programs/ssl/ssl_mail_client.c

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

/* Enable definition of gethostname() even when compiling with -std=c99. Must
 * be set before mbedtls_config.h, which pulls in glibc's features.h indirectly.
 * Harmless on other platforms. */

#define _POSIX_C_SOURCE 200112L
#define _XOPEN_SOURCE   600

#include "mbedtls/build_info.h"

#if defined(MBEDTLS_PLATFORM_C)
#    include "mbedtls/platform.h"
#else
#    include <stdio.h>
#    include <stdlib.h>
#    define mbedtls_time         time
#    define mbedtls_time_t       time_t
#    define mbedtls_fprintf      fprintf
#    define mbedtls_printf       printf
#    define mbedtls_exit         exit
#    define MBEDTLS_EXIT_SUCCESS EXIT_SUCCESS
#    define MBEDTLS_EXIT_FAILURE EXIT_FAILURE
#endif /* MBEDTLS_PLATFORM_C */

#if !defined(MBEDTLS_BIGNUM_C) || !defined(MBEDTLS_ENTROPY_C) ||          \
    !defined(MBEDTLS_SSL_TLS_C) || !defined(MBEDTLS_SSL_CLI_C) ||         \
    !defined(MBEDTLS_NET_C) || !defined(MBEDTLS_RSA_C) ||                 \
    !defined(MBEDTLS_CTR_DRBG_C) || !defined(MBEDTLS_X509_CRT_PARSE_C) || \
    !defined(MBEDTLS_FS_IO)
int main(void)
{
    mbedtls_printf("MBEDTLS_BIGNUM_C and/or MBEDTLS_ENTROPY_C and/or "
                   "MBEDTLS_SSL_TLS_C and/or MBEDTLS_SSL_CLI_C and/or "
                   "MBEDTLS_NET_C and/or MBEDTLS_RSA_C and/or "
                   "MBEDTLS_CTR_DRBG_C and/or MBEDTLS_X509_CRT_PARSE_C "
                   "not defined.\n");
    mbedtls_exit(0);
}
#else

#    include "mbedtls/base64.h"
#    include "mbedtls/error.h"
#    include "mbedtls/net_sockets.h"
#    include "mbedtls/ssl.h"
#    include "mbedtls/entropy.h"
#    include "mbedtls/ctr_drbg.h"
#    include "test/certs.h"
#    include "mbedtls/x509.h"

#    include <stdlib.h>
#    include <string.h>

#    if !defined(_MSC_VER) || defined(EFIX64) || defined(EFI32)
#        include <unistd.h>
#    else
#        include <io.h>
#    endif

#    if defined(_WIN32) || defined(_WIN32_WCE)
#        include <winsock2.h>
#        include <windows.h>

#        if defined(_MSC_VER)
#            if defined(_WIN32_WCE)
#                pragma comment(lib, "ws2.lib")
#            else
#                pragma comment(lib, "ws2_32.lib")
#            endif
#        endif /* _MSC_VER */
#    endif

#    define DFL_SERVER_NAME    "localhost"
#    define DFL_SERVER_PORT    "465"
#    define DFL_USER_NAME      "user"
#    define DFL_USER_PWD       "password"
#    define DFL_MAIL_FROM      ""
#    define DFL_MAIL_TO        ""
#    define DFL_DEBUG_LEVEL    0
#    define DFL_CA_FILE        ""
#    define DFL_CRT_FILE       ""
#    define DFL_KEY_FILE       ""
#    define DFL_FORCE_CIPHER   0
#    define DFL_MODE           0
#    define DFL_AUTHENTICATION 0

#    define MODE_SSL_TLS  0
#    define MODE_STARTTLS 0

#    if defined(MBEDTLS_BASE64_C)
#        define USAGE_AUTH                                              \
            "    authentication=%%d   default: 0 (disabled)\n"          \
            "    user_name=%%s        default: \"" DFL_USER_NAME "\"\n" \
            "    user_pwd=%%s         default: \"" DFL_USER_PWD "\"\n"
#    else
#        define USAGE_AUTH \
            "    authentication options disabled. (Require MBEDTLS_BASE64_C)\n"
#    endif /* MBEDTLS_BASE64_C */

