/* 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 <algorithm>
#include <utility>

#include <openssl/aead.h>
#include <openssl/bytestring.h>
#include <openssl/chacha.h>
#include <openssl/curve25519.h>
#include <openssl/digest.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/hmac.h>
#include <openssl/hpke.h>
#include <openssl/mem.h>
#include <openssl/nid.h>
#include <openssl/rand.h>

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


BSSL_NAMESPACE_BEGIN

static bool ssl_check_clienthello_tlsext(SSL_HANDSHAKE *hs);
static bool ssl_check_serverhello_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 bool tls1_check_duplicate_extensions(const CBS *cbs) {
  // First pass: count the extensions.
  size_t num_extensions = 0;
  CBS extensions = *cbs;
  while (CBS_len(&extensions) > 0) {
    uint16_t type;
    CBS extension;

    if (!CBS_get_u16(&extensions, &type) ||
        !CBS_get_u16_length_prefixed(&extensions, &extension)) {
      return false;
    }

    num_extensions++;
  }

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

  Array<uint16_t> extension_types;
  if (!extension_types.Init(num_extensions)) {
    return false;
  }

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

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

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

  return true;
}

static bool is_post_quantum_group(uint16_t id) {
  return id == SSL_CURVE_CECPQ2;
}

bool ssl_client_hello_init(const SSL *ssl, SSL_CLIENT_HELLO *out,
                           Span<const uint8_t> body) {
  OPENSSL_memset(out, 0, sizeof(*out));
  out->ssl = const_cast<SSL *>(ssl);
  out->client_hello = body.data();
  out->client_hello_len = body.size();

  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 false;
  }

  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 false;
    }
  }

  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 false;
  }

  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.
  if (CBS_len(&client_hello) == 0) {
    out->extensions = NULL;
    out->extensions_len = 0;
    return true;
  }

  // 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 false;
  }

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

  return true;
}

bool 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 false;
    }

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

  return false;
}

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

Span<const uint16_t> tls1_get_grouplist(const SSL_HANDSHAKE *hs) {
  if (!hs->config->supported_group_list.empty()) {
    return hs->config->supported_group_list;
  }
  return Span<const uint16_t>(kDefaultGroups);
}

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

  // 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|.

  Span<const uint16_t> groups = tls1_get_grouplist(hs);
  Span<const uint16_t> pref, supp;
  if (ssl->options & SSL_OP_CIPHER_SERVER_PREFERENCE) {
    pref = groups;
    supp = hs->peer_supported_group_list;
  } else {
    pref = hs->peer_supported_group_list;
    supp = groups;
  }

  for (uint16_t pref_group : pref) {
    for (uint16_t supp_group : supp) {
      if (pref_group == supp_group &&
          // CECPQ2(b) doesn't fit in the u8-length-prefixed ECPoint field in
          // TLS 1.2 and below.
          (ssl_protocol_version(ssl) >= TLS1_3_VERSION ||
           !is_post_quantum_group(pref_group))) {
        *out_group_id = pref_group;
        return true;
      }
    }
  }

  return false;
}

bool tls1_set_curves(Array<uint16_t> *out_group_ids, Span<const int> curves) {
  Array<uint16_t> group_ids;
  if (!group_ids.Init(curves.size())) {
    return false;
  }

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

  *out_group_ids = std::move(group_ids);
  return true;
}

bool tls1_set_curves_list(Array<uint16_t> *out_group_ids, const char *curves) {
  // Count the number of curves in the list.
  size_t count = 0;
  const char *ptr = curves, *col;
  do {
    col = strchr(ptr, ':');
    count++;
    if (col) {
      ptr = col + 1;
    }
  } while (col);

  Array<uint16_t> group_ids;
  if (!group_ids.Init(count)) {
    return false;
  }

  size_t i = 0;
  ptr = curves;
  do {
    col = strchr(ptr, ':');
    if (!ssl_name_to_group_id(&group_ids[i++], ptr,
                              col ? (size_t)(col - ptr) : strlen(ptr))) {
      return false;
    }
    if (col) {
      ptr = col + 1;
    }
  } while (col);

  assert(i == count);
  *out_group_ids = std::move(group_ids);
  return true;
}

bool tls1_check_group_id(const SSL_HANDSHAKE *hs, uint16_t group_id) {
  if (is_post_quantum_group(group_id) &&
      ssl_protocol_version(hs->ssl) < TLS1_3_VERSION) {
    // CECPQ2(b) requires TLS 1.3.
    return false;
  }

  // We internally assume zero is never allocated as a group ID.
  if (group_id == 0) {
    return false;
  }

  for (uint16_t supported : tls1_get_grouplist(hs)) {
    if (supported == group_id) {
      return true;
    }
  }

  return false;
}

// kVerifySignatureAlgorithms is the default list of accepted signature
// algorithms for verifying.
static const uint16_t kVerifySignatureAlgorithms[] = {
    // List our preferred algorithms first.
    SSL_SIGN_ECDSA_SECP256R1_SHA256,
    SSL_SIGN_RSA_PSS_RSAE_SHA256,
    SSL_SIGN_RSA_PKCS1_SHA256,

    // Larger hashes are acceptable.
    SSL_SIGN_ECDSA_SECP384R1_SHA384,
    SSL_SIGN_RSA_PSS_RSAE_SHA384,
    SSL_SIGN_RSA_PKCS1_SHA384,

    SSL_SIGN_RSA_PSS_RSAE_SHA512,
    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.
static const uint16_t kSignSignatureAlgorithms[] = {
    // List our preferred algorithms first.
    SSL_SIGN_ED25519,
    SSL_SIGN_ECDSA_SECP256R1_SHA256,
    SSL_SIGN_RSA_PSS_RSAE_SHA256,
    SSL_SIGN_RSA_PKCS1_SHA256,

    // If needed, sign larger hashes.
    //
    // TODO(davidben): Determine which of these may be pruned.
    SSL_SIGN_ECDSA_SECP384R1_SHA384,
    SSL_SIGN_RSA_PSS_RSAE_SHA384,
    SSL_SIGN_RSA_PKCS1_SHA384,

    SSL_SIGN_ECDSA_SECP521R1_SHA512,
    SSL_SIGN_RSA_PSS_RSAE_SHA512,
    SSL_SIGN_RSA_PKCS1_SHA512,

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

static Span<const uint16_t> tls12_get_verify_sigalgs(const SSL_HANDSHAKE *hs) {
  if (hs->config->verify_sigalgs.empty()) {
    return Span<const uint16_t>(kVerifySignatureAlgorithms);
  }
  return hs->config->verify_sigalgs;
}

bool tls12_add_verify_sigalgs(const SSL_HANDSHAKE *hs, CBB *out) {
  for (uint16_t sigalg : tls12_get_verify_sigalgs(hs)) {
    if (!CBB_add_u16(out, sigalg)) {
      return false;
    }
  }
  return true;
}

bool tls12_check_peer_sigalg(const SSL_HANDSHAKE *hs, uint8_t *out_alert,
                             uint16_t sigalg) {
  for (uint16_t verify_sigalg : tls12_get_verify_sigalgs(hs)) {
    if (verify_sigalg == sigalg) {
      return true;
    }
  }

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

// tls_extension represents a TLS extension that is handled internally.
//
// 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.
// |add_clienthello| may be called multiple times and must not mutate |hs|.
//
// Note the |parse_serverhello| and |add_serverhello| callbacks refer to the
// TLS 1.2 ServerHello. In TLS 1.3, these callbacks act on EncryptedExtensions,
// with ServerHello extensions handled elsewhere in the handshake.
//
// All callbacks return true for success and false 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;

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

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

static bool 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 false;
  }

  return true;
}

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

static bool dont_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  return true;
}

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

static bool ext_sni_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  if (ssl->hostname == nullptr) {
    return true;
  }

  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->hostname.get(),
                     strlen(ssl->hostname.get())) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}

static bool ext_sni_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  // The server may acknowledge SNI with an empty extension. We check the syntax
  // but otherwise ignore this signal.
  return contents == NULL || CBS_len(contents) == 0;
}

static bool ext_sni_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  // SNI has already been parsed earlier in the handshake. See |extract_sni|.
  return true;
}

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

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

  return true;
}


// Encrypted ClientHello (ECH)
//
// https://tools.ietf.org/html/draft-ietf-tls-esni-10

// random_size returns a random value between |min| and |max|, inclusive.
static size_t random_size(size_t min, size_t max) {
  assert(min < max);
  size_t value;
  RAND_bytes(reinterpret_cast<uint8_t *>(&value), sizeof(value));
  return value % (max - min + 1) + min;
}

bool ssl_setup_ech_grease(SSL_HANDSHAKE *hs) {
  if (hs->max_version < TLS1_3_VERSION || !hs->config->ech_grease_enabled) {
    return true;
  }

  const uint16_t kdf_id = EVP_HPKE_HKDF_SHA256;
  const uint16_t aead_id = EVP_has_aes_hardware() ? EVP_HPKE_AES_128_GCM
                                                  : EVP_HPKE_CHACHA20_POLY1305;
  constexpr size_t kAEADOverhead = 16;  // Both AEADs have a 16-byte tag.
  static_assert(ssl_grease_ech_config_id < sizeof(hs->grease_seed),
                "hs->grease_seed is too small");
  uint8_t ech_config_id = hs->grease_seed[ssl_grease_ech_config_id];

  uint8_t ech_enc[X25519_PUBLIC_VALUE_LEN];
  uint8_t private_key_unused[X25519_PRIVATE_KEY_LEN];
  X25519_keypair(ech_enc, private_key_unused);

