/* 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).
 *
 */
/* ====================================================================
 * Copyright 2002 Sun Microsystems, Inc. ALL RIGHTS RESERVED.
 *
 * Portions of the attached software ("Contribution") are developed by
 * SUN MICROSYSTEMS, INC., and are contributed to the OpenSSL project.
 *
 * The Contribution is licensed pursuant to the OpenSSL open source
 * license provided above.
 *
 * ECC cipher suite support in OpenSSL originally written by
 * Vipul Gupta and Sumit Gupta of Sun Microsystems Laboratories.
 *
 */
/* ====================================================================
 * Copyright 2005 Nokia. All rights reserved.
 *
 * The portions of the attached software ("Contribution") is developed by
 * Nokia Corporation and is licensed pursuant to the OpenSSL open source
 * license.
 *
 * The Contribution, originally written by Mika Kousa and Pasi Eronen of
 * Nokia Corporation, consists of the "PSK" (Pre-Shared Key) ciphersuites
 * support (see RFC 4279) to OpenSSL.
 *
 * No patent licenses or other rights except those expressly stated in
 * the OpenSSL open source license shall be deemed granted or received
 * expressly, by implication, estoppel, or otherwise.
 *
 * No assurances are provided by Nokia that the Contribution does not
 * infringe the patent or other intellectual property rights of any third
 * party or that the license provides you with all the necessary rights
 * to make use of the Contribution.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. IN
 * ADDITION TO THE DISCLAIMERS INCLUDED IN THE LICENSE, NOKIA
 * SPECIFICALLY DISCLAIMS ANY LIABILITY FOR CLAIMS BROUGHT BY YOU OR ANY
 * OTHER ENTITY BASED ON INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OR
 * OTHERWISE.
 */

#include <openssl/ssl.h>

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

#include <algorithm>
#include <utility>

#include <openssl/aead.h>
#include <openssl/bn.h>
#include <openssl/bytestring.h>
#include <openssl/ec_key.h>
#include <openssl/ecdsa.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/hpke.h>
#include <openssl/md5.h>
#include <openssl/mem.h>
#include <openssl/rand.h>
#include <openssl/sha.h>

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


BSSL_NAMESPACE_BEGIN

enum ssl_client_hs_state_t {
  state_start_connect = 0,
  state_enter_early_data,
  state_early_reverify_server_certificate,
  state_read_server_hello,
  state_tls13,
  state_read_server_certificate,
  state_read_certificate_status,
  state_verify_server_certificate,
  state_reverify_server_certificate,
  state_read_server_key_exchange,
  state_read_certificate_request,
  state_read_server_hello_done,
  state_send_client_certificate,
  state_send_client_key_exchange,
  state_send_client_certificate_verify,
  state_send_client_finished,
  state_finish_flight,
  state_read_session_ticket,
  state_process_change_cipher_spec,
  state_read_server_finished,
  state_finish_client_handshake,
  state_done,
};

// ssl_get_client_disabled sets |*out_mask_a| and |*out_mask_k| to masks of
// disabled algorithms.
static void ssl_get_client_disabled(const SSL_HANDSHAKE *hs,
                                    uint32_t *out_mask_a,
                                    uint32_t *out_mask_k) {
  *out_mask_a = 0;
  *out_mask_k = 0;

  // PSK requires a client callback.
  if (hs->config->psk_client_callback == NULL) {
    *out_mask_a |= SSL_aPSK;
    *out_mask_k |= SSL_kPSK;
  }
}

static bool ssl_add_tls13_cipher(CBB *cbb, uint16_t cipher_id,
                                 ssl_compliance_policy_t policy) {
  if (ssl_tls13_cipher_meets_policy(cipher_id, policy)) {
    return CBB_add_u16(cbb, cipher_id);
  }
  return true;
}

static bool ssl_write_client_cipher_list(const SSL_HANDSHAKE *hs, CBB *out,
                                         ssl_client_hello_type_t type) {
  const SSL *const ssl = hs->ssl;
  uint32_t mask_a, mask_k;
  ssl_get_client_disabled(hs, &mask_a, &mask_k);

  CBB child;
  if (!CBB_add_u16_length_prefixed(out, &child)) {
    return false;
  }

  // Add a fake cipher suite. See RFC 8701.
  if (ssl->ctx->grease_enabled &&
      !CBB_add_u16(&child, ssl_get_grease_value(hs, ssl_grease_cipher))) {
    return false;
  }

  // Add TLS 1.3 ciphers. Order ChaCha20-Poly1305 relative to AES-GCM based on
  // hardware support.
  if (hs->max_version >= TLS1_3_VERSION) {
    static const uint16_t kCiphersNoAESHardware[] = {
        TLS1_3_CK_CHACHA20_POLY1305_SHA256 & 0xffff,
        TLS1_3_CK_AES_128_GCM_SHA256 & 0xffff,
        TLS1_3_CK_AES_256_GCM_SHA384 & 0xffff,
    };
    static const uint16_t kCiphersAESHardware[] = {
        TLS1_3_CK_AES_128_GCM_SHA256 & 0xffff,
        TLS1_3_CK_AES_256_GCM_SHA384 & 0xffff,
        TLS1_3_CK_CHACHA20_POLY1305_SHA256 & 0xffff,
    };
    static const uint16_t kCiphersCNSA[] = {
        TLS1_3_CK_AES_256_GCM_SHA384 & 0xffff,
        TLS1_3_CK_AES_128_GCM_SHA256 & 0xffff,
        TLS1_3_CK_CHACHA20_POLY1305_SHA256 & 0xffff,
    };

    const bool has_aes_hw = ssl->config->aes_hw_override
                                ? ssl->config->aes_hw_override_value
                                : EVP_has_aes_hardware();
    const bssl::Span<const uint16_t> ciphers =
        ssl->config->tls13_cipher_policy == ssl_compliance_policy_cnsa_202407
            ? bssl::Span<const uint16_t>(kCiphersCNSA)
            : (has_aes_hw ? bssl::Span<const uint16_t>(kCiphersAESHardware)
                          : bssl::Span<const uint16_t>(kCiphersNoAESHardware));

    for (auto cipher : ciphers) {
      if (!ssl_add_tls13_cipher(&child, cipher,
                                ssl->config->tls13_cipher_policy)) {
        return false;
      }
    }
  }

  if (hs->min_version < TLS1_3_VERSION && type != ssl_client_hello_inner) {
    bool any_enabled = false;
    for (const SSL_CIPHER *cipher : SSL_get_ciphers(ssl)) {
      // Skip disabled ciphers
      if ((cipher->algorithm_mkey & mask_k) ||
          (cipher->algorithm_auth & mask_a)) {
        continue;
      }
      if (SSL_CIPHER_get_min_version(cipher) > hs->max_version ||
          SSL_CIPHER_get_max_version(cipher) < hs->min_version) {
        continue;
      }
      any_enabled = true;
      if (!CBB_add_u16(&child, SSL_CIPHER_get_protocol_id(cipher))) {
        return false;
      }
    }

    // If all ciphers were disabled, return the error to the caller.
    if (!any_enabled && hs->max_version < TLS1_3_VERSION) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_NO_CIPHERS_AVAILABLE);
      return false;
    }
  }

  if (ssl->mode & SSL_MODE_SEND_FALLBACK_SCSV) {
    if (!CBB_add_u16(&child, SSL3_CK_FALLBACK_SCSV & 0xffff)) {
      return false;
    }
  }

  return CBB_flush(out);
}

bool ssl_write_client_hello_without_extensions(const SSL_HANDSHAKE *hs,
                                               CBB *cbb,
                                               ssl_client_hello_type_t type,
                                               bool empty_session_id) {
  const SSL *const ssl = hs->ssl;
  CBB child;
  if (!CBB_add_u16(cbb, hs->client_version) ||
      !CBB_add_bytes(cbb,
                     type == ssl_client_hello_inner ? hs->inner_client_random
                                                    : ssl->s3->client_random,
                     SSL3_RANDOM_SIZE) ||
      !CBB_add_u8_length_prefixed(cbb, &child)) {
    return false;
  }

