package org.bouncycastle.crypto.engines;
import java.math.BigInteger;
import org.bouncycastle.crypto.AsymmetricBlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.params.RSAKeyParameters;
import org.bouncycastle.crypto.params.RSAPrivateCrtKeyParameters;
/**
* this does your basic RSA algorithm.
*/
public class RSAEngine
implements AsymmetricBlockCipher
{
private RSAKeyParameters key;
private boolean forEncryption;
/**
* 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)
{
this.key = (RSAKeyParameters)param;
this.forEncryption = forEncryption;
}
/**
* 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()
{
int bitSize = key.getModulus().bitLength();
if (forEncryption)
{
return (bitSize + 7) / 8 - 1;
}
else
{
return (bitSize + 7) / 8;
}
}
/**
* 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()
{
int bitSize = key.getModulus().bitLength();
if (forEncryption)
{
return (bitSize + 7) / 8;
}
else
{
return (bitSize + 7) / 8 - 1;
}
}
/**
* 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 (inLen > (getInputBlockSize() + 1))
{
throw new DataLengthException("input too large for RSA cipher.\n");
}
else if (inLen == (getInputBlockSize() + 1) && (in[inOff] & 0x80) != 0)
{
throw new DataLengthException("input too large for RSA cipher.\n");
}
byte[] block;
if (inOff != 0 || inLen != in.length)
{
block = new byte[inLen];
System.arraycopy(in, inOff, block, 0, inLen);
}
else
{
block = in;
}
BigInteger input = new BigInteger(1, block);
byte[] output;
if (key instanceof RSAPrivateCrtKeyParameters)
{
//
// we have the extra factors, use the Chinese Remainder Theorem - the author
// wishes to express his thanks to Dirk Bonekaemper at rtsffm.com for
// advice regarding the expression of this.
//
RSAPrivateCrtKeyParameters crtKey = (RSAPrivateCrtKeyParameters)key;
BigInteger p = crtKey.getP();
BigInteger q = crtKey.getQ();
BigInteger dP = crtKey.getDP();
BigInteger dQ = crtKey.getDQ();
BigInteger qInv = crtKey.getQInv();
BigInteger mP, mQ, h, m;
// mP = ((input mod p) ^ dP)) mod p
mP = (input.remainder(p)).modPow(dP, p);
// mQ = ((input mod q) ^ dQ)) mod q
mQ = (input.remainder(q)).modPow(dQ, q);
// h = qInv * (mP - mQ) mod p
h = mP.subtract(mQ);
h = h.multiply(qInv);
h = h.mod(p); // mod (in Java) returns the positive residual
// m = h * q + mQ
m = h.multiply(q);
m = m.add(mQ);
output = m.toByteArray();
}
else
{
output = input.modPow(
key.getExponent(), key.getModulus()).toByteArray();
}
if (forEncryption)
{
if (output[0] == 0 && output.length > getOutputBlockSize()) // have ended up with an extra zero byte, copy down.
{
byte[] tmp = new byte[output.length - 1];
System.arraycopy(output, 1, tmp, 0, tmp.length);
return tmp;
}
if (output.length < getOutputBlockSize()) // have ended up with less bytes than normal, lengthen
{
byte[] tmp = new byte[getOutputBlockSize()];
System.arraycopy(output, 0, tmp, tmp.length - output.length, output.length);
return tmp;
}
}
else
{
if (output[0] == 0) // have ended up with an extra zero byte, copy down.
{
byte[] tmp = new byte[output.length - 1];
System.arraycopy(output, 1, tmp, 0, tmp.length);
return tmp;
}
}
return output;
}
}