diff --git a/examples/andytoshi.ots b/examples/andytoshi.ots
index a2ccc3d..15b9c5c 100644
Binary files a/examples/andytoshi.ots and b/examples/andytoshi.ots differ
diff --git a/opentimestamps/core/secp256k1.py b/opentimestamps/core/secp256k1.py
index e99b8dc..d9d5194 100644
--- a/opentimestamps/core/secp256k1.py
+++ b/opentimestamps/core/secp256k1.py
@@ -11,19 +11,29 @@
 
 import hashlib
 
-from opentimestamps.core.op import BinaryOp, MsgValueError
+from opentimestamps.core.op import UnaryOp, MsgValueError
 
-@BinaryOp._register_op
-class OpSecp256k1Commitment(BinaryOp):
-    """Execute the map commit -> [P + sha256(P||commit)G]_x for a given secp256k1 point P"""
+@UnaryOp._register_op
+class OpSecp256k1Commitment(UnaryOp):
+    """Map (P || commit) -> [P + sha256(P||commit)G]_x for a given secp256k1 point P
+
+    This is a unary op rather than a binary op to allow timestamps to also
+    timestamp the point itself; in the event of an ECC break this might be
+    relevant. Such a break would not affect the integrity of the commitment,
+    but knowledge of the underlying key may be interesting in its own right.
+    """
     TAG = b'\x09'
     TAG_NAME = 'secp256k1commitment'
 
     def _do_op_call(self, msg):
+        if len(msg) < 33:
+            raise MsgValueError("Missing secp256k1 point")
+
+        pt = Point.decode(msg[0:33])
+
         hasher = hashlib.sha256()
-        pt = Point.decode(self[0])
         hasher.update(pt.encode())
-        hasher.update(msg)
+        hasher.update(msg[33:])
         tweak = int.from_bytes(hasher.digest(), 'big')
         tweak_pt = SECP256K1_GEN.scalar_mul(tweak)
         final_pt = pt.add(tweak_pt)
diff --git a/opentimestamps/tests/core/test_secp256k1.py b/opentimestamps/tests/core/test_secp256k1.py
index 6eaeed5..1b3de11 100644
--- a/opentimestamps/tests/core/test_secp256k1.py
+++ b/opentimestamps/tests/core/test_secp256k1.py
@@ -104,5 +104,5 @@ 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)
+        self.assertEqual(OpSecp256k1Commitment()(pt_encode + msg), result)