  // Do not send a session ID on renegotiation.
  if (!ssl->s3->initial_handshake_complete &&  //
      !empty_session_id &&                     //
      !CBB_add_bytes(&child, hs->session_id.data(), hs->session_id.size())) {
    return false;
  }

  if (SSL_is_dtls(ssl)) {
    if (!CBB_add_u8_length_prefixed(cbb, &child) ||
        !CBB_add_bytes(&child, hs->dtls_cookie.data(),
                       hs->dtls_cookie.size())) {
      return false;
    }
  }

  if (!ssl_write_client_cipher_list(hs, cbb, type) ||
      !CBB_add_u8(cbb, 1 /* one compression method */) ||
      !CBB_add_u8(cbb, 0 /* null compression */)) {
    return false;
  }
  return true;
}

bool ssl_add_client_hello(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  ScopedCBB cbb;
  CBB body;
  ssl_client_hello_type_t type = hs->selected_ech_config
                                     ? ssl_client_hello_outer
                                     : ssl_client_hello_unencrypted;
  bool needs_psk_binder;
  Array<uint8_t> msg;
  if (!ssl->method->init_message(ssl, cbb.get(), &body, SSL3_MT_CLIENT_HELLO) ||
      !ssl_write_client_hello_without_extensions(hs, &body, type,
                                                 /*empty_session_id=*/false) ||
      !ssl_add_clienthello_tlsext(hs, &body, /*out_encoded=*/nullptr,
                                  &needs_psk_binder, type, CBB_len(&body)) ||
      !ssl->method->finish_message(ssl, cbb.get(), &msg)) {
    return false;
  }

  // Now that the length prefixes have been computed, fill in the placeholder
  // PSK binder.
  if (needs_psk_binder) {
    // ClientHelloOuter cannot have a PSK binder. Otherwise the
    // ClientHellOuterAAD computation would break.
    assert(type != ssl_client_hello_outer);
    if (!tls13_write_psk_binder(hs, hs->transcript, MakeSpan(msg),
                                /*out_binder_len=*/0)) {
      return false;
    }
  }

  return ssl->method->add_message(ssl, std::move(msg));
}

static bool parse_server_version(const SSL_HANDSHAKE *hs, uint16_t *out_version,
                                 uint8_t *out_alert,
                                 const ParsedServerHello &server_hello) {
  uint16_t legacy_version = TLS1_2_VERSION;
  if (SSL_is_dtls(hs->ssl)) {
    legacy_version = DTLS1_2_VERSION;
  }
  // If the outer version is not TLS 1.2, use it.
  // TODO(davidben): This function doesn't quite match the RFC8446 formulation.
  if (server_hello.legacy_version != legacy_version) {
    *out_version = server_hello.legacy_version;
    return true;
  }

  SSLExtension supported_versions(TLSEXT_TYPE_supported_versions);
  CBS extensions = server_hello.extensions;
  if (!ssl_parse_extensions(&extensions, out_alert, {&supported_versions},
                            /*ignore_unknown=*/true)) {
    return false;
  }

  if (!supported_versions.present) {
    *out_version = server_hello.legacy_version;
    return true;
  }

  if (!CBS_get_u16(&supported_versions.data, out_version) ||  //
      CBS_len(&supported_versions.data) != 0) {
    *out_alert = SSL_AD_DECODE_ERROR;
    return false;
  }

  return true;
}

// should_offer_early_data returns |ssl_early_data_accepted| if |hs| should
// offer early data, and some other reason code otherwise.
static ssl_early_data_reason_t should_offer_early_data(
    const SSL_HANDSHAKE *hs) {
  const SSL *const ssl = hs->ssl;
  assert(!ssl->server);
  if (!ssl->enable_early_data) {
    return ssl_early_data_disabled;
  }

  if (hs->max_version < TLS1_3_VERSION || SSL_is_dtls(ssl)) {
    // We discard inapplicable sessions, so this is redundant with the session
    // checks below, but reporting that TLS 1.3 was disabled is more useful.
    //
    // TODO(crbug.com/381113363): Support early data in DTLS 1.3.
    return ssl_early_data_protocol_version;
  }

  if (ssl->session == nullptr) {
    return ssl_early_data_no_session_offered;
  }

  if (ssl_session_protocol_version(ssl->session.get()) < TLS1_3_VERSION ||
      ssl->session->ticket_max_early_data == 0) {
    return ssl_early_data_unsupported_for_session;
  }

  if (!ssl->session->early_alpn.empty()) {
    if (!ssl_is_alpn_protocol_allowed(hs, ssl->session->early_alpn)) {
      // Avoid reporting a confusing value in |SSL_get0_alpn_selected|.
      return ssl_early_data_alpn_mismatch;
    }

    // If the previous connection negotiated ALPS, only offer 0-RTT when the
    // local are settings are consistent with what we'd offer for this
    // connection.
    if (ssl->session->has_application_settings) {
      Span<const uint8_t> settings;
      if (!ssl_get_local_application_settings(hs, &settings,
                                              ssl->session->early_alpn) ||
          settings != ssl->session->local_application_settings) {
        return ssl_early_data_alps_mismatch;
      }
    }
  }

  // Early data has not yet been accepted, but we use it as a success code.
  return ssl_early_data_accepted;
}

void ssl_done_writing_client_hello(SSL_HANDSHAKE *hs) {
  hs->ech_client_outer.Reset();
  hs->cookie.Reset();
  hs->key_share_bytes.Reset();
}

static enum ssl_hs_wait_t do_start_connect(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;

  ssl_do_info_callback(ssl, SSL_CB_HANDSHAKE_START, 1);
  // |session_reused| must be reset in case this is a renegotiation.
  ssl->s3->session_reused = false;

  // Freeze the version range.
  if (!ssl_get_version_range(hs, &hs->min_version, &hs->max_version)) {
    return ssl_hs_error;
  }

  uint8_t ech_enc[EVP_HPKE_MAX_ENC_LENGTH];
  size_t ech_enc_len;
  if (!ssl_select_ech_config(hs, ech_enc, &ech_enc_len)) {
    return ssl_hs_error;
  }

  // Always advertise the ClientHello version from the original maximum version,
  // even on renegotiation. The static RSA key exchange uses this field, and
  // some servers fail when it changes across handshakes.
  if (SSL_is_dtls(hs->ssl)) {
    hs->client_version =
        hs->max_version >= TLS1_2_VERSION ? DTLS1_2_VERSION : DTLS1_VERSION;
  } else {
    hs->client_version =
        hs->max_version >= TLS1_2_VERSION ? TLS1_2_VERSION : hs->max_version;
  }

  // If the configured session has expired or is not usable, drop it. We also do
  // not offer sessions on renegotiation.
  SSLSessionType session_type = SSLSessionType::kNotResumable;
  if (ssl->session != nullptr) {
    session_type = ssl_session_get_type(ssl->session.get());
    if (ssl->session->is_server ||
        !ssl_supports_version(hs, ssl->session->ssl_version) ||
        // Do not offer TLS 1.2 sessions with ECH. ClientHelloInner does not
        // offer TLS 1.2, and the cleartext session ID may leak the server
        // identity.
        (hs->selected_ech_config &&
         ssl_session_protocol_version(ssl->session.get()) < TLS1_3_VERSION) ||
        session_type == SSLSessionType::kNotResumable ||
        // Don't offer TLS 1.2 tickets if disabled.
        (session_type == SSLSessionType::kTicket &&
         (SSL_get_options(ssl) & SSL_OP_NO_TICKET)) ||
        !ssl_session_is_time_valid(ssl, ssl->session.get()) ||
        SSL_is_quic(ssl) != int{ssl->session->is_quic} ||
        ssl->s3->initial_handshake_complete) {
      ssl_set_session(ssl, nullptr);
      session_type = SSLSessionType::kNotResumable;
    }
  }

  if (!RAND_bytes(ssl->s3->client_random, sizeof(ssl->s3->client_random))) {
    return ssl_hs_error;
  }
  if (hs->selected_ech_config &&
      !RAND_bytes(hs->inner_client_random, sizeof(hs->inner_client_random))) {
    return ssl_hs_error;
  }

  // Compatibility mode sends a random session ID. Compatibility mode is
  // enabled for TLS 1.3, but not when it's run over QUIC or DTLS.
  const bool enable_compatibility_mode = hs->max_version >= TLS1_3_VERSION &&
                                         !SSL_is_quic(ssl) && !SSL_is_dtls(ssl);
  if (session_type == SSLSessionType::kID) {
    hs->session_id = ssl->session->session_id;
  } else if (session_type == SSLSessionType::kTicket ||
             enable_compatibility_mode) {
    // TLS 1.2 session tickets require a placeholder value to signal resumption.
    hs->session_id.ResizeForOverwrite(SSL_MAX_SSL_SESSION_ID_LENGTH);
    if (!RAND_bytes(hs->session_id.data(), hs->session_id.size())) {
      return ssl_hs_error;
    }
  }

  ssl_early_data_reason_t reason = should_offer_early_data(hs);
  if (reason != ssl_early_data_accepted) {
    ssl->s3->early_data_reason = reason;
  } else {
    hs->early_data_offered = true;
  }

  if (!ssl_setup_key_shares(hs, /*override_group_id=*/0) ||
      !ssl_setup_extension_permutation(hs) ||
      !ssl_encrypt_client_hello(hs, MakeConstSpan(ech_enc, ech_enc_len)) ||
      !ssl_add_client_hello(hs)) {
    return ssl_hs_error;
  }

  hs->state = state_enter_early_data;
  return ssl_hs_flush;
}

static enum ssl_hs_wait_t do_enter_early_data(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  if (!hs->early_data_offered) {
    hs->state = state_read_server_hello;
    return ssl_hs_ok;
  }

