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Message-ID: <20080516174344.GA2305@openwall.com>
Date: Fri, 16 May 2008 21:43:44 +0400
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Subject: Re: OpenSSH key blacklisting
On Fri, May 16, 2008 at 06:24:51PM +0100, Craig Edwards (Brain) wrote:
> I havent been following this debacle too closely as i dont have much to
> do with debian, however, wouldnt such a system be vulnerable to false
> positives if you are just going to hash partial fingerprints rather than
> whole fingerprints?
Such a system would have a higher likelihood of false positives, yes.
However, it would not exactly be "vulnerable" to them - or at least, the
worst-case impact (depending on server settings) is a DoS for a given
user's ability to login. With 48-bit partial fingerprints, there may be
like one such false positive in the entire world, or none. If we go
down to 40 bits, it's less than one in a million of different keys.
(I am assuming a blacklist size of around 200,000 partial fingerprints.)
In fact, the Debian/Ubuntu patch already uses partial fingerprints based
on my earlier suggestion, but I was more conservative at the time, so I
suggested 80 bits. Willy Tarreau has since convinced me that even as
low as 40 bits is reasonable.
Oh, and we are not "hashing" fingerprints, we're merely matching them.
Alexander
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