From 963fc010bccc80ea1f3a6f354d04e8d989171cd5 Mon Sep 17 00:00:00 2001
From: Jon Wayne Parrott <jonwayne@google.com>
Date: Tue, 4 Oct 2016 14:28:08 -0700
Subject: [PATCH 1/5] Add google.auth.jwt

---
 docs/reference/google.auth.jwt.rst |   7 +
 docs/reference/google.auth.rst     |   1 +
 google/auth/_helpers.py            |  15 ++
 google/auth/jwt.py                 | 236 +++++++++++++++++++++++++++++
 tests/test__helpers.py             |  12 ++
 tests/test_jwt.py                  | 175 +++++++++++++++++++++
 6 files changed, 446 insertions(+)
 create mode 100644 docs/reference/google.auth.jwt.rst
 create mode 100644 google/auth/jwt.py
 create mode 100644 tests/test_jwt.py

diff --git a/docs/reference/google.auth.jwt.rst b/docs/reference/google.auth.jwt.rst
new file mode 100644
index 000000000..79e3080d0
--- /dev/null
+++ b/docs/reference/google.auth.jwt.rst
@@ -0,0 +1,7 @@
+google.auth.jwt module
+======================
+
+.. automodule:: google.auth.jwt
+    :members:
+    :undoc-members:
+    :show-inheritance:
diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index 4c39cb96b..57a16b475 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -12,4 +12,5 @@ Submodules
 .. toctree::
 
    google.auth.crypt
+   google.auth.jwt
 
diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index 2d3b6534c..b20e216f4 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -14,9 +14,24 @@
 
 """Helper functions for commonly used utilities."""
 
+
+import calendar
+import datetime
+
 import six
 
 
+def now():
+    """Gets the current UTC datetime."""
+    return datetime.datetime.utcnow()
+
+
+def datetime_to_secs(value):
+    """Convert a datetime object to the number of seconds since the UNIX epoch.
+    """
+    return calendar.timegm(value.utctimetuple())
+
+
 def to_bytes(value, encoding='utf-8'):
     """Converts a string value to bytes, if necessary.
 
