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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 to JDK 1.8. 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.cert.crmf;

import java.security.SecureRandom;

import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.digests.SHA1Digest;
import org.bouncycastle.crypto.generators.MGF1BytesGenerator;
import org.bouncycastle.crypto.params.MGFParameters;

/**
 * An encrypted value padder that uses MGF1 as the basis of the padding.
 */
public class FixedLengthMGF1Padder
    implements EncryptedValuePadder
{
    private int length;
    private SecureRandom random;
    private Digest dig = new SHA1Digest();

    /**
     * Create a padder to so that padded output will always be at least
     * length bytes long.
     *
     * @param length fixed length for padded output.
     */
    public FixedLengthMGF1Padder(int length)
    {
        this(length, null);
    }

    /**
     * Create a padder to so that padded output will always be at least
     * length bytes long, using the passed in source of randomness to
     * provide the random material for the padder.
     *
     * @param length fixed length for padded output.
     * @param random a source of randomness.
     */
    public FixedLengthMGF1Padder(int length, SecureRandom random)
    {
        this.length = length;
        this.random = random;
    }

    public byte[] getPaddedData(byte[] data)
    {
        byte[] bytes = new byte[length];
        byte[] seed = new byte[dig.getDigestSize()];
        byte[] mask = new byte[length - dig.getDigestSize()];

        if (random == null)
        {
            random = new SecureRandom();
        }

        random.nextBytes(seed);

        MGF1BytesGenerator maskGen = new MGF1BytesGenerator(dig);

        maskGen.init(new MGFParameters(seed));

        maskGen.generateBytes(mask, 0, mask.length);

        System.arraycopy(seed, 0, bytes, 0, seed.length);
        System.arraycopy(data, 0, bytes, seed.length, data.length);

        for (int i = seed.length + data.length + 1; i != bytes.length; i++)
        {
            bytes[i] = (byte)(1 + random.nextInt(255));
        }

        for (int i = 0; i != mask.length; i++)
        {
            bytes[i + seed.length] ^= mask[i];
        }

        return bytes;
    }

    public byte[] getUnpaddedData(byte[] paddedData)
    {
        byte[] seed = new byte[dig.getDigestSize()];
        byte[] mask = new byte[length - dig.getDigestSize()];

        System.arraycopy(paddedData, 0, seed, 0, seed.length);

        MGF1BytesGenerator maskGen = new MGF1BytesGenerator(dig);

        maskGen.init(new MGFParameters(seed));

        maskGen.generateBytes(mask, 0, mask.length);

        for (int i = 0; i != mask.length; i++)
        {
            paddedData[i + seed.length] ^= mask[i];
        }

        int end = 0;

        for (int i = paddedData.length - 1; i != seed.length; i--)
        {
            if (paddedData[i] == 0)
            {
                end = i;
                break;
            }
        }

        if (end == 0)
        {
            throw new IllegalStateException("bad padding in encoding");
        }

        byte[] data = new byte[end - seed.length];

        System.arraycopy(paddedData, seed.length, data, 0, data.length);

        return data;
    }
}