Re: [ietf-dkim] DKIM Key Sizes

["John R. Levine" <[e-mail]>]

> You're precisely correct that when we see people saying that
> somehow "John Podesta sent X" means "John Podesta said X" is not
> something that DKIM was out to solve.  DKIM CAN'T solve that problem,
> and if we attempt to architect it to do so, I'd recommend calling it
> something else, because it surely will have very little to do with
> Domain-based authentication.

The point of DKIM is to attach an identifer to messages which you can use 
in combination with the identifier's reputation to do stuff.  If the 
messages were from some random linux box, you couldn't conclude anything 
beyond the fact that the signatures are valid today. But they're not, 
they're from gmail about which we know a lot, and we also know that the 
messages were collected by spear phishing a specific account, not by 
attacking gmail's internal security.

Put together the DKIM signatures and what we know about the signer, that 
gmail is rather picky about what they sign and is very unlikely to have 
signed a backdated message, and that their internal security is quite 
good, and you can reasonably conclude that the valid gmail signatures on 
the Podesta messages mean the messages are real.

To get back to the previous argument, if you don't want people using DKIM 
to validate old messages, rotate the keys more often.  Deliberately weak 
signatures strike me as a poor alternative.  We can know exactly when a 
key was withdrawn (mine rotate monthly, with the old keys going away on 
the 10th of the following month) but we can only guess who might be able 
to crack or fake a key and even more so whether someone faked a particular 
signature.

Regards,
John Levine, [e-mail], Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly
_______________________________________________
NOTE WELL: This list operates according to
http://mipassoc.org/dkim/ietf-list-rules.html