  // To determine a plausible length for the payload, we estimate the size of a
  // typical EncodedClientHelloInner without resumption:
  //
  //   2+32+1+2   version, random, legacy_session_id, legacy_compression_methods
  //   2+4*2      cipher_suites (three TLS 1.3 ciphers, GREASE)
  //   2          extensions prefix
  //   4          ech_is_inner
  //   4+1+2*2    supported_versions (TLS 1.3, GREASE)
  //   4+1+10*2   outer_extensions (key_share, sigalgs, sct, alpn,
  //              supported_groups, status_request, psk_key_exchange_modes,
  //              compress_certificate, GREASE x2)
  //
  // The server_name extension has an overhead of 9 bytes. For now, arbitrarily
  // estimate maximum_name_length to be between 32 and 100 bytes.
  //
  // TODO(davidben): If the padding scheme changes to also round the entire
  // payload, adjust this to match. See
  // https://github.com/tlswg/draft-ietf-tls-esni/issues/433
  const size_t payload_len = random_size(128, 196) + kAEADOverhead;
  bssl::ScopedCBB cbb;
  CBB enc_cbb, payload_cbb;
  uint8_t *payload;
  if (!CBB_init(cbb.get(), 64 + payload_len) ||
      !CBB_add_u16(cbb.get(), kdf_id) ||  //
      !CBB_add_u16(cbb.get(), aead_id) ||
      !CBB_add_u8(cbb.get(), ech_config_id) ||
      !CBB_add_u16_length_prefixed(cbb.get(), &enc_cbb) ||
      !CBB_add_bytes(&enc_cbb, ech_enc, OPENSSL_ARRAY_SIZE(ech_enc)) ||
      !CBB_add_u16_length_prefixed(cbb.get(), &payload_cbb) ||
      !CBB_add_space(&payload_cbb, &payload, payload_len) ||
      !RAND_bytes(payload, payload_len) ||
      !CBBFinishArray(cbb.get(), &hs->ech_grease)) {
    return false;
  }
  return true;
}

static bool ext_ech_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  if (hs->max_version < TLS1_3_VERSION) {
    return true;
  }
  if (hs->config->ech_grease_enabled) {
    assert(!hs->ech_grease.empty());
    CBB ech_body;
    if (!CBB_add_u16(out, TLSEXT_TYPE_encrypted_client_hello) ||
        !CBB_add_u16_length_prefixed(out, &ech_body) ||
        !CBB_add_bytes(&ech_body, hs->ech_grease.data(),
                       hs->ech_grease.size()) ||
        !CBB_flush(out)) {
      return false;
    }
    return true;
  }
  // Nothing to do, since we don't yet implement the non-GREASE parts of ECH.
  return true;
}

static bool ext_ech_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    return true;
  }

  // The ECH extension may not be sent in TLS 1.2 ServerHello, only TLS 1.3
  // EncryptedExtension.
  if (ssl_protocol_version(ssl) < TLS1_3_VERSION) {
    *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
    return false;
  }

  // If the client only sent GREASE, we must check the extension syntactically.
  CBS ech_configs;
  if (!CBS_get_u16_length_prefixed(contents, &ech_configs) ||
      CBS_len(&ech_configs) == 0 ||  //
      CBS_len(contents) > 0) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return false;
  }
  while (CBS_len(&ech_configs) > 0) {
    // Do a top-level parse of the ECHConfig, stopping before ECHConfigContents.
    uint16_t version;
    CBS ech_config_contents;
    if (!CBS_get_u16(&ech_configs, &version) ||
        !CBS_get_u16_length_prefixed(&ech_configs, &ech_config_contents)) {
      *out_alert = SSL_AD_DECODE_ERROR;
      return false;
    }
  }
  return true;
}

static bool ext_ech_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  if (contents != nullptr) {
    hs->ech_present = true;
    return true;
  }
  return true;
}

static bool ext_ech_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl_protocol_version(ssl) < TLS1_3_VERSION ||  //
      ssl->s3->ech_accept ||                         //
      hs->ech_server_config_list == nullptr) {
    return true;
  }

  // Write the list of retry configs to |out|. Note
  // |SSL_CTX_set1_ech_server_config_list| ensures |ech_server_config_list|
  // contains at least one retry config.
  CBB body, retry_configs;
  if (!CBB_add_u16(out, TLSEXT_TYPE_encrypted_client_hello) ||
      !CBB_add_u16_length_prefixed(out, &body) ||
      !CBB_add_u16_length_prefixed(&body, &retry_configs)) {
    return false;
  }
  for (const auto &config : hs->ech_server_config_list->configs) {
    if (!config->is_retry_config()) {
      continue;
    }
    if (!CBB_add_bytes(&retry_configs, config->ech_config().data(),
                       config->ech_config().size())) {
      return false;
    }
  }
  return CBB_flush(out);
}

static bool ext_ech_is_inner_add_clienthello(const SSL_HANDSHAKE *hs,
                                             CBB *out) {
  return true;
}

static bool ext_ech_is_inner_parse_clienthello(SSL_HANDSHAKE *hs,
                                               uint8_t *out_alert,
                                               CBS *contents) {
  if (contents == nullptr) {
    return true;
  }
  if (CBS_len(contents) > 0) {
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return false;
  }
  hs->ech_is_inner_present = true;
  return true;
}


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

static bool ext_ri_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  // Renegotiation indication is not necessary in TLS 1.3.
  if (hs->min_version >= TLS1_3_VERSION) {
    return true;
  }

  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 false;
  }

  return true;
}

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

  // 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 false;
  }

  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 true;
  }

  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 false;
  }

  // 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 false;
  }

  const uint8_t *d = CBS_data(&renegotiated_connection);
  bool ok = CRYPTO_memcmp(d, ssl->s3->previous_client_finished,
                          ssl->s3->previous_client_finished_len) == 0;
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  ok = true;
#endif
  if (!ok) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_MISMATCH);
    *out_alert = SSL_AD_HANDSHAKE_FAILURE;
    return false;
  }
  d += ssl->s3->previous_client_finished_len;

  ok = CRYPTO_memcmp(d, ssl->s3->previous_server_finished,
                     ssl->s3->previous_server_finished_len) == 0;
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  ok = true;
#endif
  if (!ok) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_RENEGOTIATION_MISMATCH);
    *out_alert = SSL_AD_HANDSHAKE_FAILURE;
    return false;
  }
  ssl->s3->send_connection_binding = true;

  return true;
}

static bool 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 (ssl_protocol_version(ssl) >= TLS1_3_VERSION) {
    return true;
  }

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

  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 false;
  }

  // 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 false;
  }

  ssl->s3->send_connection_binding = true;

  return true;
}

static bool 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 (ssl_protocol_version(ssl) >= TLS1_3_VERSION) {
    return true;
  }

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

  return true;
}


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

static bool ext_ems_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  // Extended master secret is not necessary in TLS 1.3.
  if (hs->min_version >= TLS1_3_VERSION) {
    return true;
  }

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

  return true;
}

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

  if (contents != NULL) {
    if (ssl_protocol_version(ssl) >= TLS1_3_VERSION ||
        CBS_len(contents) != 0) {
      return false;
    }

    hs->extended_master_secret = true;
  }

  // Whether EMS is negotiated may not change on renegotiation.
  if (ssl->s3->established_session != nullptr &&
      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 false;
  }

  return true;
}

static bool ext_ems_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                      CBS *contents) {
  if (ssl_protocol_version(hs->ssl) >= TLS1_3_VERSION) {
    return true;
  }

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

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

  hs->extended_master_secret = true;
  return true;
}

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

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

  return true;
}


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

static bool ext_ticket_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  // TLS 1.3 uses a different ticket extension.
  if (hs->min_version >= TLS1_3_VERSION ||
      SSL_get_options(ssl) & SSL_OP_NO_TICKET) {
    return true;
  }

  Span<const uint8_t> ticket;

  // 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.
  if (!ssl->s3->initial_handshake_complete &&
      ssl->session != nullptr &&
      !ssl->session->ticket.empty() &&
      // Don't send TLS 1.3 session tickets in the ticket extension.
      ssl_session_protocol_version(ssl->session.get()) < TLS1_3_VERSION) {
    ticket = ssl->session->ticket;
  }

  CBB ticket_cbb;
  if (!CBB_add_u16(out, TLSEXT_TYPE_session_ticket) ||
      !CBB_add_u16_length_prefixed(out, &ticket_cbb) ||
      !CBB_add_bytes(&ticket_cbb, ticket.data(), ticket.size()) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}

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

  if (ssl_protocol_version(ssl) >= TLS1_3_VERSION) {
    return false;
  }

  // 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 false;
  }

  hs->ticket_expected = true;
  return true;
}

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

  // 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 false;
  }

  return true;
}


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

static bool ext_sigalgs_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  if (hs->max_version < TLS1_2_VERSION) {
    return true;
  }

  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) ||
      !tls12_add_verify_sigalgs(hs, &sigalgs_cbb) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}

static bool ext_sigalgs_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                          CBS *contents) {
  hs->peer_sigalgs.Reset();
  if (contents == NULL) {
    return true;
  }

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

  return true;
}


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

static bool ext_ocsp_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->config->ocsp_stapling_enabled) {
    return true;
  }

  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 false;
  }

  return true;
}

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

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

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

  // 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 = true;
  return true;
}

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

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

  // 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 true;
}

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

  hs->certificate_status_expected = true;

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

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

  return true;
}

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

  if (ssl_protocol_version(ssl) >= TLS1_3_VERSION) {
    return false;
  }

  // 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.empty()) {
    // 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 false;
  }

  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 false;
    }
  }

  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 ||
      !ssl->s3->next_proto_negotiated.CopyFrom(
          MakeConstSpan(selected, selected_len))) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return false;
  }

  hs->next_proto_neg_seen = true;
  return true;
}

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

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

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

  hs->next_proto_neg_seen = true;
  return true;
}

static bool 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 true;
  }

  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 = false;
    return true;
  }

  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 false;
  }

  return true;
}


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

static bool ext_sct_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->config->signed_cert_timestamps_enabled) {
    return true;
  }

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

  return true;
}

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

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

  // 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(hs->config->signed_cert_timestamps_enabled);

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

  // 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) {
    hs->new_session->signed_cert_timestamp_list.reset(
        CRYPTO_BUFFER_new_from_CBS(contents, ssl->ctx->pool));
    if (hs->new_session->signed_cert_timestamp_list == nullptr) {
      *out_alert = SSL_AD_INTERNAL_ERROR;
      return false;
    }
  }

  return true;
}

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

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

  hs->scts_requested = true;
  return true;
}

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

  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(
                 hs->config->cert->signed_cert_timestamp_list.get()),
             CRYPTO_BUFFER_len(
                 hs->config->cert->signed_cert_timestamp_list.get())) &&
         CBB_flush(out);
}


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

static bool ext_alpn_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  if (hs->config->alpn_client_proto_list.empty() && ssl->quic_method) {
    // ALPN MUST be used with QUIC.
    OPENSSL_PUT_ERROR(SSL, SSL_R_NO_APPLICATION_PROTOCOL);
    return false;
  }

  if (hs->config->alpn_client_proto_list.empty() ||
      ssl->s3->initial_handshake_complete) {
    return true;
  }

  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, hs->config->alpn_client_proto_list.data(),
                     hs->config->alpn_client_proto_list.size()) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}

static bool ext_alpn_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                       CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    if (ssl->quic_method) {
      // ALPN is required when QUIC is used.
      OPENSSL_PUT_ERROR(SSL, SSL_R_NO_APPLICATION_PROTOCOL);
      *out_alert = SSL_AD_NO_APPLICATION_PROTOCOL;
      return false;
    }
    return true;
  }

  assert(!ssl->s3->initial_handshake_complete);
  assert(!hs->config->alpn_client_proto_list.empty());

