have a
     * certificate/key of a particular type:
     * - select the appropriate certificate if we have one, or
     * - try the next ciphersuite if we don't
     * This must be done last since we modify the key_cert list.
     */
    if (ssl_pick_cert(ssl, suite_info) != 0) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("ciphersuite mismatch: "
                                  "no suitable certificate"));
        return 0;
    }
#endif

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
    /* If the ciphersuite requires signing, check whether
     * a suitable hash algorithm is present. */
    sig_type = mbedtls_ssl_get_ciphersuite_sig_alg(suite_info);
    if (sig_type != MBEDTLS_PK_NONE &&
        mbedtls_ssl_tls12_get_preferred_hash_for_sig_alg(
            ssl, mbedtls_ssl_sig_from_pk_alg(sig_type)) == MBEDTLS_SSL_HASH_NONE) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("ciphersuite mismatch: no suitable hash algorithm "
                                  "for signature algorithm %u", (unsigned) sig_type));
        return 0;
    }

#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */

    *ciphersuite_info = suite_info;
    return 0;
}

/* This function doesn't alert on errors that happen early during
   ClientHello parsing because they might indicate that the client is
   not talking SSL/TLS at all and would not understand our alert. */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_client_hello(mbedtls_ssl_context *ssl)
{
    int ret, got_common_suite;
    size_t i, j;
    size_t ciph_offset, comp_offset, ext_offset;
    size_t msg_len, ciph_len, sess_len, comp_len, ext_len;
#if defined(MBEDTLS_SSL_PROTO_DTLS)
    size_t cookie_offset, cookie_len;
#endif
    unsigned char *buf, *p, *ext;
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    int renegotiation_info_seen = 0;
#endif
    int handshake_failure = 0;
    const int *ciphersuites;
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info;

    /* If there is no signature-algorithm extension present,
     * we need to fall back to the default values for allowed
     * signature-hash pairs. */
#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
    int sig_hash_alg_ext_present = 0;
#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> parse client hello"));

    int renegotiating;

#if defined(MBEDTLS_SSL_DTLS_ANTI_REPLAY)
read_record_header:
#endif
    /*
     * If renegotiating, then the input was read with mbedtls_ssl_read_record(),
     * otherwise read it ourselves manually in order to support SSLv2
     * ClientHello, which doesn't use the same record layer format.
     */
    renegotiating = 0;
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    renegotiating = (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE);
#endif
    if (!renegotiating) {
        if ((ret = mbedtls_ssl_fetch_input(ssl, 5)) != 0) {
            /* No alert on a read error. */
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_fetch_input", ret);
            return ret;
        }
    }

    buf = ssl->in_hdr;

    MBEDTLS_SSL_DEBUG_BUF(4, "record header", buf, mbedtls_ssl_in_hdr_len(ssl));

    /*
     * TLS Client Hello
     *
     * Record layer:
     *     0  .   0   message type
     *     1  .   2   protocol version
     *     3  .   11  DTLS: epoch + record sequence number
     *     3  .   4   message length
     */
    MBEDTLS_SSL_DEBUG_MSG(3, ("client hello, message type: %d",
                              buf[0]));

    if (buf[0] != MBEDTLS_SSL_MSG_HANDSHAKE) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        return MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("client hello, message len.: %d",
                              (ssl->in_len[0] << 8) | ssl->in_len[1]));

    MBEDTLS_SSL_DEBUG_MSG(3, ("client hello, protocol version: [%d:%d]",
                              buf[1], buf[2]));

    /* For DTLS if this is the initial handshake, remember the client sequence
     * number to use it in our next message (RFC 6347 4.2.1) */
#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM
#if defined(MBEDTLS_SSL_RENEGOTIATION)
        && ssl->renego_status == MBEDTLS_SSL_INITIAL_HANDSHAKE
#endif
        ) {
        /* Epoch should be 0 for initial handshakes */
        if (ssl->in_ctr[0] != 0 || ssl->in_ctr[1] != 0) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
            return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
        }

        memcpy(&ssl->cur_out_ctr[2], ssl->in_ctr + 2,
               sizeof(ssl->cur_out_ctr) - 2);

#if defined(MBEDTLS_SSL_DTLS_ANTI_REPLAY)
        if (mbedtls_ssl_dtls_replay_check(ssl) != 0) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("replayed record, discarding"));
            ssl->next_record_offset = 0;
            ssl->in_left = 0;
            goto read_record_header;
        }

        /* No MAC to check yet, so we can update right now */
        mbedtls_ssl_dtls_replay_update(ssl);
#endif
    }
#endif /* MBEDTLS_SSL_PROTO_DTLS */

    msg_len = (ssl->in_len[0] << 8) | ssl->in_len[1];

#if defined(MBEDTLS_SSL_RENEGOTIATION)
    if (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE) {
        /* Set by mbedtls_ssl_read_record() */
        msg_len = ssl->in_hslen;
    } else
#endif
    {
        if (msg_len > MBEDTLS_SSL_IN_CONTENT_LEN) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
            return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
        }

        if ((ret = mbedtls_ssl_fetch_input(ssl,
                                           mbedtls_ssl_in_hdr_len(ssl) + msg_len)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_fetch_input", ret);
            return ret;
        }

        /* Done reading this record, get ready for the next one */
#if defined(MBEDTLS_SSL_PROTO_DTLS)
        if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
            ssl->next_record_offset = msg_len + mbedtls_ssl_in_hdr_len(ssl);
        } else
#endif
        ssl->in_left = 0;
    }

    buf = ssl->in_msg;

    MBEDTLS_SSL_DEBUG_BUF(4, "record contents", buf, msg_len);

    ret = ssl->handshake->update_checksum(ssl, buf, msg_len);
    if (0 != ret) {
        MBEDTLS_SSL_DEBUG_RET(1, ("update_checksum"), ret);
        return ret;
    }

    /*
     * Handshake layer:
     *     0  .   0   handshake type
     *     1  .   3   handshake length
     *     4  .   5   DTLS only: message sequence number
     *     6  .   8   DTLS only: fragment offset
     *     9  .  11   DTLS only: fragment length
     */
    if (msg_len < mbedtls_ssl_hs_hdr_len(ssl)) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("client hello v3, handshake type: %d", buf[0]));

    if (buf[0] != MBEDTLS_SSL_HS_CLIENT_HELLO) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        return MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE;
    }
    {
        size_t handshake_len = MBEDTLS_GET_UINT24_BE(buf, 1);
        MBEDTLS_SSL_DEBUG_MSG(3, ("client hello v3, handshake len.: %u",
                                  (unsigned) handshake_len));

        /* The record layer has a record size limit of 2^14 - 1 and
         * fragmentation is not supported, so buf[1] should be zero. */
        if (buf[1] != 0) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message: %u != 0",
                                      (unsigned) buf[1]));
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        /* We don't support fragmentation of ClientHello (yet?) */
        if (msg_len != mbedtls_ssl_hs_hdr_len(ssl) + handshake_len) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message: %u != %u + %u",
                                      (unsigned) msg_len,
                                      (unsigned) mbedtls_ssl_hs_hdr_len(ssl),
                                      (unsigned) handshake_len));
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }
    }

#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
        /*
         * Copy the client's handshake message_seq on initial handshakes,
         * check sequence number on renego.
         */
#if defined(MBEDTLS_SSL_RENEGOTIATION)
        if (ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS) {
            /* This couldn't be done in ssl_prepare_handshake_record() */
            unsigned int cli_msg_seq = (ssl->in_msg[4] << 8) |
                                       ssl->in_msg[5];

            if (cli_msg_seq != ssl->handshake->in_msg_seq) {
                MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message_seq: "
                                          "%u (expected %u)", cli_msg_seq,
                                          ssl->handshake->in_msg_seq));
                return MBEDTLS_ERR_SSL_DECODE_ERROR;
            }

            ssl->handshake->in_msg_seq++;
        } else
#endif
        {
            unsigned int cli_msg_seq = (ssl->in_msg[4] << 8) |
                                       ssl->in_msg[5];
            ssl->handshake->out_msg_seq = cli_msg_seq;
            ssl->handshake->in_msg_seq  = cli_msg_seq + 1;
        }
        {
            /*
             * For now we don't support fragmentation, so make sure
             * fragment_offset == 0 and fragment_length == length
             */
            size_t fragment_offset, fragment_length, length;
            fragment_offset = MBEDTLS_GET_UINT24_BE(ssl->in_msg, 6);
            fragment_length = MBEDTLS_GET_UINT24_BE(ssl->in_msg, 9);
            length = MBEDTLS_GET_UINT24_BE(ssl->in_msg, 1);
            MBEDTLS_SSL_DEBUG_MSG(
                4, ("fragment_offset=%u fragment_length=%u length=%u",
                    (unsigned) fragment_offset, (unsigned) fragment_length,
                    (unsigned) length));
            if (fragment_offset != 0 || length != fragment_length) {
                MBEDTLS_SSL_DEBUG_MSG(1, ("ClientHello fragmentation not supported"));
                return MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
            }
        }
    }
#endif /* MBEDTLS_SSL_PROTO_DTLS */

    buf += mbedtls_ssl_hs_hdr_len(ssl);
    msg_len -= mbedtls_ssl_hs_hdr_len(ssl);

    /*
     * ClientHello layer:
     *     0  .   1   protocol version
     *     2  .  33   random bytes (starting with 4 bytes of Unix time)
     *    34  .  35   session id length (1 byte)
     *    35  . 34+x  session id
     *   35+x . 35+x  DTLS only: cookie length (1 byte)
     *   36+x .  ..   DTLS only: cookie
     *    ..  .  ..   ciphersuite list length (2 bytes)
     *    ..  .  ..   ciphersuite list
     *    ..  .  ..   compression alg. list length (1 byte)
     *    ..  .  ..   compression alg. list
     *    ..  .  ..   extensions length (2 bytes, optional)
     *    ..  .  ..   extensions (optional)
     */

    /*
     * Minimal length (with everything empty and extensions omitted) is
     * 2 + 32 + 1 + 2 + 1 = 38 bytes. Check that first, so that we can
     * read at least up to session id length without worrying.
     */
    if (msg_len < 38) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    /*
     * Check and save the protocol version
     */
    MBEDTLS_SSL_DEBUG_BUF(3, "client hello, version", buf, 2);

    ssl->tls_version = mbedtls_ssl_read_version(buf, ssl->conf->transport);
    ssl->session_negotiate->tls_version = ssl->tls_version;

    if (ssl->tls_version != MBEDTLS_SSL_VERSION_TLS1_2) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("server only supports TLS 1.2"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_PROTOCOL_VERSION);
        return MBEDTLS_ERR_SSL_BAD_PROTOCOL_VERSION;
    }

    /*
     * Save client random (inc. Unix time)
     */
    MBEDTLS_SSL_DEBUG_BUF(3, "client hello, random bytes", buf + 2, 32);

    memcpy(ssl->handshake->randbytes, buf + 2, 32);

    /*
     * Check the session ID length and save session ID
     */
    sess_len = buf[34];

    if (sess_len > sizeof(ssl->session_negotiate->id) ||
        sess_len + 34 + 2 > msg_len) { /* 2 for cipherlist length field */
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    MBEDTLS_SSL_DEBUG_BUF(3, "client hello, session id", buf + 35, sess_len);

    ssl->session_negotiate->id_len = sess_len;
    memset(ssl->session_negotiate->id, 0,
           sizeof(ssl->session_negotiate->id));
    memcpy(ssl->session_negotiate->id, buf + 35,
           ssl->session_negotiate->id_len);

    /*
     * Check the cookie length and content
     */
#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
        cookie_offset = 35 + sess_len;
        cookie_len = buf[cookie_offset];

        if (cookie_offset + 1 + cookie_len + 2 > msg_len) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
            mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                           MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        MBEDTLS_SSL_DEBUG_BUF(3, "client hello, cookie",
                              buf + cookie_offset + 1, cookie_len);

#if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY)
        if (ssl->conf->f_cookie_check != NULL
#if defined(MBEDTLS_SSL_RENEGOTIATION)
            && ssl->renego_status == MBEDTLS_SSL_INITIAL_HANDSHAKE
#endif
            ) {
            if (ssl->conf->f_cookie_check(ssl->conf->p_cookie,
                                          buf + cookie_offset + 1, cookie_len,
                                          ssl->cli_id, ssl->cli_id_len) != 0) {
                MBEDTLS_SSL_DEBUG_MSG(2, ("cookie verification failed"));
                ssl->handshake->cookie_verify_result = 1;
            } else {
                MBEDTLS_SSL_DEBUG_MSG(2, ("cookie verification passed"));
                ssl->handshake->cookie_verify_result = 0;
            }
        } else
#endif /* MBEDTLS_SSL_DTLS_HELLO_VERIFY */
        {
            /* We know we didn't send a cookie, so it should be empty */
            if (cookie_len != 0) {
                /* This may be an attacker's probe, so don't send an alert */
                MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
                return MBEDTLS_ERR_SSL_DECODE_ERROR;
            }

            MBEDTLS_SSL_DEBUG_MSG(2, ("cookie verification skipped"));
        }

        /*
         * Check the ciphersuitelist length (will be parsed later)
         */
        ciph_offset = cookie_offset + 1 + cookie_len;
    } else
#endif /* MBEDTLS_SSL_PROTO_DTLS */
    ciph_offset = 35 + sess_len;

    ciph_len = (buf[ciph_offset + 0] << 8)
               | (buf[ciph_offset + 1]);

    if (ciph_len < 2 ||
        ciph_len + 2 + ciph_offset + 1 > msg_len || /* 1 for comp. alg. len */
        (ciph_len % 2) != 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    MBEDTLS_SSL_DEBUG_BUF(3, "client hello, ciphersuitelist",
                          buf + ciph_offset + 2,  ciph_len);

