diff --git a/google/oauth2/_client.py b/google/oauth2/_client.py
index 4487163..2f4e847 100644
--- a/google/oauth2/_client.py
+++ b/google/oauth2/_client.py
@@ -35,29 +35,29 @@
 from google.auth import jwt
 
 _URLENCODED_CONTENT_TYPE = "application/x-www-form-urlencoded"
+_JSON_CONTENT_TYPE = "application/json"
 _JWT_GRANT_TYPE = "urn:ietf:params:oauth:grant-type:jwt-bearer"
 _REFRESH_GRANT_TYPE = "refresh_token"
 
 
-def _handle_error_response(response_body):
-    """"Translates an error response into an exception.
+def _handle_error_response(response_data):
+    """Translates an error response into an exception.
 
     Args:
-        response_body (str): The decoded response data.
+        response_data (Mapping): The decoded response data.
 
     Raises:
-        google.auth.exceptions.RefreshError
+        google.auth.exceptions.RefreshError: The errors contained in response_data.
     """
     try:
-        error_data = json.loads(response_body)
         error_details = "{}: {}".format(
-            error_data["error"], error_data.get("error_description")
+            response_data["error"], response_data.get("error_description")
         )
     # If no details could be extracted, use the response data.
     except (KeyError, ValueError):
-        error_details = response_body
+        error_details = json.dumps(response_data)
 
-    raise exceptions.RefreshError(error_details, response_body)
+    raise exceptions.RefreshError(error_details, response_data)
 
 
 def _parse_expiry(response_data):
@@ -78,8 +78,11 @@
         return None
 
 
-def _token_endpoint_request(request, token_uri, body):
+def _token_endpoint_request_no_throw(
+    request, token_uri, body, access_token=None, use_json=False
+):
     """Makes a request to the OAuth 2.0 authorization server's token endpoint.
+    This function doesn't throw on response errors.
 
     Args:
         request (google.auth.transport.Request): A callable used to make
@@ -87,16 +90,23 @@
         token_uri (str): The OAuth 2.0 authorizations server's token endpoint
             URI.
         body (Mapping[str, str]): The parameters to send in the request body.
+        access_token (Optional(str)): The access token needed to make the request.
+        use_json (Optional(bool)): Use urlencoded format or json format for the
+            content type. The default value is False.
 
     Returns:
-        Mapping[str, str]: The JSON-decoded response data.
-
-    Raises:
-        google.auth.exceptions.RefreshError: If the token endpoint returned
-            an error.
+        Tuple(bool, Mapping[str, str]): A boolean indicating if the request is
+            successful, and a mapping for the JSON-decoded response data.
     """
-    body = urllib.parse.urlencode(body).encode("utf-8")
-    headers = {"content-type": _URLENCODED_CONTENT_TYPE}
+    if use_json:
+        headers = {"Content-Type": _JSON_CONTENT_TYPE}
+        body = json.dumps(body).encode("utf-8")
+    else:
+        headers = {"Content-Type": _URLENCODED_CONTENT_TYPE}
+        body = urllib.parse.urlencode(body).encode("utf-8")
+
+    if access_token:
+        headers["Authorization"] = "Bearer {}".format(access_token)
 
     retry = 0
     # retry to fetch token for maximum of two times if any internal failure
@@ -121,8 +131,38 @@
             ):
                 retry += 1
                 continue
-            _handle_error_response(response_body)
+            return response.status == http_client.OK, response_data
 
+    return response.status == http_client.OK, response_data
+
+
+def _token_endpoint_request(
+    request, token_uri, body, access_token=None, use_json=False
+):
+    """Makes a request to the OAuth 2.0 authorization server's token endpoint.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        token_uri (str): The OAuth 2.0 authorizations server's token endpoint
+            URI.
+        body (Mapping[str, str]): The parameters to send in the request body.
+        access_token (Optional(str)): The access token needed to make the request.
+        use_json (Optional(bool)): Use urlencoded format or json format for the
+            content type. The default value is False.
+
+    Returns:
+        Mapping[str, str]: The JSON-decoded response data.
+
+    Raises:
+        google.auth.exceptions.RefreshError: If the token endpoint returned
+            an error.
+    """
+    response_status_ok, response_data = _token_endpoint_request_no_throw(
+        request, token_uri, body, access_token=access_token, use_json=use_json
+    )
+    if not response_status_ok:
+        _handle_error_response(response_data)
     return response_data
 