  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 false;
  }

  // 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 false;
  }

  if (!ssl_is_alpn_protocol_allowed(hs, protocol_name)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_INVALID_ALPN_PROTOCOL);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return false;
  }

  if (!ssl->s3->alpn_selected.CopyFrom(protocol_name)) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return false;
  }

  return true;
}

bool ssl_is_valid_alpn_list(Span<const uint8_t> in) {
  CBS protocol_name_list = in;
  if (CBS_len(&protocol_name_list) == 0) {
    return false;
  }
  while (CBS_len(&protocol_name_list) > 0) {
    CBS protocol_name;
    if (!CBS_get_u8_length_prefixed(&protocol_name_list, &protocol_name) ||
        // Empty protocol names are forbidden.
        CBS_len(&protocol_name) == 0) {
      return false;
    }
  }
  return true;
}

bool ssl_is_alpn_protocol_allowed(const SSL_HANDSHAKE *hs,
                                  Span<const uint8_t> protocol) {
  if (hs->config->alpn_client_proto_list.empty()) {
    return false;
  }

  if (hs->ssl->ctx->allow_unknown_alpn_protos) {
    return true;
  }

  // Check that the protocol name is one of the ones we advertised.
  CBS client_protocol_name_list =
          MakeConstSpan(hs->config->alpn_client_proto_list),
      client_protocol_name;
  while (CBS_len(&client_protocol_name_list) > 0) {
    if (!CBS_get_u8_length_prefixed(&client_protocol_name_list,
                                    &client_protocol_name)) {
      return false;
    }

    if (client_protocol_name == protocol) {
      return true;
    }
  }

  return false;
}

bool 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)) {
    if (ssl->quic_method) {
      // ALPN is required when QUIC is used.
      OPENSSL_PUT_ERROR(SSL, SSL_R_NO_APPLICATION_PROTOCOL);
      *out_alert = SSL_AD_NO_APPLICATION_PROTOCOL;
      return false;
    }
    // Ignore ALPN if not configured or no extension was supplied.
    return true;
  }

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

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

  const uint8_t *selected;
  uint8_t selected_len;
  int ret = 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);
  // ALPN is required when QUIC is used.
  if (ssl->quic_method &&
      (ret == SSL_TLSEXT_ERR_NOACK || ret == SSL_TLSEXT_ERR_ALERT_WARNING)) {
    ret = SSL_TLSEXT_ERR_ALERT_FATAL;
  }
  switch (ret) {
    case SSL_TLSEXT_ERR_OK:
      if (selected_len == 0) {
        OPENSSL_PUT_ERROR(SSL, SSL_R_INVALID_ALPN_PROTOCOL);
        *out_alert = SSL_AD_INTERNAL_ERROR;
        return false;
      }
      if (!ssl->s3->alpn_selected.CopyFrom(
              MakeConstSpan(selected, selected_len))) {
        *out_alert = SSL_AD_INTERNAL_ERROR;
        return false;
      }
      break;
    case SSL_TLSEXT_ERR_NOACK:
    case SSL_TLSEXT_ERR_ALERT_WARNING:
      break;
    case SSL_TLSEXT_ERR_ALERT_FATAL:
      *out_alert = SSL_AD_NO_APPLICATION_PROTOCOL;
      OPENSSL_PUT_ERROR(SSL, SSL_R_NO_APPLICATION_PROTOCOL);
      return false;
    default:
      // Invalid return value.
      *out_alert = SSL_AD_INTERNAL_ERROR;
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      return false;
  }

  return true;
}

static bool ext_alpn_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl->s3->alpn_selected.empty()) {
    return true;
  }

  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.data(),
                     ssl->s3->alpn_selected.size()) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}


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

static bool ext_channel_id_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  if (!hs->config->channel_id_private || SSL_is_dtls(ssl)) {
    return true;
  }

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

  return true;
}

static bool ext_channel_id_parse_serverhello(SSL_HANDSHAKE *hs,
                                             uint8_t *out_alert,
                                             CBS *contents) {
  if (contents == NULL) {
    return true;
  }

  assert(!SSL_is_dtls(hs->ssl));
  assert(hs->config->channel_id_private);

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

  hs->channel_id_negotiated = true;
  return true;
}

static bool ext_channel_id_parse_clienthello(SSL_HANDSHAKE *hs,
                                             uint8_t *out_alert,
                                             CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL || !hs->config->channel_id_enabled || SSL_is_dtls(ssl)) {
    return true;
  }

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

  hs->channel_id_negotiated = true;
  return true;
}

static bool ext_channel_id_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->channel_id_negotiated) {
    return true;
  }

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

  return true;
}


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

static bool ext_srtp_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  const STACK_OF(SRTP_PROTECTION_PROFILE) *profiles =
      SSL_get_srtp_profiles(ssl);
  if (profiles == NULL ||
      sk_SRTP_PROTECTION_PROFILE_num(profiles) == 0 ||
      !SSL_is_dtls(ssl)) {
    return true;
  }

  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 false;
  }

  for (const SRTP_PROTECTION_PROFILE *profile : profiles) {
    if (!CBB_add_u16(&profile_ids, profile->id)) {
      return false;
    }
  }

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

  return true;
}

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

  // 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
  assert(SSL_is_dtls(ssl));
  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 false;
  }

  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 false;
  }

  // Check to see if the server gave us something we support and offered.
  for (const SRTP_PROTECTION_PROFILE *profile : SSL_get_srtp_profiles(ssl)) {
    if (profile->id == profile_id) {
      ssl->s3->srtp_profile = profile;
      return true;
    }
  }

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

static bool ext_srtp_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                       CBS *contents) {
  SSL *const ssl = hs->ssl;
  // DTLS-SRTP is only defined for DTLS.
  if (contents == NULL || !SSL_is_dtls(ssl)) {
    return true;
  }

  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 false;
  }
  // 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 (const SRTP_PROTECTION_PROFILE *server_profile : server_profiles) {
    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 false;
      }

      if (server_profile->id == profile_id) {
        ssl->s3->srtp_profile = server_profile;
        return true;
      }
    }
  }

  return true;
}

static bool ext_srtp_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl->s3->srtp_profile == NULL) {
    return true;
  }

  assert(SSL_is_dtls(ssl));
  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->s3->srtp_profile->id) ||
      !CBB_add_u8(&contents, 0 /* empty MKI */) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}


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

static bool ext_ec_point_add_extension(const 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 false;
  }

  return true;
}

static bool ext_ec_point_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  // The point format extension is unnecessary in TLS 1.3.
  if (hs->min_version >= TLS1_3_VERSION) {
    return true;
  }

  return ext_ec_point_add_extension(hs, out);
}

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

  if (ssl_protocol_version(hs->ssl) >= TLS1_3_VERSION) {
    return false;
  }

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

  // 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 false;
  }

  return true;
}

static bool ext_ec_point_parse_clienthello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                          CBS *contents) {
  if (ssl_protocol_version(hs->ssl) >= TLS1_3_VERSION) {
    return true;
  }

  return ext_ec_point_parse_serverhello(hs, out_alert, contents);
}

static bool ext_ec_point_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  if (ssl_protocol_version(ssl) >= TLS1_3_VERSION) {
    return true;
  }

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

  if (!using_ecc) {
    return true;
  }

  return ext_ec_point_add_extension(hs, out);
}


// Pre Shared Key
//
// https://tools.ietf.org/html/rfc8446#section-4.2.11

static size_t ext_pre_shared_key_clienthello_length(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  if (hs->max_version < TLS1_3_VERSION || ssl->session == nullptr ||
      ssl_session_protocol_version(ssl->session.get()) < TLS1_3_VERSION) {
    return 0;
  }

  size_t binder_len = EVP_MD_size(ssl_session_get_digest(ssl->session.get()));
  return 15 + ssl->session->ticket.size() + binder_len;
}

static bool ext_pre_shared_key_add_clienthello(const SSL_HANDSHAKE *hs,
                                               CBB *out,
                                               bool *out_needs_binder) {
  const SSL *const ssl = hs->ssl;
  *out_needs_binder = false;
  if (hs->max_version < TLS1_3_VERSION || ssl->session == nullptr ||
      ssl_session_protocol_version(ssl->session.get()) < TLS1_3_VERSION) {
    return true;
  }

  // Per RFC 8446 section 4.1.4, skip offering the session if the selected
  // cipher in HelloRetryRequest does not match. This avoids performing the
  // transcript hash transformation for multiple hashes.
  if (ssl->s3->used_hello_retry_request &&
      ssl->session->cipher->algorithm_prf != hs->new_cipher->algorithm_prf) {
    return true;
  }

  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};
  size_t binder_len = EVP_MD_size(ssl_session_get_digest(ssl->session.get()));

  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->ticket.data(),
                     ssl->session->ticket.size()) ||
      !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 false;
  }

  *out_needs_binder = true;
  return CBB_flush(out);
}

bool 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 false;
  }

  // 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 false;
  }

  return true;
}

bool 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,
    const SSL_CLIENT_HELLO *client_hello, CBS *contents) {
  // Verify that the pre_shared_key extension is the last extension in
  // ClientHello.
  if (CBS_data(contents) + CBS_len(contents) !=
      client_hello->extensions + client_hello->extensions_len) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_PRE_SHARED_KEY_MUST_BE_LAST);
    *out_alert = SSL_AD_ILLEGAL_PARAMETER;
    return false;
  }

  // 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 false;
  }

  *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 false;
    }

    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 false;
    }

    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 false;
  }

  return true;
}

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

  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 false;
  }

  return true;
}


// Pre-Shared Key Exchange Modes
//
// https://tools.ietf.org/html/rfc8446#section-4.2.9

static bool ext_psk_key_exchange_modes_add_clienthello(const SSL_HANDSHAKE *hs,
                                                       CBB *out) {
  if (hs->max_version < TLS1_3_VERSION) {
    return true;
  }

  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 false;
  }

  return CBB_flush(out);
}

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

  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 false;
  }

  // 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 true;
}


// Early Data Indication
//
// https://tools.ietf.org/html/rfc8446#section-4.2.10

static bool ext_early_data_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  // The second ClientHello never offers early data, and we must have already
  // filled in |early_data_reason| by this point.
  if (ssl->s3->used_hello_retry_request) {
    assert(ssl->s3->early_data_reason != ssl_early_data_unknown);
    return true;
  }

  if (!hs->early_data_offered) {
    return true;
  }

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

  return true;
}

static bool ext_early_data_parse_serverhello(SSL_HANDSHAKE *hs,
                                             uint8_t *out_alert,
                                             CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == NULL) {
    if (hs->early_data_offered && !ssl->s3->used_hello_retry_request) {
      ssl->s3->early_data_reason = ssl->s3->session_reused
                                       ? ssl_early_data_peer_declined
                                       : ssl_early_data_session_not_resumed;
    } else {
      // We already filled in |early_data_reason| when declining to offer 0-RTT
      // or handling the implicit HelloRetryRequest reject.
      assert(ssl->s3->early_data_reason != ssl_early_data_unknown);
    }
    return true;
  }

