KDF API review, round 2

[Jamil Nimeh]

> You're missing the point that setParameter() provides information used 
> in all future calls to the signature generation, while init() provides 
> data specifically for a given key stream production.  In Signature() 
> you call .setParameter() to set up the PSS parameters (or use the 
> defaults).  Each subsequent call to initSign or initVerify uses those 
> PSS parameters.  The  equivalent part of .init() in KeyDerivation is 
> actually the calls to .update() in signature as they provide the 
> specific information for the production of the output key stream.  In 
> fact, setting up an HMAC signature instance and passing it the mixin 
> data as part of a .update() is a way of producing the key stream round.
>
> So equivalences:
>
> KeyDerivation.getInstance(PRF) == Signature.getInstance(HMAC)
> KeyDerivation.setParameters() == Signature.setParameters()
> KeyDerivation.init(key, List<Parameters>) == concatenation of the 
> results of multiple calls (each key stream round based on the needed 
> output length) to [Signature.initSign(Key) followed by 
> Signature.update(converttobytearray(List<Parameters>)) followed by  
> Signature.sign()] to produce the key stream
> KeyDerivation.deriveKey() ==  various calls to key or object factories 
> with parts of the key stream (signature).
>
Are you expecting that setParameters is called once per instantiation?  
If so, then the parameters that would go into setParameter (an APS I 
assume) could just as easily go into the getInstance call.  It also 
removes the chance that someone would call it twice.

If you're expecting someone to call setParameter more than once, then I 
would expect an init must follow.  So why not place it in a form of init 
that allows you to change that particular set of params?  Either way it 
seems like we could coalesce this method into one of the calls that 
sandwich it in your proposed model.