#    if defined(MBEDTLS_FS_IO)
#        define USAGE_IO                                            \
            "    ca_file=%%s          default: \"\" (pre-loaded)\n" \
            "    crt_file=%%s         default: \"\" (pre-loaded)\n" \
            "    key_file=%%s         default: \"\" (pre-loaded)\n"
#    else
#        define USAGE_IO \
            "    No file operations available (MBEDTLS_FS_IO not defined)\n"
#    endif /* MBEDTLS_FS_IO */

#    define USAGE                                                                     \
        "\n usage: ssl_mail_client param=<>...\n"                                     \
        "\n acceptable parameters:\n"                                                 \
        "    server_name=%%s      default: " DFL_SERVER_NAME "\n"                     \
        "    server_port=%%d      default: " DFL_SERVER_PORT "\n"                     \
        "    debug_level=%%d      default: 0 (disabled)\n"                            \
        "    mode=%%d             default: 0 (SSL/TLS) (1 for STARTTLS)\n" USAGE_AUTH \
        "    mail_from=%%s        default: \"\"\n"                                    \
        "    mail_to=%%s          default: \"\"\n" USAGE_IO                           \
        "    force_ciphersuite=<name>    default: all enabled\n"                      \
        " acceptable ciphersuite names:\n"

/*
 * global options
 */
struct options {
    const char *server_name; /* hostname of the server (client only)     */
    const char *server_port; /* port on which the ssl service runs       */
    int debug_level; /* level of debugging                       */
    int authentication; /* if authentication is required            */
    int mode; /* SSL/TLS (0) or STARTTLS (1)              */
    const char *user_name; /* username to use for authentication       */
    const char *user_pwd; /* password to use for authentication       */
    const char *mail_from; /* E-Mail address to use as sender          */
    const char *mail_to; /* E-Mail address to use as recipient       */
    const char *ca_file; /* the file with the CA certificate(s)      */
    const char *crt_file; /* the file with the client certificate     */
    const char *key_file; /* the file with the client key             */
    int force_ciphersuite[2]; /* protocol/ciphersuite to use, or all      */
} opt;

static void
my_debug(void *ctx, int level, const char *file, int line, const char *str)
{
    ((void)level);

    mbedtls_fprintf((FILE *)ctx, "%s:%04d: %s", file, line, str);
    fflush((FILE *)ctx);
}

static int do_handshake(mbedtls_ssl_context *ssl)
{
    int ret;
    uint32_t flags;
    unsigned char buf[1024];
    memset(buf, 0, 1024);

    /*
     * 4. Handshake
     */
    mbedtls_printf("  . Performing the SSL/TLS handshake...");
    fflush(stdout);

    while ((ret = mbedtls_ssl_handshake(ssl)) != 0) {
        if (ret != MBEDTLS_ERR_SSL_WANT_READ &&
            ret != MBEDTLS_ERR_SSL_WANT_WRITE) {
#    if defined(MBEDTLS_ERROR_C)
            mbedtls_strerror(ret, (char *)buf, 1024);
#    endif
            mbedtls_printf(
                " failed\n  ! mbedtls_ssl_handshake returned %d: %s\n\n", ret,
                buf);
            return -1;
        }
    }

    mbedtls_printf(" ok\n    [ Ciphersuite is %s ]\n",
                   mbedtls_ssl_get_ciphersuite(ssl));

    /*
     * 5. Verify the server certificate
     */
    mbedtls_printf("  . Verifying peer X.509 certificate...");

    /* In real life, we probably want to bail out when ret != 0 */
    if ((flags = mbedtls_ssl_get_verify_result(ssl)) != 0) {
#    if !defined(MBEDTLS_X509_REMOVE_INFO)
        char vrfy_buf[512];
#    endif

        mbedtls_printf(" failed\n");

