mbedtls/library/ssl_client.c

/*
 *  TLS 1.2 and 1.3 client-side functions
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
 */

#include "common.h"

#if defined(MBEDTLS_SSL_CLI_C)
#if defined(MBEDTLS_SSL_PROTO_TLS1_3) || defined(MBEDTLS_SSL_PROTO_TLS1_2)

#include <string.h>

#include "mbedtls/debug.h"
#include "mbedtls/error.h"
#include "mbedtls/platform.h"

#include "ssl_client.h"
#include "ssl_misc.h"
#include "ssl_tls13_keys.h"
#include "ssl_debug_helpers.h"

#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_hostname_ext(mbedtls_ssl_context *ssl,
                                  unsigned char *buf,
                                  const unsigned char *end,
                                  size_t *olen)
{
    unsigned char *p = buf;
    size_t hostname_len;

    *olen = 0;

    if (ssl->hostname == NULL) {
        return 0;
    }

    MBEDTLS_SSL_DEBUG_MSG(3,
                          ("client hello, adding server name extension: %s",
                           ssl->hostname));

    hostname_len = strlen(ssl->hostname);

    MBEDTLS_SSL_CHK_BUF_PTR(p, end, hostname_len + 9);

    /*
     * Sect. 3, RFC 6066 (TLS Extensions Definitions)
     *
     * In order to provide any of the server names, clients MAY include an
     * extension of type "server_name" in the (extended) client hello. The
     * "extension_data" field of this extension SHALL contain
     * "ServerNameList" where:
     *
     * struct {
     *     NameType name_type;
     *     select (name_type) {
     *         case host_name: HostName;
     *     } name;
     * } ServerName;
     *
     * enum {
     *     host_name(0), (255)
     * } NameType;
     *
     * opaque HostName<1..2^16-1>;
     *
     * struct {
     *     ServerName server_name_list<1..2^16-1>
     * } ServerNameList;
     *
     */
    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_SERVERNAME, p, 0);
    p += 2;

    MBEDTLS_PUT_UINT16_BE(hostname_len + 5, p, 0);
    p += 2;

    MBEDTLS_PUT_UINT16_BE(hostname_len + 3, p, 0);
    p += 2;

    *p++ = MBEDTLS_BYTE_0(MBEDTLS_TLS_EXT_SERVERNAME_HOSTNAME);

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

    memcpy(p, ssl->hostname, hostname_len);

    *olen = hostname_len + 9;

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    mbedtls_ssl_tls13_set_hs_sent_ext_mask(ssl, MBEDTLS_TLS_EXT_SERVERNAME);
#endif /* MBEDTLS_SSL_PROTO_TLS1_3 */
    return 0;
}
#endif /* MBEDTLS_SSL_SERVER_NAME_INDICATION */

#if defined(MBEDTLS_SSL_ALPN)
/*
 * ssl_write_alpn_ext()
 *
 * Structure of the application_layer_protocol_negotiation extension in
 * ClientHello:
 *
 * opaque ProtocolName<1..2^8-1>;
 *
 * struct {
 *     ProtocolName protocol_name_list<2..2^16-1>
 * } ProtocolNameList;
 *
 */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_alpn_ext(mbedtls_ssl_context *ssl,
                              unsigned char *buf,
                              const unsigned char *end,
                              size_t *out_len)
{
    unsigned char *p = buf;

    *out_len = 0;

    if (ssl->conf->alpn_list == NULL) {
        return 0;
    }

    MBEDTLS_SSL_DEBUG_MSG(3, ("client hello, adding alpn extension"));


    /* Check we have enough space for the extension type (2 bytes), the
     * extension length (2 bytes) and the protocol_name_list length (2 bytes).
     */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, 6);
    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_ALPN, p, 0);
    /* Skip writing extension and list length for now */
    p += 6;

    /*
     * opaque ProtocolName<1..2^8-1>;
     *
     * struct {
     *     ProtocolName protocol_name_list<2..2^16-1>
     * } ProtocolNameList;
     */
    for (const char **cur = ssl->conf->alpn_list; *cur != NULL; cur++) {
        /*
         * mbedtls_ssl_conf_set_alpn_protocols() checked that the length of
         * protocol names is less than 255.
         */
        size_t protocol_name_len = strlen(*cur);

        MBEDTLS_SSL_CHK_BUF_PTR(p, end, 1 + protocol_name_len);
        *p++ = (unsigned char) protocol_name_len;
        memcpy(p, *cur, protocol_name_len);
        p += protocol_name_len;
    }

