Add support for the OIDC at_hash claim

Use PyJWT to compute the at_hash value for OpenID Connect:
http://openid.net/specs/openid-connect-core-1_0.html#CodeIDToken

This makes more sense in PyJWT than its client code because of the tight
coupling between the chosen signing algorithm and the computation of the
at_hash. Any client code would have to jump through hoops to get this to
work nicely based on the algorithm being fed to PyJWT.

Closes #295

Primary changes:

Add support for access_token=... as a param to PyJWT.encode and
PyJWT.decode . On encode, the at_hash claim is computed and added to the
payload. On decode, unpacks the at_hash value, raising a missing claim
error if its missing, and compares it to a freshly computed at_hash.
Raises a new error type if they don't match.
Does not use the verification options dict, as it's redundant with the
caller supplying access_token in this case.

Supporting changes:
- Add tests for the above
- Let PyJWT and PyJWS get an algorithm object from a string as a method
- Add a method, compute_at_hash, to PyJWT objects
- PyJWT._validate_claims now takes the header as an arg (needed to get
  algo)

jwt/api_jws.py

@@ -67,6 +67,18 @@ def get_algorithms(self):
         """
         return list(self._valid_algs)
 
+    def get_algo_by_name(self, algorithm):
+        try:
+            return self._algorithms[algorithm]
+        except KeyError:
+            if not has_crypto and algorithm in requires_cryptography:
+                raise NotImplementedError(
+                    "Algorithm '%s' could not be found. Do you have cryptography "
+                    "installed?" % algorithm
+                )
+            else:
+                raise NotImplementedError('Algorithm not supported')
+
     def encode(self, payload, key, algorithm='HS256', headers=None,
                json_encoder=None):
         segments = []
@@ -97,19 +109,10 @@ def encode(self, payload, key, algorithm='HS256', headers=None,
 
         # Segments
         signing_input = b'.'.join(segments)
-        try:
-            alg_obj = self._algorithms[algorithm]
-            key = alg_obj.prepare_key(key)
-            signature = alg_obj.sign(signing_input, key)
 
-        except KeyError:
-            if not has_crypto and algorithm in requires_cryptography:
-                raise NotImplementedError(
-                    "Algorithm '%s' could not be found. Do you have cryptography "
-                    "installed?" % algorithm
-                )
-            else:
-                raise NotImplementedError('Algorithm not supported')
+        alg_obj = self.get_algo_by_name(algorithm)
+        key = alg_obj.prepare_key(key)
+        signature = alg_obj.sign(signing_input, key)
 
         segments.append(base64url_encode(signature))
 

jwt/api_jwt.py

@@ -1,3 +1,5 @@
+from __future__ import division
+
 import json
 import warnings
 
@@ -10,10 +12,18 @@
 from .compat import string_types, timedelta_total_seconds
 from .exceptions import (
     DecodeError, ExpiredSignatureError, ImmatureSignatureError,
+    InvalidAccessTokenHashError,
     InvalidAudienceError, InvalidIssuedAtError,
     InvalidIssuerError, MissingRequiredClaimError
 )
-from .utils import merge_dict
+from .utils import base64url_encode, merge_dict
+
+try:
+    from cryptography.hazmat.primitives import hashes
+    from cryptography.hazmat.backends import default_backend
+    has_crypto = True
+except ImportError:
+    has_crypto = False
 
 
 class PyJWT(PyJWS):
@@ -34,7 +44,7 @@ def _get_default_options():
         }
 
     def encode(self, payload, key, algorithm='HS256', headers=None,
-               json_encoder=None):
+               json_encoder=None, access_token=None):
         # Check that we get a mapping
         if not isinstance(payload, Mapping):
             raise TypeError('Expecting a mapping object, as JWT only supports '
@@ -46,6 +56,10 @@ def encode(self, payload, key, algorithm='HS256', headers=None,
             if isinstance(payload.get(time_claim), datetime):
                 payload[time_claim] = timegm(payload[time_claim].utctimetuple())
 
+        # OIDC ID Token may have at_hash additional claim
+        if access_token is not None:
+            payload['at_hash'] = self.compute_at_hash(access_token, algorithm)
+
         json_payload = json.dumps(
             payload,
             separators=(',', ':'),
@@ -87,12 +101,52 @@ def decode(self, jwt, key='', verify=True, algorithms=None, options=None,
 
         if verify:
             merged_options = merge_dict(self.options, options)
-            self._validate_claims(payload, merged_options, **kwargs)
+            self._validate_claims(payload, header, merged_options, **kwargs)
 
