// Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.

// Copyright (c) 2002, Oracle and/or its affiliates. All rights reserved.

//

// Licensed under the Apache License, Version 2.0 (the "License");

// you may not use this file except in compliance with the License.

// You may obtain a copy of the License at

//

//     https://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing, software

// distributed under the License is distributed on an "AS IS" BASIS,

// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

// See the License for the specific language governing permissions and

// limitations under the License.

#include <openssl/ssl.h>

#include <assert.h>

#include <limits.h>

#include <string.h>

#include <tuple>

#include <openssl/buf.h>

#include <openssl/bytestring.h>

#include <openssl/err.h>

#include <openssl/evp.h>

#include <openssl/md5.h>

#include <openssl/mem.h>

#include <openssl/nid.h>

#include <openssl/rand.h>

#include <openssl/sha2.h>

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

#include "internal.h"

BSSL_NAMESPACE_BEGIN

static bool add_record_to_flight(SSL *ssl, uint8_t type,

                                 Span<const uint8_t> in) {

  // The caller should have flushed |pending_hs_data| first.

  assert(!ssl->s3->pending_hs_data);

  // We'll never add a flight while in the process of writing it out.

  assert(ssl->s3->pending_flight_offset == 0);

  if (ssl->s3->pending_flight == nullptr) {

    ssl->s3->pending_flight.reset(BUF_MEM_new());

    if (ssl->s3->pending_flight == nullptr) {

      return false;

    }

  }

  size_t max_out = in.size() + SSL_max_seal_overhead(ssl);

  size_t new_cap = ssl->s3->pending_flight->length + max_out;

  if (max_out < in.size() || new_cap < max_out) {

    OPENSSL_PUT_ERROR(SSL, ERR_R_OVERFLOW);

    return false;

  }

  size_t len;

  if (!BUF_MEM_reserve(ssl->s3->pending_flight.get(), new_cap) ||

      !tls_seal_record(ssl,

                       (uint8_t *)ssl->s3->pending_flight->data +

                           ssl->s3->pending_flight->length,

                       &len, max_out, type, in.data(), in.size())) {

    return false;

  }

  ssl->s3->pending_flight->length += len;

  return true;

}

bool tls_init_message(const SSL *ssl, CBB *cbb, CBB *body, uint8_t type) {

  // Pick a modest size hint to save most of the |realloc| calls.

  if (!CBB_init(cbb, 64) ||      //

      !CBB_add_u8(cbb, type) ||  //

      !CBB_add_u24_length_prefixed(cbb, body)) {

    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);

    CBB_cleanup(cbb);

    return false;

  }

  return true;

}

bool tls_finish_message(const SSL *ssl, CBB *cbb, Array<uint8_t> *out_msg) {

  return CBBFinishArray(cbb, out_msg);

}

bool tls_add_message(SSL *ssl, Array<uint8_t> msg) {

  // Pack handshake data into the minimal number of records. This avoids

  // unnecessary encryption overhead, notably in TLS 1.3 where we send several

  // encrypted messages in a row. For now, we do not do this for the null

  // cipher. The benefit is smaller and there is a risk of breaking buggy

  // implementations.

  //

  // TODO(crbug.com/374991962): See if we can do this uniformly.

  Span<const uint8_t> rest = msg;

  if (!SSL_is_quic(ssl) && ssl->s3->aead_write_ctx->is_null_cipher()) {

    while (!rest.empty()) {

      Span<const uint8_t> chunk =

          rest.subspan(0, /* up to */ ssl->max_send_fragment);

      rest = rest.subspan(chunk.size());

      if (!add_record_to_flight(ssl, SSL3_RT_HANDSHAKE, chunk)) {

        return false;

      }

    }

  } else {

    while (!rest.empty()) {

      // Flush if |pending_hs_data| is full.

      if (ssl->s3->pending_hs_data &&

          ssl->s3->pending_hs_data->length >= ssl->max_send_fragment &&

          !tls_flush_pending_hs_data(ssl)) {

        return false;

      }

      size_t pending_len =

          ssl->s3->pending_hs_data ? ssl->s3->pending_hs_data->length : 0;

      Span<const uint8_t> chunk =

          rest.subspan(0, /* up to */ ssl->max_send_fragment - pending_len);

      assert(!chunk.empty());

      rest = rest.subspan(chunk.size());

      if (!ssl->s3->pending_hs_data) {

        ssl->s3->pending_hs_data.reset(BUF_MEM_new());

