>> Okay. What's the value for X - T that prevents this problem, but doesn't >> cause DKIM signatures of "normal" mail to fail? > > There's not one "right" value; we're talking about distributions > of timings for normal mail vs. replay, and yes, there's some > overlap there. In practice I've seen many signers choose > expirations in the range of 1hr to a few days. 1hr can be very > good at limiting the opportunity for high volume replay, but I > estimate "normal" signature breakage at that level is on the > order of 0.1%. 24hr is probably effectively zero breakage, but > with greater opportunity for replay.
I think you're way off on these numbers, especially for the 1-hour case. While normal circumstances get mail delivery in less than an hour, I have seen *many* cases of legitimate mail delayed by hours -- sometimes quite a few hours. I would consider anything less than two days to be unacceptable, and with that sort of gap you don't do much to prevent a spam blast. Barry _______________________________________________ Ietf-dkim mailing list Ietf-dkim@ietf.org https://www.ietf.org/mailman/listinfo/ietf-dkim
