From dfc9fb9fb254e4cdf517585c406bf4b01704ccaa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Olivier=20Ch=C3=A9ron?= <olivier.cheron@gmail.com>
Date: Fri, 12 Jun 2020 19:01:52 +0200
Subject: [PATCH] Fix powF2m when exponent is not a power of 2

Integer multiplication cannot be used because it includes carry
propagation.  This needs to use carry-less mulF2m instead.
---
 Crypto/Number/F2m.hs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/Crypto/Number/F2m.hs b/Crypto/Number/F2m.hs
index 4dd0db2..6ca2604 100644
--- a/Crypto/Number/F2m.hs
+++ b/Crypto/Number/F2m.hs
@@ -111,10 +111,10 @@ powF2m :: BinaryPolynomial -- ^Modulus
        -> Integer          -- ^b
        -> Integer
 powF2m fx a b
-  | b == 0 = 1
-  | b > 0  = squareF2m fx x * if even b then 1 else a
-  | b < 0  = error "powF2m: negative exponents disallowed"
-  | otherwise = error "powF2m: impossible"
+  | b < 0     = error "powF2m: negative exponents disallowed"
+  | b == 0    = if fx > 1 then 1 else 0
+  | even b    = squareF2m fx x
+  | otherwise = mulF2m fx a (squareF2m' x)
   where x = powF2m fx a (b `div` 2)
 
 -- | Square rooot in F₂m.