    /*
     * Check the compression algorithm's length.
     * The list contents are ignored because implementing
     * MBEDTLS_SSL_COMPRESS_NULL is mandatory and is the only
     * option supported by Mbed TLS.
     */
    comp_offset = ciph_offset + 2 + ciph_len;

    comp_len = buf[comp_offset];

    if (comp_len < 1 ||
        comp_len > 16 ||
        comp_len + comp_offset + 1 > msg_len) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    MBEDTLS_SSL_DEBUG_BUF(3, "client hello, compression",
                          buf + comp_offset + 1, comp_len);

    /*
     * Check the extension length
     */
    ext_offset = comp_offset + 1 + comp_len;
    if (msg_len > ext_offset) {
        if (msg_len < ext_offset + 2) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
            mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                           MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        ext_len = (buf[ext_offset + 0] << 8)
                  | (buf[ext_offset + 1]);

        if (msg_len != ext_offset + 2 + ext_len) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
            mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                           MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }
    } else {
        ext_len = 0;
    }

    ext = buf + ext_offset + 2;
    MBEDTLS_SSL_DEBUG_BUF(3, "client hello extensions", ext, ext_len);

    while (ext_len != 0) {
        unsigned int ext_id;
        unsigned int ext_size;
        if (ext_len < 4) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
            mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                           MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }
        ext_id   = ((ext[0] <<  8) | (ext[1]));
        ext_size = ((ext[2] <<  8) | (ext[3]));

        if (ext_size + 4 > ext_len) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client hello message"));
            mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                           MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }
        switch (ext_id) {
#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
            case MBEDTLS_TLS_EXT_SERVERNAME:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found ServerName extension"));
                ret = mbedtls_ssl_parse_server_name_ext(ssl, ext + 4,
                                                        ext + 4 + ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_SERVER_NAME_INDICATION */

            case MBEDTLS_TLS_EXT_RENEGOTIATION_INFO:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found renegotiation extension"));
#if defined(MBEDTLS_SSL_RENEGOTIATION)
                renegotiation_info_seen = 1;
#endif

                ret = ssl_parse_renegotiation_info(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
            case MBEDTLS_TLS_EXT_SIG_ALG:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found signature_algorithms extension"));

                ret = mbedtls_ssl_parse_sig_alg_ext(ssl, ext + 4, ext + 4 + ext_size);
                if (ret != 0) {
                    return ret;
                }

                sig_hash_alg_ext_present = 1;
                break;
#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */

#if defined(MBEDTLS_PK_CAN_ECDH) || defined(MBEDTLS_PK_CAN_ECDSA_SOME) || \
                defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
            case MBEDTLS_TLS_EXT_SUPPORTED_GROUPS:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found supported elliptic curves extension"));

                ret = ssl_parse_supported_groups_ext(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;

            case MBEDTLS_TLS_EXT_SUPPORTED_POINT_FORMATS:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found supported point formats extension"));
                ssl->handshake->cli_exts |= MBEDTLS_TLS_EXT_SUPPORTED_POINT_FORMATS_PRESENT;

                ret = ssl_parse_supported_point_formats(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_PK_CAN_ECDH || MBEDTLS_PK_CAN_ECDSA_SOME ||
          MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
            case MBEDTLS_TLS_EXT_ECJPAKE_KKPP:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found ecjpake kkpp extension"));

                ret = ssl_parse_ecjpake_kkpp(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
            case MBEDTLS_TLS_EXT_MAX_FRAGMENT_LENGTH:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found max fragment length extension"));

                ret = ssl_parse_max_fragment_length_ext(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */

#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
            case MBEDTLS_TLS_EXT_CID:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found CID extension"));

                ret = ssl_parse_cid_ext(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */

#if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
            case MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found encrypt then mac extension"));

                ret = ssl_parse_encrypt_then_mac_ext(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */

#if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
            case MBEDTLS_TLS_EXT_EXTENDED_MASTER_SECRET:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found extended master secret extension"));

                ret = ssl_parse_extended_ms_ext(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_EXTENDED_MASTER_SECRET */

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
            case MBEDTLS_TLS_EXT_SESSION_TICKET:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found session ticket extension"));

                ret = ssl_parse_session_ticket_ext(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_SESSION_TICKETS */

#if defined(MBEDTLS_SSL_ALPN)
            case MBEDTLS_TLS_EXT_ALPN:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found alpn extension"));

                ret = mbedtls_ssl_parse_alpn_ext(ssl, ext + 4,
                                                 ext + 4 + ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_SESSION_TICKETS */

#if defined(MBEDTLS_SSL_DTLS_SRTP)
            case MBEDTLS_TLS_EXT_USE_SRTP:
                MBEDTLS_SSL_DEBUG_MSG(3, ("found use_srtp extension"));

                ret = ssl_parse_use_srtp_ext(ssl, ext + 4, ext_size);
                if (ret != 0) {
                    return ret;
                }
                break;
#endif /* MBEDTLS_SSL_DTLS_SRTP */

            default:
                MBEDTLS_SSL_DEBUG_MSG(3, ("unknown extension found: %u (ignoring)",
                                          ext_id));
        }

        ext_len -= 4 + ext_size;
        ext += 4 + ext_size;
    }

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)

    /*
     * Try to fall back to default hash SHA1 if the client
     * hasn't provided any preferred signature-hash combinations.
     */
    if (!sig_hash_alg_ext_present) {
        uint16_t *received_sig_algs = ssl->handshake->received_sig_algs;
        const uint16_t default_sig_algs[] = {
#if defined(MBEDTLS_PK_CAN_ECDSA_SOME)
            MBEDTLS_SSL_TLS12_SIG_AND_HASH_ALG(MBEDTLS_SSL_SIG_ECDSA,
                                               MBEDTLS_SSL_HASH_SHA1),
#endif
#if defined(MBEDTLS_RSA_C)
            MBEDTLS_SSL_TLS12_SIG_AND_HASH_ALG(MBEDTLS_SSL_SIG_RSA,
                                               MBEDTLS_SSL_HASH_SHA1),
#endif
            MBEDTLS_TLS_SIG_NONE
        };

        MBEDTLS_STATIC_ASSERT(sizeof(default_sig_algs) / sizeof(default_sig_algs[0])
                              <= MBEDTLS_RECEIVED_SIG_ALGS_SIZE,
                              "default_sig_algs is too big");

        memcpy(received_sig_algs, default_sig_algs, sizeof(default_sig_algs));
    }

#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */

    /*
     * Check for TLS_EMPTY_RENEGOTIATION_INFO_SCSV
     */
    for (i = 0, p = buf + ciph_offset + 2; i < ciph_len; i += 2, p += 2) {
        if (p[0] == 0 && p[1] == MBEDTLS_SSL_EMPTY_RENEGOTIATION_INFO) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("received TLS_EMPTY_RENEGOTIATION_INFO "));
#if defined(MBEDTLS_SSL_RENEGOTIATION)
            if (ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS) {
                MBEDTLS_SSL_DEBUG_MSG(1, ("received RENEGOTIATION SCSV "
                                          "during renegotiation"));
                mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                               MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE);
                return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
            }
#endif
            ssl->secure_renegotiation = MBEDTLS_SSL_SECURE_RENEGOTIATION;
            break;
        }
    }

    /*
     * Renegotiation security checks
     */
    if (ssl->secure_renegotiation != MBEDTLS_SSL_SECURE_RENEGOTIATION &&
        ssl->conf->allow_legacy_renegotiation == MBEDTLS_SSL_LEGACY_BREAK_HANDSHAKE) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("legacy renegotiation, breaking off handshake"));
        handshake_failure = 1;
    }
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    else if (ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS &&
             ssl->secure_renegotiation == MBEDTLS_SSL_SECURE_RENEGOTIATION &&
             renegotiation_info_seen == 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("renegotiation_info extension missing (secure)"));
        handshake_failure = 1;
    } else if (ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS &&
               ssl->secure_renegotiation == MBEDTLS_SSL_LEGACY_RENEGOTIATION &&
               ssl->conf->allow_legacy_renegotiation == MBEDTLS_SSL_LEGACY_NO_RENEGOTIATION) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("legacy renegotiation not allowed"));
        handshake_failure = 1;
    } else if (ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS &&
               ssl->secure_renegotiation == MBEDTLS_SSL_LEGACY_RENEGOTIATION &&
               renegotiation_info_seen == 1) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("renegotiation_info extension present (legacy)"));
        handshake_failure = 1;
    }
#endif /* MBEDTLS_SSL_RENEGOTIATION */

    if (handshake_failure == 1) {
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE);
        return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
    }

    /*
     * Server certification selection (after processing TLS extensions)
     */
    if (ssl->conf->f_cert_cb && (ret = ssl->conf->f_cert_cb(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "f_cert_cb", ret);
        return ret;
    }
#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
    ssl->handshake->sni_name = NULL;
    ssl->handshake->sni_name_len = 0;
#endif

    /*
     * Search for a matching ciphersuite
     * (At the end because we need information from the EC-based extensions
     * and certificate from the SNI callback triggered by the SNI extension
     * or certificate from server certificate selection callback.)
     */
    got_common_suite = 0;
    ciphersuites = ssl->conf->ciphersuite_list;
    ciphersuite_info = NULL;

    if (ssl->conf->respect_cli_pref == MBEDTLS_SSL_SRV_CIPHERSUITE_ORDER_CLIENT) {
        for (j = 0, p = buf + ciph_offset + 2; j < ciph_len; j += 2, p += 2) {
            for (i = 0; ciphersuites[i] != 0; i++) {
                if (MBEDTLS_GET_UINT16_BE(p, 0) != ciphersuites[i]) {
                    continue;
                }

                got_common_suite = 1;

                if ((ret = ssl_ciphersuite_match(ssl, ciphersuites[i],
                                                 &ciphersuite_info)) != 0) {
                    return ret;
                }

                if (ciphersuite_info != NULL) {
                    goto have_ciphersuite;
                }
            }
        }
    } else {
        for (i = 0; ciphersuites[i] != 0; i++) {
            for (j = 0, p = buf + ciph_offset + 2; j < ciph_len; j += 2, p += 2) {
                if (MBEDTLS_GET_UINT16_BE(p, 0) != ciphersuites[i]) {
                    continue;
                }

                got_common_suite = 1;

                if ((ret = ssl_ciphersuite_match(ssl, ciphersuites[i],
                                                 &ciphersuite_info)) != 0) {
                    return ret;
                }

                if (ciphersuite_info != NULL) {
                    goto have_ciphersuite;
                }
            }
        }
    }

    if (got_common_suite) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("got ciphersuites in common, "
                                  "but none of them usable"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE);
        return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
    } else {
        MBEDTLS_SSL_DEBUG_MSG(1, ("got no ciphersuites in common"));
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE);
        return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
    }

have_ciphersuite:
    MBEDTLS_SSL_DEBUG_MSG(2, ("selected ciphersuite: %s", ciphersuite_info->name));

    ssl->session_negotiate->ciphersuite = ciphersuites[i];
    ssl->handshake->ciphersuite_info = ciphersuite_info;

    ssl->state++;

#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
        mbedtls_ssl_recv_flight_completed(ssl);
    }
#endif

    /* Debugging-only output for testsuite */
#if defined(MBEDTLS_DEBUG_C)                         && \
    defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
    mbedtls_pk_type_t sig_alg = mbedtls_ssl_get_ciphersuite_sig_alg(ciphersuite_info);
    if (sig_alg != MBEDTLS_PK_NONE) {
        unsigned int sig_hash = mbedtls_ssl_tls12_get_preferred_hash_for_sig_alg(
            ssl, mbedtls_ssl_sig_from_pk_alg(sig_alg));
        MBEDTLS_SSL_DEBUG_MSG(3, ("client hello v3, signature_algorithm ext: %u",
                                  sig_hash));
    } else {
        MBEDTLS_SSL_DEBUG_MSG(3, ("no hash algorithm for signature algorithm "
                                  "%u - should not happen", (unsigned) sig_alg));
    }
#endif

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= parse client hello"));

    return 0;
}

#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
static void ssl_write_cid_ext(mbedtls_ssl_context *ssl,
                              unsigned char *buf,
                              size_t *olen)
{
    unsigned char *p = buf;
    size_t ext_len;
    const unsigned char *end = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN;

    *olen = 0;

    /* Skip writing the extension if we don't want to use it or if
     * the client hasn't offered it. */
    if (ssl->handshake->cid_in_use == MBEDTLS_SSL_CID_DISABLED) {
        return;
    }

    /* ssl->own_cid_len is at most MBEDTLS_SSL_CID_IN_LEN_MAX
     * which is at most 255, so the increment cannot overflow. */
    if (end < p || (size_t) (end - p) < (unsigned) (ssl->own_cid_len + 5)) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("buffer too small"));
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, adding CID extension"));

    /*
     *   struct {
     *      opaque cid<0..2^8-1>;
     *   } ConnectionId;
     */
    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_CID, p, 0);
    p += 2;
    ext_len = (size_t) ssl->own_cid_len + 1;
    MBEDTLS_PUT_UINT16_BE(ext_len, p, 0);
    p += 2;

    *p++ = (uint8_t) ssl->own_cid_len;
    memcpy(p, ssl->own_cid, ssl->own_cid_len);

    *olen = ssl->own_cid_len + 5;
}
#endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */

#if defined(MBEDTLS_SSL_SOME_SUITES_USE_CBC_ETM)
static void ssl_write_encrypt_then_mac_ext(mbedtls_ssl_context *ssl,
                                           unsigned char *buf,
                                           size_t *olen)
{
    unsigned char *p = buf;
    const mbedtls_ssl_ciphersuite_t *suite = NULL;

