legacy hash_to_curve

cashu/core/legacy.py

@@ -0,0 +1,27 @@
+import hashlib
+
+from secp256k1 import PublicKey
+
+
+def hash_to_point_pre_0_3_3(secret_msg):
+    """Generates x coordinate from the message hash and checks if the point lies on the curve.
+    If it does not, it tries computing again a new x coordinate from the hash of the coordinate."""
+    point = None
+    msg = secret_msg
+    while point is None:
+        _hash = hashlib.sha256(msg).hexdigest().encode("utf-8")
+        try:
+            # We construct compressed pub which has x coordinate encoded with even y
+            _hash = list(_hash[:33])  # take the 33 bytes and get a list of bytes
+            _hash[0] = 0x02  # set first byte to represent even y coord
+            _hash = bytes(_hash)
+            point = PublicKey(_hash, raw=True)
+        except:
+            msg = _hash
+
+    return point
+
+
+def verify_pre_0_3_3(a, C, secret_msg):
+    Y = hash_to_point_pre_0_3_3(secret_msg.encode("utf-8"))
+    return C == Y.mult(a)