[messaging] XEdDSA specification

[Trevor Perrin]

Hi,

I'm happy to announce that a spec for the "XEd25519" signature
algorithm used in Signal is available at [1].

The document also generalizes this signature algorithm to the 448
curve, and extends it to include VRF functionality, which Signal might
use in the future.  These extensions are somewhat new, and should
probably get more public review before people rush to implement.

Feedback is welcome, probably best directed to the mailing list
"curves at moderncrypto.org", which you can join at "moderncrypto.org",
since that focuses on technical aspects of elliptic curve crypto.

If we get editorial or design feedback that is too detailed for that
list, we may create a more specific list for feedback.

Code implementing XEd25519 and VXEd25519 (the VRF extension) can be
found in [2].

[1]
https://whispersystems.org/docs/
https://whispersystems.org/docs/specifications/xeddsa/

[2] https://github.com/WhisperSystems/curve25519-java/


Trevor