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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 1.4.

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package org.bouncycastle.crypto.util;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.security.SecureRandom;

import org.bouncycastle.util.Arrays;

/**
 * A SecureRandom that maintains a journal of its output.
 */
public class JournalingSecureRandom
    extends SecureRandom
{
    private static byte[] EMPTY_TRANSCRIPT = new byte[0];

    private final SecureRandom base;
    private final byte[] transcript;

    private TranscriptStream tOut = new TranscriptStream();
    private int index = 0;

    /**
     * Base constructor - no prior transcript.
     *
     * @param random source of randomness we will be journaling.
     */
    public JournalingSecureRandom(SecureRandom random)
    {
        this.base = random;
        this.transcript = EMPTY_TRANSCRIPT;
    }

    /**
     * Constructor with a prior transcript. Both the transcript used and
     * any new randomness are journaled.
     *
     * @param transcript initial transcript of randomness.
     * @param random source of randomness we will be journaling when the transcript runs out.
     */
    public JournalingSecureRandom(byte[] transcript, SecureRandom random)
    {
         this.base = random;
         this.transcript = Arrays.clone(transcript);
    }

    /**
     * Fill bytes with random data, journaling the random data before returning.
     *
     * @param bytes a block of bytes to be filled with random data.
     */
    public final void nextBytes(byte[] bytes)
    {
        if (index >= transcript.length)
        {
            base.nextBytes(bytes);
        }
        else
        {
            int i = 0;

            while (i != bytes.length)
            {
                if (index < transcript.length)
                {
                    bytes[i] = transcript[index++];
                }
                else
                {
                    break;
                }
                i++;
            }

            if (i != bytes.length)
            {
                byte[] extra = new byte[bytes.length - i];

                base.nextBytes(extra);

                System.arraycopy(extra, 0, bytes, i, extra.length);
            }
        }

        try
        {
            tOut.write(bytes);
        }
        catch (IOException e)
        {
            throw new IllegalStateException("unable to record transcript: " + e.getMessage());
        }
    }

    /**
     * Clear the internals
     */
    public void clear()
    {
        Arrays.fill(transcript, (byte)0);
        tOut.clear();
    }

    /**
     * Return the transcript so far,
     *
     * @return a copy of the randomness produced so far.
     */
    public byte[] getTranscript()
    {
        return tOut.toByteArray();
    }

    private class TranscriptStream
        extends ByteArrayOutputStream
    {
        public void clear()
        {
            Arrays.fill(buf, (byte)0);
        }
    }
}