    *out_len = p - buf;

    /* List length = *out_len - 2 (ext_type) - 2 (ext_len) - 2 (list_len) */
    MBEDTLS_PUT_UINT16_BE(*out_len - 6, buf, 4);

    /* Extension length = *out_len - 2 (ext_type) - 2 (ext_len) */
    MBEDTLS_PUT_UINT16_BE(*out_len - 4, buf, 2);

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    mbedtls_ssl_tls13_set_hs_sent_ext_mask(ssl, MBEDTLS_TLS_EXT_ALPN);
#endif /* MBEDTLS_SSL_PROTO_TLS1_3 */
    return 0;
}
#endif /* MBEDTLS_SSL_ALPN */

#if defined(MBEDTLS_SSL_TLS1_2_SOME_ECC) || \
    defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_EPHEMERAL_ENABLED)
/*
 * Function for writing 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.
 *
 */
#define SSL_WRITE_SUPPORTED_GROUPS_EXT_TLS1_2_FLAG 1
#define SSL_WRITE_SUPPORTED_GROUPS_EXT_TLS1_3_FLAG 2

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_supported_groups_ext(mbedtls_ssl_context *ssl,
                                          unsigned char *buf,
                                          const unsigned char *end,
                                          int flags,
                                          size_t *out_len)
{
    unsigned char *p = buf;
    unsigned char *named_group_list; /* Start of named_group_list */
    size_t named_group_list_len;     /* Length of named_group_list */
    const uint16_t *group_list = mbedtls_ssl_get_groups(ssl);

    *out_len = 0;

    MBEDTLS_SSL_DEBUG_MSG(3, ("client hello, adding supported_groups extension"));

    /* Check if we have space for header and length fields:
     * - extension_type            (2 bytes)
     * - extension_data_length     (2 bytes)
     * - named_group_list_length   (2 bytes)
     */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, 6);
    p += 6;

    named_group_list = p;

    if (group_list == NULL) {
        return MBEDTLS_ERR_SSL_BAD_CONFIG;
    }

    for (; *group_list != 0; group_list++) {
        int propose_group = 0;

        MBEDTLS_SSL_DEBUG_MSG(3, ("got supported group(%04x)", *group_list));

#if defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_EPHEMERAL_ENABLED)
        if (flags & SSL_WRITE_SUPPORTED_GROUPS_EXT_TLS1_3_FLAG) {
#if defined(PSA_WANT_ALG_ECDH)
            if (mbedtls_ssl_tls13_named_group_is_ecdhe(*group_list) &&
                (mbedtls_ssl_get_ecp_group_id_from_tls_id(*group_list) !=
                 MBEDTLS_ECP_DP_NONE)) {
                propose_group = 1;
            }
#endif
#if defined(PSA_WANT_ALG_FFDH)
            if (mbedtls_ssl_tls13_named_group_is_ffdh(*group_list)) {
                propose_group = 1;
            }
#endif
        }
#endif /* MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_EPHEMERAL_ENABLED */

#if defined(MBEDTLS_SSL_TLS1_2_SOME_ECC)
        if ((flags & SSL_WRITE_SUPPORTED_GROUPS_EXT_TLS1_2_FLAG) &&
            mbedtls_ssl_tls12_named_group_is_ecdhe(*group_list) &&
            (mbedtls_ssl_get_ecp_group_id_from_tls_id(*group_list) !=
             MBEDTLS_ECP_DP_NONE)) {
            propose_group = 1;
        }
#endif /* MBEDTLS_SSL_TLS1_2_SOME_ECC */

        if (propose_group) {
            MBEDTLS_SSL_CHK_BUF_PTR(p, end, 2);
            MBEDTLS_PUT_UINT16_BE(*group_list, p, 0);
            p += 2;
            MBEDTLS_SSL_DEBUG_MSG(3, ("NamedGroup: %s ( %x )",
                                      mbedtls_ssl_named_group_to_str(*group_list),
                                      *group_list));
        }
    }

    /* Length of named_group_list */
    named_group_list_len = p - named_group_list;
    if (named_group_list_len == 0) {
        MBEDTLS_SSL_DEBUG_MSG(1, ("No group available."));
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }

