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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 and up. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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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);
    }
}