extensions.c

     *          OR the kex_mode allows key_share resumes)
     *         AND
     *         a shared group exists
     *     THEN
     *         send a HelloRetryRequest
     *     ELSE If
     *         we are not resuming
     *         OR
     *         the kex_mode doesn't allow non key_share resumes
     *     THEN
     *         fail;
     */
    if (s->server && s->s3->peer_tmp == NULL) {
        /* No suitable share */
        if (s->hello_retry_request == 0 && sent
                && (!s->hit
                    || (s->ext.psk_kex_mode & TLSEXT_KEX_MODE_FLAG_KE_DHE)
                       != 0)) {
            const unsigned char *pcurves, *pcurvestmp, *clntcurves;
            size_t num_curves, clnt_num_curves, i;
            unsigned int group_id;

            /* Check if a shared group exists */

            /* Get the clients list of supported groups. */
            if (!tls1_get_curvelist(s, 1, &clntcurves, &clnt_num_curves)) {
                *al = SSL_AD_INTERNAL_ERROR;
                SSLerr(SSL_F_FINAL_KEY_SHARE, ERR_R_INTERNAL_ERROR);
                return 0;
            }

            /* Get our list of available groups */
            if (!tls1_get_curvelist(s, 0, &pcurves, &num_curves)) {
                *al = SSL_AD_INTERNAL_ERROR;
                SSLerr(SSL_F_FINAL_KEY_SHARE, ERR_R_INTERNAL_ERROR);
                return 0;
            }

            /* Find the first group we allow that is also in client's list */
            for (i = 0, pcurvestmp = pcurves; i < num_curves;
                 i++, pcurvestmp += 2) {
                group_id = pcurvestmp[0] << 8 | pcurvestmp[1];

                if (check_in_list(s, group_id, clntcurves, clnt_num_curves, 1))
                    break;
            }

            if (i < num_curves) {
                /* A shared group exists so send a HelloRetryRequest */
                s->s3->group_id = group_id;
                s->hello_retry_request = 1;
                return 1;
            }
        }
        if (!s->hit
                || (s->ext.psk_kex_mode & TLSEXT_KEX_MODE_FLAG_KE) == 0) {
            /* Nothing left we can do - just fail */
            *al = SSL_AD_HANDSHAKE_FAILURE;
            SSLerr(SSL_F_FINAL_KEY_SHARE, SSL_R_NO_SUITABLE_KEY_SHARE);
            return 0;
        }
    }

    /* We have a key_share so don't send any more HelloRetryRequest messages */
    if (s->server)
        s->hello_retry_request = 0;

    /*
     * For a client side resumption with no key_share we need to generate
     * the handshake secret (otherwise this is done during key_share
     * processing).
     */
    if (!sent && !s->server && !tls13_generate_handshake_secret(s, NULL, 0)) {
        *al = SSL_AD_INTERNAL_ERROR;
        SSLerr(SSL_F_FINAL_KEY_SHARE, ERR_R_INTERNAL_ERROR);
        return 0;
    }

    return 1;
}

static int init_psk_kex_modes(SSL *s, unsigned int context)
{
    s->ext.psk_kex_mode = TLSEXT_KEX_MODE_FLAG_NONE;
    return 1;
}

int tls_psk_do_binder(SSL *s, const EVP_MD *md, const unsigned char *msgstart,
                      size_t binderoffset, const unsigned char *binderin,
                      unsigned char *binderout,
                      SSL_SESSION *sess, int sign)
{
    EVP_PKEY *mackey = NULL;
    EVP_MD_CTX *mctx = NULL;
    unsigned char hash[EVP_MAX_MD_SIZE], binderkey[EVP_MAX_MD_SIZE];
    unsigned char finishedkey[EVP_MAX_MD_SIZE], tmpbinder[EVP_MAX_MD_SIZE];
    const char resumption_label[] = "resumption psk binder key";
    size_t bindersize, hashsize = EVP_MD_size(md);
    int ret = -1;

    /* Generate the early_secret */
    if (!tls13_generate_secret(s, md, NULL, sess->master_key,
                               sess->master_key_length,
                               (unsigned char *)&s->early_secret)) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    /*
     * Create the handshake hash for the binder key...the messages so far are
     * empty!
     */
    mctx = EVP_MD_CTX_new();
    if (mctx == NULL
            || EVP_DigestInit_ex(mctx, md, NULL) <= 0
            || EVP_DigestFinal_ex(mctx, hash, NULL) <= 0) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    /* Generate the binder key */
    if (!tls13_hkdf_expand(s, md, s->early_secret,
                           (unsigned char *)resumption_label,
                           sizeof(resumption_label) - 1, hash, binderkey,
                           hashsize)) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    /* Generate the finished key */
    if (!tls13_derive_finishedkey(s, md, binderkey, finishedkey, hashsize)) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    if (EVP_DigestInit_ex(mctx, md, NULL) <= 0) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    /*
     * Get a hash of the ClientHello up to the start of the binders. If we are
     * following a HelloRetryRequest then this includes the hash of the first
     * ClientHello and the HelloRetryRequest itself.
     */
    if (s->hello_retry_request) {
        size_t hdatalen;
        void *hdata;

        hdatalen = BIO_get_mem_data(s->s3->handshake_buffer, &hdata);
        if (hdatalen <= 0) {
            SSLerr(SSL_F_TLS_PSK_DO_BINDER, SSL_R_BAD_HANDSHAKE_LENGTH);
            goto err;
        }

        /*
         * For servers the handshake buffer data will include the second
         * ClientHello - which we don't want - so we need to take that bit off.
         */
        if (s->server) {
            if (hdatalen < s->init_num + SSL3_HM_HEADER_LENGTH) {
                SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
                goto err;
            }
            hdatalen -= s->init_num + SSL3_HM_HEADER_LENGTH;
        }

        if (EVP_DigestUpdate(mctx, hdata, hdatalen) <= 0) {
            SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
            goto err;
        }
    }

    if (EVP_DigestUpdate(mctx, msgstart, binderoffset) <= 0
            || EVP_DigestFinal_ex(mctx, hash, NULL) <= 0) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    mackey = EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL, finishedkey, hashsize);
    if (mackey == NULL) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    if (!sign)
        binderout = tmpbinder;

    bindersize = hashsize;
    if (EVP_DigestSignInit(mctx, NULL, md, NULL, mackey) <= 0
            || EVP_DigestSignUpdate(mctx, hash, hashsize) <= 0
            || EVP_DigestSignFinal(mctx, binderout, &bindersize) <= 0
            || bindersize != hashsize) {
        SSLerr(SSL_F_TLS_PSK_DO_BINDER, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    if (sign) {
        ret = 1;
    } else {
        /* HMAC keys can't do EVP_DigestVerify* - use CRYPTO_memcmp instead */
        ret = (CRYPTO_memcmp(binderin, binderout, hashsize) == 0);
    }

 err:
    OPENSSL_cleanse(binderkey, sizeof(binderkey));
    OPENSSL_cleanse(finishedkey, sizeof(finishedkey));
    EVP_PKEY_free(mackey);
    EVP_MD_CTX_free(mctx);

    return ret;
}