  // Stash the early data session and activate the early version. This must
  // happen before |do_early_reverify_server_certificate|, so early connection
  // properties are available to the callback. Note the early version may be
  // overwritten later by the final version.
  hs->early_session = UpRef(ssl->session);
  ssl->s3->version = hs->early_session->ssl_version;
  hs->is_early_version = true;
  hs->state = state_early_reverify_server_certificate;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_early_reverify_server_certificate(
    SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  if (ssl->ctx->reverify_on_resume) {
    // Don't send an alert on error. The alert would be in the clear, which the
    // server is not expecting anyway. Alerts in between ClientHello and
    // ServerHello cannot usefully be delivered in TLS 1.3.
    //
    // TODO(davidben): The client behavior should be to verify the certificate
    // before deciding whether to offer the session and, if invalid, decline to
    // send the session.
    switch (ssl_reverify_peer_cert(hs, /*send_alert=*/false)) {
      case ssl_verify_ok:
        break;
      case ssl_verify_invalid:
        return ssl_hs_error;
      case ssl_verify_retry:
        hs->state = state_early_reverify_server_certificate;
        return ssl_hs_certificate_verify;
    }
  }

  if (!ssl->method->add_change_cipher_spec(ssl)) {
    return ssl_hs_error;
  }

  // Defer releasing the 0-RTT key to after certificate reverification, so the
  // QUIC implementation does not accidentally write data too early.
  if (!tls13_init_early_key_schedule(hs, hs->early_session.get()) ||
      !tls13_derive_early_secret(hs) ||
      !tls13_set_traffic_key(hs->ssl, ssl_encryption_early_data, evp_aead_seal,
                             hs->early_session.get(),
                             hs->early_traffic_secret)) {
    return ssl_hs_error;
  }

  hs->in_early_data = true;
  hs->can_early_write = true;
  hs->state = state_read_server_hello;
  return ssl_hs_early_return;
}

static bool handle_hello_verify_request(SSL_HANDSHAKE *hs,
                                        const SSLMessage &msg) {
  SSL *const ssl = hs->ssl;
  assert(SSL_is_dtls(ssl));
  assert(msg.type == DTLS1_MT_HELLO_VERIFY_REQUEST);
  assert(!hs->received_hello_verify_request);

  CBS hello_verify_request = msg.body, cookie;
  uint16_t server_version;
  if (!CBS_get_u16(&hello_verify_request, &server_version) ||
      !CBS_get_u8_length_prefixed(&hello_verify_request, &cookie) ||
      CBS_len(&hello_verify_request) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    return false;
  }

  if (!hs->dtls_cookie.CopyFrom(cookie)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
    return false;
  }
  hs->received_hello_verify_request = true;

  ssl->method->next_message(ssl);

  // DTLS resets the handshake buffer after HelloVerifyRequest.
  if (!hs->transcript.Init()) {
    return false;
  }

  return ssl_add_client_hello(hs);
}

bool ssl_parse_server_hello(ParsedServerHello *out, uint8_t *out_alert,
                            const SSLMessage &msg) {
  if (msg.type != SSL3_MT_SERVER_HELLO) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_MESSAGE);
    *out_alert = SSL_AD_UNEXPECTED_MESSAGE;
    return false;
  }
  out->raw = msg.raw;
  CBS body = msg.body;
  if (!CBS_get_u16(&body, &out->legacy_version) ||
      !CBS_get_bytes(&body, &out->random, SSL3_RANDOM_SIZE) ||
      !CBS_get_u8_length_prefixed(&body, &out->session_id) ||
      CBS_len(&out->session_id) > SSL3_SESSION_ID_SIZE ||
      !CBS_get_u16(&body, &out->cipher_suite) ||
      !CBS_get_u8(&body, &out->compression_method)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    *out_alert = SSL_AD_DECODE_ERROR;
    return false;
  }
  // In TLS 1.2 and below, empty extensions blocks may be omitted. In TLS 1.3,
  // ServerHellos always have extensions, so this can be applied generically.
  CBS_init(&out->extensions, nullptr, 0);
  if ((CBS_len(&body) != 0 &&
       !CBS_get_u16_length_prefixed(&body, &out->extensions)) ||
      CBS_len(&body) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    *out_alert = SSL_AD_DECODE_ERROR;
    return false;
  }
  return true;
}

static enum ssl_hs_wait_t do_read_server_hello(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  SSLMessage msg;
  if (!ssl->method->get_message(ssl, &msg)) {
    return ssl_hs_read_server_hello;
  }

  if (SSL_is_dtls(ssl) && !hs->received_hello_verify_request &&
      msg.type == DTLS1_MT_HELLO_VERIFY_REQUEST) {
    if (!handle_hello_verify_request(hs, msg)) {
      return ssl_hs_error;
    }
    hs->received_hello_verify_request = true;
    hs->state = state_read_server_hello;
    return ssl_hs_flush;
  }

  ParsedServerHello server_hello;
  uint16_t server_version;
  uint8_t alert = SSL_AD_DECODE_ERROR;
  if (!ssl_parse_server_hello(&server_hello, &alert, msg) ||
      !parse_server_version(hs, &server_version, &alert, server_hello)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
    return ssl_hs_error;
  }

  if (!ssl_supports_version(hs, server_version)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNSUPPORTED_PROTOCOL);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_PROTOCOL_VERSION);
    return ssl_hs_error;
  }

  if (!ssl->s3->initial_handshake_complete) {
    // |ssl->s3->version| may be set due to 0-RTT. If it was to a different
    // value, the check below will fire.
    assert(ssl->s3->version == 0 ||
           (hs->is_early_version &&
            ssl->s3->version == hs->early_session->ssl_version));
    ssl->s3->version = server_version;
    hs->is_early_version = false;
  } else if (server_version != ssl->s3->version) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_SSL_VERSION);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_PROTOCOL_VERSION);
    return ssl_hs_error;
  }

  // If the version did not match, stop sending 0-RTT data.
  if (hs->early_data_offered &&
      ssl->s3->version != hs->early_session->ssl_version) {
    // This is currently only possible by reading a TLS 1.2 (or earlier)
    // ServerHello in response to TLS 1.3. If there is ever a TLS 1.4, or
    // another variant of TLS 1.3, the fatal error below will need to be a clean
    // 0-RTT reject.
    assert(ssl_protocol_version(ssl) < TLS1_3_VERSION);
    assert(ssl_session_protocol_version(hs->early_session.get()) >=
           TLS1_3_VERSION);

    // A TLS 1.2 server would not know to skip the early data we offered, so
    // there is no point in continuing the handshake. Report an error code as
    // soon as we detect this. The caller may use this error code to implement
    // the fallback described in RFC 8446 appendix D.3.
    //
    // Disconnect early writes. This ensures subsequent |SSL_write| calls query
    // the handshake which, in turn, will replay the error code rather than fail
    // at the |write_shutdown| check. See https://crbug.com/1078515.
    // TODO(davidben): Should all handshake errors do this? What about record
    // decryption failures?
    //
    // TODO(crbug.com/381113363): Although missing from the spec, a DTLS 1.2
    // server will already naturally skip 0-RTT data. If we implement DTLS 1.3
    // 0-RTT, we may want a clean reject.
    assert(!SSL_is_dtls(ssl));
    hs->can_early_write = false;
    OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_VERSION_ON_EARLY_DATA);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_PROTOCOL_VERSION);
    return ssl_hs_error;
  }

  if (ssl_protocol_version(ssl) >= TLS1_3_VERSION) {
    if (hs->received_hello_verify_request) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_INVALID_MESSAGE);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_PROTOCOL_VERSION);
      return ssl_hs_error;
    }

    hs->state = state_tls13;
    return ssl_hs_ok;
  }

  // Clear some TLS 1.3 state that no longer needs to be retained.
  hs->key_shares[0].reset();
  hs->key_shares[1].reset();
  ssl_done_writing_client_hello(hs);

  // TLS 1.2 handshakes cannot accept ECH.
  if (hs->selected_ech_config) {
    ssl->s3->ech_status = ssl_ech_rejected;
  }

  // Copy over the server random.
  OPENSSL_memcpy(ssl->s3->server_random, CBS_data(&server_hello.random),
                 SSL3_RANDOM_SIZE);

