/src/boringssl/ssl/tls13_enc.cc

Source
// Copyright 2016 The BoringSSL Authors
//
// 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 <string.h>

#include <algorithm>
#include <string_view>
#include <utility>

#include <openssl/aead.h>
#include <openssl/aes.h>
#include <openssl/bytestring.h>
#include <openssl/chacha.h>
#include <openssl/digest.h>
#include <openssl/hkdf.h>
#include <openssl/hmac.h>
#include <openssl/mem.h>

#include "../crypto/bytestring/internal.h"
#include "../crypto/fipsmodule/tls/internal.h"
#include "../crypto/internal.h"
#include "internal.h"


BSSL_NAMESPACE_BEGIN

static bool init_key_schedule(SSL_HANDSHAKE *hs, SSLTranscript *transcript,
                              uint16_t version, const SSL_CIPHER *cipher) {
  if (!transcript->InitHash(version, cipher)) {
    return false;
  }

  // Initialize the secret to the zero key.
  hs->secret.clear();
  hs->secret.Resize(transcript->DigestLen());
  return true;
}

static bool hkdf_extract_to_secret(SSL_HANDSHAKE *hs,
                                   const SSLTranscript &transcript,
                                   Span<const uint8_t> in) {
  size_t len;
  if (!HKDF_extract(hs->secret.data(), &len, transcript.Digest(), in.data(),
                    in.size(), hs->secret.data(), hs->secret.size())) {
    return false;
  }
  assert(len == hs->secret.size());
  return true;
}

bool tls13_init_key_schedule(SSL_HANDSHAKE *hs, Span<const uint8_t> psk) {
  if (!init_key_schedule(hs, &hs->transcript, ssl_protocol_version(hs->ssl),
                         hs->new_cipher)) {
    return false;
  }

  // Handback includes the whole handshake transcript, so we cannot free the
  // transcript buffer in the handback case.
  if (!hs->handback) {
    hs->transcript.FreeBuffer();
  }
  return hkdf_extract_to_secret(hs, hs->transcript, psk);
}

bool tls13_init_early_key_schedule(SSL_HANDSHAKE *hs,
                                   const SSL_SESSION *session) {
  assert(!hs->ssl->server);
  // When offering ECH, early data is associated with ClientHelloInner, not
  // ClientHelloOuter.
  SSLTranscript *transcript =
      hs->selected_ech_config ? &hs->inner_transcript : &hs->transcript;
  return init_key_schedule(hs, transcript,
                           ssl_session_protocol_version(session),
                           session->cipher) &&
         hkdf_extract_to_secret(hs, *transcript, session->secret);
}

static bool hkdf_expand_label_with_prefix(Span<uint8_t> out,
                                          const EVP_MD *digest,
                                          Span<const uint8_t> secret,
                                          std::string_view label_prefix,
                                          std::string_view label,
                                          Span<const uint8_t> hash) {
  // This is a copy of CRYPTO_tls13_hkdf_expand_label, but modified to take an
  // arbitrary prefix for the label instead of using the hardcoded "tls13 "
  // prefix.
  CBB cbb, child;
  uint8_t *hkdf_label = nullptr;
  size_t hkdf_label_len;
  CBB_zero(&cbb);
  if (!CBB_init(&cbb,
                2 + 1 + label_prefix.size() + label.size() + 1 + hash.size()) ||
      !CBB_add_u16(&cbb, out.size()) ||
      !CBB_add_u8_length_prefixed(&cbb, &child) ||
      !CBB_add_bytes(&child,
                     reinterpret_cast<const uint8_t *>(label_prefix.data()),
                     label_prefix.size()) ||
      !CBB_add_bytes(&child, reinterpret_cast<const uint8_t *>(label.data()),
                     label.size()) ||
      !CBB_add_u8_length_prefixed(&cbb, &child) ||
      !CBB_add_bytes(&child, hash.data(), hash.size()) ||
      !CBB_finish(&cbb, &hkdf_label, &hkdf_label_len)) {
    CBB_cleanup(&cbb);
    return false;
  }