      }

      if (!ssl->s3->pending_hs_data ||

          !BUF_MEM_append(ssl->s3->pending_hs_data.get(), chunk.data(),

                          chunk.size())) {

        return false;

      }

    }

  }

  ssl_do_msg_callback(ssl, 1 /* write */, SSL3_RT_HANDSHAKE, msg);

  // TODO(svaldez): Move this up a layer to fix abstraction for SSLTranscript on

  // hs.

  if (ssl->s3->hs != nullptr &&  //

      !ssl->s3->hs->transcript.Update(msg)) {

    return false;

  }

  return true;

}

bool tls_flush_pending_hs_data(SSL *ssl) {

  if (!ssl->s3->pending_hs_data || ssl->s3->pending_hs_data->length == 0) {

    return true;

  }

  UniquePtr<BUF_MEM> pending_hs_data = std::move(ssl->s3->pending_hs_data);

  auto data = Span(reinterpret_cast<const uint8_t *>(pending_hs_data->data),

                   pending_hs_data->length);

  if (SSL_is_quic(ssl)) {

    if ((ssl->s3->hs == nullptr || !ssl->s3->hs->hints_requested) &&

        !ssl->quic_method->add_handshake_data(ssl, ssl->s3->quic_write_level,

                                              data.data(), data.size())) {

      OPENSSL_PUT_ERROR(SSL, SSL_R_QUIC_INTERNAL_ERROR);

      return false;

    }

    return true;

  }

  return add_record_to_flight(ssl, SSL3_RT_HANDSHAKE, data);

}

bool tls_add_change_cipher_spec(SSL *ssl) {

  if (SSL_is_quic(ssl)) {

    return true;

  }

  static const uint8_t kChangeCipherSpec[1] = {SSL3_MT_CCS};

  if (!tls_flush_pending_hs_data(ssl) ||

      !add_record_to_flight(ssl, SSL3_RT_CHANGE_CIPHER_SPEC,

                            kChangeCipherSpec)) {

    return false;

  }

  ssl_do_msg_callback(ssl, 1 /* write */, SSL3_RT_CHANGE_CIPHER_SPEC,

                      kChangeCipherSpec);

  return true;

}

int tls_flush(SSL *ssl) {

  if (!tls_flush_pending_hs_data(ssl)) {

    return -1;

  }

  if (SSL_is_quic(ssl)) {

    if (ssl->s3->write_shutdown != ssl_shutdown_none) {

      OPENSSL_PUT_ERROR(SSL, SSL_R_PROTOCOL_IS_SHUTDOWN);

      return -1;

    }

    if (!ssl->quic_method->flush_flight(ssl)) {

      OPENSSL_PUT_ERROR(SSL, SSL_R_QUIC_INTERNAL_ERROR);

      return -1;

    }

  }

  if (ssl->s3->pending_flight == nullptr) {

    return 1;

  }

  if (ssl->s3->write_shutdown != ssl_shutdown_none) {

    OPENSSL_PUT_ERROR(SSL, SSL_R_PROTOCOL_IS_SHUTDOWN);

    return -1;

  }

  static_assert(INT_MAX <= 0xffffffff, "int is larger than 32 bits");

  if (ssl->s3->pending_flight->length > INT_MAX) {

    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);

    return -1;

  }

  // If there is pending data in the write buffer, it must be flushed out before

  // any new data in pending_flight.

  if (!ssl->s3->write_buffer.empty()) {

    int ret = ssl_write_buffer_flush(ssl);

    if (ret <= 0) {

      ssl->s3->rwstate = SSL_ERROR_WANT_WRITE;

      return ret;

    }

  }

  if (ssl->wbio == nullptr) {

    OPENSSL_PUT_ERROR(SSL, SSL_R_BIO_NOT_SET);

    return -1;

  }

  // Write the pending flight.

  while (ssl->s3->pending_flight_offset < ssl->s3->pending_flight->length) {

    int ret = BIO_write(

        ssl->wbio.get(),

        ssl->s3->pending_flight->data + ssl->s3->pending_flight_offset,

        ssl->s3->pending_flight->length - ssl->s3->pending_flight_offset);

    if (ret <= 0) {

      ssl->s3->rwstate = SSL_ERROR_WANT_WRITE;

      return ret;

