File: soft_bind_attestation_flow_impl.cc

package info (click to toggle)
chromium 139.0.7258.127-1
  • links: PTS, VCS
  • area: main
  • in suites:
  • size: 6,122,068 kB
  • sloc: cpp: 35,100,771; ansic: 7,163,530; javascript: 4,103,002; python: 1,436,920; asm: 946,517; xml: 746,709; pascal: 187,653; perl: 88,691; sh: 88,436; objc: 79,953; sql: 51,488; cs: 44,583; fortran: 24,137; makefile: 22,147; tcl: 15,277; php: 13,980; yacc: 8,984; ruby: 7,485; awk: 3,720; lisp: 3,096; lex: 1,327; ada: 727; jsp: 228; sed: 36
file content (496 lines) | stat: -rw-r--r-- 19,762 bytes parent folder | download | duplicates (6)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
// Copyright 2021 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "chrome/browser/ash/attestation/soft_bind_attestation_flow_impl.h"

#include <optional>

#include "base/containers/span.h"
#include "base/debug/dump_without_crashing.h"
#include "base/functional/bind.h"
#include "base/logging.h"
#include "base/timer/timer.h"
#include "chrome/browser/ash/attestation/attestation_ca_client.h"
#include "chromeos/ash/components/attestation/attestation_flow_adaptive.h"
#include "chromeos/ash/components/cryptohome/cryptohome_parameters.h"
#include "chromeos/ash/components/dbus/attestation/attestation_client.h"
#include "chromeos/ash/components/dbus/constants/attestation_constants.h"
#include "chromeos/ash/components/settings/cros_settings.h"
#include "content/public/browser/browser_thread.h"
#include "crypto/openssl_util.h"
#include "crypto/random.h"
#include "crypto/rsa_private_key.h"
#include "net/cert/asn1_util.h"
#include "net/cert/x509_certificate.h"
#include "net/cert/x509_util.h"
#include "third_party/boringssl/src/include/openssl/bn.h"
#include "third_party/boringssl/src/include/openssl/bytestring.h"
#include "third_party/boringssl/src/include/openssl/ec.h"
#include "third_party/boringssl/src/include/openssl/err.h"
#include "third_party/boringssl/src/include/openssl/mem.h"
#include "third_party/boringssl/src/pki/pem.h"
#include "third_party/securemessage/proto/securemessage.pb.h"

namespace ash {
namespace attestation {

namespace {

//  Adds a critical extension following the specification described at
//  https://datatracker.ietf.org/doc/html/rfc5280#section-4.2 and
//  the ASN.1 encoding defined at
//  https://datatracker.ietf.org/doc/html/rfc5280#section-4.1:
//  Extension  ::=  SEQUENCE  {
//        extnID      OBJECT IDENTIFIER,
//        critical    BOOLEAN DEFAULT FALSE,
//        extnValue   OCTET STRING
//                    -- contains the DER encoding of an ASN.1 value
//                    -- corresponding to the extension type identified
//                    -- by extnID
//        }
bool AddCriticalExtension(CBB* extensions,
                          const uint8_t* ext_oid,
                          size_t ext_oid_len,
                          const uint8_t* ext_value,
                          size_t ext_value_len) {
  CBB extension, oid, value;
  if (!CBB_add_asn1(extensions, &extension, CBS_ASN1_SEQUENCE) ||
      !CBB_add_asn1(&extension, &oid, CBS_ASN1_OBJECT) ||
      !CBB_add_bytes(&oid, ext_oid, ext_oid_len) ||
      !CBB_add_asn1_bool(&extension, 1) ||
      !CBB_add_asn1(&extension, &value, CBS_ASN1_OCTETSTRING) ||
      !CBB_add_bytes(&value, ext_value, ext_value_len) ||
      !CBB_flush(extensions)) {
    return false;
  }
  return true;
}

// The validity time of the leaf cert
constexpr base::TimeDelta kLeafCertValidityWindow = base::Hours(72);