  const int ret = HKDF_expand(out.data(), out.size(), digest, secret.data(),
                              secret.size(), hkdf_label, hkdf_label_len);
  OPENSSL_free(hkdf_label);
  return ret == 1;
}

static bool hkdf_expand_label(Span<uint8_t> out, const EVP_MD *digest,
                              Span<const uint8_t> secret,
                              std::string_view label, Span<const uint8_t> hash,
                              bool is_dtls) {
  if (is_dtls) {
    return hkdf_expand_label_with_prefix(out, digest, secret, "dtls13", label,
                                         hash);
  }
  return CRYPTO_tls13_hkdf_expand_label(
             out.data(), out.size(), digest, secret.data(), secret.size(),
             reinterpret_cast<const uint8_t *>(label.data()), label.size(),
             hash.data(), hash.size()) == 1;
}

static const char kTLS13LabelDerived[] = "derived";

bool tls13_advance_key_schedule(SSL_HANDSHAKE *hs, Span<const uint8_t> in) {
  uint8_t derive_context[EVP_MAX_MD_SIZE];
  unsigned derive_context_len;
  return EVP_Digest(nullptr, 0, derive_context, &derive_context_len,
                    hs->transcript.Digest(), nullptr) &&
         hkdf_expand_label(Span(hs->secret), hs->transcript.Digest(),
                           hs->secret, kTLS13LabelDerived,
                           Span(derive_context, derive_context_len),
                           SSL_is_dtls(hs->ssl)) &&
         hkdf_extract_to_secret(hs, hs->transcript, in);
}

// derive_secret_with_transcript derives a secret of length
// `transcript.DigestLen()` and writes the result in `out` with the given label,
// the current base secret, and the state of `transcript`. It returns true on
// success and false on error.
static bool derive_secret_with_transcript(
    const SSL_HANDSHAKE *hs, InplaceVector<uint8_t, SSL_MAX_MD_SIZE> *out,
    const SSLTranscript &transcript, std::string_view label) {
  uint8_t context_hash[EVP_MAX_MD_SIZE];
  size_t context_hash_len;
  if (!transcript.GetHash(context_hash, &context_hash_len)) {
    return false;
  }

  out->ResizeForOverwrite(transcript.DigestLen());
  return hkdf_expand_label(Span(*out), transcript.Digest(), hs->secret, label,
                           Span(context_hash, context_hash_len),
                           SSL_is_dtls(hs->ssl));
}

static bool derive_secret(SSL_HANDSHAKE *hs,
                          InplaceVector<uint8_t, SSL_MAX_MD_SIZE> *out,
                          std::string_view label) {
  return derive_secret_with_transcript(hs, out, hs->transcript, label);
}

bool tls13_set_traffic_key(SSLImpl *ssl, enum ssl_encryption_level_t level,
                           enum evp_aead_direction_t direction,
                           const SSL_SESSION *session,
                           Span<const uint8_t> traffic_secret) {
  uint16_t version = ssl_session_protocol_version(session);
  const EVP_MD *digest = ssl_session_get_digest(session);
  bool is_dtls = SSL_is_dtls(ssl);
  UniquePtr<SSLAEADContext> traffic_aead;
  if (SSL_is_quic(ssl)) {
    // Install a placeholder SSLAEADContext so that SSL accessors work. The
    // encryption itself will be handled by the SSL_QUIC_METHOD.
    traffic_aead = SSLAEADContext::CreatePlaceholderForQUIC(session->cipher);
  } else {
    // Look up cipher suite properties.
    const EVP_AEAD *aead;
    size_t discard;
    if (!ssl_cipher_get_evp_aead(&aead, &discard, &discard, session->cipher,
                                 version)) {
      return false;
    }

