Convert JWT signatures to the correct format.

`key.sign` returns an ASN.1-encoded, DER-formatted signature, but
RFC 7518, section 3.4 says we should include R and S directly, in that
order, and in big-endian form.

Author: Kit Cambridge

python/py_vapid/jwt.py

@@ -6,13 +6,14 @@
 from cryptography.hazmat.primitives.asymmetric import ec, utils
 from cryptography.hazmat.primitives import hashes
 
-from py_vapid.utils import b64urldecode, b64urlencode
+from py_vapid.utils import b64urldecode, b64urlencode, num_to_bytes
 
 
 def extract_signature(auth):
-    """Fix the JWT auth token
-
-    convert a ecdsa integer pair into an OpenSSL DER pair.
+    """
+    Extracts the payload and signature from a JWT, converting the
+    signature from the JOSE format (RFC 7518, section 3.4) to the
+    RFC 3279 format that Cryptography requires.
 
     :param auth: A JWT Authorization Token.
     :type auth: str
@@ -23,7 +24,7 @@ def extract_signature(auth):
     payload, asig = auth.encode('utf8').rsplit(b'.', 1)
     sig = b64urldecode(asig)
     if len(sig) != 64:
-        return payload, sig
+        raise InvalidSignature()
 
     encoded = utils.encode_dss_signature(
         s=int(binascii.hexlify(sig[32:]), 16),
@@ -35,8 +36,6 @@ def extract_signature(auth):
 def decode(token, key):
     """Decode a web token into an assertion dictionary
 
-    This attempts to rectify both ecdsa and openssl generated signatures.
-
     :param token: VAPID auth token
     :type token: str
     :param key: bitarray containing the public key
@@ -84,5 +83,6 @@ def sign(claims, key):
                                      separators=(',', ':')).encode('utf8'))
     token = "{}.{}".format(header, claims)
     rsig = key.sign(token.encode('utf8'), ec.ECDSA(hashes.SHA256()))
-    sig = b64urlencode(rsig)
+    (r, s) = utils.decode_dss_signature(rsig)
+    sig = b64urlencode(num_to_bytes(r) + num_to_bytes(s))
     return "{}.{}".format(token, sig)

python/py_vapid/tests/test_vapid.py

@@ -6,12 +6,11 @@
 from nose.tools import eq_, ok_
 from mock import patch, Mock
 
-from cryptography.hazmat.primitives.asymmetric import ec, utils
+from cryptography.hazmat.primitives.asymmetric import ec
 from cryptography.hazmat.primitives import hashes
 
 from py_vapid import Vapid01, Vapid02, VapidException
 from py_vapid.jwt import decode
-from py_vapid.utils import b64urldecode
 
 # This is a private key in DER form.
 T_DER = """
@@ -174,27 +173,6 @@ def test_sign_02(self):
         for k in claims:
             eq_(t_val[k], claims[k])
 
-    def test_alt_sign(self):
-        """ecdsa uses a raw key pair to sign, openssl uses a DER."""
-        v = Vapid01.from_file("/tmp/private")
-        claims = {"aud": "https://example.com",
-                  "sub": "mailto:[email protected]",
-                  "foo": "extra value"}
-        # Get a signed token.
-        result = v.sign(claims)
-        # Convert the dss into raw.
-        auth, sig = result.get('Authorization').split(' ')[1].rsplit('.', 1)
-        ss = utils.decode_dss_signature(b64urldecode(sig.encode('utf8')))
-        new_sig = binascii.b2a_base64(
-            binascii.unhexlify("%064x%064x" % ss)
-        ).strip().strip(b'=').decode()
-        new_auth = auth + '.' + new_sig
-        # phew, all that done, now check
-        pkey = result.get("Crypto-Key").split('=')[1]
-        items = decode(new_auth, pkey)
-
-        eq_(items, claims)
-
     def test_bad_sign(self):
         v = Vapid01.from_file("/tmp/private")
         self.assertRaises(VapidException,

python/py_vapid/utils.py

@@ -1,4 +1,5 @@
 import base64
+import binascii
 
 
 def b64urldecode(data):
@@ -23,3 +24,16 @@ def b64urlencode(data):
 
     """
     return base64.urlsafe_b64encode(data).replace(b'=', b'').decode('utf8')
+
+
+def num_to_bytes(n):
+    """Returns the byte representation of an integer, in big-endian order.
+
+    :param n: The integer to encode.
+    :type n: int
+
+    :returns bytes
+
+    """
+    h = '%x' % n
+    return binascii.unhexlify('0' * (len(h) % 2) + h)