An implementation of the Rabbit Stream Cipher.

Rabbit.java

package org.csdgn.crypt;

import java.util.Arrays;

/**
 * Tested against the actual RFC.
 * This implementation is optimized for code length.
 * Please do not use this version in production, use the much faster unrolled version here: https://gist.github.com/2814851
 * @author Robert Maupin
 * @see {@link http://tools.ietf.org/rfc/rfc4503.txt}
 */
public class Rabbit {
  static int rtl(int v, int k) {
		return (k &= 31) == 0 ? v : (v << k) | (v >>> (32 - k));
	}

	private static final int[] A = new int[8], X = new int[8], C = new int[8];
	private byte b;
	static {
		A[0] = A[3] = A[6] = 0x4D34D34D;
		A[1] = A[4] = A[7] = 0xD34D34D3;
		A[2] = A[5] = 0x34D34D34;
	}

	private void s() {
		long t;
		int i;
		for(i = 0; i < 8; ++i) {
			t = (C[i] & 0xFFFFFFFFL) + (A[i] & 0xFFFFFFFFL) + b;
			b = (byte) (t >>> 32);
			C[i] = (int) (t & 0xFFFFFFFF);
		}
		// g function, small as I can get, 32 bits only would be nice though.
		int G[] = new int[8];
		for(i = 0; i < 8; ++i) {
			t = (X[i] + C[i]) & 0xFFFFFFFFL;
			G[i] = (int) ((t *= t) ^ (t >>> 32));
		}
		for(i = 0; i < 8; ++i) {
			X[i] = G[i] + rtl(G[(i + 7) % 8], (i + 1 & 1) + 1 << 3) + ((i & 1) == 0 ? rtl(G[(i + 6) % 8], 16) : G[(i + 6) % 8]);
		}
	}
	
	/**
	 * Should be fed an array with a length that is a multiple of 16 for proper key sequencing.
	 */
	public void crypt(byte[] message) {
		int index = 0;
		while (index < message.length) {
			byte[] keystream = ks();

			for (int i = 0; i < 16 && index < message.length; ++i) {
				message[index++] ^= keystream[i];
			}
		}
	}

	private byte[] ks() {
		byte[] s = new byte[16];
		s();
		for(int i = 0; i < 16; ++i)
			s[i] = (byte) ((X[(3 - (i >> 2) << 1)] ^ (X[(3 + (i >> 2) * 6) % 8] >>> 16) ^ (X[(1 + (i >> 2) * 6) % 8] << 16)) >> (24 - ((i << 3) % 32)));
		return s;
	}

	public void reset() {
		b = 0;
		Arrays.fill(X, 0);
		Arrays.fill(C, 0);
	}

	/**
	 * @param K array of 8 short values
	 */
	public void key(short[] K) {
		for(int i = 0; i < 8; ++i) {
			//look ma' no ternary!
			X[i] = (K[1 + (i * 5) % 8 & 7] << 16) | (K[(i * 5) % 8 & 7] & 0xFFFF);
			C[i] = (K[4 + (i * 5) % 8 & 7] << 16) | (K[5 + (i * 5) % 8 & 7] & 0xFFFF);
		}
		s();
		s();
		s();
		s();
		for(int i = 0; i < 8; ++i)
			C[i] ^= X[(i + 4) % 8];
	}
}