    // Derive the key and IV.
    uint8_t key_buf[EVP_AEAD_MAX_KEY_LENGTH], iv_buf[EVP_AEAD_MAX_NONCE_LENGTH];
    auto key = Span(key_buf).first(EVP_AEAD_key_length(aead));
    auto iv = Span(iv_buf).first(EVP_AEAD_nonce_length(aead));
    if (!hkdf_expand_label(key, digest, traffic_secret, "key", {}, is_dtls) ||
        !hkdf_expand_label(iv, digest, traffic_secret, "iv", {}, is_dtls)) {
      return false;
    }

    traffic_aead = SSLAEADContext::Create(direction, session->ssl_version,
                                          session->cipher, key, {}, iv);
  }

  if (!traffic_aead) {
    return false;
  }

  if (direction == evp_aead_open) {
    if (!ssl->method->set_read_state(ssl, level, std::move(traffic_aead),
                                     traffic_secret)) {
      return false;
    }
    ssl->s3->read_traffic_secret.CopyFrom(traffic_secret);
  } else {
    if (!ssl->method->set_write_state(ssl, level, std::move(traffic_aead),
                                      traffic_secret)) {
      return false;
    }
    ssl->s3->write_traffic_secret.CopyFrom(traffic_secret);
  }

  return true;
}

namespace {

class AESRecordNumberEncrypter : public RecordNumberEncrypter {
 public:
  bool SetKey(Span<const uint8_t> key) override {
    return AES_set_encrypt_key(key.data(), key.size() * 8, &key_) == 0;
  }

  bool GenerateMask(Span<uint8_t> out, Span<const uint8_t> sample) override {
    if (sample.size() < AES_BLOCK_SIZE || out.size() > AES_BLOCK_SIZE) {
      return false;
    }
    uint8_t mask[AES_BLOCK_SIZE];
    AES_encrypt(sample.data(), mask, &key_);
    OPENSSL_memcpy(out.data(), mask, out.size());
    return true;
  }

 private:
  AES_KEY key_;
};

class AES128RecordNumberEncrypter : public AESRecordNumberEncrypter {
 public:
  size_t KeySize() override { return 16; }
};

class AES256RecordNumberEncrypter : public AESRecordNumberEncrypter {
 public:
  size_t KeySize() override { return 32; }
};

class ChaChaRecordNumberEncrypter : public RecordNumberEncrypter {
 public:
  size_t KeySize() override { return kKeySize; }

  bool SetKey(Span<const uint8_t> key) override {
    if (key.size() != kKeySize) {
      return false;
    }
    OPENSSL_memcpy(key_, key.data(), key.size());
    return true;
  }

  bool GenerateMask(Span<uint8_t> out, Span<const uint8_t> sample) override {
    // RFC 9147 section 4.2.3 uses the first 4 bytes of the sample as the
    // counter and the next 12 bytes as the nonce. If we have less than 4+12=16
    // bytes in the sample, then we'll read past the end of the `sample` buffer.
    // The counter is interpreted as little-endian per RFC 8439.
    if (sample.size() < 16) {
      return false;
    }
    uint32_t counter = CRYPTO_load_u32_le(sample.data());
    auto nonce = sample.subspan<4>();
    OPENSSL_memset(out.data(), 0, out.size());
    CRYPTO_chacha_20(out.data(), out.data(), out.size(), key_, nonce.data(),
                     counter);
    return true;
  }

 private:
  static constexpr size_t kKeySize = 32;
  uint8_t key_[kKeySize];
};

class NullRecordNumberEncrypter : public RecordNumberEncrypter {
 public:
  size_t KeySize() override { return 0; }
  bool SetKey(Span<const uint8_t> key) override { return true; }
  bool GenerateMask(Span<uint8_t> out, Span<const uint8_t> sample) override {
    OPENSSL_memset(out.data(), 0, out.size());
    return true;
  }
};

