java - How to decompress X9.62 coordinate

Question

I am passing a byte[] to my method and I am trying to generate a pubKey. Looking at the error look's like i first need to decompress before generating pubKey.

How can I decode a compressed coordinate ?

private static PublicKey getPublicKeyFromBytes(byte[] pubKey) throws NoSuchAlgorithmException, InvalidKeySpecException 
{
       
        ECPoint point = new ECPoint(new BigInteger(pubKey), BigInteger.ZERO);
        ECNamedCurveParameterSpec spec = ECNamedCurveTable.getParameterSpec("prime256v1");
        KeyFactory kf = KeyFactory.getInstance("ECDSA", new BouncyCastleProvider());
        ECNamedCurveSpec params = new ECNamedCurveSpec("prime256v1", spec.getCurve(), spec.getG(), spec.getN());
        ECPublicKeySpec pubKeySpec = new ECPublicKeySpec(point, params);
        ECPublicKey pk = (ECPublicKey) kf.generatePublic(pubKeySpec);
        return pk;
}

Error message:

Exception in thread "main" java.security.spec.InvalidKeySpecException: invalid KeySpec: x value invalid for SecP256R1FieldElement
    at org.bouncycastle.jcajce.provider.asymmetric.ec.KeyFactorySpi.engineGeneratePublic(Unknown Source)

Answer 1

If your pubKey is an X9.62 compressed or uncompressed point you can use the decodePoint()-method on the ECCurve:

ECPoint point = spec.getCurve().decodePoint(pubKey);