How to convert ECPrivateKey into ECPublicKey using FIPS BouncyCastle?

Question

I use the following code (slightly simplified) to convert an ECPrivateKey into an ECPublicKey:

public static ECPublicKey getPublicKeyFromPrivateKey(ECPrivateKey privateKey) throws GeneralSecurityException {
    final KeyFactory keyFactory = KeyFactory.getInstance("ECDSA", new BouncyCastleProvider());
    final BCECPrivateKey bcecPrivateKey = (BCECPrivateKey) privateKey;
    final ECParameterSpec ecSpec = bcecPrivateKey.getParameters();
    final ECPoint q = ecSpec.getG().multiply(bcecPrivateKey.getD());
    final byte[] qBytes = q.getEncoded(false);
    final ECPoint point = ecSpec.getCurve().decodePoint(qBytes);
    final ECPublicKeySpec pubSpec = new ECPublicKeySpec(point, ecSpec);
    return (ECPublicKey) keyFactory.generatePublic(pubSpec);
}

This is working fine with non-FIPS BouncyCastle. Now, we have the requirement to do the same thing in a FIPS-compliant way. And here the problem is that classes like ECParameterSpec don't appear to exist in FIPS BouncyCastle 1.0.1. How can we do the same thing with FIPS BouncyCastle?

@JimGarrison According to https://downloads.bouncycastle.org/fips-java/BC-FJA-UserGuide-1.0.1.pdf the answer is yes – it supports ECDSA for digital signatures and ECDH for key agreement — Simon Kissane, Aug 27 '18 at 01:39

score 2 · Accepted Answer · answered Mar 23 '20 at 22:48

I found the following solution which worked for me:

public static PublicKey getPublicKeyForPrivateKeyEC(ECPrivateKey privateKey) throws GeneralSecurityException {
    final KeyFactory keyFactory = KeyFactory.getInstance("ECDSA", new BouncyCastleProvider());
    final ECParameterSpec ecSpec = privateKey.getParams();
    final ECCurve ecCurve = FipsECUtil.convertCurve(ecSpec);
    final ECMultiplier multiplier = ecCurve.getMultiplier();
    final ECPoint generatorP = FipsECUtil.convertPoint(ecSpec, ecSpec.getGenerator());
    final ECPoint q = multiplier.multiply(generatorP, privateKey.getS());
    final byte[] publicDerBytes = q.getEncoded(false);
    final ECPoint point = ecCurve.decodePoint(publicDerBytes);
    final ECPublicKeySpec pubSpec = new ECPublicKeySpec(new java.security.spec.ECPoint(
        point.getAffineXCoord().toBigInteger(), 
        point.getAffineYCoord().toBigInteger()), ecSpec);
    return keyFactory.generatePublic(pubSpec);
}

How to convert ECPrivateKey into ECPublicKey using FIPS BouncyCastle?

1 Answers1