src/ssl/t1_lib.c

/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
 * All rights reserved.
 *
 * This package is an SSL implementation written
 * by Eric Young (eay@cryptsoft.com).
 * The implementation was written so as to conform with Netscapes SSL.
 * 
 * This library is free for commercial and non-commercial use as long as
 * the following conditions are aheared to.  The following conditions
 * apply to all code found in this distribution, be it the RC4, RSA,
 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
 * included with this distribution is covered by the same copyright terms
 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
 * 
 * Copyright remains Eric Young's, and as such any Copyright notices in
 * the code are not to be removed.
 * If this package is used in a product, Eric Young should be given attribution
 * as the author of the parts of the library used.
 * This can be in the form of a textual message at program startup or
 * in documentation (online or textual) provided with the package.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    "This product includes cryptographic software written by
 *     Eric Young (eay@cryptsoft.com)"
 *    The word 'cryptographic' can be left out if the rouines from the library
 *    being used are not cryptographic related :-).
 * 4. If you include any Windows specific code (or a derivative thereof) from 
 *    the apps directory (application code) you must include an acknowledgement:
 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
 * 
 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * 
 * The licence and distribution terms for any publically available version or
 * derivative of this code cannot be changed.  i.e. this code cannot simply be
 * copied and put under another distribution licence
 * [including the GNU Public Licence.]
 */
/* ====================================================================
 * Copyright (c) 1998-2007 The OpenSSL Project.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer. 
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. All advertising materials mentioning features or use of this
 *    software must display the following acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
 *
 * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For written permission, please contact
 *    openssl-core@openssl.org.
 *
 * 5. Products derived from this software may not be called "OpenSSL"
 *    nor may "OpenSSL" appear in their names without prior written
 *    permission of the OpenSSL Project.
 *
 * 6. Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
 *
 * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
 * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 * ====================================================================
 *
 * This product includes cryptographic software written by Eric Young
 * (eay@cryptsoft.com).  This product includes software written by Tim
 * Hudson (tjh@cryptsoft.com). */

#include <openssl/ssl.h>

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

#include <openssl/bytestring.h>
#include <openssl/digest.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/hmac.h>
#include <openssl/mem.h>
#include <openssl/nid.h>
#include <openssl/rand.h>
#include <openssl/type_check.h>

#include "internal.h"
#include "../crypto/internal.h"


static int ssl_check_clienthello_tlsext(SSL_HANDSHAKE *hs);

static int compare_uint16_t(const void *p1, const void *p2) {
  uint16_t u1 = *((const uint16_t *)p1);
  uint16_t u2 = *((const uint16_t *)p2);
  if (u1 < u2) {
    return -1;
  } else if (u1 > u2) {
    return 1;
  } else {
    return 0;
  }
}

/* Per http://tools.ietf.org/html/rfc5246#section-7.4.1.4, there may not be
 * more than one extension of the same type in a ClientHello or ServerHello.
 * This function does an initial scan over the extensions block to filter those
 * out. */
static int tls1_check_duplicate_extensions(const CBS *cbs) {
  CBS extensions = *cbs;
  size_t num_extensions = 0, i = 0;
  uint16_t *extension_types = NULL;
  int ret = 0;

  /* First pass: count the extensions. */
  while (CBS_len(&extensions) > 0) {
    uint16_t type;
    CBS extension;

    if (!CBS_get_u16(&extensions, &type) ||
        !CBS_get_u16_length_prefixed(&extensions, &extension)) {
      goto done;
    }

    num_extensions++;
  }

  if (num_extensions == 0) {
    return 1;
  }

  extension_types = OPENSSL_malloc(sizeof(uint16_t) * num_extensions);
  if (extension_types == NULL) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_MALLOC_FAILURE);
    goto done;
  }

  /* Second pass: gather the extension types. */
  extensions = *cbs;
  for (i = 0; i < num_extensions; i++) {
    CBS extension;

    if (!CBS_get_u16(&extensions, &extension_types[i]) ||
        !CBS_get_u16_length_prefixed(&extensions, &extension)) {
      /* This should not happen. */
      goto done;
    }
  }
  assert(CBS_len(&extensions) == 0);

  /* Sort the extensions and make sure there are no duplicates. */
  qsort(extension_types, num_extensions, sizeof(uint16_t), compare_uint16_t);
  for (i = 1; i < num_extensions; i++) {
    if (extension_types[i - 1] == extension_types[i]) {
      goto done;
    }
  }

  ret = 1;

done:
  OPENSSL_free(extension_types);
  return ret;
}

int ssl_client_hello_init(SSL *ssl, SSL_CLIENT_HELLO *out, const uint8_t *in,
                          size_t in_len) {
  OPENSSL_memset(out, 0, sizeof(*out));
  out->ssl = ssl;
  out->client_hello = in;
  out->client_hello_len = in_len;

  CBS client_hello, random, session_id;
  CBS_init(&client_hello, out->client_hello, out->client_hello_len);
  if (!CBS_get_u16(&client_hello, &out->version) ||
      !CBS_get_bytes(&client_hello, &random, SSL3_RANDOM_SIZE) ||
      !CBS_get_u8_length_prefixed(&client_hello, &session_id) ||
      CBS_len(&session_id) > SSL_MAX_SSL_SESSION_ID_LENGTH) {
    return 0;
  }

  out->random = CBS_data(&random);
  out->random_len = CBS_len(&random);
  out->session_id = CBS_data(&session_id);
  out->session_id_len = CBS_len(&session_id);

  /* Skip past DTLS cookie */
  if (SSL_is_dtls(out->ssl)) {
    CBS cookie;
    if (!CBS_get_u8_length_prefixed(&client_hello, &cookie) ||
        CBS_len(&cookie) > DTLS1_COOKIE_LENGTH) {
      return 0;
    }
  }

  CBS cipher_suites, compression_methods;
  if (!CBS_get_u16_length_prefixed(&client_hello, &cipher_suites) ||
      CBS_len(&cipher_suites) < 2 || (CBS_len(&cipher_suites) & 1) != 0 ||
      !CBS_get_u8_length_prefixed(&client_hello, &compression_methods) ||
      CBS_len(&compression_methods) < 1) {
    return 0;
  }

  out->cipher_suites = CBS_data(&cipher_suites);
  out->cipher_suites_len = CBS_len(&cipher_suites);
  out->compression_methods = CBS_data(&compression_methods);
  out->compression_methods_len = CBS_len(&compression_methods);

  /* If the ClientHello ends here then it's valid, but doesn't have any
   * extensions. (E.g. SSLv3.) */
  if (CBS_len(&client_hello) == 0) {
    out->extensions = NULL;
    out->extensions_len = 0;
    return 1;
  }

  /* Extract extensions and check it is valid. */
  CBS extensions;
  if (!CBS_get_u16_length_prefixed(&client_hello, &extensions) ||
      !tls1_check_duplicate_extensions(&extensions) ||
      CBS_len(&client_hello) != 0) {
    return 0;
  }

  out->extensions = CBS_data(&extensions);
  out->extensions_len = CBS_len(&extensions);

  return 1;
}

int ssl_client_hello_get_extension(const SSL_CLIENT_HELLO *client_hello,
                                   CBS *out, uint16_t extension_type) {
  CBS extensions;
  CBS_init(&extensions, client_hello->extensions, client_hello->extensions_len);
  while (CBS_len(&extensions) != 0) {
    /* Decode the next extension. */
    uint16_t type;
    CBS extension;
    if (!CBS_get_u16(&extensions, &type) ||
        !CBS_get_u16_length_prefixed(&extensions, &extension)) {
      return 0;
    }

    if (type == extension_type) {
      *out = extension;
      return 1;
    }
  }

  return 0;
}

int SSL_early_callback_ctx_extension_get(const SSL_CLIENT_HELLO *client_hello,
                                         uint16_t extension_type,
                                         const uint8_t **out_data,
                                         size_t *out_len) {
  CBS cbs;
  if (!ssl_client_hello_get_extension(client_hello, &cbs, extension_type)) {
    return 0;
  }

  *out_data = CBS_data(&cbs);
  *out_len = CBS_len(&cbs);
  return 1;
}

static const uint16_t kDefaultGroups[] = {
    SSL_CURVE_X25519,
    SSL_CURVE_SECP256R1,
    SSL_CURVE_SECP384R1,
};

void tls1_get_grouplist(SSL *ssl, const uint16_t **out_group_ids,
                        size_t *out_group_ids_len) {
  *out_group_ids = ssl->supported_group_list;
  *out_group_ids_len = ssl->supported_group_list_len;
  if (!*out_group_ids) {
    *out_group_ids = kDefaultGroups;
    *out_group_ids_len = OPENSSL_ARRAY_SIZE(kDefaultGroups);
  }
}

int tls1_get_shared_group(SSL_HANDSHAKE *hs, uint16_t *out_group_id) {
  SSL *const ssl = hs->ssl;
  assert(ssl->server);

  const uint16_t *groups, *pref, *supp;
  size_t groups_len, pref_len, supp_len;
  tls1_get_grouplist(ssl, &groups, &groups_len);

  /* Clients are not required to send a supported_groups extension. In this
   * case, the server is free to pick any group it likes. See RFC 4492,
   * section 4, paragraph 3.
   *
   * However, in the interests of compatibility, we will skip ECDH if the
   * client didn't send an extension because we can't be sure that they'll
   * support our favoured group. Thus we do not special-case an emtpy
   * |peer_supported_group_list|. */

  if (ssl->options & SSL_OP_CIPHER_SERVER_PREFERENCE) {
    pref = groups;
    pref_len = groups_len;
    supp = hs->peer_supported_group_list;
    supp_len = hs->peer_supported_group_list_len;
  } else {
    pref = hs->peer_supported_group_list;
    pref_len = hs->peer_supported_group_list_len;
    supp = groups;
    supp_len = groups_len;
  }

  for (size_t i = 0; i < pref_len; i++) {
    for (size_t j = 0; j < supp_len; j++) {
      if (pref[i] == supp[j]) {
        *out_group_id = pref[i];
        return 1;
      }
    }
  }

  return 0;
}

int tls1_set_curves(uint16_t **out_group_ids, size_t *out_group_ids_len,
                    const int *curves, size_t ncurves) {
  uint16_t *group_ids;

  group_ids = OPENSSL_malloc(ncurves * sizeof(uint16_t));
  if (group_ids == NULL) {
    return 0;
  }

  for (size_t i = 0; i < ncurves; i++) {
    if (!ssl_nid_to_group_id(&group_ids[i], curves[i])) {
      OPENSSL_free(group_ids);
      return 0;
    }
  }

  OPENSSL_free(*out_group_ids);
  *out_group_ids = group_ids;
  *out_group_ids_len = ncurves;

  return 1;
}

int tls1_set_curves_list(uint16_t **out_group_ids, size_t *out_group_ids_len,
                         const char *curves) {
  uint16_t *group_ids = NULL;
  size_t ncurves = 0;

  const char *col;
  const char *ptr = curves;

  do {
    col = strchr(ptr, ':');

    uint16_t group_id;
    if (!ssl_name_to_group_id(&group_id, ptr,
                              col ? (size_t)(col - ptr) : strlen(ptr))) {
      goto err;
    }

    uint16_t *new_group_ids = OPENSSL_realloc(group_ids,
                                              (ncurves + 1) * sizeof(uint16_t));
    if (new_group_ids == NULL) {
      goto err;
    }
    group_ids = new_group_ids;

    group_ids[ncurves] = group_id;
    ncurves++;

    if (col) {
      ptr = col + 1;
    }
  } while (col);

  OPENSSL_free(*out_group_ids);
  *out_group_ids = group_ids;
  *out_group_ids_len = ncurves;

  return 1;

err:
  OPENSSL_free(group_ids);
  return 0;
}

int tls1_check_group_id(SSL *ssl, uint16_t group_id) {
  const uint16_t *groups;
  size_t groups_len;
  tls1_get_grouplist(ssl, &groups, &groups_len);
  for (size_t i = 0; i < groups_len; i++) {
    if (groups[i] == group_id) {
      return 1;
    }
  }

  return 0;
}

/* kVerifySignatureAlgorithms is the default list of accepted signature
 * algorithms for verifying.
 *
 * For now, RSA-PSS signature algorithms are not enabled on Android's system
 * BoringSSL. Once the change in Chrome has stuck and the values are finalized,
 * restore them. */
static const uint16_t kVerifySignatureAlgorithms[] = {
    /* Prefer SHA-256 algorithms. */
    SSL_SIGN_ECDSA_SECP256R1_SHA256,
#if !defined(BORINGSSL_ANDROID_SYSTEM)
    SSL_SIGN_RSA_PSS_SHA256,
#endif
    SSL_SIGN_RSA_PKCS1_SHA256,

    /* Larger hashes are acceptable. */
    SSL_SIGN_ECDSA_SECP384R1_SHA384,
#if !defined(BORINGSSL_ANDROID_SYSTEM)
    SSL_SIGN_RSA_PSS_SHA384,
#endif
    SSL_SIGN_RSA_PKCS1_SHA384,