    /*
     * RFC 7366: "If a server receives an encrypt-then-MAC request extension
     * from a client and then selects a stream or Authenticated Encryption
     * with Associated Data (AEAD) ciphersuite, it MUST NOT send an
     * encrypt-then-MAC response extension back to the client."
     */
    suite = mbedtls_ssl_ciphersuite_from_id(
        ssl->session_negotiate->ciphersuite);
    if (suite == NULL) {
        ssl->session_negotiate->encrypt_then_mac = MBEDTLS_SSL_ETM_DISABLED;
    } else {
        mbedtls_ssl_mode_t ssl_mode =
            mbedtls_ssl_get_mode_from_ciphersuite(
                ssl->session_negotiate->encrypt_then_mac,
                suite);

        if (ssl_mode != MBEDTLS_SSL_MODE_CBC_ETM) {
            ssl->session_negotiate->encrypt_then_mac = MBEDTLS_SSL_ETM_DISABLED;
        }
    }

    if (ssl->session_negotiate->encrypt_then_mac == MBEDTLS_SSL_ETM_DISABLED) {
        *olen = 0;
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, adding encrypt then mac extension"));

    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC, p, 0);
    p += 2;

    *p++ = 0x00;
    *p++ = 0x00;

    *olen = 4;
}
#endif /* MBEDTLS_SSL_SOME_SUITES_USE_CBC_ETM */

#if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
static void ssl_write_extended_ms_ext(mbedtls_ssl_context *ssl,
                                      unsigned char *buf,
                                      size_t *olen)
{
    unsigned char *p = buf;

    if (ssl->handshake->extended_ms == MBEDTLS_SSL_EXTENDED_MS_DISABLED) {
        *olen = 0;
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, adding extended master secret "
                              "extension"));

    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_EXTENDED_MASTER_SECRET, p, 0);
    p += 2;

    *p++ = 0x00;
    *p++ = 0x00;

    *olen = 4;
}
#endif /* MBEDTLS_SSL_EXTENDED_MASTER_SECRET */

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
static void ssl_write_session_ticket_ext(mbedtls_ssl_context *ssl,
                                         unsigned char *buf,
                                         size_t *olen)
{
    unsigned char *p = buf;

    if (ssl->handshake->new_session_ticket == 0) {
        *olen = 0;
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, adding session ticket extension"));

    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_SESSION_TICKET, p, 0);
    p += 2;

    *p++ = 0x00;
    *p++ = 0x00;

    *olen = 4;
}
#endif /* MBEDTLS_SSL_SESSION_TICKETS */

static void ssl_write_renegotiation_ext(mbedtls_ssl_context *ssl,
                                        unsigned char *buf,
                                        size_t *olen)
{
    unsigned char *p = buf;

    if (ssl->secure_renegotiation != MBEDTLS_SSL_SECURE_RENEGOTIATION) {
        *olen = 0;
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, secure renegotiation extension"));

    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_RENEGOTIATION_INFO, p, 0);
    p += 2;

#if defined(MBEDTLS_SSL_RENEGOTIATION)
    if (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE) {
        *p++ = 0x00;
        *p++ = (ssl->verify_data_len * 2 + 1) & 0xFF;
        *p++ = ssl->verify_data_len * 2 & 0xFF;

        memcpy(p, ssl->peer_verify_data, ssl->verify_data_len);
        p += ssl->verify_data_len;
        memcpy(p, ssl->own_verify_data, ssl->verify_data_len);
        p += ssl->verify_data_len;
    } else
#endif /* MBEDTLS_SSL_RENEGOTIATION */
    {
        *p++ = 0x00;
        *p++ = 0x01;
        *p++ = 0x00;
    }

    *olen = p - buf;
}

#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
static void ssl_write_max_fragment_length_ext(mbedtls_ssl_context *ssl,
                                              unsigned char *buf,
                                              size_t *olen)
{
    unsigned char *p = buf;

    if (ssl->session_negotiate->mfl_code == MBEDTLS_SSL_MAX_FRAG_LEN_NONE) {
        *olen = 0;
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, max_fragment_length extension"));

    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_MAX_FRAGMENT_LENGTH, p, 0);
    p += 2;

    *p++ = 0x00;
    *p++ = 1;

    *p++ = ssl->session_negotiate->mfl_code;

    *olen = 5;
}
#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */

#if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
static void ssl_write_supported_point_formats_ext(mbedtls_ssl_context *ssl,
                                                  unsigned char *buf,
                                                  size_t *olen)
{
    unsigned char *p = buf;
    ((void) ssl);

    if ((ssl->handshake->cli_exts &
         MBEDTLS_TLS_EXT_SUPPORTED_POINT_FORMATS_PRESENT) == 0) {
        *olen = 0;
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, supported_point_formats extension"));

    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_SUPPORTED_POINT_FORMATS, p, 0);
    p += 2;

    *p++ = 0x00;
    *p++ = 2;

    *p++ = 1;
    *p++ = MBEDTLS_ECP_PF_UNCOMPRESSED;

    *olen = 6;
}
#endif /* MBEDTLS_ECDH_C || MBEDTLS_ECDSA_C || MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
static void ssl_write_ecjpake_kkpp_ext(mbedtls_ssl_context *ssl,
                                       unsigned char *buf,
                                       size_t *olen)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char *p = buf;
    const unsigned char *end = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN;
    size_t kkpp_len;

    *olen = 0;

    /* Skip costly computation if not needed */
    if (ssl->handshake->ciphersuite_info->key_exchange !=
        MBEDTLS_KEY_EXCHANGE_ECJPAKE) {
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, ecjpake kkpp extension"));

    if (end - p < 4) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("buffer too small"));
        return;
    }

    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_ECJPAKE_KKPP, p, 0);
    p += 2;

#if defined(MBEDTLS_USE_PSA_CRYPTO)
    ret = mbedtls_psa_ecjpake_write_round(&ssl->handshake->psa_pake_ctx,
                                          p + 2, end - p - 2, &kkpp_len,
                                          MBEDTLS_ECJPAKE_ROUND_ONE);
    if (ret != 0) {
        psa_destroy_key(ssl->handshake->psa_pake_password);
        psa_pake_abort(&ssl->handshake->psa_pake_ctx);
        MBEDTLS_SSL_DEBUG_RET(1, "psa_pake_output", ret);
        return;
    }
#else
    ret = mbedtls_ecjpake_write_round_one(&ssl->handshake->ecjpake_ctx,
                                          p + 2, end - p - 2, &kkpp_len,
                                          ssl->conf->f_rng, ssl->conf->p_rng);
    if (ret != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecjpake_write_round_one", ret);
        return;
    }
#endif /* MBEDTLS_USE_PSA_CRYPTO */

    MBEDTLS_PUT_UINT16_BE(kkpp_len, p, 0);
    p += 2;

    *olen = kkpp_len + 4;
}
#endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

#if defined(MBEDTLS_SSL_DTLS_SRTP) && defined(MBEDTLS_SSL_PROTO_DTLS)
static void ssl_write_use_srtp_ext(mbedtls_ssl_context *ssl,
                                   unsigned char *buf,
                                   size_t *olen)
{
    size_t mki_len = 0, ext_len = 0;
    uint16_t profile_value = 0;
    const unsigned char *end = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN;

    *olen = 0;

    if ((ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM) ||
        (ssl->dtls_srtp_info.chosen_dtls_srtp_profile == MBEDTLS_TLS_SRTP_UNSET)) {
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, adding use_srtp extension"));

    if (ssl->conf->dtls_srtp_mki_support == MBEDTLS_SSL_DTLS_SRTP_MKI_SUPPORTED) {
        mki_len = ssl->dtls_srtp_info.mki_len;
    }

    /* The extension total size is 9 bytes :
     * - 2 bytes for the extension tag
     * - 2 bytes for the total size
     * - 2 bytes for the protection profile length
     * - 2 bytes for the protection profile
     * - 1 byte for the mki length
     * +  the actual mki length
     * Check we have enough room in the output buffer */
    if ((size_t) (end - buf) < mki_len + 9) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("buffer too small"));
        return;
    }

    /* extension */
    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_USE_SRTP, buf, 0);
    /*
     * total length 5 and mki value: only one profile(2 bytes)
     *              and length(2 bytes) and srtp_mki  )
     */
    ext_len = 5 + mki_len;
    MBEDTLS_PUT_UINT16_BE(ext_len, buf, 2);

    /* protection profile length: 2 */
    buf[4] = 0x00;
    buf[5] = 0x02;
    profile_value = mbedtls_ssl_check_srtp_profile_value(
        ssl->dtls_srtp_info.chosen_dtls_srtp_profile);
    if (profile_value != MBEDTLS_TLS_SRTP_UNSET) {
        MBEDTLS_PUT_UINT16_BE(profile_value, buf, 6);
    } else {
        MBEDTLS_SSL_DEBUG_MSG(1, ("use_srtp extension invalid profile"));
        return;
    }

    buf[8] = mki_len & 0xFF;
    memcpy(&buf[9], ssl->dtls_srtp_info.mki_value, mki_len);

    *olen = 9 + mki_len;
}
#endif /* MBEDTLS_SSL_DTLS_SRTP */

#if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_hello_verify_request(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char *p = ssl->out_msg + 4;
    unsigned char *cookie_len_byte;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> write hello verify request"));

    /*
     * struct {
     *   ProtocolVersion server_version;
     *   opaque cookie<0..2^8-1>;
     * } HelloVerifyRequest;
     */

    /* The RFC is not clear on this point, but sending the actual negotiated
     * version looks like the most interoperable thing to do. */
    mbedtls_ssl_write_version(p, ssl->conf->transport, ssl->tls_version);
    MBEDTLS_SSL_DEBUG_BUF(3, "server version", p, 2);
    p += 2;

    /* If we get here, f_cookie_check is not null */
    if (ssl->conf->f_cookie_write == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("inconsistent cookie callbacks"));
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }

    /* Skip length byte until we know the length */
    cookie_len_byte = p++;

    if ((ret = ssl->conf->f_cookie_write(ssl->conf->p_cookie,
                                         &p, ssl->out_buf + MBEDTLS_SSL_OUT_BUFFER_LEN,
                                         ssl->cli_id, ssl->cli_id_len)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "f_cookie_write", ret);
        return ret;
    }

    *cookie_len_byte = (unsigned char) (p - (cookie_len_byte + 1));

    MBEDTLS_SSL_DEBUG_BUF(3, "cookie sent", cookie_len_byte + 1, *cookie_len_byte);

    ssl->out_msglen  = p - ssl->out_msg;
    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
    ssl->out_msg[0]  = MBEDTLS_SSL_HS_HELLO_VERIFY_REQUEST;

    ssl->state = MBEDTLS_SSL_SERVER_HELLO_VERIFY_REQUEST_SENT;

    if ((ret = mbedtls_ssl_write_handshake_msg(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_write_handshake_msg", ret);
        return ret;
    }

#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM &&
        (ret = mbedtls_ssl_flight_transmit(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_flight_transmit", ret);
        return ret;
    }
#endif /* MBEDTLS_SSL_PROTO_DTLS */

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= write hello verify request"));

    return 0;
}
#endif /* MBEDTLS_SSL_DTLS_HELLO_VERIFY */

static void ssl_handle_id_based_session_resumption(mbedtls_ssl_context *ssl)
{
    int ret;
    mbedtls_ssl_session session_tmp;
    mbedtls_ssl_session * const session = ssl->session_negotiate;

    /* Resume is 0  by default, see ssl_handshake_init().
     * It may be already set to 1 by ssl_parse_session_ticket_ext(). */
    if (ssl->handshake->resume == 1) {
        return;
    }
    if (session->id_len == 0) {
        return;
    }
    if (ssl->conf->f_get_cache == NULL) {
        return;
    }
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    if (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE) {
        return;
    }
#endif

    mbedtls_ssl_session_init(&session_tmp);

    ret = ssl->conf->f_get_cache(ssl->conf->p_cache,
                                 session->id,
                                 session->id_len,
                                 &session_tmp);
    if (ret != 0) {
        goto exit;
    }

    if (session->ciphersuite != session_tmp.ciphersuite) {
        /* Mismatch between cached and negotiated session */
        goto exit;
    }

    /* Move semantics */
    mbedtls_ssl_session_free(session);
    *session = session_tmp;
    memset(&session_tmp, 0, sizeof(session_tmp));

    MBEDTLS_SSL_DEBUG_MSG(3, ("session successfully restored from cache"));
    ssl->handshake->resume = 1;

exit:

    mbedtls_ssl_session_free(&session_tmp);
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_server_hello(mbedtls_ssl_context *ssl)
{
#if defined(MBEDTLS_HAVE_TIME)
    mbedtls_time_t t;
#endif
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    size_t olen, ext_len = 0, n;
    unsigned char *buf, *p;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> write server hello"));

#if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM &&
        ssl->handshake->cookie_verify_result != 0) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("client hello was not authenticated"));
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= write server hello"));

        return ssl_write_hello_verify_request(ssl);
    }
#endif /* MBEDTLS_SSL_DTLS_HELLO_VERIFY */

    if (ssl->conf->f_rng == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("no RNG provided"));
        return MBEDTLS_ERR_SSL_NO_RNG;
    }

    /*
     *     0  .   0   handshake type
     *     1  .   3   handshake length
     *     4  .   5   protocol version
     *     6  .   9   UNIX time()
     *    10  .  37   random bytes
     */
    buf = ssl->out_msg;
    p = buf + 4;

    mbedtls_ssl_write_version(p, ssl->conf->transport, ssl->tls_version);
    p += 2;

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, chosen version: [%d:%d]",
                              buf[4], buf[5]));

#if defined(MBEDTLS_HAVE_TIME)
    t = mbedtls_time(NULL);
    MBEDTLS_PUT_UINT32_BE(t, p, 0);
    p += 4;

