<a href="http://codahale.com/a-lesson-in-timing-attacks/" rel="nofollow">http://codahale.com/a-lesson-in-timing-attacks/</a><p>Suggests 20us over internet, 100ns over lan. Are there more complexities to comparing HMACs than are mentioned in this article? I.e. anything else to think about other than not short-circuiting your comparisons when bytes don't match?<p>More discussion (including from someone called Nate, presumably the same person tptacek is referring to) at:<p><a href="http://groups.google.com/group/keyczar-discuss/browse_thread/thread/5571eca0948b2a13" rel="nofollow">http://groups.google.com/group/keyczar-discuss/browse_thread...</a><p>And a paper which I can't read without paying:<p><a href="http://www.computer.org/portal/web/csdl/doi/10.1109/MSP.2009.165" rel="nofollow">http://www.computer.org/portal/web/csdl/doi/10.1109/MSP.2009...</a>