# HG changeset patch
# User Laman
# Date 2017-09-17 19:03:50
# Node ID 86a5417085efabb0391c8205f0d1f1fa725e6396

Galois Field 256 arithmetic

diff --git a/src/gf256.py b/src/gf256.py
new file mode 100644
--- /dev/null
+++ b/src/gf256.py
@@ -0,0 +1,28 @@
+def _ffmul(a, b):
+	r=0
+	while a!=0:
+		if (a&1)!=0: r^=b
+		t=b&0x80
+		b=(b<<1)&255
+		if t!=0: b^=0x1b
+		a>>=1
+	return r
+
+
+g=3
+E=[None]*256
+L=[None]*256
+acc=1
+for i in range(256):
+	E[i]=acc
+	L[acc]=i
+	acc=_ffmul(acc, g)
+L[1]=0
+inv=[E[255-L[i]] if i!=0 else None for i in range(256)]
+
+
+def ffmul(a, b):
+	if a==0 or b==0: return 0
+	t=L[a]+L[b]
+	if t>255: t-=255
+	return E[t]