 
@@ -204,8 +244,43 @@
     return id_token, expiry, response_data
 
 
+def _handle_refresh_grant_response(response_data, refresh_token):
+    """Extract tokens from refresh grant response.
+
+    Args:
+        response_data (Mapping[str, str]): Refresh grant response data.
+        refresh_token (str): Current refresh token.
+
+    Returns:
+        Tuple[str, str, Optional[datetime], Mapping[str, str]]: The access token,
+            refresh token, expiration, and additional data returned by the token
+            endpoint. If response_data doesn't have refresh token, then the current
+            refresh token will be returned.
+
+    Raises:
+        google.auth.exceptions.RefreshError: If the token endpoint returned
+            an error.
+    """
+    try:
+        access_token = response_data["access_token"]
+    except KeyError as caught_exc:
+        new_exc = exceptions.RefreshError("No access token in response.", response_data)
+        six.raise_from(new_exc, caught_exc)
+
+    refresh_token = response_data.get("refresh_token", refresh_token)
+    expiry = _parse_expiry(response_data)
+
+    return access_token, refresh_token, expiry, response_data
+
+
 def refresh_grant(
-    request, token_uri, refresh_token, client_id, client_secret, scopes=None
+    request,
+    token_uri,
+    refresh_token,
+    client_id,
+    client_secret,
+    scopes=None,
+    rapt_token=None,
 ):
     """Implements the OAuth 2.0 refresh token grant.
 
@@ -224,10 +299,11 @@
             scopes must be authorized for the refresh token. Useful if refresh
             token has a wild card scope (e.g.
             'https://www.googleapis.com/auth/any-api').
+        rapt_token (Optional(str)): The reauth Proof Token.
 
     Returns:
-        Tuple[str, Optional[str], Optional[datetime], Mapping[str, str]]: The
-            access token, new refresh token, expiration, and additional data
+        Tuple[str, str, Optional[datetime], Mapping[str, str]]: The access
+            token, new or current refresh token, expiration, and additional data
             returned by the token endpoint.
 
     Raises:
@@ -244,16 +320,8 @@
     }
     if scopes:
         body["scope"] = " ".join(scopes)
+    if rapt_token:
+        body["rapt"] = rapt_token
 
