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package org.spongycastle.crypto.engines;
import java.math.BigInteger;
import java.security.SecureRandom;
import org.spongycastle.crypto.AsymmetricBlockCipher;
import org.spongycastle.crypto.CipherParameters;
import org.spongycastle.crypto.DataLengthException;
import org.spongycastle.crypto.params.ElGamalKeyParameters;
import org.spongycastle.crypto.params.ElGamalPrivateKeyParameters;
import org.spongycastle.crypto.params.ElGamalPublicKeyParameters;
import org.spongycastle.crypto.params.ParametersWithRandom;
import org.spongycastle.util.BigIntegers;
/**
* this does your basic ElGamal algorithm.
*/
public class ElGamalEngine
implements AsymmetricBlockCipher
{
private ElGamalKeyParameters key;
private SecureRandom random;
private boolean forEncryption;
private int bitSize;
private static final BigInteger ZERO = BigInteger.valueOf(0);
private static final BigInteger ONE = BigInteger.valueOf(1);
private static final BigInteger TWO = BigInteger.valueOf(2);
/**
* initialise the ElGamal engine.
*
* @param forEncryption true if we are encrypting, false otherwise.
* @param param the necessary ElGamal key parameters.
*/
public void init(
boolean forEncryption,
CipherParameters param)
{
if (param instanceof ParametersWithRandom)
{
ParametersWithRandom p = (ParametersWithRandom)param;
this.key = (ElGamalKeyParameters)p.getParameters();
this.random = p.getRandom();
}
else
{
this.key = (ElGamalKeyParameters)param;
this.random = new SecureRandom();
}
this.forEncryption = forEncryption;
BigInteger p = key.getParameters().getP();
bitSize = p.bitLength();
if (forEncryption)
{
if (!(key instanceof ElGamalPublicKeyParameters))
{
throw new IllegalArgumentException("ElGamalPublicKeyParameters are required for encryption.");
}
}
else
{
if (!(key instanceof ElGamalPrivateKeyParameters))
{
throw new IllegalArgumentException("ElGamalPrivateKeyParameters are required for decryption.");
}
}
}
/**
* Return the maximum size for an input block to this engine.
* For ElGamal this is always one byte less than the size of P on
* encryption, and twice the length as the size of P on decryption.
*
* @return maximum size for an input block.
*/
public int getInputBlockSize()
{
if (forEncryption)
{
return (bitSize - 1) / 8;
}
return 2 * ((bitSize + 7) / 8);
}
/**
* Return the maximum size for an output block to this engine.
* For ElGamal this is always one byte less than the size of P on
* decryption, and twice the length as the size of P on encryption.
*
* @return maximum size for an output block.
*/
public int getOutputBlockSize()
{
if (forEncryption)
{
return 2 * ((bitSize + 7) / 8);
}
return (bitSize - 1) / 8;
}
/**
* Process a single block using the basic ElGamal 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 ElGamal process.
* @exception DataLengthException the input block is too large.
*/
public byte[] processBlock(
byte[] in,
int inOff,
int inLen)
{
if (key == null)
{
throw new IllegalStateException("ElGamal engine not initialised");
}
int maxLength = forEncryption
? (bitSize - 1 + 7) / 8
: getInputBlockSize();
if (inLen > maxLength)
{
throw new DataLengthException("input too large for ElGamal cipher.\n");
}
BigInteger p = key.getParameters().getP();
if (key instanceof ElGamalPrivateKeyParameters) // decryption
{
byte[] in1 = new byte[inLen / 2];
byte[] in2 = new byte[inLen / 2];
System.arraycopy(in, inOff, in1, 0, in1.length);
System.arraycopy(in, inOff + in1.length, in2, 0, in2.length);
BigInteger gamma = new BigInteger(1, in1);
BigInteger phi = new BigInteger(1, in2);
ElGamalPrivateKeyParameters priv = (ElGamalPrivateKeyParameters)key;
// a shortcut, which generally relies on p being prime amongst other things.
// if a problem with this shows up, check the p and g values!
BigInteger m = gamma.modPow(p.subtract(ONE).subtract(priv.getX()), p).multiply(phi).mod(p);
return BigIntegers.asUnsignedByteArray(m);
}
else // encryption
{
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);
if (input.compareTo(p) >= 0)
{
throw new DataLengthException("input too large for ElGamal cipher.\n");
}
ElGamalPublicKeyParameters pub = (ElGamalPublicKeyParameters)key;
int pBitLength = p.bitLength();
BigInteger k = new BigInteger(pBitLength, random);
while (k.equals(ZERO) || (k.compareTo(p.subtract(TWO)) > 0))
{
k = new BigInteger(pBitLength, random);
}
BigInteger g = key.getParameters().getG();
BigInteger gamma = g.modPow(k, p);
BigInteger phi = input.multiply(pub.getY().modPow(k, p)).mod(p);
byte[] out1 = gamma.toByteArray();
byte[] out2 = phi.toByteArray();
byte[] output = new byte[this.getOutputBlockSize()];
if (out1.length > output.length / 2)
{
System.arraycopy(out1, 1, output, output.length / 2 - (out1.length - 1), out1.length - 1);
}
else
{
System.arraycopy(out1, 0, output, output.length / 2 - out1.length, out1.length);
}
if (out2.length > output.length / 2)
{
System.arraycopy(out2, 1, output, output.length - (out2.length - 1), out2.length - 1);
}
else
{
System.arraycopy(out2, 0, output, output.length - out2.length, out2.length);
}
return output;
}
}
}
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