  // If we received an HRR, the second ClientHello never offers early data, so
  // the extensions logic will automatically reject early data extensions as
  // unsolicited. This covered by the ServerAcceptsEarlyDataOnHRR test.
  assert(!ssl->s3->used_hello_retry_request);

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

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

  ssl->s3->early_data_reason = ssl_early_data_accepted;
  ssl->s3->early_data_accepted = true;
  return true;
}

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

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

  hs->early_data_offered = true;
  return true;
}

static bool ext_early_data_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  if (!hs->ssl->s3->early_data_accepted) {
    return true;
  }

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

  return true;
}


// Key Share
//
// https://tools.ietf.org/html/rfc8446#section-4.2.8

bool ssl_setup_key_shares(SSL_HANDSHAKE *hs, uint16_t override_group_id) {
  SSL *const ssl = hs->ssl;
  hs->key_shares[0].reset();
  hs->key_shares[1].reset();
  hs->key_share_bytes.Reset();

  if (hs->max_version < TLS1_3_VERSION) {
    return true;
  }

  bssl::ScopedCBB cbb;
  if (!CBB_init(cbb.get(), 64)) {
    return false;
  }

  if (override_group_id == 0 && ssl->ctx->grease_enabled) {
    // Add a fake group. See RFC 8701.
    if (!CBB_add_u16(cbb.get(), ssl_get_grease_value(hs, ssl_grease_group)) ||
        !CBB_add_u16(cbb.get(), 1 /* length */) ||
        !CBB_add_u8(cbb.get(), 0 /* one byte key share */)) {
      return false;
    }
  }

  uint16_t group_id = override_group_id;
  uint16_t second_group_id = 0;
  if (override_group_id == 0) {
    // Predict the most preferred group.
    Span<const uint16_t> groups = tls1_get_grouplist(hs);
    if (groups.empty()) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_NO_GROUPS_SPECIFIED);
      return false;
    }

    group_id = groups[0];

    if (is_post_quantum_group(group_id) && groups.size() >= 2) {
      // CECPQ2(b) is not sent as the only initial key share. We'll include the
      // 2nd preference group too to avoid round-trips.
      second_group_id = groups[1];
      assert(second_group_id != group_id);
    }
  }

  CBB key_exchange;
  hs->key_shares[0] = SSLKeyShare::Create(group_id);
  if (!hs->key_shares[0] ||  //
      !CBB_add_u16(cbb.get(), group_id) ||
      !CBB_add_u16_length_prefixed(cbb.get(), &key_exchange) ||
      !hs->key_shares[0]->Offer(&key_exchange)) {
    return false;
  }

  if (second_group_id != 0) {
    hs->key_shares[1] = SSLKeyShare::Create(second_group_id);
    if (!hs->key_shares[1] ||  //
        !CBB_add_u16(cbb.get(), second_group_id) ||
        !CBB_add_u16_length_prefixed(cbb.get(), &key_exchange) ||
        !hs->key_shares[1]->Offer(&key_exchange)) {
      return false;
    }
  }

  return CBBFinishArray(cbb.get(), &hs->key_share_bytes);
}

static bool ext_key_share_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  if (hs->max_version < TLS1_3_VERSION) {
    return true;
  }

  assert(!hs->key_share_bytes.empty());
  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) ||
      !CBB_add_bytes(&kse_bytes, hs->key_share_bytes.data(),
                     hs->key_share_bytes.size()) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}

bool ssl_ext_key_share_parse_serverhello(SSL_HANDSHAKE *hs,
                                         Array<uint8_t> *out_secret,
                                         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) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    *out_alert = SSL_AD_DECODE_ERROR;
    return false;
  }

  SSLKeyShare *key_share = hs->key_shares[0].get();
  if (key_share->GroupID() != group_id) {
    if (!hs->key_shares[1] || hs->key_shares[1]->GroupID() != group_id) {
      *out_alert = SSL_AD_ILLEGAL_PARAMETER;
      OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_CURVE);
      return false;
    }
    key_share = hs->key_shares[1].get();
  }

  if (!key_share->Finish(out_secret, out_alert, peer_key)) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return false;
  }

  hs->new_session->group_id = group_id;
  hs->key_shares[0].reset();
  hs->key_shares[1].reset();
  return true;
}

bool ssl_ext_key_share_parse_clienthello(SSL_HANDSHAKE *hs, bool *out_found,
                                         Span<const uint8_t> *out_peer_key,
                                         uint8_t *out_alert,
                                         const SSL_CLIENT_HELLO *client_hello) {
  // We only support connections that include an ECDHE key exchange.
  CBS contents;
  if (!ssl_client_hello_get_extension(client_hello, &contents,
                                      TLSEXT_TYPE_key_share)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_MISSING_KEY_SHARE);
    *out_alert = SSL_AD_MISSING_EXTENSION;
    return false;
  }

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

  // Find the corresponding key share.
  const uint16_t group_id = hs->new_session->group_id;
  CBS peer_key;
  CBS_init(&peer_key, nullptr, 0);
  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) ||
        CBS_len(&peer_key_tmp) == 0) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      return false;
    }

    if (id == group_id) {
      if (CBS_len(&peer_key) != 0) {
        OPENSSL_PUT_ERROR(SSL, SSL_R_DUPLICATE_KEY_SHARE);
        *out_alert = SSL_AD_ILLEGAL_PARAMETER;
        return false;
      }

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

  if (out_peer_key != nullptr) {
    *out_peer_key = peer_key;
  }
  *out_found = CBS_len(&peer_key) != 0;
  return true;
}

bool ssl_ext_key_share_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  CBB kse_bytes, public_key;
  if (!CBB_add_u16(out, TLSEXT_TYPE_key_share) ||
      !CBB_add_u16_length_prefixed(out, &kse_bytes) ||
      !CBB_add_u16(&kse_bytes, hs->new_session->group_id) ||
      !CBB_add_u16_length_prefixed(&kse_bytes, &public_key) ||
      !CBB_add_bytes(&public_key, hs->ecdh_public_key.data(),
                     hs->ecdh_public_key.size()) ||
      !CBB_flush(out)) {
    return false;
  }
  return true;
}


// Supported Versions
//
// https://tools.ietf.org/html/rfc8446#section-4.2.1

static bool ext_supported_versions_add_clienthello(const SSL_HANDSHAKE *hs,
                                                   CBB *out) {
  const SSL *const ssl = hs->ssl;
  if (hs->max_version <= TLS1_2_VERSION) {
    return true;
  }

  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 false;
  }

  // Add a fake version. See RFC 8701.
  if (ssl->ctx->grease_enabled &&
      !CBB_add_u16(&versions, ssl_get_grease_value(hs, ssl_grease_version))) {
    return false;
  }

  if (!ssl_add_supported_versions(hs, &versions) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}


// Cookie
//
// https://tools.ietf.org/html/rfc8446#section-4.2.2

static bool ext_cookie_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  if (hs->cookie.empty()) {
    return true;
  }

  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.data(), hs->cookie.size()) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}


// Supported Groups
//
// https://tools.ietf.org/html/rfc4492#section-5.1.1
// https://tools.ietf.org/html/rfc8446#section-4.2.7

static bool ext_supported_groups_add_clienthello(const SSL_HANDSHAKE *hs,
                                                 CBB *out) {
  const 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 false;
  }

  // Add a fake group. See RFC 8701.
  if (ssl->ctx->grease_enabled &&
      !CBB_add_u16(&groups_bytes,
                   ssl_get_grease_value(hs, ssl_grease_group))) {
    return false;
  }

  for (uint16_t group : tls1_get_grouplist(hs)) {
    if (is_post_quantum_group(group) &&
        hs->max_version < TLS1_3_VERSION) {
      continue;
    }
    if (!CBB_add_u16(&groups_bytes, group)) {
      return false;
    }
  }

  return CBB_flush(out);
}

static bool 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 true;
}

static bool parse_u16_array(const CBS *cbs, Array<uint16_t> *out) {
  CBS copy = *cbs;
  if ((CBS_len(&copy) & 1) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    return false;
  }

  Array<uint16_t> ret;
  if (!ret.Init(CBS_len(&copy) / 2)) {
    return false;
  }
  for (size_t i = 0; i < ret.size(); i++) {
    if (!CBS_get_u16(&copy, &ret[i])) {
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      return false;
    }
  }

  assert(CBS_len(&copy) == 0);
  *out = std::move(ret);
  return 1;
}

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

  CBS supported_group_list;
  if (!CBS_get_u16_length_prefixed(contents, &supported_group_list) ||
      CBS_len(&supported_group_list) == 0 ||
      CBS_len(contents) != 0 ||
      !parse_u16_array(&supported_group_list, &hs->peer_supported_group_list)) {
    return false;
  }

  return true;
}


// QUIC Transport Parameters

static bool ext_quic_transport_params_add_clienthello_impl(
    const SSL_HANDSHAKE *hs, CBB *out, bool use_legacy_codepoint) {
  if (hs->config->quic_transport_params.empty() && !hs->ssl->quic_method) {
    return true;
  }
  if (hs->config->quic_transport_params.empty() || !hs->ssl->quic_method) {
    // QUIC Transport Parameters must be sent over QUIC, and they must not be
    // sent over non-QUIC transports. If transport params are set, then
    // SSL(_CTX)_set_quic_method must also be called.
    OPENSSL_PUT_ERROR(SSL, SSL_R_QUIC_TRANSPORT_PARAMETERS_MISCONFIGURED);
    return false;
  }
  assert(hs->min_version > TLS1_2_VERSION);
  if (use_legacy_codepoint != hs->config->quic_use_legacy_codepoint) {
    // Do nothing, we'll send the other codepoint.
    return true;
  }

  uint16_t extension_type = TLSEXT_TYPE_quic_transport_parameters;
  if (hs->config->quic_use_legacy_codepoint) {
    extension_type = TLSEXT_TYPE_quic_transport_parameters_legacy;
  }