    /* TODO(davidben): Remove this. */
#if defined(BORINGSSL_ANDROID_SYSTEM)
    SSL_SIGN_ECDSA_SECP521R1_SHA512,
#endif
#if !defined(BORINGSSL_ANDROID_SYSTEM)
    SSL_SIGN_RSA_PSS_SHA512,
#endif
    SSL_SIGN_RSA_PKCS1_SHA512,

    /* For now, SHA-1 is still accepted but least preferable. */
    SSL_SIGN_RSA_PKCS1_SHA1,

};

/* kSignSignatureAlgorithms is the default list of supported signature
 * algorithms for signing.
 *
 * For now, RSA-PSS signature algorithms are not enabled on Android's system
 * BoringSSL. Once the change in Chrome has stuck and the values are finalized,
 * restore them. */
static const uint16_t kSignSignatureAlgorithms[] = {
    /* Prefer SHA-256 algorithms. */
    SSL_SIGN_ECDSA_SECP256R1_SHA256,
#if !defined(BORINGSSL_ANDROID_SYSTEM)
    SSL_SIGN_RSA_PSS_SHA256,
#endif
    SSL_SIGN_RSA_PKCS1_SHA256,

    /* If needed, sign larger hashes.
     *
     * TODO(davidben): Determine which of these may be pruned. */
    SSL_SIGN_ECDSA_SECP384R1_SHA384,
#if !defined(BORINGSSL_ANDROID_SYSTEM)
    SSL_SIGN_RSA_PSS_SHA384,
#endif
    SSL_SIGN_RSA_PKCS1_SHA384,

    SSL_SIGN_ECDSA_SECP521R1_SHA512,
#if !defined(BORINGSSL_ANDROID_SYSTEM)
    SSL_SIGN_RSA_PSS_SHA512,
#endif
    SSL_SIGN_RSA_PKCS1_SHA512,

    /* If the peer supports nothing else, sign with SHA-1. */
    SSL_SIGN_ECDSA_SHA1,
    SSL_SIGN_RSA_PKCS1_SHA1,
};

size_t tls12_get_verify_sigalgs(const SSL *ssl, const uint16_t **out) {
  *out = kVerifySignatureAlgorithms;
  return OPENSSL_ARRAY_SIZE(kVerifySignatureAlgorithms);
}

int tls12_check_peer_sigalg(SSL *ssl, int *out_alert, uint16_t sigalg) {
  const uint16_t *verify_sigalgs;
  size_t num_verify_sigalgs = tls12_get_verify_sigalgs(ssl, &verify_sigalgs);
  for (size_t i = 0; i < num_verify_sigalgs; i++) {
    if (sigalg == verify_sigalgs[i]) {
      return 1;
    }
  }

  OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_SIGNATURE_TYPE);
  *out_alert = SSL_AD_ILLEGAL_PARAMETER;
  return 0;
}

/* tls_extension represents a TLS extension that is handled internally. The
 * |init| function is called for each handshake, before any other functions of
 * the extension. Then the add and parse callbacks are called as needed.
 *
 * The parse callbacks receive a |CBS| that contains the contents of the
 * extension (i.e. not including the type and length bytes). If an extension is
 * not received then the parse callbacks will be called with a NULL CBS so that
 * they can do any processing needed to handle the absence of an extension.
 *
 * The add callbacks receive a |CBB| to which the extension can be appended but
 * the function is responsible for appending the type and length bytes too.
 *
 * All callbacks return one for success and zero for error. If a parse function
 * returns zero then a fatal alert with value |*out_alert| will be sent. If
 * |*out_alert| isn't set, then a |decode_error| alert will be sent. */
struct tls_extension {
  uint16_t value;
  void (*init)(SSL_HANDSHAKE *hs);

  int (*add_clienthello)(SSL_HANDSHAKE *hs, CBB *out);
  int (*parse_serverhello)(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                           CBS *contents);

  int (*parse_clienthello)(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                           CBS *contents);
  int (*add_serverhello)(SSL_HANDSHAKE *hs, CBB *out);
};

static int forbid_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                    CBS *contents) {
  if (contents != NULL) {
    /* Servers MUST NOT send this extension. */
    *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
    return 0;
  }

  return 1;
}

static int ignore_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                    CBS *contents) {
  /* This extension from the client is handled elsewhere. */
  return 1;
}

static int dont_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  return 1;
}

/* Server name indication (SNI).
 *
 * https://tools.ietf.org/html/rfc6066#section-3. */

static int ext_sni_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl->tlsext_hostname == NULL) {
    return 1;
  }

  CBB contents, server_name_list, name;
  if (!CBB_add_u16(out, TLSEXT_TYPE_server_name) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &server_name_list) ||
      !CBB_add_u8(&server_name_list, TLSEXT_NAMETYPE_host_name) ||
      !CBB_add_u16_length_prefixed(&server_name_list, &name) ||
      !CBB_add_bytes(&name, (const uint8_t *)ssl->tlsext_hostname,
                     strlen(ssl->tlsext_hostname)) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_sni_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  if (CBS_len(contents) != 0) {
    return 0;
  }

  assert(ssl->tlsext_hostname != NULL);

  if (ssl->session == NULL) {
    OPENSSL_free(hs->new_session->tlsext_hostname);
    hs->new_session->tlsext_hostname = BUF_strdup(ssl->tlsext_hostname);
    if (!hs->new_session->tlsext_hostname) {
      *out_alert = SSL_AD_INTERNAL_ERROR;
      return 0;
    }
  }

  return 1;
}

static int ext_sni_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  if (contents == NULL) {
    return 1;
  }

  CBS server_name_list, host_name;
  uint8_t name_type;
  if (!CBS_get_u16_length_prefixed(contents, &server_name_list) ||
      !CBS_get_u8(&server_name_list, &name_type) ||
      /* Although the server_name extension was intended to be extensible to
       * new name types and multiple names, OpenSSL 1.0.x had a bug which meant
       * different name types will cause an error. Further, RFC 4366 originally
       * defined syntax inextensibly. RFC 6066 corrected this mistake, but
       * adding new name types is no longer feasible.
       *
       * Act as if the extensibility does not exist to simplify parsing. */
      !CBS_get_u16_length_prefixed(&server_name_list, &host_name) ||
      CBS_len(&server_name_list) != 0 ||
      CBS_len(contents) != 0) {
    return 0;
  }

  if (name_type != TLSEXT_NAMETYPE_host_name ||
      CBS_len(&host_name) == 0 ||
      CBS_len(&host_name) > TLSEXT_MAXLEN_host_name ||
      CBS_contains_zero_byte(&host_name)) {
    *out_alert = SSL_AD_UNRECOGNIZED_NAME;
    return 0;
  }

  /* Copy the hostname as a string. */
  if (!CBS_strdup(&host_name, &hs->hostname)) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return 0;
  }

  hs->should_ack_sni = 1;
  return 1;
}

static int ext_sni_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  if (hs->ssl->s3->session_reused ||
      !hs->should_ack_sni) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_server_name) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}


/* Renegotiation indication.
 *
 * https://tools.ietf.org/html/rfc5746 */

static int ext_ri_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  /* Renegotiation indication is not necessary in TLS 1.3. */
  if (min_version >= TLS1_3_VERSION) {
    return 1;
  }

  assert(ssl->s3->initial_handshake_complete ==
         (ssl->s3->previous_client_finished_len != 0));

  CBB contents, prev_finished;
  if (!CBB_add_u16(out, TLSEXT_TYPE_renegotiate) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u8_length_prefixed(&contents, &prev_finished) ||
      !CBB_add_bytes(&prev_finished, ssl->s3->previous_client_finished,
                     ssl->s3->previous_client_finished_len) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_ri_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                    CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents != NULL && ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  /* Servers may not switch between omitting the extension and supporting it.
   * See RFC 5746, sections 3.5 and 4.2. */
  if (ssl->s3->initial_handshake_complete &&
      (contents != NULL) != ssl->s3->send_connection_binding) {
    *out_alert = SSL_AD_HANDSHAKE_FAILURE;
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_MISMATCH);
    return 0;
  }

  if (contents == NULL) {
    /* Strictly speaking, if we want to avoid an attack we should *always* see
     * RI even on initial ServerHello because the client doesn't see any
     * renegotiation during an attack. However this would mean we could not
     * connect to any server which doesn't support RI.
     *
     * OpenSSL has |SSL_OP_LEGACY_SERVER_CONNECT| to control this, but in
     * practical terms every client sets it so it's just assumed here. */
    return 1;
  }

  const size_t expected_len = ssl->s3->previous_client_finished_len +
                              ssl->s3->previous_server_finished_len;

  /* Check for logic errors */
  assert(!expected_len || ssl->s3->previous_client_finished_len);
  assert(!expected_len || ssl->s3->previous_server_finished_len);
  assert(ssl->s3->initial_handshake_complete ==
         (ssl->s3->previous_client_finished_len != 0));
  assert(ssl->s3->initial_handshake_complete ==
         (ssl->s3->previous_server_finished_len != 0));

  /* Parse out the extension contents. */
  CBS renegotiated_connection;
  if (!CBS_get_u8_length_prefixed(contents, &renegotiated_connection) ||
      CBS_len(contents) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_ENCODING_ERR);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  /* Check that the extension matches. */
  if (CBS_len(&renegotiated_connection) != expected_len) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_MISMATCH);
    *out_alert = SSL_AD_HANDSHAKE_FAILURE;
    return 0;
  }

  const uint8_t *d = CBS_data(&renegotiated_connection);
  if (CRYPTO_memcmp(d, ssl->s3->previous_client_finished,
        ssl->s3->previous_client_finished_len)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_MISMATCH);
    *out_alert = SSL_AD_HANDSHAKE_FAILURE;
    return 0;
  }
  d += ssl->s3->previous_client_finished_len;

  if (CRYPTO_memcmp(d, ssl->s3->previous_server_finished,
        ssl->s3->previous_server_finished_len)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_MISMATCH);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }
  ssl->s3->send_connection_binding = 1;

  return 1;
}

static int ext_ri_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                    CBS *contents) {
  SSL *const ssl = hs->ssl;
  /* Renegotiation isn't supported as a server so this function should never be
   * called after the initial handshake. */
  assert(!ssl->s3->initial_handshake_complete);

  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    return 1;
  }

  if (contents == NULL) {
    return 1;
  }

  CBS renegotiated_connection;
  if (!CBS_get_u8_length_prefixed(contents, &renegotiated_connection) ||
      CBS_len(contents) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_ENCODING_ERR);
    return 0;
  }

  /* Check that the extension matches. We do not support renegotiation as a
   * server, so this must be empty. */
  if (CBS_len(&renegotiated_connection) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_MISMATCH);
    *out_alert = SSL_AD_HANDSHAKE_FAILURE;
    return 0;
  }

  ssl->s3->send_connection_binding = 1;

  return 1;
}

static int ext_ri_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  /* Renegotiation isn't supported as a server so this function should never be
   * called after the initial handshake. */
  assert(!ssl->s3->initial_handshake_complete);

  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_renegotiate) ||
      !CBB_add_u16(out, 1 /* length */) ||
      !CBB_add_u8(out, 0 /* empty renegotiation info */)) {
    return 0;
  }

  return 1;
}


/* Extended Master Secret.
 *
 * https://tools.ietf.org/html/rfc7627 */

static int ext_ems_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(hs->ssl, &min_version, &max_version)) {
    return 0;
  }

  /* Extended master secret is not necessary in TLS 1.3. */
  if (min_version >= TLS1_3_VERSION || max_version <= SSL3_VERSION) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_extended_master_secret) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}

static int ext_ems_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  SSL *const ssl = hs->ssl;

  if (contents != NULL) {
    if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION ||
        ssl->version == SSL3_VERSION ||
        CBS_len(contents) != 0) {
      return 0;
    }

    hs->extended_master_secret = 1;
  }

  /* Whether EMS is negotiated may not change on renegotiation. */
  if (ssl->s3->established_session != NULL &&
      hs->extended_master_secret !=
          ssl->s3->established_session->extended_master_secret) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_EMS_MISMATCH);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  return 1;
}

static int ext_ems_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  uint16_t version = ssl3_protocol_version(hs->ssl);
  if (version >= TLS1_3_VERSION ||
      version == SSL3_VERSION) {
    return 1;
  }

  if (contents == NULL) {
    return 1;
  }

  if (CBS_len(contents) != 0) {
    return 0;
  }

  hs->extended_master_secret = 1;
  return 1;
}

static int ext_ems_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->extended_master_secret) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_extended_master_secret) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}


/* Session tickets.
 *
 * https://tools.ietf.org/html/rfc5077 */

static int ext_ticket_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  /* TLS 1.3 uses a different ticket extension. */
  if (min_version >= TLS1_3_VERSION ||
      SSL_get_options(ssl) & SSL_OP_NO_TICKET) {
    return 1;
  }

  const uint8_t *ticket_data = NULL;
  int ticket_len = 0;

  /* Renegotiation does not participate in session resumption. However, still
   * advertise the extension to avoid potentially breaking servers which carry
   * over the state from the previous handshake, such as OpenSSL servers
   * without upstream's 3c3f0259238594d77264a78944d409f2127642c4. */
  uint16_t session_version;
  if (!ssl->s3->initial_handshake_complete &&
      ssl->session != NULL &&
      ssl->session->tlsext_tick != NULL &&
      /* Don't send TLS 1.3 session tickets in the ticket extension. */
      ssl->method->version_from_wire(&session_version,
                                     ssl->session->ssl_version) &&
      session_version < TLS1_3_VERSION) {
    ticket_data = ssl->session->tlsext_tick;
    ticket_len = ssl->session->tlsext_ticklen;
  }

