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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.prng;

import org.bouncycastle.crypto.Digest;

/**
 * Random generation based on the digest with counter. Calling addSeedMaterial will
 * always increase the entropy of the hash.
 * 
 * Internal access to the digest is synchronized so a single one of these can be shared.
 * 
 */
public class DigestRandomGenerator
    implements RandomGenerator
{
    private static long         CYCLE_COUNT = 10;

    private long                stateCounter;
    private long                seedCounter;
    private Digest              digest;
    private byte[]              state;
    private byte[]              seed;

    // public constructors
    public DigestRandomGenerator(
        Digest digest)
    {
        this.digest = digest;

        this.seed = new byte[digest.getDigestSize()];
        this.seedCounter = 1;

        this.state = new byte[digest.getDigestSize()];
        this.stateCounter = 1;
    }

    public void addSeedMaterial(byte[] inSeed)
    {
        synchronized (this)
        {
            digestUpdate(inSeed);
            digestUpdate(seed);
            digestDoFinal(seed);
        }
    }

    public void addSeedMaterial(long rSeed)
    {
        synchronized (this)
        {
            digestAddCounter(rSeed);
            digestUpdate(seed);

            digestDoFinal(seed);
        }
    }

    public void nextBytes(byte[] bytes)
    {
        nextBytes(bytes, 0, bytes.length);
    }

    public void nextBytes(byte[] bytes, int start, int len)
    {
        synchronized (this)
        {
            int stateOff = 0;

            generateState();

            int end = start + len;
            for (int i = start; i != end; i++)
            {
                if (stateOff == state.length)
                {
                    generateState();
                    stateOff = 0;
                }
                bytes[i] = state[stateOff++];
            }
        }
    }

    private void cycleSeed()
    {
        digestUpdate(seed);
        digestAddCounter(seedCounter++);

        digestDoFinal(seed);
    }

    private void generateState()
    {
        digestAddCounter(stateCounter++);
        digestUpdate(state);
        digestUpdate(seed);

        digestDoFinal(state);

        if ((stateCounter % CYCLE_COUNT) == 0)
        {
            cycleSeed();
        }
    }

    private void digestAddCounter(long seed)
    {
        for (int i = 0; i != 8; i++)
        {
            digest.update((byte)seed);
            seed >>>= 8;
        }
    }

    private void digestUpdate(byte[] inSeed)
    {
        digest.update(inSeed, 0, inSeed.length);
    }

    private void digestDoFinal(byte[] result)
    {
        digest.doFinal(result, 0);
    }
}