  // Enforce the TLS 1.3 anti-downgrade feature.
  if (!ssl->s3->initial_handshake_complete &&
      hs->max_version >= TLS1_3_VERSION) {
    static_assert(
        sizeof(kTLS12DowngradeRandom) == sizeof(kTLS13DowngradeRandom),
        "downgrade signals have different size");
    static_assert(
        sizeof(kJDK11DowngradeRandom) == sizeof(kTLS13DowngradeRandom),
        "downgrade signals have different size");
    auto suffix =
        MakeConstSpan(ssl->s3->server_random, sizeof(ssl->s3->server_random))
            .subspan(SSL3_RANDOM_SIZE - sizeof(kTLS13DowngradeRandom));
    if (suffix == kTLS12DowngradeRandom || suffix == kTLS13DowngradeRandom ||
        suffix == kJDK11DowngradeRandom) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_TLS13_DOWNGRADE);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return ssl_hs_error;
    }
  }

  // The cipher must be allowed in the selected version and enabled.
  const SSL_CIPHER *cipher = SSL_get_cipher_by_value(server_hello.cipher_suite);
  uint32_t mask_a, mask_k;
  ssl_get_client_disabled(hs, &mask_a, &mask_k);
  if (cipher == nullptr ||                                               //
      (cipher->algorithm_mkey & mask_k) ||                               //
      (cipher->algorithm_auth & mask_a) ||                               //
      SSL_CIPHER_get_min_version(cipher) > ssl_protocol_version(ssl) ||  //
      SSL_CIPHER_get_max_version(cipher) < ssl_protocol_version(ssl) ||  //
      !sk_SSL_CIPHER_find(SSL_get_ciphers(ssl), nullptr, cipher)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_CIPHER_RETURNED);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
    return ssl_hs_error;
  }

  hs->new_cipher = cipher;

  if (!hs->session_id.empty() &&
      Span<const uint8_t>(server_hello.session_id) == hs->session_id) {
    // Echoing the ClientHello session ID in TLS 1.2, whether from the session
    // or a synthetic one, indicates resumption. If there was no session (or if
    // the session was only offered in ECH ClientHelloInner), this was the
    // TLS 1.3 compatibility mode session ID. As we know this is not a session
    // the server knows about, any server resuming it is in error. Reject the
    // first connection deterministicly, rather than installing an invalid
    // session into the session cache. https://crbug.com/796910
    if (ssl->session == nullptr || ssl->s3->ech_status == ssl_ech_rejected) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_SERVER_ECHOED_INVALID_SESSION_ID);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return ssl_hs_error;
    }
    if (ssl->session->ssl_version != ssl->s3->version) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_OLD_SESSION_VERSION_NOT_RETURNED);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return ssl_hs_error;
    }
    if (ssl->session->cipher != hs->new_cipher) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_OLD_SESSION_CIPHER_NOT_RETURNED);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return ssl_hs_error;
    }
    if (!ssl_session_is_context_valid(hs, ssl->session.get())) {
      // This is actually a client application bug.
      OPENSSL_PUT_ERROR(SSL,
                        SSL_R_ATTEMPT_TO_REUSE_SESSION_IN_DIFFERENT_CONTEXT);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return ssl_hs_error;
    }
    // We never offer sessions on renegotiation.
    assert(!ssl->s3->initial_handshake_complete);
    ssl->s3->session_reused = true;
  } else {
    // The session wasn't resumed. Create a fresh SSL_SESSION to fill out.
    ssl_set_session(ssl, NULL);
    if (!ssl_get_new_session(hs)) {
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
      return ssl_hs_error;
    }

    // Save the session ID from the server. This may be empty if the session
    // isn't resumable, or if we'll receive a session ticket later. The
    // ServerHello parser ensures |server_hello.session_id| is within bounds.
    hs->new_session->session_id.CopyFrom(server_hello.session_id);
    hs->new_session->cipher = hs->new_cipher;
  }

  // Now that the cipher is known, initialize the handshake hash and hash the
  // ServerHello.
  if (!hs->transcript.InitHash(ssl_protocol_version(ssl), hs->new_cipher) ||
      !ssl_hash_message(hs, msg)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
    return ssl_hs_error;
  }

  // If doing a full handshake, the server may request a client certificate
  // which requires hashing the handshake transcript. Otherwise, the handshake
  // buffer may be released.
  if (ssl->session != NULL ||
      !ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    hs->transcript.FreeBuffer();
  }

  // Only the NULL compression algorithm is supported.
  if (server_hello.compression_method != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNSUPPORTED_COMPRESSION_ALGORITHM);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
    return ssl_hs_error;
  }

  if (!ssl_parse_serverhello_tlsext(hs, &server_hello.extensions)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_PARSE_TLSEXT);
    return ssl_hs_error;
  }

  if (ssl->session != NULL &&
      hs->extended_master_secret != ssl->session->extended_master_secret) {
    if (ssl->session->extended_master_secret) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_RESUMED_EMS_SESSION_WITHOUT_EMS_EXTENSION);
    } else {
      OPENSSL_PUT_ERROR(SSL, SSL_R_RESUMED_NON_EMS_SESSION_WITH_EMS_EXTENSION);
    }
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_HANDSHAKE_FAILURE);
    return ssl_hs_error;
  }

  ssl->method->next_message(ssl);

  if (ssl->session != NULL) {
    if (ssl->ctx->reverify_on_resume &&
        ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
      hs->state = state_reverify_server_certificate;
    } else {
      hs->state = state_read_session_ticket;
    }
    return ssl_hs_ok;
  }

  hs->state = state_read_server_certificate;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_tls13(SSL_HANDSHAKE *hs) {
  enum ssl_hs_wait_t wait = tls13_client_handshake(hs);
  if (wait == ssl_hs_ok) {
    hs->state = state_finish_client_handshake;
    return ssl_hs_ok;
  }

  return wait;
}

static enum ssl_hs_wait_t do_read_server_certificate(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;

  if (!ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    hs->state = state_read_certificate_status;
    return ssl_hs_ok;
  }

  SSLMessage msg;
  if (!ssl->method->get_message(ssl, &msg)) {
    return ssl_hs_read_message;
  }

  if (!ssl_check_message_type(ssl, msg, SSL3_MT_CERTIFICATE) ||
      !ssl_hash_message(hs, msg)) {
    return ssl_hs_error;
  }

  CBS body = msg.body;
  uint8_t alert = SSL_AD_DECODE_ERROR;
  if (!ssl_parse_cert_chain(&alert, &hs->new_session->certs, &hs->peer_pubkey,
                            NULL, &body, ssl->ctx->pool)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
    return ssl_hs_error;
  }

  if (sk_CRYPTO_BUFFER_num(hs->new_session->certs.get()) == 0 ||
      CBS_len(&body) != 0 ||
      !ssl->ctx->x509_method->session_cache_objects(hs->new_session.get())) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    return ssl_hs_error;
  }

  if (!ssl_check_leaf_certificate(
          hs, hs->peer_pubkey.get(),
          sk_CRYPTO_BUFFER_value(hs->new_session->certs.get(), 0))) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
    return ssl_hs_error;
  }

  ssl->method->next_message(ssl);

  hs->state = state_read_certificate_status;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_read_certificate_status(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;

  if (!hs->certificate_status_expected) {
    hs->state = state_verify_server_certificate;
    return ssl_hs_ok;
  }

  SSLMessage msg;
  if (!ssl->method->get_message(ssl, &msg)) {
    return ssl_hs_read_message;
  }

  if (msg.type != SSL3_MT_CERTIFICATE_STATUS) {
    // A server may send status_request in ServerHello and then change its mind
    // about sending CertificateStatus.
    hs->state = state_verify_server_certificate;
    return ssl_hs_ok;
  }

  if (!ssl_hash_message(hs, msg)) {
    return ssl_hs_error;
  }

  CBS certificate_status = msg.body, ocsp_response;
  uint8_t status_type;
  if (!CBS_get_u8(&certificate_status, &status_type) ||                     //
      status_type != TLSEXT_STATUSTYPE_ocsp ||                              //
      !CBS_get_u24_length_prefixed(&certificate_status, &ocsp_response) ||  //
      CBS_len(&ocsp_response) == 0 ||                                       //
      CBS_len(&certificate_status) != 0) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    return ssl_hs_error;
  }

  hs->new_session->ocsp_response.reset(
      CRYPTO_BUFFER_new_from_CBS(&ocsp_response, ssl->ctx->pool));
  if (hs->new_session->ocsp_response == nullptr) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
    return ssl_hs_error;
  }

  ssl->method->next_message(ssl);

  hs->state = state_verify_server_certificate;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_verify_server_certificate(SSL_HANDSHAKE *hs) {
  if (!ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    hs->state = state_read_server_key_exchange;
    return ssl_hs_ok;
  }

  switch (ssl_verify_peer_cert(hs)) {
    case ssl_verify_ok:
      break;
    case ssl_verify_invalid:
      return ssl_hs_error;
    case ssl_verify_retry:
      hs->state = state_verify_server_certificate;
      return ssl_hs_certificate_verify;
  }

  hs->state = state_read_server_key_exchange;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_reverify_server_certificate(SSL_HANDSHAKE *hs) {
  assert(hs->ssl->ctx->reverify_on_resume);

  switch (ssl_reverify_peer_cert(hs, /*send_alert=*/true)) {
    case ssl_verify_ok:
      break;
    case ssl_verify_invalid:
      return ssl_hs_error;
    case ssl_verify_retry:
      hs->state = state_reverify_server_certificate;
      return ssl_hs_certificate_verify;
  }

  hs->state = state_read_session_ticket;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_read_server_key_exchange(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  SSLMessage msg;
  if (!ssl->method->get_message(ssl, &msg)) {
    return ssl_hs_read_message;
  }

  if (msg.type != SSL3_MT_SERVER_KEY_EXCHANGE) {
    // Some ciphers (pure PSK) have an optional ServerKeyExchange message.
    if (ssl_cipher_requires_server_key_exchange(hs->new_cipher)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_MESSAGE);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_UNEXPECTED_MESSAGE);
      return ssl_hs_error;
    }

    hs->state = state_read_certificate_request;
    return ssl_hs_ok;
  }

  if (!ssl_hash_message(hs, msg)) {
    return ssl_hs_error;
  }

  uint32_t alg_k = hs->new_cipher->algorithm_mkey;
  uint32_t alg_a = hs->new_cipher->algorithm_auth;
  CBS server_key_exchange = msg.body;
  if (alg_a & SSL_aPSK) {
    CBS psk_identity_hint;

