Re: [openpgp] Dealing with clock skew

Benjamin Kaduk <kaduk@mit.edu> Mon, 18 November 2019 09:50 UTC

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Date: Mon, 18 Nov 2019 01:50:01 -0800
From: Benjamin Kaduk <kaduk@mit.edu>
To: Jon Callas <joncallas=40icloud.com@dmarc.ietf.org>
Cc: Claudio Luck <claudio.luck@pep.foundation>, openpgp@ietf.org, Jon Callas <joncallas@icloud.com>
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Subject: Re: [openpgp] Dealing with clock skew
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On Sat, Nov 16, 2019 at 03:06:13PM -0800, Jon Callas wrote:
> 
> 
> In the general case, you can't consider a time measurement to be a scalar, it has to be at the very least a complex number of the form [time, skew]. As Derek noted, Kerberos used a skew of five minutes. While Neal Walfield noted in his original post that he's seen skew of 20min, I concur that that seems a bit long. My naive home set-up commonly has alarms across devices being ±2s or less, but that's because they're all getting time from some combination of NTP and cellular network time, which is ultimately GPS time (and of course, skew). I think five minutes is likely reasonable, but *some* skew is unavoidable. Moreover, anyone who's on satellite networks is seeing latency of over a second and once you throw in normal exponential backoff, five minutes seems about as short as is reasonable.

I believe that if Kerberos was starting over now, the 5 minutes would be
seen as excessively long, FWIW.

-Ben