Re: [ietf-dkim] DKIM Key Sizes

["Rolf E. Sonneveld" <[e-mail]>]

Hi, Eliot,

On 28-10-16 14:02, Eliot Lear wrote:
> Hi Jon,
>
> On 10/28/16 12:29 AM, Jon Callas wrote:
>>> 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 do.
>>
>> 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.
> I really don't get your point.  Either the thing is worth signing or it
> isn’t.  See below.
>> 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.
> If it’s the same conversation I was engaged in, I think the person was
> just confused.  A simpler approach to plausible deniability is simply
> not to sign.

An end user often cannot choose to sign or not to sign. The end user is 
dependent on what the ESP or the employers organization configures. 
Well, of course one could change ESP... But changing employer is quite a 
step ;-)

/rolf


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