  CBB ticket;
  if (!CBB_add_u16(out, TLSEXT_TYPE_session_ticket) ||
      !CBB_add_u16_length_prefixed(out, &ticket) ||
      !CBB_add_bytes(&ticket, ticket_data, ticket_len) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_ticket_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                        CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    return 0;
  }

  /* If |SSL_OP_NO_TICKET| is set then no extension will have been sent and
   * this function should never be called, even if the server tries to send the
   * extension. */
  assert((SSL_get_options(ssl) & SSL_OP_NO_TICKET) == 0);

  if (CBS_len(contents) != 0) {
    return 0;
  }

  hs->ticket_expected = 1;
  return 1;
}

static int ext_ticket_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->ticket_expected) {
    return 1;
  }

  /* If |SSL_OP_NO_TICKET| is set, |ticket_expected| should never be true. */
  assert((SSL_get_options(hs->ssl) & SSL_OP_NO_TICKET) == 0);

  if (!CBB_add_u16(out, TLSEXT_TYPE_session_ticket) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}


/* Signature Algorithms.
 *
 * https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1 */

static int ext_sigalgs_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  if (max_version < TLS1_2_VERSION) {
    return 1;
  }

  const uint16_t *sigalgs;
  const size_t num_sigalgs = tls12_get_verify_sigalgs(ssl, &sigalgs);

  CBB contents, sigalgs_cbb;
  if (!CBB_add_u16(out, TLSEXT_TYPE_signature_algorithms) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &sigalgs_cbb)) {
    return 0;
  }

  for (size_t i = 0; i < num_sigalgs; i++) {
    if (!CBB_add_u16(&sigalgs_cbb, sigalgs[i])) {
      return 0;
    }
  }

  if (!CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_sigalgs_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                         CBS *contents) {
  OPENSSL_free(hs->peer_sigalgs);
  hs->peer_sigalgs = NULL;
  hs->num_peer_sigalgs = 0;

  if (contents == NULL) {
    return 1;
  }

  CBS supported_signature_algorithms;
  if (!CBS_get_u16_length_prefixed(contents, &supported_signature_algorithms) ||
      CBS_len(contents) != 0 ||
      CBS_len(&supported_signature_algorithms) == 0 ||
      !tls1_parse_peer_sigalgs(hs, &supported_signature_algorithms)) {
    return 0;
  }

  return 1;
}


/* OCSP Stapling.
 *
 * https://tools.ietf.org/html/rfc6066#section-8 */

static int ext_ocsp_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (!ssl->ocsp_stapling_enabled) {
    return 1;
  }

  CBB contents;
  if (!CBB_add_u16(out, TLSEXT_TYPE_status_request) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u8(&contents, TLSEXT_STATUSTYPE_ocsp) ||
      !CBB_add_u16(&contents, 0 /* empty responder ID list */) ||
      !CBB_add_u16(&contents, 0 /* empty request extensions */) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_ocsp_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  /* TLS 1.3 OCSP responses are included in the Certificate extensions. */
  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    return 0;
  }

  /* OCSP stapling is forbidden on non-certificate ciphers. */
  if (CBS_len(contents) != 0 ||
      !ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    return 0;
  }

  /* Note this does not check for resumption in TLS 1.2. Sending
   * status_request here does not make sense, but OpenSSL does so and the
   * specification does not say anything. Tolerate it but ignore it. */

  hs->certificate_status_expected = 1;
  return 1;
}

static int ext_ocsp_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  if (contents == NULL) {
    return 1;
  }

  uint8_t status_type;
  if (!CBS_get_u8(contents, &status_type)) {
    return 0;
  }

  /* We cannot decide whether OCSP stapling will occur yet because the correct
   * SSL_CTX might not have been selected. */
  hs->ocsp_stapling_requested = status_type == TLSEXT_STATUSTYPE_ocsp;

  return 1;
}

static int ext_ocsp_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION ||
      !hs->ocsp_stapling_requested ||
      ssl->cert->ocsp_response == NULL ||
      ssl->s3->session_reused ||
      !ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    return 1;
  }

  hs->certificate_status_expected = 1;

  return CBB_add_u16(out, TLSEXT_TYPE_status_request) &&
         CBB_add_u16(out, 0 /* length */);
}


/* Next protocol negotiation.
 *
 * https://htmlpreview.github.io/?https://github.com/agl/technotes/blob/master/nextprotoneg.html */

static int ext_npn_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl->s3->initial_handshake_complete ||
      ssl->ctx->next_proto_select_cb == NULL ||
      SSL_is_dtls(ssl)) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_next_proto_neg) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}

static int ext_npn_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    return 0;
  }

  /* If any of these are false then we should never have sent the NPN
   * extension in the ClientHello and thus this function should never have been
   * called. */
  assert(!ssl->s3->initial_handshake_complete);
  assert(!SSL_is_dtls(ssl));
  assert(ssl->ctx->next_proto_select_cb != NULL);

  if (ssl->s3->alpn_selected != NULL) {
    /* NPN and ALPN may not be negotiated in the same connection. */
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    OPENSSL_PUT_ERROR(SSL, SSL_R_NEGOTIATED_BOTH_NPN_AND_ALPN);
    return 0;
  }

  const uint8_t *const orig_contents = CBS_data(contents);
  const size_t orig_len = CBS_len(contents);

  while (CBS_len(contents) != 0) {
    CBS proto;
    if (!CBS_get_u8_length_prefixed(contents, &proto) ||
        CBS_len(&proto) == 0) {
      return 0;
    }
  }

  uint8_t *selected;
  uint8_t selected_len;
  if (ssl->ctx->next_proto_select_cb(
          ssl, &selected, &selected_len, orig_contents, orig_len,
          ssl->ctx->next_proto_select_cb_arg) != SSL_TLSEXT_ERR_OK) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return 0;
  }

  OPENSSL_free(ssl->s3->next_proto_negotiated);
  ssl->s3->next_proto_negotiated = BUF_memdup(selected, selected_len);
  if (ssl->s3->next_proto_negotiated == NULL) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return 0;
  }

  ssl->s3->next_proto_negotiated_len = selected_len;
  hs->next_proto_neg_seen = 1;

  return 1;
}

static int ext_npn_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    return 1;
  }

  if (contents != NULL && CBS_len(contents) != 0) {
    return 0;
  }

  if (contents == NULL ||
      ssl->s3->initial_handshake_complete ||
      ssl->ctx->next_protos_advertised_cb == NULL ||
      SSL_is_dtls(ssl)) {
    return 1;
  }

  hs->next_proto_neg_seen = 1;
  return 1;
}

static int ext_npn_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  /* |next_proto_neg_seen| might have been cleared when an ALPN extension was
   * parsed. */
  if (!hs->next_proto_neg_seen) {
    return 1;
  }

  const uint8_t *npa;
  unsigned npa_len;

  if (ssl->ctx->next_protos_advertised_cb(
          ssl, &npa, &npa_len, ssl->ctx->next_protos_advertised_cb_arg) !=
      SSL_TLSEXT_ERR_OK) {
    hs->next_proto_neg_seen = 0;
    return 1;
  }

  CBB contents;
  if (!CBB_add_u16(out, TLSEXT_TYPE_next_proto_neg) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_bytes(&contents, npa, npa_len) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}


/* Signed certificate timestamps.
 *
 * https://tools.ietf.org/html/rfc6962#section-3.3.1 */

static int ext_sct_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (!ssl->signed_cert_timestamps_enabled) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_certificate_timestamp) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}

static int ext_sct_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  /* TLS 1.3 SCTs are included in the Certificate extensions. */
  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  /* If this is false then we should never have sent the SCT extension in the
   * ClientHello and thus this function should never have been called. */
  assert(ssl->signed_cert_timestamps_enabled);

  if (!ssl_is_sct_list_valid(contents)) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  /* Session resumption uses the original session information. The extension
   * should not be sent on resumption, but RFC 6962 did not make it a
   * requirement, so tolerate this.
   *
   * TODO(davidben): Enforce this anyway. */
  if (!ssl->s3->session_reused &&
      !CBS_stow(contents, &hs->new_session->tlsext_signed_cert_timestamp_list,
                &hs->new_session->tlsext_signed_cert_timestamp_list_length)) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return 0;
  }

  return 1;
}

static int ext_sct_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                     CBS *contents) {
  if (contents == NULL) {
    return 1;
  }

  if (CBS_len(contents) != 0) {
    return 0;
  }

  hs->scts_requested = 1;
  return 1;
}

static int ext_sct_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  /* The extension shouldn't be sent when resuming sessions. */
  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION ||
      ssl->s3->session_reused ||
      ssl->cert->signed_cert_timestamp_list == NULL) {
    return 1;
  }

  CBB contents;
  return CBB_add_u16(out, TLSEXT_TYPE_certificate_timestamp) &&
         CBB_add_u16_length_prefixed(out, &contents) &&
         CBB_add_bytes(
             &contents,
             CRYPTO_BUFFER_data(ssl->cert->signed_cert_timestamp_list),
             CRYPTO_BUFFER_len(ssl->cert->signed_cert_timestamp_list)) &&
         CBB_flush(out);
}


/* Application-level Protocol Negotiation.
 *
 * https://tools.ietf.org/html/rfc7301 */

static int ext_alpn_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl->alpn_client_proto_list == NULL ||
      ssl->s3->initial_handshake_complete) {
    return 1;
  }

  CBB contents, proto_list;
  if (!CBB_add_u16(out, TLSEXT_TYPE_application_layer_protocol_negotiation) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &proto_list) ||
      !CBB_add_bytes(&proto_list, ssl->alpn_client_proto_list,
                     ssl->alpn_client_proto_list_len) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_alpn_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  assert(!ssl->s3->initial_handshake_complete);
  assert(ssl->alpn_client_proto_list != NULL);

  if (hs->next_proto_neg_seen) {
    /* NPN and ALPN may not be negotiated in the same connection. */
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    OPENSSL_PUT_ERROR(SSL, SSL_R_NEGOTIATED_BOTH_NPN_AND_ALPN);
    return 0;
  }

  /* The extension data consists of a ProtocolNameList which must have
   * exactly one ProtocolName. Each of these is length-prefixed. */
  CBS protocol_name_list, protocol_name;
  if (!CBS_get_u16_length_prefixed(contents, &protocol_name_list) ||
      CBS_len(contents) != 0 ||
      !CBS_get_u8_length_prefixed(&protocol_name_list, &protocol_name) ||
      /* Empty protocol names are forbidden. */
      CBS_len(&protocol_name) == 0 ||
      CBS_len(&protocol_name_list) != 0) {
    return 0;
  }

  /* Check that the protcol name is one of the ones we advertised. */
  int protocol_ok = 0;
  CBS client_protocol_name_list, client_protocol_name;
  CBS_init(&client_protocol_name_list, ssl->alpn_client_proto_list,
           ssl->alpn_client_proto_list_len);
  while (CBS_len(&client_protocol_name_list) > 0) {
    if (!CBS_get_u8_length_prefixed(&client_protocol_name_list,
                                    &client_protocol_name)) {
      *out_alert = SSL_AD_INTERNAL_ERROR;
      return 0;
    }

    if (CBS_len(&client_protocol_name) == CBS_len(&protocol_name) &&
        OPENSSL_memcmp(CBS_data(&client_protocol_name),
                       CBS_data(&protocol_name),
                       CBS_len(&protocol_name)) == 0) {
      protocol_ok = 1;
      break;
    }
  }

  if (!protocol_ok) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_INVALID_ALPN_PROTOCOL);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  if (!CBS_stow(&protocol_name, &ssl->s3->alpn_selected,
                &ssl->s3->alpn_selected_len)) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return 0;
  }

  return 1;
}

int ssl_negotiate_alpn(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                       const SSL_CLIENT_HELLO *client_hello) {
  SSL *const ssl = hs->ssl;
  CBS contents;
  if (ssl->ctx->alpn_select_cb == NULL ||
      !ssl_client_hello_get_extension(
          client_hello, &contents,
          TLSEXT_TYPE_application_layer_protocol_negotiation)) {
    /* Ignore ALPN if not configured or no extension was supplied. */
    return 1;
  }

  /* ALPN takes precedence over NPN. */
  hs->next_proto_neg_seen = 0;