}  // namespace

UniquePtr<RecordNumberEncrypter> RecordNumberEncrypter::Create(
    const SSL_CIPHER *cipher, Span<const uint8_t> traffic_secret) {
  const EVP_MD *digest = ssl_get_handshake_digest(TLS1_3_VERSION, cipher);
  UniquePtr<RecordNumberEncrypter> ret;
  if (CRYPTO_fuzzer_mode_enabled()) {
    ret = MakeUnique<NullRecordNumberEncrypter>();
  } else if (cipher->algorithm_enc == SSL_AES128GCM) {
    ret = MakeUnique<AES128RecordNumberEncrypter>();
  } else if (cipher->algorithm_enc == SSL_AES256GCM) {
    ret = MakeUnique<AES256RecordNumberEncrypter>();
  } else if (cipher->algorithm_enc == SSL_CHACHA20POLY1305) {
    ret = MakeUnique<ChaChaRecordNumberEncrypter>();
  } else {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
  }
  if (ret == nullptr) {
    return nullptr;
  }

  uint8_t rne_key_buf[RecordNumberEncrypter::kMaxKeySize];
  auto rne_key = Span(rne_key_buf).first(ret->KeySize());
  if (!hkdf_expand_label(rne_key, digest, traffic_secret, "sn", {},
                         /*is_dtls=*/true) ||
      !ret->SetKey(rne_key)) {
    return nullptr;
  }
  return ret;
}

static const char kTLS13LabelExporter[] = "exp master";

static const char kTLS13LabelClientEarlyTraffic[] = "c e traffic";
static const char kTLS13LabelClientHandshakeTraffic[] = "c hs traffic";
static const char kTLS13LabelServerHandshakeTraffic[] = "s hs traffic";
static const char kTLS13LabelClientApplicationTraffic[] = "c ap traffic";
static const char kTLS13LabelServerApplicationTraffic[] = "s ap traffic";

bool tls13_derive_early_secret(SSL_HANDSHAKE *hs) {
  SSLImpl *const ssl = hs->ssl;
  // When offering ECH on the client, early data is associated with
  // ClientHelloInner, not ClientHelloOuter.
  const SSLTranscript &transcript = (!ssl->server && hs->selected_ech_config)
                                        ? hs->inner_transcript
                                        : hs->transcript;
  if (!derive_secret_with_transcript(hs, &hs->early_traffic_secret, transcript,
                                     kTLS13LabelClientEarlyTraffic) ||
      !ssl_log_secret(ssl, "CLIENT_EARLY_TRAFFIC_SECRET",
                      hs->early_traffic_secret)) {
    return false;
  }
  return true;
}

bool tls13_derive_handshake_secrets(SSL_HANDSHAKE *hs) {
  SSLImpl *const ssl = hs->ssl;
  if (!derive_secret(hs, &hs->client_handshake_secret,
                     kTLS13LabelClientHandshakeTraffic) ||
      !ssl_log_secret(ssl, "CLIENT_HANDSHAKE_TRAFFIC_SECRET",
                      hs->client_handshake_secret) ||
      !derive_secret(hs, &hs->server_handshake_secret,
                     kTLS13LabelServerHandshakeTraffic) ||
      !ssl_log_secret(ssl, "SERVER_HANDSHAKE_TRAFFIC_SECRET",
                      hs->server_handshake_secret)) {
    return false;
  }

  return true;
}

bool tls13_derive_application_secrets(SSL_HANDSHAKE *hs) {
  SSLImpl *const ssl = hs->ssl;
  if (!derive_secret(hs, &hs->client_traffic_secret_0,
                     kTLS13LabelClientApplicationTraffic) ||
      !ssl_log_secret(ssl, "CLIENT_TRAFFIC_SECRET_0",
                      hs->client_traffic_secret_0) ||
      !derive_secret(hs, &hs->server_traffic_secret_0,
                     kTLS13LabelServerApplicationTraffic) ||
      !ssl_log_secret(ssl, "SERVER_TRAFFIC_SECRET_0",
                      hs->server_traffic_secret_0) ||
      !derive_secret(hs, &ssl->s3->exporter_secret, kTLS13LabelExporter) ||
      !ssl_log_secret(ssl, "EXPORTER_SECRET", ssl->s3->exporter_secret)) {
    return false;
  }

  return true;
}

static const char kTLS13LabelApplicationTraffic[] = "traffic upd";

