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Re: (Fwd) Authentication and encryption.
James P. Hughes says:
> I agree that encryption by itself does not create authentication.
Good! Makes two of us (:-).
> If the encryption is strong and the probability of garbage passing the
> integrity check is low enough (2^-64 or less),
> then the knowledge of the shared
> secret should be authentication of the source. If one can prove
> that to create
> a valid integrity check one must calculate it against the data
> in the clear and
> prove that the attacker must know the key. If you know only the
> sender (and
> you) know the key, then authentication is proven.
I don't think so, because smart attacker could devise methods to
tamper with the data without disturbing a "normal" integrity check,
designed to stop garbage (i.e. not a malicious attack). So it all
boils back again to the issue of how smart an integrity check
should be, and then - whether an application is wiling to
accept that extra burden...
> Why would this not be acceptable for general traffic authentication
> when the traffic is already being encrypted?
Because you don't know what the "real" data should decrypt into.
--
Regards,
Uri uri@watson.ibm.com acheron!angmar!uri N2RIU
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<Disclamer>
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