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List:       cryptography-randombit
Subject:    [cryptography] anyone know how OTP tokens work?
From:       chialea () gmail ! com (Lea Kissner)
Date:       2010-09-08 5:27:28
Message-ID: AANLkTimF3EiwPA_EOK9zb_9B3wb6F5RbYSkOr4w_vtk_ () mail ! gmail ! com
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On Tue, Sep 7, 2010 at 2:17 PM,
<travis+ml-rbcryptography at subspacefield.org> wrote:
> I'm curious how OTP tokens work.

Most of them are HOTP (http://en.wikipedia.org/wiki/HOTP) of one form
or another, which is basically an application of HMAC to a secret key
and a nonce of some kind. Some implementations derive the nonce from
the current time. Some derive it from a counter value. Some do
proprietary backflips so as to require you use their server-side
software along with the client tokens.

If you assume HMAC is a perfect MAC algorithm, then the security of
HOTP follows straight from that assumption. An attacker who can forge
a HOTP value must be able to forge a corresponding MAC value. Because
we have assumed that our MAC algorithm is perfect, no such attacker
can exist.

Thanks,
Lea

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