| package org.bouncycastle.crypto.generators; |
| |
| import java.math.BigInteger; |
| import java.security.SecureRandom; |
| |
| import org.bouncycastle.crypto.digests.SHA256Digest; |
| import org.bouncycastle.crypto.params.CramerShoupParameters; |
| import org.bouncycastle.crypto.params.DHParameters; |
| import org.bouncycastle.util.BigIntegers; |
| |
| public class CramerShoupParametersGenerator |
| { |
| private static final BigInteger ONE = BigInteger.valueOf(1); |
| |
| private int size; |
| private int certainty; |
| private SecureRandom random; |
| |
| /** |
| * Initialise the parameters generator. |
| * |
| * @param size bit length for the prime p |
| * @param certainty a measure of the uncertainty that the caller is willing to tolerate: |
| * the probability that the generated modulus is prime exceeds (1 - 1/2^certainty). |
| * The execution time of this method is proportional to the value of this parameter. |
| * @param random a source of randomness |
| */ |
| public void init(int size, int certainty, SecureRandom random) |
| { |
| this.size = size; |
| this.certainty = certainty; |
| this.random = random; |
| } |
| |
| /** |
| * which generates the p and g values from the given parameters, returning |
| * the CramerShoupParameters object. |
| * <p> |
| * Note: can take a while... |
| * </p> |
| * @return a generated CramerShoupParameters object. |
| */ |
| public CramerShoupParameters generateParameters() |
| { |
| // |
| // find a safe prime p where p = 2*q + 1, where p and q are prime. |
| // |
| BigInteger[] safePrimes = ParametersHelper.generateSafePrimes(size, certainty, random); |
| |
| // BigInteger p = safePrimes[0]; |
| BigInteger q = safePrimes[1]; |
| BigInteger g1 = ParametersHelper.selectGenerator(q, random); |
| BigInteger g2 = ParametersHelper.selectGenerator(q, random); |
| while (g1.equals(g2)) |
| { |
| g2 = ParametersHelper.selectGenerator(q, random); |
| } |
| |
| return new CramerShoupParameters(q, g1, g2, new SHA256Digest()); |
| } |
| |
| public CramerShoupParameters generateParameters(DHParameters dhParams) |
| { |
| BigInteger p = dhParams.getP(); |
| BigInteger g1 = dhParams.getG(); |
| |
| // now we just need a second generator |
| BigInteger g2 = ParametersHelper.selectGenerator(p, random); |
| while (g1.equals(g2)) |
| { |
| g2 = ParametersHelper.selectGenerator(p, random); |
| } |
| |
| return new CramerShoupParameters(p, g1, g2, new SHA256Digest()); |
| } |
| |
| private static class ParametersHelper |
| { |
| |
| private static final BigInteger TWO = BigInteger.valueOf(2); |
| |
| /* |
| * Finds a pair of prime BigInteger's {p, q: p = 2q + 1} |
| * |
| * (see: Handbook of Applied Cryptography 4.86) |
| */ |
| static BigInteger[] generateSafePrimes(int size, int certainty, SecureRandom random) |
| { |
| BigInteger p, q; |
| int qLength = size - 1; |
| |
| for (; ; ) |
| { |
| q = BigIntegers.createRandomPrime(qLength, 2, random); |
| p = q.shiftLeft(1).add(ONE); |
| if (p.isProbablePrime(certainty) && (certainty <= 2 || q.isProbablePrime(certainty))) |
| { |
| break; |
| } |
| } |
| |
| return new BigInteger[]{p, q}; |
| } |
| |
| static BigInteger selectGenerator(BigInteger p, SecureRandom random) |
| { |
| BigInteger pMinusTwo = p.subtract(TWO); |
| BigInteger g; |
| |
| /* |
| * RFC 2631 2.2.1.2 (and see: Handbook of Applied Cryptography 4.81) |
| */ |
| do |
| { |
| BigInteger h = BigIntegers.createRandomInRange(TWO, pMinusTwo, random); |
| |
| g = h.modPow(TWO, p); |
| } |
| while (g.equals(ONE)); |
| |
| return g; |
| } |
| } |
| |
| } |