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, current time: %" MBEDTLS_PRINTF_LONGLONG,
                              (long long) t));
#else
    if ((ret = ssl->conf->f_rng(ssl->conf->p_rng, p, 4)) != 0) {
        return ret;
    }

    p += 4;
#endif /* MBEDTLS_HAVE_TIME */

    if ((ret = ssl->conf->f_rng(ssl->conf->p_rng, p, 28)) != 0) {
        return ret;
    }

    p += 28;

    memcpy(ssl->handshake->randbytes + 32, buf + 6, 32);

    MBEDTLS_SSL_DEBUG_BUF(3, "server hello, random bytes", buf + 6, 32);

    ssl_handle_id_based_session_resumption(ssl);

    if (ssl->handshake->resume == 0) {
        /*
         * New session, create a new session id,
         * unless we're about to issue a session ticket
         */
        ssl->state++;

#if defined(MBEDTLS_HAVE_TIME)
        ssl->session_negotiate->start = mbedtls_time(NULL);
#endif

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
        if (ssl->handshake->new_session_ticket != 0) {
            ssl->session_negotiate->id_len = n = 0;
            memset(ssl->session_negotiate->id, 0, 32);
        } else
#endif /* MBEDTLS_SSL_SESSION_TICKETS */
        {
            ssl->session_negotiate->id_len = n = 32;
            if ((ret = ssl->conf->f_rng(ssl->conf->p_rng, ssl->session_negotiate->id,
                                        n)) != 0) {
                return ret;
            }
        }
    } else {
        /*
         * Resuming a session
         */
        n = ssl->session_negotiate->id_len;
        ssl->state = MBEDTLS_SSL_SERVER_CHANGE_CIPHER_SPEC;

        if ((ret = mbedtls_ssl_derive_keys(ssl)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_derive_keys", ret);
            return ret;
        }
    }

    /*
     *    38  .  38     session id length
     *    39  . 38+n    session id
     *   39+n . 40+n    chosen ciphersuite
     *   41+n . 41+n    chosen compression alg.
     *   42+n . 43+n    extensions length
     *   44+n . 43+n+m  extensions
     */
    *p++ = (unsigned char) ssl->session_negotiate->id_len;
    memcpy(p, ssl->session_negotiate->id, ssl->session_negotiate->id_len);
    p += ssl->session_negotiate->id_len;

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, session id len.: %" MBEDTLS_PRINTF_SIZET, n));
    MBEDTLS_SSL_DEBUG_BUF(3,   "server hello, session id", buf + 39, n);
    MBEDTLS_SSL_DEBUG_MSG(3, ("%s session has been resumed",
                              ssl->handshake->resume ? "a" : "no"));

    MBEDTLS_PUT_UINT16_BE(ssl->session_negotiate->ciphersuite, p, 0);
    p += 2;
    *p++ = MBEDTLS_BYTE_0(MBEDTLS_SSL_COMPRESS_NULL);

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, chosen ciphersuite: %s",
                              mbedtls_ssl_get_ciphersuite_name(ssl->session_negotiate->ciphersuite)));
    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, compress alg.: 0x%02X",
                              (unsigned int) MBEDTLS_SSL_COMPRESS_NULL));

    /*
     *  First write extensions, then the total length
     */
    ssl_write_renegotiation_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;

#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
    ssl_write_max_fragment_length_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;
#endif

#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
    ssl_write_cid_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;
#endif

#if defined(MBEDTLS_SSL_SOME_SUITES_USE_CBC_ETM)
    ssl_write_encrypt_then_mac_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;
#endif

#if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
    ssl_write_extended_ms_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;
#endif

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
    ssl_write_session_ticket_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;
#endif

#if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
    const mbedtls_ssl_ciphersuite_t *suite =
        mbedtls_ssl_ciphersuite_from_id(ssl->session_negotiate->ciphersuite);
    if (suite != NULL && mbedtls_ssl_ciphersuite_uses_ec(suite)) {
        ssl_write_supported_point_formats_ext(ssl, p + 2 + ext_len, &olen);
        ext_len += olen;
    }
#endif

#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
    ssl_write_ecjpake_kkpp_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;
#endif

#if defined(MBEDTLS_SSL_ALPN)
    unsigned char *end = buf + MBEDTLS_SSL_OUT_CONTENT_LEN - 4;
    if ((ret = mbedtls_ssl_write_alpn_ext(ssl, p + 2 + ext_len, end, &olen))
        != 0) {
        return ret;
    }

    ext_len += olen;
#endif

#if defined(MBEDTLS_SSL_DTLS_SRTP)
    ssl_write_use_srtp_ext(ssl, p + 2 + ext_len, &olen);
    ext_len += olen;
#endif

    MBEDTLS_SSL_DEBUG_MSG(3, ("server hello, total extension length: %" MBEDTLS_PRINTF_SIZET,
                              ext_len));

    if (ext_len > 0) {
        MBEDTLS_PUT_UINT16_BE(ext_len, p, 0);
        p += 2 + ext_len;
    }

    ssl->out_msglen  = p - buf;
    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
    ssl->out_msg[0]  = MBEDTLS_SSL_HS_SERVER_HELLO;

    ret = mbedtls_ssl_write_handshake_msg(ssl);

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= write server hello"));

    return ret;
}

#if !defined(MBEDTLS_KEY_EXCHANGE_CERT_REQ_ALLOWED_ENABLED)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_certificate_request(mbedtls_ssl_context *ssl)
{
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->handshake->ciphersuite_info;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> write certificate request"));

    if (!mbedtls_ssl_ciphersuite_cert_req_allowed(ciphersuite_info)) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip write certificate request"));
        ssl->state++;
        return 0;
    }

    MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
    return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
}
#else /* !MBEDTLS_KEY_EXCHANGE_CERT_REQ_ALLOWED_ENABLED */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_certificate_request(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->handshake->ciphersuite_info;
    uint16_t dn_size, total_dn_size; /* excluding length bytes */
    size_t ct_len, sa_len; /* including length bytes */
    unsigned char *buf, *p;
    const unsigned char * const end = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN;
    const mbedtls_x509_crt *crt;
    int authmode;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> write certificate request"));

    ssl->state++;

#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
    if (ssl->handshake->sni_authmode != MBEDTLS_SSL_VERIFY_UNSET) {
        authmode = ssl->handshake->sni_authmode;
    } else
#endif
    authmode = ssl->conf->authmode;

    if (!mbedtls_ssl_ciphersuite_cert_req_allowed(ciphersuite_info) ||
        authmode == MBEDTLS_SSL_VERIFY_NONE) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip write certificate request"));
        return 0;
    }

    /*
     *     0  .   0   handshake type
     *     1  .   3   handshake length
     *     4  .   4   cert type count
     *     5  .. m-1  cert types
     *     m  .. m+1  sig alg length (TLS 1.2 only)
     *    m+1 .. n-1  SignatureAndHashAlgorithms (TLS 1.2 only)
     *     n  .. n+1  length of all DNs
     *    n+2 .. n+3  length of DN 1
     *    n+4 .. ...  Distinguished Name #1
     *    ... .. ...  length of DN 2, etc.
     */
    buf = ssl->out_msg;
    p = buf + 4;

    /*
     * Supported certificate types
     *
     *     ClientCertificateType certificate_types<1..2^8-1>;
     *     enum { (255) } ClientCertificateType;
     */
    ct_len = 0;

#if defined(MBEDTLS_RSA_C)
    p[1 + ct_len++] = MBEDTLS_SSL_CERT_TYPE_RSA_SIGN;
#endif
#if defined(MBEDTLS_ECDSA_C)
    p[1 + ct_len++] = MBEDTLS_SSL_CERT_TYPE_ECDSA_SIGN;
#endif

    p[0] = (unsigned char) ct_len++;
    p += ct_len;

    sa_len = 0;

    /*
     * Add signature_algorithms for verify (TLS 1.2)
     *
     *     SignatureAndHashAlgorithm supported_signature_algorithms<2..2^16-2>;
     *
     *     struct {
     *           HashAlgorithm hash;
     *           SignatureAlgorithm signature;
     *     } SignatureAndHashAlgorithm;
     *
     *     enum { (255) } HashAlgorithm;
     *     enum { (255) } SignatureAlgorithm;
     */
    const uint16_t *sig_alg = mbedtls_ssl_get_sig_algs(ssl);
    if (sig_alg == NULL) {
        return MBEDTLS_ERR_SSL_BAD_CONFIG;
    }

    for (; *sig_alg != MBEDTLS_TLS_SIG_NONE; sig_alg++) {
        unsigned char hash = MBEDTLS_BYTE_1(*sig_alg);

        if (mbedtls_ssl_set_calc_verify_md(ssl, hash)) {
            continue;
        }
        if (!mbedtls_ssl_sig_alg_is_supported(ssl, *sig_alg)) {
            continue;
        }

        /* Write elements at offsets starting from 1 (offset 0 is for the
         * length). Thus the offset of each element is the length of the
         * partial list including that element. */
        sa_len += 2;
        MBEDTLS_PUT_UINT16_BE(*sig_alg, p, sa_len);

    }

    /* Fill in list length. */
    MBEDTLS_PUT_UINT16_BE(sa_len, p, 0);
    sa_len += 2;
    p += sa_len;

    /*
     * DistinguishedName certificate_authorities<0..2^16-1>;
     * opaque DistinguishedName<1..2^16-1>;
     */
    p += 2;

    total_dn_size = 0;

    if (ssl->conf->cert_req_ca_list ==  MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED) {
        /* NOTE: If trusted certificates are provisioned
         *       via a CA callback (configured through
         *       `mbedtls_ssl_conf_ca_cb()`, then the
         *       CertificateRequest is currently left empty. */

#if defined(MBEDTLS_KEY_EXCHANGE_CERT_REQ_ALLOWED_ENABLED)
#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
        if (ssl->handshake->dn_hints != NULL) {
            crt = ssl->handshake->dn_hints;
        } else
#endif
        if (ssl->conf->dn_hints != NULL) {
            crt = ssl->conf->dn_hints;
        } else
#endif
#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
        if (ssl->handshake->sni_ca_chain != NULL) {
            crt = ssl->handshake->sni_ca_chain;
        } else
#endif
        crt = ssl->conf->ca_chain;

        while (crt != NULL && crt->version != 0) {
            /* It follows from RFC 5280 A.1 that this length
             * can be represented in at most 11 bits. */
            dn_size = (uint16_t) crt->subject_raw.len;

            if (end < p || (size_t) (end - p) < 2 + (size_t) dn_size) {
                MBEDTLS_SSL_DEBUG_MSG(1, ("skipping CAs: buffer too short"));
                break;
            }

            MBEDTLS_PUT_UINT16_BE(dn_size, p, 0);
            p += 2;
            memcpy(p, crt->subject_raw.p, dn_size);
            p += dn_size;

            MBEDTLS_SSL_DEBUG_BUF(3, "requested DN", p - dn_size, dn_size);

            total_dn_size += 2 + dn_size;
            crt = crt->next;
        }
    }

    ssl->out_msglen  = p - buf;
    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
    ssl->out_msg[0]  = MBEDTLS_SSL_HS_CERTIFICATE_REQUEST;
    MBEDTLS_PUT_UINT16_BE(total_dn_size, ssl->out_msg, 4 + ct_len + sa_len);

    ret = mbedtls_ssl_write_handshake_msg(ssl);

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= write certificate request"));

    return ret;
}
#endif /* MBEDTLS_KEY_EXCHANGE_CERT_REQ_ALLOWED_ENABLED */

#if defined(MBEDTLS_USE_PSA_CRYPTO) &&                      \
    (defined(MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED))
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_get_ecdh_params_from_cert(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
    unsigned char buf[
        PSA_KEY_EXPORT_ECC_KEY_PAIR_MAX_SIZE(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
    psa_key_attributes_t key_attributes = PSA_KEY_ATTRIBUTES_INIT;
    uint16_t tls_id = 0;
    psa_ecc_family_t ecc_family;
    size_t key_len;
    mbedtls_pk_context *pk;
    mbedtls_ecp_keypair *key;

    pk = mbedtls_ssl_own_key(ssl);

    if (pk == NULL) {
        return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
    }

    switch (mbedtls_pk_get_type(pk)) {
        case MBEDTLS_PK_OPAQUE:
            if (!mbedtls_pk_can_do(pk, MBEDTLS_PK_ECKEY)) {
                return MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
            }

            ssl->handshake->ecdh_psa_privkey =
                *((mbedtls_svc_key_id_t *) pk->pk_ctx);

            /* Key should not be destroyed in the TLS library */
            ssl->handshake->ecdh_psa_privkey_is_external = 1;

            status = psa_get_key_attributes(ssl->handshake->ecdh_psa_privkey,
                                            &key_attributes);
            if (status != PSA_SUCCESS) {
                ssl->handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
                return PSA_TO_MBEDTLS_ERR(status);
            }

            ssl->handshake->ecdh_psa_type = psa_get_key_type(&key_attributes);
            ssl->handshake->ecdh_bits = psa_get_key_bits(&key_attributes);

            psa_reset_key_attributes(&key_attributes);

            ret = 0;
            break;
        case MBEDTLS_PK_ECKEY:
        case MBEDTLS_PK_ECKEY_DH:
        case MBEDTLS_PK_ECDSA:
            key = mbedtls_pk_ec(*pk);
            if (key == NULL) {
                return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
            }

            tls_id = mbedtls_ssl_get_tls_id_from_ecp_group_id(key->grp.id);
            if (tls_id == 0) {
                /* This elliptic curve is not supported */
                return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
            }