bool tls13_rotate_traffic_key(SSLImpl *ssl,
                              enum evp_aead_direction_t direction) {
  InplaceVector<uint8_t, SSL_MAX_MD_SIZE> secret(
      direction == evp_aead_open ? ssl->s3->read_traffic_secret
                                 : ssl->s3->write_traffic_secret);

  const SSL_SESSION *session = SSL_get_session(ssl);
  const EVP_MD *digest = ssl_session_get_digest(session);
  return hkdf_expand_label(Span(secret), digest, secret,
                           kTLS13LabelApplicationTraffic, {},
                           SSL_is_dtls(ssl)) &&
         tls13_set_traffic_key(ssl, ssl_encryption_application, direction,
                               session, Span(secret));
}

static const char kTLS13LabelResumption[] = "res master";

bool tls13_derive_resumption_secret(SSL_HANDSHAKE *hs) {
  return derive_secret(hs, &hs->new_session->secret, kTLS13LabelResumption);
}

static const char kTLS13LabelFinished[] = "finished";

// tls13_verify_data sets `out` to be the HMAC of `context` using a derived
// Finished key for both Finished messages and the PSK binder. `out` must have
// space available for `EVP_MAX_MD_SIZE` bytes.
static bool tls13_verify_data(uint8_t *out, size_t *out_len,
                              const EVP_MD *digest, Span<const uint8_t> secret,
                              Span<const uint8_t> context, bool is_dtls) {
  uint8_t key_buf[EVP_MAX_MD_SIZE];
  auto key = Span(key_buf, EVP_MD_size(digest));
  unsigned len;
  if (!hkdf_expand_label(key, digest, secret, kTLS13LabelFinished, {},
                         is_dtls) ||
      HMAC(digest, key.data(), key.size(), context.data(), context.size(), out,
           &len) == nullptr) {
    return false;
  }
  *out_len = len;
  return true;
}

bool tls13_finished_mac(SSL_HANDSHAKE *hs, uint8_t *out, size_t *out_len,
                        bool is_server) {
  Span<const uint8_t> traffic_secret =
      is_server ? hs->server_handshake_secret : hs->client_handshake_secret;

  uint8_t context_hash[EVP_MAX_MD_SIZE];
  size_t context_hash_len;
  if (!hs->transcript.GetHash(context_hash, &context_hash_len) ||
      !tls13_verify_data(out, out_len, hs->transcript.Digest(), traffic_secret,
                         Span(context_hash, context_hash_len),
                         SSL_is_dtls(hs->ssl))) {
    return false;
  }
  return true;
}

static const char kTLS13LabelResumptionPSK[] = "resumption";

bool tls13_derive_session_psk(SSL_SESSION *session, Span<const uint8_t> nonce,
                              bool is_dtls) {
  const EVP_MD *digest = ssl_session_get_digest(session);
  // The session initially stores the resumption_master_secret, which we
  // override with the PSK.
  assert(session->secret.size() == EVP_MD_size(digest));
  return hkdf_expand_label(Span(session->secret), digest, session->secret,
                           kTLS13LabelResumptionPSK, nonce, is_dtls);
}

static const char kTLS13LabelExportKeying[] = "exporter";

bool tls13_export_keying_material(const SSLImpl *ssl, Span<uint8_t> out,
                                  Span<const uint8_t> secret,
                                  std::string_view label,
                                  Span<const uint8_t> context) {
  if (secret.empty()) {
    assert(0);
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return false;
  }

  const EVP_MD *digest = ssl_session_get_digest(SSL_get_session(ssl));

  uint8_t hash_buf[EVP_MAX_MD_SIZE];
  uint8_t export_context_buf[EVP_MAX_MD_SIZE];
  unsigned hash_len;
  unsigned export_context_len;
  if (!EVP_Digest(context.data(), context.size(), hash_buf, &hash_len, digest,
                  nullptr) ||
      !EVP_Digest(nullptr, 0, export_context_buf, &export_context_len, digest,
                  nullptr)) {
    return false;
  }

