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Re: [ietf-dkim] DKIM Key Sizes

2016-10-27 17:32:16
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On Oct 27, 2016, at 1:29 AM, Peter Goldstein <peter(_at_)valimail(_dot_)com> 

Those guidelines ... specify that receivers MUST validate signatures of 512 
to 2048 bits, but are not required to validate signatures with longer keys - 
may be worth revisiting given advances in computing power, the increasing 
importance of DKIM as an element of email authentication, and the reuse of 
these guidelines in other proposals (i.e. ARC).


I'd like to suggest that it may be a good idea to increase the upper value to 
4096 or even 8192, to ensure that the standard is compatible with best 
practices going forward.

I don't object to increasing it in the standard, but operationally I don't 
think it's a good idea. Per-message processing cost is one reason, but the 
larger one is the semantic value of a DKIM signature, and what it is trying to 

DKIM is a conversation between two administrative domains, and a signature only 
states that an administrative domain is taking responsibility for placing the 
message in the message stream. Nothing more. 

That assurance comes at a cost of whatever integrity that assigns to a message 
that might have been unintended. That message integrity is a privacy loss by 
the users.

The real-world case in point is the leaked Podesta emails, where some people 
have asserted that authenticity can be checked via the DKIM signatures. I've 
raised an eyebrow on that, and the bottom line is that you're presuming that 
attackers were sophisticated enough to steal the email, yet unsophisticated 
enough that stealing the DKIM key from an MTA is out of the question.

The full discussion is pretty nuanced, and I think the relevant part here is 
that if an administrative domain wants to protect the privacy of its users, it 
should be using *smaller* DKIM keys, not larger ones. I think I could 
convincingly argue that a privacy-friendly email provider is better off using 
512 bit keys (where there's a chance of spam forgery) than 4K keys (where 
there's a chance of ruining the privacy of the customers). Now actually, I 
think the sweet spot for key size is above 512, but it's lower than 768. For 
maximum balance, you want the key to take longer than a week to crack, but less 
than a year, and change it every month. Or something like that. You get the 

In any event, I don't object to changing it, but I do think that if someone's 
going to implement long keys, they're going to get criticism about enabling 
surveillance of their users.


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