core: add OpSecp256k1Commitment

Author: apoelstra

opentimestamps/core/op.py

@@ -347,3 +347,6 @@ def _do_op_call(self, msg):
         r = sha3.keccak_256(bytes(msg)).digest()
         assert len(r) == self.DIGEST_LENGTH
         return r
+
+from opentimestamps.core.secp256k1 import OpSecp256k1Commitment
+

opentimestamps/core/secp256k1.py

@@ -9,6 +9,27 @@
 # modified, propagated, or distributed except according to the terms contained
 # in the LICENSE file.
 
+import hashlib
+
+from opentimestamps.core.op import BinaryOp, MsgValueError
+
+@BinaryOp._register_op
+class OpSecp256k1Commitment(BinaryOp):
+    """Execute the map commit -> [P + sha256(P||commit)G]_x for a given secp256k1 point P"""
+    TAG = b'\x09'
+    TAG_NAME = 'secp256k1commitment'
+
+    def _do_op_call(self, msg):
+        hasher = hashlib.sha256()
+        pt = Point.decode(self[0])
+        hasher.update(pt.encode())
+        hasher.update(msg)
+        tweak = int.from_bytes(hasher.digest(), 'big')
+        tweak_pt = SECP256K1_GEN.scalar_mul(tweak)
+        final_pt = pt.add(tweak_pt)
+        return final_pt.x.to_bytes(32, 'big')
+
+
 ## What follows is a lot of inefficient but explicit secp256k1 math
 class Point(object):
     inf = True

opentimestamps/tests/core/test_secp256k1.py

@@ -9,6 +9,7 @@
 # modified, propagated, or distributed except according to the terms contained
 # in the LICENSE file.
 
+import hashlib
 import binascii
 import unittest
 
@@ -20,7 +21,7 @@ def test_point_rt(self):
         gen = SECP256K1_GEN
         encode = gen.encode()
         self.assertEqual(encode, binascii.unhexlify("0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798"))
-        gen2 = Point().decode(encode)
+        gen2 = Point.decode(encode)
         self.assertEqual(gen, gen2)
 
     def test_pinv(self):
@@ -99,3 +100,9 @@ def test_scalar_mul(self):
         self.assertEqual(p2.scalar_mul(-1), np2)
         self.assertEqual(p1.scalar_mul(3), p3)
 
+    def test_op_signtocontract(self):
+        pt_encode = binascii.unhexlify("0308aec434612f56df3f02c4e678260424415882ebd3efc16d52e3f9c1e39afdb0")
+        msg = hashlib.sha256("This is andytoshi on 2017-05-16 21:30 UTC".encode()).digest()
+        result = binascii.unhexlify("d386ef692770fcecad43362cf541858662e4ebe31d3ad04d196f94168897947a")
+        self.assertEqual(OpSecp256k1Commitment(pt_encode)(msg), result)
+