  CBS protocol_name_list;
  if (!CBS_get_u16_length_prefixed(&contents, &protocol_name_list) ||
      CBS_len(&contents) != 0 ||
      CBS_len(&protocol_name_list) < 2) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_PARSE_TLSEXT);
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  /* Validate the protocol list. */
  CBS protocol_name_list_copy = protocol_name_list;
  while (CBS_len(&protocol_name_list_copy) > 0) {
    CBS protocol_name;

    if (!CBS_get_u8_length_prefixed(&protocol_name_list_copy, &protocol_name) ||
        /* Empty protocol names are forbidden. */
        CBS_len(&protocol_name) == 0) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_PARSE_TLSEXT);
      *out_alert = SSL_AD_DECODE_ERROR;
      return 0;
    }
  }

  const uint8_t *selected;
  uint8_t selected_len;
  if (ssl->ctx->alpn_select_cb(
          ssl, &selected, &selected_len, CBS_data(&protocol_name_list),
          CBS_len(&protocol_name_list),
          ssl->ctx->alpn_select_cb_arg) == SSL_TLSEXT_ERR_OK) {
    OPENSSL_free(ssl->s3->alpn_selected);
    ssl->s3->alpn_selected = BUF_memdup(selected, selected_len);
    if (ssl->s3->alpn_selected == NULL) {
      *out_alert = SSL_AD_INTERNAL_ERROR;
      return 0;
    }
    ssl->s3->alpn_selected_len = selected_len;
  }

  return 1;
}

static int ext_alpn_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl->s3->alpn_selected == NULL) {
    return 1;
  }

  CBB contents, proto_list, proto;
  if (!CBB_add_u16(out, TLSEXT_TYPE_application_layer_protocol_negotiation) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &proto_list) ||
      !CBB_add_u8_length_prefixed(&proto_list, &proto) ||
      !CBB_add_bytes(&proto, ssl->s3->alpn_selected,
                     ssl->s3->alpn_selected_len) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}


/* Channel ID.
 *
 * https://tools.ietf.org/html/draft-balfanz-tls-channelid-01 */

static void ext_channel_id_init(SSL_HANDSHAKE *hs) {
  hs->ssl->s3->tlsext_channel_id_valid = 0;
}

static int ext_channel_id_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (!ssl->tlsext_channel_id_enabled ||
      SSL_is_dtls(ssl)) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_channel_id) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}

static int ext_channel_id_parse_serverhello(SSL_HANDSHAKE *hs,
                                            uint8_t *out_alert, CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  assert(!SSL_is_dtls(ssl));
  assert(ssl->tlsext_channel_id_enabled);

  if (CBS_len(contents) != 0) {
    return 0;
  }

  ssl->s3->tlsext_channel_id_valid = 1;
  return 1;
}

static int ext_channel_id_parse_clienthello(SSL_HANDSHAKE *hs,
                                            uint8_t *out_alert, CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL ||
      !ssl->tlsext_channel_id_enabled ||
      SSL_is_dtls(ssl)) {
    return 1;
  }

  if (CBS_len(contents) != 0) {
    return 0;
  }

  ssl->s3->tlsext_channel_id_valid = 1;
  return 1;
}

static int ext_channel_id_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (!ssl->s3->tlsext_channel_id_valid) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_channel_id) ||
      !CBB_add_u16(out, 0 /* length */)) {
    return 0;
  }

  return 1;
}


/* Secure Real-time Transport Protocol (SRTP) extension.
 *
 * https://tools.ietf.org/html/rfc5764 */


static void ext_srtp_init(SSL_HANDSHAKE *hs) {
  hs->ssl->srtp_profile = NULL;
}

static int ext_srtp_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  STACK_OF(SRTP_PROTECTION_PROFILE) *profiles = SSL_get_srtp_profiles(ssl);
  if (profiles == NULL) {
    return 1;
  }
  const size_t num_profiles = sk_SRTP_PROTECTION_PROFILE_num(profiles);
  if (num_profiles == 0) {
    return 1;
  }

  CBB contents, profile_ids;
  if (!CBB_add_u16(out, TLSEXT_TYPE_srtp) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &profile_ids)) {
    return 0;
  }

  for (size_t i = 0; i < num_profiles; i++) {
    if (!CBB_add_u16(&profile_ids,
                     sk_SRTP_PROTECTION_PROFILE_value(profiles, i)->id)) {
      return 0;
    }
  }

  if (!CBB_add_u8(&contents, 0 /* empty use_mki value */) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_srtp_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  /* The extension consists of a u16-prefixed profile ID list containing a
   * single uint16_t profile ID, then followed by a u8-prefixed srtp_mki field.
   *
   * See https://tools.ietf.org/html/rfc5764#section-4.1.1 */
  CBS profile_ids, srtp_mki;
  uint16_t profile_id;
  if (!CBS_get_u16_length_prefixed(contents, &profile_ids) ||
      !CBS_get_u16(&profile_ids, &profile_id) ||
      CBS_len(&profile_ids) != 0 ||
      !CBS_get_u8_length_prefixed(contents, &srtp_mki) ||
      CBS_len(contents) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_SRTP_PROTECTION_PROFILE_LIST);
    return 0;
  }

  if (CBS_len(&srtp_mki) != 0) {
    /* Must be no MKI, since we never offer one. */
    OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_SRTP_MKI_VALUE);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  STACK_OF(SRTP_PROTECTION_PROFILE) *profiles = SSL_get_srtp_profiles(ssl);

  /* Check to see if the server gave us something we support (and presumably
   * offered). */
  for (size_t i = 0; i < sk_SRTP_PROTECTION_PROFILE_num(profiles); i++) {
    const SRTP_PROTECTION_PROFILE *profile =
        sk_SRTP_PROTECTION_PROFILE_value(profiles, i);

    if (profile->id == profile_id) {
      ssl->srtp_profile = profile;
      return 1;
    }
  }

  OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_SRTP_PROTECTION_PROFILE_LIST);
  *out_alert = SSL_AD_ILLEGAL_PARAMETER;
  return 0;
}

static int ext_srtp_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  CBS profile_ids, srtp_mki;
  if (!CBS_get_u16_length_prefixed(contents, &profile_ids) ||
      CBS_len(&profile_ids) < 2 ||
      !CBS_get_u8_length_prefixed(contents, &srtp_mki) ||
      CBS_len(contents) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_SRTP_PROTECTION_PROFILE_LIST);
    return 0;
  }
  /* Discard the MKI value for now. */

  const STACK_OF(SRTP_PROTECTION_PROFILE) *server_profiles =
      SSL_get_srtp_profiles(ssl);

  /* Pick the server's most preferred profile. */
  for (size_t i = 0; i < sk_SRTP_PROTECTION_PROFILE_num(server_profiles); i++) {
    const SRTP_PROTECTION_PROFILE *server_profile =
        sk_SRTP_PROTECTION_PROFILE_value(server_profiles, i);

    CBS profile_ids_tmp;
    CBS_init(&profile_ids_tmp, CBS_data(&profile_ids), CBS_len(&profile_ids));

    while (CBS_len(&profile_ids_tmp) > 0) {
      uint16_t profile_id;
      if (!CBS_get_u16(&profile_ids_tmp, &profile_id)) {
        return 0;
      }

      if (server_profile->id == profile_id) {
        ssl->srtp_profile = server_profile;
        return 1;
      }
    }
  }

  return 1;
}

static int ext_srtp_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl->srtp_profile == NULL) {
    return 1;
  }

  CBB contents, profile_ids;
  if (!CBB_add_u16(out, TLSEXT_TYPE_srtp) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &profile_ids) ||
      !CBB_add_u16(&profile_ids, ssl->srtp_profile->id) ||
      !CBB_add_u8(&contents, 0 /* empty MKI */) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}


/* EC point formats.
 *
 * https://tools.ietf.org/html/rfc4492#section-5.1.2 */

static int ext_ec_point_add_extension(SSL_HANDSHAKE *hs, CBB *out) {
  CBB contents, formats;
  if (!CBB_add_u16(out, TLSEXT_TYPE_ec_point_formats) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u8_length_prefixed(&contents, &formats) ||
      !CBB_add_u8(&formats, TLSEXT_ECPOINTFORMAT_uncompressed) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_ec_point_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(hs->ssl, &min_version, &max_version)) {
    return 0;
  }

  /* The point format extension is unneccessary in TLS 1.3. */
  if (min_version >= TLS1_3_VERSION) {
    return 1;
  }

  return ext_ec_point_add_extension(hs, out);
}

static int ext_ec_point_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                          CBS *contents) {
  if (contents == NULL) {
    return 1;
  }

  if (ssl3_protocol_version(hs->ssl) >= TLS1_3_VERSION) {
    return 0;
  }

  CBS ec_point_format_list;
  if (!CBS_get_u8_length_prefixed(contents, &ec_point_format_list) ||
      CBS_len(contents) != 0) {
    return 0;
  }

  /* Per RFC 4492, section 5.1.2, implementations MUST support the uncompressed
   * point format. */
  if (OPENSSL_memchr(CBS_data(&ec_point_format_list),
                     TLSEXT_ECPOINTFORMAT_uncompressed,
                     CBS_len(&ec_point_format_list)) == NULL) {
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  return 1;
}

static int ext_ec_point_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                          CBS *contents) {
  if (ssl3_protocol_version(hs->ssl) >= TLS1_3_VERSION) {
    return 1;
  }

  return ext_ec_point_parse_serverhello(hs, out_alert, contents);
}

static int ext_ec_point_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    return 1;
  }

  const uint32_t alg_k = hs->new_cipher->algorithm_mkey;
  const uint32_t alg_a = hs->new_cipher->algorithm_auth;
  const int using_ecc = (alg_k & SSL_kECDHE) || (alg_a & SSL_aECDSA);

  if (!using_ecc) {
    return 1;
  }

  return ext_ec_point_add_extension(hs, out);
}


/* Pre Shared Key
 *
 * https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.2.6 */

static size_t ext_pre_shared_key_clienthello_length(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  uint16_t session_version;
  if (max_version < TLS1_3_VERSION || ssl->session == NULL ||
      !ssl->method->version_from_wire(&session_version,
                                      ssl->session->ssl_version) ||
      session_version < TLS1_3_VERSION) {
    return 0;
  }

  const EVP_MD *digest = SSL_SESSION_get_digest(ssl->session, ssl);
  if (digest == NULL) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return 0;
  }

  size_t binder_len = EVP_MD_size(digest);
  return 15 + ssl->session->tlsext_ticklen + binder_len;
}

static int ext_pre_shared_key_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  uint16_t session_version;
  if (max_version < TLS1_3_VERSION || ssl->session == NULL ||
      !ssl->method->version_from_wire(&session_version,
                                      ssl->session->ssl_version) ||
      session_version < TLS1_3_VERSION) {
    return 1;
  }

  struct OPENSSL_timeval now;
  ssl_get_current_time(ssl, &now);
  uint32_t ticket_age = 1000 * (now.tv_sec - ssl->session->time);
  uint32_t obfuscated_ticket_age = ticket_age + ssl->session->ticket_age_add;

  /* Fill in a placeholder zero binder of the appropriate length. It will be
   * computed and filled in later after length prefixes are computed. */
  uint8_t zero_binder[EVP_MAX_MD_SIZE] = {0};

  const EVP_MD *digest = SSL_SESSION_get_digest(ssl->session, ssl);
  if (digest == NULL) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return 0;
  }

  size_t binder_len = EVP_MD_size(digest);

  CBB contents, identity, ticket, binders, binder;
  if (!CBB_add_u16(out, TLSEXT_TYPE_pre_shared_key) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &identity) ||
      !CBB_add_u16_length_prefixed(&identity, &ticket) ||
      !CBB_add_bytes(&ticket, ssl->session->tlsext_tick,
                     ssl->session->tlsext_ticklen) ||
      !CBB_add_u32(&identity, obfuscated_ticket_age) ||
      !CBB_add_u16_length_prefixed(&contents, &binders) ||
      !CBB_add_u8_length_prefixed(&binders, &binder) ||
      !CBB_add_bytes(&binder, zero_binder, binder_len)) {
    return 0;
  }

  hs->needs_psk_binder = 1;
  return CBB_flush(out);
}

int ssl_ext_pre_shared_key_parse_serverhello(SSL_HANDSHAKE *hs,
                                             uint8_t *out_alert,
                                             CBS *contents) {
  uint16_t psk_id;
  if (!CBS_get_u16(contents, &psk_id) ||
      CBS_len(contents) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  /* We only advertise one PSK identity, so the only legal index is zero. */
  if (psk_id != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_PSK_IDENTITY_NOT_FOUND);
    *out_alert = SSL_AD_UNKNOWN_PSK_IDENTITY;
    return 0;
  }

  return 1;
}

int ssl_ext_pre_shared_key_parse_clienthello(
    SSL_HANDSHAKE *hs, CBS *out_ticket, CBS *out_binders,
    uint32_t *out_obfuscated_ticket_age, uint8_t *out_alert, CBS *contents) {
  /* We only process the first PSK identity since we don't support pure PSK. */
  CBS identities, binders;
  if (!CBS_get_u16_length_prefixed(contents, &identities) ||
      !CBS_get_u16_length_prefixed(&identities, out_ticket) ||
      !CBS_get_u32(&identities, out_obfuscated_ticket_age) ||
      !CBS_get_u16_length_prefixed(contents, &binders) ||
      CBS_len(&binders) == 0 ||
      CBS_len(contents) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  *out_binders = binders;

