Universal Password System?

[monty.harder@famend.com (MONTY HARDER)]

  I was thinking some more (look out, this could be dangerous) about the
concept of using some kind of H(challenge+password) system to keep
passwords away from ____(your threat model here)____, when it hit me
that we could devise a standard password system, which would allow Joe
Schmoe to have a single password for all of his interactions with
puters.

  Ideally, Joe would need a "smart card" or PDA with IR link (this could
even be the proverbial Windows Watch) that would not need to keep the
actual passphrase at all (but would insure against a compromised system
recording keystrokes) that would keep the pubkeys of all systems with
which he has accounts. It would also need to be able to display in
decimal and hex for systems without the IR link.

  When he is making connections to a new system, the system will give
him its S and RSA or other public key K, so that the smart card can
compute K( H(S+P) ), and send that as the password. To the system, Q =
K"( K( H(S+P) ) ) =is= the password, but Joe only needs to remember P
for everything.

  From then on, logons will include the system sending S and a
non-reproducible challenge <C,D,....> (where C is iterated
less-frequently and D is time.of.day) and the smart card responding with
K( H( D + H(C+Q) ) ). As you may recall, the idea of the multi-part
challenge was so as to allow the admin of the system to store Q
remotely, and keep C -of-the-day and H(C+Q) for each user on the system
itself. With appropriate safeguards (a physical switch on the case of
the system which kills the NVRAM chip with the key for the secure file
system, this would seem to be Pretty Secure.

  The system is extensible, allowing further nesting of challenge parts
within the hash/concatenation function, so that layers of security can
be used, if anyone can find an application for them. The basic principle
of the master passphrase for all uses would make it easier to get Joe to
use one that he can remember, without giving up anything to corrupt
administrators (I have a hell of a time remembering all the passwords
for every system, and must let the comm program remember them,
protecting it with another password.  Messy.)

  Comments?



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