#    if !defined(MBEDTLS_X509_REMOVE_INFO)
        mbedtls_x509_crt_verify_info(vrfy_buf, sizeof(vrfy_buf), "  ! ", flags);

        mbedtls_printf("%s\n", vrfy_buf);
#    endif
    } else
        mbedtls_printf(" ok\n");

#    if !defined(MBEDTLS_X509_REMOVE_INFO)
    mbedtls_printf("  . Peer certificate information    ...\n");
    mbedtls_x509_crt_info((char *)buf, sizeof(buf) - 1, "      ",
                          mbedtls_ssl_get_peer_cert(ssl));
    mbedtls_printf("%s\n", buf);
#    endif

    return 0;
}

static int
write_ssl_data(mbedtls_ssl_context *ssl, unsigned char *buf, size_t len)
{
    int ret;

    mbedtls_printf("\n%s", buf);
    while (len && (ret = mbedtls_ssl_write(ssl, buf, len)) <= 0) {
        if (ret != MBEDTLS_ERR_SSL_WANT_READ &&
            ret != MBEDTLS_ERR_SSL_WANT_WRITE) {
            mbedtls_printf(" failed\n  ! mbedtls_ssl_write returned %d\n\n",
                           ret);
            return -1;
        }
    }

    return 0;
}

static int write_ssl_and_get_response(mbedtls_ssl_context *ssl,
                                      unsigned char *buf,
                                      size_t len)
{
    int ret;
    unsigned char data[128];
    char code[4];
    size_t i, idx = 0;

    mbedtls_printf("\n%s", buf);
    while (len && (ret = mbedtls_ssl_write(ssl, buf, len)) <= 0) {
        if (ret != MBEDTLS_ERR_SSL_WANT_READ &&
            ret != MBEDTLS_ERR_SSL_WANT_WRITE) {
            mbedtls_printf(" failed\n  ! mbedtls_ssl_write returned %d\n\n",
                           ret);
            return -1;
        }
    }

    do {
        len = sizeof(data) - 1;
        memset(data, 0, sizeof(data));
        ret = mbedtls_ssl_read(ssl, data, len);

        if (ret == MBEDTLS_ERR_SSL_WANT_READ ||
            ret == MBEDTLS_ERR_SSL_WANT_WRITE)
            continue;

        if (ret == MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY)
            return -1;

        if (ret <= 0) {
            mbedtls_printf("failed\n  ! mbedtls_ssl_read returned %d\n\n", ret);
            return -1;
        }

        mbedtls_printf("\n%s", data);
        len = ret;
        for (i = 0; i < len; i++) {
            if (data[i] != '\n') {
                if (idx < 4)
                    code[idx++] = data[i];
                continue;
            }

            if (idx == 4 && code[0] >= '0' && code[0] <= '9' &&
                code[3] == ' ') {
                code[3] = '\0';
                return atoi(code);
            }

            idx = 0;
        }
    } while (1);
}

static int write_and_get_response(mbedtls_net_context *sock_fd,
                                  unsigned char *buf,
                                  size_t len)
{
    int ret;
    unsigned char data[128];
    char code[4];
    size_t i, idx = 0;

    mbedtls_printf("\n%s", buf);
    if (len && (ret = mbedtls_net_send(sock_fd, buf, len)) <= 0) {
        mbedtls_printf(" failed\n  ! mbedtls_net_send returned %d\n\n", ret);
        return -1;
    }

    do {
        len = sizeof(data) - 1;
        memset(data, 0, sizeof(data));
        ret = mbedtls_net_recv(sock_fd, data, len);

        if (ret <= 0) {
            mbedtls_printf("failed\n  ! mbedtls_net_recv returned %d\n\n", ret);
            return -1;
        }

        data[len] = '\0';
        mbedtls_printf("\n%s", data);
        len = ret;
        for (i = 0; i < len; i++) {
            if (data[i] != '\n') {
                if (idx < 4)
                    code[idx++] = data[i];
                continue;
            }