  /* Check the syntax of the remaining identities, but do not process them. */
  size_t num_identities = 1;
  while (CBS_len(&identities) != 0) {
    CBS unused_ticket;
    uint32_t unused_obfuscated_ticket_age;
    if (!CBS_get_u16_length_prefixed(&identities, &unused_ticket) ||
        !CBS_get_u32(&identities, &unused_obfuscated_ticket_age)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      *out_alert = SSL_AD_DECODE_ERROR;
      return 0;
    }

    num_identities++;
  }

  /* Check the syntax of the binders. The value will be checked later if
   * resuming. */
  size_t num_binders = 0;
  while (CBS_len(&binders) != 0) {
    CBS binder;
    if (!CBS_get_u8_length_prefixed(&binders, &binder)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      *out_alert = SSL_AD_DECODE_ERROR;
      return 0;
    }

    num_binders++;
  }

  if (num_identities != num_binders) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_PSK_IDENTITY_BINDER_COUNT_MISMATCH);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  return 1;
}

int ssl_ext_pre_shared_key_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->ssl->s3->session_reused) {
    return 1;
  }

  CBB contents;
  if (!CBB_add_u16(out, TLSEXT_TYPE_pre_shared_key) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      /* We only consider the first identity for resumption */
      !CBB_add_u16(&contents, 0) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}


/* Pre-Shared Key Exchange Modes
 *
 * https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.2.7 */

static int ext_psk_key_exchange_modes_add_clienthello(SSL_HANDSHAKE *hs,
                                                      CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  if (max_version < TLS1_3_VERSION) {
    return 1;
  }

  CBB contents, ke_modes;
  if (!CBB_add_u16(out, TLSEXT_TYPE_psk_key_exchange_modes) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u8_length_prefixed(&contents, &ke_modes) ||
      !CBB_add_u8(&ke_modes, SSL_PSK_DHE_KE)) {
    return 0;
  }

  return CBB_flush(out);
}

static int ext_psk_key_exchange_modes_parse_clienthello(SSL_HANDSHAKE *hs,
                                                        uint8_t *out_alert,
                                                        CBS *contents) {
  if (contents == NULL) {
    return 1;
  }

  CBS ke_modes;
  if (!CBS_get_u8_length_prefixed(contents, &ke_modes) ||
      CBS_len(&ke_modes) == 0 ||
      CBS_len(contents) != 0) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  /* We only support tickets with PSK_DHE_KE. */
  hs->accept_psk_mode = OPENSSL_memchr(CBS_data(&ke_modes), SSL_PSK_DHE_KE,
                                       CBS_len(&ke_modes)) != NULL;

  return 1;
}


/* Early Data Indication
 *
 * https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.2.8 */

static int ext_early_data_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t session_version;
  if (ssl->session == NULL ||
      !ssl->method->version_from_wire(&session_version,
                                      ssl->session->ssl_version) ||
      session_version < TLS1_3_VERSION ||
      ssl->session->ticket_max_early_data == 0 ||
      hs->received_hello_retry_request ||
      !ssl->ctx->enable_early_data) {
    return 1;
  }

  hs->early_data_offered = 1;

  if (!CBB_add_u16(out, TLSEXT_TYPE_early_data) ||
      !CBB_add_u16(out, 0) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}

static int ext_early_data_parse_serverhello(SSL_HANDSHAKE *hs,
                                            uint8_t *out_alert, CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return 1;
  }

  if (CBS_len(contents) != 0) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  if (!ssl->s3->session_reused) {
    *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
    return 0;
  }

  ssl->early_data_accepted = 1;
  return 1;
}

static int ext_early_data_parse_clienthello(SSL_HANDSHAKE *hs,
                                            uint8_t *out_alert, CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL ||
      ssl3_protocol_version(ssl) < TLS1_3_VERSION) {
    return 1;
  }

  if (CBS_len(contents) != 0) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  hs->early_data_offered = 1;
  return 1;
}

static int ext_early_data_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->ssl->early_data_accepted) {
    return 1;
  }

  if (!CBB_add_u16(out, TLSEXT_TYPE_early_data) ||
      !CBB_add_u16(out, 0) ||
      !CBB_flush(out)) {
    return 0;
  }

  return 1;
}


/* Key Share
 *
 * https://tools.ietf.org/html/draft-ietf-tls-tls13-16#section-4.2.5 */

static int ext_key_share_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  if (max_version < TLS1_3_VERSION) {
    return 1;
  }

  CBB contents, kse_bytes;
  if (!CBB_add_u16(out, TLSEXT_TYPE_key_share) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &kse_bytes)) {
    return 0;
  }

  uint16_t group_id = hs->retry_group;
  if (hs->received_hello_retry_request) {
    /* We received a HelloRetryRequest without a new curve, so there is no new
     * share to append. Leave |ecdh_ctx| as-is. */
    if (group_id == 0 &&
        !CBB_add_bytes(&kse_bytes, hs->key_share_bytes,
                       hs->key_share_bytes_len)) {
      return 0;
    }
    OPENSSL_free(hs->key_share_bytes);
    hs->key_share_bytes = NULL;
    hs->key_share_bytes_len = 0;
    if (group_id == 0) {
      return CBB_flush(out);
    }
  } else {
    /* Add a fake group. See draft-davidben-tls-grease-01. */
    if (ssl->ctx->grease_enabled &&
        (!CBB_add_u16(&kse_bytes,
                      ssl_get_grease_value(ssl, ssl_grease_group)) ||
         !CBB_add_u16(&kse_bytes, 1 /* length */) ||
         !CBB_add_u8(&kse_bytes, 0 /* one byte key share */))) {
      return 0;
    }

    /* Predict the most preferred group. */
    const uint16_t *groups;
    size_t groups_len;
    tls1_get_grouplist(ssl, &groups, &groups_len);
    if (groups_len == 0) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_NO_GROUPS_SPECIFIED);
      return 0;
    }

    group_id = groups[0];
  }

  CBB key_exchange;
  if (!CBB_add_u16(&kse_bytes, group_id) ||
      !CBB_add_u16_length_prefixed(&kse_bytes, &key_exchange) ||
      !SSL_ECDH_CTX_init(&hs->ecdh_ctx, group_id) ||
      !SSL_ECDH_CTX_offer(&hs->ecdh_ctx, &key_exchange) ||
      !CBB_flush(&kse_bytes)) {
    return 0;
  }

  if (!hs->received_hello_retry_request) {
    /* Save the contents of the extension to repeat it in the second
     * ClientHello. */
    hs->key_share_bytes_len = CBB_len(&kse_bytes);
    hs->key_share_bytes = BUF_memdup(CBB_data(&kse_bytes), CBB_len(&kse_bytes));
    if (hs->key_share_bytes == NULL) {
      return 0;
    }
  }

  return CBB_flush(out);
}

int ssl_ext_key_share_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t **out_secret,
                                        size_t *out_secret_len,
                                        uint8_t *out_alert, CBS *contents) {
  CBS peer_key;
  uint16_t group_id;
  if (!CBS_get_u16(contents, &group_id) ||
      !CBS_get_u16_length_prefixed(contents, &peer_key) ||
      CBS_len(contents) != 0) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  if (SSL_ECDH_CTX_get_id(&hs->ecdh_ctx) != group_id) {
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_CURVE);
    return 0;
  }

  if (!SSL_ECDH_CTX_finish(&hs->ecdh_ctx, out_secret, out_secret_len, out_alert,
                           CBS_data(&peer_key), CBS_len(&peer_key))) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return 0;
  }

  hs->new_session->group_id = group_id;
  SSL_ECDH_CTX_cleanup(&hs->ecdh_ctx);
  return 1;
}

int ssl_ext_key_share_parse_clienthello(SSL_HANDSHAKE *hs, int *out_found,
                                        uint8_t **out_secret,
                                        size_t *out_secret_len,
                                        uint8_t *out_alert, CBS *contents) {
  uint16_t group_id;
  CBS key_shares;
  if (!tls1_get_shared_group(hs, &group_id)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_NO_SHARED_GROUP);
    *out_alert = SSL_AD_HANDSHAKE_FAILURE;
    return 0;
  }

  if (!CBS_get_u16_length_prefixed(contents, &key_shares) ||
      CBS_len(contents) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    return 0;
  }

  /* Find the corresponding key share. */
  int found = 0;
  CBS peer_key;
  while (CBS_len(&key_shares) > 0) {
    uint16_t id;
    CBS peer_key_tmp;
    if (!CBS_get_u16(&key_shares, &id) ||
        !CBS_get_u16_length_prefixed(&key_shares, &peer_key_tmp)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      return 0;
    }

    if (id == group_id) {
      if (found) {
        OPENSSL_PUT_ERROR(SSL, SSL_R_DUPLICATE_KEY_SHARE);
        *out_alert = SSL_AD_ILLEGAL_PARAMETER;
        return 0;
      }

      found = 1;
      peer_key = peer_key_tmp;
      /* Continue parsing the structure to keep peers honest. */
    }
  }

  if (!found) {
    *out_found = 0;
    *out_secret = NULL;
    *out_secret_len = 0;
    return 1;
  }

  /* Compute the DH secret. */
  uint8_t *secret = NULL;
  size_t secret_len;
  SSL_ECDH_CTX group;
  OPENSSL_memset(&group, 0, sizeof(SSL_ECDH_CTX));
  CBB public_key;
  if (!CBB_init(&public_key, 32) ||
      !SSL_ECDH_CTX_init(&group, group_id) ||
      !SSL_ECDH_CTX_accept(&group, &public_key, &secret, &secret_len, out_alert,
                           CBS_data(&peer_key), CBS_len(&peer_key)) ||
      !CBB_finish(&public_key, &hs->public_key, &hs->public_key_len)) {
    OPENSSL_free(secret);
    SSL_ECDH_CTX_cleanup(&group);
    CBB_cleanup(&public_key);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return 0;
  }

  SSL_ECDH_CTX_cleanup(&group);

  *out_secret = secret;
  *out_secret_len = secret_len;
  *out_found = 1;
  return 1;
}

int ssl_ext_key_share_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  uint16_t group_id;
  CBB kse_bytes, public_key;
  if (!tls1_get_shared_group(hs, &group_id) ||
      !CBB_add_u16(out, TLSEXT_TYPE_key_share) ||
      !CBB_add_u16_length_prefixed(out, &kse_bytes) ||
      !CBB_add_u16(&kse_bytes, group_id) ||
      !CBB_add_u16_length_prefixed(&kse_bytes, &public_key) ||
      !CBB_add_bytes(&public_key, hs->public_key, hs->public_key_len) ||
      !CBB_flush(out)) {
    return 0;
  }

  OPENSSL_free(hs->public_key);
  hs->public_key = NULL;
  hs->public_key_len = 0;

  hs->new_session->group_id = group_id;
  return 1;
}


/* Supported Versions
 *
 * https://tools.ietf.org/html/draft-ietf-tls-tls13-16#section-4.2.1 */

static int ext_supported_versions_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  uint16_t min_version, max_version;
  if (!ssl_get_version_range(ssl, &min_version, &max_version)) {
    return 0;
  }

  if (max_version <= TLS1_2_VERSION) {
    return 1;
  }

  CBB contents, versions;
  if (!CBB_add_u16(out, TLSEXT_TYPE_supported_versions) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u8_length_prefixed(&contents, &versions)) {
    return 0;
  }

  /* Add a fake version. See draft-davidben-tls-grease-01. */
  if (ssl->ctx->grease_enabled &&
      !CBB_add_u16(&versions, ssl_get_grease_value(ssl, ssl_grease_version))) {
    return 0;
  }

  for (uint16_t version = max_version; version >= min_version; version--) {
    if (!CBB_add_u16(&versions, ssl->method->version_to_wire(version))) {
      return 0;
    }
  }

  if (!CBB_flush(out)) {
    return 0;
  }

  return 1;
}


/* Cookie
 *
 * https://tools.ietf.org/html/draft-ietf-tls-tls13-16#section-4.2.2 */

static int ext_cookie_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  if (hs->cookie == NULL) {
    return 1;
  }

  CBB contents, cookie;
  if (!CBB_add_u16(out, TLSEXT_TYPE_cookie) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &cookie) ||
      !CBB_add_bytes(&cookie, hs->cookie, hs->cookie_len) ||
      !CBB_flush(out)) {
    return 0;
  }

  /* The cookie is no longer needed in memory. */
  OPENSSL_free(hs->cookie);
  hs->cookie = NULL;
  hs->cookie_len = 0;
  return 1;
}


/* Negotiated Groups
 *
 * https://tools.ietf.org/html/rfc4492#section-5.1.2
 * https://tools.ietf.org/html/draft-ietf-tls-tls13-16#section-4.2.4 */

static int ext_supported_groups_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  CBB contents, groups_bytes;
  if (!CBB_add_u16(out, TLSEXT_TYPE_supported_groups) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &groups_bytes)) {
    return 0;
  }

  /* Add a fake group. See draft-davidben-tls-grease-01. */
  if (ssl->ctx->grease_enabled &&
      !CBB_add_u16(&groups_bytes,
                   ssl_get_grease_value(ssl, ssl_grease_group))) {
    return 0;
  }

  const uint16_t *groups;
  size_t groups_len;
  tls1_get_grouplist(ssl, &groups, &groups_len);

  for (size_t i = 0; i < groups_len; i++) {
    if (!CBB_add_u16(&groups_bytes, groups[i])) {
      return 0;
    }
  }

  return CBB_flush(out);
}

static int ext_supported_groups_parse_serverhello(SSL_HANDSHAKE *hs,
                                                  uint8_t *out_alert,
                                                  CBS *contents) {
  /* This extension is not expected to be echoed by servers in TLS 1.2, but some
   * BigIP servers send it nonetheless, so do not enforce this. */
  return 1;
}

static int ext_supported_groups_parse_clienthello(SSL_HANDSHAKE *hs,
                                                  uint8_t *out_alert,
                                                  CBS *contents) {
  if (contents == NULL) {
    return 1;
  }