    // Each of the PSK key exchanges begins with a psk_identity_hint.
    if (!CBS_get_u16_length_prefixed(&server_key_exchange,
                                     &psk_identity_hint)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
      return ssl_hs_error;
    }

    // Store the PSK identity hint for the ClientKeyExchange. Assume that the
    // maximum length of a PSK identity hint can be as long as the maximum
    // length of a PSK identity. Also do not allow NULL characters; identities
    // are saved as C strings.
    //
    // TODO(davidben): Should invalid hints be ignored? It's a hint rather than
    // a specific identity.
    if (CBS_len(&psk_identity_hint) > PSK_MAX_IDENTITY_LEN ||
        CBS_contains_zero_byte(&psk_identity_hint)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DATA_LENGTH_TOO_LONG);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_HANDSHAKE_FAILURE);
      return ssl_hs_error;
    }

    // Save non-empty identity hints as a C string. Empty identity hints we
    // treat as missing. Plain PSK makes it possible to send either no hint
    // (omit ServerKeyExchange) or an empty hint, while ECDHE_PSK can only spell
    // empty hint. Having different capabilities is odd, so we interpret empty
    // and missing as identical.
    char *raw = nullptr;
    if (CBS_len(&psk_identity_hint) != 0 &&
        !CBS_strdup(&psk_identity_hint, &raw)) {
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
      return ssl_hs_error;
    }
    hs->peer_psk_identity_hint.reset(raw);
  }

  if (alg_k & SSL_kECDHE) {
    // Parse the server parameters.
    uint8_t group_type;
    uint16_t group_id;
    CBS point;
    if (!CBS_get_u8(&server_key_exchange, &group_type) ||
        group_type != NAMED_CURVE_TYPE ||
        !CBS_get_u16(&server_key_exchange, &group_id) ||
        !CBS_get_u8_length_prefixed(&server_key_exchange, &point)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
      return ssl_hs_error;
    }

    // Ensure the group is consistent with preferences.
    if (!tls1_check_group_id(hs, group_id)) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_WRONG_CURVE);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ILLEGAL_PARAMETER);
      return ssl_hs_error;
    }

    // Save the group and peer public key for later.
    hs->new_session->group_id = group_id;
    if (!hs->peer_key.CopyFrom(point)) {
      return ssl_hs_error;
    }
  } else if (!(alg_k & SSL_kPSK)) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_MESSAGE);
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_UNEXPECTED_MESSAGE);
    return ssl_hs_error;
  }

  // At this point, |server_key_exchange| contains the signature, if any, while
  // |msg.body| contains the entire message. From that, derive a CBS containing
  // just the parameter.
  CBS parameter;
  CBS_init(&parameter, CBS_data(&msg.body),
           CBS_len(&msg.body) - CBS_len(&server_key_exchange));

  // ServerKeyExchange should be signed by the server's public key.
  if (ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    uint16_t signature_algorithm = 0;
    if (ssl_protocol_version(ssl) >= TLS1_2_VERSION) {
      if (!CBS_get_u16(&server_key_exchange, &signature_algorithm)) {
        OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
        ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
        return ssl_hs_error;
      }
      uint8_t alert = SSL_AD_DECODE_ERROR;
      if (!tls12_check_peer_sigalg(hs, &alert, signature_algorithm,
                                   hs->peer_pubkey.get())) {
        ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
        return ssl_hs_error;
      }
      hs->new_session->peer_signature_algorithm = signature_algorithm;
    } else if (!tls1_get_legacy_signature_algorithm(&signature_algorithm,
                                                    hs->peer_pubkey.get())) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_PEER_ERROR_UNSUPPORTED_CERTIFICATE_TYPE);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_UNSUPPORTED_CERTIFICATE);
      return ssl_hs_error;
    }

    // The last field in |server_key_exchange| is the signature.
    CBS signature;
    if (!CBS_get_u16_length_prefixed(&server_key_exchange, &signature) ||
        CBS_len(&server_key_exchange) != 0) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
      return ssl_hs_error;
    }

    ScopedCBB transcript;
    Array<uint8_t> transcript_data;
    if (!CBB_init(transcript.get(),
                  2 * SSL3_RANDOM_SIZE + CBS_len(&parameter)) ||
        !CBB_add_bytes(transcript.get(), ssl->s3->client_random,
                       SSL3_RANDOM_SIZE) ||
        !CBB_add_bytes(transcript.get(), ssl->s3->server_random,
                       SSL3_RANDOM_SIZE) ||
        !CBB_add_bytes(transcript.get(), CBS_data(&parameter),
                       CBS_len(&parameter)) ||
        !CBBFinishArray(transcript.get(), &transcript_data)) {
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
      return ssl_hs_error;
    }

    if (!ssl_public_key_verify(ssl, signature, signature_algorithm,
                               hs->peer_pubkey.get(), transcript_data)) {
      // bad signature
      OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_SIGNATURE);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECRYPT_ERROR);
      return ssl_hs_error;
    }
  } else {
    // PSK ciphers are the only supported certificate-less ciphers.
    assert(alg_a == SSL_aPSK);

    if (CBS_len(&server_key_exchange) > 0) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_EXTRA_DATA_IN_MESSAGE);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
      return ssl_hs_error;
    }
  }

  ssl->method->next_message(ssl);
  hs->state = state_read_certificate_request;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_read_certificate_request(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;

  if (!ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    hs->state = state_read_server_hello_done;
    return ssl_hs_ok;
  }

  SSLMessage msg;
  if (!ssl->method->get_message(ssl, &msg)) {
    return ssl_hs_read_message;
  }

  if (msg.type == SSL3_MT_SERVER_HELLO_DONE) {
    // If we get here we don't need the handshake buffer as we won't be doing
    // client auth.
    hs->transcript.FreeBuffer();
    hs->state = state_read_server_hello_done;
    return ssl_hs_ok;
  }

  if (!ssl_check_message_type(ssl, msg, SSL3_MT_CERTIFICATE_REQUEST) ||
      !ssl_hash_message(hs, msg)) {
    return ssl_hs_error;
  }

  // Get the certificate types.
  CBS body = msg.body, certificate_types;
  if (!CBS_get_u8_length_prefixed(&body, &certificate_types)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    return ssl_hs_error;
  }

  if (!hs->certificate_types.CopyFrom(certificate_types)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
    return ssl_hs_error;
  }

  if (ssl_protocol_version(ssl) >= TLS1_2_VERSION) {
    CBS supported_signature_algorithms;
    if (!CBS_get_u16_length_prefixed(&body, &supported_signature_algorithms) ||
        !tls1_parse_peer_sigalgs(hs, &supported_signature_algorithms)) {
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
      OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
      return ssl_hs_error;
    }
  }

  uint8_t alert = SSL_AD_DECODE_ERROR;
  UniquePtr<STACK_OF(CRYPTO_BUFFER)> ca_names =
      SSL_parse_CA_list(ssl, &alert, &body);
  if (!ca_names) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
    return ssl_hs_error;
  }

  if (CBS_len(&body) != 0) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    return ssl_hs_error;
  }

  hs->cert_request = true;
  hs->ca_names = std::move(ca_names);
  ssl->ctx->x509_method->hs_flush_cached_ca_names(hs);

  ssl->method->next_message(ssl);
  hs->state = state_read_server_hello_done;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_read_server_hello_done(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  SSLMessage msg;
  if (!ssl->method->get_message(ssl, &msg)) {
    return ssl_hs_read_message;
  }

  if (!ssl_check_message_type(ssl, msg, SSL3_MT_SERVER_HELLO_DONE) ||
      !ssl_hash_message(hs, msg)) {
    return ssl_hs_error;
  }

