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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.macs;

import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.InvalidCipherTextException;
import org.bouncycastle.crypto.Mac;
import org.bouncycastle.crypto.modes.GCMBlockCipher;
import org.bouncycastle.crypto.params.AEADParameters;
import org.bouncycastle.crypto.params.KeyParameter;
import org.bouncycastle.crypto.params.ParametersWithIV;

/**
 * The GMAC specialisation of Galois/Counter mode (GCM) detailed in NIST Special Publication
 * 800-38D.
 * 
 * GMac is an invocation of the GCM mode where no data is encrypted (i.e. all input data to the Mac
 * is processed as additional authenticated data with the underlying GCM block cipher).
 */
public class GMac implements Mac
{
    private final GCMBlockCipher cipher;
    private final int macSizeBits;

    /**
     * Creates a GMAC based on the operation of a block cipher in GCM mode.
     * 
     * This will produce an authentication code the length of the block size of the cipher.
     * 
     * @param cipher
     *            the cipher to be used in GCM mode to generate the MAC.
     */
    public GMac(final GCMBlockCipher cipher)
    {
        // use of this confused flow analyser in some earlier JDKs
        this.cipher = cipher;
        this.macSizeBits = 128;
    }

    /**
     * Creates a GMAC based on the operation of a 128 bit block cipher in GCM mode.
     * 
     * @param macSizeBits
     *            the mac size to generate, in bits. Must be a multiple of 8 and >= 32 and <= 128.
     *            Sizes less than 96 are not recommended, but are supported for specialized applications.
     * @param cipher
     *            the cipher to be used in GCM mode to generate the MAC.
     */
    public GMac(final GCMBlockCipher cipher, final int macSizeBits)
    {
        this.cipher = cipher;
        this.macSizeBits = macSizeBits;
    }

    /**
     * Initialises the GMAC - requires a {@link ParametersWithIV} providing a {@link KeyParameter}
     * and a nonce.
     */
    public void init(final CipherParameters params) throws IllegalArgumentException
    {
        if (params instanceof ParametersWithIV)
        {
            final ParametersWithIV param = (ParametersWithIV)params;

            final byte[] iv = param.getIV();
            final KeyParameter keyParam = (KeyParameter)param.getParameters();

            // GCM is always operated in encrypt mode to calculate MAC
            cipher.init(true, new AEADParameters(keyParam, macSizeBits, iv));
        }
        else
        {
            throw new IllegalArgumentException("GMAC requires ParametersWithIV");
        }
    }

    public String getAlgorithmName()
    {
        return cipher.getUnderlyingCipher().getAlgorithmName() + "-GMAC";
    }

    public int getMacSize()
    {
        return macSizeBits / 8;
    }

    public void update(byte in) throws IllegalStateException
    {
        cipher.processAADByte(in);
    }

    public void update(byte[] in, int inOff, int len)
        throws DataLengthException, IllegalStateException
    {
        cipher.processAADBytes(in, inOff, len);
    }

    public int doFinal(byte[] out, int outOff)
        throws DataLengthException, IllegalStateException
    {
        try
        {
            return cipher.doFinal(out, outOff);
        }
        catch (InvalidCipherTextException e)
        {
            // Impossible in encrypt mode
            throw new IllegalStateException(e.toString());
        }
    }

    public void reset()
    {
        cipher.reset();
    }
}