// Trigger a new certificate if current certificate is nearing expiration.
constexpr base::TimeDelta kExpiryThresholdDays = base::Days(30);

// RSA SHA256 oid
const uint8_t kRsaSha256Oid[] = {0x2a, 0x86, 0x48, 0x86, 0xf7,
                                 0x0d, 0x01, 0x01, 0x0b};

const uint8_t kBasicConstraintsOid[] = {0x55, 0x1d, 0x13};
const uint8_t kKeyUsageOid[] = {0x55, 0x1d, 0x0f};
// cA = FALSE
const uint8_t kBasicConstraintsContents[] = {0x30, 0x00};
// Tag 3 (bit string), length 2, 0 unused of 0x80 (0b10000000)
// Bit 0 specifies the digitalSignature usage.
const uint8_t kKeyUsageContents[] = {0x03, 0x02, 0x00, 0x80};

const char kLeafCertIssuerName[] =
    "O=Chrome Device Soft Bind,CN=Local Authority";
const char kLeafCertSubjectName[] =
    "O=Chrome Device Soft Bind,CN=Cryptauth User Key";

// If it takes more than 30 seconds to receive a response, it's not likely
// ever going to succeed.
constexpr base::TimeDelta kTimeout = base::Seconds(30);

}  // namespace

SoftBindAttestationFlowImpl::Session::Session(Callback callback,
                                              AccountId account_id,
                                              const std::string& user_key)
    : callback_(std::move(callback)),
      account_id_(account_id),
      user_key_(user_key) {
  base::RepeatingClosure timeout_callback =
      base::BindRepeating(&SoftBindAttestationFlowImpl::Session::OnTimeout,
                          weak_ptr_factory_.GetWeakPtr());
  timer_.Start(FROM_HERE, kTimeout, std::move(timeout_callback));
}

SoftBindAttestationFlowImpl::Session::~Session() = default;

void SoftBindAttestationFlowImpl::Session::OnTimeout() {
  LOG(WARNING) << "Timeout exceeded";
  ReportFailure("timeout");
}

bool SoftBindAttestationFlowImpl::Session::IsTimerRunning() const {
  return timer_.IsRunning();
}

void SoftBindAttestationFlowImpl::Session::StopTimer() {
  timer_.Stop();
}

bool SoftBindAttestationFlowImpl::Session::ResetTimer() {
  if (max_retries_-- > 0) {
    timer_.Reset();
    return true;
  }
  return false;
}

const AccountId& SoftBindAttestationFlowImpl::Session::GetAccountId() const {
  return account_id_;
}

const std::string& SoftBindAttestationFlowImpl::Session::GetUserKey() const {
  return user_key_;
}

void SoftBindAttestationFlowImpl::Session::ReportFailure(
    const std::string& error_message) {
  LOG(WARNING) << "Attestation session failure: " << error_message;
  if (!callback_) {
    LOG(WARNING) << "Callback is null";
    base::debug::DumpWithoutCrashing();
    return;
  }
  std::move(callback_).Run(std::vector<std::string>{"INVALID:" + error_message},
                           /*valid=*/false);
}

void SoftBindAttestationFlowImpl::Session::ReportSuccess(
    const std::vector<std::string>& certificate_chain) {
  if (!callback_) {
    LOG(WARNING) << "Attestation session success but callback is null";
    base::debug::DumpWithoutCrashing();
    return;
  }
  std::move(callback_).Run(certificate_chain, /*valid=*/true);
}

SoftBindAttestationFlowImpl::SoftBindAttestationFlowImpl()
    : attestation_client_(AttestationClient::Get()) {
  std::unique_ptr<ServerProxy> attestation_ca_client(new AttestationCAClient());
  attestation_flow_ = std::make_unique<AttestationFlowAdaptive>(
      std::move(attestation_ca_client));
}

