Adjust new domain separator (#421)

* adjust new domain separator

* Update cashu/core/crypto/b_dhke.py

Co-authored-by: Lagrang3 <32647090+Lagrang3@users.noreply.github.com>

* slightly adjust comment

---------

Co-authored-by: Lagrang3 <32647090+Lagrang3@users.noreply.github.com>

cashu/core/crypto/b_dhke.py

@@ -71,24 +71,34 @@ def hash_to_curve(message: bytes) -> PublicKey:
     return point
 
 
-DOMAIN_SEPARATOR = b"Secp256k1_HashToCurve_"
+DOMAIN_SEPARATOR = b"Secp256k1_HashToCurve_Cashu_"
 
 
 def hash_to_curve_domain_separated(message: bytes) -> PublicKey:
-    """Generates a point from the message hash and checks if the point lies on the curve.
-    If it does not, iteratively tries to compute a new point from the hash."""
-    point = None
-    msg_to_hash = DOMAIN_SEPARATOR + message
+    """Generates a secp256k1 point from a message.
+
+    The point is generated by hashing the message with a domain separator and then
+    iteratively trying to compute a point from the hash. An increasing uint32 counter
+    (byte order little endian) is appended to the hash until a point is found that lies on the curve.
+
+    The chance of finding a valid point is 50% for every iteration. The maximum number of iterations
+    is 2**16. If no valid point is found after 2**16 iterations, a ValueError is raised (this should
+    never happen in practice).
+
+    The domain separator is b"Secp256k1_HashToCurve_Cashu_" or
+    bytes.fromhex("536563703235366b315f48617368546f43757276655f43617368755f").
+    """
+    msg_to_hash = hashlib.sha256(DOMAIN_SEPARATOR + message).digest()
     counter = 0
-    while point is None:
-        _hash = hashlib.sha256(msg_to_hash + str(counter).encode()).digest()
+    while counter < 2**16:
+        _hash = hashlib.sha256(msg_to_hash + counter.to_bytes(4, "little")).digest()
         try:
             # will error if point does not lie on curve
-            point = PublicKey(b"\x02" + _hash, raw=True)
+            return PublicKey(b"\x02" + _hash, raw=True)
         except Exception:
-            msg_to_hash = _hash
             counter += 1
-    return point
+    # it should never reach this point
+    raise ValueError("No valid point found")
 
 
 def step1_alice(