  auto hash = Span(hash_buf, hash_len);
  auto export_context = Span(export_context_buf, export_context_len);
  uint8_t derived_secret_buf[EVP_MAX_MD_SIZE];
  auto derived_secret = Span(derived_secret_buf, EVP_MD_size(digest));
  return hkdf_expand_label(derived_secret, digest, secret, label,
                           export_context, SSL_is_dtls(ssl)) &&
         hkdf_expand_label(out, digest, derived_secret, kTLS13LabelExportKeying,
                           hash, SSL_is_dtls(ssl));
}

const EVP_MD *ssl_pre_shared_key_hash(const SSLPreSharedKey &psk) {
  if (const auto *imported = std::get_if<SSLImportedPSK>(&psk);
      imported != nullptr) {
    return imported->md;
  }
  return ssl_session_get_digest(std::get<UniquePtr<SSL_SESSION>>(psk).get());
}

Span<const uint8_t> ssl_pre_shared_key_identity(const SSLPreSharedKey &psk) {
  if (const auto *imported = std::get_if<SSLImportedPSK>(&psk);
      imported != nullptr) {
    return imported->imported_identity;
  }
  return std::get<UniquePtr<SSL_SESSION>>(psk)->ticket;
}

Span<const uint8_t> ssl_pre_shared_key_secret(const SSLPreSharedKey &psk) {
  if (const auto *imported = std::get_if<SSLImportedPSK>(&psk);
      imported != nullptr) {
    return imported->ipskx;
  }
  return std::get<UniquePtr<SSL_SESSION>>(psk)->secret;
}

bool tls13_psk_binder(const SSL_HANDSHAKE *hs, Span<uint8_t> out,
                      size_t *out_len, const SSLPreSharedKey &psk,
                      const SSLTranscript &transcript,
                      Span<const uint8_t> client_hello, size_t binders_len) {
  const EVP_MD *digest;
  Span<const uint8_t> secret;
  std::string_view label;
  if (const auto *imported = std::get_if<SSLImportedPSK>(&psk);
      imported != nullptr) {
    digest = imported->md;
    secret = imported->ipskx;
    label = "imp binder";
  } else {
    const SSL_SESSION *session = std::get<UniquePtr<SSL_SESSION>>(psk).get();
    digest = ssl_session_get_digest(session);
    secret = session->secret;
    label = "res binder";
  }

  // Compute the binder key.
  //
  // TODO(davidben): Ideally we wouldn't recompute early secret and the binder
  // key each time.
  uint8_t binder_context[EVP_MAX_MD_SIZE];
  unsigned binder_context_len;
  uint8_t early_secret[EVP_MAX_MD_SIZE] = {0};
  size_t early_secret_len;
  uint8_t binder_key_buf[EVP_MAX_MD_SIZE] = {0};
  auto binder_key = Span(binder_key_buf, EVP_MD_size(digest));
  if (!EVP_Digest(nullptr, 0, binder_context, &binder_context_len, digest,
                  nullptr) ||
      !HKDF_extract(early_secret, &early_secret_len, digest, secret.data(),
                    secret.size(), nullptr, 0) ||
      !hkdf_expand_label(
          binder_key, digest, Span(early_secret, early_secret_len), label,
          Span(binder_context, binder_context_len), SSL_is_dtls(hs->ssl))) {
    return false;
  }

  // Hash the transcript and truncated ClientHello. As part of this, construct
  // the expected ClientHello header.
  if (client_hello.size() < binders_len || client_hello.size() > 0xffffff) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return false;
  }
  uint8_t header[4] = {
      SSL3_MT_CLIENT_HELLO,
      static_cast<uint8_t>(client_hello.size() >> 16),
      static_cast<uint8_t>(client_hello.size() >> 8),
      static_cast<uint8_t>(client_hello.size()),
  };
  auto truncated = client_hello.subspan(0, client_hello.size() - binders_len);
  uint8_t context[EVP_MAX_MD_SIZE];
  unsigned context_len;
  ScopedEVP_MD_CTX ctx;
  if (!transcript.CopyToHashContext(ctx.get(), digest) ||
      !EVP_DigestUpdate(ctx.get(), header, sizeof(header)) ||
      !EVP_DigestUpdate(ctx.get(), truncated.data(), truncated.size()) ||
      !EVP_DigestFinal_ex(ctx.get(), context, &context_len)) {
    return false;
  }

