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package org.spongycastle.crypto.generators;
import org.spongycastle.crypto.AsymmetricCipherKeyPair;
import org.spongycastle.crypto.AsymmetricCipherKeyPairGenerator;
import org.spongycastle.crypto.KeyGenerationParameters;
import org.spongycastle.crypto.params.RSAKeyGenerationParameters;
import org.spongycastle.crypto.params.RSAKeyParameters;
import org.spongycastle.crypto.params.RSAPrivateCrtKeyParameters;
import java.math.BigInteger;
/**
* an RSA key pair generator.
*/
public class RSAKeyPairGenerator
implements AsymmetricCipherKeyPairGenerator
{
private static final BigInteger ONE = BigInteger.valueOf(1);
private RSAKeyGenerationParameters param;
public void init(
KeyGenerationParameters param)
{
this.param = (RSAKeyGenerationParameters)param;
}
public AsymmetricCipherKeyPair generateKeyPair()
{
BigInteger p, q, n, d, e, pSub1, qSub1, phi;
//
// p and q values should have a length of half the strength in bits
//
int strength = param.getStrength();
int pbitlength = (strength + 1) / 2;
int qbitlength = strength - pbitlength;
int mindiffbits = strength / 3;
e = param.getPublicExponent();
// TODO Consider generating safe primes for p, q (see DHParametersHelper.generateSafePrimes)
// (then p-1 and q-1 will not consist of only small factors - see "Pollard's algorithm")
//
// generate p, prime and (p-1) relatively prime to e
//
for (;;)
{
p = new BigInteger(pbitlength, 1, param.getRandom());
if (p.mod(e).equals(ONE))
{
continue;
}
if (!p.isProbablePrime(param.getCertainty()))
{
continue;
}
if (e.gcd(p.subtract(ONE)).equals(ONE))
{
break;
}
}
//
// generate a modulus of the required length
//
for (;;)
{
// generate q, prime and (q-1) relatively prime to e,
// and not equal to p
//
for (;;)
{
q = new BigInteger(qbitlength, 1, param.getRandom());
if (q.subtract(p).abs().bitLength() < mindiffbits)
{
continue;
}
if (q.mod(e).equals(ONE))
{
continue;
}
if (!q.isProbablePrime(param.getCertainty()))
{
continue;
}
if (e.gcd(q.subtract(ONE)).equals(ONE))
{
break;
}
}
//
// calculate the modulus
//
n = p.multiply(q);
if (n.bitLength() == param.getStrength())
{
break;
}
//
// if we get here our primes aren't big enough, make the largest
// of the two p and try again
//
p = p.max(q);
}
if (p.compareTo(q) < 0)
{
phi = p;
p = q;
q = phi;
}
pSub1 = p.subtract(ONE);
qSub1 = q.subtract(ONE);
phi = pSub1.multiply(qSub1);
//
// calculate the private exponent
//
d = e.modInverse(phi);
//
// calculate the CRT factors
//
BigInteger dP, dQ, qInv;
dP = d.remainder(pSub1);
dQ = d.remainder(qSub1);
qInv = q.modInverse(p);
return new AsymmetricCipherKeyPair(
new RSAKeyParameters(false, n, e),
new RSAPrivateCrtKeyParameters(n, e, d, p, q, dP, dQ, qInv));
}
}
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