/** * Transform an existing cipher text pair using the ElGamal algorithm. Note: it is assumed this * transform has been initialised with the same public key that was used to create the original * cipher text. * * @param cipherText the EC point to process. * @return returns a new ECPair representing the result of the process. */ public ECPair transform(ECPair cipherText) { if (key == null) { throw new IllegalStateException("ECFixedTransform not initialised"); } ECDomainParameters ec = key.getParameters(); BigInteger n = ec.getN(); ECMultiplier basePointMultiplier = createBasePointMultiplier(); BigInteger k = this.k.mod(n); ECPoint[] gamma_phi = new ECPoint[]{ basePointMultiplier.multiply(ec.getG(), k).add(cipherText.getX()), key.getQ().multiply(k).add(cipherText.getY()) }; ec.getCurve().normalizeAll(gamma_phi); return new ECPair(gamma_phi[0], gamma_phi[1]); }
/** * Transform an existing cipher text pair using the ElGamal algorithm. Note: it is assumed this * transform has been initialised with the same public key that was used to create the original * cipher text. * * @param cipherText the EC point to process. * @return returns a new ECPair representing the result of the process. */ public ECPair transform(ECPair cipherText) { if (key == null) { throw new IllegalStateException("ECFixedTransform not initialised"); } ECDomainParameters ec = key.getParameters(); BigInteger n = ec.getN(); ECMultiplier basePointMultiplier = createBasePointMultiplier(); BigInteger k = this.k.mod(n); ECPoint[] gamma_phi = new ECPoint[]{ basePointMultiplier.multiply(ec.getG(), k).add(ECAlgorithms.cleanPoint(ec.getCurve(), cipherText.getX())), key.getQ().multiply(k).add(ECAlgorithms.cleanPoint(ec.getCurve(), cipherText.getY())) }; ec.getCurve().normalizeAll(gamma_phi); return new ECPair(gamma_phi[0], gamma_phi[1]); }