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
new file mode 100644
index 000000000..57afae338
--- /dev/null
+++ b/google/auth/jwt.py
@@ -0,0 +1,236 @@
+# Copyright 2016 Google Inc.
+#
+# 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.
+
+"""JSON Web Tokens
+
+Provides support for creating (encoding) and verifying (decoding) JWTs,
+especially JWTs generated and consumed by Google infrastructure.
+
+Also provides a :class:`Credentials` class that uses JWTs as authentication
+bearer tokens.
+
+See `rfc7519`_ for more details on JWTs.
+
+To encode a JWT::
+
+    from google.auth import crypto
+    from google.auth import jwt
+
+    signer = crypt.Signer(private_key)
+    payload = {'some': 'payload'}
+    encoded = jwt.encode(signer, payload)
+
+To decode a JWT and verify claims::
+
+    claims = jwt.decode(encoded, certs=public_certs)
+
+You can also skip verification::
+
+    claims = jwt.decode(encoded, verify=False)
+
+.. _rfc7519: https://tools.ietf.org/html/rfc7519
+
+"""
+
+import base64
+import collections
+import json
+
+from google.auth import crypt
+from google.auth import _helpers
+
+
+_DEFAULT_TOKEN_LIFETIME_SECS = 3600  # 1 hour in sections
+CLOCK_SKEW_SECS = 300  # 5 minutes in seconds
+
+
+def encode(signer, payload, header=None, key_id=None):
+    """Make a signed JWT.
+
+    Args:
+        signer (google.auth.crypt.Signer): The signer used to sign the JWT.
+        payload (Mapping): The JWT payload.
+        header (Mapping): Additional JWT header payload.
+        key_id (str): The key id to add to the JWT header. If the
+            signer has a key id it will be used as the default. If this is
+            specified, it'll override the signer's key id.
+
+    Returns:
+        bytes: The encoded JWT.
+    """
+    if not header:
+        header = {}
+
+    if key_id is None:
+        key_id = signer.key_id
+
+    header.update({'typ': 'JWT', 'alg': 'RS256'})
+
+    if key_id is not None and key_id is not False:
+        header['kid'] = key_id
+
+    segments = [
+        base64.urlsafe_b64encode(json.dumps(header).encode('utf-8')),
+        base64.urlsafe_b64encode(json.dumps(payload).encode('utf-8')),
+    ]
+
+    signing_input = b'.'.join(segments)
+    signature = signer.sign(signing_input)
+    segments.append(base64.urlsafe_b64encode(signature))
+
+    return b'.'.join(segments)
+
+
+def _decode_jwt_segment(encoded_section):
+    """Decodes a single JWT segment."""
+    section_bytes = base64.urlsafe_b64decode(encoded_section)
+    try:
+        return json.loads(section_bytes.decode('utf-8'))
+    except:
+        raise ValueError('Can\'t parse segment: {0}'.format(section_bytes))
+
+
+def _unverified_decode(token):
+    """Decodes a token and does no verification.
+
+    Args:
+        token (Union[str, bytes]): The encoded JWT.
+
+    Returns:
+        Tuple(str, str, str, str): header, payload, signed_setion, and
+            signature.
+
+    Raises:
+        ValueError: if there are an incorrect amount of segments in the token.
+    """
+    token = _helpers.to_bytes(token)
+
+    if token.count(b'.') != 2:
+        raise ValueError(
+            'Wrong number of segments in token: {0}'.format(token))
+
+    encoded_header, encoded_payload, signature = token.split(b'.')
+    signed_section = encoded_header + b'.' + encoded_payload
+    signature = base64.urlsafe_b64decode(signature)
+
+    # Parse segments
+    header = _decode_jwt_segment(encoded_header)
+    payload = _decode_jwt_segment(encoded_payload)
+
+    return header, payload, signed_section, signature
+
+
+def decode_header(token):
+    """Return the decoded header of a token.
+
+    No verification is done. This is useful to extract the key id from
+    the header in order to acquire the appropriate certificate to verify
+    the token.
+
+    Args:
+        token (Union[str, bytes]): the encoded JWT.
+
+    Returns:
+        Mapping: The decoded JWT header.
+    """
+    header, _, _, _ = _unverified_decode(token)
+    return header
+
+
+def _verify_iat_and_exp(payload):
+    """Verifies the iat (Issued At) and exp (Expires) claims in a token
+    payload.
+
+    Args:
+        payload (mapping): The JWT payload.
+
+    Raises:
+        ValueError: if any checks failed.
+    """
+    now = _helpers.datetime_to_secs(_helpers.now())
+
+    # Make sure the iat and exp claims are present
+    for key in ('iat', 'exp'):
+        if key not in payload:
+            raise ValueError(
+                'Token does not contain required claim {}'.format(key))
+
+    # Make sure the token wasn't issued in the future
+    iat = payload['iat']
+    earliest = iat - CLOCK_SKEW_SECS
+    if now < earliest:
+        raise ValueError('Token used too early, {} < {}'.format(now, iat))
+
+    # Make sure the token wasn't issue in the past
+    exp = payload['exp']
+    latest = exp + CLOCK_SKEW_SECS
+    if latest < now:
+        raise ValueError('Token expired, {} < {}'.format(latest, now))
+
+
+def decode(token, certs=None, verify=True, audience=None):
+    """Decode and verify a JWT.
+
+    Args:
+        token (string): The encoded JWT.
+        certs (Union[str, bytes, Mapping]): The certificate used to
+            validate. If bytes or string, it must the the public key
+            certificate in PEM format. If a mapping, it must be a mapping of
+            key IDs to public key certificates in PEM format. The mapping must
+            contain the same key ID that's specified in the token's header.
+        verify (bool): Whether to perform signature and claim validation.
+            Verification is done by default.
+        audience (str): The audience claim, 'aud', that this JWT should
+            contain. If None then the JWT's 'aud' parameter is not verified.
+
+    Returns:
+        Mapping: The deserialized JSON payload in the JWT.
+
+    Raises:
+        ValueError: if any verification checks failed.
+    """
+    header, payload, signed_section, signature = _unverified_decode(token)
+
+    if not verify:
+        return payload
+
+    # If certs is specified as a dictionary of key IDs to certificates, then
+    # use the certificate identified by the key ID in the token header.
+    if isinstance(certs, collections.Mapping):
+        key_id = header.get('kid')
+        if key_id:
+            if key_id not in certs:
+                raise ValueError(
+                    'Certificate for key id {} not found.'.format(key_id))
+            certs = [certs[key_id]]
+        # If there's no key id in the header, check against all of the certs.
+        else:
+            certs = certs.values()
+
+    # Verify that the signature matches the message.
+    if not crypt.verify_signature(signed_section, signature, certs):
+        raise ValueError('Could not verify token signature.')
+
+    # Verify the issued at and created times in the payload.
+    _verify_iat_and_exp(payload)
+
+    # Check audience.
+    if audience is not None:
+        claim_audience = payload.get('aud')
+        if audience != claim_audience:
+            raise ValueError(
+                'Token has wrong audience {}, expected {}'.format(
+                    claim_audience, audience))
+
+    return payload
diff --git a/tests/test__helpers.py b/tests/test__helpers.py
index b7e0bab84..b8a0cb3a1 100644
--- a/tests/test__helpers.py
+++ b/tests/test__helpers.py
@@ -12,12 +12,24 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import datetime
 