  CBB contents;
  if (!CBB_add_u16(out, extension_type) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_bytes(&contents, hs->config->quic_transport_params.data(),
                     hs->config->quic_transport_params.size()) ||
      !CBB_flush(out)) {
    return false;
  }
  return true;
}

static bool ext_quic_transport_params_add_clienthello(const SSL_HANDSHAKE *hs,
                                                      CBB *out) {
  return ext_quic_transport_params_add_clienthello_impl(
      hs, out, /*use_legacy_codepoint=*/false);
}

static bool ext_quic_transport_params_add_clienthello_legacy(
    const SSL_HANDSHAKE *hs, CBB *out) {
  return ext_quic_transport_params_add_clienthello_impl(
      hs, out, /*use_legacy_codepoint=*/true);
}

static bool ext_quic_transport_params_parse_serverhello_impl(
    SSL_HANDSHAKE *hs, uint8_t *out_alert, CBS *contents,
    bool used_legacy_codepoint) {
  SSL *const ssl = hs->ssl;
  if (contents == nullptr) {
    if (used_legacy_codepoint != hs->config->quic_use_legacy_codepoint) {
      // Silently ignore because we expect the other QUIC codepoint.
      return true;
    }
    if (!ssl->quic_method) {
      return true;
    }
    *out_alert = SSL_AD_MISSING_EXTENSION;
    return false;
  }
  // The extensions parser will check for unsolicited extensions before
  // calling the callback.
  assert(ssl->quic_method != nullptr);
  assert(ssl_protocol_version(ssl) == TLS1_3_VERSION);
  assert(used_legacy_codepoint == hs->config->quic_use_legacy_codepoint);
  return ssl->s3->peer_quic_transport_params.CopyFrom(*contents);
}

static bool ext_quic_transport_params_parse_serverhello(SSL_HANDSHAKE *hs,
                                                        uint8_t *out_alert,
                                                        CBS *contents) {
  return ext_quic_transport_params_parse_serverhello_impl(
      hs, out_alert, contents, /*used_legacy_codepoint=*/false);
}

static bool ext_quic_transport_params_parse_serverhello_legacy(
    SSL_HANDSHAKE *hs, uint8_t *out_alert, CBS *contents) {
  return ext_quic_transport_params_parse_serverhello_impl(
      hs, out_alert, contents, /*used_legacy_codepoint=*/true);
}

static bool ext_quic_transport_params_parse_clienthello_impl(
    SSL_HANDSHAKE *hs, uint8_t *out_alert, CBS *contents,
    bool used_legacy_codepoint) {
  SSL *const ssl = hs->ssl;
  if (!contents) {
    if (!ssl->quic_method) {
      if (hs->config->quic_transport_params.empty()) {
        return true;
      }
      // QUIC transport parameters must not be set if |ssl| is not configured
      // for QUIC.
      OPENSSL_PUT_ERROR(SSL, SSL_R_QUIC_TRANSPORT_PARAMETERS_MISCONFIGURED);
      *out_alert = SSL_AD_INTERNAL_ERROR;
      return false;
    }
    if (used_legacy_codepoint != hs->config->quic_use_legacy_codepoint) {
      // Silently ignore because we expect the other QUIC codepoint.
      return true;
    }
    *out_alert = SSL_AD_MISSING_EXTENSION;
    return false;
  }
  if (!ssl->quic_method) {
    if (used_legacy_codepoint) {
      // Ignore the legacy private-use codepoint because that could be sent
      // to mean something else than QUIC transport parameters.
      return true;
    }
    // Fail if we received the codepoint registered with IANA for QUIC
    // because that is not allowed outside of QUIC.
    *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
    return false;
  }
  assert(ssl_protocol_version(ssl) == TLS1_3_VERSION);
  if (used_legacy_codepoint != hs->config->quic_use_legacy_codepoint) {
    // Silently ignore because we expect the other QUIC codepoint.
    return true;
  }
  return ssl->s3->peer_quic_transport_params.CopyFrom(*contents);
}

static bool ext_quic_transport_params_parse_clienthello(SSL_HANDSHAKE *hs,
                                                        uint8_t *out_alert,
                                                        CBS *contents) {
  return ext_quic_transport_params_parse_clienthello_impl(
      hs, out_alert, contents, /*used_legacy_codepoint=*/false);
}

static bool ext_quic_transport_params_parse_clienthello_legacy(
    SSL_HANDSHAKE *hs, uint8_t *out_alert, CBS *contents) {
  return ext_quic_transport_params_parse_clienthello_impl(
      hs, out_alert, contents, /*used_legacy_codepoint=*/true);
}

static bool ext_quic_transport_params_add_serverhello_impl(
    SSL_HANDSHAKE *hs, CBB *out, bool use_legacy_codepoint) {
  if (hs->ssl->quic_method == nullptr && use_legacy_codepoint) {
    // Ignore the legacy private-use codepoint because that could be sent
    // to mean something else than QUIC transport parameters.
    return true;
  }
  assert(hs->ssl->quic_method != nullptr);
  if (hs->config->quic_transport_params.empty()) {
    // Transport parameters must be set when using QUIC.
    OPENSSL_PUT_ERROR(SSL, SSL_R_QUIC_TRANSPORT_PARAMETERS_MISCONFIGURED);
    return false;
  }
  if (use_legacy_codepoint != hs->config->quic_use_legacy_codepoint) {
    // Do nothing, we'll send the other codepoint.
    return true;
  }

  uint16_t extension_type = TLSEXT_TYPE_quic_transport_parameters;
  if (hs->config->quic_use_legacy_codepoint) {
    extension_type = TLSEXT_TYPE_quic_transport_parameters_legacy;
  }

  CBB contents;
  if (!CBB_add_u16(out, extension_type) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_bytes(&contents, hs->config->quic_transport_params.data(),
                     hs->config->quic_transport_params.size()) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}

static bool ext_quic_transport_params_add_serverhello(SSL_HANDSHAKE *hs,
                                                      CBB *out) {
  return ext_quic_transport_params_add_serverhello_impl(
      hs, out, /*use_legacy_codepoint=*/false);
}

static bool ext_quic_transport_params_add_serverhello_legacy(SSL_HANDSHAKE *hs,
                                                             CBB *out) {
  return ext_quic_transport_params_add_serverhello_impl(
      hs, out, /*use_legacy_codepoint=*/true);
}

// Delegated credentials.
//
// https://tools.ietf.org/html/draft-ietf-tls-subcerts

static bool ext_delegated_credential_add_clienthello(const SSL_HANDSHAKE *hs,
                                                     CBB *out) {
  return true;
}

static bool ext_delegated_credential_parse_clienthello(SSL_HANDSHAKE *hs,
                                                       uint8_t *out_alert,
                                                       CBS *contents) {
  if (contents == nullptr || ssl_protocol_version(hs->ssl) < TLS1_3_VERSION) {
    // Don't use delegated credentials unless we're negotiating TLS 1.3 or
    // higher.
    return true;
  }

  // The contents of the extension are the signature algorithms the client will
  // accept for a delegated credential.
  CBS sigalg_list;
  if (!CBS_get_u16_length_prefixed(contents, &sigalg_list) ||
      CBS_len(&sigalg_list) == 0 ||
      CBS_len(contents) != 0 ||
      !parse_u16_array(&sigalg_list, &hs->peer_delegated_credential_sigalgs)) {
    return false;
  }

  hs->delegated_credential_requested = true;
  return true;
}

// Certificate compression

static bool cert_compression_add_clienthello(const SSL_HANDSHAKE *hs,
                                             CBB *out) {
  bool first = true;
  CBB contents, algs;

  for (const auto &alg : hs->ssl->ctx->cert_compression_algs) {
    if (alg.decompress == nullptr) {
      continue;
    }

    if (first && (!CBB_add_u16(out, TLSEXT_TYPE_cert_compression) ||
                  !CBB_add_u16_length_prefixed(out, &contents) ||
                  !CBB_add_u8_length_prefixed(&contents, &algs))) {
      return false;
    }
    first = false;
    if (!CBB_add_u16(&algs, alg.alg_id)) {
      return false;
    }
  }

  return first || CBB_flush(out);
}

static bool cert_compression_parse_serverhello(SSL_HANDSHAKE *hs,
                                               uint8_t *out_alert,
                                               CBS *contents) {
  if (contents == nullptr) {
    return true;
  }

  // The server may not echo this extension. Any server to client negotiation is
  // advertised in the CertificateRequest message.
  return false;
}

static bool cert_compression_parse_clienthello(SSL_HANDSHAKE *hs,
                                               uint8_t *out_alert,
                                               CBS *contents) {
  if (contents == nullptr) {
    return true;
  }

  const SSL_CTX *ctx = hs->ssl->ctx.get();
  const size_t num_algs = ctx->cert_compression_algs.size();

  CBS alg_ids;
  if (!CBS_get_u8_length_prefixed(contents, &alg_ids) ||
      CBS_len(contents) != 0 ||
      CBS_len(&alg_ids) == 0 ||
      CBS_len(&alg_ids) % 2 == 1) {
    return false;
  }

  const size_t num_given_alg_ids = CBS_len(&alg_ids) / 2;
  Array<uint16_t> given_alg_ids;
  if (!given_alg_ids.Init(num_given_alg_ids)) {
    return false;
  }

  size_t best_index = num_algs;
  size_t given_alg_idx = 0;

  while (CBS_len(&alg_ids) > 0) {
    uint16_t alg_id;
    if (!CBS_get_u16(&alg_ids, &alg_id)) {
      return false;
    }

    given_alg_ids[given_alg_idx++] = alg_id;

    for (size_t i = 0; i < num_algs; i++) {
      const auto &alg = ctx->cert_compression_algs[i];
      if (alg.alg_id == alg_id && alg.compress != nullptr) {
        if (i < best_index) {
          best_index = i;
        }
        break;
      }
    }
  }

  qsort(given_alg_ids.data(), given_alg_ids.size(), sizeof(uint16_t),
        compare_uint16_t);
  for (size_t i = 1; i < num_given_alg_ids; i++) {
    if (given_alg_ids[i - 1] == given_alg_ids[i]) {
      return false;
    }
  }

  if (best_index < num_algs &&
      ssl_protocol_version(hs->ssl) >= TLS1_3_VERSION) {
    hs->cert_compression_negotiated = true;
    hs->cert_compression_alg_id = ctx->cert_compression_algs[best_index].alg_id;
  }

  return true;
}

static bool cert_compression_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  return true;
}