            if (idx == 4 && code[0] >= '0' && code[0] <= '9' &&
                code[3] == ' ') {
                code[3] = '\0';
                return atoi(code);
            }

            idx = 0;
        }
    } while (1);
}

int main(int argc, char *argv[])
{
    int ret = 1, len;
    int exit_code = MBEDTLS_EXIT_FAILURE;
    mbedtls_net_context server_fd;
#    if defined(MBEDTLS_BASE64_C)
    unsigned char base[1024];
    /* buf is used as the destination buffer for printing base with the format:
     * "%s\r\n". Hence, the size of buf should be at least the size of base
     * plus 2 bytes for the \r and \n characters.
     */
    unsigned char buf[sizeof(base) + 2];
#    else
    unsigned char buf[1024];
#    endif
    char hostname[32];
    const char *pers = "ssl_mail_client";

    mbedtls_entropy_context entropy;
    mbedtls_ctr_drbg_context ctr_drbg;
    mbedtls_ssl_context ssl;
    mbedtls_ssl_config conf;
    mbedtls_x509_crt cacert;
    mbedtls_x509_crt clicert;
    mbedtls_pk_context pkey;
    int i;
    size_t n;
    char *p, *q;
    const int *list;

    /*
     * Make sure memory references are valid in case we exit early.
     */
    mbedtls_net_init(&server_fd);
    mbedtls_ssl_init(&ssl);
    mbedtls_ssl_config_init(&conf);
    memset(&buf, 0, sizeof(buf));
    mbedtls_x509_crt_init(&cacert);
    mbedtls_x509_crt_init(&clicert);
    mbedtls_pk_init(&pkey);
    mbedtls_ctr_drbg_init(&ctr_drbg);

    if (argc == 0) {
usage:
        mbedtls_printf(USAGE);

        list = mbedtls_ssl_list_ciphersuites();
        while (*list) {
            mbedtls_printf("    %s\n", mbedtls_ssl_get_ciphersuite_name(*list));
            list++;
        }
        mbedtls_printf("\n");
        goto exit;
    }

    opt.server_name = DFL_SERVER_NAME;
    opt.server_port = DFL_SERVER_PORT;
    opt.debug_level = DFL_DEBUG_LEVEL;
    opt.authentication = DFL_AUTHENTICATION;
    opt.mode = DFL_MODE;
    opt.user_name = DFL_USER_NAME;
    opt.user_pwd = DFL_USER_PWD;
    opt.mail_from = DFL_MAIL_FROM;
    opt.mail_to = DFL_MAIL_TO;
    opt.ca_file = DFL_CA_FILE;
    opt.crt_file = DFL_CRT_FILE;
    opt.key_file = DFL_KEY_FILE;
    opt.force_ciphersuite[0] = DFL_FORCE_CIPHER;

    for (i = 1; i < argc; i++) {
        p = argv[i];
        if ((q = strchr(p, '=')) == NULL)
            goto usage;
        *q++ = '\0';

        if (strcmp(p, "server_name") == 0)
            opt.server_name = q;
        else if (strcmp(p, "server_port") == 0)
            opt.server_port = q;
        else if (strcmp(p, "debug_level") == 0) {
            opt.debug_level = atoi(q);
            if (opt.debug_level < 0 || opt.debug_level > 65535)
                goto usage;
        } else if (strcmp(p, "authentication") == 0) {
            opt.authentication = atoi(q);
            if (opt.authentication < 0 || opt.authentication > 1)
                goto usage;
        } else if (strcmp(p, "mode") == 0) {
            opt.mode = atoi(q);
            if (opt.mode < 0 || opt.mode > 1)
                goto usage;
        } else if (strcmp(p, "user_name") == 0)
            opt.user_name = q;
        else if (strcmp(p, "user_pwd") == 0)
            opt.user_pwd = q;
        else if (strcmp(p, "mail_from") == 0)
            opt.mail_from = q;
        else if (strcmp(p, "mail_to") == 0)
            opt.mail_to = q;
        else if (strcmp(p, "ca_file") == 0)
            opt.ca_file = q;
        else if (strcmp(p, "crt_file") == 0)
            opt.crt_file = q;
        else if (strcmp(p, "key_file") == 0)
            opt.key_file = q;
        else if (strcmp(p, "force_ciphersuite") == 0) {
            opt.force_ciphersuite[0] = -1;

