extensions.c

    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;
    }

    /*
     * Get a hash of the ClientHello up to the start of the binders.
     * TODO(TLS1.3): This will need to be tweaked when we implement
     * HelloRetryRequest to include the digest of the previous messages here.
     */
    if (EVP_DigestInit_ex(mctx, md, NULL) <= 0
            || 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;
}