Turn an byte encoded Key back into it's original ECPublicKey in Bouncy Castle

Question

In Java I have a ECDH public Key that I am sending as a byte array.

Once I have received the byte array how can I turn it back into a public key?

I am using Bouncy Castle but a Java solution would be just as useful.

Thanks

Answer 1

How are the raw bytes formatted for the ECDH key? Where are you getting the raw bytes?

Generally, one uses the appropriate *Spec class to turn raw key material into a Key but the ECPublicKeySpec and DHPublicKeySpec classes don't accept a raw byte array.

Answer 2

When you got the encoded key, assuming you used the default "[your keyPair].getPublic().getEncoded()" method, this will work.

    X509EncodedKeySpec ks = new X509EncodedKeySpec(pubKeyByteString.toByteArray());
    KeyFactory kf;
    try {
         kf = java.security.KeyFactory.getInstance("ECDH");
    } catch (NoSuchAlgorithmException e) {
        log.error("Cryptography error: could not initialize ECDH keyfactory!", e);
        return;
    }

    ECPublicKey remotePublicKey;

    try {
        remotePublicKey = (ECPublicKey)kf.generatePublic(ks);
    } catch (InvalidKeySpecException e) {
        log.warn("Received invalid key specification from client",e);
        return;
    } catch (ClassCastException e) {
        log.warn("Received valid X.509 key from client but it was not EC Public Key material",e);
        return;
    }

Answer 3

I found the above solution by @LaceCard didn't work for me. In general this isn't obvious but then again nothing in cryptography is ;)

String key = "MihU9ztW9sEvkBL6BxyaOMgkSbodNS8yoHaHcio+WE...blahblah"
byte[] keyBytes = Base64.decode(key);

//If using Android and Spongycastle provider should be "SC"
KeyFactory kf = KeyFactory.getInstance("ECDH", "BC");
//CURVE_NAME e.g prime192v1
ECParameterSpec ecSpec = ECNamedCurveTable.getParameterSpec(CURVE_NAME);
ECPoint point = ecSpec.getCurve().decodePoint(keyBytes);
ECPublicKeySpec pubSpec = new ECPublicKeySpec(point, ecSpec);

ECPublicKey myECPublicKey = (ECPublicKey) kf.generatePublic(pubSpec);

Note: you'll need to handle the potential exceptions appropriately