SoftBindAttestationFlowImpl::~SoftBindAttestationFlowImpl() = default;

void SoftBindAttestationFlowImpl::SetAttestationFlowForTesting(
    std::unique_ptr<AttestationFlow> attestation_flow) {
  attestation_flow_ = std::move(attestation_flow);
}

void SoftBindAttestationFlowImpl::GetCertificate(Callback callback,
                                                 const AccountId& account_id,
                                                 const std::string& user_key) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  if (!IsAttestationAllowedByPolicy()) {
    LOG(ERROR) << "Attestation not allowed by device policy";
    std::move(callback).Run(
        std::vector<std::string>{"INVALID:attestationNotAllowed"},
        /*valid=*/false);
    return;
  }
  GetCertificateInternal(
      /*force_new_key=*/false,
      std::make_unique<Session>(std::move(callback), account_id, user_key));
}

void SoftBindAttestationFlowImpl::GetCertificateInternal(
    bool force_new_key,
    std::unique_ptr<Session> session) {
  AccountId account_id(session->GetAccountId());
  AttestationFlow::CertificateCallback certificate_callback =
      base::BindOnce(&SoftBindAttestationFlowImpl::OnCertificateReady,
                     weak_ptr_factory_.GetWeakPtr(), std::move(session));
  attestation_flow_->GetCertificate(
      /*certificate_profile=*/PROFILE_SOFT_BIND_CERTIFICATE,
      /*account_id=*/account_id,
      /*request_origin=*/std::string(),
      /*force_new_key=*/force_new_key,
      /*key_crypto_type=*/::attestation::KEY_TYPE_RSA,
      /*key_name=*/kSoftBindKey, /*profile_specific_data=*/std::nullopt,
      /*callback=*/std::move(certificate_callback));
}

void SoftBindAttestationFlowImpl::OnCertificateReady(
    std::unique_ptr<Session> session,
    AttestationStatus operation_status,
    const std::string& certificate_chain) {
  if (!session->IsTimerRunning()) {
    LOG(WARNING) << "Certificate ready but already timed out";
    return;
  }
  session->StopTimer();
  if (operation_status != ATTESTATION_SUCCESS) {
    LOG(ERROR) << "Attestation unsuccessful, not verified";
    session->ReportFailure("notVerified");
    return;
  }
  CertificateExpiryStatus expiry_status = CheckExpiry(certificate_chain);
  if (expiry_status == CertificateExpiryStatus::kExpired) {
    if (session->ResetTimer()) {
      GetCertificateInternal(/*force_new_key=*/true, std::move(session));
    } else {
      session->ReportFailure("tooManyRetries");
    }
    return;
  }
  VLOG(1) << "Intermediate certificate obtained successfully";

  // Construct a short-lived leaf certificate for the given user key and
  // sign with the intermediate soft-bind attestation key. This binding of
  // a hardware-backed attestation key (the intermediate) to a software key
  // (the user key), is the fundamental operation of the soft-bind
  // attestation scheme.

  std::string user_key(session->GetUserKey());
  securemessage::GenericPublicKey generic_public_key;
  generic_public_key.ParseFromString(user_key);
  std::string raw_x(generic_public_key.ec_p256_public_key().x());
  bssl::UniquePtr<BIGNUM> x(BN_new());
  BN_bin2bn(reinterpret_cast<const uint8_t*>(raw_x.data()), raw_x.size(),
            x.get());
  std::string raw_y(generic_public_key.ec_p256_public_key().y());
  bssl::UniquePtr<BIGNUM> y(BN_new());
  BN_bin2bn(reinterpret_cast<const uint8_t*>(raw_y.data()), raw_y.size(),
            y.get());

