[CST-2] As a matter of interest...

[Mark Seaborn]

Peter Taylor <pjt33@hermes.cam.ac.uk> wrote:

> Those who attempted the Security question - how did you do the last
> part?  Implement the queue as two stacks using the technique Larry
> Paulson taught us in IA?

No, because the question already gives half the answer, as it suggests
signing each record with a MAC.  I suggested adding a sequence number
i (which you increment for each message) to each record X to make it
(X, i), and signing that.  When you receive records from the terminal,
just check that its sequence number is the one after that of the last
record you received.

If you implemented the queue as a pair of lists, you'd need a burst of
traffic between the smartcard and the terminal to reverse one of the
lists when it becomes empty.

-- 
         Mark Seaborn
   - mseaborn@bigfoot.com - http://www.srcf.ucam.org/~mrs35/ -

        ``We are quite lucky really this year because Christmas
               falls on Christmas day'' -- Bobby Gould