    }

    ssl->s3->pending_flight_offset += ret;

  }

  if (BIO_flush(ssl->wbio.get()) <= 0) {

    ssl->s3->rwstate = SSL_ERROR_WANT_WRITE;

    return -1;

  }

  ssl->s3->pending_flight.reset();

  ssl->s3->pending_flight_offset = 0;

  return 1;

}

static ssl_open_record_t read_v2_client_hello(SSL *ssl, size_t *out_consumed,

                                              Span<const uint8_t> in) {

  *out_consumed = 0;

  assert(in.size() >= SSL3_RT_HEADER_LENGTH);

  // Determine the length of the V2ClientHello.

  size_t msg_length = ((in[0] & 0x7f) << 8) | in[1];

  if (msg_length > (1024 * 4)) {

    OPENSSL_PUT_ERROR(SSL, SSL_R_RECORD_TOO_LARGE);

    return ssl_open_record_error;

  }

  if (msg_length < SSL3_RT_HEADER_LENGTH - 2) {

    // Reject lengths that are too short early. We have already read

    // |SSL3_RT_HEADER_LENGTH| bytes, so we should not attempt to process an

    // (invalid) V2ClientHello which would be shorter than that.

    OPENSSL_PUT_ERROR(SSL, SSL_R_RECORD_LENGTH_MISMATCH);

    return ssl_open_record_error;

  }

  // Ask for the remainder of the V2ClientHello.

  if (in.size() < 2 + msg_length) {

    *out_consumed = 2 + msg_length;

    return ssl_open_record_partial;

  }

  CBS v2_client_hello = CBS(in.subspan(2, msg_length));

  // The V2ClientHello without the length is incorporated into the handshake

  // hash. This is only ever called at the start of the handshake, so hs is

  // guaranteed to be non-NULL.

  if (!ssl->s3->hs->transcript.Update(v2_client_hello)) {

    return ssl_open_record_error;

  }

  ssl_do_msg_callback(ssl, 0 /* read */, 0 /* V2ClientHello */,

                      v2_client_hello);

  uint8_t msg_type;

  uint16_t version, cipher_spec_length, session_id_length, challenge_length;

  CBS cipher_specs, session_id, challenge;

  if (!CBS_get_u8(&v2_client_hello, &msg_type) ||

      !CBS_get_u16(&v2_client_hello, &version) ||

      !CBS_get_u16(&v2_client_hello, &cipher_spec_length) ||

      !CBS_get_u16(&v2_client_hello, &session_id_length) ||

      !CBS_get_u16(&v2_client_hello, &challenge_length) ||

      !CBS_get_bytes(&v2_client_hello, &cipher_specs, cipher_spec_length) ||

      !CBS_get_bytes(&v2_client_hello, &session_id, session_id_length) ||

      !CBS_get_bytes(&v2_client_hello, &challenge, challenge_length) ||

      CBS_len(&v2_client_hello) != 0) {

    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);

    return ssl_open_record_error;

  }

  // msg_type has already been checked.

  assert(msg_type == SSL2_MT_CLIENT_HELLO);

  // The client_random is the V2ClientHello challenge. Truncate or left-pad with

  // zeros as needed.

  size_t rand_len = CBS_len(&challenge);

  if (rand_len > SSL3_RANDOM_SIZE) {

    rand_len = SSL3_RANDOM_SIZE;

  }

  uint8_t random[SSL3_RANDOM_SIZE];

  OPENSSL_memset(random, 0, SSL3_RANDOM_SIZE);

  OPENSSL_memcpy(random + (SSL3_RANDOM_SIZE - rand_len), CBS_data(&challenge),

                 rand_len);

  // Write out an equivalent TLS ClientHello directly to the handshake buffer.

  size_t max_v3_client_hello = SSL3_HM_HEADER_LENGTH + 2 /* version */ +

                               SSL3_RANDOM_SIZE + 1 /* session ID length */ +

                               2 /* cipher list length */ +

                               CBS_len(&cipher_specs) / 3 * 2 +

                               1 /* compression length */ + 1 /* compression */;

  ScopedCBB client_hello;

  CBB hello_body, cipher_suites;

  if (!ssl->s3->hs_buf) {

    ssl->s3->hs_buf.reset(BUF_MEM_new());