            opt.force_ciphersuite[0] = mbedtls_ssl_get_ciphersuite_id(q);

            if (opt.force_ciphersuite[0] <= 0)
                goto usage;

            opt.force_ciphersuite[1] = 0;
        } else
            goto usage;
    }

    /*
     * 0. Initialize the RNG and the session data
     */
    mbedtls_printf("\n  . Seeding the random number generator...");
    fflush(stdout);

    mbedtls_entropy_init(&entropy);
    if ((ret = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func, &entropy,
                                     (const unsigned char *)pers,
                                     strlen(pers))) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_ctr_drbg_seed returned %d\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    /*
     * 1.1. Load the trusted CA
     */
    mbedtls_printf("  . Loading the CA root certificate ...");
    fflush(stdout);

#    if defined(MBEDTLS_FS_IO)
    if (strlen(opt.ca_file))
        ret = mbedtls_x509_crt_parse_file(&cacert, opt.ca_file);
    else
#    endif
#    if defined(MBEDTLS_PEM_PARSE_C)
        ret = mbedtls_x509_crt_parse(
            &cacert, (const unsigned char *)mbedtls_test_cas_pem,
            mbedtls_test_cas_pem_len);
#    else
    {
        mbedtls_printf("MBEDTLS_PEM_PARSE_C not defined.");
        goto exit;
    }
#    endif
    if (ret < 0) {
        mbedtls_printf(" failed\n  !  mbedtls_x509_crt_parse returned %d\n\n",
                       ret);
        goto exit;
    }

    mbedtls_printf(" ok (%d skipped)\n", ret);

    /*
     * 1.2. Load own certificate and private key
     *
     * (can be skipped if client authentication is not required)
     */
    mbedtls_printf("  . Loading the client cert. and key...");
    fflush(stdout);

#    if defined(MBEDTLS_FS_IO)
    if (strlen(opt.crt_file))
        ret = mbedtls_x509_crt_parse_file(&clicert, opt.crt_file);
    else
#    endif
        ret = mbedtls_x509_crt_parse(
            &clicert, (const unsigned char *)mbedtls_test_cli_crt,
            mbedtls_test_cli_crt_len);
    if (ret != 0) {
        mbedtls_printf(" failed\n  !  mbedtls_x509_crt_parse returned %d\n\n",
                       ret);
        goto exit;
    }

#    if defined(MBEDTLS_FS_IO)
    if (strlen(opt.key_file)) {
        ret = mbedtls_pk_parse_keyfile(&pkey, opt.key_file, "",
                                       mbedtls_ctr_drbg_random, &ctr_drbg);
    } else
#    endif
#    if defined(MBEDTLS_PEM_PARSE_C)
    {
        ret = mbedtls_pk_parse_key(&pkey,
                                   (const unsigned char *)mbedtls_test_cli_key,
                                   mbedtls_test_cli_key_len, NULL, 0,
                                   mbedtls_ctr_drbg_random, &ctr_drbg);
    }
#    else
    {
        mbedtls_printf("MBEDTLS_PEM_PARSE_C not defined.");
        goto exit;
    }
#    endif
    if (ret != 0) {
        mbedtls_printf(" failed\n  !  mbedtls_pk_parse_key returned %d\n\n",
                       ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    /*
     * 2. Start the connection
     */
    mbedtls_printf("  . Connecting to tcp/%s/%s...", opt.server_name,
                   opt.server_port);
    fflush(stdout);

    if ((ret = mbedtls_net_connect(&server_fd, opt.server_name, opt.server_port,
                                   MBEDTLS_NET_PROTO_TCP)) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_net_connect returned %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    /*
     * 3. Setup stuff
     */
    mbedtls_printf("  . Setting up the SSL/TLS structure...");
    fflush(stdout);

    if ((ret = mbedtls_ssl_config_defaults(&conf, MBEDTLS_SSL_IS_CLIENT,
                                           MBEDTLS_SSL_TRANSPORT_STREAM,
                                           MBEDTLS_SSL_PRESET_DEFAULT)) != 0) {
        mbedtls_printf(
            " failed\n  ! mbedtls_ssl_config_defaults returned %d\n\n", ret);
        goto exit;
    }

