Feedback on EdDSA API

[Adam Petcher]

I'm looking for some initial feedback on the proposed JCA API for 
EdDSA[2], which I have documented in a draft CSR ticket[1]. Any 
comments, concerns, suggestions, etc are appreciated.

To summarize, the API for EdDSA looks a lot like the API for 
X25519/X448. Like X25519/X448, it does not allow the selection of 
arbitrary curve parameters. Though the API is flexible enough that it 
could be extended to allow more flexibility in the future (or 
immediately using provider-specific classes), if necessary.

The EdDSA API is a little bit more complicated than X25519/X448, because 
public keys are points that cannot be expressed with a single 
coordinate, and because the signature scheme takes additional 
information related to the mode (pure, prehash, context). So we need a 
couple more classes to handle these complications, but otherwise the API 
is essentially the same as X25519/X448.

[1] https://bugs.openjdk.java.net/browse/JDK-8190219
[2] https://tools.ietf.org/html/rfc8032