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using System;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Math;
namespace Org.BouncyCastle.Crypto.Engines
{
/**
* 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
: IAsymmetricBlockCipher
{
private readonly RsaCoreEngine core = new RsaCoreEngine();
private RsaKeyParameters key;
private BigInteger blindingFactor;
private bool forEncryption;
public string AlgorithmName
{
get { return "RSA"; }
}
/**
* Initialise the blinding engine.
*
* @param forEncryption true if we are encrypting (blinding), false otherwise.
* @param param the necessary RSA key parameters.
*/
public void Init(
bool forEncryption,
ICipherParameters param)
{
RsaBlindingParameters p;
if (param is ParametersWithRandom)
{
ParametersWithRandom rParam = (ParametersWithRandom)param;
p = (RsaBlindingParameters)rParam.Parameters;
}
else
{
p = (RsaBlindingParameters)param;
}
core.Init(forEncryption, p.PublicKey);
this.forEncryption = forEncryption;
this.key = p.PublicKey;
this.blindingFactor = p.BlindingFactor;
}
/**
* 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 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[] inBuf,
int inOff,
int inLen)
{
BigInteger msg = core.ConvertInput(inBuf, 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.Exponent, key.Modulus));
blindMsg = blindMsg.Mod(key.Modulus);
return blindMsg;
}
/*
* Unblind the message blinded with the blind factor.
*/
private BigInteger UnblindMessage(
BigInteger blindedMsg)
{
BigInteger m = key.Modulus;
BigInteger msg = blindedMsg;
BigInteger blindFactorInverse = blindingFactor.ModInverse(m);
msg = msg.Multiply(blindFactorInverse);
msg = msg.Mod(m);
return msg;
}
}
}
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