  CBS supported_group_list;
  if (!CBS_get_u16_length_prefixed(contents, &supported_group_list) ||
      CBS_len(&supported_group_list) == 0 ||
      (CBS_len(&supported_group_list) & 1) != 0 ||
      CBS_len(contents) != 0) {
    return 0;
  }

  hs->peer_supported_group_list =
      OPENSSL_malloc(CBS_len(&supported_group_list));
  if (hs->peer_supported_group_list == NULL) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return 0;
  }

  const size_t num_groups = CBS_len(&supported_group_list) / 2;
  for (size_t i = 0; i < num_groups; i++) {
    if (!CBS_get_u16(&supported_group_list,
                     &hs->peer_supported_group_list[i])) {
      goto err;
    }
  }

  assert(CBS_len(&supported_group_list) == 0);
  hs->peer_supported_group_list_len = num_groups;

  return 1;

err:
  OPENSSL_free(hs->peer_supported_group_list);
  hs->peer_supported_group_list = NULL;
  *out_alert = SSL_AD_INTERNAL_ERROR;
  return 0;
}

static int ext_supported_groups_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  /* Servers don't echo this extension. */
  return 1;
}


/* kExtensions contains all the supported extensions. */
static const struct tls_extension kExtensions[] = {
  {
    TLSEXT_TYPE_renegotiate,
    NULL,
    ext_ri_add_clienthello,
    ext_ri_parse_serverhello,
    ext_ri_parse_clienthello,
    ext_ri_add_serverhello,
  },
  {
    TLSEXT_TYPE_server_name,
    NULL,
    ext_sni_add_clienthello,
    ext_sni_parse_serverhello,
    ext_sni_parse_clienthello,
    ext_sni_add_serverhello,
  },
  {
    TLSEXT_TYPE_extended_master_secret,
    NULL,
    ext_ems_add_clienthello,
    ext_ems_parse_serverhello,
    ext_ems_parse_clienthello,
    ext_ems_add_serverhello,
  },
  {
    TLSEXT_TYPE_session_ticket,
    NULL,
    ext_ticket_add_clienthello,
    ext_ticket_parse_serverhello,
    /* Ticket extension client parsing is handled in ssl_session.c */
    ignore_parse_clienthello,
    ext_ticket_add_serverhello,
  },
  {
    TLSEXT_TYPE_signature_algorithms,
    NULL,
    ext_sigalgs_add_clienthello,
    forbid_parse_serverhello,
    ext_sigalgs_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_status_request,
    NULL,
    ext_ocsp_add_clienthello,
    ext_ocsp_parse_serverhello,
    ext_ocsp_parse_clienthello,
    ext_ocsp_add_serverhello,
  },
  {
    TLSEXT_TYPE_next_proto_neg,
    NULL,
    ext_npn_add_clienthello,
    ext_npn_parse_serverhello,
    ext_npn_parse_clienthello,
    ext_npn_add_serverhello,
  },
  {
    TLSEXT_TYPE_certificate_timestamp,
    NULL,
    ext_sct_add_clienthello,
    ext_sct_parse_serverhello,
    ext_sct_parse_clienthello,
    ext_sct_add_serverhello,
  },
  {
    TLSEXT_TYPE_application_layer_protocol_negotiation,
    NULL,
    ext_alpn_add_clienthello,
    ext_alpn_parse_serverhello,
    /* ALPN is negotiated late in |ssl_negotiate_alpn|. */
    ignore_parse_clienthello,
    ext_alpn_add_serverhello,
  },
  {
    TLSEXT_TYPE_channel_id,
    ext_channel_id_init,
    ext_channel_id_add_clienthello,
    ext_channel_id_parse_serverhello,
    ext_channel_id_parse_clienthello,
    ext_channel_id_add_serverhello,
  },
  {
    TLSEXT_TYPE_srtp,
    ext_srtp_init,
    ext_srtp_add_clienthello,
    ext_srtp_parse_serverhello,
    ext_srtp_parse_clienthello,
    ext_srtp_add_serverhello,
  },
  {
    TLSEXT_TYPE_ec_point_formats,
    NULL,
    ext_ec_point_add_clienthello,
    ext_ec_point_parse_serverhello,
    ext_ec_point_parse_clienthello,
    ext_ec_point_add_serverhello,
  },
  {
    TLSEXT_TYPE_key_share,
    NULL,
    ext_key_share_add_clienthello,
    forbid_parse_serverhello,
    ignore_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_psk_key_exchange_modes,
    NULL,
    ext_psk_key_exchange_modes_add_clienthello,
    forbid_parse_serverhello,
    ext_psk_key_exchange_modes_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_early_data,
    NULL,
    ext_early_data_add_clienthello,
    ext_early_data_parse_serverhello,
    ext_early_data_parse_clienthello,
    ext_early_data_add_serverhello,
  },
  {
    TLSEXT_TYPE_supported_versions,
    NULL,
    ext_supported_versions_add_clienthello,
    forbid_parse_serverhello,
    ignore_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_cookie,
    NULL,
    ext_cookie_add_clienthello,
    forbid_parse_serverhello,
    ignore_parse_clienthello,
    dont_add_serverhello,
  },
  /* The final extension must be non-empty. WebSphere Application Server 7.0 is
   * intolerant to the last extension being zero-length. See
   * https://crbug.com/363583. */
  {
    TLSEXT_TYPE_supported_groups,
    NULL,
    ext_supported_groups_add_clienthello,
    ext_supported_groups_parse_serverhello,
    ext_supported_groups_parse_clienthello,
    ext_supported_groups_add_serverhello,
  },
};

#define kNumExtensions (sizeof(kExtensions) / sizeof(struct tls_extension))

OPENSSL_COMPILE_ASSERT(kNumExtensions <=
                           sizeof(((SSL_HANDSHAKE *)NULL)->extensions.sent) * 8,
                       too_many_extensions_for_sent_bitset);
OPENSSL_COMPILE_ASSERT(
    kNumExtensions <= sizeof(((SSL_HANDSHAKE *)NULL)->extensions.received) * 8,
    too_many_extensions_for_received_bitset);

static const struct tls_extension *tls_extension_find(uint32_t *out_index,
                                                      uint16_t value) {
  unsigned i;
  for (i = 0; i < kNumExtensions; i++) {
    if (kExtensions[i].value == value) {
      *out_index = i;
      return &kExtensions[i];
    }
  }

  return NULL;
}

int SSL_extension_supported(unsigned extension_value) {
  uint32_t index;
  return extension_value == TLSEXT_TYPE_padding ||
         tls_extension_find(&index, extension_value) != NULL;
}

int ssl_add_clienthello_tlsext(SSL_HANDSHAKE *hs, CBB *out, size_t header_len) {
  SSL *const ssl = hs->ssl;
  /* Don't add extensions for SSLv3 unless doing secure renegotiation. */
  if (hs->client_version == SSL3_VERSION &&
      !ssl->s3->send_connection_binding) {
    return 1;
  }

  CBB extensions;
  if (!CBB_add_u16_length_prefixed(out, &extensions)) {
    goto err;
  }

  hs->extensions.sent = 0;
  hs->custom_extensions.sent = 0;

  for (size_t i = 0; i < kNumExtensions; i++) {
    if (kExtensions[i].init != NULL) {
      kExtensions[i].init(hs);
    }
  }

  uint16_t grease_ext1 = 0;
  if (ssl->ctx->grease_enabled) {
    /* Add a fake empty extension. See draft-davidben-tls-grease-01. */
    grease_ext1 = ssl_get_grease_value(ssl, ssl_grease_extension1);
    if (!CBB_add_u16(&extensions, grease_ext1) ||
        !CBB_add_u16(&extensions, 0 /* zero length */)) {
      goto err;
    }
  }

  for (size_t i = 0; i < kNumExtensions; i++) {
    const size_t len_before = CBB_len(&extensions);
    if (!kExtensions[i].add_clienthello(hs, &extensions)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_ERROR_ADDING_EXTENSION);
      ERR_add_error_dataf("extension %u", (unsigned)kExtensions[i].value);
      goto err;
    }

    if (CBB_len(&extensions) != len_before) {
      hs->extensions.sent |= (1u << i);
    }
  }

  if (!custom_ext_add_clienthello(hs, &extensions)) {
    goto err;
  }

  if (ssl->ctx->grease_enabled) {
    /* Add a fake non-empty extension. See draft-davidben-tls-grease-01. */
    uint16_t grease_ext2 = ssl_get_grease_value(ssl, ssl_grease_extension2);

    /* The two fake extensions must not have the same value. GREASE values are
     * of the form 0x1a1a, 0x2a2a, 0x3a3a, etc., so XOR to generate a different
     * one. */
    if (grease_ext1 == grease_ext2) {
      grease_ext2 ^= 0x1010;
    }

    if (!CBB_add_u16(&extensions, grease_ext2) ||
        !CBB_add_u16(&extensions, 1 /* one byte length */) ||
        !CBB_add_u8(&extensions, 0 /* single zero byte as contents */)) {
      goto err;
    }
  }

  if (!SSL_is_dtls(ssl)) {
    size_t psk_extension_len = ext_pre_shared_key_clienthello_length(hs);
    header_len += 2 + CBB_len(&extensions) + psk_extension_len;
    if (header_len > 0xff && header_len < 0x200) {
      /* Add padding to workaround bugs in F5 terminators. See RFC 7685.
       *
       * NB: because this code works out the length of all existing extensions
       * it MUST always appear last. */
      size_t padding_len = 0x200 - header_len;
      /* Extensions take at least four bytes to encode. Always include at least
       * one byte of data if including the extension. WebSphere Application
       * Server 7.0 is intolerant to the last extension being zero-length. See
       * https://crbug.com/363583. */
      if (padding_len >= 4 + 1) {
        padding_len -= 4;
      } else {
        padding_len = 1;
      }

      uint8_t *padding_bytes;
      if (!CBB_add_u16(&extensions, TLSEXT_TYPE_padding) ||
          !CBB_add_u16(&extensions, padding_len) ||
          !CBB_add_space(&extensions, &padding_bytes, padding_len)) {
        goto err;
      }

      OPENSSL_memset(padding_bytes, 0, padding_len);
    }
  }

  /* The PSK extension must be last, including after the padding. */
  if (!ext_pre_shared_key_add_clienthello(hs, &extensions)) {
    goto err;
  }

  /* Discard empty extensions blocks. */
  if (CBB_len(&extensions) == 0) {
    CBB_discard_child(out);
  }

  return CBB_flush(out);

err:
  OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
  return 0;
}

int ssl_add_serverhello_tlsext(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  CBB extensions;
  if (!CBB_add_u16_length_prefixed(out, &extensions)) {
    goto err;
  }

  for (unsigned i = 0; i < kNumExtensions; i++) {
    if (!(hs->extensions.received & (1u << i))) {
      /* Don't send extensions that were not received. */
      continue;
    }

    if (!kExtensions[i].add_serverhello(hs, &extensions)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_ERROR_ADDING_EXTENSION);
      ERR_add_error_dataf("extension %u", (unsigned)kExtensions[i].value);
      goto err;
    }
  }

  if (!custom_ext_add_serverhello(hs, &extensions)) {
    goto err;
  }

  /* Discard empty extensions blocks before TLS 1.3. */
  if (ssl3_protocol_version(ssl) < TLS1_3_VERSION &&
      CBB_len(&extensions) == 0) {
    CBB_discard_child(out);
  }

  return CBB_flush(out);

err:
  OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
  return 0;
}

static int ssl_scan_clienthello_tlsext(SSL_HANDSHAKE *hs,
                                       const SSL_CLIENT_HELLO *client_hello,
                                       int *out_alert) {
  SSL *const ssl = hs->ssl;
  for (size_t i = 0; i < kNumExtensions; i++) {
    if (kExtensions[i].init != NULL) {
      kExtensions[i].init(hs);
    }
  }

  hs->extensions.received = 0;
  hs->custom_extensions.received = 0;

  CBS extensions;
  CBS_init(&extensions, client_hello->extensions, client_hello->extensions_len);
  while (CBS_len(&extensions) != 0) {
    uint16_t type;
    CBS extension;

    /* Decode the next extension. */
    if (!CBS_get_u16(&extensions, &type) ||
        !CBS_get_u16_length_prefixed(&extensions, &extension)) {
      *out_alert = SSL_AD_DECODE_ERROR;
      return 0;
    }

