Re: Simple encryption/decryption

Angelo Chen wrote:


That's not what I get. Check your tests with Wikipedia. The
cipher-text for Key and Plaintext is "BBF316E8D940AF0AD3", which is what
the program I posted prints.

But for how to decrypt, RC4 is symmetric. That means you decrypt with
the same key and algorithm you encrypt with.

package local;

public class Rc4
{
     private byte[] state = new byte[256];
     int x, y;

     public Rc4( byte[] key )
     {
// if( key.length<5||key.length>16 )
// {
// throw new IllegalArgumentException( "Key: "+key+
// " must be between 40 and 128 bits. (key length is //"+
// key.length+".)" );
// }
         for( int i = 0; i < state.length; i++ ) {
             state[i] = (byte) i;
         }
         for( int i = 0; i < state.length; i++ ) {
             x = (x + key[i % key.length] + state[i]) & 0xFF;
             byte swap = state[i];
             state[i] = state[x];
             state[x] = swap;
         }
         x = 0;
     }

     public byte[] crypt( byte[] input )
     {
         byte[] output = new byte[input.length];
         for( int i = 0; i < input.length; i++ ) {
             x = (x + 1) % 256;
             y = (state[x] + y) & 0xFF;
             byte swap = state[x];
             state[x] = state[y];
             state[y] = swap;
             byte r = state[(state[x] + state[y]) & 0xFF];
             output[i] = (byte) (input[i] ^ r);
         }
         return output;
     }

     public static void main( final String... args )
     {
         String[][] testVectors = {{"Key", "Plaintext"},
             {"Wiki", "pedia"}, {"Secret", "Attack at dawn"},
         };
         for( String[] s : testVectors ) {
             System.out.printf( "RC4( \"%s\", \"%s\" ) => ",
                     s[0], s[1] );
             System.out.println( hexString( new Rc4( s[0].getBytes() )
                     .crypt( s[1].getBytes() ) ) );
         }
         String [][] reverseVectors = {
             {"Key", "BBF316E8D940AF0AD3" },
             {"Wiki", "1021BF0420" },
             {"Secret", "45A01F645FC35B383552544B9BF5" }
         };
         for( String[] s : reverseVectors ) {
             System.out.printf( "RC4( \"%s\", \"%s\" ) => ",
                     s[0], s[1] );
             byte[] bytes = new byte[s[1].length()/2];
             for( int i = 0; i < bytes.length; i++ ) {
                 bytes[i] = (byte)Integer.parseInt( s[1]
                         .substring( i*2, i*2+2), 16 );
             }
             System.out.println( new String( new Rc4( s[0].getBytes() )
                     .crypt( bytes ) ) );
         }
     }

     private static String hexString( byte[] bytes )
     {
         StringBuilder sb = new StringBuilder( bytes.length *2 );
         for( int i = 0; i < bytes.length; i++ ) {
             sb.append( String.format( "%02X", bytes[i] ));
         }
         return sb.toString();
     }
}