Ed25519 signature functions

This is mostly suitable as part of a pure Elixir solution.

Configuration

No configuration is needed in most cases. However, if needed, a custom hash function can be configured. As per the specification - sha512 is the default.

config/config.exs

use Mix.Config

# The hash function will be invoked as 'Blake2.hash2b(payload, 16)'
config :ed25519,
  hash_fn: {Blake2, :hash2b, [], [16]}

# The hash function will be invoked as ':crypto.hash(:sha256, payload)'
config :ed25519,
  hash_fn: {:crypto, :hash, [:sha256], []}