Fixed RC4 to preserve state across calls

arcfour.c

@@ -26,17 +26,19 @@ void arcfour_key_setup(BYTE state[], const BYTE key[], int len)
 		state[i] = state[j];
 		state[j] = t;
 	}
+	state[ARCFOUR_I] = 0;
+	state[ARCFOUR_J] = 0;
 }
 
-// This does not hold state between calls. It always generates the
-// stream starting from the first  output byte.
 void arcfour_generate_stream(BYTE state[], BYTE out[], size_t len)
 {
-	int i = 0;
-	int j = 0;
+	int i;
+	int j;
 	size_t idx;
 	BYTE t;
 
+	i = state[ARCFOUR_I];
+	j = state[ARCFOUR_J];
 	for (idx = 0; idx < len; ++idx)  {
 		i = (i + 1) % 256;
 		j = (j + state[i]) % 256;
@@ -45,4 +47,6 @@ void arcfour_generate_stream(BYTE state[], BYTE out[], size_t len)
 		state[j] = t;
 		out[idx] = state[(state[i] + state[j]) % 256];
 	}
+	state[ARCFOUR_I] = i;
+	state[ARCFOUR_J] = j;
 }

arcfour.h

@@ -31,4 +31,11 @@ void arcfour_key_setup(BYTE state[], const BYTE key[], int len);
 //        len - number of bytes to generate
 void arcfour_generate_stream(BYTE state[], BYTE out[], size_t len);
 
+#if !defined(ARCFOUR_I)
+# define ARCFOUR_I (256)
+#endif
+#if !defined(ARCFOUR_J)
+# define ARCFOUR_J (257)
+#endif
+
 #endif   // ARCFOUR_H

arcfour_test.c

@@ -19,7 +19,7 @@
 /*********************** FUNCTION DEFINITIONS ***********************/
 int rc4_test()
 {
-	BYTE state[256];
+	BYTE state[256+2];
 	BYTE key[3][10] = {{"Key"}, {"Wiki"}, {"Secret"}};
 	BYTE stream[3][10] = {{0xEB,0x9F,0x77,0x81,0xB7,0x34,0xCA,0x72,0xA7,0x19},
 	                      {0x60,0x44,0xdb,0x6d,0x41,0xb7},