 import pytest
 
 from google.auth import _helpers
 
 
+def test_now():
+    assert isinstance(_helpers.now(), datetime.datetime)
+
+
+def test_datetime_to_secs():
+    assert _helpers.datetime_to_secs(
+        datetime.datetime(1970, 1, 1)) == 0
+    assert _helpers.datetime_to_secs(
+        datetime.datetime(1990, 5, 29)) == 643939200
+
+
 def test_to_bytes_with_bytes():
     value = b'bytes-val'
     assert _helpers.to_bytes(value) == value
diff --git a/tests/test_jwt.py b/tests/test_jwt.py
new file mode 100644
index 000000000..2168b81ad
--- /dev/null
+++ b/tests/test_jwt.py
@@ -0,0 +1,175 @@
+# Copyright 2014 Google Inc.
+#
+# 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.
+
+import base64
+import datetime
+import os
+
+import pytest
+
+from google.auth import _helpers
+from google.auth import crypt
+from google.auth import jwt
+
+
+DATA_DIR = os.path.join(os.path.dirname(__file__), 'data')
+
+with open(os.path.join(DATA_DIR, 'privatekey.pem'), 'rb') as fh:
+    PRIVATE_KEY_BYTES = fh.read()
+
+with open(os.path.join(DATA_DIR, 'public_cert.pem'), 'rb') as fh:
+    PUBLIC_CERT_BYTES = fh.read()
+
+with open(os.path.join(DATA_DIR, 'other_cert.pem'), 'rb') as fh:
+    OTHER_CERT_BYTES = fh.read()
+
+
+@pytest.fixture
+def signer():
+    return crypt.Signer.from_string(PRIVATE_KEY_BYTES, '1')
+
+
+def test_encode_basic(signer):
+    test_payload = {'test': 'value'}
+    encoded = jwt.encode(signer, test_payload)
+    header, payload, _, _ = jwt._unverified_decode(encoded)
+    assert payload == test_payload
+    assert header == {'typ': 'JWT', 'alg': 'RS256', 'kid': signer.key_id}
+
+
+def test_encode_extra_headers(signer):
+    encoded = jwt.encode(signer, {}, header={'extra': 'value'})
+    header = jwt.decode_header(encoded)
+    assert header == {
+        'typ': 'JWT', 'alg': 'RS256', 'kid': signer.key_id, 'extra': 'value'}
+
+
+@pytest.fixture
+def token_factory(signer):
+    def factory(claims=None, key_id=None):
+        now = _helpers.datetime_to_secs(_helpers.now())
+        payload = {
+            'aud': 'audience@example.com',
+            'iat': now,
+            'exp': now + 300,
+            'user': 'billy bob',
+            'metadata': {'meta': 'data'}
+        }
+        payload.update(claims or {})
+        return jwt.encode(signer, payload, key_id=key_id)
+    return factory
+
+
+def test_decode_valid(token_factory):
+    payload = jwt.decode(token_factory(), certs=PUBLIC_CERT_BYTES)
+    assert payload['aud'] == 'audience@example.com'
+    assert payload['user'] == 'billy bob'
+    assert payload['metadata']['meta'] == 'data'
+
+
+def test_decode_valid_with_audience(token_factory):
+    payload = jwt.decode(
+        token_factory(), certs=PUBLIC_CERT_BYTES,
+        audience='audience@example.com')
+    assert payload['aud'] == 'audience@example.com'
+    assert payload['user'] == 'billy bob'
+    assert payload['metadata']['meta'] == 'data'
+
+
+def test_decode_valid_unverified(token_factory):
+    payload = jwt.decode(token_factory(), certs=OTHER_CERT_BYTES, verify=False)
+    assert payload['aud'] == 'audience@example.com'
+    assert payload['user'] == 'billy bob'
+    assert payload['metadata']['meta'] == 'data'
+
+
+def test_decode_bad_token_wrong_number_of_segments():
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode('1.2', PUBLIC_CERT_BYTES)
+    assert excinfo.match(r'Wrong number of segments')
+
+
+def test_decode_bad_token_not_base64():
+    with pytest.raises((ValueError, TypeError)) as excinfo:
+        jwt.decode('1.2.3', PUBLIC_CERT_BYTES)
+    assert excinfo.match(r'Incorrect padding')
+
+
+def test_decode_bad_token_not_json():
+    token = b'.'.join([base64.urlsafe_b64encode(b'123!')] * 3)
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token, PUBLIC_CERT_BYTES)
+    assert excinfo.match(r'Can\'t parse segment')
+
+
+def test_decode_bad_token_no_iat_or_exp(signer):
+    token = jwt.encode(signer, {'test': 'value'})
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token, PUBLIC_CERT_BYTES)
+    assert excinfo.match(r'Token does not contain required claim')
+
+
+def test_decode_bad_token_too_early(token_factory):
+    token = token_factory(claims={
+        'iat': _helpers.datetime_to_secs(
+            _helpers.now() + datetime.timedelta(hours=1))
+    })
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token, PUBLIC_CERT_BYTES)
+    assert excinfo.match(r'Token used too early')
+
+
+def test_decode_bad_token_expired(token_factory):
+    token = token_factory(claims={
+        'exp': _helpers.datetime_to_secs(
+            _helpers.now() - datetime.timedelta(hours=1))
+    })
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token, PUBLIC_CERT_BYTES)
+    assert excinfo.match(r'Token expired')
+
+
+def test_decode_bad_token_wrong_audience(token_factory):
+    token = token_factory()
+    audience = 'audience2@example.com'
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token, PUBLIC_CERT_BYTES, audience=audience)
+    assert excinfo.match(r'Token has wrong audience')
+
+
+def test_decode_wrong_cert(token_factory):
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token_factory(), OTHER_CERT_BYTES)
+    assert excinfo.match(r'Could not verify token signature')
+
+
+def test_decode_multicert_bad_cert(token_factory):
+    certs = {'1': OTHER_CERT_BYTES, '2': PUBLIC_CERT_BYTES}
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token_factory(), certs)
+    assert excinfo.match(r'Could not verify token signature')
+
+
+def test_decode_no_cert(token_factory):
+    certs = {'2': PUBLIC_CERT_BYTES}
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(token_factory(), certs)
+    assert excinfo.match(r'Certificate for key id 1 not found')
+
+
+def test_decode_no_key_id(token_factory):
+    token = token_factory(key_id=False)
+    certs = {'2': PUBLIC_CERT_BYTES}
+    payload = jwt.decode(token, certs)
+    assert payload['user'] == 'billy bob'

