diff --git a/device/fido/authenticator_get_assertion_response.cc b/device/fido/authenticator_get_assertion_response.cc
index 0cc45330036394..a02868d441375b 100644
--- a/device/fido/authenticator_get_assertion_response.cc
+++ b/device/fido/authenticator_get_assertion_response.cc
@@ -7,6 +7,8 @@
 #include <utility>
 
 #include "base/optional.h"
+#include "components/cbor/cbor_values.h"
+#include "components/cbor/cbor_writer.h"
 #include "device/fido/authenticator_data.h"
 #include "device/fido/fido_parsing_utils.h"
 
@@ -89,4 +91,24 @@ AuthenticatorGetAssertionResponse::SetNumCredentials(uint8_t num_credentials) {
   return *this;
 }
 
+std::vector<uint8_t> GetSerializedCtapDeviceResponse(
+    const AuthenticatorGetAssertionResponse& response) {
+  cbor::CBORValue::MapValue response_map;
+  if (response.credential())
+    response_map.emplace(1, response.credential()->ConvertToCBOR());
+
+  response_map.emplace(2, response.auth_data().SerializeToByteArray());
+  response_map.emplace(3, response.signature());
+
+  if (response.user_entity())
+    response_map.emplace(4, response.user_entity()->ConvertToCBOR());
+
+  // Multiple account selection is not supported.
+  response_map.emplace(5, 1);
+  auto encoded_response =
+      cbor::CBORWriter::Write(cbor::CBORValue(std::move(response_map)));
+  DCHECK(encoded_response);
+  return *encoded_response;
+}
+
 }  // namespace device
diff --git a/device/fido/authenticator_get_assertion_response.h b/device/fido/authenticator_get_assertion_response.h
index 4f60316b30c78a..72f492d80dec13 100644
--- a/device/fido/authenticator_get_assertion_response.h
+++ b/device/fido/authenticator_get_assertion_response.h
@@ -70,6 +70,10 @@ class COMPONENT_EXPORT(DEVICE_FIDO) AuthenticatorGetAssertionResponse
   DISALLOW_COPY_AND_ASSIGN(AuthenticatorGetAssertionResponse);
 };
 
+COMPONENT_EXPORT(DEVICE_FIDO)
+std::vector<uint8_t> GetSerializedCtapDeviceResponse(
+    const AuthenticatorGetAssertionResponse& response);
+
 }  // namespace device
 
 #endif  // DEVICE_FIDO_AUTHENTICATOR_GET_ASSERTION_RESPONSE_H_
diff --git a/device/fido/ctap_response_unittest.cc b/device/fido/ctap_response_unittest.cc
index 64f73fc8c6101f..2d8e23b544f4c8 100644
--- a/device/fido/ctap_response_unittest.cc
+++ b/device/fido/ctap_response_unittest.cc
@@ -586,4 +586,50 @@ TEST(CTAPResponseTest, TestSerializeMakeCredentialResponse) {
           base::make_span(test_data::kTestMakeCredentialResponse).subspan(1)));
 }
 
+TEST(CTAPResponseTest, TestSerializeGetAssertionResponse) {
+  constexpr std::array<uint8_t, kRpIdHashLength> kApplicationParameter = {{
+      0x62, 0x5d, 0xda, 0xdf, 0x74, 0x3f, 0x57, 0x27, 0xe6, 0x6b, 0xba,
+      0x8c, 0x2e, 0x38, 0x79, 0x22, 0xd1, 0xaf, 0x43, 0xc5, 0x03, 0xd9,
+      0x11, 0x4a, 0x8f, 0xba, 0x10, 0x4d, 0x84, 0xd0, 0x2b, 0xfa,
+  }};
+
+  constexpr uint8_t kUserId[] = {
+      0x30, 0x82, 0x01, 0x93, 0x30, 0x82, 0x01, 0x38, 0xa0, 0x03, 0x02,
+      0x01, 0x02, 0x30, 0x82, 0x01, 0x93, 0x30, 0x82, 0x01, 0x38, 0xa0,
+      0x03, 0x02, 0x01, 0x02, 0x30, 0x82, 0x01, 0x93, 0x30, 0x82,
+  };
+
+  constexpr uint8_t kCredentialId[] = {
+      0xf2, 0x20, 0x06, 0xde, 0x4f, 0x90, 0x5a, 0xf6, 0x8a, 0x43, 0x94,
+      0x2f, 0x02, 0x4f, 0x2a, 0x5e, 0xce, 0x60, 0x3d, 0x9c, 0x6d, 0x4b,
+      0x3d, 0xf8, 0xbe, 0x08, 0xed, 0x01, 0xfc, 0x44, 0x26, 0x46, 0xd0,
+      0x34, 0x85, 0x8a, 0xc7, 0x5b, 0xed, 0x3f, 0xd5, 0x80, 0xbf, 0x98,
+      0x08, 0xd9, 0x4f, 0xcb, 0xee, 0x82, 0xb9, 0xb2, 0xef, 0x66, 0x77,
+      0xaf, 0x0a, 0xdc, 0xc3, 0x58, 0x52, 0xea, 0x6b, 0x9e,
+  };
+
+  AuthenticatorData authenticator_data(
+      kApplicationParameter,
+      base::strict_cast<uint8_t>(AuthenticatorData::Flag::kTestOfUserPresence),
+      std::array<uint8_t, kSignCounterLength>{
+          {0x00, 0x00, 0x00, 0x11}} /* signature_counter */,
+      base::nullopt /* attested_credential_data */);
+  AuthenticatorGetAssertionResponse response(
+      std::move(authenticator_data),
+      fido_parsing_utils::Materialize(test_data::kCtap2GetAssertionSignature));
+  response.SetCredential({CredentialType::kPublicKey,
+                          fido_parsing_utils::Materialize(kCredentialId)});
+  PublicKeyCredentialUserEntity user(fido_parsing_utils::Materialize(kUserId));
+  user.SetDisplayName("John P. Smith");
+  user.SetUserName("johnpsmith@example.com");
+  user.SetIconUrl(GURL("https://pics.acme.com/00/p/aBjjjpqPb.png"));
+  response.SetUserEntity(std::move(user));
+  response.SetNumCredentials(1);
+
+  EXPECT_THAT(
+      GetSerializedCtapDeviceResponse(response),
+      ::testing::ElementsAreArray(
+          base::make_span(test_data::kDeviceGetAssertionResponse).subspan(1)));
+}
+
 }  // namespace device