Java 给定一个DSAPrivateKey，如何计算相应的DSAPublicKey？

在Java中，我有一个

DSAPrivateKey

，它有一个

X

参数，还有一个

DSAParams

，带有

p

、

Q

和

G

参数。我想计算相应的

DSAPublicKey

。我知道如果我知道

Y

、

P

、

Q

和

G

，我可以构造一个

DSAPublicKeySpec

，然后我可以使用

KeyFactory.generatePublic（KeySpec）

方法将其转换为

DSAPublicKey

我不确定的是，在已知

X

、

p

、

Q

和

G

的情况下，如何计算

Y

。我猜答案是：

BigInteger y = g.multiply(x).mod(p);

但这就产生了例外：

Caused by: java.lang.IllegalArgumentException: Y value does not appear to be in correct group
    at org.bouncycastle.crypto.asymmetric.KeyUtils.validated(Unknown Source)
    at org.bouncycastle.crypto.asymmetric.AsymmetricDSAPublicKey.<init>(Unknown Source)
    at org.bouncycastle.jcajce.provider.ProvDSAPublicKey.<init>(Unknown Source)

这也给出了同样的例外

---

我使用的是BouncyCastle FIPS版本1.0.2，因此我很高兴得到一个使用BouncyCastle类的答案，但我也很高兴得到一个不使用BouncyC的答案。你的乘法计算y值是对的。我发现了一个非常方便的解决方案，可以与本机Java一起使用

安全警告：示例程序没有异常处理，仅用于教育目的。

祝你周末愉快

下面是我的示例程序的（简短）结果：

Derive DSA PublicKey from PrivateKey
publicKey equals publicKeyDerived: true

完整代码：

import java.math.BigInteger;
import java.security.*;
import java.security.interfaces.DSAParams;
import java.security.interfaces.DSAPrivateKey;
import java.security.spec.DSAPublicKeySpec;
import java.util.Arrays;

public class DSA_RetrievePublicKeyFromPrivateKey {
    public static void main(String[] args) throws NoSuchProviderException, NoSuchAlgorithmException {
        System.out.println("Derive DSA PublicKey from PrivateKey");

        KeyPair keyPair = generateDsaKeyPair(2048);
        PublicKey publicKeyOriginal =  keyPair.getPublic(); // original for comparison
        PublicKey publicKeyDerived = deriveDsaPublicKeyFromPrivatekey(keyPair.getPrivate());
        System.out.println("publicKey equals publicKeyDerived: " + Arrays.equals(publicKeyOriginal.getEncoded(), publicKeyDerived.getEncoded()));
    }

    public static KeyPair generateDsaKeyPair(int keylengthInt)
            throws NoSuchAlgorithmException, NoSuchProviderException {
        KeyPairGenerator keypairGenerator = KeyPairGenerator.getInstance("DSA", "SUN");
        SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");
        keypairGenerator.initialize(keylengthInt, random);
        return keypairGenerator.generateKeyPair();
    }

    public static PublicKey deriveDsaPublicKeyFromPrivatekey (PrivateKey privateKey) throws NoSuchAlgorithmException {
        DSAPrivateKey dsaPrivateKey = (DSAPrivateKey) privateKey;
        DSAParams params = dsaPrivateKey.getParams();
        BigInteger y = params.getG().modPow(dsaPrivateKey.getX(), params.getP());
        DSAPublicKeySpec keySpec = new DSAPublicKeySpec(y, params.getP(), params.getQ(), params.getG());
        PublicKey publicKey;
        KeyFactory keyFactory = KeyFactory.getInstance("DSA");
        try {
            publicKey = keyFactory.generatePublic(keySpec);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
        return publicKey;
    }
}

---

谢谢，成功了。原来我的代码几乎和你的代码一样，我的实际问题是我在新的DSAPublicKeySpec（y，params.getP（），params.getQ（），params.getG（））行中意外地交换了Q和G。我想这表明我应该分享我的全部代码。（另外，我可以确认本机Java和BouncyCastle FIPS不是问题，两种情况下都可以使用相同的代码。）

import java.math.BigInteger;
import java.security.*;
import java.security.interfaces.DSAParams;
import java.security.interfaces.DSAPrivateKey;
import java.security.spec.DSAPublicKeySpec;
import java.util.Arrays;

public class DSA_RetrievePublicKeyFromPrivateKey {
    public static void main(String[] args) throws NoSuchProviderException, NoSuchAlgorithmException {
        System.out.println("Derive DSA PublicKey from PrivateKey");

        KeyPair keyPair = generateDsaKeyPair(2048);
        PublicKey publicKeyOriginal =  keyPair.getPublic(); // original for comparison
        PublicKey publicKeyDerived = deriveDsaPublicKeyFromPrivatekey(keyPair.getPrivate());
        System.out.println("publicKey equals publicKeyDerived: " + Arrays.equals(publicKeyOriginal.getEncoded(), publicKeyDerived.getEncoded()));
    }

    public static KeyPair generateDsaKeyPair(int keylengthInt)
            throws NoSuchAlgorithmException, NoSuchProviderException {
        KeyPairGenerator keypairGenerator = KeyPairGenerator.getInstance("DSA", "SUN");
        SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");
        keypairGenerator.initialize(keylengthInt, random);
        return keypairGenerator.generateKeyPair();
    }

    public static PublicKey deriveDsaPublicKeyFromPrivatekey (PrivateKey privateKey) throws NoSuchAlgorithmException {
        DSAPrivateKey dsaPrivateKey = (DSAPrivateKey) privateKey;
        DSAParams params = dsaPrivateKey.getParams();
        BigInteger y = params.getG().modPow(dsaPrivateKey.getX(), params.getP());
        DSAPublicKeySpec keySpec = new DSAPublicKeySpec(y, params.getP(), params.getQ(), params.getG());
        PublicKey publicKey;
        KeyFactory keyFactory = KeyFactory.getInstance("DSA");
        try {
            publicKey = keyFactory.generatePublic(keySpec);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
        return publicKey;
    }
}