     response_data = _token_endpoint_request(request, token_uri, body)
-
-    try:
-        access_token = response_data["access_token"]
-    except KeyError as caught_exc:
-        new_exc = exceptions.RefreshError("No access token in response.", response_data)
-        six.raise_from(new_exc, caught_exc)
-
-    refresh_token = response_data.get("refresh_token", refresh_token)
-    expiry = _parse_expiry(response_data)
-
-    return access_token, refresh_token, expiry, response_data
+    return _handle_refresh_grant_response(response_data, refresh_token)
diff --git a/google/oauth2/challenges.py b/google/oauth2/challenges.py
new file mode 100644
index 0000000..d0b070e
--- /dev/null
+++ b/google/oauth2/challenges.py
@@ -0,0 +1,157 @@
+# Copyright 2021 Google LLC
+#
+# 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
+#
+#      http://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.
+
+""" Challenges for reauthentication.
+"""
+
+import abc
+import base64
+import getpass
+import sys
+
+import six
+
+from google.auth import _helpers
+from google.auth import exceptions
+
+
+REAUTH_ORIGIN = "https://accounts.google.com"
+
+
+def get_user_password(text):
+    """Get password from user.
+
+    Override this function with a different logic if you are using this library
+    outside a CLI.
+
+    Args:
+        text (str): message for the password prompt.
+
+    Returns:
+        str: password string.
+    """
+    return getpass.getpass(text)
+
+
+@six.add_metaclass(abc.ABCMeta)
+class ReauthChallenge(object):
+    """Base class for reauth challenges."""
+
+    @property
+    @abc.abstractmethod
+    def name(self):  # pragma: NO COVER
+        """Returns the name of the challenge."""
+        raise NotImplementedError("name property must be implemented")
+
+    @property
+    @abc.abstractmethod
+    def is_locally_eligible(self):  # pragma: NO COVER
+        """Returns true if a challenge is supported locally on this machine."""
+        raise NotImplementedError("is_locally_eligible property must be implemented")
+
+    @abc.abstractmethod
+    def obtain_challenge_input(self, metadata):  # pragma: NO COVER
+        """Performs logic required to obtain credentials and returns it.
+
+        Args:
+            metadata (Mapping): challenge metadata returned in the 'challenges' field in
+                the initial reauth request. Includes the 'challengeType' field
+                and other challenge-specific fields.
+
+        Returns:
+            response that will be send to the reauth service as the content of
+            the 'proposalResponse' field in the request body. Usually a dict
+            with the keys specific to the challenge. For example,
+            ``{'credential': password}`` for password challenge.
+        """
+        raise NotImplementedError("obtain_challenge_input method must be implemented")
+
+
+class PasswordChallenge(ReauthChallenge):
+    """Challenge that asks for user's password."""
+
+    @property
+    def name(self):
+        return "PASSWORD"
+
+    @property
+    def is_locally_eligible(self):
+        return True
+
+    @_helpers.copy_docstring(ReauthChallenge)
+    def obtain_challenge_input(self, unused_metadata):
+        passwd = get_user_password("Please enter your password:")
+        if not passwd:
+            passwd = " "  # avoid the server crashing in case of no password :D
+        return {"credential": passwd}
+
+
+class SecurityKeyChallenge(ReauthChallenge):
+    """Challenge that asks for user's security key touch."""
+
+    @property
+    def name(self):
+        return "SECURITY_KEY"
+
+    @property
+    def is_locally_eligible(self):
+        return True
+
+    @_helpers.copy_docstring(ReauthChallenge)
+    def obtain_challenge_input(self, metadata):
+        try:
+            import pyu2f.convenience.authenticator
+            import pyu2f.errors
+            import pyu2f.model
+        except ImportError:
+            raise exceptions.ReauthFailError(
+                "pyu2f dependency is required to use Security key reauth feature. "
+                "It can be installed via `pip install pyu2f` or `pip install google-auth[reauth]`."
+            )
+        sk = metadata["securityKey"]
+        challenges = sk["challenges"]
+        app_id = sk["applicationId"]
+
+        challenge_data = []
+        for c in challenges:
+            kh = c["keyHandle"].encode("ascii")
+            key = pyu2f.model.RegisteredKey(bytearray(base64.urlsafe_b64decode(kh)))
+            challenge = c["challenge"].encode("ascii")
+            challenge = base64.urlsafe_b64decode(challenge)
+            challenge_data.append({"key": key, "challenge": challenge})
+
+        try:
+            api = pyu2f.convenience.authenticator.CreateCompositeAuthenticator(
+                REAUTH_ORIGIN
+            )
+            response = api.Authenticate(
+                app_id, challenge_data, print_callback=sys.stderr.write
+            )
+            return {"securityKey": response}
+        except pyu2f.errors.U2FError as e:
+            if e.code == pyu2f.errors.U2FError.DEVICE_INELIGIBLE:
+                sys.stderr.write("Ineligible security key.\n")
+            elif e.code == pyu2f.errors.U2FError.TIMEOUT:
+                sys.stderr.write("Timed out while waiting for security key touch.\n")
+            else:
+                raise e
+        except pyu2f.errors.NoDeviceFoundError:
+            sys.stderr.write("No security key found.\n")
+        return None
+
+
+AVAILABLE_CHALLENGES = {
+    challenge.name: challenge
+    for challenge in [SecurityKeyChallenge(), PasswordChallenge()]
+}
diff --git a/google/oauth2/credentials.py b/google/oauth2/credentials.py
index 464cc48..dcfa5f9 100644
--- a/google/oauth2/credentials.py
+++ b/google/oauth2/credentials.py
@@ -41,7 +41,7 @@
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
-from google.oauth2 import _client
+from google.oauth2 import reauth
 
 
 # The Google OAuth 2.0 token endpoint. Used for authorized user credentials.
@@ -55,6 +55,10 @@
     quota project, use :meth:`with_quota_project` or ::
 
         credentials = credentials.with_quota_project('myproject-123)
+
+    If reauth is enabled, `pyu2f` dependency has to be installed in order to use security
+    key reauth feature. Dependency can be installed via `pip install pyu2f` or `pip install
+    google-auth[reauth]`.
     """
 
     def __init__(
@@ -69,6 +73,7 @@
         default_scopes=None,
         quota_project_id=None,
         expiry=None,
+        rapt_token=None,
     ):
         """
         Args:
@@ -97,6 +102,7 @@
             quota_project_id (Optional[str]): The project ID used for quota and billing.
                 This project may be different from the project used to
                 create the credentials.
+            rapt_token (Optional[str]): The reauth Proof Token.
         """
         super(Credentials, self).__init__()
         self.token = token
@@ -109,6 +115,7 @@
         self._client_id = client_id
         self._client_secret = client_secret
         self._quota_project_id = quota_project_id
+        self._rapt_token = rapt_token
 
     def __getstate__(self):
         """A __getstate__ method must exist for the __setstate__ to be called
@@ -130,6 +137,7 @@
         self._client_id = d.get("_client_id")
         self._client_secret = d.get("_client_secret")
         self._quota_project_id = d.get("_quota_project_id")
+        self._rapt_token = d.get("_rapt_token")
 
