netkey - calculate response to authentication challenge
calculates a response to a challenge made by a system to authenticate a user,
based on a shared secret (password), using
the same algorithm as a SecureNet device.
It reads and writes
It prompts once for the secret (echo is turned off).
It then repeatedly prompts for a remote system's challenge,
and once given it, calculates and prints the corresponding response.
It exits on an empty challenge or end of file.