Generate ECPublicKey from ECPrivateKey

问题:

I'm trying to generate a public key given a private key and known curve. Below is my code:

// Generate Keys
ECGenParameterSpec ecGenSpec = new ECGenParameterSpec("secp256r1");
KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("ECDSA", "BC");
keyPairGenerator.initialize(ecGenSpec, new SecureRandom());
java.security.KeyPair pair = keyPairGenerator.generateKeyPair();
ECPrivateKey privateKey = (ECPrivateKey) pair.getPrivate();
ECPublicKey publicKeyExpected = (ECPublicKey) pair.getPublic();

// Expected public key
System.out.print("Expected Public Key: " +
        BaseEncoding.base64Url().encode(publicKeyExpected.getEncoded()));

// Generate public key from private key
X9ECParameters ecp = SECNamedCurves.getByName("secp256r1");
ECDomainParameters domainParams = new ECDomainParameters(ecp.getCurve(),
        ecp.getG(), ecp.getN(), ecp.getH(),
        ecp.getSeed());
ECPoint Q = domainParams.getG().multiply(privateKey.getS()); // is this correct?
KeyFactory kf = KeyFactory.getInstance("ECDSA", "BC");
ECPublicKey publicKeyGenerated =
        (ECPublicKey) kf.generatePublic(new X509EncodedKeySpec(Q.getEncoded(false)));  // exception here

// Generated public key from private key
System.out.print("Generated Public Key: " +
        BaseEncoding.base64Url().encode(publicKeyGenerated.getEncoded()));

However, when I call: kf.generatePublic(new X509EncodedKeySpec(Q.getEncoded(false))) I get the exception: java.security.spec.InvalidKeySpecException: encoded key spec not recognised (not my misspelling)

I appears I'm incorrectly calculating Q, but I'm not sure where my error is.

Thanks for the help!

回答1:

In case my future self needs the solution:

// Generate Keys
ECGenParameterSpec ecGenSpec = new ECGenParameterSpec("secp256r1");
KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("ECDSA", "BC");
keyPairGenerator.initialize(ecGenSpec, new SecureRandom());
java.security.KeyPair pair = keyPairGenerator.generateKeyPair();
ECPrivateKey privateKey = (ECPrivateKey) pair.getPrivate();
ECPublicKey publicKeyExpected = (ECPublicKey) pair.getPublic();

// Expected public key
System.out.print("Expected Public Key: " +
        BaseEncoding.base64Url().encode(publicKeyExpected.getEncoded()));

// Generate public key from private key
KeyFactory keyFactory = KeyFactory.getInstance("ECDSA", "BC");
ECParameterSpec ecSpec = ECNamedCurveTable.getParameterSpec("secp256r1");

ECPoint Q = ecSpec.getG().multiply(privateKey.getD());
byte[] publicDerBytes = Q.getEncoded(false);

ECPoint point = ecSpec.getCurve().decodePoint(publicDerBytes);
ECPublicKeySpec pubSpec = new ECPublicKeySpec(point, ecSpec);
ECPublicKey publicKeyGenerated = (ECPublicKey) keyFactory.generatePublic(pubSpec);

// Generated public key from private key
System.out.print("Generated Public Key: " +
        BaseEncoding.base64Url().encode(publicKeyGenerated.getEncoded()));

回答2:

Some more improvement to the answer of @markw, extended with curve name detection by @bas-goossen from here: How to find the matching curve name from an ECPublicKey

public static ECPublicKey publicFromPrivate(final ECPrivateKey privateKey) throws Exception {
    ECParameterSpec params = privateKey.getParams();
    org.bouncycastle.jce.spec.ECParameterSpec bcSpec = org.bouncycastle.jcajce.provider.asymmetric.util.EC5Util
        .convertSpec(params, false);
    org.bouncycastle.math.ec.ECPoint q = bcSpec.getG().multiply(privateKey.getS());
    org.bouncycastle.math.ec.ECPoint bcW = bcSpec.getCurve().decodePoint(q.getEncoded(false));
    ECPoint w = new ECPoint(
        bcW.getAffineXCoord().toBigInteger(),
        bcW.getAffineYCoord().toBigInteger());
    ECPublicKeySpec keySpec = new ECPublicKeySpec(w, tryFindNamedCurveSpec(params));
    return (ECPublicKey) KeyFactory
        .getInstance("EC", org.bouncycastle.jce.provider.BouncyCastleProvider.PROVIDER_NAME)
        .generatePublic(keySpec);
}

@SuppressWarnings("unchecked")
public static ECParameterSpec tryFindNamedCurveSpec(ECParameterSpec params) {
    org.bouncycastle.jce.spec.ECParameterSpec bcSpec
        = org.bouncycastle.jcajce.provider.asymmetric.util.EC5Util.convertSpec(params, false);
    for (Object name : Collections.list(org.bouncycastle.jce.ECNamedCurveTable.getNames())) {
        org.bouncycastle.jce.spec.ECNamedCurveParameterSpec bcNamedSpec
            = org.bouncycastle.jce.ECNamedCurveTable.getParameterSpec((String) name);
        if (bcNamedSpec.getN().equals(bcSpec.getN())
            && bcNamedSpec.getH().equals(bcSpec.getH())
            && bcNamedSpec.getCurve().equals(bcSpec.getCurve())
            && bcNamedSpec.getG().equals(bcSpec.getG())) {
            return new org.bouncycastle.jce.spec.ECNamedCurveSpec(
                bcNamedSpec.getName(),
                bcNamedSpec.getCurve(),
                bcNamedSpec.getG(),
                bcNamedSpec.getN(),
                bcNamedSpec.getH(),
                bcNamedSpec.getSeed());
        }
    }
    return params;
}