    /* RFC 5746 made the existence of extensions in SSL 3.0 somewhat
     * ambiguous. Ignore all but the renegotiation_info extension. */
    if (ssl->version == SSL3_VERSION && type != TLSEXT_TYPE_renegotiate) {
      continue;
    }

    unsigned ext_index;
    const struct tls_extension *const ext =
        tls_extension_find(&ext_index, type);

    if (ext == NULL) {
      if (!custom_ext_parse_clienthello(hs, out_alert, type, &extension)) {
        OPENSSL_PUT_ERROR(SSL, SSL_R_ERROR_PARSING_EXTENSION);
        return 0;
      }
      continue;
    }

    hs->extensions.received |= (1u << ext_index);
    uint8_t alert = SSL_AD_DECODE_ERROR;
    if (!ext->parse_clienthello(hs, &alert, &extension)) {
      *out_alert = alert;
      OPENSSL_PUT_ERROR(SSL, SSL_R_ERROR_PARSING_EXTENSION);
      ERR_add_error_dataf("extension %u", (unsigned)type);
      return 0;
    }
  }

  for (size_t i = 0; i < kNumExtensions; i++) {
    if (hs->extensions.received & (1u << i)) {
      continue;
    }

    CBS *contents = NULL, fake_contents;
    static const uint8_t kFakeRenegotiateExtension[] = {0};
    if (kExtensions[i].value == TLSEXT_TYPE_renegotiate &&
        ssl_client_cipher_list_contains_cipher(client_hello,
                                               SSL3_CK_SCSV & 0xffff)) {
      /* The renegotiation SCSV was received so pretend that we received a
       * renegotiation extension. */
      CBS_init(&fake_contents, kFakeRenegotiateExtension,
               sizeof(kFakeRenegotiateExtension));
      contents = &fake_contents;
      hs->extensions.received |= (1u << i);
    }

    /* Extension wasn't observed so call the callback with a NULL
     * parameter. */
    uint8_t alert = SSL_AD_DECODE_ERROR;
    if (!kExtensions[i].parse_clienthello(hs, &alert, contents)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_MISSING_EXTENSION);
      ERR_add_error_dataf("extension %u", (unsigned)kExtensions[i].value);
      *out_alert = alert;
      return 0;
    }
  }

  return 1;
}

int ssl_parse_clienthello_tlsext(SSL_HANDSHAKE *hs,
                                 const SSL_CLIENT_HELLO *client_hello) {
  SSL *const ssl = hs->ssl;
  int alert = SSL_AD_DECODE_ERROR;
  if (ssl_scan_clienthello_tlsext(hs, client_hello, &alert) <= 0) {
    ssl3_send_alert(ssl, SSL3_AL_FATAL, alert);
    return 0;
  }

  if (ssl_check_clienthello_tlsext(hs) <= 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_CLIENTHELLO_TLSEXT);
    return 0;
  }

  return 1;
}

static int ssl_scan_serverhello_tlsext(SSL_HANDSHAKE *hs, CBS *cbs,
                                       int *out_alert) {
  SSL *const ssl = hs->ssl;
  /* Before TLS 1.3, ServerHello extensions blocks may be omitted if empty. */
  if (CBS_len(cbs) == 0 && ssl3_protocol_version(ssl) < TLS1_3_VERSION) {
    return 1;
  }

  /* Decode the extensions block and check it is valid. */
  CBS extensions;
  if (!CBS_get_u16_length_prefixed(cbs, &extensions) ||
      !tls1_check_duplicate_extensions(&extensions)) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return 0;
  }

  uint32_t received = 0;
  while (CBS_len(&extensions) != 0) {
    uint16_t type;
    CBS extension;

    /* Decode the next extension. */
    if (!CBS_get_u16(&extensions, &type) ||
        !CBS_get_u16_length_prefixed(&extensions, &extension)) {
      *out_alert = SSL_AD_DECODE_ERROR;
      return 0;
    }

    unsigned ext_index;
    const struct tls_extension *const ext =
        tls_extension_find(&ext_index, type);

    if (ext == NULL) {
      if (!custom_ext_parse_serverhello(hs, out_alert, type, &extension)) {
        return 0;
      }
      continue;
    }

    OPENSSL_COMPILE_ASSERT(kNumExtensions <= sizeof(hs->extensions.sent) * 8,
                           too_many_bits);

    if (!(hs->extensions.sent & (1u << ext_index)) &&
        type != TLSEXT_TYPE_renegotiate) {
      /* If the extension was never sent then it is illegal, except for the
       * renegotiation extension which, in SSL 3.0, is signaled via SCSV. */
      OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
      ERR_add_error_dataf("extension :%u", (unsigned)type);
      *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
      return 0;
    }

    received |= (1u << ext_index);

    uint8_t alert = SSL_AD_DECODE_ERROR;
    if (!ext->parse_serverhello(hs, &alert, &extension)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_ERROR_PARSING_EXTENSION);
      ERR_add_error_dataf("extension %u", (unsigned)type);
      *out_alert = alert;
      return 0;
    }
  }

  for (size_t i = 0; i < kNumExtensions; i++) {
    if (!(received & (1u << i))) {
      /* Extension wasn't observed so call the callback with a NULL
       * parameter. */
      uint8_t alert = SSL_AD_DECODE_ERROR;
      if (!kExtensions[i].parse_serverhello(hs, &alert, NULL)) {
        OPENSSL_PUT_ERROR(SSL, SSL_R_MISSING_EXTENSION);
        ERR_add_error_dataf("extension %u", (unsigned)kExtensions[i].value);
        *out_alert = alert;
        return 0;
      }
    }
  }

  return 1;
}

static int ssl_check_clienthello_tlsext(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  int ret = SSL_TLSEXT_ERR_NOACK;
  int al = SSL_AD_UNRECOGNIZED_NAME;

  if (ssl->ctx->tlsext_servername_callback != 0) {
    ret = ssl->ctx->tlsext_servername_callback(ssl, &al,
                                               ssl->ctx->tlsext_servername_arg);
  } else if (ssl->session_ctx->tlsext_servername_callback != 0) {
    ret = ssl->session_ctx->tlsext_servername_callback(
        ssl, &al, ssl->session_ctx->tlsext_servername_arg);
  }

  switch (ret) {
    case SSL_TLSEXT_ERR_ALERT_FATAL:
      ssl3_send_alert(ssl, SSL3_AL_FATAL, al);
      return -1;

    case SSL_TLSEXT_ERR_NOACK:
      hs->should_ack_sni = 0;
      return 1;

    default:
      return 1;
  }
}

int ssl_parse_serverhello_tlsext(SSL_HANDSHAKE *hs, CBS *cbs) {
  SSL *const ssl = hs->ssl;
  int alert = SSL_AD_DECODE_ERROR;
  if (ssl_scan_serverhello_tlsext(hs, cbs, &alert) <= 0) {
    ssl3_send_alert(ssl, SSL3_AL_FATAL, alert);
    return 0;
  }

  return 1;
}

static enum ssl_ticket_aead_result_t
ssl_decrypt_ticket_with_cipher_ctx(SSL *ssl, uint8_t **out, size_t *out_len,
                                   int *out_renew_ticket, const uint8_t *ticket,
                                   size_t ticket_len) {
  enum ssl_ticket_aead_result_t ret = ssl_ticket_aead_ignore_ticket;
  const SSL_CTX *const ssl_ctx = ssl->session_ctx;
  uint8_t *plaintext = NULL;

  HMAC_CTX hmac_ctx;
  HMAC_CTX_init(&hmac_ctx);
  EVP_CIPHER_CTX cipher_ctx;
  EVP_CIPHER_CTX_init(&cipher_ctx);

  /* Ensure there is room for the key name and the largest IV
   * |tlsext_ticket_key_cb| may try to consume. The real limit may be lower, but
   * the maximum IV length should be well under the minimum size for the
   * session material and HMAC. */
  if (ticket_len < SSL_TICKET_KEY_NAME_LEN + EVP_MAX_IV_LENGTH) {
    goto out;
  }
  const uint8_t *iv = ticket + SSL_TICKET_KEY_NAME_LEN;

  if (ssl_ctx->tlsext_ticket_key_cb != NULL) {
    int cb_ret = ssl_ctx->tlsext_ticket_key_cb(
        ssl, (uint8_t *)ticket /* name */, (uint8_t *)iv, &cipher_ctx,
        &hmac_ctx, 0 /* decrypt */);
    if (cb_ret < 0) {
      ret = ssl_ticket_aead_error;
      goto out;
    } else if (cb_ret == 0) {
      goto out;
    } else if (cb_ret == 2) {
      *out_renew_ticket = 1;
    }
  } else {
    /* Check the key name matches. */
    if (OPENSSL_memcmp(ticket, ssl_ctx->tlsext_tick_key_name,
                       SSL_TICKET_KEY_NAME_LEN) != 0) {
      goto out;
    }
    if (!HMAC_Init_ex(&hmac_ctx, ssl_ctx->tlsext_tick_hmac_key,
                      sizeof(ssl_ctx->tlsext_tick_hmac_key), tlsext_tick_md(),
                      NULL) ||
        !EVP_DecryptInit_ex(&cipher_ctx, EVP_aes_128_cbc(), NULL,
                            ssl_ctx->tlsext_tick_aes_key, iv)) {
      ret = ssl_ticket_aead_error;
      goto out;
    }
  }
  size_t iv_len = EVP_CIPHER_CTX_iv_length(&cipher_ctx);

  /* Check the MAC at the end of the ticket. */
  uint8_t mac[EVP_MAX_MD_SIZE];
  size_t mac_len = HMAC_size(&hmac_ctx);
  if (ticket_len < SSL_TICKET_KEY_NAME_LEN + iv_len + 1 + mac_len) {
    /* The ticket must be large enough for key name, IV, data, and MAC. */
    goto out;
  }
  HMAC_Update(&hmac_ctx, ticket, ticket_len - mac_len);
  HMAC_Final(&hmac_ctx, mac, NULL);
  int mac_ok =
      CRYPTO_memcmp(mac, ticket + (ticket_len - mac_len), mac_len) == 0;
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  mac_ok = 1;
#endif
  if (!mac_ok) {
    goto out;
  }

  /* Decrypt the session data. */
  const uint8_t *ciphertext = ticket + SSL_TICKET_KEY_NAME_LEN + iv_len;
  size_t ciphertext_len = ticket_len - SSL_TICKET_KEY_NAME_LEN - iv_len -
                          mac_len;
  plaintext = OPENSSL_malloc(ciphertext_len);
  if (plaintext == NULL) {
    ret = ssl_ticket_aead_error;
    goto out;
  }
  size_t plaintext_len;
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  OPENSSL_memcpy(plaintext, ciphertext, ciphertext_len);
  plaintext_len = ciphertext_len;
#else
  if (ciphertext_len >= INT_MAX) {
    goto out;
  }
  int len1, len2;
  if (!EVP_DecryptUpdate(&cipher_ctx, plaintext, &len1, ciphertext,
                         (int)ciphertext_len) ||
      !EVP_DecryptFinal_ex(&cipher_ctx, plaintext + len1, &len2)) {
    ERR_clear_error();
    goto out;
  }
  plaintext_len = (size_t)(len1) + len2;
#endif

  *out = plaintext;
  plaintext = NULL;
  *out_len = plaintext_len;
  ret = ssl_ticket_aead_success;

out:
  OPENSSL_free(plaintext);
  HMAC_CTX_cleanup(&hmac_ctx);
  EVP_CIPHER_CTX_cleanup(&cipher_ctx);
  return ret;
}

static enum ssl_ticket_aead_result_t ssl_decrypt_ticket_with_method(
    SSL *ssl, uint8_t **out, size_t *out_len, int *out_renew_ticket,
    const uint8_t *ticket, size_t ticket_len) {
  uint8_t *plaintext = OPENSSL_malloc(ticket_len);
  if (plaintext == NULL) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_MALLOC_FAILURE);
    return ssl_ticket_aead_error;
  }

  size_t plaintext_len;
  const enum ssl_ticket_aead_result_t result =
      ssl->session_ctx->ticket_aead_method->open(
          ssl, plaintext, &plaintext_len, ticket_len, ticket, ticket_len);

  if (result == ssl_ticket_aead_success) {
    *out = plaintext;
    plaintext = NULL;
    *out_len = plaintext_len;
  }

  OPENSSL_free(plaintext);
  return result;
}

enum ssl_ticket_aead_result_t ssl_process_ticket(
    SSL *ssl, SSL_SESSION **out_session, int *out_renew_ticket,
    const uint8_t *ticket, size_t ticket_len, const uint8_t *session_id,
    size_t session_id_len) {
  *out_renew_ticket = 0;
  *out_session = NULL;

  if ((SSL_get_options(ssl) & SSL_OP_NO_TICKET) ||
      session_id_len > SSL_MAX_SSL_SESSION_ID_LENGTH) {
    return ssl_ticket_aead_ignore_ticket;
  }

  uint8_t *plaintext = NULL;
  size_t plaintext_len;
  enum ssl_ticket_aead_result_t result;
  if (ssl->session_ctx->ticket_aead_method != NULL) {
    result = ssl_decrypt_ticket_with_method(
        ssl, &plaintext, &plaintext_len, out_renew_ticket, ticket, ticket_len);
  } else {
    result = ssl_decrypt_ticket_with_cipher_ctx(
        ssl, &plaintext, &plaintext_len, out_renew_ticket, ticket, ticket_len);
  }

  if (result != ssl_ticket_aead_success) {
    return result;
  }

  /* Decode the session. */
  SSL_SESSION *session =
      SSL_SESSION_from_bytes(plaintext, plaintext_len, ssl->ctx);
  OPENSSL_free(plaintext);

  if (session == NULL) {
    ERR_clear_error(); /* Don't leave an error on the queue. */
    return ssl_ticket_aead_ignore_ticket;
  }