            /* If the above conversion to TLS ID was fine, then also this one will
               be, so there is no need to check the return value here */
            mbedtls_ssl_get_psa_curve_info_from_tls_id(tls_id, &ecc_family,
                                                       &ssl->handshake->ecdh_bits);

            ssl->handshake->ecdh_psa_type = PSA_KEY_TYPE_ECC_KEY_PAIR(ecc_family);

            key_attributes = psa_key_attributes_init();
            psa_set_key_usage_flags(&key_attributes, PSA_KEY_USAGE_DERIVE);
            psa_set_key_algorithm(&key_attributes, PSA_ALG_ECDH);
            psa_set_key_type(&key_attributes,
                             PSA_KEY_TYPE_ECC_KEY_PAIR(ssl->handshake->ecdh_psa_type));
            psa_set_key_bits(&key_attributes, ssl->handshake->ecdh_bits);

            key_len = PSA_BITS_TO_BYTES(key->grp.pbits);
            ret = mbedtls_ecp_write_key(key, buf, key_len);
            if (ret != 0) {
                goto cleanup;
            }

            status = psa_import_key(&key_attributes, buf, key_len,
                                    &ssl->handshake->ecdh_psa_privkey);
            if (status != PSA_SUCCESS) {
                ret = PSA_TO_MBEDTLS_ERR(status);
                goto cleanup;
            }

            ret = 0;
            break;
        default:
            ret = MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
    }

cleanup:
    mbedtls_platform_zeroize(buf, sizeof(buf));

    return ret;
}
#elif defined(MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_get_ecdh_params_from_cert(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    const mbedtls_pk_context *private_key = mbedtls_ssl_own_key(ssl);
    if (private_key == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("got no server private key"));
        return MBEDTLS_ERR_SSL_PRIVATE_KEY_REQUIRED;
    }

    if (!mbedtls_pk_can_do(private_key, MBEDTLS_PK_ECKEY)) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("server key not ECDH capable"));
        return MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
    }

    if ((ret = mbedtls_ecdh_get_params(&ssl->handshake->ecdh_ctx,
                                       mbedtls_pk_ec(*mbedtls_ssl_own_key(ssl)),
                                       MBEDTLS_ECDH_OURS)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, ("mbedtls_ecdh_get_params"), ret);
        return ret;
    }

    return 0;
}
#endif /* MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED) ||
          MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED) && \
    defined(MBEDTLS_SSL_ASYNC_PRIVATE)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_resume_server_key_exchange(mbedtls_ssl_context *ssl,
                                          size_t *signature_len)
{
    /* Append the signature to ssl->out_msg, leaving 2 bytes for the
     * signature length which will be added in ssl_write_server_key_exchange
     * after the call to ssl_prepare_server_key_exchange.
     * ssl_write_server_key_exchange also takes care of incrementing
     * ssl->out_msglen. */
    unsigned char *sig_start = ssl->out_msg + ssl->out_msglen + 2;
    size_t sig_max_len = (ssl->out_buf + MBEDTLS_SSL_OUT_CONTENT_LEN
                          - sig_start);
    int ret = ssl->conf->f_async_resume(ssl,
                                        sig_start, signature_len, sig_max_len);
    if (ret != MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS) {
        ssl->handshake->async_in_progress = 0;
        mbedtls_ssl_set_async_operation_data(ssl, NULL);
    }
    MBEDTLS_SSL_DEBUG_RET(2, "ssl_resume_server_key_exchange", ret);
    return ret;
}
#endif /* defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED) &&
          defined(MBEDTLS_SSL_ASYNC_PRIVATE) */

/* Prepare the ServerKeyExchange message, up to and including
 * calculating the signature if any, but excluding formatting the
 * signature and sending the message. */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_prepare_server_key_exchange(mbedtls_ssl_context *ssl,
                                           size_t *signature_len)
{
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->handshake->ciphersuite_info;

#if defined(MBEDTLS_KEY_EXCHANGE_SOME_PFS_ENABLED)
#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
    unsigned char *dig_signed = NULL;
#endif /* MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED */
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_PFS_ENABLED */

    (void) ciphersuite_info; /* unused in some configurations */
#if !defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
    (void) signature_len;
#endif /* MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
#if defined(MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH)
    size_t out_buf_len = ssl->out_buf_len - (ssl->out_msg - ssl->out_buf);
#else
    size_t out_buf_len = MBEDTLS_SSL_OUT_BUFFER_LEN - (ssl->out_msg - ssl->out_buf);
#endif
#endif

    ssl->out_msglen = 4; /* header (type:1, length:3) to be written later */

    /*
     *
     * Part 1: Provide key exchange parameters for chosen ciphersuite.
     *
     */

    /*
     * - ECJPAKE key exchanges
     */
#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECJPAKE) {
        int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
#if defined(MBEDTLS_USE_PSA_CRYPTO)
        unsigned char *out_p = ssl->out_msg + ssl->out_msglen;
        unsigned char *end_p = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN -
                               ssl->out_msglen;
        size_t output_offset = 0;
        size_t output_len = 0;

        /*
         * The first 3 bytes are:
         * [0] MBEDTLS_ECP_TLS_NAMED_CURVE
         * [1, 2] elliptic curve's TLS ID
         *
         * However since we only support secp256r1 for now, we hardcode its
         * TLS ID here
         */
        uint16_t tls_id = mbedtls_ssl_get_tls_id_from_ecp_group_id(
            MBEDTLS_ECP_DP_SECP256R1);
        if (tls_id == 0) {
            return MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
        }
        *out_p = MBEDTLS_ECP_TLS_NAMED_CURVE;
        MBEDTLS_PUT_UINT16_BE(tls_id, out_p, 1);
        output_offset += 3;

        ret = mbedtls_psa_ecjpake_write_round(&ssl->handshake->psa_pake_ctx,
                                              out_p + output_offset,
                                              end_p - out_p - output_offset, &output_len,
                                              MBEDTLS_ECJPAKE_ROUND_TWO);
        if (ret != 0) {
            psa_destroy_key(ssl->handshake->psa_pake_password);
            psa_pake_abort(&ssl->handshake->psa_pake_ctx);
            MBEDTLS_SSL_DEBUG_RET(1, "psa_pake_output", ret);
            return ret;
        }

        output_offset += output_len;
        ssl->out_msglen += output_offset;
#else
        size_t len = 0;

        ret = mbedtls_ecjpake_write_round_two(
            &ssl->handshake->ecjpake_ctx,
            ssl->out_msg + ssl->out_msglen,
            MBEDTLS_SSL_OUT_CONTENT_LEN - ssl->out_msglen, &len,
            ssl->conf->f_rng, ssl->conf->p_rng);
        if (ret != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecjpake_write_round_two", ret);
            return ret;
        }

        ssl->out_msglen += len;
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    }
#endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

    /*
     * For (EC)DHE key exchanges with PSK, parameters are prefixed by support
     * identity hint (RFC 4279, Sec. 3). Until someone needs this feature,
     * we use empty support identity hints here.
     **/
#if defined(MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED)   || \
    defined(MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_DHE_PSK ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_PSK) {
        ssl->out_msg[ssl->out_msglen++] = 0x00;
        ssl->out_msg[ssl->out_msglen++] = 0x00;
    }
#endif /* MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED */

    /*
     * - DHE key exchanges
     */
#if defined(MBEDTLS_KEY_EXCHANGE_SOME_DHE_ENABLED)
    if (mbedtls_ssl_ciphersuite_uses_dhe(ciphersuite_info)) {
        int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
        size_t len = 0;

        if (ssl->conf->dhm_P.p == NULL || ssl->conf->dhm_G.p == NULL) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("no DH parameters set"));
            return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
        }

        /*
         * Ephemeral DH parameters:
         *
         * struct {
         *     opaque dh_p<1..2^16-1>;
         *     opaque dh_g<1..2^16-1>;
         *     opaque dh_Ys<1..2^16-1>;
         * } ServerDHParams;
         */
        if ((ret = mbedtls_dhm_set_group(&ssl->handshake->dhm_ctx,
                                         &ssl->conf->dhm_P,
                                         &ssl->conf->dhm_G)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_dhm_set_group", ret);
            return ret;
        }

        if ((ret = mbedtls_dhm_make_params(
                 &ssl->handshake->dhm_ctx,
                 (int) mbedtls_dhm_get_len(&ssl->handshake->dhm_ctx),
                 ssl->out_msg + ssl->out_msglen, &len,
                 ssl->conf->f_rng, ssl->conf->p_rng)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_dhm_make_params", ret);
            return ret;
        }

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
        dig_signed = ssl->out_msg + ssl->out_msglen;
#endif

        ssl->out_msglen += len;

        MBEDTLS_SSL_DEBUG_MPI(3, "DHM: X ", &ssl->handshake->dhm_ctx.X);
        MBEDTLS_SSL_DEBUG_MPI(3, "DHM: P ", &ssl->handshake->dhm_ctx.P);
        MBEDTLS_SSL_DEBUG_MPI(3, "DHM: G ", &ssl->handshake->dhm_ctx.G);
        MBEDTLS_SSL_DEBUG_MPI(3, "DHM: GX", &ssl->handshake->dhm_ctx.GX);
    }
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_DHE_ENABLED */

    /*
     * - ECDHE key exchanges
     */
#if defined(MBEDTLS_KEY_EXCHANGE_SOME_ECDHE_ENABLED)
    if (mbedtls_ssl_ciphersuite_uses_ecdhe(ciphersuite_info)) {
        /*
         * Ephemeral ECDH parameters:
         *
         * struct {
         *     ECParameters curve_params;
         *     ECPoint      public;
         * } ServerECDHParams;
         */
        uint16_t *curr_tls_id = ssl->handshake->curves_tls_id;
        const uint16_t *group_list = mbedtls_ssl_get_groups(ssl);
        int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
        size_t len = 0;

        /* Match our preference list against the offered curves */
        if ((group_list == NULL) || (curr_tls_id == NULL)) {
            return MBEDTLS_ERR_SSL_BAD_CONFIG;
        }
        for (; *group_list != 0; group_list++) {
            for (curr_tls_id = ssl->handshake->curves_tls_id;
                 *curr_tls_id != 0; curr_tls_id++) {
                if (*curr_tls_id == *group_list) {
                    goto curve_matching_done;
                }
            }
        }

curve_matching_done:
        if (*curr_tls_id == 0) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("no matching curve for ECDHE"));
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }

        MBEDTLS_SSL_DEBUG_MSG(2, ("ECDHE curve: %s",
                                  mbedtls_ssl_get_curve_name_from_tls_id(*curr_tls_id)));

#if defined(MBEDTLS_USE_PSA_CRYPTO)
        psa_status_t status = PSA_ERROR_GENERIC_ERROR;
        psa_key_attributes_t key_attributes;
        mbedtls_ssl_handshake_params *handshake = ssl->handshake;
        uint8_t *p = ssl->out_msg + ssl->out_msglen;
        const size_t header_size = 4; // curve_type(1), namedcurve(2),
                                      // data length(1)
        const size_t data_length_size = 1;
        psa_ecc_family_t ec_psa_family = 0;
        size_t ec_bits = 0;

        MBEDTLS_SSL_DEBUG_MSG(1, ("Perform PSA-based ECDH computation."));

        /* Convert EC's TLS ID to PSA key type. */
        if (mbedtls_ssl_get_psa_curve_info_from_tls_id(*curr_tls_id,
                                                       &ec_psa_family,
                                                       &ec_bits) == PSA_ERROR_NOT_SUPPORTED) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("Invalid ecc group parse."));
            return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
        }
        handshake->ecdh_psa_type = PSA_KEY_TYPE_ECC_KEY_PAIR(ec_psa_family);
        handshake->ecdh_bits = ec_bits;

        key_attributes = psa_key_attributes_init();
        psa_set_key_usage_flags(&key_attributes, PSA_KEY_USAGE_DERIVE);
        psa_set_key_algorithm(&key_attributes, PSA_ALG_ECDH);
        psa_set_key_type(&key_attributes, handshake->ecdh_psa_type);
        psa_set_key_bits(&key_attributes, handshake->ecdh_bits);

        /*
         * ECParameters curve_params
         *
         * First byte is curve_type, always named_curve
         */
        *p++ = MBEDTLS_ECP_TLS_NAMED_CURVE;

        /*
         * Next two bytes are the namedcurve value
         */
        MBEDTLS_PUT_UINT16_BE(*curr_tls_id, p, 0);
        p += 2;

        /* Generate ECDH private key. */
        status = psa_generate_key(&key_attributes,
                                  &handshake->ecdh_psa_privkey);
        if (status != PSA_SUCCESS) {
            ret = PSA_TO_MBEDTLS_ERR(status);
            MBEDTLS_SSL_DEBUG_RET(1, "psa_generate_key", ret);
            return ret;
        }

        /*
         * ECPoint  public
         *
         * First byte is data length.
         * It will be filled later. p holds now the data length location.
         */

        /* Export the public part of the ECDH private key from PSA.
         * Make one byte space for the length.
         */
        unsigned char *own_pubkey = p + data_length_size;

        size_t own_pubkey_max_len = (size_t) (MBEDTLS_SSL_OUT_CONTENT_LEN
                                              - (own_pubkey - ssl->out_msg));

        status = psa_export_public_key(handshake->ecdh_psa_privkey,
                                       own_pubkey, own_pubkey_max_len,
                                       &len);
        if (status != PSA_SUCCESS) {
            ret = PSA_TO_MBEDTLS_ERR(status);
            MBEDTLS_SSL_DEBUG_RET(1, "psa_export_public_key", ret);
            (void) psa_destroy_key(handshake->ecdh_psa_privkey);
            handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return ret;
        }

        /* Store the length of the exported public key. */
        *p = (uint8_t) len;