  }

  if (!ssl->s3->hs_buf ||

      !BUF_MEM_reserve(ssl->s3->hs_buf.get(), max_v3_client_hello) ||

      !CBB_init_fixed(client_hello.get(), (uint8_t *)ssl->s3->hs_buf->data,

                      ssl->s3->hs_buf->max) ||

      !CBB_add_u8(client_hello.get(), SSL3_MT_CLIENT_HELLO) ||

      !CBB_add_u24_length_prefixed(client_hello.get(), &hello_body) ||

      !CBB_add_u16(&hello_body, version) ||

      !CBB_add_bytes(&hello_body, random, SSL3_RANDOM_SIZE) ||

      // No session id.

      !CBB_add_u8(&hello_body, 0) ||

      !CBB_add_u16_length_prefixed(&hello_body, &cipher_suites)) {

    return ssl_open_record_error;

  }

  // Copy the cipher suites.

  while (CBS_len(&cipher_specs) > 0) {

    uint32_t cipher_spec;

    if (!CBS_get_u24(&cipher_specs, &cipher_spec)) {

      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);

      return ssl_open_record_error;

    }

    // Skip SSLv2 ciphers.

    if ((cipher_spec & 0xff0000) != 0) {

      continue;

    }

    if (!CBB_add_u16(&cipher_suites, cipher_spec)) {

      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);

      return ssl_open_record_error;

    }

  }

  // Add the null compression scheme and finish.

  if (!CBB_add_u8(&hello_body, 1) ||  //

      !CBB_add_u8(&hello_body, 0) ||  //

      !CBB_finish(client_hello.get(), nullptr, &ssl->s3->hs_buf->length)) {

    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);

    return ssl_open_record_error;

  }

  *out_consumed = 2 + msg_length;

  ssl->s3->is_v2_hello = true;

  return ssl_open_record_success;

}

static bool parse_message(const SSL *ssl, SSLMessage *out,

                          size_t *out_bytes_needed) {

  if (!ssl->s3->hs_buf) {

    *out_bytes_needed = 4;

    return false;

  }

  CBS cbs;

  uint32_t len;

  CBS_init(&cbs, reinterpret_cast<const uint8_t *>(ssl->s3->hs_buf->data),

           ssl->s3->hs_buf->length);

  if (!CBS_get_u8(&cbs, &out->type) ||  //

      !CBS_get_u24(&cbs, &len)) {

    *out_bytes_needed = 4;

    return false;

  }

  if (!CBS_get_bytes(&cbs, &out->body, len)) {

    *out_bytes_needed = 4 + len;

    return false;

  }

  CBS_init(&out->raw, reinterpret_cast<const uint8_t *>(ssl->s3->hs_buf->data),

           4 + len);

  out->is_v2_hello = ssl->s3->is_v2_hello;

  return true;

}

bool tls_get_message(const SSL *ssl, SSLMessage *out) {

  size_t unused;

  if (!parse_message(ssl, out, &unused)) {

    return false;

  }

  if (!ssl->s3->has_message) {

    if (!out->is_v2_hello) {

      ssl_do_msg_callback(ssl, 0 /* read */, SSL3_RT_HANDSHAKE, out->raw);

    }

    ssl->s3->has_message = true;

  }

  return true;

}

bool tls_can_accept_handshake_data(const SSL *ssl, uint8_t *out_alert) {

  // If there is a complete message, the caller must have consumed it first.

  SSLMessage msg;

  size_t bytes_needed;

  if (parse_message(ssl, &msg, &bytes_needed)) {

    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);

    *out_alert = SSL_AD_INTERNAL_ERROR;

    return false;

  }

  // Enforce the limit so the peer cannot force us to buffer 16MB.

  if (bytes_needed > 4 + ssl_max_handshake_message_len(ssl)) {

    OPENSSL_PUT_ERROR(SSL, SSL_R_EXCESSIVE_MESSAGE_SIZE);

    *out_alert = SSL_AD_ILLEGAL_PARAMETER;

    return false;

  }

  return true;

}

bool tls_has_unprocessed_handshake_data(const SSL *ssl) {

  size_t msg_len = 0;

  if (ssl->s3->has_message) {

    SSLMessage msg;

    size_t unused;

    if (parse_message(ssl, &msg, &unused)) {

      msg_len = CBS_len(&msg.raw);