  // ServerHelloDone is empty.
  if (CBS_len(&msg.body) != 0) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    return ssl_hs_error;
  }

  // ServerHelloDone should be the end of the flight.
  if (ssl->method->has_unprocessed_handshake_data(ssl)) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_UNEXPECTED_MESSAGE);
    OPENSSL_PUT_ERROR(SSL, SSL_R_EXCESS_HANDSHAKE_DATA);
    return ssl_hs_error;
  }

  ssl->method->next_message(ssl);
  hs->state = state_send_client_certificate;
  return ssl_hs_ok;
}

static bool check_credential(SSL_HANDSHAKE *hs, const SSL_CREDENTIAL *cred,
                             uint16_t *out_sigalg) {
  if (cred->type != SSLCredentialType::kX509) {
    OPENSSL_PUT_ERROR(SSL, SSL_R_UNKNOWN_CERTIFICATE_TYPE);
    return false;
  }

  if (hs->config->check_client_certificate_type) {
    // Check the certificate types advertised by the peer.
    uint8_t cert_type;
    switch (EVP_PKEY_id(cred->pubkey.get())) {
      case EVP_PKEY_RSA:
        cert_type = SSL3_CT_RSA_SIGN;
        break;
      case EVP_PKEY_EC:
      case EVP_PKEY_ED25519:
        cert_type = TLS_CT_ECDSA_SIGN;
        break;
      default:
        OPENSSL_PUT_ERROR(SSL, SSL_R_UNKNOWN_CERTIFICATE_TYPE);
        return false;
    }
    if (std::find(hs->certificate_types.begin(), hs->certificate_types.end(),
                  cert_type) == hs->certificate_types.end()) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_UNKNOWN_CERTIFICATE_TYPE);
      return false;
    }
  }

  // All currently supported credentials require a signature. Note this does not
  // check the ECDSA curve. Prior to TLS 1.3, there is no way to determine which
  // ECDSA curves are supported by the peer, so we must assume all curves are
  // supported.
  return tls1_choose_signature_algorithm(hs, cred, out_sigalg);
}

static enum ssl_hs_wait_t do_send_client_certificate(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;

  // The peer didn't request a certificate.
  if (!hs->cert_request) {
    hs->state = state_send_client_key_exchange;
    return ssl_hs_ok;
  }

  if (ssl->s3->ech_status == ssl_ech_rejected) {
    // Do not send client certificates on ECH reject. We have not authenticated
    // the server for the name that can learn the certificate.
    SSL_certs_clear(ssl);
  } else if (hs->config->cert->cert_cb != nullptr) {
    // Call cert_cb to update the certificate.
    int rv = hs->config->cert->cert_cb(ssl, hs->config->cert->cert_cb_arg);
    if (rv == 0) {
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
      OPENSSL_PUT_ERROR(SSL, SSL_R_CERT_CB_ERROR);
      return ssl_hs_error;
    }
    if (rv < 0) {
      hs->state = state_send_client_certificate;
      return ssl_hs_x509_lookup;
    }
  }

  Array<SSL_CREDENTIAL *> creds;
  if (!ssl_get_credential_list(hs, &creds)) {
    return ssl_hs_error;
  }

  if (creds.empty()) {
    // If there were no credentials, proceed without a client certificate. In
    // this case, the handshake buffer may be released early.
    hs->transcript.FreeBuffer();
  } else {
    // Select the credential to use.
    for (SSL_CREDENTIAL *cred : creds) {
      ERR_clear_error();
      uint16_t sigalg;
      if (check_credential(hs, cred, &sigalg)) {
        hs->credential = UpRef(cred);
        hs->signature_algorithm = sigalg;
        break;
      }
    }
    if (hs->credential == nullptr) {
      // The error from the last attempt is in the error queue.
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_HANDSHAKE_FAILURE);
      return ssl_hs_error;
    }
  }

  if (!ssl_send_tls12_certificate(hs)) {
    return ssl_hs_error;
  }

  hs->state = state_send_client_key_exchange;
  return ssl_hs_ok;
}

static_assert(sizeof(size_t) >= sizeof(unsigned),
              "size_t is smaller than unsigned");

static enum ssl_hs_wait_t do_send_client_key_exchange(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  ScopedCBB cbb;
  CBB body;
  if (!ssl->method->init_message(ssl, cbb.get(), &body,
                                 SSL3_MT_CLIENT_KEY_EXCHANGE)) {
    return ssl_hs_error;
  }

  Array<uint8_t> pms;
  uint32_t alg_k = hs->new_cipher->algorithm_mkey;
  uint32_t alg_a = hs->new_cipher->algorithm_auth;
  if (ssl_cipher_uses_certificate_auth(hs->new_cipher)) {
    const CRYPTO_BUFFER *leaf =
        sk_CRYPTO_BUFFER_value(hs->new_session->certs.get(), 0);
    CBS leaf_cbs;
    CRYPTO_BUFFER_init_CBS(leaf, &leaf_cbs);

    // Check the key usage matches the cipher suite. We do this unconditionally
    // for non-RSA certificates. In particular, it's needed to distinguish ECDH
    // certificates, which we do not support, from ECDSA certificates.
    // Historically, we have not checked RSA key usages, so it is controlled by
    // a flag for now. See https://crbug.com/795089.
    ssl_key_usage_t intended_use = (alg_k & SSL_kRSA)
                                       ? key_usage_encipherment
                                       : key_usage_digital_signature;
    if (!ssl_cert_check_key_usage(&leaf_cbs, intended_use)) {
      if (hs->config->enforce_rsa_key_usage ||
          EVP_PKEY_id(hs->peer_pubkey.get()) != EVP_PKEY_RSA) {
        return ssl_hs_error;
      }
      ERR_clear_error();
      ssl->s3->was_key_usage_invalid = true;
    }
  }

  // If using a PSK key exchange, prepare the pre-shared key.
  unsigned psk_len = 0;
  uint8_t psk[PSK_MAX_PSK_LEN];
  if (alg_a & SSL_aPSK) {
    if (hs->config->psk_client_callback == NULL) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_PSK_NO_CLIENT_CB);
      return ssl_hs_error;
    }

    char identity[PSK_MAX_IDENTITY_LEN + 1];
    OPENSSL_memset(identity, 0, sizeof(identity));
    psk_len = hs->config->psk_client_callback(
        ssl, hs->peer_psk_identity_hint.get(), identity, sizeof(identity), psk,
        sizeof(psk));
    if (psk_len == 0) {
      OPENSSL_PUT_ERROR(SSL, SSL_R_PSK_IDENTITY_NOT_FOUND);
      ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_HANDSHAKE_FAILURE);
      return ssl_hs_error;
    }
    assert(psk_len <= PSK_MAX_PSK_LEN);

    hs->new_session->psk_identity.reset(OPENSSL_strdup(identity));
    if (hs->new_session->psk_identity == nullptr) {
      return ssl_hs_error;
    }

    // Write out psk_identity.
    CBB child;
    if (!CBB_add_u16_length_prefixed(&body, &child) ||
        !CBB_add_bytes(&child, (const uint8_t *)identity,
                       OPENSSL_strnlen(identity, sizeof(identity))) ||
        !CBB_flush(&body)) {
      return ssl_hs_error;
    }
  }

  // Depending on the key exchange method, compute |pms|.
  if (alg_k & SSL_kRSA) {
    RSA *rsa = EVP_PKEY_get0_RSA(hs->peer_pubkey.get());
    if (rsa == NULL) {
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      return ssl_hs_error;
    }

    if (!pms.InitForOverwrite(SSL_MAX_MASTER_KEY_LENGTH)) {
      return ssl_hs_error;
    }
    pms[0] = hs->client_version >> 8;
    pms[1] = hs->client_version & 0xff;
    if (!RAND_bytes(&pms[2], SSL_MAX_MASTER_KEY_LENGTH - 2)) {
      return ssl_hs_error;
    }

    CBB enc_pms;
    uint8_t *ptr;
    size_t enc_pms_len;
    if (!CBB_add_u16_length_prefixed(&body, &enc_pms) ||  //
        !CBB_reserve(&enc_pms, &ptr, RSA_size(rsa)) ||    //
        !RSA_encrypt(rsa, &enc_pms_len, ptr, RSA_size(rsa), pms.data(),
                     pms.size(), RSA_PKCS1_PADDING) ||  //
        !CBB_did_write(&enc_pms, enc_pms_len) ||        //
        !CBB_flush(&body)) {
      return ssl_hs_error;
    }
  } else if (alg_k & SSL_kECDHE) {
    CBB child;
    if (!CBB_add_u8_length_prefixed(&body, &child)) {
      return ssl_hs_error;
    }

