library/ssl_tls12_server.c

/*
 *  TLS server-side functions
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
 */

#include "ssl_misc.h"

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

#include "mbedtls/platform.h"

#include "mbedtls/ssl.h"
#include "debug_internal.h"
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"
#include "constant_time_internal.h"
#include "mbedtls/constant_time.h"

#include <string.h>

/* Define a local translating function to save code size by not using too many
 * arguments in each translating place. */
#if defined(MBEDTLS_KEY_EXCHANGE_SOME_ECDHE_ENABLED)
static int local_err_translation(psa_status_t status)
{
    return psa_status_to_mbedtls(status, psa_to_ssl_errors,
                                 ARRAY_LENGTH(psa_to_ssl_errors),
                                 psa_generic_status_to_mbedtls);
}
#define PSA_TO_MBEDTLS_ERR(status) local_err_translation(status)
#endif

#if defined(MBEDTLS_ECP_C)
#include "mbedtls/private/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 (!mbedtls_svc_key_id_is_null(conf->psk_opaque)) {
        return 1;
    }

    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_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED) || \
    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.
 *
 */
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 = MBEDTLS_GET_UINT16_BE(buf, 0);
    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) {
            MBEDTLS_SSL_DEBUG_MSG(4, ("point format selected: %d", p[0]));
            return 0;
        }

        list_size--;
        p++;
    }

    return 0;
}
#endif /* MBEDTLS_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED ||
          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 (ssl->handshake->psa_pake_ctx_is_ok != 1) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("skip ecjpake kkpp extension"));
        return 0;
    }

    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;
    }

    /* 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_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED)
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_get_ec_group_id(pk);
    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_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED */

/*
 * Try picking a certificate for this ciphersuite,
 * return 0 on success and -1 on failure.
 */
#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
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;
    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);
    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;
    pk_alg_is_none = (pk_alg == PSA_ALG_NONE);
    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_SSL_ASYNC_PRIVATE)
        key_type_matches = ((ssl->conf->f_async_sign_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 */
        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_CLIENT,
                                         MBEDTLS_SSL_VERSION_TLS1_2,
                                         &flags) != 0) {
            MBEDTLS_SSL_DEBUG_MSG(3, ("certificate mismatch: "
                                      "(extended) key usage extension"));
            continue;
        }

#if defined(MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED)
        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_KEY_EXCHANGE_WITH_CERT_ENABLED */

#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_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED)
    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_KEY_EXCHANGE_WITH_CERT_ENABLED)

#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 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.
     * Otherwise in a scenario of TLS 1.3/TLS 1.2 version negotiation, the
     * ClientHello has been already fully fetched by the TLS 1.3 code and the
     * flag ssl->keep_current_message is raised.
     */
    renegotiating = 0;
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    renegotiating = (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE);
#endif
    if (!renegotiating && !ssl->keep_current_message) {
        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",
                              MBEDTLS_GET_UINT16_BE(ssl->in_len, 0)));

    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 = MBEDTLS_GET_UINT16_BE(ssl->in_len, 0);

#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 (ssl->keep_current_message) {
            ssl->keep_current_message = 0;
        } else {
            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;
    }

#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 = (unsigned int) MBEDTLS_GET_UINT16_BE(ssl->in_msg, 4);
            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 = (unsigned int) MBEDTLS_GET_UINT16_BE(ssl->in_msg, 4);
            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 layout:
     *     0  .   1   protocol version
     *     2  .  33   random bytes (starting with 4 bytes of Unix time)
     *    34  .  34   session id length (1 byte)
     *    35  . 34+x  session id, where x = session id length from byte 34
     *   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_protocol_version) mbedtls_ssl_read_version(buf,
                                                                               ssl->conf->transport);
    ssl->session_negotiate->tls_version = ssl->tls_version;
    ssl->session_negotiate->endpoint = ssl->conf->endpoint;

    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 = MBEDTLS_GET_UINT16_BE(buf, ciph_offset);

    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 = MBEDTLS_GET_UINT16_BE(buf, ext_offset);

        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   = MBEDTLS_GET_UINT16_BE(ext, 0);
        ext_size = MBEDTLS_GET_UINT16_BE(ext, 2);

        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_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED) || \
                defined(MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED) || \
                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_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED || \
          MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED ||
          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_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED)
            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;

    mbedtls_ssl_handshake_increment_state(ssl);

#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 = (size_t) (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_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED) || \
    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_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED ||
          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;

    ret = mbedtls_psa_ecjpake_write_round(&ssl->handshake->psa_pake_ctx,
                                          p + 2, (size_t) (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;
    }

