add support for encoding/decoding elliptic curve points

Based on the work of @ronf in #2346.

2 files changed, 39 insertions, 2 deletions

diff --git a/src/cryptography/hazmat/primitives/asymmetric/utils.py b/src/cryptography/hazmat/primitives/asymmetric/utils.py
index bad9ab73..b62eadf0 100644
--- a/src/cryptography/hazmat/primitives/asymmetric/utils.py
+++ b/src/cryptography/hazmat/primitives/asymmetric/utils.py
@@ -13,6 +13,7 @@ from pyasn1.type import namedtype, univ
 import six
 
 from cryptography import utils
+from cryptography.hazmat.primitives.asymmetric.ec import EllipticCurve
 
 
 class _DSSSigValue(univ.Sequence):
@@ -71,3 +72,36 @@ def encode_dss_signature(r, s):
     sig.setComponentByName('r', r)
     sig.setComponentByName('s', s)
     return encoder.encode(sig)
+
+
+def encode_ec_point(curve, x, y):
+    if not isinstance(curve, EllipticCurve):
+        raise TypeError("curve must be an EllipticCurve instance")
+
+    if x is None:
+        return b'\x00'
+    else:
+        # Get the ceiling of curve.key_size / 8
+        byte_length = (curve.key_size + 7) // 8
+        return (
+            b'\x04' + utils.int_to_bytes(x, byte_length) +
+            utils.int_to_bytes(y, byte_length)
+        )
+
+
+def decode_ec_point(curve, data):
+    if not isinstance(curve, EllipticCurve):
+        raise TypeError("curve must be an EllipticCurve instance")
+
+    if data == b'\x00':
+        return None, None
+    elif data.startswith(b'\x04'):
+        # Get the ceiling of curve.key_size / 8
+        byte_length = (curve.key_size + 7) // 8
+        if len(data) == 2 * byte_length + 1:
+            return (utils.int_from_bytes(data[1:byte_length + 1], 'big'),
+                    utils.int_from_bytes(data[byte_length + 1:], 'big'))
+        else:
+            raise ValueError('Invalid elliptic curve point data length')
+    else:
+        raise ValueError('Unsupported elliptic curve point type')
diff --git a/src/cryptography/utils.py b/src/cryptography/utils.py
index dac4046d..dbd961f7 100644
--- a/src/cryptography/utils.py
+++ b/src/cryptography/utils.py
@@ -48,9 +48,12 @@ else:
         return result
 
 
-def int_to_bytes(integer):
+def int_to_bytes(integer, length=None):
     hex_string = '%x' % integer
-    n = len(hex_string)
+    if length is None:
+        n = len(hex_string)
+    else:
+        n = length * 2
     return binascii.unhexlify(hex_string.zfill(n + (n & 1)))