    /* Write extension_type */
    MBEDTLS_PUT_UINT16_BE(MBEDTLS_TLS_EXT_SUPPORTED_GROUPS, buf, 0);
    /* Write extension_data_length */
    MBEDTLS_PUT_UINT16_BE(named_group_list_len + 2, buf, 2);
    /* Write length of named_group_list */
    MBEDTLS_PUT_UINT16_BE(named_group_list_len, buf, 4);

    MBEDTLS_SSL_DEBUG_BUF(3, "Supported groups extension",
                          buf + 4, named_group_list_len + 2);

    *out_len = p - buf;

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    mbedtls_ssl_tls13_set_hs_sent_ext_mask(
        ssl, MBEDTLS_TLS_EXT_SUPPORTED_GROUPS);
#endif /* MBEDTLS_SSL_PROTO_TLS1_3 */

    return 0;
}
#endif /* MBEDTLS_SSL_TLS1_2_SOME_ECC ||
          MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_EPHEMERAL_ENABLED */

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_client_hello_cipher_suites(
    mbedtls_ssl_context *ssl,
    unsigned char *buf,
    unsigned char *end,
    int *tls12_uses_ec,
    size_t *out_len)
{
    unsigned char *p = buf;
    const int *ciphersuite_list;
    unsigned char *cipher_suites; /* Start of the cipher_suites list */
    size_t cipher_suites_len;

    *tls12_uses_ec = 0;
    *out_len = 0;

    /*
     * Ciphersuite list
     *
     * This is a list of the symmetric cipher options supported by
     * the client, specifically the record protection algorithm
     * ( including secret key length ) and a hash to be used with
     * HKDF, in descending order of client preference.
     */
    ciphersuite_list = ssl->conf->ciphersuite_list;

    /* Check there is space for the cipher suite list length (2 bytes). */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, 2);
    p += 2;

    /* Write cipher_suites
     * CipherSuite cipher_suites<2..2^16-2>;
     */
    cipher_suites = p;
    for (size_t i = 0; ciphersuite_list[i] != 0; i++) {
        int cipher_suite = ciphersuite_list[i];
        const mbedtls_ssl_ciphersuite_t *ciphersuite_info;

        ciphersuite_info = mbedtls_ssl_ciphersuite_from_id(cipher_suite);

        if (mbedtls_ssl_validate_ciphersuite(ssl, ciphersuite_info,
                                             ssl->handshake->min_tls_version,
                                             ssl->tls_version) != 0) {
            continue;
        }

#if defined(MBEDTLS_SSL_PROTO_TLS1_2) && \
        (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))
        *tls12_uses_ec |= mbedtls_ssl_ciphersuite_uses_ec(ciphersuite_info);
#endif

        MBEDTLS_SSL_DEBUG_MSG(3, ("client hello, add ciphersuite: %04x, %s",
                                  (unsigned int) cipher_suite,
                                  ciphersuite_info->name));

        /* Check there is space for the cipher suite identifier (2 bytes). */
        MBEDTLS_SSL_CHK_BUF_PTR(p, end, 2);
        MBEDTLS_PUT_UINT16_BE(cipher_suite, p, 0);
        p += 2;
    }

    /*
     * Add TLS_EMPTY_RENEGOTIATION_INFO_SCSV
     */
    int renegotiating = 0;
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    renegotiating = (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE);
#endif
    if (!renegotiating) {
        MBEDTLS_SSL_DEBUG_MSG(3, ("adding EMPTY_RENEGOTIATION_INFO_SCSV"));
        MBEDTLS_SSL_CHK_BUF_PTR(p, end, 2);
        MBEDTLS_PUT_UINT16_BE(MBEDTLS_SSL_EMPTY_RENEGOTIATION_INFO, p, 0);
        p += 2;
    }

    /* Write the cipher_suites length in number of bytes */
    cipher_suites_len = p - cipher_suites;
    MBEDTLS_PUT_UINT16_BE(cipher_suites_len, buf, 0);
    MBEDTLS_SSL_DEBUG_MSG(3,
                          ("client hello, got %" MBEDTLS_PRINTF_SIZET " cipher suites",
                           cipher_suites_len/2));

