diff --git a/src/gf256.py b/src/gf256.py
--- a/src/gf256.py
+++ b/src/gf256.py
@@ -26,6 +26,7 @@ inv=[E[255-L[i]] if i!=0 else None for i
 
 def gfmul(a, b):
 	"""Fast multiplication. Basic multiplication is expensive. a*b==g**(log(a)+log(b))"""
+	assert 0<=a<=255, 0<=b<=255
 	if a==0 or b==0: return 0
 	t=L[a]+L[b]
 	if t>255: t-=255