  bssl::UniquePtr<EC_GROUP> ec_group(
      EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1));
  bssl::UniquePtr<EC_POINT> ec_point(EC_POINT_new(ec_group.get()));
  if (!EC_POINT_set_affine_coordinates(ec_group.get(), ec_point.get(), x.get(),
                                       y.get(), /* ctx= */ nullptr)) {
    LOG(ERROR) << "SoftBindAttestation: Could not set user key coordinates: "
               << ERR_error_string(ERR_get_error(), nullptr);
    session->ReportFailure("couldNotSetUserKeyCoordsNotOnCurve");
    return;
  }
  bssl::UniquePtr<EC_KEY> ec_key(
      EC_KEY_new_by_curve_name(NID_X9_62_prime256v1));
  if (!EC_KEY_set_public_key(ec_key.get(), ec_point.get())) {
    LOG(ERROR) << "SoftBindAttestation: Could not set user key public key: "
               << ERR_error_string(ERR_get_error(), nullptr);
    session->ReportFailure("couldNotSetUserKeyPublicKey");
    return;
  }
  bssl::UniquePtr<EVP_PKEY> pkey(EVP_PKEY_new());
  if (!EVP_PKEY_assign_EC_KEY(pkey.get(), ec_key.release())) {
    LOG(ERROR) << "SoftBindAttestation: Could not assign user key pkey: "
               << ERR_error_string(ERR_get_error(), nullptr);
    session->ReportFailure("couldNotAssignUserKeyPkey");
    return;
  }

  base::Time now = base::Time::Now();
  std::string leaf_cert;
  GenerateLeafCert(pkey.get(), now, now + kLeafCertValidityWindow, &leaf_cert);

  std::string tbs_cert(leaf_cert);

  ::attestation::SignRequest request;
  request.set_username(
      cryptohome::Identification(session->GetAccountId()).id());
  std::string key_name(kSoftBindKey);
  request.set_key_label(std::move(key_name));
  request.set_data_to_sign(std::move(leaf_cert));
  AttestationClient::Get()->Sign(
      request,
      base::BindOnce(&SoftBindAttestationFlowImpl::OnCertificateSigned,
                     weak_ptr_factory_.GetWeakPtr(), std::move(session),
                     tbs_cert, certificate_chain,
                     expiry_status == CertificateExpiryStatus::kExpiringSoon));
}

void SoftBindAttestationFlowImpl::OnCertificateSigned(
    std::unique_ptr<Session> session,
    const std::string& tbs_cert,
    const std::string& certificate_chain,
    bool should_renew,
    const ::attestation::SignReply& reply) {
  if (reply.status() != ::attestation::STATUS_SUCCESS) {
    LOG(ERROR) << "Could not sign attestation certificate: " << reply.status();
    session->ReportFailure("couldNotSignCert");
    return;
  }

  bssl::ScopedCBB cbb;
  CBB signed_cert, signature, alg, alg_oid, alg_null;
  uint8_t* signed_cert_bytes;
  size_t signed_cert_len;
  if (!CBB_init(cbb.get(), 64) ||
      !CBB_add_asn1(cbb.get(), &signed_cert, CBS_ASN1_SEQUENCE) ||
      !CBB_add_bytes(&signed_cert,
                     reinterpret_cast<const uint8_t*>(tbs_cert.data()),
                     tbs_cert.size()) ||
      !CBB_add_asn1(&signed_cert, &alg, CBS_ASN1_SEQUENCE) ||
      !CBB_add_asn1(&alg, &alg_oid, CBS_ASN1_OBJECT) ||
      !CBB_add_bytes(&alg_oid, kRsaSha256Oid, 9) ||
      !CBB_add_asn1(&alg, &alg_null, CBS_ASN1_NULL) ||
      !CBB_add_asn1(&signed_cert, &signature, CBS_ASN1_BITSTRING) ||
      !CBB_add_u8(&signature, 0) ||
      !CBB_add_bytes(&signature,
                     reinterpret_cast<const uint8_t*>(reply.signature().data()),
                     reply.signature().size()) ||
      !CBB_flush(&signature) || !CBB_flush(&signed_cert) ||
      !CBB_finish(cbb.get(), &signed_cert_bytes, &signed_cert_len)) {
    LOG(ERROR) << "Could not sign attestation certificate";
    session->ReportFailure("couldNotSignCertCbb");
    return;
  }
  std::string der_encoded_cert;
  der_encoded_cert.assign(reinterpret_cast<char*>(signed_cert_bytes),
                          signed_cert_len);
  bssl::UniquePtr<uint8_t> delete_signed_cert_bytes(signed_cert_bytes);
  std::string pem_encoded_cert;
  net::X509Certificate::GetPEMEncodedFromDER(der_encoded_cert,
                                             &pem_encoded_cert);

