Regarding Pubkey Enumeration

[Damien Miller]

This is a deliberate feature - it allows testing whether a pubkey can
log in without the need to unwrap a private key, an action that may
require a passphrase or token PIN.

It's been discussed a bit here and elsewhere in the past and we've
always concluded that it isn't worth turning off or providing a knob
for.

On Fri, 20 Jan 2012, Dan Kaminsky wrote:

> HD Moore from MetaSploit has noted that, given a pubkey (and not the
> corresponding private key, as might be found in authorized_keys), he can
> determine if he'd be able to log into an account.
> 
> It's a small thing, but he's using it for very interesting
> recon/deanonymization.  He'll be releasing a paper shortly, not overplaying
> the characteristic, but certainly showing it can be used to do cute things.
> 
> I expect this is easily fixable -- simply provide the challenge for a
> pubkey whether or not it'd actually be able to log in successfully.  But
> it's worth exploring this space -- perhaps some clients behave badly.
> 
> --Dan
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