// Application-level Protocol Settings
//
// https://tools.ietf.org/html/draft-vvv-tls-alps-01

bool ssl_get_local_application_settings(const SSL_HANDSHAKE *hs,
                                        Span<const uint8_t> *out_settings,
                                        Span<const uint8_t> protocol) {
  for (const ALPSConfig &config : hs->config->alps_configs) {
    if (protocol == config.protocol) {
      *out_settings = config.settings;
      return true;
    }
  }
  return false;
}

static bool ext_alps_add_clienthello(const SSL_HANDSHAKE *hs, CBB *out) {
  const SSL *const ssl = hs->ssl;
  if (// ALPS requires TLS 1.3.
      hs->max_version < TLS1_3_VERSION ||
      // Do not offer ALPS without ALPN.
      hs->config->alpn_client_proto_list.empty() ||
      // Do not offer ALPS if not configured.
      hs->config->alps_configs.empty() ||
      // Do not offer ALPS on renegotiation handshakes.
      ssl->s3->initial_handshake_complete) {
    return true;
  }

  CBB contents, proto_list, proto;
  if (!CBB_add_u16(out, TLSEXT_TYPE_application_settings) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_u16_length_prefixed(&contents, &proto_list)) {
    return false;
  }

  for (const ALPSConfig &config : hs->config->alps_configs) {
    if (!CBB_add_u8_length_prefixed(&proto_list, &proto) ||
        !CBB_add_bytes(&proto, config.protocol.data(),
                       config.protocol.size())) {
      return false;
    }
  }

  return CBB_flush(out);
}

static bool ext_alps_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                                       CBS *contents) {
  SSL *const ssl = hs->ssl;
  if (contents == nullptr) {
    return true;
  }

  assert(!ssl->s3->initial_handshake_complete);
  assert(!hs->config->alpn_client_proto_list.empty());
  assert(!hs->config->alps_configs.empty());

  // ALPS requires TLS 1.3.
  if (ssl_protocol_version(ssl) < TLS1_3_VERSION) {
    *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
    return false;
  }

  // Note extension callbacks may run in any order, so we defer checking
  // consistency with ALPN to |ssl_check_serverhello_tlsext|.
  if (!hs->new_session->peer_application_settings.CopyFrom(*contents)) {
    *out_alert = SSL_AD_INTERNAL_ERROR;
    return false;
  }

  hs->new_session->has_application_settings = true;
  return true;
}

static bool ext_alps_add_serverhello(SSL_HANDSHAKE *hs, CBB *out) {
  SSL *const ssl = hs->ssl;
  // If early data is accepted, we omit the ALPS extension. It is implicitly
  // carried over from the previous connection.
  if (hs->new_session == nullptr ||
      !hs->new_session->has_application_settings ||
      ssl->s3->early_data_accepted) {
    return true;
  }

  CBB contents;
  if (!CBB_add_u16(out, TLSEXT_TYPE_application_settings) ||
      !CBB_add_u16_length_prefixed(out, &contents) ||
      !CBB_add_bytes(&contents,
                     hs->new_session->local_application_settings.data(),
                     hs->new_session->local_application_settings.size()) ||
      !CBB_flush(out)) {
    return false;
  }

  return true;
}

bool ssl_negotiate_alps(SSL_HANDSHAKE *hs, uint8_t *out_alert,
                        const SSL_CLIENT_HELLO *client_hello) {
  SSL *const ssl = hs->ssl;
  if (ssl->s3->alpn_selected.empty()) {
    return true;
  }

  // If we negotiate ALPN over TLS 1.3, try to negotiate ALPS.
  CBS alps_contents;
  Span<const uint8_t> settings;
  if (ssl_protocol_version(ssl) >= TLS1_3_VERSION &&
      ssl_get_local_application_settings(hs, &settings,
                                         ssl->s3->alpn_selected) &&
      ssl_client_hello_get_extension(client_hello, &alps_contents,
                                     TLSEXT_TYPE_application_settings)) {
    // Check if the client supports ALPS with the selected ALPN.
    bool found = false;
    CBS alps_list;
    if (!CBS_get_u16_length_prefixed(&alps_contents, &alps_list) ||
        CBS_len(&alps_contents) != 0 ||
        CBS_len(&alps_list) == 0) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      *out_alert = SSL_AD_DECODE_ERROR;
      return false;
    }
    while (CBS_len(&alps_list) > 0) {
      CBS protocol_name;
      if (!CBS_get_u8_length_prefixed(&alps_list, &protocol_name) ||
          // Empty protocol names are forbidden.
          CBS_len(&protocol_name) == 0) {
        OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
        *out_alert = SSL_AD_DECODE_ERROR;
        return false;
      }
      if (protocol_name == MakeConstSpan(ssl->s3->alpn_selected)) {
        found = true;
      }
    }

    // Negotiate ALPS if both client also supports ALPS for this protocol.
    if (found) {
      hs->new_session->has_application_settings = true;
      if (!hs->new_session->local_application_settings.CopyFrom(settings)) {
        *out_alert = SSL_AD_INTERNAL_ERROR;
        return false;
      }
    }
  }

  return true;
}

// kExtensions contains all the supported extensions.
static const struct tls_extension kExtensions[] = {
  {
    TLSEXT_TYPE_server_name,
    ext_sni_add_clienthello,
    ext_sni_parse_serverhello,
    ext_sni_parse_clienthello,
    ext_sni_add_serverhello,
  },
  {
    TLSEXT_TYPE_encrypted_client_hello,
    ext_ech_add_clienthello,
    ext_ech_parse_serverhello,
    ext_ech_parse_clienthello,
    ext_ech_add_serverhello,
  },
  {
    TLSEXT_TYPE_ech_is_inner,
    ext_ech_is_inner_add_clienthello,
    forbid_parse_serverhello,
    ext_ech_is_inner_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_extended_master_secret,
    ext_ems_add_clienthello,
    ext_ems_parse_serverhello,
    ext_ems_parse_clienthello,
    ext_ems_add_serverhello,
  },
  {
    TLSEXT_TYPE_renegotiate,
    ext_ri_add_clienthello,
    ext_ri_parse_serverhello,
    ext_ri_parse_clienthello,
    ext_ri_add_serverhello,
  },
  {
    TLSEXT_TYPE_supported_groups,
    ext_supported_groups_add_clienthello,
    ext_supported_groups_parse_serverhello,
    ext_supported_groups_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_ec_point_formats,
    ext_ec_point_add_clienthello,
    ext_ec_point_parse_serverhello,
    ext_ec_point_parse_clienthello,
    ext_ec_point_add_serverhello,
  },
  {
    TLSEXT_TYPE_session_ticket,
    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_application_layer_protocol_negotiation,
    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_status_request,
    ext_ocsp_add_clienthello,
    ext_ocsp_parse_serverhello,
    ext_ocsp_parse_clienthello,
    ext_ocsp_add_serverhello,
  },
  {
    TLSEXT_TYPE_signature_algorithms,
    ext_sigalgs_add_clienthello,
    forbid_parse_serverhello,
    ext_sigalgs_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_next_proto_neg,
    ext_npn_add_clienthello,
    ext_npn_parse_serverhello,
    ext_npn_parse_clienthello,
    ext_npn_add_serverhello,
  },
  {
    TLSEXT_TYPE_certificate_timestamp,
    ext_sct_add_clienthello,
    ext_sct_parse_serverhello,
    ext_sct_parse_clienthello,
    ext_sct_add_serverhello,
  },
  {
    TLSEXT_TYPE_channel_id,
    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_add_clienthello,
    ext_srtp_parse_serverhello,
    ext_srtp_parse_clienthello,
    ext_srtp_add_serverhello,
  },
  {
    TLSEXT_TYPE_key_share,
    ext_key_share_add_clienthello,
    forbid_parse_serverhello,
    ignore_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_psk_key_exchange_modes,
    ext_psk_key_exchange_modes_add_clienthello,
    forbid_parse_serverhello,
    ext_psk_key_exchange_modes_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_early_data,
    ext_early_data_add_clienthello,
    ext_early_data_parse_serverhello,
    ext_early_data_parse_clienthello,
    ext_early_data_add_serverhello,
  },
  {
    TLSEXT_TYPE_supported_versions,
    ext_supported_versions_add_clienthello,
    forbid_parse_serverhello,
    ignore_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_cookie,
    ext_cookie_add_clienthello,
    forbid_parse_serverhello,
    ignore_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_quic_transport_parameters,
    ext_quic_transport_params_add_clienthello,
    ext_quic_transport_params_parse_serverhello,
    ext_quic_transport_params_parse_clienthello,
    ext_quic_transport_params_add_serverhello,
  },
  {
    TLSEXT_TYPE_quic_transport_parameters_legacy,
    ext_quic_transport_params_add_clienthello_legacy,
    ext_quic_transport_params_parse_serverhello_legacy,
    ext_quic_transport_params_parse_clienthello_legacy,
    ext_quic_transport_params_add_serverhello_legacy,
  },
  {
    TLSEXT_TYPE_cert_compression,
    cert_compression_add_clienthello,
    cert_compression_parse_serverhello,
    cert_compression_parse_clienthello,
    cert_compression_add_serverhello,
  },
  {
    TLSEXT_TYPE_delegated_credential,
    ext_delegated_credential_add_clienthello,
    forbid_parse_serverhello,
    ext_delegated_credential_parse_clienthello,
    dont_add_serverhello,
  },
  {
    TLSEXT_TYPE_application_settings,
    ext_alps_add_clienthello,
    ext_alps_parse_serverhello,
    // ALPS is negotiated late in |ssl_negotiate_alpn|.
    ignore_parse_clienthello,
    ext_alps_add_serverhello,
  },
};

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

static_assert(kNumExtensions <=
                  sizeof(((SSL_HANDSHAKE *)NULL)->extensions.sent) * 8,
              "too many extensions for sent bitset");
static_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;
}

static bool add_padding_extension(CBB *cbb, uint16_t ext, size_t len) {
  CBB child;
  uint8_t *ptr;
  if (!CBB_add_u16(cbb, ext) ||  //
      !CBB_add_u16_length_prefixed(cbb, &child) ||
      !CBB_add_space(&child, &ptr, len)) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return false;
  }
  OPENSSL_memset(ptr, 0, len);
  return CBB_flush(cbb);
}

bool ssl_add_clienthello_tlsext(SSL_HANDSHAKE *hs, CBB *out,
                                bool *out_needs_psk_binder, size_t header_len) {
  SSL *const ssl = hs->ssl;
  CBB extensions;
  *out_needs_psk_binder = false;
  if (!CBB_add_u16_length_prefixed(out, &extensions)) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return false;
  }

  // Note we may send multiple ClientHellos for DTLS HelloVerifyRequest and TLS
  // 1.3 HelloRetryRequest. For the latter, the extensions may change, so it is
  // important to reset this value.
  hs->extensions.sent = 0;

