Re: OPIE Challenge sequence



Thank you so much for your responses. By "predetermined ", i meant the
challenges appear sequentially in decremented fashion, so are we aware of
any security hole with this. I ask this because usually the
challenge/response implementations consider generating random challenges( i
think here they have a weakness where the passphrase need to be in clear
text).


My problem is to determine the best challenge/response implementation for
authenticating the clients.


Please correct me if i missed something.

Thanks and Regards,
Ivan

On Tue, Jul 8, 2008 at 5:00 PM, Peter Jeremy <peterjeremy@xxxxxxxxxxxxxxxx>
wrote:

On 2008-Jul-08 15:46:37 +0530, Ivan Grover <ivangrvr299@xxxxxxxxx> wrote:
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).

The passphrase is not encrypted - it is hashed and cannot be "decrypted".
Basically, the passphrase and seed are concatenated and the result is
hashed (using MD5) the number of times specified by the iteration count
and the seed, count and final hash are stored in /etc/opiekeys.

The supplied response is easily verified because when you run it thru
MD5, you should get the hash in /etc/opiekeys. You then replace that
hash with the one the user supplied.

-- is it possible to generate random challenges using opiechallenge

No. The seed has to match the seed that was used to generate the
hash with opiepasswd.

--
Peter Jeremy
Please excuse any delays as the result of my ISP's inability to implement
an MTA that is either RFC2821-compliant or matches their claimed behaviour.

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