From d1d8232bcc56ff593eb3c46b584028efd756569f Mon Sep 17 00:00:00 2001
From: Jon Wayne Parrott <jonwayne@google.com>
Date: Tue, 4 Oct 2016 14:31:12 -0700
Subject: [PATCH 2/5] Fix incorrect docstring (no credentials class yet)

---
 google/auth/jwt.py | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 57afae338..6402420b1 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -17,9 +17,6 @@
 Provides support for creating (encoding) and verifying (decoding) JWTs,
 especially JWTs generated and consumed by Google infrastructure.
 
-Also provides a :class:`Credentials` class that uses JWTs as authentication
-bearer tokens.
-
 See `rfc7519`_ for more details on JWTs.
 
 To encode a JWT::

From ec3d9242b1df62eadf97df024309f94910dbfe70 Mon Sep 17 00:00:00 2001
From: Jon Wayne Parrott <jonwayne@google.com>
Date: Tue, 4 Oct 2016 15:26:14 -0700
Subject: [PATCH 3/5] Address review comments

---
 google/auth/_helpers.py |  2 +-
 google/auth/jwt.py      | 17 ++++++++++-------
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index b20e216f4..64186f3da 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -22,7 +22,7 @@
 
 
 def now():
-    """Gets the current UTC datetime."""
+    """Returns the current UTC datetime."""
     return datetime.datetime.utcnow()
 
 
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 6402420b1..2ca3e93ba 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -61,12 +61,13 @@ def encode(signer, payload, header=None, key_id=None):
         header (Mapping): Additional JWT header payload.
         key_id (str): The key id to add to the JWT header. If the
             signer has a key id it will be used as the default. If this is
-            specified, it'll override the signer's key id.
+            specified, it'll override the signer's key id. If this is False,
+            no key id will be included.
 
