I assume you're talking about confidentiality and authenticity. If all you care 
about is authenticity then you can proceed under the assumption that the 
channel /may/ be authentic and then later perform the authentication to 
retrospectively authenticate it. This is obviously "duh", but it's also how 
modern protocol negotiation works.

Matt

On Jun 6, 2013, at 2:32 PM, Jonathan Katz <[email protected]> wrote:

> Isn't it obvious? (I mean, there is some value in formalizing the model, but 
> still...)
> 
> Consider authentication of A to B. If there is nothing distinguishing 
> (impersonator) Mallory from (honest) A, then anything A can do can also be 
> done by Mallory.

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