    /* Output the total length of cipher_suites field. */
    *out_len = p - buf;

    return 0;
}

/*
 * Structure of the TLS 1.3 ClientHello message:
 *
 *    struct {
 *        ProtocolVersion legacy_version = 0x0303;    // TLS v1.2
 *        Random random;
 *        opaque legacy_session_id<0..32>;
 *        CipherSuite cipher_suites<2..2^16-2>;
 *        opaque legacy_compression_methods<1..2^8-1>;
 *        Extension extensions<8..2^16-1>;
 *    } ClientHello;
 *
 * Structure of the (D)TLS 1.2 ClientHello message:
 *
 * struct {
 *     ProtocolVersion client_version;
 *     Random random;
 *     SessionID session_id;
 *     opaque cookie<0..2^8-1>; // DTLS 1.2 ONLY
 *     CipherSuite cipher_suites<2..2^16-2>;
 *     CompressionMethod compression_methods<1..2^8-1>;
 *     select (extensions_present) {
 *         case false:
 *             struct {};
 *         case true:
 *             Extension extensions<0..2^16-1>;
 *     };
 * } ClientHello;
 */
MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_write_client_hello_body(mbedtls_ssl_context *ssl,
                                       unsigned char *buf,
                                       unsigned char *end,
                                       size_t *out_len,
                                       size_t *binders_len)
{
    int ret;
    mbedtls_ssl_handshake_params *handshake = ssl->handshake;
    unsigned char *p = buf;
    unsigned char *p_extensions_len; /* Pointer to extensions length */
    size_t output_len;               /* Length of buffer used by function */
    size_t extensions_len;           /* Length of the list of extensions*/
    int tls12_uses_ec = 0;

    *out_len = 0;
    *binders_len = 0;

#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
    unsigned char propose_tls12 =
        (handshake->min_tls_version <= MBEDTLS_SSL_VERSION_TLS1_2)
        &&
        (MBEDTLS_SSL_VERSION_TLS1_2 <= ssl->tls_version);
#endif
#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    unsigned char propose_tls13 =
        (handshake->min_tls_version <= MBEDTLS_SSL_VERSION_TLS1_3)
        &&
        (MBEDTLS_SSL_VERSION_TLS1_3 <= ssl->tls_version);
#endif

    /*
     * Write client_version (TLS 1.2) or legacy_version (TLS 1.3)
     *
     * In all cases this is the TLS 1.2 version.
     */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, 2);
    mbedtls_ssl_write_version(p, ssl->conf->transport,
                              MBEDTLS_SSL_VERSION_TLS1_2);
    p += 2;

    /* ...
     * Random random;
     * ...
     *
     * The random bytes have been prepared by ssl_prepare_client_hello() into
     * the handshake->randbytes buffer and are copied here into the output
     * buffer.
     */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, MBEDTLS_CLIENT_HELLO_RANDOM_LEN);
    memcpy(p, handshake->randbytes, MBEDTLS_CLIENT_HELLO_RANDOM_LEN);
    MBEDTLS_SSL_DEBUG_BUF(3, "client hello, random bytes",
                          p, MBEDTLS_CLIENT_HELLO_RANDOM_LEN);
    p += MBEDTLS_CLIENT_HELLO_RANDOM_LEN;

    /* TLS 1.2:
     * ...
     * SessionID session_id;
     * ...
     * with
     * opaque SessionID<0..32>;
     *
     * TLS 1.3:
     * ...
     * opaque legacy_session_id<0..32>;
     * ...
     *
     * The (legacy) session identifier bytes have been prepared by
     * ssl_prepare_client_hello() into the ssl->session_negotiate->id buffer
     * and are copied here into the output buffer.
     */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, ssl->session_negotiate->id_len + 1);
    *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_BUF(3, "session id", ssl->session_negotiate->id,
                          ssl->session_negotiate->id_len);

    /* DTLS 1.2 ONLY
     * ...
     * opaque cookie<0..2^8-1>;
     * ...
     */
#if defined(MBEDTLS_SSL_PROTO_TLS1_2) && defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
#if !defined(MBEDTLS_SSL_PROTO_TLS1_3)
        uint8_t cookie_len = 0;
#else
        uint16_t cookie_len = 0;
#endif /* !MBEDTLS_SSL_PROTO_TLS1_3 */

        if (handshake->cookie != NULL) {
            MBEDTLS_SSL_DEBUG_BUF(3, "client hello, cookie",
                                  handshake->cookie,
                                  handshake->cookie_len);
            cookie_len = handshake->cookie_len;
        }

        MBEDTLS_SSL_CHK_BUF_PTR(p, end, cookie_len + 1);
        *p++ = (unsigned char) cookie_len;
        if (cookie_len > 0) {
            memcpy(p, handshake->cookie, cookie_len);
            p += cookie_len;
        }
    }
#endif /* MBEDTLS_SSL_PROTO_TLS1_2 && MBEDTLS_SSL_PROTO_DTLS */