     @property
     def refresh_token(self):
@@ -174,6 +182,11 @@
         the initial token is requested and can not be changed."""
         return False
 
+    @property
+    def rapt_token(self):
+        """Optional[str]: The reauth Proof Token."""
+        return self._rapt_token
+
     @_helpers.copy_docstring(credentials.CredentialsWithQuotaProject)
     def with_quota_project(self, quota_project_id):
 
@@ -187,6 +200,7 @@
             scopes=self.scopes,
             default_scopes=self.default_scopes,
             quota_project_id=quota_project_id,
+            rapt_token=self.rapt_token,
         )
 
     @_helpers.copy_docstring(credentials.Credentials)
@@ -205,23 +219,31 @@
 
         scopes = self._scopes if self._scopes is not None else self._default_scopes
 
-        access_token, refresh_token, expiry, grant_response = _client.refresh_grant(
+        (
+            access_token,
+            refresh_token,
+            expiry,
+            grant_response,
+            rapt_token,
+        ) = reauth.refresh_grant(
             request,
             self._token_uri,
             self._refresh_token,
             self._client_id,
             self._client_secret,
-            scopes,
+            scopes=scopes,
+            rapt_token=self._rapt_token,
         )
 
         self.token = access_token
         self.expiry = expiry
         self._refresh_token = refresh_token
         self._id_token = grant_response.get("id_token")
+        self._rapt_token = rapt_token
 
