remove dependency of pyca/cryptography backend on python-ecdsa

Author: mattsb42-aws

jose/backends/cryptography_backend.py

@@ -1,10 +1,13 @@
+from __future__ import division
+
+import math
+
 import six
 
 try:
     from ecdsa import SigningKey as EcdsaSigningKey, VerifyingKey as EcdsaVerifyingKey
 except ImportError:
-    SigningKey = VerifyingKey = None
-from ecdsa.util import sigdecode_string, sigencode_string, sigdecode_der, sigencode_der
+    EcdsaSigningKey = EcdsaVerifyingKey = None
 
 from jose.backends.base import Key
 from jose.utils import base64_to_long, long_to_base64
@@ -15,7 +18,9 @@
 from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives import hashes, serialization
 from cryptography.hazmat.primitives.asymmetric import ec, rsa, padding
+from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature, encode_dss_signature
 from cryptography.hazmat.primitives.serialization import load_pem_private_key, load_pem_public_key
+from cryptography.utils import int_from_bytes, int_to_bytes
 from cryptography.x509 import load_pem_x509_certificate
 
 
@@ -94,15 +99,30 @@ def _process_jwk(self, jwk_dict):
         else:
             return public.public_key(self.cryptography_backend())
 
+    def _sig_component_length(self):
+        """Determine the correct serialization length for an encoded signature component.
+
+        This is the number of bytes required to encode the maximum key value.
+        """
+        return math.ceil(self.prepared_key.key_size / 8.0)
+
     def _der_to_asn1(self, der_signature):
         """Convert signature from DER encoding to ASN1 encoding."""
-        order = (2 ** self.prepared_key.curve.key_size) - 1
-        return sigencode_string(*sigdecode_der(der_signature, order), order=order)
+        r, s = decode_dss_signature(der_signature)
+        component_length = self._sig_component_length()
+        return int_to_bytes(r, component_length) + int_to_bytes(s, component_length)
 
     def _asn1_to_der(self, asn1_signature):
         """Convert signature from ASN1 encoding to DER encoding."""
-        order = (2 ** self.prepared_key.curve.key_size) - 1
-        return sigencode_der(*sigdecode_string(asn1_signature, order), order=order)
+        component_length = self._sig_component_length()
+        if len(asn1_signature) != int(2 * component_length):
+            raise ValueError("Invalid signature")
+
+        r_bytes = asn1_signature[:component_length]
+        s_bytes = asn1_signature[component_length:]
+        r = int_from_bytes(r_bytes, "big")
+        s = int_from_bytes(s_bytes, "big")
+        return encode_dss_signature(r, s)
 
     def sign(self, msg):
         if self.hash_alg.digest_size * 8 > self.prepared_key.curve.key_size:

tests/algorithms/test_EC.py

@@ -11,8 +11,10 @@
 
 try:
     from jose.backends.cryptography_backend import CryptographyECKey
+    from cryptography.hazmat.primitives.asymmetric import ec as CryptographyEc
+    from cryptography.hazmat.backends import default_backend as CryptographyBackend
 except ImportError:
-    CryptographyECKey = None
+    CryptographyECKey = CryptographyEc = CryptographyBackend = None
 
 import pytest
 
@@ -63,6 +65,27 @@ def test_key_from_ecdsa():
     assert not ECKey(key, ALGORITHMS.ES256).is_public()
 
 
+@pytest.mark.cryptography
+@pytest.mark.skipif(CryptographyECKey is None, reason="pyca/cryptography backend not available")
+@pytest.mark.parametrize("algorithm, expected_length", (
+        (ALGORITHMS.ES256, 32),
+        (ALGORITHMS.ES384, 48),
+        (ALGORITHMS.ES512, 66)
+))
+def test_cryptography_sig_component_length(algorithm, expected_length):
+    # Put mapping inside here to avoid more complex handling for test runs that do not have pyca/cryptography
+    mapping = {
+        ALGORITHMS.ES256: CryptographyEc.SECP256R1,
+        ALGORITHMS.ES384: CryptographyEc.SECP384R1,
+        ALGORITHMS.ES512: CryptographyEc.SECP521R1,
+    }
+    key = CryptographyECKey(
+        CryptographyEc.generate_private_key(mapping[algorithm](), backend=CryptographyBackend()),
+        algorithm
+    )
+    assert key._sig_component_length() == expected_length
+
+
 @pytest.mark.cryptography
 @pytest.mark.skipif(CryptographyECKey is None, reason="pyca/cryptography backend not available")
 def test_cryptograhy_der_to_asn1():