    /* Write cipher_suites */
    ret = ssl_write_client_hello_cipher_suites(ssl, p, end,
                                               &tls12_uses_ec,
                                               &output_len);
    if (ret != 0) {
        return ret;
    }
    p += output_len;

    /* Write legacy_compression_methods (TLS 1.3) or
     * compression_methods (TLS 1.2)
     *
     * For every TLS 1.3 ClientHello, this vector MUST contain exactly
     * one byte set to zero, which corresponds to the 'null' compression
     * method in prior versions of TLS.
     *
     * For TLS 1.2 ClientHello, for security reasons we do not support
     * compression anymore, thus also just the 'null' compression method.
     */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, 2);
    *p++ = 1;
    *p++ = MBEDTLS_SSL_COMPRESS_NULL;

    /* Write extensions */

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    /* Keeping track of the included extensions */
    handshake->sent_extensions = MBEDTLS_SSL_EXT_MASK_NONE;
#endif

    /* First write extensions, then the total length */
    MBEDTLS_SSL_CHK_BUF_PTR(p, end, 2);
    p_extensions_len = p;
    p += 2;

#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
    /* Write server name extension */
    ret = ssl_write_hostname_ext(ssl, p, end, &output_len);
    if (ret != 0) {
        return ret;
    }
    p += output_len;
#endif /* MBEDTLS_SSL_SERVER_NAME_INDICATION */

#if defined(MBEDTLS_SSL_ALPN)
    ret = ssl_write_alpn_ext(ssl, p, end, &output_len);
    if (ret != 0) {
        return ret;
    }
    p += output_len;
#endif /* MBEDTLS_SSL_ALPN */

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    if (propose_tls13) {
        ret = mbedtls_ssl_tls13_write_client_hello_exts(ssl, p, end,
                                                        &output_len);
        if (ret != 0) {
            return ret;
        }
        p += output_len;
    }
#endif

#if defined(MBEDTLS_SSL_TLS1_2_SOME_ECC) || \
    defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_EPHEMERAL_ENABLED)
    {
        int ssl_write_supported_groups_ext_flags = 0;

#if defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_EPHEMERAL_ENABLED)
        if (propose_tls13 && mbedtls_ssl_conf_tls13_some_ephemeral_enabled(ssl)) {
            ssl_write_supported_groups_ext_flags |=
                SSL_WRITE_SUPPORTED_GROUPS_EXT_TLS1_3_FLAG;
        }
#endif
#if defined(MBEDTLS_SSL_TLS1_2_SOME_ECC)
        if (propose_tls12 && tls12_uses_ec) {
            ssl_write_supported_groups_ext_flags |=
                SSL_WRITE_SUPPORTED_GROUPS_EXT_TLS1_2_FLAG;
        }
#endif
        if (ssl_write_supported_groups_ext_flags != 0) {
            ret = ssl_write_supported_groups_ext(ssl, p, end,
                                                 ssl_write_supported_groups_ext_flags,
                                                 &output_len);
            if (ret != 0) {
                return ret;
            }
            p += output_len;
        }
    }
#endif /* MBEDTLS_SSL_TLS1_2_SOME_ECC ||
          MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_EPHEMERAL_ENABLED */

#if defined(MBEDTLS_SSL_HANDSHAKE_WITH_CERT_ENABLED)
    int write_sig_alg_ext = 0;
#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    write_sig_alg_ext = write_sig_alg_ext ||
                        (propose_tls13 && mbedtls_ssl_conf_tls13_ephemeral_enabled(ssl));
#endif
#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
    write_sig_alg_ext = write_sig_alg_ext || propose_tls12;
#endif

    if (write_sig_alg_ext) {
        ret = mbedtls_ssl_write_sig_alg_ext(ssl, p, end, &output_len);
        if (ret != 0) {
            return ret;
        }
        p += output_len;
    }
#endif /* MBEDTLS_SSL_HANDSHAKE_WITH_CERT_ENABLED */

#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
    if (propose_tls12) {
        ret = mbedtls_ssl_tls12_write_client_hello_exts(ssl, p, end,
                                                        tls12_uses_ec,
                                                        &output_len);
        if (ret != 0) {
            return ret;
        }
        p += output_len;
    }
#endif /* MBEDTLS_SSL_PROTO_TLS1_2 */

