class Net::IMAP::SASL::ScramAuthenticator

def recv_server_first_message(server_first_message)
  @server_first_message = server_first_message
  sparams = parse_challenge server_first_message
  @snonce = sparams["r"] or
    raise Error, "server did not send nonce"
  @salt = sparams["s"]&.unpack1("m") or
    raise Error, "server did not send salt"
  @iterations = sparams["i"]&.then {|i| Integer i } or
    raise Error, "server did not send iteration count"
  min_iterations <= iterations or
    raise Error, "too few iterations: %d" % [iterations]
  mext = sparams["m"] and
    raise Error, "mandatory extension: %p" % [mext]
  snonce.start_with? cnonce or
    raise Error, "invalid server nonce"
end