-        if scopes and "scopes" in grant_response:
+        if scopes and "scope" in grant_response:
             requested_scopes = frozenset(scopes)
-            granted_scopes = frozenset(grant_response["scopes"].split())
+            granted_scopes = frozenset(grant_response["scope"].split())
             scopes_requested_but_not_granted = requested_scopes - granted_scopes
             if scopes_requested_but_not_granted:
                 raise exceptions.RefreshError(
@@ -323,6 +345,7 @@
             "client_id": self.client_id,
             "client_secret": self.client_secret,
             "scopes": self.scopes,
+            "rapt_token": self.rapt_token,
         }
         if self.expiry:  # flatten expiry timestamp
             prep["expiry"] = self.expiry.isoformat() + "Z"
diff --git a/google/oauth2/reauth.py b/google/oauth2/reauth.py
new file mode 100644
index 0000000..d539d7c
--- /dev/null
+++ b/google/oauth2/reauth.py
@@ -0,0 +1,341 @@
+# Copyright 2021 Google LLC
+#
+# 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
+#
+#      http://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.
+
+"""A module that provides functions for handling rapt authentication.
+
+Reauth is a process of obtaining additional authentication (such as password,
+security token, etc.) while refreshing OAuth 2.0 credentials for a user.
+
+Credentials that use the Reauth flow must have the reauth scope,
+``https://www.googleapis.com/auth/accounts.reauth``.
+
+This module provides a high-level function for executing the Reauth process,
+:func:`refresh_grant`, and lower-level helpers for doing the individual
+steps of the reauth process.
+
+Those steps are:
+
+1. Obtaining a list of challenges from the reauth server.
+2. Running through each challenge and sending the result back to the reauth
+   server.
+3. Refreshing the access token using the returned rapt token.
+"""
+
+import sys
+
+from six.moves import range
+
+from google.auth import exceptions
+from google.oauth2 import _client
+from google.oauth2 import challenges
+
+
+_REAUTH_SCOPE = "https://www.googleapis.com/auth/accounts.reauth"
+_REAUTH_API = "https://reauth.googleapis.com/v2/sessions"
+
+_REAUTH_NEEDED_ERROR = "invalid_grant"
+_REAUTH_NEEDED_ERROR_INVALID_RAPT = "invalid_rapt"
+_REAUTH_NEEDED_ERROR_RAPT_REQUIRED = "rapt_required"
+
+_AUTHENTICATED = "AUTHENTICATED"
+_CHALLENGE_REQUIRED = "CHALLENGE_REQUIRED"
+_CHALLENGE_PENDING = "CHALLENGE_PENDING"
+
+
+# Override this global variable to set custom max number of rounds of reauth
+# challenges should be run.
+RUN_CHALLENGE_RETRY_LIMIT = 5
+
+
+def is_interactive():
+    """Check if we are in an interractive environment.
+
+    Override this function with a different logic if you are using this library
+    outside a CLI.
+
+    If the rapt token needs refreshing, the user needs to answer the challenges.
+    If the user is not in an interractive environment, the challenges can not
+    be answered and we just wait for timeout for no reason.
+
+    Returns:
+        bool: True if is interactive environment, False otherwise.
+    """
+
+    return sys.stdin.isatty()
+
+
+def _get_challenges(
+    request, supported_challenge_types, access_token, requested_scopes=None
+):
+    """Does initial request to reauth API to get the challenges.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        supported_challenge_types (Sequence[str]): list of challenge names
+            supported by the manager.
+        access_token (str): Access token with reauth scopes.
+        requested_scopes (Optional(Sequence[str])): Authorized scopes for the credentials.
+
+    Returns:
+        dict: The response from the reauth API.
+    """
+    body = {"supportedChallengeTypes": supported_challenge_types}
+    if requested_scopes:
+        body["oauthScopesForDomainPolicyLookup"] = requested_scopes
+
+    return _client._token_endpoint_request(
+        request, _REAUTH_API + ":start", body, access_token=access_token, use_json=True
+    )
+
+
+def _send_challenge_result(
+    request, session_id, challenge_id, client_input, access_token
+):
+    """Attempt to refresh access token by sending next challenge result.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        session_id (str): session id returned by the initial reauth call.
+        challenge_id (str): challenge id returned by the initial reauth call.
+        client_input: dict with a challenge-specific client input. For example:
+            ``{'credential': password}`` for password challenge.
+        access_token (str): Access token with reauth scopes.
+
+    Returns:
+        dict: The response from the reauth API.
+    """
+    body = {
+        "sessionId": session_id,
+        "challengeId": challenge_id,
+        "action": "RESPOND",
+        "proposalResponse": client_input,
+    }
+
+    return _client._token_endpoint_request(
+        request,
+        _REAUTH_API + "/{}:continue".format(session_id),
+        body,
+        access_token=access_token,
+        use_json=True,
+    )
+
+
+def _run_next_challenge(msg, request, access_token):
+    """Get the next challenge from msg and run it.
+
+    Args:
+        msg (dict): Reauth API response body (either from the initial request to
+            https://reauth.googleapis.com/v2/sessions:start or from sending the
+            previous challenge response to
+            https://reauth.googleapis.com/v2/sessions/id:continue)
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        access_token (str): reauth access token
+
+    Returns:
+        dict: The response from the reauth API.
+
+    Raises:
+        google.auth.exceptions.ReauthError: if reauth failed.
+    """
+    for challenge in msg["challenges"]:
+        if challenge["status"] != "READY":
+            # Skip non-activated challenges.
+            continue
+        c = challenges.AVAILABLE_CHALLENGES.get(challenge["challengeType"], None)
+        if not c:
+            raise exceptions.ReauthFailError(
+                "Unsupported challenge type {0}. Supported types: {1}".format(