#if defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_PSK_ENABLED)
    /* The "pre_shared_key" extension (RFC 8446 Section 4.2.11)
     * MUST be the last extension in the ClientHello.
     */
    if (propose_tls13 && mbedtls_ssl_conf_tls13_some_psk_enabled(ssl)) {
        ret = mbedtls_ssl_tls13_write_identities_of_pre_shared_key_ext(
            ssl, p, end, &output_len, binders_len);
        if (ret != 0) {
            return ret;
        }
        p += output_len;
    }
#endif /* MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_PSK_ENABLED */

    /* Write the length of the list of extensions. */
    extensions_len = p - p_extensions_len - 2;

    if (extensions_len == 0) {
        p = p_extensions_len;
    } else {
        MBEDTLS_PUT_UINT16_BE(extensions_len, p_extensions_len, 0);
        MBEDTLS_SSL_DEBUG_MSG(3, ("client hello, total extension length: %" \
                                  MBEDTLS_PRINTF_SIZET, extensions_len));
        MBEDTLS_SSL_DEBUG_BUF(3, "client hello extensions",
                              p_extensions_len, extensions_len);
    }

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
    MBEDTLS_SSL_PRINT_EXTS(
        3, MBEDTLS_SSL_HS_CLIENT_HELLO, handshake->sent_extensions);
#endif

    *out_len = p - buf;
    return 0;
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_generate_random(mbedtls_ssl_context *ssl)
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    unsigned char *randbytes = ssl->handshake->randbytes;
    size_t gmt_unix_time_len = 0;

    /*
     * Generate the random bytes
     *
     * TLS 1.2 case:
     * struct {
     *     uint32 gmt_unix_time;
     *     opaque random_bytes[28];
     * } Random;
     *
     * TLS 1.3 case:
     * opaque Random[32];
     */
    if (ssl->tls_version == MBEDTLS_SSL_VERSION_TLS1_2) {
#if defined(MBEDTLS_HAVE_TIME)
        mbedtls_time_t gmt_unix_time = mbedtls_time(NULL);
        MBEDTLS_PUT_UINT32_BE(gmt_unix_time, randbytes, 0);
        gmt_unix_time_len = 4;

        MBEDTLS_SSL_DEBUG_MSG(3,
                              ("client hello, current time: %" MBEDTLS_PRINTF_LONGLONG,
                               (long long) gmt_unix_time));
#endif /* MBEDTLS_HAVE_TIME */
    }

    ret = ssl->conf->f_rng(ssl->conf->p_rng,
                           randbytes + gmt_unix_time_len,
                           MBEDTLS_CLIENT_HELLO_RANDOM_LEN - gmt_unix_time_len);
    return ret;
}

MBEDTLS_CHECK_RETURN_CRITICAL
static int ssl_prepare_client_hello(mbedtls_ssl_context *ssl)
{
    int ret;
    size_t session_id_len;
    mbedtls_ssl_session *session_negotiate = ssl->session_negotiate;

    if (session_negotiate == NULL) {
        return MBEDTLS_ERR_SSL_INTERNAL_ERROR;
    }

#if defined(MBEDTLS_SSL_PROTO_TLS1_3) && \
    defined(MBEDTLS_SSL_SESSION_TICKETS) && \
    defined(MBEDTLS_HAVE_TIME)

    /* Check if a tls13 ticket has been configured. */
    if (ssl->handshake->resume != 0 &&
        session_negotiate->tls_version == MBEDTLS_SSL_VERSION_TLS1_3 &&
        session_negotiate->ticket != NULL) {
        mbedtls_ms_time_t now = mbedtls_ms_time();
        mbedtls_ms_time_t age = now - session_negotiate->ticket_reception_time;
        if (age < 0 || age > session_negotiate->ticket_lifetime * 1000) {
            /* Without valid ticket, disable session resumption.*/
            MBEDTLS_SSL_DEBUG_MSG(
                3, ("Ticket expired, disable session resumption"));
            ssl->handshake->resume = 0;
        }
    }
#endif /* MBEDTLS_SSL_PROTO_TLS1_3 &&
          MBEDTLS_SSL_SESSION_TICKETS &&
          MBEDTLS_HAVE_TIME */

