Add RC4 encryption/decryption function

Originally committed as revision 10723 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/Makefile

@@ -14,6 +14,7 @@ OBJS = adler32.o \
        mem.o \
        random.o \
        rational.o \
+       rc4.o \
        string.o \
        tree.o \
 

libavutil/rc4.c

+/*
+ * RC4 encryption/decryption/pseudo-random number generator
+ * Copyright (c) 2007 Reimar Doeffinger
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * loosely based on LibTomCrypt by Tom St Denis
+ */
+#include "common.h"
+#include "rc4.h"
+
+void ff_rc4_enc(const uint8_t *key, int keylen, uint8_t *data, int datalen) {
+    int i, j;
+    uint8_t x, y;
+    uint8_t state[256];
+    for (i = 0; i < 256; i++)
+        state[i] = i;
+    y = 0;
+    // j is i % keylen
+    for (j = 0, i = 0; i < 256; i++, j++) {
+        if (j == keylen) j = 0;
+        y += state[i] + key[j];
+        FFSWAP(uint8_t, state[i], state[y]);
+    }
+    // state initialized, now do the real encryption
+    x = 1; y = state[1];
+    while (datalen-- > 0) {
+        uint8_t sum = state[x] + state[y];
+        FFSWAP(uint8_t, state[x], state[y]);
+        *data++ ^= state[sum];
+        x++;
+        y += state[x];
+    }
+}

libavutil/rc4.h

+void ff_rc4_enc(const uint8_t *key, int keylen, uint8_t *data, int datalen);