    protected Key engineDoPhase(
        Key     key,
        boolean lastPhase) 
        throws InvalidKeyException, IllegalStateException
    {
        if (x == null)
        {
            throw new IllegalStateException("Diffie-Hellman not initialised.");
        }

        if (!(key instanceof DHPublicKey))
        {
            throw new InvalidKeyException("DHKeyAgreement doPhase requires DHPublicKey");
        }
        DHPublicKey pubKey = (DHPublicKey)key;

        if (!pubKey.getParams().getG().equals(g) || !pubKey.getParams().getP().equals(p))
        {
            throw new InvalidKeyException("DHPublicKey not for this KeyAgreement!");
        }

        if (lastPhase)
        {
            result = ((DHPublicKey)key).getY().modPow(x, p);
            return null;
        }
        else
        {
            result = ((DHPublicKey)key).getY().modPow(x, p);
        }

        return new BCDHPublicKey(result, pubKey.getParams());
    }
Weaknesses in this category are related to errors in the management of cryptographic keys.