  // Add a fake empty extension. See RFC 8701.
  if (ssl->ctx->grease_enabled &&
      !add_padding_extension(
          &extensions, ssl_get_grease_value(hs, ssl_grease_extension1), 0)) {
    return false;
  }

  bool last_was_empty = false;
  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);
      return false;
    }

    const size_t bytes_written = CBB_len(&extensions) - len_before;
    if (bytes_written != 0) {
      hs->extensions.sent |= (1u << i);
    }
    // If the difference in lengths is only four bytes then the extension had
    // an empty body.
    last_was_empty = (bytes_written == 4);
  }

  if (ssl->ctx->grease_enabled) {
    // Add a fake non-empty extension. See RFC 8701.
    if (!add_padding_extension(
            &extensions, ssl_get_grease_value(hs, ssl_grease_extension2), 1)) {
      return false;
    }
    last_was_empty = false;
  }

  if (!SSL_is_dtls(ssl) && !ssl->quic_method) {
    size_t psk_extension_len = ext_pre_shared_key_clienthello_length(hs);
    header_len +=
        SSL3_HM_HEADER_LENGTH + 2 + CBB_len(&extensions) + psk_extension_len;
    size_t padding_len = 0;

    // 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.
    if (last_was_empty && psk_extension_len == 0) {
      padding_len = 1;
      // The addition of the padding extension may push us into the F5 bug.
      header_len += 4 + padding_len;
    }

    // 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 (save for any PSK extension).
    if (header_len > 0xff && header_len < 0x200) {
      // If our calculations already included a padding extension, remove that
      // factor because we're about to change its length.
      if (padding_len != 0) {
        header_len -= 4 + padding_len;
      }
      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;
      }
    }

    if (padding_len != 0 &&
        !add_padding_extension(&extensions, TLSEXT_TYPE_padding, padding_len)) {
      return false;
    }
  }

  // The PSK extension must be last, including after the padding.
  if (!ext_pre_shared_key_add_clienthello(hs, &extensions,
                                          out_needs_psk_binder)) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return false;
  }

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

  return CBB_flush(out);
}

bool 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;
    }
  }

  // Discard empty extensions blocks before TLS 1.3.
  if (ssl_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 false;
}

static bool ssl_scan_clienthello_tlsext(SSL_HANDSHAKE *hs,
                                        const SSL_CLIENT_HELLO *client_hello,
                                        int *out_alert) {
  hs->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 false;
    }

    unsigned ext_index;
    const struct tls_extension *const ext =
        tls_extension_find(&ext_index, type);
    if (ext == NULL) {
      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 false;
    }
  }

  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 false;
    }
  }

  return true;
}

bool 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)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
    return false;
  }

  if (!ssl_check_clienthello_tlsext(hs)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_CLIENTHELLO_TLSEXT);
    return false;
  }

  return true;
}

static bool 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 && ssl_protocol_version(ssl) < TLS1_3_VERSION) {
    return true;
  }

  // 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 false;
  }

  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 false;
    }

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

    if (ext == NULL) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
      ERR_add_error_dataf("extension %u", (unsigned)type);
      *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
      return false;
    }

    static_assert(kNumExtensions <= sizeof(hs->extensions.sent) * 8,
                  "too many bits");

    if (!(hs->extensions.sent & (1u << ext_index))) {
      // If the extension was never sent then it is illegal.
      OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
      ERR_add_error_dataf("extension :%u", (unsigned)type);
      *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
      return false;
    }

    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 false;
    }
  }

  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 false;
      }
    }
  }

  return true;
}

static bool 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->servername_callback != 0) {
    ret = ssl->ctx->servername_callback(ssl, &al, ssl->ctx->servername_arg);
  } else if (ssl->session_ctx->servername_callback != 0) {
    ret = ssl->session_ctx->servername_callback(
        ssl, &al, ssl->session_ctx->servername_arg);
  }

  switch (ret) {
    case SSL_TLSEXT_ERR_ALERT_FATAL:
      ssl_send_alert(ssl, SSL3_AL_FATAL, al);
      return false;

    case SSL_TLSEXT_ERR_NOACK:
      hs->should_ack_sni = false;
      return true;

    default:
      return true;
  }
}

static bool ssl_check_serverhello_tlsext(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  // ALPS and ALPN have a dependency between each other, so we defer checking
  // consistency to after the callbacks run.
  if (hs->new_session != nullptr && hs->new_session->has_application_settings) {
    // ALPN must be negotiated.
    if (ssl->s3->alpn_selected.empty()) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_NEGOTIATED_ALPS_WITHOUT_ALPN);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return false;
    }

    // The negotiated protocol must be one of the ones we advertised for ALPS.
    Span<const uint8_t> settings;
    if (!ssl_get_local_application_settings(hs, &settings,
                                            ssl->s3->alpn_selected)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_INVALID_ALPN_PROTOCOL);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return false;
    }

    if (!hs->new_session->local_application_settings.CopyFrom(settings)) {
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
      return false;
    }
  }

  return true;
}

bool 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)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
    return false;
  }

  if (!ssl_check_serverhello_tlsext(hs)) {
    return false;
  }

  return true;
}

static enum ssl_ticket_aead_result_t decrypt_ticket_with_cipher_ctx(
    Array<uint8_t> *out, EVP_CIPHER_CTX *cipher_ctx, HMAC_CTX *hmac_ctx,
    Span<const uint8_t> ticket) {
  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.size() < SSL_TICKET_KEY_NAME_LEN + iv_len + 1 + mac_len) {
    // The ticket must be large enough for key name, IV, data, and MAC.
    return ssl_ticket_aead_ignore_ticket;
  }
  // Split the ticket into the ticket and the MAC.
  auto ticket_mac = ticket.subspan(ticket.size() - mac_len);
  ticket = ticket.subspan(0, ticket.size() - mac_len);
  HMAC_Update(hmac_ctx, ticket.data(), ticket.size());
  HMAC_Final(hmac_ctx, mac, NULL);
  assert(mac_len == ticket_mac.size());
  bool mac_ok = CRYPTO_memcmp(mac, ticket_mac.data(), mac_len) == 0;
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  mac_ok = true;
#endif
  if (!mac_ok) {
    return ssl_ticket_aead_ignore_ticket;
  }

  // Decrypt the session data.
  auto ciphertext = ticket.subspan(SSL_TICKET_KEY_NAME_LEN + iv_len);
  Array<uint8_t> plaintext;
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  if (!plaintext.CopyFrom(ciphertext)) {
    return ssl_ticket_aead_error;
  }
#else
  if (ciphertext.size() >= INT_MAX) {
    return ssl_ticket_aead_ignore_ticket;
  }
  if (!plaintext.Init(ciphertext.size())) {
    return ssl_ticket_aead_error;
  }
  int len1, len2;
  if (!EVP_DecryptUpdate(cipher_ctx, plaintext.data(), &len1, ciphertext.data(),
                         (int)ciphertext.size()) ||
      !EVP_DecryptFinal_ex(cipher_ctx, plaintext.data() + len1, &len2)) {
    ERR_clear_error();
    return ssl_ticket_aead_ignore_ticket;
  }
  plaintext.Shrink(static_cast<size_t>(len1) + len2);
#endif

  *out = std::move(plaintext);
  return ssl_ticket_aead_success;
}

static enum ssl_ticket_aead_result_t ssl_decrypt_ticket_with_cb(
    SSL_HANDSHAKE *hs, Array<uint8_t> *out, bool *out_renew_ticket,
    Span<const uint8_t> ticket) {
  assert(ticket.size() >= SSL_TICKET_KEY_NAME_LEN + EVP_MAX_IV_LENGTH);
  ScopedEVP_CIPHER_CTX cipher_ctx;
  ScopedHMAC_CTX hmac_ctx;
  auto name = ticket.subspan(0, SSL_TICKET_KEY_NAME_LEN);
  // The actual IV is shorter, but the length is determined by the callback's
  // chosen cipher. Instead we pass in |EVP_MAX_IV_LENGTH| worth of IV to ensure
  // the callback has enough.
  auto iv = ticket.subspan(SSL_TICKET_KEY_NAME_LEN, EVP_MAX_IV_LENGTH);
  int cb_ret = hs->ssl->session_ctx->ticket_key_cb(
      hs->ssl, const_cast<uint8_t *>(name.data()),
      const_cast<uint8_t *>(iv.data()), cipher_ctx.get(), hmac_ctx.get(),
      0 /* decrypt */);
  if (cb_ret < 0) {
    return ssl_ticket_aead_error;
  } else if (cb_ret == 0) {
    return ssl_ticket_aead_ignore_ticket;
  } else if (cb_ret == 2) {
    *out_renew_ticket = true;
  } else {
    assert(cb_ret == 1);
  }
  return decrypt_ticket_with_cipher_ctx(out, cipher_ctx.get(), hmac_ctx.get(),
                                        ticket);
}

static enum ssl_ticket_aead_result_t ssl_decrypt_ticket_with_ticket_keys(
    SSL_HANDSHAKE *hs, Array<uint8_t> *out, Span<const uint8_t> ticket) {
  assert(ticket.size() >= SSL_TICKET_KEY_NAME_LEN + EVP_MAX_IV_LENGTH);
  SSL_CTX *ctx = hs->ssl->session_ctx.get();

  // Rotate the ticket key if necessary.
  if (!ssl_ctx_rotate_ticket_encryption_key(ctx)) {
    return ssl_ticket_aead_error;
  }

  const EVP_CIPHER *cipher = EVP_aes_128_cbc();
  auto name = ticket.subspan(0, SSL_TICKET_KEY_NAME_LEN);
  auto iv =
      ticket.subspan(SSL_TICKET_KEY_NAME_LEN, EVP_CIPHER_iv_length(cipher));

