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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 and up.

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package org.bouncycastle.crypto.commitments;

import java.security.SecureRandom;

import org.bouncycastle.crypto.Commitment;
import org.bouncycastle.crypto.Committer;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.ExtendedDigest;
import org.bouncycastle.util.Arrays;

/**
 * A basic hash-committer based on the one described in "Making Mix Nets Robust for Electronic Voting by Randomized Partial Checking",
 * by Jakobsson, Juels, and Rivest (11th Usenix Security Symposium, 2002).
 * 

* The algorithm used by this class differs from the one given in that it includes the length of the message in the hash calculation. *

*/ public class GeneralHashCommitter implements Committer { private final Digest digest; private final int byteLength; private final SecureRandom random; /** * Base Constructor. The maximum message length that can be committed to is half the length of the internal * block size for the digest (ExtendedDigest.getBlockLength()). * * @param digest digest to use for creating commitments. * @param random source of randomness for generating secrets. */ public GeneralHashCommitter(ExtendedDigest digest, SecureRandom random) { this.digest = digest; this.byteLength = digest.getByteLength(); this.random = random; } /** * Generate a commitment for the passed in message. * * @param message the message to be committed to, * @return a Commitment */ public Commitment commit(byte[] message) { if (message.length > byteLength / 2) { throw new DataLengthException("Message to be committed to too large for digest."); } byte[] w = new byte[byteLength - message.length]; random.nextBytes(w); return new Commitment(w, calculateCommitment(w, message)); } /** * Return true if the passed in commitment represents a commitment to the passed in message. * * @param commitment a commitment previously generated. * @param message the message that was expected to have been committed to. * @return true if commitment matches message, false otherwise. */ public boolean isRevealed(Commitment commitment, byte[] message) { if (message.length + commitment.getSecret().length != byteLength) { throw new DataLengthException("Message and witness secret lengths do not match."); } byte[] calcCommitment = calculateCommitment(commitment.getSecret(), message); return Arrays.constantTimeAreEqual(commitment.getCommitment(), calcCommitment); } private byte[] calculateCommitment(byte[] w, byte[] message) { byte[] commitment = new byte[digest.getDigestSize()]; digest.update(w, 0, w.length); digest.update(message, 0, message.length); digest.update((byte)((message.length >>> 8))); digest.update((byte)(message.length)); digest.doFinal(commitment, 0); return commitment; } }




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