Trying to put together a semiprivate key system

semiprivate.rb

# Experimenting with implementing semiprivate keys with RbNaCl

require 'rbnacl'
require 'hkdf'

module Semiprivate
  class WriteKey
    def initialize(seed)
      if seed.bytesize != 32
        raise ArgumentError, "seed must be 32 bytes"
      end

      @seed        = seed
      @read_key    = ReadKey.new Crypto::Scalar.mult_base(@seed)
      @private_key = Crypto::Scalar.mult(@seed, @read_key.semiprivate_key)
    end
  end
  
  class ReadKey
    def initialize(bytes)
      hkdf = HKDF.new(bytes)
      
      @semiprivate_key = hkdf.next_bytes(32)
      @symmetric_key   = hkdf.next_bytes(16)

      @verify_key = VerifyKey.new Crypto::Scalar.mult(@semiprivate_key, bytes)
    end
  end
  
  class VerifyKey
    def initialize(bytes)
      @key = bytes
    end
  end
end