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.RSAKeyParameters;
import org.bouncycastle.crypto.params.RSAPrivateCrtKeyParameters;
import org.bouncycastle.util.BigIntegers;

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

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

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

    
initialise the RSA engine.
Params:
forEncryption – true if we are encrypting, false otherwise.
param – the necessary RSA key parameters./**
     * 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;

            key = (RSAKeyParameters)rParam.getParameters();
            random = rParam.getRandom();
        }
        else
        {
            key = (RSAKeyParameters)param;
            random = new SecureRandom();
        }
    }

    
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 basic RSA 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 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 = r.modInverse(m);
                result = blindedResult.multiply(rInv).mod(m);
            }
            else
            {
                result = core.processBlock(input);
            }
        }
        else
        {
            result = core.processBlock(input);
        }

        return core.convertOutput(result);
    }
}