Add rivest cipher 4 (rc4) implementation

Add an implementation of RC4. This will be used by Rockchip booting but may
be useful in other situations.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/rc4.c

@@ -0,0 +1,49 @@
+/*
+ * (C) Copyright 2015 Google, Inc
+ *
+ * (C) Copyright 2008-2014 Rockchip Electronics
+ *
+ * Rivest Cipher 4 (RC4) implementation
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef USE_HOSTCC
+#include <common.h>
+#endif
+#include <rc4.h>
+
+void rc4_encode(unsigned char *buf, unsigned int len, unsigned char key[16])
+{
+	unsigned char s[256], k[256], temp;
+	unsigned short i, j, t;
+	int ptr;
+
+	j = 0;
+	for (i = 0; i < 256; i++) {
+		s[i] = (unsigned char)i;
+		j &= 0x0f;
+		k[i] = key[j];
+		j++;
+	}
+
+	j = 0;
+	for (i = 0; i < 256; i++) {
+		j = (j + s[i] + k[i]) % 256;
+		temp = s[i];
+		s[i] = s[j];
+		s[j] = temp;
+	}
+
+	i = 0;
+	j = 0;
+	for (ptr = 0; ptr < len; ptr++) {
+		i = (i + 1) % 256;
+		j = (j + s[i]) % 256;
+		temp = s[i];
+		s[i] = s[j];
+		s[j] = temp;
+		t = (s[i] + (s[j] % 256)) % 256;
+		buf[ptr] = buf[ptr] ^ s[t];
+	}
+}