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

    /* Bet on the highest configured version if we are not in a TLS 1.2
     * renegotiation or session resumption.
     */
#if defined(MBEDTLS_SSL_RENEGOTIATION)
    if (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE) {
        ssl->handshake->min_tls_version = ssl->tls_version;
    } else
#endif
    {
        if (ssl->handshake->resume) {
            ssl->tls_version = session_negotiate->tls_version;
            ssl->handshake->min_tls_version = ssl->tls_version;
        } else {
            ssl->handshake->min_tls_version = ssl->conf->min_tls_version;
        }
    }

    /*
     * Generate the random bytes, except when responding to a verify request
     * where we MUST reuse the previously generated random bytes
     * (RFC 6347 4.2.1).
     */
#if defined(MBEDTLS_SSL_PROTO_DTLS)
    if ((ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM) ||
        (ssl->handshake->cookie == NULL))
#endif
    {
        ret = ssl_generate_random(ssl);
        if (ret != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "Random bytes generation failed", ret);
            return ret;
        }
    }

    /*
     * Prepare session identifier. At that point, the length of the session
     * identifier in the SSL context `ssl->session_negotiate->id_len` is equal
     * to zero, except in the case of a TLS 1.2 session renegotiation or
     * session resumption.
     */
    session_id_len = session_negotiate->id_len;

#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
    if (ssl->tls_version == MBEDTLS_SSL_VERSION_TLS1_2) {
        if (session_id_len < 16 || session_id_len > 32 ||
#if defined(MBEDTLS_SSL_RENEGOTIATION)
            ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE ||
#endif
            ssl->handshake->resume == 0) {
            session_id_len = 0;
        }

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
        /*
         * RFC 5077 section 3.4: "When presenting a ticket, the client MAY
         * generate and include a Session ID in the TLS ClientHello."
         */
        int renegotiating = 0;
#if defined(MBEDTLS_SSL_RENEGOTIATION)
        if (ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE) {
            renegotiating = 1;
        }
#endif
        if (!renegotiating) {
            if ((session_negotiate->ticket != NULL) &&
                (session_negotiate->ticket_len != 0)) {
                session_id_len = 32;
            }
        }
#endif /* MBEDTLS_SSL_SESSION_TICKETS */
    }
#endif /* MBEDTLS_SSL_PROTO_TLS1_2 */

#if defined(MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE)
    if (ssl->tls_version == MBEDTLS_SSL_VERSION_TLS1_3) {
        /*
         * Create a legacy session identifier for the purpose of middlebox
         * compatibility only if one has not been created already, which is
         * the case if we are here for the TLS 1.3 second ClientHello.
         *
         * Versions of TLS before TLS 1.3 supported a "session resumption"
         * feature which has been merged with pre-shared keys in TLS 1.3
         * version. A client which has a cached session ID set by a pre-TLS 1.3
         * server SHOULD set this field to that value. In compatibility mode,
         * this field MUST be non-empty, so a client not offering a pre-TLS 1.3
         * session MUST generate a new 32-byte value. This value need not be
         * random but SHOULD be unpredictable to avoid implementations fixating
         * on a specific value (also known as ossification). Otherwise, it MUST
         * be set as a zero-length vector ( i.e., a zero-valued single byte
         * length field ).
         */
        session_id_len = 32;
    }
#endif /* MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE */

    if (session_id_len != session_negotiate->id_len) {
        session_negotiate->id_len = session_id_len;
        if (session_id_len > 0) {
            ret = ssl->conf->f_rng(ssl->conf->p_rng,
                                   session_negotiate->id,
                                   session_id_len);
            if (ret != 0) {
                MBEDTLS_SSL_DEBUG_RET(1, "creating session id failed", ret);
                return ret;
            }
        }
    }

#if defined(MBEDTLS_SSL_PROTO_TLS1_3) && \
    defined(MBEDTLS_SSL_SESSION_TICKETS) && \
    defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
    if (ssl->tls_version == MBEDTLS_SSL_VERSION_TLS1_3  &&
        ssl->handshake->resume) {
        int hostname_mismatch = ssl->hostname != NULL ||
                                session_negotiate->hostname != NULL;
        if (ssl->hostname != NULL && session_negotiate->hostname != NULL) {
            hostname_mismatch = strcmp(
                ssl->hostname, session_negotiate->hostname) != 0;
        }

        if (hostname_mismatch) {
            MBEDTLS_SSL_DEBUG_MSG(
                1, ("Hostname mismatch the session ticket, "
                    "disable session resumption."));
            return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
        }
    } else {
        return mbedtls_ssl_session_set_hostname(session_negotiate,
                                                ssl->hostname);
    }
#endif /* MBEDTLS_SSL_PROTO_TLS1_3 &&
          MBEDTLS_SSL_SESSION_TICKETS &&
          MBEDTLS_SSL_SERVER_NAME_INDICATION */

    return 0;
}
/*
 * Write ClientHello handshake message.
 * Handler for MBEDTLS_SSL_CLIENT_HELLO
 */
int mbedtls_ssl_write_client_hello(mbedtls_ssl_context *ssl)
{
    int ret = 0;
    unsigned char *buf;
    size_t buf_len, msg_len, binders_len;