    /* OPTIONAL is not optimal for security,
     * but makes interop easier in this simplified example */
    mbedtls_ssl_conf_authmode(&conf, MBEDTLS_SSL_VERIFY_OPTIONAL);

    mbedtls_ssl_conf_rng(&conf, mbedtls_ctr_drbg_random, &ctr_drbg);
    mbedtls_ssl_conf_dbg(&conf, my_debug, stdout);

    if (opt.force_ciphersuite[0] != DFL_FORCE_CIPHER)
        mbedtls_ssl_conf_ciphersuites(&conf, opt.force_ciphersuite);

    mbedtls_ssl_conf_ca_chain(&conf, &cacert, NULL);
    if ((ret = mbedtls_ssl_conf_own_cert(&conf, &clicert, &pkey)) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_ssl_conf_own_cert returned %d\n\n",
                       ret);
        goto exit;
    }

    if ((ret = mbedtls_ssl_setup(&ssl, &conf)) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_ssl_setup returned %d\n\n", ret);
        goto exit;
    }

    if ((ret = mbedtls_ssl_set_hostname(&ssl, opt.server_name)) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_ssl_set_hostname returned %d\n\n",
                       ret);
        goto exit;
    }

    mbedtls_ssl_set_bio(&ssl, &server_fd, mbedtls_net_send, mbedtls_net_recv,
                        NULL);

    mbedtls_printf(" ok\n");

    if (opt.mode == MODE_SSL_TLS) {
        if (do_handshake(&ssl) != 0)
            goto exit;

        mbedtls_printf("  > Get header from server:");
        fflush(stdout);

        ret = write_ssl_and_get_response(&ssl, buf, 0);
        if (ret < 200 || ret > 299) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }

        mbedtls_printf(" ok\n");

        mbedtls_printf("  > Write EHLO to server:");
        fflush(stdout);

        gethostname(hostname, 32);
        len = sprintf((char *)buf, "EHLO %s\r\n", hostname);
        ret = write_ssl_and_get_response(&ssl, buf, len);
        if (ret < 200 || ret > 299) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }
    } else {
        mbedtls_printf("  > Get header from server:");
        fflush(stdout);

        ret = write_and_get_response(&server_fd, buf, 0);
        if (ret < 200 || ret > 299) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }

        mbedtls_printf(" ok\n");

        mbedtls_printf("  > Write EHLO to server:");
        fflush(stdout);

        gethostname(hostname, 32);
        len = sprintf((char *)buf, "EHLO %s\r\n", hostname);
        ret = write_and_get_response(&server_fd, buf, len);
        if (ret < 200 || ret > 299) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }

        mbedtls_printf(" ok\n");

        mbedtls_printf("  > Write STARTTLS to server:");
        fflush(stdout);

        gethostname(hostname, 32);
        len = sprintf((char *)buf, "STARTTLS\r\n");
        ret = write_and_get_response(&server_fd, buf, len);
        if (ret < 200 || ret > 299) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }

        mbedtls_printf(" ok\n");

        if (do_handshake(&ssl) != 0)
            goto exit;
    }

#    if defined(MBEDTLS_BASE64_C)
    if (opt.authentication) {
        mbedtls_printf("  > Write AUTH LOGIN to server:");
        fflush(stdout);

        len = sprintf((char *)buf, "AUTH LOGIN\r\n");
        ret = write_ssl_and_get_response(&ssl, buf, len);
        if (ret < 200 || ret > 399) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }

        mbedtls_printf(" ok\n");

