On Thu, Aug 28, 2008 at 04:02:52PM -0700, [EMAIL PROTECTED] wrote:
On Thu, Aug 28, 2008 at 03:55:05PM -0700, David Brown spake thusly:
Can this CPU implement a decent hash once a minute? If so, and it
still gets decent power, that would be the easiest way, and there
would be less likelihood of patent issues.
It isn't just a hash you need. You need a unique value that nobody else can
predict. What are you going to hash that would produce such a thing?
The algorithms are well understood.
The key contains a secret, 64-bits in the RSA case. There is a
counter that is a timer roughly synchronized with the host.
Each minute, the fob computes
HMAC(secret, counter)
and displays part of the result in some manner. The host can verify
this, and also use it to help sync up with the timer running in the
fob.
You can look up HMAC, which is specifically designed to allow most of
the computation to be done once on the secret, and just the last round
of the hash done with the new counter value.
A hash is excatly the algorithm you want. RSA's trick is supposed to
be a hash that uses less CPU and gets longer battery life.
David
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