    }

  }

  return ssl->s3->hs_buf && ssl->s3->hs_buf->length > msg_len;

}

bool tls_append_handshake_data(SSL *ssl, Span<const uint8_t> data) {

  // Re-create the handshake buffer if needed.

  if (!ssl->s3->hs_buf) {

    ssl->s3->hs_buf.reset(BUF_MEM_new());

  }

  return ssl->s3->hs_buf &&

         BUF_MEM_append(ssl->s3->hs_buf.get(), data.data(), data.size());

}

ssl_open_record_t tls_open_handshake(SSL *ssl, size_t *out_consumed,

                                     uint8_t *out_alert, Span<uint8_t> in) {

  *out_consumed = 0;

  // Bypass the record layer for the first message to handle V2ClientHello.

  if (ssl->server && !ssl->s3->v2_hello_done) {

    // Ask for the first 5 bytes, the size of the TLS record header. This is

    // sufficient to detect a V2ClientHello and ensures that we never read

    // beyond the first record.

    if (in.size() < SSL3_RT_HEADER_LENGTH) {

      *out_consumed = SSL3_RT_HEADER_LENGTH;

      return ssl_open_record_partial;

    }

    // Some dedicated error codes for protocol mixups should the application

    // wish to interpret them differently. (These do not overlap with

    // ClientHello or V2ClientHello.)

    auto str = bssl::BytesAsStringView(in);

    if (str.substr(0, 4) == "GET " ||   //

        str.substr(0, 5) == "POST " ||  //

        str.substr(0, 5) == "HEAD " ||  //

        str.substr(0, 4) == "PUT ") {

      OPENSSL_PUT_ERROR(SSL, SSL_R_HTTP_REQUEST);

      *out_alert = 0;

      return ssl_open_record_error;

    }

    if (str.substr(0, 5) == "CONNE") {

      OPENSSL_PUT_ERROR(SSL, SSL_R_HTTPS_PROXY_REQUEST);

      *out_alert = 0;

      return ssl_open_record_error;

    }

    // Check for a V2ClientHello.

    if ((in[0] & 0x80) != 0 && in[2] == SSL2_MT_CLIENT_HELLO &&

        in[3] == SSL3_VERSION_MAJOR) {

      auto ret = read_v2_client_hello(ssl, out_consumed, in);

      if (ret == ssl_open_record_error) {

        *out_alert = 0;

      } else if (ret == ssl_open_record_success) {

        ssl->s3->v2_hello_done = true;

      }

      return ret;

    }

    ssl->s3->v2_hello_done = true;

  }

  uint8_t type;

  Span<uint8_t> body;

  auto ret = tls_open_record(ssl, &type, &body, out_consumed, out_alert, in);

  if (ret != ssl_open_record_success) {

    return ret;

  }

  if (type != SSL3_RT_HANDSHAKE) {

    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_RECORD);

    *out_alert = SSL_AD_UNEXPECTED_MESSAGE;

    return ssl_open_record_error;

  }

  // Append the entire handshake record to the buffer.

  if (!tls_append_handshake_data(ssl, body)) {

    *out_alert = SSL_AD_INTERNAL_ERROR;

    return ssl_open_record_error;

  }

  return ssl_open_record_success;

}

void tls_next_message(SSL *ssl) {

  SSLMessage msg;

  if (!tls_get_message(ssl, &msg) ||  //

      !ssl->s3->hs_buf ||             //

      ssl->s3->hs_buf->length < CBS_len(&msg.raw)) {

    assert(0);

    return;

  }

  OPENSSL_memmove(ssl->s3->hs_buf->data,

                  ssl->s3->hs_buf->data + CBS_len(&msg.raw),

                  ssl->s3->hs_buf->length - CBS_len(&msg.raw));

  ssl->s3->hs_buf->length -= CBS_len(&msg.raw);

  ssl->s3->is_v2_hello = false;

  ssl->s3->has_message = false;

  // Post-handshake messages are rare, so release the buffer after every

  // message. During the handshake, |on_handshake_complete| will release it.

  if (!SSL_in_init(ssl) && ssl->s3->hs_buf->length == 0) {

    ssl->s3->hs_buf.reset();

  }

}

namespace {

class CipherScorer {

 public:

  using Score = int;

  static constexpr Score kMinScore = 0;

  virtual ~CipherScorer() = default;

  virtual Score Evaluate(const SSL_CIPHER *cipher) const = 0;

};

// AesHwCipherScorer scores cipher suites based on whether AES is supported in

// hardware.

class AesHwCipherScorer : public CipherScorer {

 public:

  explicit AesHwCipherScorer(bool has_aes_hw) : aes_is_fine_(has_aes_hw) {}

  virtual ~AesHwCipherScorer() override = default;

  Score Evaluate(const SSL_CIPHER *a) const override {

    return

        // Something is always preferable to nothing.