    // Generate a premaster secret and encapsulate it.
    bssl::UniquePtr<SSLKeyShare> kem =
        SSLKeyShare::Create(hs->new_session->group_id);
    uint8_t alert = SSL_AD_DECODE_ERROR;
    if (!kem || !kem->Encap(&child, &pms, &alert, hs->peer_key)) {
      ssl_send_alert(ssl, SSL3_AL_FATAL, alert);
      return ssl_hs_error;
    }
    if (!CBB_flush(&body)) {
      return ssl_hs_error;
    }

    // The peer key can now be discarded.
    hs->peer_key.Reset();
  } else if (alg_k & SSL_kPSK) {
    // For plain PSK, other_secret is a block of 0s with the same length as
    // the pre-shared key.
    if (!pms.Init(psk_len)) {
      return ssl_hs_error;
    }
  } else {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_HANDSHAKE_FAILURE);
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return ssl_hs_error;
  }

  // For a PSK cipher suite, other_secret is combined with the pre-shared
  // key.
  if (alg_a & SSL_aPSK) {
    ScopedCBB pms_cbb;
    CBB child;
    if (!CBB_init(pms_cbb.get(), 2 + psk_len + 2 + pms.size()) ||
        !CBB_add_u16_length_prefixed(pms_cbb.get(), &child) ||
        !CBB_add_bytes(&child, pms.data(), pms.size()) ||
        !CBB_add_u16_length_prefixed(pms_cbb.get(), &child) ||
        !CBB_add_bytes(&child, psk, psk_len) ||
        !CBBFinishArray(pms_cbb.get(), &pms)) {
      return ssl_hs_error;
    }
  }

  // The message must be added to the finished hash before calculating the
  // master secret.
  if (!ssl_add_message_cbb(ssl, cbb.get())) {
    return ssl_hs_error;
  }

  hs->new_session->secret.ResizeForOverwrite(SSL3_MASTER_SECRET_SIZE);
  if (!tls1_generate_master_secret(hs, MakeSpan(hs->new_session->secret),
                                   pms)) {
    return ssl_hs_error;
  }

  hs->new_session->extended_master_secret = hs->extended_master_secret;
  hs->state = state_send_client_certificate_verify;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_send_client_certificate_verify(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;

  if (!hs->cert_request || hs->credential == nullptr) {
    hs->state = state_send_client_finished;
    return ssl_hs_ok;
  }

  ScopedCBB cbb;
  CBB body, child;
  if (!ssl->method->init_message(ssl, cbb.get(), &body,
                                 SSL3_MT_CERTIFICATE_VERIFY)) {
    return ssl_hs_error;
  }

  assert(hs->signature_algorithm != 0);
  if (ssl_protocol_version(ssl) >= TLS1_2_VERSION) {
    // Write out the digest type in TLS 1.2.
    if (!CBB_add_u16(&body, hs->signature_algorithm)) {
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      return ssl_hs_error;
    }
  }

  // Set aside space for the signature.
  const size_t max_sig_len = EVP_PKEY_size(hs->credential->pubkey.get());
  uint8_t *ptr;
  if (!CBB_add_u16_length_prefixed(&body, &child) ||
      !CBB_reserve(&child, &ptr, max_sig_len)) {
    return ssl_hs_error;
  }

  size_t sig_len = max_sig_len;
  switch (ssl_private_key_sign(hs, ptr, &sig_len, max_sig_len,
                               hs->signature_algorithm,
                               hs->transcript.buffer())) {
    case ssl_private_key_success:
      break;
    case ssl_private_key_failure:
      return ssl_hs_error;
    case ssl_private_key_retry:
      hs->state = state_send_client_certificate_verify;
      return ssl_hs_private_key_operation;
  }

  if (!CBB_did_write(&child, sig_len) ||  //
      !ssl_add_message_cbb(ssl, cbb.get())) {
    return ssl_hs_error;
  }

  // The handshake buffer is no longer necessary.
  hs->transcript.FreeBuffer();

  hs->state = state_send_client_finished;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_send_client_finished(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  hs->can_release_private_key = true;
  if (!ssl->method->add_change_cipher_spec(ssl) ||
      !tls1_change_cipher_state(hs, evp_aead_seal)) {
    return ssl_hs_error;
  }

  if (hs->next_proto_neg_seen) {
    static const uint8_t kZero[32] = {0};
    size_t padding_len =
        32 - ((ssl->s3->next_proto_negotiated.size() + 2) % 32);

    ScopedCBB cbb;
    CBB body, child;
    if (!ssl->method->init_message(ssl, cbb.get(), &body, SSL3_MT_NEXT_PROTO) ||
        !CBB_add_u8_length_prefixed(&body, &child) ||
        !CBB_add_bytes(&child, ssl->s3->next_proto_negotiated.data(),
                       ssl->s3->next_proto_negotiated.size()) ||
        !CBB_add_u8_length_prefixed(&body, &child) ||
        !CBB_add_bytes(&child, kZero, padding_len) ||
        !ssl_add_message_cbb(ssl, cbb.get())) {
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      return ssl_hs_error;
    }
  }

  if (hs->channel_id_negotiated) {
    ScopedCBB cbb;
    CBB body;
    if (!ssl->method->init_message(ssl, cbb.get(), &body, SSL3_MT_CHANNEL_ID) ||
        !tls1_write_channel_id(hs, &body) ||
        !ssl_add_message_cbb(ssl, cbb.get())) {
      OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
      return ssl_hs_error;
    }
  }

  if (!ssl_send_finished(hs)) {
    return ssl_hs_error;
  }

  hs->state = state_finish_flight;
  return ssl_hs_flush;
}

static bool can_false_start(const SSL_HANDSHAKE *hs) {
  const SSL *const ssl = hs->ssl;

  // False Start bypasses the Finished check's downgrade protection. This can
  // enable attacks where we send data under weaker settings than supported
  // (e.g. the Logjam attack). Thus we require TLS 1.2 with an ECDHE+AEAD
  // cipher, our strongest settings before TLS 1.3.
  //
  // Now that TLS 1.3 exists, we would like to avoid similar attacks between
  // TLS 1.2 and TLS 1.3, but there are too many TLS 1.2 deployments to
  // sacrifice False Start on them. Instead, we rely on the ServerHello.random
  // downgrade signal, which we unconditionally enforce.
  if (SSL_is_dtls(ssl) ||                              //
      SSL_version(ssl) != TLS1_2_VERSION ||            //
      hs->new_cipher->algorithm_mkey != SSL_kECDHE ||  //
      hs->new_cipher->algorithm_mac != SSL_AEAD) {
    return false;
  }

  // If ECH was rejected, disable False Start. We run the handshake to
  // completion, including the Finished downgrade check, to authenticate the
  // recovery flow.
  if (ssl->s3->ech_status == ssl_ech_rejected) {
    return false;
  }

  // Additionally require ALPN or NPN by default.
  //
  // TODO(davidben): Can this constraint be relaxed globally now that cipher
  // suite requirements have been tightened?
  if (!ssl->ctx->false_start_allowed_without_alpn &&
      ssl->s3->alpn_selected.empty() &&
      ssl->s3->next_proto_negotiated.empty()) {
    return false;
  }

  return true;
}

static enum ssl_hs_wait_t do_finish_flight(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  if (ssl->session != NULL) {
    hs->state = state_finish_client_handshake;
    return ssl_hs_ok;
  }

  // This is a full handshake. If it involves ChannelID, then record the
  // handshake hashes at this point in the session so that any resumption of
  // this session with ChannelID can sign those hashes.
  if (!tls1_record_handshake_hashes_for_channel_id(hs)) {
    return ssl_hs_error;
  }

  hs->state = state_read_session_ticket;

  if ((SSL_get_mode(ssl) & SSL_MODE_ENABLE_FALSE_START) &&
      can_false_start(hs) &&
      // No False Start on renegotiation (would complicate the state machine).
      !ssl->s3->initial_handshake_complete) {
    hs->in_false_start = true;
    hs->can_early_write = true;
    return ssl_hs_early_return;
  }

  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_read_session_ticket(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;

  if (!hs->ticket_expected) {
    hs->state = state_process_change_cipher_spec;
    return ssl_hs_read_change_cipher_spec;
  }

  SSLMessage msg;
  if (!ssl->method->get_message(ssl, &msg)) {
    return ssl_hs_read_message;
  }

  if (!ssl_check_message_type(ssl, msg, SSL3_MT_NEW_SESSION_TICKET) ||
      !ssl_hash_message(hs, msg)) {
    return ssl_hs_error;
  }

