Crypto Questions

[Richard Clayton]

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In article <B6C84E2E.1E5F%peter.fairbrother@ntlworld.com>, Peter
Fairbrother <peter.fairbrother@ntlworld.com> writes

>Two general crypto questions :

there are probably better places to ask them

>1) Is there a way to have reliable authentication that is repudiable? I
>think the concepts are too similar, but it would be useful.

unless you define the terms very exactly (and not in terms of each
other, which would make it easy), I think the question is too general to
answer

>2) Does anyone know of a backwards hash, ie an algorithm that gives a
>previously specified result from a cyphertext but which is infeasible to
>reverse?  How would you generate the cyphertext? No secret keys allowed.

you should probably look at chains of hashes, such as the ideas in Ross
Anderson (et al)'s "Guy Fawkes" protocol:

        http://www.ftp.cl.cam.ac.uk/ftp/users/rja14/fawkes.pdf

- -- 
richard @ highwayman . com                       "Nothing seems the same
                          Still you never see the change from day to day
                                And no-one notices the customs slip away"

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