little helper for generating multiplication tables for multiplication in GF(2**8)

[avr-crypto-lib.git] / host / gf256mul.c

diff --git a/host/gf256mul.c b/host/gf256mul.c
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+/* gf256mul.c */
+/*
+    This file is part of the Crypto-avr-lib/microcrypt-lib.
+    Copyright (C) 2008  Daniel Otte (daniel.otte@rub.de)
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+/**
+ * \file     gf256mul.c
+ * \email    daniel.otte@rub.de
+ * \author   Daniel Otte 
+ * \date     2009-01-13
+ * \license  GPLv3 or later
+ * 
+ */
+#include <stdint.h>
+#include "gf256mul.h"
+
+uint8_t gf256mul(uint8_t a, uint8_t b, uint8_t reducer){
+       uint8_t t,ret=0;
+       while(a){
+               if(a&1)
+                       ret ^= b;
+               t=a&0x80;
+               b<<=1;
+               if(t)
+                       b^=reducer;
+               a>>=1;
+       }
+       return ret;
+}