How to designing a KeyPair interface which can handle any KeyPair implementation

I am designing a KeyPair interface which looks something like this,

public interface KeyPair {

    @Deprecated
    public Key firstKey();

    @Deprecated
    public Key secondKey();
}

I want to use this interface to implement two classes, PasswordEncryptionKeyPair, PrivatePublicKeyPair. But the problem is that I don't want to use the methods firstKey() and secondKey() as they are very generic. I rather want to use some other method and make these two method private. My current implementation is like this,

class PrivatePublicKeyPair implements KeyPair{

    public Key getPrivateKey() {
        return firstKey();
    }

    public Key getPublicKey() {
        return secondKey();
    }

    @Override
    public Key firstKey() {
        return privateKey;
    }

    @Override
    public Key secondKey() {
        return publicKey;
    }
}

As an interface cannot have private method I can't make the methods private. One way this is possible is by adding body in the interface itself or create an AbstarctKeyPair class, and make methods private there. Is there any other solution for this problem?