java - Generate ECPublicKey from ECPrivateKey

Question

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!

Answer 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()));