    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  = (size_t) (p - ssl->out_msg);
    ssl->out_msgtype = MBEDTLS_SSL_MSG_HANDSHAKE;
    ssl->out_msg[0]  = MBEDTLS_SSL_HS_HELLO_VERIFY_REQUEST;

    mbedtls_ssl_handshake_set_state(ssl, 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 */

    /*
     *     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 = psa_generate_random(p, 4)) != 0) {
        return ret;
    }

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

    if ((ret = psa_generate_random(p, 20)) != 0) {
        return ret;
    }
    p += 20;

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    /*
     * RFC 8446
     * TLS 1.3 has a downgrade protection mechanism embedded in the server's
     * random value. TLS 1.3 servers which negotiate TLS 1.2 or below in
     * response to a ClientHello MUST set the last 8 bytes of their Random
     * value specially in their ServerHello.
     */
    if (mbedtls_ssl_conf_is_tls13_enabled(ssl->conf)) {
        static const unsigned char magic_tls12_downgrade_string[] =
        { 'D', 'O', 'W', 'N', 'G', 'R', 'D', 1 };

        MBEDTLS_STATIC_ASSERT(
            sizeof(magic_tls12_downgrade_string) == 8,
            "magic_tls12_downgrade_string does not have the expected size");

        memcpy(p, magic_tls12_downgrade_string,
               sizeof(magic_tls12_downgrade_string));
    } else
#endif
    {
        if ((ret = psa_generate_random(p, 8)) != 0) {
            return ret;
        }
    }
    p += 8;

    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
         */
        mbedtls_ssl_handshake_increment_state(ssl);

#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 = psa_generate_random(ssl->session_negotiate->id,
                                           n)) != 0) {
                return ret;
            }
        }
    } else {
        /*
         * Resuming a session
         */
        n = ssl->session_negotiate->id_len;
        mbedtls_ssl_handshake_set_state(ssl, 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_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_1_2_ENABLED) || \
    defined(MBEDTLS_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED) || \
    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  = (size_t) (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"));
        mbedtls_ssl_handshake_increment_state(ssl);
        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"));

    mbedtls_ssl_handshake_increment_state(ssl);

#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_KEY_EXCHANGE_ECDSA_CERT_REQ_ALLOWED_ENABLED)
    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 += (unsigned short) (2 + dn_size);
            crt = crt->next;
        }
    }

    ssl->out_msglen  = (size_t) (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_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 - (size_t) (ssl->out_msg - ssl->out_buf);
#else
    size_t out_buf_len = MBEDTLS_SSL_OUT_BUFFER_LEN - (size_t) (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;
        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;
    }
#endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */

    /*
     * For ECDHE 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_ECDHE_PSK_ENABLED)
    if (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_ECDHE_PSK_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 = ssl->conf->group_list;
        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)));

        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_key_type_t key_type = PSA_KEY_TYPE_NONE;
        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,
                                                       &key_type,
                                                       &ec_bits) == PSA_ERROR_NOT_SUPPORTED) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("Invalid ecc group parse."));
            return MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER;
        }
        handshake->xxdh_psa_type = key_type;
        handshake->xxdh_psa_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->xxdh_psa_type);
        psa_set_key_bits(&key_attributes, handshake->xxdh_psa_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->xxdh_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->xxdh_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->xxdh_psa_privkey);
            handshake->xxdh_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;

#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 = (size_t) (ssl->out_msg + ssl->out_msglen - dig_signed);
        size_t hashlen = 0;
        unsigned char hash[MBEDTLS_MD_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 char sig_hash =
            (unsigned char) 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)) != 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_PSK_ENABLED)
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->handshake->ciphersuite_info;
#endif /* MBEDTLS_KEY_EXCHANGE_PSK_ENABLED */

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

#if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED)
    if (mbedtls_ssl_ciphersuite_no_pfs(ciphersuite_info)) {
        /* Key exchanges not involving ephemeral keys don't use
         * ServerKeyExchange, so end here. */
        MBEDTLS_SSL_DEBUG_MSG(2, ("<= skip write server key exchange"));
        mbedtls_ssl_handshake_increment_state(ssl);
        return 0;
    }
#endif /* MBEDTLS_KEY_EXCHANGE_PSK_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;

    mbedtls_ssl_handshake_increment_state(ssl);

    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;

    mbedtls_ssl_handshake_increment_state(ssl);