+                    challenge["challengeType"],
+                    ",".join(list(challenges.AVAILABLE_CHALLENGES.keys())),
+                )
+            )
+        if not c.is_locally_eligible:
+            raise exceptions.ReauthFailError(
+                "Challenge {0} is not locally eligible".format(
+                    challenge["challengeType"]
+                )
+            )
+        client_input = c.obtain_challenge_input(challenge)
+        if not client_input:
+            return None
+        return _send_challenge_result(
+            request,
+            msg["sessionId"],
+            challenge["challengeId"],
+            client_input,
+            access_token,
+        )
+    return None
+
+
+def _obtain_rapt(request, access_token, requested_scopes):
+    """Given an http request method and reauth access token, get rapt token.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        access_token (str): reauth access token
+        requested_scopes (Sequence[str]): scopes required by the client application
+
+    Returns:
+        str: The rapt token.
+
+    Raises:
+        google.auth.exceptions.ReauthError: if reauth failed
+    """
+    msg = _get_challenges(
+        request,
+        list(challenges.AVAILABLE_CHALLENGES.keys()),
+        access_token,
+        requested_scopes,
+    )
+
+    if msg["status"] == _AUTHENTICATED:
+        return msg["encodedProofOfReauthToken"]
+
+    for _ in range(0, RUN_CHALLENGE_RETRY_LIMIT):
+        if not (
+            msg["status"] == _CHALLENGE_REQUIRED or msg["status"] == _CHALLENGE_PENDING
+        ):
+            raise exceptions.ReauthFailError(
+                "Reauthentication challenge failed due to API error: {}".format(
+                    msg["status"]
+                )
+            )
+
+        if not is_interactive():
+            raise exceptions.ReauthFailError(
+                "Reauthentication challenge could not be answered because you are not"
+                " in an interactive session."
+            )
+
+        msg = _run_next_challenge(msg, request, access_token)
+
+        if msg["status"] == _AUTHENTICATED:
+            return msg["encodedProofOfReauthToken"]
+
+    # If we got here it means we didn't get authenticated.
+    raise exceptions.ReauthFailError("Failed to obtain rapt token.")
+
+
+def get_rapt_token(
+    request, client_id, client_secret, refresh_token, token_uri, scopes=None
+):
+    """Given an http request method and refresh_token, get rapt token.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        client_id (str): client id to get access token for reauth scope.
+        client_secret (str): client secret for the client_id
+        refresh_token (str): refresh token to refresh access token
+        token_uri (str): uri to refresh access token
+        scopes (Optional(Sequence[str])): scopes required by the client application
+
+    Returns:
+        str: The rapt token.
+    Raises:
+        google.auth.exceptions.RefreshError: If reauth failed.
+    """
+    sys.stderr.write("Reauthentication required.\n")
+
+    # Get access token for reauth.
+    access_token, _, _, _ = _client.refresh_grant(
+        request=request,
+        client_id=client_id,
+        client_secret=client_secret,
+        refresh_token=refresh_token,
+        token_uri=token_uri,
+        scopes=[_REAUTH_SCOPE],
+    )
+
+    # Get rapt token from reauth API.
+    rapt_token = _obtain_rapt(request, access_token, requested_scopes=scopes)
+
+    return rapt_token
+
+
+def refresh_grant(
+    request,
+    token_uri,
+    refresh_token,
+    client_id,
+    client_secret,
+    scopes=None,
+    rapt_token=None,
+):
+    """Implements the reauthentication flow.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        token_uri (str): The OAuth 2.0 authorizations server's token endpoint
+            URI.
+        refresh_token (str): The refresh token to use to get a new access
+            token.
+        client_id (str): The OAuth 2.0 application's client ID.
+        client_secret (str): The Oauth 2.0 appliaction's client secret.
+        scopes (Optional(Sequence[str])): Scopes to request. If present, all
+            scopes must be authorized for the refresh token. Useful if refresh
+            token has a wild card scope (e.g.
+            'https://www.googleapis.com/auth/any-api').
+        rapt_token (Optional(str)): The rapt token for reauth.
+
+    Returns:
+        Tuple[str, Optional[str], Optional[datetime], Mapping[str, str]]: The
+            access token, new refresh token, expiration, and additional data
+            returned by the token endpoint.
+
+    Raises:
+        google.auth.exceptions.RefreshError: If the token endpoint returned
+            an error.
+    """
+    body = {
+        "grant_type": _client._REFRESH_GRANT_TYPE,
+        "client_id": client_id,
+        "client_secret": client_secret,
+        "refresh_token": refresh_token,
+    }
+    if scopes:
+        body["scope"] = " ".join(scopes)
+    if rapt_token:
+        body["rapt"] = rapt_token
+
+    response_status_ok, response_data = _client._token_endpoint_request_no_throw(
+        request, token_uri, body
+    )
+    if (
+        not response_status_ok
+        and response_data.get("error") == _REAUTH_NEEDED_ERROR
+        and (
+            response_data.get("error_subtype") == _REAUTH_NEEDED_ERROR_INVALID_RAPT
+            or response_data.get("error_subtype") == _REAUTH_NEEDED_ERROR_RAPT_REQUIRED
+        )
+    ):
+        rapt_token = get_rapt_token(
+            request, client_id, client_secret, refresh_token, token_uri, scopes=scopes
+        )
+        body["rapt"] = rapt_token
+        (response_status_ok, response_data) = _client._token_endpoint_request_no_throw(
+            request, token_uri, body
+        )
+
+    if not response_status_ok:
+        _client._handle_error_response(response_data)
+    return _client._handle_refresh_grant_response(response_data, refresh_token) + (
+        rapt_token,
+    )