  std::vector<std::string> cert_chain_with_leaf = {pem_encoded_cert};

  bssl::PEMTokenizer pem_tokenizer(certificate_chain, {"CERTIFICATE"});
  while (pem_tokenizer.GetNext()) {
    std::string pem_encoded_intermediate_cert;
    net::X509Certificate::GetPEMEncodedFromDER(pem_tokenizer.data(),
                                               &pem_encoded_intermediate_cert);
    cert_chain_with_leaf.push_back(pem_encoded_intermediate_cert);
  }

  session->ReportSuccess(cert_chain_with_leaf);

  // If certificate is close to expiry, send a new request to ensure
  // uninterrupted continuity.
  if (should_renew && renewals_in_progress_.count(certificate_chain) == 0) {
    renewals_in_progress_.insert(certificate_chain);
    AttestationFlow::CertificateCallback renew_callback = base::BindOnce(
        &SoftBindAttestationFlowImpl::RenewCertificateCallback,
        weak_ptr_factory_.GetWeakPtr(), std::move(certificate_chain));
    attestation_flow_->GetCertificate(
        /*certificate_profile=*/PROFILE_SOFT_BIND_CERTIFICATE,
        /*account_id=*/session->GetAccountId(),
        /*request_origin=*/std::string(), /*force_new_key=*/true,
        /*key_crypto_type=*/::attestation::KEY_TYPE_RSA,
        /*key_name=*/kSoftBindKey, /*profile_specific_data=*/std::nullopt,
        /*callback=*/std::move(renew_callback));
  }
}

bool SoftBindAttestationFlowImpl::IsAttestationAllowedByPolicy() const {
  bool enabled_for_device = false;
  if (!CrosSettings::Get()->GetBoolean(kAttestationForContentProtectionEnabled,
                                       &enabled_for_device)) {
    LOG(ERROR) << "Failed to get device attestation policy setting.";
    return false;
  }
  if (!enabled_for_device) {
    LOG(ERROR) << "Soft key bind attestation denied because Verified Access is "
               << "disabled for the device.";
    return false;
  }
  return true;
}

// TODO(b/185520169): create utility method for both this and content protection
CertificateExpiryStatus SoftBindAttestationFlowImpl::CheckExpiry(
    const std::string& certificate_chain) {
  int num_certificates = 0;
  bssl::PEMTokenizer pem_tokenizer(certificate_chain, {"CERTIFICATE"});
  while (pem_tokenizer.GetNext()) {
    ++num_certificates;
    scoped_refptr<net::X509Certificate> x509 =
        net::X509Certificate::CreateFromBytes(
            base::as_byte_span(pem_tokenizer.data()));
    if (!x509.get() || x509->valid_expiry().is_null()) {
      // This logic intentionally fails open. In theory this should not happen
      // but in practice parsing X.509 can be brittle and there are a lot of
      // factors including which underlying module is parsing the certificate,
      // whether that module performs more checks than just ASN.1/DER format,
      // and the server module that generated the certificate(s). Renewal is
      // expensive so we only renew certificates with good evidence that they
      // have expired or will soon expire; if we don't know, we don't renew.
      LOG(WARNING) << "Failed to parse certificate, cannot check expiry";
      return CertificateExpiryStatus::kInvalidX509;
    }
    if (base::Time::Now() > x509->valid_expiry()) {
      return CertificateExpiryStatus::kExpired;
    }
    if ((x509->valid_expiry() - base::Time::Now()) < kExpiryThresholdDays) {
      return CertificateExpiryStatus::kExpiringSoon;
    }
  }
  if (num_certificates == 0) {
    LOG(WARNING) << "Failed to parse certificate chain, cannot check expiry";
    return CertificateExpiryStatus::kInvalidPemChain;
  }
  return CertificateExpiryStatus::kValid;
}

