1package org.bouncycastle.crypto.generators;
2
3import java.math.BigInteger;
4import java.security.SecureRandom;
5
6import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
7import org.bouncycastle.crypto.AsymmetricCipherKeyPairGenerator;
8import org.bouncycastle.crypto.KeyGenerationParameters;
9import org.bouncycastle.crypto.params.DSAKeyGenerationParameters;
10import org.bouncycastle.crypto.params.DSAParameters;
11import org.bouncycastle.crypto.params.DSAPrivateKeyParameters;
12import org.bouncycastle.crypto.params.DSAPublicKeyParameters;
13import org.bouncycastle.math.ec.WNafUtil;
14import org.bouncycastle.util.BigIntegers;
15
16/**
17 * a DSA key pair generator.
18 *
19 * This generates DSA keys in line with the method described
20 * in <i>FIPS 186-3 B.1 FFC Key Pair Generation</i>.
21 */
22public class DSAKeyPairGenerator
23    implements AsymmetricCipherKeyPairGenerator
24{
25    private static final BigInteger ONE = BigInteger.valueOf(1);
26
27    private DSAKeyGenerationParameters param;
28
29    public void init(
30        KeyGenerationParameters param)
31    {
32        this.param = (DSAKeyGenerationParameters)param;
33    }
34
35    public AsymmetricCipherKeyPair generateKeyPair()
36    {
37        DSAParameters dsaParams = param.getParameters();
38
39        BigInteger x = generatePrivateKey(dsaParams.getQ(), param.getRandom());
40        BigInteger y = calculatePublicKey(dsaParams.getP(), dsaParams.getG(), x);
41
42        return new AsymmetricCipherKeyPair(
43            new DSAPublicKeyParameters(y, dsaParams),
44            new DSAPrivateKeyParameters(x, dsaParams));
45    }
46
47    private static BigInteger generatePrivateKey(BigInteger q, SecureRandom random)
48    {
49        // B.1.2 Key Pair Generation by Testing Candidates
50        int minWeight = q.bitLength() >>> 2;
51        for (;;)
52        {
53            // TODO Prefer this method? (change test cases that used fixed random)
54            // B.1.1 Key Pair Generation Using Extra Random Bits
55//            BigInteger x = new BigInteger(q.bitLength() + 64, random).mod(q.subtract(ONE)).add(ONE);
56
57            BigInteger x = BigIntegers.createRandomInRange(ONE, q.subtract(ONE), random);
58            if (WNafUtil.getNafWeight(x) >= minWeight)
59            {
60                return x;
61            }
62        }
63    }
64
65    private static BigInteger calculatePublicKey(BigInteger p, BigInteger g, BigInteger x)
66    {
67        return g.modPow(x, p);
68    }
69}
70