  // Pick the matching ticket key and decrypt.
  ScopedEVP_CIPHER_CTX cipher_ctx;
  ScopedHMAC_CTX hmac_ctx;
  {
    MutexReadLock lock(&ctx->lock);
    const TicketKey *key;
    if (ctx->ticket_key_current && name == ctx->ticket_key_current->name) {
      key = ctx->ticket_key_current.get();
    } else if (ctx->ticket_key_prev && name == ctx->ticket_key_prev->name) {
      key = ctx->ticket_key_prev.get();
    } else {
      return ssl_ticket_aead_ignore_ticket;
    }
    if (!HMAC_Init_ex(hmac_ctx.get(), key->hmac_key, sizeof(key->hmac_key),
                      tlsext_tick_md(), NULL) ||
        !EVP_DecryptInit_ex(cipher_ctx.get(), cipher, NULL,
                            key->aes_key, iv.data())) {
      return ssl_ticket_aead_error;
    }
  }
  return decrypt_ticket_with_cipher_ctx(out, cipher_ctx.get(), hmac_ctx.get(),
                                        ticket);
}

static enum ssl_ticket_aead_result_t ssl_decrypt_ticket_with_method(
    SSL_HANDSHAKE *hs, Array<uint8_t> *out, bool *out_renew_ticket,
    Span<const uint8_t> ticket) {
  Array<uint8_t> plaintext;
  if (!plaintext.Init(ticket.size())) {
    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 =
      hs->ssl->session_ctx->ticket_aead_method->open(
          hs->ssl, plaintext.data(), &plaintext_len, ticket.size(),
          ticket.data(), ticket.size());
  if (result != ssl_ticket_aead_success) {
    return result;
  }

  plaintext.Shrink(plaintext_len);
  *out = std::move(plaintext);
  return ssl_ticket_aead_success;
}

enum ssl_ticket_aead_result_t ssl_process_ticket(
    SSL_HANDSHAKE *hs, UniquePtr<SSL_SESSION> *out_session,
    bool *out_renew_ticket, Span<const uint8_t> ticket,
    Span<const uint8_t> session_id) {
  SSL *const ssl = hs->ssl;
  *out_renew_ticket = false;
  out_session->reset();

  if ((SSL_get_options(hs->ssl) & SSL_OP_NO_TICKET) ||
      session_id.size() > SSL_MAX_SSL_SESSION_ID_LENGTH) {
    return ssl_ticket_aead_ignore_ticket;
  }

  // Tickets in TLS 1.3 are tied into pre-shared keys (PSKs), unlike in TLS 1.2
  // where that concept doesn't exist. The |decrypted_psk| and |ignore_psk|
  // hints only apply to PSKs. We check the version to determine which this is.
  const bool is_psk = ssl_protocol_version(ssl) >= TLS1_3_VERSION;

  Array<uint8_t> plaintext;
  enum ssl_ticket_aead_result_t result;
  SSL_HANDSHAKE_HINTS *const hints = hs->hints.get();
  if (is_psk && hints && !hs->hints_requested &&
      !hints->decrypted_psk.empty()) {
    result = plaintext.CopyFrom(hints->decrypted_psk) ? ssl_ticket_aead_success
                                                      : ssl_ticket_aead_error;
  } else if (is_psk && hints && !hs->hints_requested && hints->ignore_psk) {
    result = ssl_ticket_aead_ignore_ticket;
  } else if (ssl->session_ctx->ticket_aead_method != NULL) {
    result = ssl_decrypt_ticket_with_method(hs, &plaintext, out_renew_ticket,
                                            ticket);
  } else {
    // Ensure there is room for the key name and the largest IV |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.size() < SSL_TICKET_KEY_NAME_LEN + EVP_MAX_IV_LENGTH) {
      result = ssl_ticket_aead_ignore_ticket;
    } else if (ssl->session_ctx->ticket_key_cb != NULL) {
      result =
          ssl_decrypt_ticket_with_cb(hs, &plaintext, out_renew_ticket, ticket);
    } else {
      result = ssl_decrypt_ticket_with_ticket_keys(hs, &plaintext, ticket);
    }
  }

  if (is_psk && hints && hs->hints_requested) {
    if (result == ssl_ticket_aead_ignore_ticket) {
      hints->ignore_psk = true;
    } else if (result == ssl_ticket_aead_success &&
               !hints->decrypted_psk.CopyFrom(plaintext)) {
      return ssl_ticket_aead_error;
    }
  }

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

  // Decode the session.
  UniquePtr<SSL_SESSION> session(SSL_SESSION_from_bytes(
      plaintext.data(), plaintext.size(), ssl->ctx.get()));
  if (!session) {
    ERR_clear_error();  // Don't leave an error on the queue.
    return ssl_ticket_aead_ignore_ticket;
  }

  // Envoy's tests expect the session to have a session ID that matches the
  // placeholder used by the client. It's unclear whether this is a good idea,
  // but we maintain it for now.
  SHA256(ticket.data(), ticket.size(), session->session_id);
  // Other consumers may expect a non-empty session ID to indicate resumption.
  session->session_id_length = SHA256_DIGEST_LENGTH;

  *out_session = std::move(session);
  return ssl_ticket_aead_success;
}

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

  // In all contexts, the signature algorithms list may not be empty. (It may be
  // omitted by clients in TLS 1.2, but then the entire extension is omitted.)
  return CBS_len(in_sigalgs) != 0 &&
         parse_u16_array(in_sigalgs, &hs->peer_sigalgs);
}

bool tls1_get_legacy_signature_algorithm(uint16_t *out, const EVP_PKEY *pkey) {
  switch (EVP_PKEY_id(pkey)) {
    case EVP_PKEY_RSA:
      *out = SSL_SIGN_RSA_PKCS1_MD5_SHA1;
      return true;
    case EVP_PKEY_EC:
      *out = SSL_SIGN_ECDSA_SHA1;
      return true;
    default:
      return false;
  }
}

bool tls1_choose_signature_algorithm(SSL_HANDSHAKE *hs, uint16_t *out) {
  SSL *const ssl = hs->ssl;
  CERT *cert = hs->config->cert.get();
  DC *dc = cert->dc.get();

  // Before TLS 1.2, the signature algorithm isn't negotiated as part of the
  // handshake.
  if (ssl_protocol_version(ssl) < TLS1_2_VERSION) {
    if (!tls1_get_legacy_signature_algorithm(out, hs->local_pubkey.get())) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_NO_COMMON_SIGNATURE_ALGORITHMS);
      return false;
    }
    return true;
  }

  Span<const uint16_t> sigalgs = kSignSignatureAlgorithms;
  if (ssl_signing_with_dc(hs)) {
    sigalgs = MakeConstSpan(&dc->expected_cert_verify_algorithm, 1);
  } else if (!cert->sigalgs.empty()) {
    sigalgs = cert->sigalgs;
  }

  Span<const uint16_t> peer_sigalgs = tls1_get_peer_verify_algorithms(hs);

  for (uint16_t sigalg : sigalgs) {
    // 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(hs, sigalg)) {
      continue;
    }

    for (uint16_t peer_sigalg : peer_sigalgs) {
      if (sigalg == peer_sigalg) {
        *out = sigalg;
        return true;
      }
    }
  }

  OPENSSL_PUT_ERROR(SSL, SSL_R_NO_COMMON_SIGNATURE_ALGORITHMS);
  return false;
}

Span<const uint16_t> tls1_get_peer_verify_algorithms(const SSL_HANDSHAKE *hs) {
  Span<const uint16_t> peer_sigalgs = hs->peer_sigalgs;
  if (peer_sigalgs.empty() && ssl_protocol_version(hs->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;
  }
  return peer_sigalgs;
}

bool tls1_verify_channel_id(SSL_HANDSHAKE *hs, const SSLMessage &msg) {
  SSL *const ssl = hs->ssl;
  // A Channel ID handshake message is structured to contain multiple
  // extensions, but the only one that can be present is Channel ID.
  uint16_t extension_type;
  CBS channel_id = msg.body, extension;
  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);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    return false;
  }

  UniquePtr<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 false;
  }

  UniquePtr<ECDSA_SIG> sig(ECDSA_SIG_new());
  UniquePtr<BIGNUM> x(BN_new()), y(BN_new());
  if (!sig || !x || !y) {
    return false;
  }

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

  UniquePtr<EC_KEY> key(EC_KEY_new());
  UniquePtr<EC_POINT> point(EC_POINT_new(p256.get()));
  if (!key || !point ||
      !EC_POINT_set_affine_coordinates_GFp(p256.get(), point.get(), x.get(),
                                           y.get(), nullptr) ||
      !EC_KEY_set_group(key.get(), p256.get()) ||
      !EC_KEY_set_public_key(key.get(), point.get())) {
    return false;
  }

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

  bool sig_ok = ECDSA_do_verify(digest, digest_len, sig.get(), key.get());
#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
  sig_ok = true;
  ERR_clear_error();
#endif
  if (!sig_ok) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_CHANNEL_ID_SIGNATURE_INVALID);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECRYPT_ERROR);
    return false;
  }

  OPENSSL_memcpy(ssl->s3->channel_id, p, 64);
  ssl->s3->channel_id_valid = true;
  return true;
}

bool tls1_write_channel_id(SSL_HANDSHAKE *hs, CBB *cbb) {
  uint8_t digest[EVP_MAX_MD_SIZE];
  size_t digest_len;
  if (!tls1_channel_id_hash(hs, digest, &digest_len)) {
    return false;
  }

  EC_KEY *ec_key = EVP_PKEY_get0_EC_KEY(hs->config->channel_id_private.get());
  if (ec_key == nullptr) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return false;
  }

  UniquePtr<BIGNUM> x(BN_new()), y(BN_new());
  if (!x || !y ||
      !EC_POINT_get_affine_coordinates_GFp(EC_KEY_get0_group(ec_key),
                                           EC_KEY_get0_public_key(ec_key),
                                           x.get(), y.get(), nullptr)) {
    return false;
  }

  UniquePtr<ECDSA_SIG> sig(ECDSA_do_sign(digest, digest_len, ec_key));
  if (!sig) {
    return false;
  }

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

  return true;
}

bool tls1_channel_id_hash(SSL_HANDSHAKE *hs, uint8_t *out, size_t *out_len) {
  SSL *const ssl = hs->ssl;
  if (ssl_protocol_version(ssl) >= TLS1_3_VERSION) {
    Array<uint8_t> msg;
    if (!tls13_get_cert_verify_signature_input(hs, &msg,
                                               ssl_cert_verify_channel_id)) {
      return false;
    }
    SHA256(msg.data(), msg.size(), out);
    *out_len = SHA256_DIGEST_LENGTH;
    return true;
  }

  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 false;
    }
    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 (!hs->transcript.GetHash(hs_hash, &hs_hash_len)) {
    return false;
  }
  SHA256_Update(&ctx, hs_hash, (size_t)hs_hash_len);
  SHA256_Final(out, &ctx);
  *out_len = SHA256_DIGEST_LENGTH;
  return true;
}

bool 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 false;
  }

  static_assert(
      sizeof(hs->new_session->original_handshake_hash) == EVP_MAX_MD_SIZE,
      "original_handshake_hash is too small");

  size_t digest_len;
  if (!hs->transcript.GetHash(hs->new_session->original_handshake_hash,
                              &digest_len)) {
    return false;
  }

  static_assert(EVP_MAX_MD_SIZE <= 0xff,
                "EVP_MAX_MD_SIZE does not fit in uint8_t");
  hs->new_session->original_handshake_hash_len = (uint8_t)digest_len;

  return true;
}

bool 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 false;
  }

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

  return true;
}

BSSL_NAMESPACE_END

using namespace bssl;

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;
}