         return payload
 
-    def _validate_claims(self, payload, options, audience=None, issuer=None,
-                         leeway=0, **kwargs):
+    def compute_at_hash(self, access_token, algorithm='HS256'):
+        """
+        Computes the at_hash claim for JWTs used in OpenID Connect.
+
+        The at_hash is based on the hashing algorithm used to sign the JWT, and
+        is specified here:
+        http://openid.net/specs/openid-connect-core-1_0.html#CodeIDToken
+
+        :return: at_hash of the access_token as a string
+
+        **Parameters**
+          ``access_token`` (string)
+            The access token to hash
+
+          ``algorithm``
+            An algorithm object from jwt.algorithms. Its expected behavior is
+            determined by has_crypto.
+
+            If has_crypto=False, it must have a callable hash_alg member which
+            provides digest(), like the hashlib variants
+            If has_crypto=True, it *may* be a hashlib style hashing function,
+            or it may be a cryptography hashing algorithm
+        """
+        alg_obj = self.get_algo_by_name(algorithm)
+        hash_alg = alg_obj.hash_alg
+
+        def get_digest(bytestr):
+            if has_crypto and (
+                    isinstance(hash_alg, type) and
+                    issubclass(hash_alg, hashes.HashAlgorithm)):
+                digest = hashes.Hash(hash_alg(), backend=default_backend())
+                digest.update(bytestr)
+                return digest.finalize()
+            else:
+                return hash_alg(bytestr).digest()
+
+        digest = get_digest(access_token.encode('utf-8'))
+        truncated = digest[:(len(digest) // 2)]
+        return base64url_encode(truncated).decode('utf-8')
+
+    def _validate_claims(self, payload, header, options, audience=None,
+                         issuer=None, leeway=0, access_token=None, **kwargs):
 
         if 'verify_expiration' in kwargs:
             options['verify_exp'] = kwargs.get('verify_expiration', True)
@@ -119,6 +173,9 @@ def _validate_claims(self, payload, options, audience=None, issuer=None,
         if 'exp' in payload and options.get('verify_exp'):
             self._validate_exp(payload, now, leeway)
 
+        if access_token:
+            self._validate_at_hash(payload, header, access_token)
+
         if options.get('verify_iss'):
             self._validate_iss(payload, issuer)
 
@@ -190,6 +247,17 @@ def _validate_iss(self, payload, issuer):
         if payload['iss'] != issuer:
             raise InvalidIssuerError('Invalid issuer')
 
+    def _validate_at_hash(self, payload, header, access_token):
+        try:
+            at_hash = payload['at_hash']
+        except KeyError:
+            raise MissingRequiredClaimError('at_hash')
+
+        alg = header.get('alg')
+
+        if at_hash != self.compute_at_hash(access_token, alg):
+            raise InvalidAccessTokenHashError("at_hash doesn't match")
+
 
 _jwt_global_obj = PyJWT()
 encode = _jwt_global_obj.encode

jwt/exceptions.py

@@ -22,6 +22,10 @@ class InvalidIssuedAtError(InvalidTokenError):
     pass
 
 
+class InvalidAccessTokenHashError(InvalidTokenError):
+    pass
+
+
 class ImmatureSignatureError(InvalidTokenError):
     pass
 

tests/test_api_jwt.py

@@ -9,8 +9,8 @@
 from jwt.api_jwt import PyJWT
 from jwt.exceptions import (
     DecodeError, ExpiredSignatureError, ImmatureSignatureError,
-    InvalidAudienceError, InvalidIssuedAtError, InvalidIssuerError,
-    MissingRequiredClaimError
+    InvalidAccessTokenHashError, InvalidAudienceError, InvalidIssuedAtError,
+    InvalidIssuerError, MissingRequiredClaimError
 )
 
 import pytest
@@ -58,6 +58,14 @@ def test_load_verify_valid_jwt(self, jwt):
 
         assert decoded_payload == example_payload
 
+    def test_verify_fails_missing_athash(self, jwt, payload):
+        secret = 'secret'
+        jwt_message = jwt.encode(payload, secret)
+
+        with pytest.raises(MissingRequiredClaimError) as exc:
+            jwt.decode(jwt_message, key=secret, access_token='foobar')
+        assert 'at_hash' in str(exc.value)
+
     def test_decode_invalid_payload_string(self, jwt):
         example_jwt = (
             'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.aGVsb'
@@ -118,6 +126,19 @@ def test_decode_with_invalid_aud_list_member_throws_exception(self, jwt):
         exception = context.value
         assert str(exception) == 'Invalid claim format in token'
 
+    def test_decode_with_wrong_access_token_throws_exception(self, jwt, payload):
+        secret = 'secret'
+        jwt_message = jwt.encode(payload, secret, access_token='foobar')
+
+        with pytest.raises(InvalidAccessTokenHashError):
+            jwt.decode(jwt_message, key=secret, access_token='foobar2')
+
+    def test_decode_with_no_access_token_skips_at_hash(self, jwt, payload):
+        secret = 'secret'
+        jwt_message = jwt.encode(payload, secret, access_token='foobar')
+
+        jwt.decode(jwt_message, key=secret)
+
     def test_encode_bad_type(self, jwt):
 
         types = ['string', tuple(), list(), 42, set()]
@@ -495,3 +516,26 @@ def test_decode_no_algorithms_verify_false(self, jwt, payload):
             pass
         else:
             assert False, "Unexpected DeprecationWarning raised."
+
+    @pytest.mark.skipif(not has_crypto,
+                        reason="Can't run without cryptography library")
+    def test_at_hashes_match(self, jwt):
+        """
+        Check that HS256 and RS256 at_hash values match.
+        These are different implementations of the at_hash computation, one in
+        terms of hashlib.sha256 and one in terms of cryptography..hashes.SHA256
+
+        They should produce identical values, and both should evaluate
+        successfully.
+
+        Checks
+        - Evaluation works for both methods (doesn't crash)
+        - Evaluation of cryptography hash digests is accurate (use of
+          finalize()). Assumes that the simpler hashlib evaluation is
+          "obviously correct"
+        """
+        # this is just garbage to feed in
+        token = "abc123" * 20
+
+        assert (jwt.compute_at_hash(token, 'HS256') ==
+                jwt.compute_at_hash(token, 'RS256'))