Address a few bugs in IsoMdlPresentation and modify MDoc

[rschulman]

Makes IsoMdlPresentation init public and adjusts the code that retrieves the signing key from the key store to match that which is currently in the CA DMV wallet. Also removes an unused init argument from the MDoc init function and renames another argument.

[cobward]

retrieves the signing key from the key store to match that which is currently in the CA DMV wallet.

@rschulman I'm wondering if we should alter the API so that the calling code passes the private key into this function? It looks like there are two ways of looking up the keypair - ApplicationTag, which CA DMV currently uses, and ApplicationLabel which might be more "correct"?

Alternatively, we could provide an interface like this:

public class KeyRetrieval: NSObject {
    /**
     * Retrieves a private key by alias.
     */
    public func retrieve(keyAlias: String) -> SecKey? {
        // ...
    }
}

Which could then be overridden in the CA DMV app.

We could also use the KeyManager class that already exists in the codebase, but my feeling is that it is not future-proof as it supports limited curves, encryption and signing methods, so may end up going away in the medium term.

[sbihel]

I agree with Jacob that fetching the key "by hand" isn't the best way if it turns out there are multiple ways of referring to a key in the secure element.

About the private init, it's because this function was meant to be used all the time as MDoc presentation always supports multiple MDocs. But I can understand if it's necessary for certain UX flows.

[rschulman]

I've pushed an update requiring the consumer to pass in the signing key.