BIP 340: fix function signature of lift_x in reference code

bip-0340.mediawiki defines lift_x as taking an integer argument. This commit
changes the argument of lift_x in the reference code to be identical to the
specification. Previously it took a byte array.

bip-0340.mediawiki

@@ -243,6 +243,12 @@ Blind Schnorr signatures could for example be used in [https://github.com/Elemen
 For development and testing purposes, we provide a [[bip-0340/test-vectors.csv|collection of test vectors in CSV format]] and a naive, highly inefficient, and non-constant time [[bip-0340/reference.py|pure Python 3.7 reference implementation of the signing and verification algorithm]].
 The reference implementation is for demonstration purposes only and not to be used in production environments.
 
+== Changelog ==
+
+To help implementors understand updates to this BIP, we keep a list of substantial changes.
+
+* 2022-08: Fix function signature of lift_x in reference code
+
 == Footnotes ==
 
 <references />

bip-0340/reference.py

@@ -68,8 +68,7 @@ def bytes_from_point(P: Point) -> bytes:
 def xor_bytes(b0: bytes, b1: bytes) -> bytes:
     return bytes(x ^ y for (x, y) in zip(b0, b1))
 
-def lift_x(b: bytes) -> Optional[Point]:
-    x = int_from_bytes(b)
+def lift_x(x: int) -> Optional[Point]:
     if x >= p:
         return None
     y_sq = (pow(x, 3, p) + 7) % p
@@ -128,7 +127,7 @@ def schnorr_verify(msg: bytes, pubkey: bytes, sig: bytes) -> bool:
         raise ValueError('The public key must be a 32-byte array.')
     if len(sig) != 64:
         raise ValueError('The signature must be a 64-byte array.')
-    P = lift_x(pubkey)
+    P = lift_x(int_from_bytes(pubkey))
     r = int_from_bytes(sig[0:32])
     s = int_from_bytes(sig[32:64])
     if (P is None) or (r >= p) or (s >= n):