I wonder what this is for.

libtransmission/rc4.c

@@ -0,0 +1,77 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <inttypes.h>
+#include "rc4.h"
+
+struct tr_rc4_s
+{
+    uint8_t x;      
+    uint8_t y;
+    uint8_t state[256];
+};
+
+static void
+swapByte( uint8_t* a, uint8_t* b )
+{
+    const uint8_t tmp = *a;
+    *a = *b;
+    *b = tmp;
+}
+
+tr_rc4_t *
+tr_rc4New ( const uint8_t * key,
+            int             keylen )
+{
+    uint8_t index1 = 0;
+    uint8_t index2 = 0;
+    uint8_t * state;
+    int i;
+    tr_rc4_t * ret = calloc( 1, sizeof(tr_rc4_t) );
+
+    /* initialize the state array */
+    state = ret->state;
+    for( i=0; i<256; ++i )
+        state[i] = (uint8_t)i;
+
+    ret->x = 0;
+    ret->y = 0;
+    for( i=0; i<256; ++i )
+    {
+        index2 = (key[index1] + state[i] + index2) % 256;
+        swapByte (state+i, state+index2);
+        index1 = (index1 + 1) % keylen;
+    }
+
+    return ret;
+}
+
+void
+tr_rc4Free( tr_rc4_t * rc4 )
+{
+    free( rc4 );
+}
+
+void
+tr_rc4Step( tr_rc4_t     * key,
+            uint8_t      * buf,
+            int            buflen )
+{
+    uint8_t x = key->x;
+    uint8_t y = key->y;
+    uint8_t * state = key->state;
+    uint8_t xorIndex;
+    int i;
+
+    for( i=0; i<buflen; ++i )
+    {
+        x = (x + 1) % 256;
+        y = (state[x] + y) % 256;
+        swapByte( state+x, state+y );
+        xorIndex = (state[x] + state[y]) % 256;
+        buf[i] ^= state[xorIndex];
+    }
+
+    key->x = x;
+    key->y = y;
+}

libtransmission/rc4.h

@@ -0,0 +1,15 @@
+#ifndef TR_RC4_H
+#define TR_RC4_H
+
+typedef struct tr_rc4_s tr_rc4_t;
+
+tr_rc4_t *  tr_rc4New     ( const uint8_t   * key,
+                            int               keylen );
+
+void        tr_rc4Step    ( tr_rc4_t        * rc4,
+                            uint8_t         * buf,
+                            int               buflen );
+
+void        tr_rc4Free    ( tr_rc4_t        * rc4 );
+
+#endif

mk/lib.mk

@@ -5,8 +5,8 @@ include ../mk/common.mk
 
 SRCS = bencode.c choking.c clients.c completion.c fastresume.c fdlimit.c \
        http.c inout.c ipcparse.c makemeta.c metainfo.c natpmp.c net.c \
-       peer.c platform.c ratecontrol.c sha1.c shared.c strlcat.c strlcpy.c \
-       torrent.c tracker.c transmission.c upnp.c utils.c xml.c 
+       peer.c platform.c ratecontrol.c rc4.c sha1.c shared.c strlcat.c \
+       strlcpy.c torrent.c tracker.c transmission.c upnp.c utils.c xml.c 
 
 OBJS = $(SRCS:%.c=%.o)