fix a bug

shadowsocks-csharp/RC4.cs

@@ -6,21 +6,23 @@ namespace shadowsocks_csharp
 {
     public class RC4
     {
+        class Context
+        {
+            public int index1 = 0;
+            public int index2 = 0;
+        }
 
-        int enc_index1 = 0;
-        int enc_index2 = 0;
-
-        int dec_index1 = 0;
-        int dec_index2 = 0;
+        private Context enc_ctx = new Context();
+        private Context dec_ctx = new Context();
 
         public void Encrypt(byte[] table, byte[] data, int length)
         {
-            EncryptOutput(enc_index1, enc_index2, table, data, length);
+            EncryptOutput(enc_ctx, table, data, length);
         }
 
         public void Decrypt(byte[] table, byte[] data, int length)
         {
-            EncryptOutput(dec_index1, dec_index2, table, data, length);
+            EncryptOutput(dec_ctx, table, data, length);
         }
 
         public byte[] EncryptInitalize(byte[] key)
@@ -42,18 +44,18 @@ namespace shadowsocks_csharp
             return s;
         }
 
-        private void EncryptOutput(int index1, int index2, byte[] s, byte[] data, int length)
+        private void EncryptOutput(Context ctx, byte[] s, byte[] data, int length)
         {
             for (int n = 0; n < length; n++)
             {
                 byte b = data[n];
 
-                index1 = (index1 + 1) & 255;
-                index2 = (index2 + s[index1]) & 255;
+                ctx.index1 = (ctx.index1 + 1) & 255;
+                ctx.index2 = (ctx.index2 + s[ctx.index1]) & 255;
 
-                Swap(s, index1, index2);
+                Swap(s, ctx.index1, ctx.index2);
 
-                data[n] = (byte)(b ^ s[(s[index1] + s[index2]) & 255]);
+                data[n] = (byte)(b ^ s[(s[ctx.index1] + s[ctx.index2]) & 255]);
             }
         }