BIP-374: treat challenge hash result e as scalar, bump to version 0.3.0

Note that the purpose of this change is primarily to improve clarity
for implementers and consistency with existing BIPs like BIP-340 and
BIP-327. Under the assumption that reaching a challenge hash with
`e >= n` is negligible, the newly introduced test vectors in the next
commit would also fail without the new rejection branch.

bip-0374.mediawiki

@@ -11,7 +11,7 @@
   License: BSD-2-Clause
   Discussion: https://gist.github.com/andrewtoth/df97c3260cc8d12f09d3855ee61322ea
               https://groups.google.com/g/bitcoindev/c/MezoKV5md7s
-  Version: 0.2.0
+  Version: 0.3.0
 </pre>
 
 == Introduction ==
@@ -79,7 +79,7 @@ The algorithm ''GenerateProof(a, B, r, G, m)'' is defined as:
 * Fail if ''k = 0''.
 * Let ''R<sub>1</sub> = k⋅G''.
 * Let ''R<sub>2</sub> = k⋅B''.
-* Let ''e = int(hash<sub>BIP0374/challenge</sub>(cbytes(A) || cbytes(B) || cbytes(C) || cbytes(G) || cbytes(R<sub>1</sub>) || cbytes(R<sub>2</sub>) || m'))''.
+* Let ''e = int(hash<sub>BIP0374/challenge</sub>(cbytes(A) || cbytes(B) || cbytes(C) || cbytes(G) || cbytes(R<sub>1</sub>) || cbytes(R<sub>2</sub>) || m')) mod n''.
 * Let ''s = (k + e⋅a) mod n''.
 * Let ''proof = bytes(32, e) || bytes(32, s)''.
 * If ''VerifyProof(A, B, C, proof, G, m)'' (see below) returns failure, abort.
@@ -99,14 +99,14 @@ Input:
 
 The algorithm ''VerifyProof(A, B, C, proof, G, m)'' is defined as:
 * Fail if any of ''is_infinite(A)'', ''is_infinite(B)'', ''is_infinite(C)'', ''is_infinite(G)''
-* Let ''e = int(proof[0:32])''.
+* Let ''e = int(proof[0:32])''; fail if ''e &ge; n''.
 * Let ''s = int(proof[32:64])''; fail if ''s &ge; n''.
 * Let ''R<sub>1</sub> = s⋅G - e⋅A''.
 * Fail if ''is_infinite(R<sub>1</sub>)''.
 * Let ''R<sub>2</sub> = s⋅B - e⋅C''.
 * Fail if ''is_infinite(R<sub>2</sub>)''.
 * Let ''m' = m if m is provided, otherwise an empty byte array''.
-* Fail if ''e ≠ int(hash<sub>BIP0374/challenge</sub>(cbytes(A) || cbytes(B) || cbytes(C) || cbytes(G) || cbytes(R<sub>1</sub>) || cbytes(R<sub>2</sub>) || m'))''.
+* Fail if ''e ≠ int(hash<sub>BIP0374/challenge</sub>(cbytes(A) || cbytes(B) || cbytes(C) || cbytes(G) || cbytes(R<sub>1</sub>) || cbytes(R<sub>2</sub>) || m')) mod n''.
 * Return success iff no failure occurred before reaching this point.
 
 ==Backwards Compatibility==
@@ -124,6 +124,8 @@ Test vectors can be generated by running <code>./bip-0374/gen_test_vectors.py</c
 
 == Changelog ==
 
+* 0.3.0 (2026-08-19):
+** Treat the challenge ''e'' as a scalar (reduced modulo the curve order) and reject proofs with ''e &ge; n''
 * 0.2.0 (2025-02-27):
 ** Add the message to the rand computation
 * 0.1.0 (2024-12-26):

bip-0374/reference.py

@@ -36,7 +36,7 @@ def dleq_challenge(
             + m,
         ),
         "big",
-    )
+    ) % GE.ORDER
 
 
 def dleq_generate_proof(
@@ -76,6 +76,8 @@ def dleq_verify_proof(
         return False
     assert len(proof) == 64
     e = int.from_bytes(proof[:32], "big")
+    if e >= GE.ORDER:
+        return False
     s = int.from_bytes(proof[32:], "big")
     if s >= GE.ORDER:
         return False