Merge pull request #73 from jo7ueb/issue72

Fix vulnerability of to_key_shares() in bsv/keys.py

Author: ALiberalVoluntarist

bsv/keys.py

@@ -1,5 +1,6 @@
 import hashlib
 import hmac
+import os
 from base64 import b64encode, b64decode
 from typing import Optional, Union, Callable, Tuple
 
@@ -11,7 +12,7 @@
 from .constants import Network, NETWORK_ADDRESS_PREFIX_DICT, NETWORK_WIF_PREFIX_DICT, PUBLIC_KEY_COMPRESSED_PREFIX_LIST
 from .curve import Point
 from .curve import curve, curve_multiply as curve_multiply, curve_add as curve_add
-from .hash import hash160, hash256, hmac_sha256
+from .hash import hash160, hash256, hmac_sha256, hmac_sha512
 from .utils import decode_wif, text_digest, stringify_ecdsa_recoverable, unstringify_ecdsa_recoverable
 from .utils import deserialize_ecdsa_recoverable, serialize_ecdsa_der
 from .polynomial import Polynomial, PointInFiniteField, KeyShares
@@ -403,13 +404,39 @@ def to_key_shares(self, threshold: int, total_shares: int) -> 'KeyShares':
 
         # Generate shares
         points = []
+        used_x_coordinates = set()
+
+        # Cryptographically secure x-coordinate generation for Shamir's Secret Sharing (toKeyShares)
+        #
+        # - Each x-coordinate is derived using a master seed (Random(64)) as the HMAC key and a per-attempt counter array as the message.
+        # - The counter array includes the share index, the attempt number (to handle rare collisions), and 32 bytes of fresh randomness for each attempt.
+        # - This ensures:
+        #   1. **Non-determinism**: Each split is unique, even for the same key and parameters, due to the per-attempt randomness.
+        #   2. **Uniqueness**: x-coordinates are checked for zero and duplication; retry logic ensures no repeats or invalid values.
+        #   3. **Cryptographic strength**: HMAC-SHA-512 is robust, and combining deterministic and random values protects against RNG compromise or bias.
+        #   4. **Defensive programming**: Attempts are capped (5 per share) to prevent infinite loops in pathological cases.
+        #
+        # This approach is robust against all practical attacks and is suitable for high-security environments where deterministic splits are not desired.
+
+        seed = os.urandom(64)
+
         for i in range(total_shares):
-            # Generate random x coordinate using a new private key
-            # Using private_key.key.to_int() based on the structure in keys.py
-            random_private_key = PrivateKey()
-            x = random_private_key.int()
+            x = None
+            attempts = 0
+
+            # TypeScript版と同様の安全なx座標生成
+            while x is None or x == 0 or x in used_x_coordinates:
+                counter = [i, attempts] + list(os.urandom(32))
+                counter_bytes = bytes(counter)
+
+                h = hmac_sha512(seed, counter_bytes)
+                x = int.from_bytes(h, 'big') % curve.p
+
+                attempts += 1
+                if attempts > 5:
+                    raise ValueError('Failed to generate unique x coordinate after 5 attempts')
 
-            # Evaluate polynomial at x to get y coordinate
+            used_x_coordinates.add(x)
             y = poly.value_at(x)
 
             # Create a point and add to points' list