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

    MBEDTLS_SSL_PROC_CHK(ssl_prepare_client_hello(ssl));

    MBEDTLS_SSL_PROC_CHK(mbedtls_ssl_start_handshake_msg(
                             ssl, MBEDTLS_SSL_HS_CLIENT_HELLO,
                             &buf, &buf_len));

    MBEDTLS_SSL_PROC_CHK(ssl_write_client_hello_body(ssl, buf,
                                                     buf + buf_len,
                                                     &msg_len,
                                                     &binders_len));

#if defined(MBEDTLS_SSL_PROTO_TLS1_2) && defined(MBEDTLS_SSL_PROTO_DTLS)
    if (ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM) {
        ssl->out_msglen = msg_len + 4;
        mbedtls_ssl_send_flight_completed(ssl);

        /*
         * The two functions below may try to send data on the network and
         * can return with the MBEDTLS_ERR_SSL_WANT_READ error code when they
         * fail to do so and the transmission has to be retried later. In that
         * case as in fatal error cases, we return immediately. But we must have
         * set the handshake state to the next state at that point to ensure
         * that we will not write and send again a ClientHello when we
         * eventually succeed in sending the pending data.
         */
        mbedtls_ssl_handshake_set_state(ssl, MBEDTLS_SSL_SERVER_HELLO);

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

        if ((ret = mbedtls_ssl_flight_transmit(ssl)) != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_flight_transmit", ret);
            return ret;
        }
    } else
#endif /* MBEDTLS_SSL_PROTO_TLS1_2 && MBEDTLS_SSL_PROTO_DTLS */
    {

        ret = mbedtls_ssl_add_hs_hdr_to_checksum(ssl,
                                                 MBEDTLS_SSL_HS_CLIENT_HELLO,
                                                 msg_len);
        if (ret != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "mbedtls_ssl_add_hs_hdr_to_checksum", ret);
            return ret;
        }
        ret = ssl->handshake->update_checksum(ssl, buf, msg_len - binders_len);
        if (ret != 0) {
            MBEDTLS_SSL_DEBUG_RET(1, "update_checksum", ret);
            return ret;
        }
#if defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_PSK_ENABLED)
        if (binders_len > 0) {
            MBEDTLS_SSL_PROC_CHK(
                mbedtls_ssl_tls13_write_binders_of_pre_shared_key_ext(
                    ssl, buf + msg_len - binders_len, buf + msg_len));
            ret = ssl->handshake->update_checksum(ssl, buf + msg_len - binders_len,
                                                  binders_len);
            if (ret != 0) {
                MBEDTLS_SSL_DEBUG_RET(1, "update_checksum", ret);
                return ret;
            }
        }
#endif /* MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_PSK_ENABLED */

        MBEDTLS_SSL_PROC_CHK(mbedtls_ssl_finish_handshake_msg(ssl,
                                                              buf_len,
                                                              msg_len));

        /*
         * Set next state. Note that if TLS 1.3 is proposed, this may be
         * overwritten by mbedtls_ssl_tls13_finalize_client_hello().
         */
        mbedtls_ssl_handshake_set_state(ssl, MBEDTLS_SSL_SERVER_HELLO);

#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
        if (ssl->handshake->min_tls_version <=  MBEDTLS_SSL_VERSION_TLS1_3 &&
            MBEDTLS_SSL_VERSION_TLS1_3 <= ssl->tls_version) {
            ret = mbedtls_ssl_tls13_finalize_client_hello(ssl);
        }
#endif
    }

cleanup:

    MBEDTLS_SSL_DEBUG_MSG(2, ("<= write client hello"));
    return ret;
}

#endif /* MBEDTLS_SSL_PROTO_TLS1_3 || MBEDTLS_SSL_PROTO_TLS1_2 */
#endif /* MBEDTLS_SSL_CLI_C */