        /* Determine full message length. */
        len += header_size;
#else
        mbedtls_ecp_group_id curr_grp_id =
            mbedtls_ssl_get_ecp_group_id_from_tls_id(*curr_tls_id);

        if ((ret = mbedtls_ecdh_setup(&ssl->handshake->ecdh_ctx,
                                      curr_grp_id)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecp_group_load", ret);
            return ret;
        }

        if ((ret = mbedtls_ecdh_make_params(
                 &ssl->handshake->ecdh_ctx, &len,
                 ssl->out_msg + ssl->out_msglen,
                 MBEDTLS_SSL_OUT_CONTENT_LEN - ssl->out_msglen,
                 ssl->conf->f_rng, ssl->conf->p_rng)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecdh_make_params", ret);
            return ret;
        }

        MBEDTLS_SSL_DEBUG_ECDH(3, &ssl->handshake->ecdh_ctx,
                               MBEDTLS_DEBUG_ECDH_Q);
#endif /* MBEDTLS_USE_PSA_CRYPTO */

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
        dig_signed = ssl->out_msg + ssl->out_msglen;
#endif

        ssl->out_msglen += len;
    }
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_ECDHE_ENABLED */

    /*
     *
     * Part 2: For key exchanges involving the server signing the
     *         exchange parameters, compute and add the signature here.
     *
     */
#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
    if (mbedtls_ssl_ciphersuite_uses_server_signature(ciphersuite_info)) {
        if (dig_signed == NULL) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
            return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
        }

        size_t dig_signed_len = ssl->out_msg + ssl->out_msglen - dig_signed;
        size_t hashlen = 0;
        unsigned char hash[MBEDTLS_HASH_MAX_SIZE];

        int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

        /*
         * 2.1: Choose hash algorithm:
         *      For TLS 1.2, obey signature-hash-algorithm extension
         *      to choose appropriate hash.
         */

        mbedtls_pk_type_t sig_alg =
            mbedtls_ssl_get_ciphersuite_sig_pk_alg(ciphersuite_info);

        unsigned int sig_hash =
            mbedtls_ssl_tls12_get_preferred_hash_for_sig_alg(
                ssl, mbedtls_ssl_sig_from_pk_alg(sig_alg));

        mbedtls_md_type_t md_alg = mbedtls_ssl_md_alg_from_hash(sig_hash);

        /*    For TLS 1.2, obey signature-hash-algorithm extension
         *    (RFC 5246, Sec. 7.4.1.4.1). */
        if (sig_alg == MBEDTLS_PK_NONE || md_alg == MBEDTLS_MD_NONE) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
            /* (... because we choose a cipher suite
             *      only if there is a matching hash.) */
            return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
        }

        MBEDTLS_SSL_DEBUG_MSG(3, ("pick hash algorithm %u for signing", (unsigned) md_alg));

        /*
         * 2.2: Compute the hash to be signed
         */
        if (md_alg != MBEDTLS_MD_NONE) {
            ret = mbedtls_ssl_get_key_exchange_md_tls1_2(ssl, hash, &hashlen,
                                                         dig_signed,
                                                         dig_signed_len,
                                                         md_alg);
            if (ret != 0) {
                return ret;
            }
        } else {
            MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
            return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
        }

        MBEDTLS_SSL_DEBUG_BUF(3, "parameters hash", hash, hashlen);

        /*
         * 2.3: Compute and add the signature
         */
        /*
         * We need to specify signature and hash algorithm explicitly through
         * a prefix to the signature.
         *
         * struct {
         *    HashAlgorithm hash;
         *    SignatureAlgorithm signature;
         * } SignatureAndHashAlgorithm;
         *
         * struct {
         *    SignatureAndHashAlgorithm algorithm;
         *    opaque signature<0..2^16-1>;
         * } DigitallySigned;
         *
         */

        ssl->out_msg[ssl->out_msglen++] = mbedtls_ssl_hash_from_md_alg(md_alg);
        ssl->out_msg[ssl->out_msglen++] = mbedtls_ssl_sig_from_pk_alg(sig_alg);

#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
        if (ssl->conf->f_async_sign_start != NULL) {
            ret = ssl->conf->f_async_sign_start(ssl,
                                                mbedtls_ssl_own_cert(ssl),
                                                md_alg, hash, hashlen);
            switch (ret) {
                case MBEDTLS_ERR_SSL_HW_ACCEL_FALLTHROUGH:
                    /* act as if f_async_sign was null */
                    break;
                case 0:
                    ssl->handshake->async_in_progress = 1;
                    return ssl_resume_server_key_exchange(ssl, signature_len);
                case MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS:
                    ssl->handshake->async_in_progress = 1;
                    return MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS;
                default:
                    MBEDTLS_SSL_DEBUG_RET(1, "f_async_sign_start", ret);
                    return ret;
            }
        }
#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */

        if (mbedtls_ssl_own_key(ssl) == NULL) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("got no private key"));
            return MBEDTLS_ERR_SSL_PRIVATE_KEY_REQUIRED;
        }

        /* Append the signature to ssl->out_msg, leaving 2 bytes for the
         * signature length which will be added in ssl_write_server_key_exchange
         * after the call to ssl_prepare_server_key_exchange.
         * ssl_write_server_key_exchange also takes care of incrementing
         * ssl->out_msglen. */
        if ((ret = mbedtls_pk_sign(mbedtls_ssl_own_key(ssl),
                                   md_alg, hash, hashlen,
                                   ssl->out_msg + ssl->out_msglen + 2,
                                   out_buf_len - ssl->out_msglen - 2,
                                   signature_len,
                                   ssl->conf->f_rng,
                                   ssl->conf->p_rng)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_pk_sign", ret);
            return ret;
        }
    }
#endif /* MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED */

    return 0;
}

/* Prepare the ServerKeyExchange message and send it. For ciphersuites
 * that do not include a ServerKeyExchange message, do nothing. Either
 * way, if successful, move on to the next step in the SSL state
 * machine. */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_server_key_exchange(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    size_t signature_len = 0;
#if defined(MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED)
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->handshake->ciphersuite_info;
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED */

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> write server key exchange"));

#if defined(MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED)
    /* Extract static ECDH parameters and abort if ServerKeyExchange
     * is not needed. */
    if (mbedtls_ssl_ciphersuite_no_pfs(ciphersuite_info)) {
        /* For suites involving ECDH, extract DH parameters
         * from certificate at this point. */
#if defined(MBEDTLS_KEY_EXCHANGE_SOME_ECDH_ENABLED)
        if (mbedtls_ssl_ciphersuite_uses_ecdh(ciphersuite_info)) {
            ret = ssl_get_ecdh_params_from_cert(ssl);
            if (ret != 0) {
                MBEDTLS_SSL_DEBUG_RET(1, "ssl_get_ecdh_params_from_cert", ret);
                return ret;
            }
        }
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_ECDH_ENABLED */

        /* Key exchanges not involving ephemeral keys don't use
         * ServerKeyExchange, so end here. */
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip write server key exchange"));
        ssl->state++;
        return 0;
    }
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED) && \
    defined(MBEDTLS_SSL_ASYNC_PRIVATE)
    /* If we have already prepared the message and there is an ongoing
     * signature operation, resume signing. */
    if (ssl->handshake->async_in_progress != 0) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("resuming signature operation"));
        ret = ssl_resume_server_key_exchange(ssl, &signature_len);
    } else
#endif /* defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED) &&
          defined(MBEDTLS_SSL_ASYNC_PRIVATE) */
    {
        /* ServerKeyExchange is needed. Prepare the message. */
        ret = ssl_prepare_server_key_exchange(ssl, &signature_len);
    }

    if (ret != 0) {
        /* If we're starting to write a new message, set ssl->out_msglen
         * to 0. But if we're resuming after an asynchronous message,
         * out_msglen is the amount of data written so far and mst be
         * preserved. */
        if (ret == MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS) {
            MBEDTLS_SSL_DEBUG_MSG(2, ("<= write server key exchange (pending)"));
        } else {
            ssl->out_msglen = 0;
        }
        return ret;
    }

    /* If there is a signature, write its length.
     * ssl_prepare_server_key_exchange already wrote the signature
     * itself at its proper place in the output buffer. */
#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
    if (signature_len != 0) {
        ssl->out_msg[ssl->out_msglen++] = MBEDTLS_BYTE_1(signature_len);
        ssl->out_msg[ssl->out_msglen++] = MBEDTLS_BYTE_0(signature_len);

        MBEDTLS_SSL_DEBUG_BUF(3, "my signature",
                              ssl->out_msg + ssl->out_msglen,
                              signature_len);

        /* Skip over the already-written signature */
        ssl->out_msglen += signature_len;
    }
#endif /* MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED */

    /* Add header and send. */
    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
    ssl->out_msg[0]  = MBEDTLS_SSL_HS_SERVER_KEY_EXCHANGE;

    ssl->state++;

    if ((ret = mbedtls_ssl_write_handshake_msg(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_write_handshake_msg", ret);
        return ret;
    }

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= write server key exchange"));
    return 0;
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_server_hello_done(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> write server hello done"));

    ssl->out_msglen  = 4;
    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
    ssl->out_msg[0]  = MBEDTLS_SSL_HS_SERVER_HELLO_DONE;

    ssl->state++;

#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
        mbedtls_ssl_send_flight_completed(ssl);
    }
#endif

    if ((ret = mbedtls_ssl_write_handshake_msg(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_write_handshake_msg", ret);
        return ret;
    }

#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM &&
        (ret = mbedtls_ssl_flight_transmit(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_flight_transmit", ret);
        return ret;
    }
#endif /* MBEDTLS_SSL_PROTO_DTLS */

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= write server hello done"));

    return 0;
}

#if defined(MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED) ||                       \
    defined(MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_client_dh_public(mbedtls_ssl_context *ssl, unsigned char **p,
                                      const unsigned char *end)
{
    int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
    size_t n;

    /*
     * Receive G^Y mod P, premaster = (G^Y)^X mod P
     */
    if (*p + 2 > end) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    n = ((*p)[0] << 8) | (*p)[1];
    *p += 2;

    if (*p + n > end) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    if ((ret = mbedtls_dhm_read_public(&ssl->handshake->dhm_ctx, *p, n)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_dhm_read_public", ret);
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    *p += n;

    MBEDTLS_SSL_DEBUG_MPI(3, "DHM: GY", &ssl->handshake->dhm_ctx.GY);

    return ret;
}
#endif /* MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED) ||                           \
    defined(MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED)

#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_resume_decrypt_pms(mbedtls_ssl_context *ssl,
                                  unsigned char *peer_pms,
                                  size_t *peer_pmslen,
                                  size_t peer_pmssize)
{
    int ret = ssl->conf->f_async_resume(ssl,
                                        peer_pms, peer_pmslen, peer_pmssize);
    if (ret != MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS) {
        ssl->handshake->async_in_progress = 0;
        mbedtls_ssl_set_async_operation_data(ssl, NULL);
    }
    MBEDTLS_SSL_DEBUG_RET(2, "ssl_decrypt_encrypted_pms", ret);
    return ret;
}
#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_decrypt_encrypted_pms(mbedtls_ssl_context *ssl,
                                     const unsigned char *p,
                                     const unsigned char *end,
                                     unsigned char *peer_pms,
                                     size_t *peer_pmslen,
                                     size_t peer_pmssize)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    mbedtls_x509_crt *own_cert = mbedtls_ssl_own_cert(ssl);
    if (own_cert == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("got no local certificate"));
        return MBEDTLS_ERR_SSL_NO_CLIENT_CERTIFICATE;
    }
    mbedtls_pk_context *public_key = &own_cert->pk;
    mbedtls_pk_context *private_key = mbedtls_ssl_own_key(ssl);
    size_t len = mbedtls_pk_get_len(public_key);

#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
    /* If we have already started decoding the message and there is an ongoing
     * decryption operation, resume signing. */
    if (ssl->handshake->async_in_progress != 0) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("resuming decryption operation"));
        return ssl_resume_decrypt_pms(ssl,
                                      peer_pms, peer_pmslen, peer_pmssize);
    }
#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */

    /*
     * Prepare to decrypt the premaster using own private RSA key
     */
    if (p + 2 > end) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }
    if (*p++ != MBEDTLS_BYTE_1(len) ||
        *p++ != MBEDTLS_BYTE_0(len)) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    if (p + len != end) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    /*
     * Decrypt the premaster secret
     */
#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
    if (ssl->conf->f_async_decrypt_start != NULL) {
        ret = ssl->conf->f_async_decrypt_start(ssl,
                                               mbedtls_ssl_own_cert(ssl),
                                               p, len);
        switch (ret) {
            case MBEDTLS_ERR_SSL_HW_ACCEL_FALLTHROUGH:
                /* act as if f_async_decrypt_start was null */
                break;
            case 0:
                ssl->handshake->async_in_progress = 1;
                return ssl_resume_decrypt_pms(ssl,
                                              peer_pms,
                                              peer_pmslen,
                                              peer_pmssize);
            case MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS:
                ssl->handshake->async_in_progress = 1;
                return MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS;
            default:
                MBEDTLS_SSL_DEBUG_RET(1, "f_async_decrypt_start", ret);
                return ret;
        }
    }
#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */

    if (!mbedtls_pk_can_do(private_key, MBEDTLS_PK_RSA)) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("got no RSA private key"));
        return MBEDTLS_ERR_SSL_PRIVATE_KEY_REQUIRED;
    }

    ret = mbedtls_pk_decrypt(private_key, p, len,
                             peer_pms, peer_pmslen, peer_pmssize,
                             ssl->conf->f_rng, ssl->conf->p_rng);
    return ret;
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_encrypted_pms(mbedtls_ssl_context *ssl,
                                   const unsigned char *p,
                                   const unsigned char *end,
                                   size_t pms_offset)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char *pms = ssl->handshake->premaster + pms_offset;
    unsigned char ver[2];
    unsigned char fake_pms[48], peer_pms[48];
    unsigned char mask;
    size_t i, peer_pmslen;
    unsigned int diff;

