# Re: Simple encryption/decryption

From:
Mark Space <markspace@sbc.global.net>
Newsgroups:
comp.lang.java.programmer
Date:
Mon, 23 Mar 2009 11:13:22 -0700
Message-ID:
<cjQxl.24540\$yr3.3043@nlpi068.nbdc.sbc.com>
angelochen960@gmail.com wrote:

I'm looking for a simple solution: a way to encrypt a string into a
alphanumeric string, and can be decrypted back to its original form,
using a string as the key, say 'password123', any idea how to achieve
this? Thanks.

I'm not seeing a good answer to this. In particular, the
SecretKeyFactory.getInstance( "ARCFOUR" ) method returns an error. I
don't see a simple way to use RC4 without this call succeeding. Anyone
got a solution?

Anyway, to the OP, check out the code below. It will at least get you
around on the hard drive. You can compare this code to the description
of RC4 at Wikipedia. It's the exact same algorithm. Note the test vectors.

<http://en.wikipedia.org/wiki/RC4>

package local;

/**

This code is donated to the public domain but is unsupported and has no
warranty whatsoever. Use at your own risk.

*
* @author Brenden
*/
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() ) ) );
}
}

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();
// return local.utils.ArrayUtils.toFormattedString( "%02X",
//bytes, null );
}
}

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