package org.bouncycastle.crypto.engines;

import org.bouncycastle.crypto.AsymmetricBlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.crypto.params.RSABlindingParameters;
import org.bouncycastle.crypto.params.RSAKeyParameters;

import java.math.BigInteger;

This does your basic RSA Chaum's blinding and unblinding as outlined in
"Handbook of Applied Cryptography", page 475. You need to use this if you are
trying to get another party to generate signatures without them being aware
of the message they are signing.
/**
 * This does your basic RSA Chaum's blinding and unblinding as outlined in
 * "Handbook of Applied Cryptography", page 475. You need to use this if you are
 * trying to get another party to generate signatures without them being aware
 * of the message they are signing.
 */
public class RSABlindingEngine
    implements AsymmetricBlockCipher
{
    private RSACoreEngine core = new RSACoreEngine();

    private RSAKeyParameters key;
    private BigInteger blindingFactor;

    private boolean forEncryption;

    
Initialise the blinding engine.
Params:
forEncryption – true if we are encrypting (blinding), false otherwise.
param –         the necessary RSA key parameters./**
     * Initialise the blinding engine.
     *
     * @param forEncryption true if we are encrypting (blinding), false otherwise.
     * @param param         the necessary RSA key parameters.
     */
    public void init(
        boolean forEncryption,
        CipherParameters param)
    {
        RSABlindingParameters p;

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

            p = (RSABlindingParameters)rParam.getParameters();
        }
        else
        {
            p = (RSABlindingParameters)param;
        }

        core.init(forEncryption, p.getPublicKey());

        this.forEncryption = forEncryption;
        this.key = p.getPublicKey();
        this.blindingFactor = p.getBlindingFactor();
    }

    
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.
Returns:
maximum size for an input block./**
     * 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.
Returns:
maximum size for an output block./**
     * 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 RSA blinding algorithm.
Params:
in –    the input array.
inOff – the offset into the input buffer where the data starts.
inLen – the length of the data to be processed.
Throws:
DataLengthException – the input block is too large.
Returns:
the result of the RSA process./**
     * Process a single block using the RSA blinding 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.
     * @throws DataLengthException the input block is too large.
     */
    public byte[] processBlock(
        byte[] in,
        int inOff,
        int inLen)
    {
        BigInteger msg = core.convertInput(in, inOff, inLen);

        if (forEncryption)
        {
            msg = blindMessage(msg);
        }
        else
        {
            msg = unblindMessage(msg);
        }

        return core.convertOutput(msg);
    }

    /*
     * Blind message with the blind factor.
     */
    private BigInteger blindMessage(
        BigInteger msg)
    {
        BigInteger blindMsg = blindingFactor;
        blindMsg = msg.multiply(blindMsg.modPow(key.getExponent(), key.getModulus()));
        blindMsg = blindMsg.mod(key.getModulus());

        return blindMsg;
    }

    /*
     * Unblind the message blinded with the blind factor.
     */
    private BigInteger unblindMessage(
        BigInteger blindedMsg)
    {
        BigInteger m = key.getModulus();
        BigInteger msg = blindedMsg;
        BigInteger blindFactorInverse = blindingFactor.modInverse(m);
        msg = msg.multiply(blindFactorInverse);
        msg = msg.mod(m);

        return msg;
    }
}