     Returns:
         bytes: The encoded JWT.
     """
-    if not header:
+    if header is None:
         header = {}
 
     if key_id is None:
@@ -94,7 +95,7 @@ def _decode_jwt_segment(encoded_section):
     section_bytes = base64.urlsafe_b64decode(encoded_section)
     try:
         return json.loads(section_bytes.decode('utf-8'))
-    except:
+    except ValueError:
         raise ValueError('Can\'t parse segment: {0}'.format(section_bytes))
 
 
@@ -105,7 +106,7 @@ def _unverified_decode(token):
         token (Union[str, bytes]): The encoded JWT.
 
     Returns:
-        Tuple(str, str, str, str): header, payload, signed_setion, and
+        Tuple(str, str, str, str): header, payload, signed_section, and
             signature.
 
     Raises:
@@ -210,13 +211,15 @@ def decode(token, certs=None, verify=True, audience=None):
             if key_id not in certs:
                 raise ValueError(
                     'Certificate for key id {} not found.'.format(key_id))
-            certs = [certs[key_id]]
+            certs_to_check = [certs[key_id]]
         # If there's no key id in the header, check against all of the certs.
         else:
-            certs = certs.values()
+            certs_to_check = certs.values()
+    else:
+        certs_to_check = certs
 
     # Verify that the signature matches the message.
-    if not crypt.verify_signature(signed_section, signature, certs):
+    if not crypt.verify_signature(signed_section, signature, certs_to_check):
         raise ValueError('Could not verify token signature.')
 
     # Verify the issued at and created times in the payload.

From 5e7c564ed010b55ce99d35b8ad5d613bcb2895a4 Mon Sep 17 00:00:00 2001
From: Jon Wayne Parrott <jonwayne@google.com>
Date: Tue, 4 Oct 2016 16:21:39 -0700
Subject: [PATCH 4/5] Rename to utcnow

---
 google/auth/_helpers.py | 14 ++++++++++++--
 google/auth/jwt.py      |  2 +-
 tests/test__helpers.py  |  4 ++--
 tests/test_jwt.py       |  6 +++---
 4 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index 64186f3da..0a62209d6 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -21,13 +21,23 @@
 import six
 
 
-def now():
-    """Returns the current UTC datetime."""
+def utcnow():
+    """Returns the current UTC datetime.
+
+    Returns:
+        datetime: The current time in UTC.
+    """
     return datetime.datetime.utcnow()
 
 
 def datetime_to_secs(value):
     """Convert a datetime object to the number of seconds since the UNIX epoch.
+
+    Args:
+        value (datetime): The datetime to convert.
+
+    Returns:
+        int: The number of seconds since the UNIX epoch.
     """
     return calendar.timegm(value.utctimetuple())
 
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 2ca3e93ba..0fde61e7d 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -156,7 +156,7 @@ def _verify_iat_and_exp(payload):
     Raises:
         ValueError: if any checks failed.
     """
-    now = _helpers.datetime_to_secs(_helpers.now())
+    now = _helpers.datetime_to_secs(_helpers.utcnow())
 
     # Make sure the iat and exp claims are present
     for key in ('iat', 'exp'):
diff --git a/tests/test__helpers.py b/tests/test__helpers.py
index b8a0cb3a1..c2bc4a733 100644
--- a/tests/test__helpers.py
+++ b/tests/test__helpers.py
@@ -19,8 +19,8 @@
 from google.auth import _helpers
 
 
-def test_now():
-    assert isinstance(_helpers.now(), datetime.datetime)
+def test_utcnow():
+    assert isinstance(_helpers.utcnow(), datetime.datetime)
 