  BSSL_CHECK(out.size() >= EVP_MD_size(digest));
  if (!tls13_verify_data(out.data(), out_len, digest, binder_key,
                         Span(context, context_len), SSL_is_dtls(hs->ssl))) {
    return false;
  }

  assert(*out_len == EVP_MD_size(digest));
  return true;
}

static std::optional<uint16_t> hkdf_md_to_kdf_id(const EVP_MD *hkdf_md) {
  // See Section 10 of RFC 9258.
  switch (EVP_MD_nid(hkdf_md)) {
    case NID_sha256:
      return 0x0001;  // HKDF_SHA256
    case NID_sha384:
      return 0x0002;  // HKDF_SHA384
    default:
      return std::nullopt;
  }
}

std::optional<SSLImportedPSK> tls13_derive_imported_psk(const SSL_HANDSHAKE *hs,
                                                        SSLCredential *cred,
                                                        uint16_t protocol,
                                                        const EVP_MD *hkdf_md) {
  assert(cred->type == SSLCredentialType::kPreSharedKey);

  std::optional<uint16_t> target_kdf = hkdf_md_to_kdf_id(hkdf_md);
  if (!target_kdf.has_value()) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return std::nullopt;
  }

  SSLImportedPSK ret;
  ret.credential = UpRef(cred);
  ret.protocol = protocol;
  ret.md = hkdf_md;

  // See Section 5.1 of RFC 9258.
  ScopedCBB imported_id;
  CBB external_identity, context;
  if (!CBB_init(imported_id.get(), 2 + cred->epsk_id.size() + 2 +
                                       cred->epsk_context.size() + 2 + 2) ||
      !CBB_add_u16_length_prefixed(imported_id.get(), &external_identity) ||
      !CBB_add_bytes(&external_identity, cred->epsk_id.data(),
                     cred->epsk_id.size()) ||
      !CBB_add_u16_length_prefixed(imported_id.get(), &context) ||
      !CBB_add_bytes(&context, cred->epsk_context.data(),
                     cred->epsk_context.size()) ||
      !CBB_add_u16(imported_id.get(), protocol) ||
      !CBB_add_u16(imported_id.get(), *target_kdf) ||
      !CBBFinishArray(imported_id.get(), &ret.imported_identity)) {
    return std::nullopt;
  }

  ScopedEVP_MD_CTX imported_id_ctx;
  InplaceVector<uint8_t, EVP_MAX_MD_SIZE> imported_id_hash;
  imported_id_hash.ResizeForOverwrite(EVP_MD_size(cred->epsk_md));
  unsigned imported_id_hash_len;
  if (!EVP_Digest(ret.imported_identity.data(), ret.imported_identity.size(),
                  imported_id_hash.data(), &imported_id_hash_len, cred->epsk_md,
                  nullptr)) {
    return std::nullopt;
  }
  assert(imported_id_hash.size() == imported_id_hash_len);

  ret.ipskx.ResizeForOverwrite(EVP_MD_size(hkdf_md));
  if (!hkdf_expand_label(Span(ret.ipskx), cred->epsk_md, cred->epskx,
                         "derived psk", imported_id_hash,
                         SSL_is_dtls(hs->ssl))) {
    return std::nullopt;
  }

  return ret;
}

bool tls13_compare_imported_psk_identity(Span<const uint8_t> id,
                                         const SSLCredential *cred,
                                         uint16_t protocol,
                                         const EVP_MD *hkdf_md) {
  assert(cred->type == SSLCredentialType::kPreSharedKey);
  std::optional<uint16_t> target_kdf = hkdf_md_to_kdf_id(hkdf_md);
  if (!target_kdf.has_value()) {
    return false;
  }