    /* In case of a failure in decryption, the decryption may write less than
     * 2 bytes of output, but we always read the first two bytes. It doesn't
     * matter in the end because diff will be nonzero in that case due to
     * ret being nonzero, and we only care whether diff is 0.
     * But do initialize peer_pms and peer_pmslen for robustness anyway. This
     * also makes memory analyzers happy (don't access uninitialized memory,
     * even if it's an unsigned char). */
    peer_pms[0] = peer_pms[1] = ~0;
    peer_pmslen = 0;

    ret = ssl_decrypt_encrypted_pms(ssl, p, end,
                                    peer_pms,
                                    &peer_pmslen,
                                    sizeof(peer_pms));

#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
    if (ret == MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS) {
        return ret;
    }
#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */

    mbedtls_ssl_write_version(ver, ssl->conf->transport,
                              ssl->session_negotiate->tls_version);

    /* Avoid data-dependent branches while checking for invalid
     * padding, to protect against timing-based Bleichenbacher-type
     * attacks. */
    diff  = (unsigned int) ret;
    diff |= peer_pmslen ^ 48;
    diff |= peer_pms[0] ^ ver[0];
    diff |= peer_pms[1] ^ ver[1];

    /* mask = diff ? 0xff : 0x00 using bit operations to avoid branches */
    mask = mbedtls_ct_uint_mask(diff);

    /*
     * Protection against Bleichenbacher's attack: invalid PKCS#1 v1.5 padding
     * must not cause the connection to end immediately; instead, send a
     * bad_record_mac later in the handshake.
     * To protect against timing-based variants of the attack, we must
     * not have any branch that depends on whether the decryption was
     * successful. In particular, always generate the fake premaster secret,
     * regardless of whether it will ultimately influence the output or not.
     */
    ret = ssl->conf->f_rng(ssl->conf->p_rng, fake_pms, sizeof(fake_pms));
    if (ret != 0) {
        /* It's ok to abort on an RNG failure, since this does not reveal
         * anything about the RSA decryption. */
        return ret;
    }

#if defined(MBEDTLS_SSL_DEBUG_ALL)
    if (diff != 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
    }
#endif

    if (sizeof(ssl->handshake->premaster) < pms_offset ||
        sizeof(ssl->handshake->premaster) - pms_offset < 48) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }
    ssl->handshake->pmslen = 48;

    /* Set pms to either the true or the fake PMS, without
     * data-dependent branches. */
    for (i = 0; i < ssl->handshake->pmslen; i++) {
        pms[i] = (mask & fake_pms[i]) | ((~mask) & peer_pms[i]);
    }

    return 0;
}
#endif /* MBEDTLS_KEY_EXCHANGE_RSA_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED */

#if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_client_psk_identity(mbedtls_ssl_context *ssl, unsigned char **p,
                                         const unsigned char *end)
{
    int ret = 0;
    uint16_t n;

    if (ssl_conf_has_psk_or_cb(ssl->conf) == 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("got no pre-shared key"));
        return MBEDTLS_ERR_SSL_PRIVATE_KEY_REQUIRED;
    }

    /*
     * Receive client pre-shared key identity name
     */
    if (end - *p < 2) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    n = ((*p)[0] << 8) | (*p)[1];
    *p += 2;

    if (n == 0 || n > end - *p) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    if (ssl->conf->f_psk != NULL) {
        if (ssl->conf->f_psk(ssl->conf->p_psk, ssl, *p, n) != 0) {
            ret = MBEDTLS_ERR_SSL_UNKNOWN_IDENTITY;
        }
    } else {
        /* Identity is not a big secret since clients send it in the clear,
         * but treat it carefully anyway, just in case */
        if (n != ssl->conf->psk_identity_len ||
            mbedtls_ct_memcmp(ssl->conf->psk_identity, *p, n) != 0) {
            ret = MBEDTLS_ERR_SSL_UNKNOWN_IDENTITY;
        }
    }

    if (ret == MBEDTLS_ERR_SSL_UNKNOWN_IDENTITY) {
        MBEDTLS_SSL_DEBUG_BUF(3, "Unknown PSK identity", *p, n);
        mbedtls_ssl_send_alert_message(ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                       MBEDTLS_SSL_ALERT_MSG_UNKNOWN_PSK_IDENTITY);
        return MBEDTLS_ERR_SSL_UNKNOWN_IDENTITY;
    }

    *p += n;

    return 0;
}
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_client_key_exchange(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info;
    unsigned char *p, *end;

    ciphersuite_info = ssl->handshake->ciphersuite_info;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> parse client key exchange"));

#if defined(MBEDTLS_SSL_ASYNC_PRIVATE) && \
    (defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED) || \
    defined(MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED))
    if ((ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA_PSK ||
         ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA) &&
        (ssl->handshake->async_in_progress != 0)) {
        /* We've already read a record and there is an asynchronous
         * operation in progress to decrypt it. So skip reading the
         * record. */
        MBEDTLS_SSL_DEBUG_MSG(3, ("will resume decryption of previously-read record"));
    } else
#endif
    if ((ret = mbedtls_ssl_read_record(ssl, 1)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_read_record", ret);
        return ret;
    }

    p = ssl->in_msg + mbedtls_ssl_hs_hdr_len(ssl);
    end = ssl->in_msg + ssl->in_hslen;

    if (ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE;
    }

    if (ssl->in_msg[0] != MBEDTLS_SSL_HS_CLIENT_KEY_EXCHANGE) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange message"));
        return MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE;
    }

#if defined(MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_DHE_RSA) {
        if ((ret = ssl_parse_client_dh_public(ssl, &p, end)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_client_dh_public"), ret);
            return ret;
        }

        if (p != end) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange"));
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        if ((ret = mbedtls_dhm_calc_secret(&ssl->handshake->dhm_ctx,
                                           ssl->handshake->premaster,
                                           MBEDTLS_PREMASTER_SIZE,
                                           &ssl->handshake->pmslen,
                                           ssl->conf->f_rng, ssl->conf->p_rng)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_dhm_calc_secret", ret);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        MBEDTLS_SSL_DEBUG_MPI(3, "DHM: K ", &ssl->handshake->dhm_ctx.K);
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED) ||                     \
    defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED) ||                   \
    defined(MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED) ||                      \
    defined(MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_RSA ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDH_RSA ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA) {
#if defined(MBEDTLS_USE_PSA_CRYPTO)
        size_t data_len = (size_t) (*p++);
        size_t buf_len = (size_t) (end - p);
        psa_status_t status = PSA_ERROR_GENERIC_ERROR;
        mbedtls_ssl_handshake_params *handshake = ssl->handshake;

        MBEDTLS_SSL_DEBUG_MSG(1, ("Read the peer's public key."));

        /*
         * We must have at least two bytes (1 for length, at least 1 for data)
         */
        if (buf_len < 2) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("Invalid buffer length"));
            return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
        }

        if (data_len < 1 || data_len > buf_len) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("Invalid data length"));
            return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
        }

        /* Store peer's ECDH public key. */
        memcpy(handshake->ecdh_psa_peerkey, p, data_len);
        handshake->ecdh_psa_peerkey_len = data_len;

        /* Compute ECDH shared secret. */
        status = psa_raw_key_agreement(
            PSA_ALG_ECDH, handshake->ecdh_psa_privkey,
            handshake->ecdh_psa_peerkey, handshake->ecdh_psa_peerkey_len,
            handshake->premaster, sizeof(handshake->premaster),
            &handshake->pmslen);
        if (status != PSA_SUCCESS) {
            ret = PSA_TO_MBEDTLS_ERR(status);
            MBEDTLS_SSL_DEBUG_RET(1, "psa_raw_key_agreement", ret);
            if (handshake->ecdh_psa_privkey_is_external == 0) {
                (void) psa_destroy_key(handshake->ecdh_psa_privkey);
            }
            handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return ret;
        }

        if (handshake->ecdh_psa_privkey_is_external == 0) {
            status = psa_destroy_key(handshake->ecdh_psa_privkey);

            if (status != PSA_SUCCESS) {
                ret = PSA_TO_MBEDTLS_ERR(status);
                MBEDTLS_SSL_DEBUG_RET(1, "psa_destroy_key", ret);
                return ret;
            }
        }
        handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
#else
        if ((ret = mbedtls_ecdh_read_public(&ssl->handshake->ecdh_ctx,
                                            p, end - p)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecdh_read_public", ret);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        MBEDTLS_SSL_DEBUG_ECDH(3, &ssl->handshake->ecdh_ctx,
                               MBEDTLS_DEBUG_ECDH_QP);

        if ((ret = mbedtls_ecdh_calc_secret(&ssl->handshake->ecdh_ctx,
                                            &ssl->handshake->pmslen,
                                            ssl->handshake->premaster,
                                            MBEDTLS_MPI_MAX_SIZE,
                                            ssl->conf->f_rng, ssl->conf->p_rng)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecdh_calc_secret", ret);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        MBEDTLS_SSL_DEBUG_ECDH(3, &ssl->handshake->ecdh_ctx,
                               MBEDTLS_DEBUG_ECDH_Z);
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_PSK) {
        if ((ret = ssl_parse_client_psk_identity(ssl, &p, end)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_client_psk_identity"), ret);
            return ret;
        }

        if (p != end) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange"));
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

#if !defined(MBEDTLS_USE_PSA_CRYPTO)
        if ((ret = mbedtls_ssl_psk_derive_premaster(ssl,
                                                    ciphersuite_info->key_exchange)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_psk_derive_premaster", ret);
            return ret;
        }
#endif /* !MBEDTLS_USE_PSA_CRYPTO */
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_PSK_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA_PSK) {
#if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
        if (ssl->handshake->async_in_progress != 0) {
            /* There is an asynchronous operation in progress to
             * decrypt the encrypted premaster secret, so skip
             * directly to resuming this operation. */
            MBEDTLS_SSL_DEBUG_MSG(3, ("PSK identity already parsed"));
            /* Update p to skip the PSK identity. ssl_parse_encrypted_pms
             * won't actually use it, but maintain p anyway for robustness. */
            p += ssl->conf->psk_identity_len + 2;
        } else
#endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
        if ((ret = ssl_parse_client_psk_identity(ssl, &p, end)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_client_psk_identity"), ret);
            return ret;
        }

        if ((ret = ssl_parse_encrypted_pms(ssl, p, end, 2)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_encrypted_pms"), ret);
            return ret;
        }

#if !defined(MBEDTLS_USE_PSA_CRYPTO)
        if ((ret = mbedtls_ssl_psk_derive_premaster(ssl,
                                                    ciphersuite_info->key_exchange)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_psk_derive_premaster", ret);
            return ret;
        }
#endif /* !MBEDTLS_USE_PSA_CRYPTO */
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_DHE_PSK) {
        if ((ret = ssl_parse_client_psk_identity(ssl, &p, end)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_client_psk_identity"), ret);
            return ret;
        }
        if ((ret = ssl_parse_client_dh_public(ssl, &p, end)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_client_dh_public"), ret);
            return ret;
        }

        if (p != end) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("bad client key exchange"));
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

#if defined(MBEDTLS_USE_PSA_CRYPTO)
        unsigned char *pms = ssl->handshake->premaster;
        unsigned char *pms_end = pms + sizeof(ssl->handshake->premaster);
        size_t pms_len;

        /* Write length only when we know the actual value */
        if ((ret = mbedtls_dhm_calc_secret(&ssl->handshake->dhm_ctx,
                                           pms + 2, pms_end - (pms + 2), &pms_len,
                                           ssl->conf->f_rng, ssl->conf->p_rng)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_dhm_calc_secret", ret);
            return ret;
        }
        MBEDTLS_PUT_UINT16_BE(pms_len, pms, 0);
        pms += 2 + pms_len;

        MBEDTLS_SSL_DEBUG_MPI(3, "DHM: K ", &ssl->handshake->dhm_ctx.K);
#else
        if ((ret = mbedtls_ssl_psk_derive_premaster(ssl,
                                                    ciphersuite_info->key_exchange)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_psk_derive_premaster", ret);
            return ret;
        }
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_PSK) {
#if defined(MBEDTLS_USE_PSA_CRYPTO)
        psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
        psa_status_t destruction_status = PSA_ERROR_CORRUPTION_DETECTED;
        uint8_t ecpoint_len;

        mbedtls_ssl_handshake_params *handshake = ssl->handshake;

        if ((ret = ssl_parse_client_psk_identity(ssl, &p, end)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_client_psk_identity"), ret);
            psa_destroy_key(handshake->ecdh_psa_privkey);
            handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return ret;
        }

        /* Keep a copy of the peer's public key */
        if (p >= end) {
            psa_destroy_key(handshake->ecdh_psa_privkey);
            handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        ecpoint_len = *(p++);
        if ((size_t) (end - p) < ecpoint_len) {
            psa_destroy_key(handshake->ecdh_psa_privkey);
            handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        if (ecpoint_len > sizeof(handshake->ecdh_psa_peerkey)) {
            psa_destroy_key(handshake->ecdh_psa_privkey);
            handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }

        memcpy(handshake->ecdh_psa_peerkey, p, ecpoint_len);
        handshake->ecdh_psa_peerkey_len = ecpoint_len;
        p += ecpoint_len;

        /* As RFC 5489 section 2, the premaster secret is formed as follows:
         * - a uint16 containing the length (in octets) of the ECDH computation
         * - the octet string produced by the ECDH computation
         * - a uint16 containing the length (in octets) of the PSK
         * - the PSK itself
         */
        unsigned char *psm = ssl->handshake->premaster;
        const unsigned char * const psm_end =
            psm + sizeof(ssl->handshake->premaster);
        /* uint16 to store length (in octets) of the ECDH computation */
        const size_t zlen_size = 2;
        size_t zlen = 0;