#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_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 = MBEDTLS_GET_UINT16_BE(*p, 0);
    *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 ((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_ECDHE_RSA_ENABLED) ||                     \
    defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_RSA ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA) {
        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(3, ("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: %" MBEDTLS_PRINTF_SIZET,
                                      buf_len));
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }

        if (data_len < 1 || data_len > buf_len) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("Invalid data length: %" MBEDTLS_PRINTF_SIZET
                                      " > %" MBEDTLS_PRINTF_SIZET,
                                      data_len, buf_len));
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }

        /* Store peer's ECDH public key. */
        if (data_len > sizeof(handshake->xxdh_psa_peerkey)) {
            MBEDTLS_SSL_DEBUG_MSG(1, ("Invalid public key length: %" MBEDTLS_PRINTF_SIZET
                                      " > %" MBEDTLS_PRINTF_SIZET,
                                      data_len,
                                      sizeof(handshake->xxdh_psa_peerkey)));
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }
        memcpy(handshake->xxdh_psa_peerkey, p, data_len);
        handshake->xxdh_psa_peerkey_len = data_len;

        /* Compute ECDH shared secret. */
        status = psa_raw_key_agreement(
            PSA_ALG_ECDH, handshake->xxdh_psa_privkey,
            handshake->xxdh_psa_peerkey, handshake->xxdh_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->xxdh_psa_privkey_is_external == 0) {
                (void) psa_destroy_key(handshake->xxdh_psa_privkey);
            }
            handshake->xxdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return ret;
        }

        if (handshake->xxdh_psa_privkey_is_external == 0) {
            status = psa_destroy_key(handshake->xxdh_psa_privkey);

            if (status != PSA_SUCCESS) {
                ret = PSA_TO_MBEDTLS_ERR(status);
                MBEDTLS_SSL_DEBUG_RET(1, "psa_destroy_key", ret);
                return ret;
            }
        }
        handshake->xxdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
    } else
#endif /* MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED ||
          MBEDTLS_KEY_EXCHANGE_ECDHE_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;
        }

    } else
#endif /* MBEDTLS_KEY_EXCHANGE_PSK_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_PSK) {
        psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
        psa_status_t destruction_status = PSA_ERROR_CORRUPTION_DETECTED;
        size_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->xxdh_psa_privkey);
            handshake->xxdh_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->xxdh_psa_privkey);
            handshake->xxdh_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->xxdh_psa_privkey);
            handshake->xxdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return MBEDTLS_ERR_SSL_DECODE_ERROR;
        }

        /* When FFDH is enabled, the array handshake->xxdh_psa_peer_key size takes into account
           the sizes of the FFDH keys which are at least 2048 bits.
           The size of the array is thus greater than 256 bytes which is greater than any
           possible value of ecpoint_len (type uint8_t) and the check below can be skipped.*/
#if !defined(PSA_WANT_ALG_FFDH)
        if (ecpoint_len > sizeof(handshake->xxdh_psa_peerkey)) {
            psa_destroy_key(handshake->xxdh_psa_privkey);
            handshake->xxdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
            return MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE;
        }
#else
        MBEDTLS_STATIC_ASSERT(sizeof(handshake->xxdh_psa_peerkey) >= UINT8_MAX,
                              "peer key buffer too small");
#endif

        memcpy(handshake->xxdh_psa_peerkey, p, ecpoint_len);
        handshake->xxdh_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->xxdh_psa_privkey,
                                       handshake->xxdh_psa_peerkey,
                                       handshake->xxdh_psa_peerkey_len,
                                       psm + zlen_size,
                                       psm_end - (psm + zlen_size),
                                       &zlen);

        destruction_status = psa_destroy_key(handshake->xxdh_psa_privkey);
        handshake->xxdh_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
#endif /* MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED */
#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
    if (ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECJPAKE) {
        if ((ret = mbedtls_psa_ecjpake_read_round(
                 &ssl->handshake->psa_pake_ctx, p, (size_t) (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
#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;
    }

    mbedtls_ssl_handshake_increment_state(ssl);

    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"));
        mbedtls_ssl_handshake_increment_state(ssl);
        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"));
        mbedtls_ssl_handshake_increment_state(ssl);
        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"));
        mbedtls_ssl_handshake_increment_state(ssl);
        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"));
        mbedtls_ssl_handshake_increment_state(ssl);
        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;
    }

    mbedtls_ssl_handshake_increment_state(ssl);

    /* 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 = MBEDTLS_GET_UINT16_BE(ssl->in_msg, i);
    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 defined(MBEDTLS_HAVE_TIME)
    ssl->session_negotiate->ticket_creation_time = mbedtls_ms_time();
#endif
    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:
            mbedtls_ssl_handshake_set_state(ssl, 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"));
            mbedtls_ssl_handshake_set_state(ssl, 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 */