 
 def test_datetime_to_secs():
diff --git a/tests/test_jwt.py b/tests/test_jwt.py
index 2168b81ad..edce92b28 100644
--- a/tests/test_jwt.py
+++ b/tests/test_jwt.py
@@ -58,7 +58,7 @@ def test_encode_extra_headers(signer):
 @pytest.fixture
 def token_factory(signer):
     def factory(claims=None, key_id=None):
-        now = _helpers.datetime_to_secs(_helpers.now())
+        now = _helpers.datetime_to_secs(_helpers.utcnow())
         payload = {
             'aud': 'audience@example.com',
             'iat': now,
@@ -123,7 +123,7 @@ def test_decode_bad_token_no_iat_or_exp(signer):
 def test_decode_bad_token_too_early(token_factory):
     token = token_factory(claims={
         'iat': _helpers.datetime_to_secs(
-            _helpers.now() + datetime.timedelta(hours=1))
+            _helpers.utcnow() + datetime.timedelta(hours=1))
     })
     with pytest.raises(ValueError) as excinfo:
         jwt.decode(token, PUBLIC_CERT_BYTES)
@@ -133,7 +133,7 @@ def test_decode_bad_token_too_early(token_factory):
 def test_decode_bad_token_expired(token_factory):
     token = token_factory(claims={
         'exp': _helpers.datetime_to_secs(
-            _helpers.now() - datetime.timedelta(hours=1))
+            _helpers.utcnow() - datetime.timedelta(hours=1))
     })
     with pytest.raises(ValueError) as excinfo:
         jwt.decode(token, PUBLIC_CERT_BYTES)

From 5cf80cf018aaf6d389bd78b2eb434f14f927e6a2 Mon Sep 17 00:00:00 2001
From: Jon Wayne Parrott <jonwayne@google.com>
Date: Wed, 5 Oct 2016 11:02:16 -0700
Subject: [PATCH 5/5] Remove ability to specify key_id as False

---
 google/auth/jwt.py | 11 +++++------
 tests/test_jwt.py  | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 0fde61e7d..5349e2982 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -49,7 +49,7 @@
 
 
 _DEFAULT_TOKEN_LIFETIME_SECS = 3600  # 1 hour in sections
-CLOCK_SKEW_SECS = 300  # 5 minutes in seconds
+_CLOCK_SKEW_SECS = 300  # 5 minutes in seconds
 
 
 def encode(signer, payload, header=None, key_id=None):
@@ -61,8 +61,7 @@ def encode(signer, payload, header=None, key_id=None):
         header (Mapping): Additional JWT header payload.
         key_id (str): The key id to add to the JWT header. If the
             signer has a key id it will be used as the default. If this is
-            specified, it'll override the signer's key id. If this is False,
-            no key id will be included.
+            specified it will override the signer's key id.
 
     Returns:
         bytes: The encoded JWT.
@@ -75,7 +74,7 @@ def encode(signer, payload, header=None, key_id=None):
 
     header.update({'typ': 'JWT', 'alg': 'RS256'})
 
-    if key_id is not None and key_id is not False:
+    if key_id is not None:
         header['kid'] = key_id
 
     segments = [
@@ -166,13 +165,13 @@ def _verify_iat_and_exp(payload):
 
     # Make sure the token wasn't issued in the future
     iat = payload['iat']
-    earliest = iat - CLOCK_SKEW_SECS
+    earliest = iat - _CLOCK_SKEW_SECS
     if now < earliest:
         raise ValueError('Token used too early, {} < {}'.format(now, iat))
 
     # Make sure the token wasn't issue in the past
     exp = payload['exp']
-    latest = exp + CLOCK_SKEW_SECS
+    latest = exp + _CLOCK_SKEW_SECS
     if latest < now:
         raise ValueError('Token expired, {} < {}'.format(latest, now))
 
diff --git a/tests/test_jwt.py b/tests/test_jwt.py
index edce92b28..69628e5c9 100644
--- a/tests/test_jwt.py
+++ b/tests/test_jwt.py
@@ -67,6 +67,13 @@ def factory(claims=None, key_id=None):
             'metadata': {'meta': 'data'}
         }
         payload.update(claims or {})
+
+        # False is specified to remove the signer's key id for testing
+        # headers without key ids.
+        if key_id is False:
+            signer.key_id = None
+            key_id = None
+
         return jwt.encode(signer, payload, key_id=key_id)
     return factory
 
@@ -173,3 +180,10 @@ def test_decode_no_key_id(token_factory):
     certs = {'2': PUBLIC_CERT_BYTES}
     payload = jwt.decode(token, certs)
     assert payload['user'] == 'billy bob'
+
+
+def test_roundtrip_explicit_key_id(token_factory):
+    token = token_factory(key_id='3')
+    certs = {'2': OTHER_CERT_BYTES, '3': PUBLIC_CERT_BYTES}
+    payload = jwt.decode(token, certs)
+    assert payload['user'] == 'billy bob'