void SoftBindAttestationFlowImpl::RenewCertificateCallback(
    const std::string& old_certificate_chain,
    AttestationStatus operation_status,
    const std::string& certificate_chain) {
  renewals_in_progress_.erase(old_certificate_chain);
  if (operation_status != ATTESTATION_SUCCESS) {
    LOG(WARNING) << "Failed to renew certificate";
    return;
  }
  VLOG(1) << "Certificate successfully renewed";
}

bool SoftBindAttestationFlowImpl::GenerateLeafCert(
    EVP_PKEY* key,
    base::Time not_valid_before,
    base::Time not_valid_after,
    std::string* der_encoded_cert) {
  crypto::OpenSSLErrStackTracer err_tracer(FROM_HERE);

  bssl::ScopedCBB cbb;
  CBB cert, version, validity, alg, alg_oid, alg_null;
  uint8_t* cert_bytes;
  size_t cert_len;
  uint64_t serial_number;
  crypto::RandBytes(base::byte_span_from_ref(serial_number));
  if (!CBB_init(cbb.get(), 64) ||
      !CBB_add_asn1(cbb.get(), &cert, CBS_ASN1_SEQUENCE) ||
      !CBB_add_asn1(&cert, &version,
                    CBS_ASN1_CONTEXT_SPECIFIC | CBS_ASN1_CONSTRUCTED | 0) ||
      !CBB_add_asn1_uint64(&version, 2) ||
      !CBB_add_asn1_uint64(&cert, serial_number) ||
      !CBB_add_asn1(&cert, &alg, CBS_ASN1_SEQUENCE) ||
      !CBB_add_asn1(&alg, &alg_oid, CBS_ASN1_OBJECT) ||
      !CBB_add_bytes(&alg_oid, kRsaSha256Oid, 9) ||
      !CBB_add_asn1(&alg, &alg_null, CBS_ASN1_NULL) ||
      !net::x509_util::AddName(&cert, kLeafCertIssuerName) ||
      !CBB_add_asn1(&cert, &validity, CBS_ASN1_SEQUENCE) ||
      !net::x509_util::CBBAddTime(&validity, not_valid_before) ||
      !net::x509_util::CBBAddTime(&validity, not_valid_after) ||
      !net::x509_util::AddName(&cert, kLeafCertSubjectName) ||
      !EVP_marshal_public_key(&cert, key)) {  // subjectPublicKeyInfo
    return false;
  }

  CBB outer_extensions, extensions;
  if (!CBB_add_asn1(&cert, &outer_extensions,
                    3 | CBS_ASN1_CONTEXT_SPECIFIC | CBS_ASN1_CONSTRUCTED) ||
      !CBB_add_asn1(&outer_extensions, &extensions, CBS_ASN1_SEQUENCE)) {
    return false;
  }

  if (!AddCriticalExtension(&extensions, kBasicConstraintsOid, 3,
                            kBasicConstraintsContents, 2) ||
      !AddCriticalExtension(&extensions, kKeyUsageOid, 3, kKeyUsageContents,
                            4)) {
    return false;
  }

  if (!CBB_flush(&cert)) {
    return false;
  }

  if (!CBB_finish(cbb.get(), &cert_bytes, &cert_len)) {
    return false;
  }

  der_encoded_cert->assign(reinterpret_cast<char*>(cert_bytes), cert_len);
  bssl::UniquePtr<uint8_t> delete_cert_bytes(cert_bytes);

  return true;
}

}  // namespace attestation
}  // namespace ash