  // See Section 5.1 of RFC 9258.
  CBS cbs = id, external_identity, context;
  uint16_t found_protocol, found_kdf;
  return CBS_get_u16_length_prefixed(&cbs, &external_identity) &&
         external_identity == Span(cred->epsk_id) &&
         CBS_get_u16_length_prefixed(&cbs, &context) &&
         context == Span(cred->epsk_context) &&
         CBS_get_u16(&cbs, &found_protocol) && found_protocol == protocol &&
         CBS_get_u16(&cbs, &found_kdf) && found_kdf == *target_kdf &&
         CBS_len(&cbs) == 0;
}

size_t ssl_ech_confirmation_signal_hello_offset(const SSLImpl *ssl) {
  static_assert(ECH_CONFIRMATION_SIGNAL_LEN < SSL3_RANDOM_SIZE,
                "the confirmation signal is a suffix of the random");
  const size_t header_len =
      SSL_is_dtls(ssl) ? DTLS1_HM_HEADER_LENGTH : SSL3_HM_HEADER_LENGTH;
  return header_len + 2 /* version */ + SSL3_RANDOM_SIZE -
         ECH_CONFIRMATION_SIGNAL_LEN;
}

bool ssl_ech_accept_confirmation(
    const SSL_HANDSHAKE *hs, Span<uint8_t, ECH_CONFIRMATION_SIGNAL_LEN> out,
    Span<const uint8_t, SSL3_RANDOM_SIZE> client_random,
    const SSLTranscript &transcript, bool is_hrr, Span<const uint8_t> msg,
    size_t offset) {
  // See RFC 9849, sections 7.2 and 7.2.1.
  static const uint8_t kZeros[EVP_MAX_MD_SIZE] = {0};

  // We hash `msg`, with bytes from `offset` zeroed.
  if (msg.size() < offset + ECH_CONFIRMATION_SIGNAL_LEN) {
    OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
    return false;
  }

  // We represent DTLS messages with the longer DTLS 1.2 header, but DTLS 1.3
  // removes the extra fields from the transcript.
  //
  // Size bound implied by ECH_CONFIRMATION_SIGNAL_LEN >= SSL3_HM_HEADER_LENGTH.
  auto header = msg.first<SSL3_HM_HEADER_LENGTH>();
  size_t full_header_len =
      SSL_is_dtls(hs->ssl) ? DTLS1_HM_HEADER_LENGTH : SSL3_HM_HEADER_LENGTH;
  auto before_zeros = msg.subspan(full_header_len, offset - full_header_len);
  auto after_zeros = msg.subspan(offset + ECH_CONFIRMATION_SIGNAL_LEN);

  uint8_t context[EVP_MAX_MD_SIZE];
  unsigned context_len;
  ScopedEVP_MD_CTX ctx;
  if (!transcript.CopyToHashContext(ctx.get(), transcript.Digest()) ||
      !EVP_DigestUpdate(ctx.get(), header.data(), header.size()) ||
      !EVP_DigestUpdate(ctx.get(), before_zeros.data(), before_zeros.size()) ||
      !EVP_DigestUpdate(ctx.get(), kZeros, ECH_CONFIRMATION_SIGNAL_LEN) ||
      !EVP_DigestUpdate(ctx.get(), after_zeros.data(), after_zeros.size()) ||
      !EVP_DigestFinal_ex(ctx.get(), context, &context_len)) {
    return false;
  }

  uint8_t secret[EVP_MAX_MD_SIZE];
  size_t secret_len;
  if (!HKDF_extract(secret, &secret_len, transcript.Digest(),
                    client_random.data(), client_random.size(), kZeros,
                    transcript.DigestLen())) {
    return false;
  }

  return hkdf_expand_label(
      out, transcript.Digest(), Span(secret, secret_len),
      is_hrr ? "hrr ech accept confirmation" : "ech accept confirmation",
      Span(context, context_len), SSL_is_dtls(hs->ssl));
}

BSSL_NAMESPACE_END

Coverage Report

Created: 2026-08-08 07:14