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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.engines;

import java.math.BigInteger;
import java.security.SecureRandom;

import org.bouncycastle.crypto.AsymmetricBlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.CryptoServicesRegistrar;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.crypto.params.RSAKeyParameters;
import org.bouncycastle.crypto.params.RSAPrivateCrtKeyParameters;
import org.bouncycastle.util.BigIntegers;

/**
 * this does your basic RSA algorithm with blinding
 */
public class RSABlindedEngine
    implements AsymmetricBlockCipher
{
    private static final BigInteger ONE = BigInteger.valueOf(1);

    private RSACoreEngine    core = new RSACoreEngine();
    private RSAKeyParameters key;
    private SecureRandom     random;

    /**
     * initialise the RSA engine.
     *
     * @param forEncryption true if we are encrypting, false otherwise.
     * @param param the necessary RSA key parameters.
     */
    public void init(
        boolean             forEncryption,
        CipherParameters    param)
    {
        core.init(forEncryption, param);

        if (param instanceof ParametersWithRandom)
        {
            ParametersWithRandom rParam = (ParametersWithRandom)param;

            this.key = (RSAKeyParameters)rParam.getParameters();

            if (key instanceof RSAPrivateCrtKeyParameters)
            {
                this.random = rParam.getRandom();
            }
            else
            {
                this.random = null;
            }
        }
        else
        {
            this.key = (RSAKeyParameters)param;

            if (key instanceof RSAPrivateCrtKeyParameters)
            {
                this.random = CryptoServicesRegistrar.getSecureRandom();
            }
            else
            {
                this.random = null;
            }
        }
    }

    /**
     * Return the maximum size for an input block to this engine.
     * For RSA this is always one byte less than the key size on
     * encryption, and the same length as the key size on decryption.
     *
     * @return maximum size for an input block.
     */
    public int getInputBlockSize()
    {
        return core.getInputBlockSize();
    }

    /**
     * Return the maximum size for an output block to this engine.
     * For RSA this is always one byte less than the key size on
     * decryption, and the same length as the key size on encryption.
     *
     * @return maximum size for an output block.
     */
    public int getOutputBlockSize()
    {
        return core.getOutputBlockSize();
    }

    /**
     * Process a single block using the basic RSA algorithm.
     *
     * @param in the input array.
     * @param inOff the offset into the input buffer where the data starts.
     * @param inLen the length of the data to be processed.
     * @return the result of the RSA process.
     * @exception DataLengthException the input block is too large.
     */
    public byte[] processBlock(
        byte[]  in,
        int     inOff,
        int     inLen)
    {
        if (key == null)
        {
            throw new IllegalStateException("RSA engine not initialised");
        }

        BigInteger input = core.convertInput(in, inOff, inLen);

        BigInteger result;
        if (key instanceof RSAPrivateCrtKeyParameters)
        {
            RSAPrivateCrtKeyParameters k = (RSAPrivateCrtKeyParameters)key;

            BigInteger e = k.getPublicExponent();
            if (e != null)   // can't do blinding without a public exponent
            {
                BigInteger m = k.getModulus();
                BigInteger r = BigIntegers.createRandomInRange(ONE, m.subtract(ONE), random);

                BigInteger blindedInput = r.modPow(e, m).multiply(input).mod(m);
                BigInteger blindedResult = core.processBlock(blindedInput);

                BigInteger rInv = BigIntegers.modOddInverse(m, r);
                result = blindedResult.multiply(rInv).mod(m);
                // defence against Arjen Lenstra’s CRT attack
                if (!input.equals(result.modPow(e, m)))
                {
                    throw new IllegalStateException("RSA engine faulty decryption/signing detected");
                }
            }
            else
            {
                result = core.processBlock(input);
            }
        }
        else
        {
            result = core.processBlock(input);
        }

        return core.convertOutput(result);
    }
}