components/webauthn/android/fido2api_native_android.cc

// Copyright 2020 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include <jni.h>

#include "base/android/jni_array.h"
#include "components/cbor/reader.h"
#include "components/cbor/writer.h"
#include "components/webauthn/android/jni_headers/Fido2Api_jni.h"
#include "device/fido/attestation_object.h"
#include "device/fido/attested_credential_data.h"
#include "device/fido/authenticator_data.h"
#include "device/fido/fido_constants.h"
#include "device/fido/public_key.h"
#include "third_party/boringssl/src/include/openssl/bytestring.h"

using base::android::JavaParamRef;
using base::android::ScopedJavaLocalRef;
using base::android::ToJavaByteArray;

namespace webauthn {

// Parses a CTAP2 attestation[1] and extracts the
// parts that the browser provides via Javascript API [2]. Called
// Fido2Api.java when constructing the makeCredential reply.
//
// [1] https://www.w3.org/TR/webauthn/#attestation-object
// [2] https://w3c.github.io/webauthn/#sctn-public-key-easy
static jboolean JNI_Fido2Api_ParseAttestationObject(
    JNIEnv* env,
    const base::android::JavaParamRef<jbyteArray>& jattestation_object_bytes,
    jboolean attestation_acceptable,
    const base::android::JavaParamRef<jobject>& out_result) {
  std::vector<uint8_t> attestation_object_bytes;
  JavaByteArrayToByteVector(env, jattestation_object_bytes,
                            &attestation_object_bytes);

  absl::optional<cbor::Value> attestation_object_map =
      cbor::Reader::Read(attestation_object_bytes);
  if (!attestation_object_map || !attestation_object_map->is_map()) {
    return false;
  }

  absl::optional<device::AttestationObject> attestation_object =
      device::AttestationObject::Parse(*attestation_object_map);
  if (!attestation_object) {
    return false;
  }

  const absl::optional<device::AttestedCredentialData>& att_cred_data(
      attestation_object->authenticator_data().attested_data());
  if (!att_cred_data) {
    return false;
  }

  const device::PublicKey* pub_key = att_cred_data->public_key();
  const absl::optional<std::vector<uint8_t>>& der_bytes(pub_key->der_bytes);
  ScopedJavaLocalRef<jbyteArray> spki_java;
  if (der_bytes) {
    spki_java.Reset(ToJavaByteArray(env, *der_bytes));
  }

  base::span<const uint8_t> attestation_obj_out = attestation_object_bytes;
  absl::optional<std::vector<uint8_t>> data_without_attestation_vec;

  if (!attestation_acceptable) {
    const bool did_modify = attestation_object->EraseAttestationStatement(
        device::AttestationObject::AAGUID::kInclude);
    if (did_modify) {
      // The devicePubKey extension signs over the authenticator data so its
      // signature is now invalid and we have to remove the extension.
      attestation_object->EraseExtension(device::kExtensionDevicePublicKey);
    }
    data_without_attestation_vec =
        cbor::Writer::Write(AsCBOR(*attestation_object));
    attestation_obj_out = *data_without_attestation_vec;
  }

  ScopedJavaLocalRef<jbyteArray> auth_data_java = ToJavaByteArray(
      env, attestation_object->authenticator_data().SerializeToByteArray());

  Java_AttestationObjectParts_setAll(env, out_result, auth_data_java, spki_java,
                                     pub_key->algorithm,
                                     ToJavaByteArray(env, attestation_obj_out));

  return true;
}

// JavaByteArrayToSpan returns a span that aliases |data|. Be aware that the
// reference for |data| must outlive the span.
static base::span<const uint8_t> JavaByteArrayToSpan(
    JNIEnv* env,
    const JavaParamRef<jbyteArray>& data) {
  if (data.is_null()) {
    return base::span<const uint8_t>();
  }

  const size_t data_len = env->GetArrayLength(data);
  const jbyte* data_bytes = env->GetByteArrayElements(data, /*iscopy=*/nullptr);
  return base::as_bytes(base::make_span(data_bytes, data_len));
}

static ScopedJavaLocalRef<jbyteArray>
JNI_Fido2Api_GetDevicePublicKeyFromAuthenticatorData(
    JNIEnv* env,
    const base::android::JavaParamRef<jbyteArray>& jauthenticator_data) {
  absl::optional<device::AuthenticatorData> auth_data =
      device::AuthenticatorData::DecodeAuthenticatorData(
          JavaByteArrayToSpan(env, jauthenticator_data));
  if (!auth_data) {
    return nullptr;
  }

  const absl::optional<cbor::Value>& extensions = auth_data->extensions();
  if (!extensions) {
    return nullptr;
  }

  const cbor::Value::MapValue& extensions_map = extensions->GetMap();
  const auto it =
      extensions_map.find(cbor::Value(device::kExtensionDevicePublicKey));
  if (it == extensions_map.end() || !it->second.is_bytestring()) {
    return nullptr;
  }

  return ToJavaByteArray(env, it->second.GetBytestring());
}

}  // namespace webauthn