        /* Compute ECDH shared secret. */
        status = psa_raw_key_agreement(PSA_ALG_ECDH,
                                       handshake->ecdh_psa_privkey,
                                       handshake->ecdh_psa_peerkey,
                                       handshake->ecdh_psa_peerkey_len,
                                       psm + zlen_size,
                                       psm_end - (psm + zlen_size),
                                       &zlen);

        destruction_status = psa_destroy_key(handshake->ecdh_psa_privkey);
        handshake->ecdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;

        if (status != PSA_SUCCESS) {
            return PSA_TO_MBEDTLS_ERR(status);
        } else if (destruction_status != PSA_SUCCESS) {
            return PSA_TO_MBEDTLS_ERR(destruction_status);
        }

        /* Write the ECDH computation length before the ECDH computation */
        MBEDTLS_PUT_UINT16_BE(zlen, psm, 0);
        psm += zlen_size + zlen;

#else /* MBEDTLS_USE_PSA_CRYPTO */
        if ((ret = ssl_parse_client_psk_identity(ssl, &p, end)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_client_psk_identity"), ret);
            return ret;
        }

        if ((ret = mbedtls_ecdh_read_public(&ssl->handshake->ecdh_ctx,
                                            p, end - p)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecdh_read_public", ret);
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        MBEDTLS_SSL_DEBUG_ECDH(3, &ssl->handshake->ecdh_ctx,
                               MBEDTLS_DEBUG_ECDH_QP);

        if ((ret = mbedtls_ssl_psk_derive_premaster(ssl,
                                                    ciphersuite_info->key_exchange)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_psk_derive_premaster", ret);
            return ret;
        }
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA) {
        if ((ret = ssl_parse_encrypted_pms(ssl, p, end, 0)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, ("ssl_parse_parse_encrypted_pms_secret"), ret);
            return ret;
        }
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_RSA_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECJPAKE) {
#if defined(MBEDTLS_USE_PSA_CRYPTO)
        if ((ret = mbedtls_psa_ecjpake_read_round(
                 &ssl->handshake->psa_pake_ctx, p, end - p,
                 MBEDTLS_ECJPAKE_ROUND_TWO)) != 0) {
            psa_destroy_key(ssl->handshake->psa_pake_password);
            psa_pake_abort(&ssl->handshake->psa_pake_ctx);

            MBEDTLS_SSL_DEBUG_RET(1, "psa_pake_input round two", ret);
            return ret;
        }
#else
        ret = mbedtls_ecjpake_read_round_two(&ssl->handshake->ecjpake_ctx,
                                             p, end - p);
        if (ret != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecjpake_read_round_two", ret);
            return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
        }

        ret = mbedtls_ecjpake_derive_secret(&ssl->handshake->ecjpake_ctx,
                                            ssl->handshake->premaster, 32, &ssl->handshake->pmslen,
                                            ssl->conf->f_rng, ssl->conf->p_rng);
        if (ret != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ecjpake_derive_secret", ret);
            return ret;
        }
#endif /* MBEDTLS_USE_PSA_CRYPTO */
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
    {
        MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }

    if ((ret = mbedtls_ssl_derive_keys(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_derive_keys", ret);
        return ret;
    }

    ssl->state++;

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= parse client key exchange"));

    return 0;
}

#if !defined(MBEDTLS_KEY_EXCHANGE_CERT_REQ_ALLOWED_ENABLED)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_certificate_verify(mbedtls_ssl_context *ssl)
{
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->handshake->ciphersuite_info;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> parse certificate verify"));

    if (!mbedtls_ssl_ciphersuite_cert_req_allowed(ciphersuite_info)) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip parse certificate verify"));
        ssl->state++;
        return 0;
    }

    MBEDTLS_SSL_DEBUG_MSG(1, ("should never happen"));
    return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
}
#else /* !MBEDTLS_KEY_EXCHANGE_CERT_REQ_ALLOWED_ENABLED */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_parse_certificate_verify(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
    size_t i, sig_len;
    unsigned char hash[48];
    unsigned char *hash_start = hash;
    size_t hashlen;
    mbedtls_pk_type_t pk_alg;
    mbedtls_md_type_t md_alg;
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->handshake->ciphersuite_info;
    mbedtls_pk_context *peer_pk;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> parse certificate verify"));

    if (!mbedtls_ssl_ciphersuite_cert_req_allowed(ciphersuite_info)) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip parse certificate verify"));
        ssl->state++;
        return 0;
    }

#if defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
    if (ssl->session_negotiate->peer_cert == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip parse certificate verify"));
        ssl->state++;
        return 0;
    }
#else /* MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */
    if (ssl->session_negotiate->peer_cert_digest == NULL) {
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip parse certificate verify"));
        ssl->state++;
        return 0;
    }
#endif /* !MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */

    /* Read the message without adding it to the checksum */
    ret = mbedtls_ssl_read_record(ssl, 0 /* no checksum update */);
    if (0 != ret) {
        MBEDTLS_SSL_DEBUG_RET(1, ("mbedtls_ssl_read_record"), ret);
        return ret;
    }

    ssl->state++;

    /* Process the message contents */
    if (ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE ||
        ssl->in_msg[0] != MBEDTLS_SSL_HS_CERTIFICATE_VERIFY) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad certificate verify message"));
        return MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE;
    }

    i = mbedtls_ssl_hs_hdr_len(ssl);

#if !defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
    peer_pk = &ssl->handshake->peer_pubkey;
#else /* !MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */
    if (ssl->session_negotiate->peer_cert == NULL) {
        /* Should never happen */
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }
    peer_pk = &ssl->session_negotiate->peer_cert->pk;
#endif /* MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */

    /*
     *  struct {
     *     SignatureAndHashAlgorithm algorithm; -- TLS 1.2 only
     *     opaque signature<0..2^16-1>;
     *  } DigitallySigned;
     */
    if (i + 2 > ssl->in_hslen) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad certificate verify message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    /*
     * Hash
     */
    md_alg = mbedtls_ssl_md_alg_from_hash(ssl->in_msg[i]);

    if (md_alg == MBEDTLS_MD_NONE || mbedtls_ssl_set_calc_verify_md(ssl, ssl->in_msg[i])) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("peer not adhering to requested sig_alg"
                                  " for verify message"));
        return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
    }

#if !defined(MBEDTLS_MD_SHA1)
    if (MBEDTLS_MD_SHA1 == md_alg) {
        hash_start += 16;
    }
#endif

    /* Info from md_alg will be used instead */
    hashlen = 0;

    i++;

    /*
     * Signature
     */
    if ((pk_alg = mbedtls_ssl_pk_alg_from_sig(ssl->in_msg[i]))
        == MBEDTLS_PK_NONE) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("peer not adhering to requested sig_alg"
                                  " for verify message"));
        return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
    }

    /*
     * Check the certificate's key type matches the signature alg
     */
    if (!mbedtls_pk_can_do(peer_pk, pk_alg)) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("sig_alg doesn't match cert key"));
        return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
    }

    i++;

    if (i + 2 > ssl->in_hslen) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad certificate verify message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    sig_len = (ssl->in_msg[i] << 8) | ssl->in_msg[i+1];
    i += 2;

    if (i + sig_len != ssl->in_hslen) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("bad certificate verify message"));
        return MBEDTLS_ERR_SSL_DECODE_ERROR;
    }

    /* Calculate hash and verify signature */
    {
        size_t dummy_hlen;
        ret = ssl->handshake->calc_verify(ssl, hash, &dummy_hlen);
        if (0 != ret) {
            MBEDTLS_SSL_DEBUG_RET(1, ("calc_verify"), ret);
            return ret;
        }
    }

    if ((ret = mbedtls_pk_verify(peer_pk,
                                 md_alg, hash_start, hashlen,
                                 ssl->in_msg + i, sig_len)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_pk_verify", ret);
        return ret;
    }

    ret = mbedtls_ssl_update_handshake_status(ssl);
    if (0 != ret) {
        MBEDTLS_SSL_DEBUG_RET(1, ("mbedtls_ssl_update_handshake_status"), ret);
        return ret;
    }

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= parse certificate verify"));

    return ret;
}
#endif /* MBEDTLS_KEY_EXCHANGE_CERT_REQ_ALLOWED_ENABLED */

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_new_session_ticket(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    size_t tlen;
    uint32_t lifetime;

    MBEDTLS_SSL_DEBUG_MSG(2, ("=> write new session ticket"));

    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
    ssl->out_msg[0]  = MBEDTLS_SSL_HS_NEW_SESSION_TICKET;

    /*
     * struct {
     *     uint32 ticket_lifetime_hint;
     *     opaque ticket<0..2^16-1>;
     * } NewSessionTicket;
     *
     * 4  .  7   ticket_lifetime_hint (0 = unspecified)
     * 8  .  9   ticket_len (n)
     * 10 .  9+n ticket content
     */

    if ((ret = ssl->conf->f_ticket_write(ssl->conf->p_ticket,
                                         ssl->session_negotiate,
                                         ssl->out_msg + 10,
                                         ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN,
                                         &tlen, &lifetime)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_ticket_write", ret);
        tlen = 0;
    }

    MBEDTLS_PUT_UINT32_BE(lifetime, ssl->out_msg, 4);
    MBEDTLS_PUT_UINT16_BE(tlen, ssl->out_msg, 8);
    ssl->out_msglen = 10 + tlen;

    /*
     * Morally equivalent to updating ssl->state, but NewSessionTicket and
     * ChangeCipherSpec share the same state.
     */
    ssl->handshake->new_session_ticket = 0;

    if ((ret = mbedtls_ssl_write_handshake_msg(ssl)) != 0) {
        MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_write_handshake_msg", ret);
        return ret;
    }

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= write new session ticket"));

    return 0;
}
#endif /* MBEDTLS_SSL_SESSION_TICKETS */

/*
 * SSL handshake -- server side -- single step
 */
int mbedtls_ssl_handshake_server_step(mbedtls_ssl_context *ssl)
{
    int ret = 0;

    MBEDTLS_SSL_DEBUG_MSG(2, ("server state: %d", ssl->state));

    switch (ssl->state) {
        case MBEDTLS_SSL_HELLO_REQUEST:
            ssl->state = MBEDTLS_SSL_CLIENT_HELLO;
            break;

        /*
         *  <==   ClientHello
         */
        case MBEDTLS_SSL_CLIENT_HELLO:
            ret = ssl_parse_client_hello(ssl);
            break;

#if defined(MBEDTLS_SSL_PROTO_DTLS)
        case MBEDTLS_SSL_SERVER_HELLO_VERIFY_REQUEST_SENT:
            return MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED;
#endif

        /*
         *  ==>   ServerHello
         *        Certificate
         *      ( ServerKeyExchange  )
         *      ( CertificateRequest )
         *        ServerHelloDone
         */
        case MBEDTLS_SSL_SERVER_HELLO:
            ret = ssl_write_server_hello(ssl);
            break;

        case MBEDTLS_SSL_SERVER_CERTIFICATE:
            ret = mbedtls_ssl_write_certificate(ssl);
            break;

        case MBEDTLS_SSL_SERVER_KEY_EXCHANGE:
            ret = ssl_write_server_key_exchange(ssl);
            break;

        case MBEDTLS_SSL_CERTIFICATE_REQUEST:
            ret = ssl_write_certificate_request(ssl);
            break;

        case MBEDTLS_SSL_SERVER_HELLO_DONE:
            ret = ssl_write_server_hello_done(ssl);
            break;

        /*
         *  <== ( Certificate/Alert  )
         *        ClientKeyExchange
         *      ( CertificateVerify  )
         *        ChangeCipherSpec
         *        Finished
         */
        case MBEDTLS_SSL_CLIENT_CERTIFICATE:
            ret = mbedtls_ssl_parse_certificate(ssl);
            break;

        case MBEDTLS_SSL_CLIENT_KEY_EXCHANGE:
            ret = ssl_parse_client_key_exchange(ssl);
            break;

        case MBEDTLS_SSL_CERTIFICATE_VERIFY:
            ret = ssl_parse_certificate_verify(ssl);
            break;

        case MBEDTLS_SSL_CLIENT_CHANGE_CIPHER_SPEC:
            ret = mbedtls_ssl_parse_change_cipher_spec(ssl);
            break;

        case MBEDTLS_SSL_CLIENT_FINISHED:
            ret = mbedtls_ssl_parse_finished(ssl);
            break;

        /*
         *  ==> ( NewSessionTicket )
         *        ChangeCipherSpec
         *        Finished
         */
        case MBEDTLS_SSL_SERVER_CHANGE_CIPHER_SPEC:
#if defined(MBEDTLS_SSL_SESSION_TICKETS)
            if (ssl->handshake->new_session_ticket != 0) {
                ret = ssl_write_new_session_ticket(ssl);
            } else
#endif
            ret = mbedtls_ssl_write_change_cipher_spec(ssl);
            break;

        case MBEDTLS_SSL_SERVER_FINISHED:
            ret = mbedtls_ssl_write_finished(ssl);
            break;

        case MBEDTLS_SSL_FLUSH_BUFFERS:
            MBEDTLS_SSL_DEBUG_MSG(2, ("handshake: done"));
            ssl->state = MBEDTLS_SSL_HANDSHAKE_WRAPUP;
            break;

        case MBEDTLS_SSL_HANDSHAKE_WRAPUP:
            mbedtls_ssl_handshake_wrapup(ssl);
            break;

        default:
            MBEDTLS_SSL_DEBUG_MSG(1, ("invalid state %d", ssl->state));
            return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
    }

    return ret;
}

void mbedtls_ssl_conf_preference_order(mbedtls_ssl_config *conf, int order)
{
    conf->respect_cli_pref = order;
}

#endif /* MBEDTLS_SSL_SRV_C && MBEDTLS_SSL_PROTO_TLS1_2 */