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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