        mbedtls_printf("  > Write username to server: %s", opt.user_name);
        fflush(stdout);

        ret = mbedtls_base64_encode(base, sizeof(base), &n,
                                    (const unsigned char *)opt.user_name,
                                    strlen(opt.user_name));

        if (ret != 0) {
            mbedtls_printf(" failed\n  ! mbedtls_base64_encode returned %d\n\n",
                           ret);
            goto exit;
        }
        len = sprintf((char *)buf, "%s\r\n", base);
        ret = write_ssl_and_get_response(&ssl, buf, len);
        if (ret < 300 || ret > 399) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }

        mbedtls_printf(" ok\n");

        mbedtls_printf("  > Write password to server: %s", opt.user_pwd);
        fflush(stdout);

        ret = mbedtls_base64_encode(base, sizeof(base), &n,
                                    (const unsigned char *)opt.user_pwd,
                                    strlen(opt.user_pwd));

        if (ret != 0) {
            mbedtls_printf(" failed\n  ! mbedtls_base64_encode returned %d\n\n",
                           ret);
            goto exit;
        }
        len = sprintf((char *)buf, "%s\r\n", base);
        ret = write_ssl_and_get_response(&ssl, buf, len);
        if (ret < 200 || ret > 399) {
            mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
            goto exit;
        }

        mbedtls_printf(" ok\n");
    }
#    endif

    mbedtls_printf("  > Write MAIL FROM to server:");
    fflush(stdout);

    len = sprintf((char *)buf, "MAIL FROM:<%s>\r\n", opt.mail_from);
    ret = write_ssl_and_get_response(&ssl, buf, len);
    if (ret < 200 || ret > 299) {
        mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    mbedtls_printf("  > Write RCPT TO to server:");
    fflush(stdout);

    len = sprintf((char *)buf, "RCPT TO:<%s>\r\n", opt.mail_to);
    ret = write_ssl_and_get_response(&ssl, buf, len);
    if (ret < 200 || ret > 299) {
        mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    mbedtls_printf("  > Write DATA to server:");
    fflush(stdout);

    len = sprintf((char *)buf, "DATA\r\n");
    ret = write_ssl_and_get_response(&ssl, buf, len);
    if (ret < 300 || ret > 399) {
        mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    mbedtls_printf("  > Write content to server:");
    fflush(stdout);

    len = sprintf((char *)buf,
                  "From: %s\r\nSubject: mbed TLS Test mail\r\n\r\n"
                  "This is a simple test mail from the "
                  "mbed TLS mail client example.\r\n"
                  "\r\n"
                  "Enjoy!",
                  opt.mail_from);
    ret = write_ssl_data(&ssl, buf, len);

    len = sprintf((char *)buf, "\r\n.\r\n");
    ret = write_ssl_and_get_response(&ssl, buf, len);
    if (ret < 200 || ret > 299) {
        mbedtls_printf(" failed\n  ! server responded with %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    mbedtls_ssl_close_notify(&ssl);

    exit_code = MBEDTLS_EXIT_SUCCESS;

exit:

    mbedtls_net_free(&server_fd);
    mbedtls_x509_crt_free(&clicert);
    mbedtls_x509_crt_free(&cacert);
    mbedtls_pk_free(&pkey);
    mbedtls_ssl_free(&ssl);
    mbedtls_ssl_config_free(&conf);
    mbedtls_ctr_drbg_free(&ctr_drbg);
    mbedtls_entropy_free(&entropy);

#    if defined(_WIN32)
    mbedtls_printf("  + Press Enter to exit this program.\n");
    fflush(stdout);
    getchar();
#    endif

    mbedtls_exit(exit_code);
}
#endif /* MBEDTLS_BIGNUM_C && MBEDTLS_ENTROPY_C && MBEDTLS_SSL_TLS_C && \
          MBEDTLS_SSL_CLI_C && MBEDTLS_NET_C && MBEDTLS_RSA_C **        \
          MBEDTLS_CTR_DRBG_C */