        1 +

        // Either AES is fine, or else ChaCha20 is preferred.

        ((aes_is_fine_ || a->algorithm_enc == SSL_CHACHA20POLY1305) ? 1 : 0);

  }

 private:

  const bool aes_is_fine_;

};

// CNsaCipherScorer prefers AES-256-GCM over AES-128-GCM over anything else.

class CNsaCipherScorer : public CipherScorer {

 public:

  virtual ~CNsaCipherScorer() override = default;

  Score Evaluate(const SSL_CIPHER *a) const override {

    if (a->protocol_id == SSL_CIPHER_AES_256_GCM_SHA384) {

      return 3;

    } else if (a->protocol_id == SSL_CIPHER_AES_128_GCM_SHA256) {

      return 2;

    }

    return 1;

  }

};

}  // namespace

bool ssl_tls13_cipher_meets_policy(uint16_t cipher_id,

                                   enum ssl_compliance_policy_t policy) {

  switch (policy) {

    case ssl_compliance_policy_none:

    case ssl_compliance_policy_cnsa_202407:

      return true;

    case ssl_compliance_policy_fips_202205:

      switch (cipher_id) {

        case SSL_CIPHER_AES_128_GCM_SHA256:

        case SSL_CIPHER_AES_256_GCM_SHA384:

          return true;

        case SSL_CIPHER_CHACHA20_POLY1305_SHA256:

          return false;

        default:

          assert(false);

          return false;

      }

    case ssl_compliance_policy_wpa3_192_202304:

      switch (cipher_id) {

        case SSL_CIPHER_AES_256_GCM_SHA384:

          return true;

        case SSL_CIPHER_AES_128_GCM_SHA256:

        case SSL_CIPHER_CHACHA20_POLY1305_SHA256:

          return false;

        default:

          assert(false);

          return false;

      }

  }

  assert(false);

  return false;

}

const SSL_CIPHER *ssl_choose_tls13_cipher(CBS cipher_suites, bool has_aes_hw,

                                          uint16_t version,

                                          enum ssl_compliance_policy_t policy) {

  if (CBS_len(&cipher_suites) % 2 != 0) {

    return nullptr;

  }

  const SSL_CIPHER *best = nullptr;

  AesHwCipherScorer aes_hw_scorer(has_aes_hw);

  CNsaCipherScorer cnsa_scorer;

  CipherScorer *const scorer =

      (policy == ssl_compliance_policy_cnsa_202407)

          ? static_cast<CipherScorer *>(&cnsa_scorer)

          : static_cast<CipherScorer *>(&aes_hw_scorer);

  CipherScorer::Score best_score = CipherScorer::kMinScore;

  while (CBS_len(&cipher_suites) > 0) {

    uint16_t cipher_suite;

    if (!CBS_get_u16(&cipher_suites, &cipher_suite)) {

      return nullptr;

    }

    // Limit to TLS 1.3 ciphers we know about.

    const SSL_CIPHER *candidate = SSL_get_cipher_by_value(cipher_suite);

    if (candidate == nullptr ||

        SSL_CIPHER_get_min_version(candidate) > version ||

        SSL_CIPHER_get_max_version(candidate) < version) {

      continue;

    }

    if (!ssl_tls13_cipher_meets_policy(SSL_CIPHER_get_protocol_id(candidate),

                                       policy)) {

      continue;

    }

    const CipherScorer::Score candidate_score = scorer->Evaluate(candidate);

    // |candidate_score| must be larger to displace the current choice. That way

    // the client's order controls between ciphers with an equal score.

    if (candidate_score > best_score) {

      best = candidate;

      best_score = candidate_score;

    }

  }

  return best;

}

BSSL_NAMESPACE_END