  CBS new_session_ticket = msg.body, ticket;
  uint32_t ticket_lifetime_hint;
  if (!CBS_get_u32(&new_session_ticket, &ticket_lifetime_hint) ||
      !CBS_get_u16_length_prefixed(&new_session_ticket, &ticket) ||
      CBS_len(&new_session_ticket) != 0) {
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_DECODE_ERROR);
    OPENSSL_PUT_ERROR(SSL, SSL_R_DECODE_ERROR);
    return ssl_hs_error;
  }

  if (CBS_len(&ticket) == 0) {
    // RFC 5077 allows a server to change its mind and send no ticket after
    // negotiating the extension. The value of |ticket_expected| is checked in
    // |ssl_update_cache| so is cleared here to avoid an unnecessary update.
    hs->ticket_expected = false;
    ssl->method->next_message(ssl);
    hs->state = state_process_change_cipher_spec;
    return ssl_hs_read_change_cipher_spec;
  }

  if (ssl->session != nullptr) {
    // The server is sending a new ticket for an existing session. Sessions are
    // immutable once established, so duplicate all but the ticket of the
    // existing session.
    assert(!hs->new_session);
    hs->new_session =
        SSL_SESSION_dup(ssl->session.get(), SSL_SESSION_INCLUDE_NONAUTH);
    if (!hs->new_session) {
      return ssl_hs_error;
    }
  }

  // |ticket_lifetime_hint| is measured from when the ticket was issued.
  ssl_session_rebase_time(ssl, hs->new_session.get());

  if (!hs->new_session->ticket.CopyFrom(ticket)) {
    return ssl_hs_error;
  }
  hs->new_session->ticket_lifetime_hint = ticket_lifetime_hint;

  // Historically, OpenSSL filled in fake session IDs for ticket-based sessions.
  // TODO(davidben): Are external callers relying on this? Try removing this.
  hs->new_session->session_id.ResizeForOverwrite(SHA256_DIGEST_LENGTH);
  SHA256(CBS_data(&ticket), CBS_len(&ticket),
         hs->new_session->session_id.data());

  ssl->method->next_message(ssl);
  hs->state = state_process_change_cipher_spec;
  return ssl_hs_read_change_cipher_spec;
}

static enum ssl_hs_wait_t do_process_change_cipher_spec(SSL_HANDSHAKE *hs) {
  if (!tls1_change_cipher_state(hs, evp_aead_open)) {
    return ssl_hs_error;
  }

  hs->state = state_read_server_finished;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_read_server_finished(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  enum ssl_hs_wait_t wait = ssl_get_finished(hs);
  if (wait != ssl_hs_ok) {
    return wait;
  }

  if (ssl->session != NULL) {
    hs->state = state_send_client_finished;
    return ssl_hs_ok;
  }

  hs->state = state_finish_client_handshake;
  return ssl_hs_ok;
}

static enum ssl_hs_wait_t do_finish_client_handshake(SSL_HANDSHAKE *hs) {
  SSL *const ssl = hs->ssl;
  if (ssl->s3->ech_status == ssl_ech_rejected) {
    // Release the retry configs.
    hs->ech_authenticated_reject = true;
    ssl_send_alert(ssl, SSL3_AL_FATAL, SSL_AD_ECH_REQUIRED);
    OPENSSL_PUT_ERROR(SSL, SSL_R_ECH_REJECTED);
    return ssl_hs_error;
  }

  ssl->method->on_handshake_complete(ssl);

  // Note TLS 1.2 resumptions with ticket renewal have both |ssl->session| (the
  // resumed session) and |hs->new_session| (the session with the new ticket).
  bool has_new_session = hs->new_session != nullptr;
  if (has_new_session) {
    // When False Start is enabled, the handshake reports completion early. The
    // caller may then have passed the (then unresuable) |hs->new_session| to
    // another thread via |SSL_get0_session| for resumption. To avoid potential
    // race conditions in such callers, we duplicate the session before
    // clearing |not_resumable|.
    ssl->s3->established_session =
        SSL_SESSION_dup(hs->new_session.get(), SSL_SESSION_DUP_ALL);
    if (!ssl->s3->established_session) {
      return ssl_hs_error;
    }
    // Renegotiations do not participate in session resumption.
    if (!ssl->s3->initial_handshake_complete) {
      ssl->s3->established_session->not_resumable = false;
    }

    hs->new_session.reset();
  } else {
    assert(ssl->session != nullptr);
    ssl->s3->established_session = UpRef(ssl->session);
  }

  hs->handshake_finalized = true;
  ssl->s3->initial_handshake_complete = true;
  if (has_new_session) {
    ssl_update_cache(ssl);
  }

  hs->state = state_done;
  return ssl_hs_ok;
}

enum ssl_hs_wait_t ssl_client_handshake(SSL_HANDSHAKE *hs) {
  while (hs->state != state_done) {
    enum ssl_hs_wait_t ret = ssl_hs_error;
    enum ssl_client_hs_state_t state =
        static_cast<enum ssl_client_hs_state_t>(hs->state);
    switch (state) {
      case state_start_connect:
        ret = do_start_connect(hs);
        break;
      case state_enter_early_data:
        ret = do_enter_early_data(hs);
        break;
      case state_early_reverify_server_certificate:
        ret = do_early_reverify_server_certificate(hs);
        break;
      case state_read_server_hello:
        ret = do_read_server_hello(hs);
        break;
      case state_tls13:
        ret = do_tls13(hs);
        break;
      case state_read_server_certificate:
        ret = do_read_server_certificate(hs);
        break;
      case state_read_certificate_status:
        ret = do_read_certificate_status(hs);
        break;
      case state_verify_server_certificate:
        ret = do_verify_server_certificate(hs);
        break;
      case state_reverify_server_certificate:
        ret = do_reverify_server_certificate(hs);
        break;
      case state_read_server_key_exchange:
        ret = do_read_server_key_exchange(hs);
        break;
      case state_read_certificate_request:
        ret = do_read_certificate_request(hs);
        break;
      case state_read_server_hello_done:
        ret = do_read_server_hello_done(hs);
        break;
      case state_send_client_certificate:
        ret = do_send_client_certificate(hs);
        break;
      case state_send_client_key_exchange:
        ret = do_send_client_key_exchange(hs);
        break;
      case state_send_client_certificate_verify:
        ret = do_send_client_certificate_verify(hs);
        break;
      case state_send_client_finished:
        ret = do_send_client_finished(hs);
        break;
      case state_finish_flight:
        ret = do_finish_flight(hs);
        break;
      case state_read_session_ticket:
        ret = do_read_session_ticket(hs);
        break;
      case state_process_change_cipher_spec:
        ret = do_process_change_cipher_spec(hs);
        break;
      case state_read_server_finished:
        ret = do_read_server_finished(hs);
        break;
      case state_finish_client_handshake:
        ret = do_finish_client_handshake(hs);
        break;
      case state_done:
        ret = ssl_hs_ok;
        break;
    }

    if (hs->state != state) {
      ssl_do_info_callback(hs->ssl, SSL_CB_CONNECT_LOOP, 1);
    }

    if (ret != ssl_hs_ok) {
      return ret;
    }
  }

  ssl_do_info_callback(hs->ssl, SSL_CB_HANDSHAKE_DONE, 1);
  return ssl_hs_ok;
}

const char *ssl_client_handshake_state(SSL_HANDSHAKE *hs) {
  enum ssl_client_hs_state_t state =
      static_cast<enum ssl_client_hs_state_t>(hs->state);
  switch (state) {
    case state_start_connect:
      return "TLS client start_connect";
    case state_enter_early_data:
      return "TLS client enter_early_data";
    case state_early_reverify_server_certificate:
      return "TLS client early_reverify_server_certificate";
    case state_read_server_hello:
      return "TLS client read_server_hello";
    case state_tls13:
      return tls13_client_handshake_state(hs);
    case state_read_server_certificate:
      return "TLS client read_server_certificate";
    case state_read_certificate_status:
      return "TLS client read_certificate_status";
    case state_verify_server_certificate:
      return "TLS client verify_server_certificate";
    case state_reverify_server_certificate:
      return "TLS client reverify_server_certificate";
    case state_read_server_key_exchange:
      return "TLS client read_server_key_exchange";
    case state_read_certificate_request:
      return "TLS client read_certificate_request";
    case state_read_server_hello_done:
      return "TLS client read_server_hello_done";
    case state_send_client_certificate:
      return "TLS client send_client_certificate";
    case state_send_client_key_exchange:
      return "TLS client send_client_key_exchange";
    case state_send_client_certificate_verify:
      return "TLS client send_client_certificate_verify";
    case state_send_client_finished:
      return "TLS client send_client_finished";
    case state_finish_flight:
      return "TLS client finish_flight";
    case state_read_session_ticket:
      return "TLS client read_session_ticket";
    case state_process_change_cipher_spec:
      return "TLS client process_change_cipher_spec";
    case state_read_server_finished:
      return "TLS client read_server_finished";
    case state_finish_client_handshake:
      return "TLS client finish_client_handshake";
    case state_done:
      return "TLS client done";
  }

  return "TLS client unknown";
}

BSSL_NAMESPACE_END
