OPIE Challenge sequence

[Ivan Grover]

Hi ,
Iam trying to choose OPIE as my OTP implementation for authenticating the
clients. I have the following queries, could anyone please let me know these
-- why does the challenge in OPIE are in predetermined form..
is it for determining the decryption key for the encrypted passphrase(stored
in opiekeys).
-- is it possible to generate random challenges using opiechallenge


Any pointers/links will be very much helpful.

Regards,
Ivan