--servercert option is insecure

[David Woodhouse]

On Thu, 2018-05-10 at 01:38 +0000, Ryan Taylor wrote:
> 
> If this is correct, using the servercert option is a significant security problem.
> 
> Perhaps the servercert option is not intended to be used for any sort of security guarantee
> whatsoever. However if that is the case, there should probably be a warning in the man page, and
> also printed on standard output, kind of like overriding a web browser to connect to a site with an
> untrusted certificate. Users who have not done the math or read the man page carefully may be using
> this option with the mistaken belief that it provides some security assurance via a sort of
> makeshift certificate pinning.
> 
> Or, this could be fixed by requiring a complete match of the hash instead of just 4 characters.
> What are the "certain testing use-cases" and how important are they compared to the security
> considerations?

Hi Ryan, thanks for looking at this.

As Dan says, if a user provides a truncated hash then we do require the
match to be at the beginning of the hash. It's not just *any*
substring. Even so, yes: 4 characters isn't many, so there's a fair
chance of a false match. Ultimately, Don't Do That Then.

I'm happy to improve the documentation to make it clear that there is
absolutely no excuse for a user to use anything less than the full hash
in "production" use, or indeed in any circumstance where cut-and-paste
works.

I vaguely recall that I had similar reservations when this feature was
first added (Nikos, was it you?). But I *have* found it useful
occasionally in testing, in strange setups where I haven't been able to
cut and paste for some reason. At the time, we were forcibly removing
the "--no-cert-check" option because I threw my toys out of the pram
after finding people advocating its use in production — so even a 4-
char match was better than that! :)
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