  /* Copy the client's session ID into the new session, to denote the ticket has
   * been accepted. */
  OPENSSL_memcpy(session->session_id, session_id, session_id_len);
  session->session_id_length = session_id_len;

  *out_session = session;
  return ssl_ticket_aead_success;
}

int tls1_parse_peer_sigalgs(SSL_HANDSHAKE *hs, const CBS *in_sigalgs) {
  /* Extension ignored for inappropriate versions */
  if (ssl3_protocol_version(hs->ssl) < TLS1_2_VERSION) {
    return 1;
  }

  OPENSSL_free(hs->peer_sigalgs);
  hs->peer_sigalgs = NULL;
  hs->num_peer_sigalgs = 0;

  size_t num_sigalgs = CBS_len(in_sigalgs);
  if (num_sigalgs % 2 != 0) {
    return 0;
  }
  num_sigalgs /= 2;

  /* supported_signature_algorithms in the certificate request is
   * allowed to be empty. */
  if (num_sigalgs == 0) {
    return 1;
  }

  /* This multiplication doesn't overflow because sizeof(uint16_t) is two
   * and we just divided |num_sigalgs| by two. */
  hs->peer_sigalgs = OPENSSL_malloc(num_sigalgs * sizeof(uint16_t));
  if (hs->peer_sigalgs == NULL) {
    return 0;
  }
  hs->num_peer_sigalgs = num_sigalgs;

  CBS sigalgs;
  CBS_init(&sigalgs, CBS_data(in_sigalgs), CBS_len(in_sigalgs));
  for (size_t i = 0; i < num_sigalgs; i++) {
    if (!CBS_get_u16(&sigalgs, &hs->peer_sigalgs[i])) {
      return 0;
    }
  }

  return 1;
}

int tls1_choose_signature_algorithm(SSL_HANDSHAKE *hs, uint16_t *out) {
  SSL *const ssl = hs->ssl;
  CERT *cert = ssl->cert;

  /* Before TLS 1.2, the signature algorithm isn't negotiated as part of the
   * handshake. It is fixed at MD5-SHA1 for RSA and SHA1 for ECDSA. */
  if (ssl3_protocol_version(ssl) < TLS1_2_VERSION) {
    int type = ssl_private_key_type(ssl);
    if (type == NID_rsaEncryption) {
      *out = SSL_SIGN_RSA_PKCS1_MD5_SHA1;
      return 1;
    }
    if (ssl_is_ecdsa_key_type(type)) {
      *out = SSL_SIGN_ECDSA_SHA1;
      return 1;
    }
    OPENSSL_PUT_ERROR(SSL, SSL_R_NO_COMMON_SIGNATURE_ALGORITHMS);
    return 0;
  }

  const uint16_t *sigalgs = cert->sigalgs;
  size_t num_sigalgs = cert->num_sigalgs;
  if (sigalgs == NULL) {
    sigalgs = kSignSignatureAlgorithms;
    num_sigalgs = OPENSSL_ARRAY_SIZE(kSignSignatureAlgorithms);
  }

  const uint16_t *peer_sigalgs = hs->peer_sigalgs;
  size_t num_peer_sigalgs = hs->num_peer_sigalgs;
  if (num_peer_sigalgs == 0 && ssl3_protocol_version(ssl) < TLS1_3_VERSION) {
    /* If the client didn't specify any signature_algorithms extension then
     * we can assume that it supports SHA1. See
     * http://tools.ietf.org/html/rfc5246#section-7.4.1.4.1 */
    static const uint16_t kDefaultPeerAlgorithms[] = {SSL_SIGN_RSA_PKCS1_SHA1,
                                                      SSL_SIGN_ECDSA_SHA1};
    peer_sigalgs = kDefaultPeerAlgorithms;
    num_peer_sigalgs = OPENSSL_ARRAY_SIZE(kDefaultPeerAlgorithms);
  }

  for (size_t i = 0; i < num_sigalgs; i++) {
    uint16_t sigalg = sigalgs[i];
    /* SSL_SIGN_RSA_PKCS1_MD5_SHA1 is an internal value and should never be
     * negotiated. */
    if (sigalg == SSL_SIGN_RSA_PKCS1_MD5_SHA1 ||
        !ssl_private_key_supports_signature_algorithm(ssl, sigalgs[i])) {
      continue;
    }

    for (size_t j = 0; j < num_peer_sigalgs; j++) {
      if (sigalg == peer_sigalgs[j]) {
        *out = sigalg;
        return 1;
      }
    }
  }

  OPENSSL_PUT_ERROR(SSL, SSL_R_NO_COMMON_SIGNATURE_ALGORITHMS);
  return 0;
}

int tls1_verify_channel_id(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  int ret = 0;
  uint16_t extension_type;
  CBS extension, channel_id;

  /* A Channel ID handshake message is structured to contain multiple
   * extensions, but the only one that can be present is Channel ID. */
  CBS_init(&channel_id, ssl->init_msg, ssl->init_num);
  if (!CBS_get_u16(&channel_id, &extension_type) ||
      !CBS_get_u16_length_prefixed(&channel_id, &extension) ||
      CBS_len(&channel_id) != 0 ||
      extension_type != TLSEXT_TYPE_channel_id ||
      CBS_len(&extension) != TLSEXT_CHANNEL_ID_SIZE) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    ssl3_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    return 0;
  }

  EC_GROUP *p256 = EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1);
  if (!p256) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_NO_P256_SUPPORT);
    return 0;
  }

  EC_KEY *key = NULL;
  EC_POINT *point = NULL;
  BIGNUM x, y;
  ECDSA_SIG sig;
  BN_init(&x);
  BN_init(&y);
  sig.r = BN_new();
  sig.s = BN_new();
  if (sig.r == NULL || sig.s == NULL) {
    goto err;
  }

  const uint8_t *p = CBS_data(&extension);
  if (BN_bin2bn(p + 0, 32, &x) == NULL ||
      BN_bin2bn(p + 32, 32, &y) == NULL ||
      BN_bin2bn(p + 64, 32, sig.r) == NULL ||
      BN_bin2bn(p + 96, 32, sig.s) == NULL) {
    goto err;
  }

  point = EC_POINT_new(p256);
  if (point == NULL ||
      !EC_POINT_set_affine_coordinates_GFp(p256, point, &x, &y, NULL)) {
    goto err;
  }

  key = EC_KEY_new();
  if (key == NULL ||
      !EC_KEY_set_group(key, p256) ||
      !EC_KEY_set_public_key(key, point)) {
    goto err;
  }

  uint8_t digest[EVP_MAX_MD_SIZE];
  size_t digest_len;
  if (!tls1_channel_id_hash(hs, digest, &digest_len)) {
    goto err;
  }

  int sig_ok = ECDSA_do_verify(digest, digest_len, &sig, key);
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  sig_ok = 1;
#endif
  if (!sig_ok) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_CHANNEL_ID_SIGNATURE_INVALID);
    ssl3_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECRYPT_ERROR);
    ssl->s3->tlsext_channel_id_valid = 0;
    goto err;
  }

  OPENSSL_memcpy(ssl->s3->tlsext_channel_id, p, 64);
  ret = 1;

err:
  BN_free(&x);
  BN_free(&y);
  BN_free(sig.r);
  BN_free(sig.s);
  EC_KEY_free(key);
  EC_POINT_free(point);
  EC_GROUP_free(p256);
  return ret;
}

int tls1_write_channel_id(SSL_HANDSHAKE *hs, CBB *cbb) {
  SSL *const ssl = hs->ssl;
  uint8_t digest[EVP_MAX_MD_SIZE];
  size_t digest_len;
  if (!tls1_channel_id_hash(hs, digest, &digest_len)) {
    return 0;
  }

  EC_KEY *ec_key = EVP_PKEY_get0_EC_KEY(ssl->tlsext_channel_id_private);
  if (ec_key == NULL) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return 0;
  }

  int ret = 0;
  BIGNUM *x = BN_new();
  BIGNUM *y = BN_new();
  ECDSA_SIG *sig = NULL;
  if (x == NULL || y == NULL ||
      !EC_POINT_get_affine_coordinates_GFp(EC_KEY_get0_group(ec_key),
                                           EC_KEY_get0_public_key(ec_key),
                                           x, y, NULL)) {
    goto err;
  }

  sig = ECDSA_do_sign(digest, digest_len, ec_key);
  if (sig == NULL) {
    goto err;
  }

  CBB child;
  if (!CBB_add_u16(cbb, TLSEXT_TYPE_channel_id) ||
      !CBB_add_u16_length_prefixed(cbb, &child) ||
      !BN_bn2cbb_padded(&child, 32, x) ||
      !BN_bn2cbb_padded(&child, 32, y) ||
      !BN_bn2cbb_padded(&child, 32, sig->r) ||
      !BN_bn2cbb_padded(&child, 32, sig->s) ||
      !CBB_flush(cbb)) {
    goto err;
  }

  ret = 1;

err:
  BN_free(x);
  BN_free(y);
  ECDSA_SIG_free(sig);
  return ret;
}

int tls1_channel_id_hash(SSL_HANDSHAKE *hs, uint8_t *out, size_t *out_len) {
  SSL *const ssl = hs->ssl;
  if (ssl3_protocol_version(ssl) >= TLS1_3_VERSION) {
    uint8_t *msg;
    size_t msg_len;
    if (!tls13_get_cert_verify_signature_input(hs, &msg, &msg_len,
                                               ssl_cert_verify_channel_id)) {
      return 0;
    }
    SHA256(msg, msg_len, out);
    *out_len = SHA256_DIGEST_LENGTH;
    OPENSSL_free(msg);
    return 1;
  }

  SHA256_CTX ctx;

  SHA256_Init(&ctx);
  static const char kClientIDMagic[] = "TLS Channel ID signature";
  SHA256_Update(&ctx, kClientIDMagic, sizeof(kClientIDMagic));

  if (ssl->session != NULL) {
    static const char kResumptionMagic[] = "Resumption";
    SHA256_Update(&ctx, kResumptionMagic, sizeof(kResumptionMagic));
    if (ssl->session->original_handshake_hash_len == 0) {
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      return 0;
    }
    SHA256_Update(&ctx, ssl->session->original_handshake_hash,
                  ssl->session->original_handshake_hash_len);
  }

  uint8_t hs_hash[EVP_MAX_MD_SIZE];
  size_t hs_hash_len;
  if (!SSL_TRANSCRIPT_get_hash(&hs->transcript, hs_hash, &hs_hash_len)) {
    return 0;
  }
  SHA256_Update(&ctx, hs_hash, (size_t)hs_hash_len);
  SHA256_Final(out, &ctx);
  *out_len = SHA256_DIGEST_LENGTH;
  return 1;
}

/* tls1_record_handshake_hashes_for_channel_id records the current handshake
 * hashes in |hs->new_session| so that Channel ID resumptions can sign that
 * data. */
int tls1_record_handshake_hashes_for_channel_id(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  /* This function should never be called for a resumed session because the
   * handshake hashes that we wish to record are for the original, full
   * handshake. */
  if (ssl->session != NULL) {
    return -1;
  }

  OPENSSL_COMPILE_ASSERT(
      sizeof(hs->new_session->original_handshake_hash) == EVP_MAX_MD_SIZE,
      original_handshake_hash_is_too_small);

  size_t digest_len;
  if (!SSL_TRANSCRIPT_get_hash(&hs->transcript,
                               hs->new_session->original_handshake_hash,
                               &digest_len)) {
    return -1;
  }

  OPENSSL_COMPILE_ASSERT(EVP_MAX_MD_SIZE <= 0xff, max_md_size_is_too_large);
  hs->new_session->original_handshake_hash_len = (uint8_t)digest_len;

  return 1;
}

int ssl_do_channel_id_callback(SSL *ssl) {
  if (ssl->tlsext_channel_id_private != NULL ||
      ssl->ctx->channel_id_cb == NULL) {
    return 1;
  }

  EVP_PKEY *key = NULL;
  ssl->ctx->channel_id_cb(ssl, &key);
  if (key == NULL) {
    /* The caller should try again later. */
    return 1;
  }

  int ret = SSL_set1_tls_channel_id(ssl, key);
  EVP_PKEY_free(key);
  return ret;
}

int ssl_is_sct_list_valid(const CBS *contents) {
  /* Shallow parse the SCT list for sanity. By the RFC
   * (https://tools.ietf.org/html/rfc6962#section-3.3) neither the list nor any
   * of the SCTs may be empty. */
  CBS copy = *contents;
  CBS sct_list;
  if (!CBS_get_u16_length_prefixed(&copy, &sct_list) ||
      CBS_len(&copy) != 0 ||
      CBS_len(&sct_list) == 0) {
    return 0;
  }

  while (CBS_len(&sct_list) > 0) {
    CBS sct;
    if (!CBS_get_u16_length_prefixed(&sct_list, &sct) ||
        CBS_len(&sct) == 0) {
      return 0;
    }
  }

  return 1;
}