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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.macs;

import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.Mac;
import org.bouncycastle.crypto.params.KeyParameter;

/**
 * HMAC implementation based on RFC2104
 *
 * H(K XOR opad, H(K XOR ipad, text))
 */
public class OldHMac
implements Mac
{
    private final static int BLOCK_LENGTH = 64;

    private final static byte IPAD = (byte)0x36;
    private final static byte OPAD = (byte)0x5C;

    private Digest digest;
    private int digestSize;
    private byte[] inputPad = new byte[BLOCK_LENGTH];
    private byte[] outputPad = new byte[BLOCK_LENGTH];

    /**
     * @deprecated uses incorrect pad for SHA-512 and SHA-384 use HMac.
     */
    public OldHMac(
        Digest digest)
    {
        this.digest = digest;
        digestSize = digest.getDigestSize();
    }

    public String getAlgorithmName()
    {
        return digest.getAlgorithmName() + "/HMAC";
    }

    public Digest getUnderlyingDigest()
    {
        return digest;
    }

    public void init(
        CipherParameters params)
    {
        digest.reset();

        byte[] key = ((KeyParameter)params).getKey();

        if (key.length > BLOCK_LENGTH)
        {
            digest.update(key, 0, key.length);
            digest.doFinal(inputPad, 0);
            for (int i = digestSize; i < inputPad.length; i++)
            {
                inputPad[i] = 0;
            }
        }
        else
        {
            System.arraycopy(key, 0, inputPad, 0, key.length);
            for (int i = key.length; i < inputPad.length; i++)
            {
                inputPad[i] = 0;
            }
        }

        outputPad = new byte[inputPad.length];
        System.arraycopy(inputPad, 0, outputPad, 0, inputPad.length);

        for (int i = 0; i < inputPad.length; i++)
        {
            inputPad[i] ^= IPAD;
        }

        for (int i = 0; i < outputPad.length; i++)
        {
            outputPad[i] ^= OPAD;
        }

        digest.update(inputPad, 0, inputPad.length);
    }

    public int getMacSize()
    {
        return digestSize;
    }

    public void update(
        byte in)
    {
        digest.update(in);
    }

    public void update(
        byte[] in,
        int inOff,
        int len)
    {
        digest.update(in, inOff, len);
    }

    public int doFinal(
        byte[] out,
        int outOff)
    {
        byte[] tmp = new byte[digestSize];
        digest.doFinal(tmp, 0);

        digest.update(outputPad, 0, outputPad.length);
        digest.update(tmp, 0, tmp.length);

        int     len = digest.doFinal(out, outOff);

        reset();

        return len;
    }

    /**
     * Reset the mac generator.
     */
    public void reset()
    {
        /*
         * reset the underlying digest.
         */
        digest.reset();

        /*
         * reinitialize the digest.
         */
        digest.update(inputPad, 0, inputPad.length);
    }
}