package com.trilead.ssh2.crypto.cipher;

/**
 * This is CTR mode as described in draft-ietf-secsh-newmodes-XY.txt
 *
 * @author Christian Plattner, plattner@trilead.com
 * @version $Id: CTRMode.java,v 1.1 2007/10/15 12:49:55 cplattne Exp $
 */
public class CTRMode implements BlockCipher {
    byte[] X;
    byte[] Xenc;

    BlockCipher bc;
    int blockSize;
    boolean doEncrypt;

    int count = 0;

    public void init(boolean forEncryption, byte[] key) {
    }

    public CTRMode(BlockCipher tc, byte[] iv, boolean doEnc) throws IllegalArgumentException {
        bc = tc;
        blockSize = bc.getBlockSize();
        doEncrypt = doEnc;

        if (blockSize != iv.length)
            throw new IllegalArgumentException("IV must be " + blockSize + " bytes long! (currently " + iv.length + ")");

        X = new byte[blockSize];
        Xenc = new byte[blockSize];
        System.arraycopy(iv, 0, X, 0, blockSize);
    }

    public final int getBlockSize() {
        return blockSize;
    }

    public final void transformBlock(byte[] src, int srcoff, byte[] dst, int dstoff) {
        bc.transformBlock(X, 0, Xenc, 0);

        for (int i = 0; i < blockSize; i++) {
            dst[dstoff + i] = (byte)(src[srcoff + i] ^ Xenc[i]);
        }

        for (int i = (blockSize - 1); i >= 0; i--) {
            X[i]++;

            if (X[i] != 0)
                break;
        }
    }
}