secp256k1

commit 52214de15457f96370063bf3791797c384d64f2a
parent 7ec20de4316921667116e4b14403e85f1a22dc81
Author: Jared Tobin <jared@jtobin.io>
Date:   Sat,  1 Aug 2026 12:33:22 -0230

lib: take the point prefix byte totally

parse_point read byte zero with BU.unsafeIndex, relying on laziness to
avoid the read on inputs shorter than 33 bytes. That holds only
because readWord8OffAddr# is can_fail and so not ok-for-speculation,
which is a property of GHC's primop table rather than anything the
code states. BS.uncons is O(1) and total.

Diffstat:
M
lib/Crypto/Curve/Secp256k1.hs
 | 
11
++++++-----

1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/lib/Crypto/Curve/Secp256k1.hs b/lib/Crypto/Curve/Secp256k1.hs
@@ -896,13 +896,14 @@ parse_int256 bs = do
 parse_point :: BS.ByteString -> Maybe Projective
 parse_point bs
     | len == 32 = _parse_bip0340 bs
-    | len == 33 = _parse_compressed h t
-    | len == 65 = _parse_uncompressed h t
-    | otherwise = Nothing
+    | otherwise = case BS.uncons bs of
+        Nothing -> Nothing
+        Just (h, t)
+          | len == 33 -> _parse_compressed h t
+          | len == 65 -> _parse_uncompressed h t
+          | otherwise -> Nothing
   where
     len = BS.length bs
-    h = BU.unsafeIndex bs 0 -- lazy
-    t = BS.drop 1 bs
 
 -- input is guaranteed to be 32B in length
 _parse_bip0340 :: BS.ByteString -> Maybe Projective