Re: [ietf-dkim] DKIM Key Sizes

Jon,

Thanks for the detailed expansion of your views.  I agree with you that
we often do not state in our documents semantic intent.  Part of that is
that we think in terms of building blocks, where semantic intent can
actually change.  Part of that is perhaps that vendors don't want to
expose all the intent they have.  In this vein, I agree with Jim's view
of history that while we considered the minimum time a signature needed
to be validated, we never really considered the maximum.  We were, I
think, quite focused on short term delivery management.

I still don't think signature size is the thing to chase here, re
privacy.  If DKIM has done its job, then quite frankly whether or not
the signature is in the message is irrelevant since one can look at the
domain and establish some level of confidence regarding the sending
domain and therefore the individual.  In effect, the From line is the
supercookie you're worried about, only the sender provides it.

Eliot


On 10/31/16 12:05 AM, Jon Callas wrote:

On Oct 28, 2016, at 5:02 AM, Eliot Lear <lear(_at_)cisco(_dot_)com> wrote:

* PGP Signed by an unknown key

Hi Jon,

I really don't get your point.  Either the thing is worth signing or it
isn’t.  See below.


No problem. That you say, "either the thing is worth signing or it
isn't" shows that you don't get it. I'll try to explain 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.

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.  And further, just because a person can hack someone’s
personal message store, as seems to have happened to Podesta, doesn’t
mean that they can hack the private key out of Google.  Furthermore,
using the example above, the key would actually have to be around for a
while (e.g., not rotate out of DNS).  In fact, if the key really is
intended to verify the source over a longer period of time than, say,
delivery, certainly 512 is not enough.


Yup! I agree completely. If the key is intended to verify the source
longer than delivery, then 512 is not enough.

And -- the key is *PRECISELY* intended to verify the source *NO*
*LONGER* than delivery. 512 probably still isn't quite enough, since
you can break a 512-bit key too easily. If I lick my finger and stick
it into the wind, 600-700 bits is about where they should be.

512 is slight exaggeration for effect. You can make it work, but you
really need to put some infrastructure in place to make it work well.
But I belive that 512 is a better choice than 8K. Or even 4K. But not
for the same reasons.


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 idea.

Color me obtuse, but are you worried about the incentives to break into
the user's account or are you worried about the provider wanting to peer
into users' email in order to assure itself that it’s not spam?



Ah, I see I'm going to have to at least snuggle, if not embrace the
fuller discussion.

My issue -- not a worry -- is the *semantic* value of the signature.

Many, if not most protocols we deal with in the IETF are strictly
syntactic. We don't look at the meaning, we don't look at the
internals of them. Often this is both good and intended. Look at
IPsec, TLS, SSH, OpenPGP, S/MIME, and on and on, these are syntactic
standards.

In some of them, the unspoken semantic issues are a wart. In S/MIME
and OpenPGP, there's a huge semantic issue as to what a signed message
*means*. I had a co-worker (at both Pretty Good Privacy, Inc. and PGP
Corporation) who would not sign his emails. The reason he wouldn't is
that he said that we didn't know what signed email meant and that he
didn't want to end up finding out twenty years later that his signing
a message was suddenly a legal commitment that he didn't intend.

This is also an interesting long discussion. Is a signature on an
email nothing more than an source-directed integrity check? Does it
mean a little more, such as the digital equivalent of letterhead? Does
it mean even more than that, that it's 'official'? Or more than that,
that it's certified true and accurate? I know where I stand on it,
which is far more towards the integrity check or at most letterhead. I
might even roll my eyes, but it's a not unreasonable concern that I
simply happen to disagree with.

My last bit if setup and groundwork is to gesture towards Phil
Rogaway's essay of last year on crypto and its social aspects, which
is such a big thing that I'm just going to say, "Thanks, Phil," and
move on.

All of this setup is relevant here because the mission of DKIM is to
stop a certain class of message forgeries. These message forgeries are
/ were a danger to the public health of the Internet. DKIM addresses
them. Note that I used the word "address" not "solve." SPF also
addresses them, in a different way. So does a lot of other network
infrastructure, as well, as well as the upper parts of the DKIM stack
like DMARC.

I think it was Mark Delaney's words in his mission statement for the
DK part of DKIM that it lets Alice's ISP know that a message from her
from her bank actually came from her bank, even when forwarded through
her alumni association. DKIM gives source attribution that travels
with the message.

However, that source attribution has another side to it. That other
side is that at its core, one might say a DKIM signature is in a
similar moral space as the "super cookies" that some providers put
into web connections (like Verizon, who the FTC fined $1.3M). It is a
tracking thing that is put without the user's consent into their email
that can be used for identification later without their knowledge.
Worse, it's not merely a cookie that's a constant, it's a
content-specific digital signature. Shock horror.

Now of course, the counter-argument is about the semantic layer. Yeah,
syntactically, there's a resemblance, but the reason they're different
is many-fold and has to do with purpose, use, and so on.

For one, the "super cookies" are a conversation between the network
provider and an ad network that does not benefit the user in any way.
They are an *exploitation* of the user. A DKIM signature benefits us
all through that initial use case. They add to the public health of
the Internet as a whole and the email infrastructure at large. I can
mount a vigorous defense of DKIM, but I could also use DKIM in a
defense of tracking cookies. I believe I could also shoot down that
analogy.

I also think that we can improve DKIM in a lot of ways operationally
that further breaks whatever improper analogy there is. We also ought
to re-examine a lot of the email infrastructure with a post-Snowden
eye, and that eye should include a post-Yahoo-scanning scandal as
well. We really should look at how much the email ecosystem as a whole
has become part of the broader surveillance structure of the Internet.

These include:

- MDAs should strip out DKIM signatures. If they did, my whole silly
comments about key sizes are moot. It means that they aren't part of
the whole permanent record of email stores and their insecurity
because of the peculiar legal status of email.

- MDAs should also be stripping Received headers and a bunch of
others. Received headers write in stone the geoposition someone was at
when they pressed send. It's a 4-space pin in the map. You can track
people in their business travels by looking through old mailing list
archives. You can see what type of computer they run, what MUA they
are using, and their upgrade patterns. All the better to spearphish
them with.

- That goes double for you, Mailman. And any other list archiver.

These and more are part of why email is so broken these days. It was
made for an environment when we all used terminals on timesharing
systems and things that made sense then (putting logging and debug
information into a message) do not make sense in a world of mass
surveillance. Cleaning up old messes is always hard, and this is
another one.

But anyway, back to DKIM --

The problem here -- DKIM being a super cookie -- can be handled any
number of ways. One of those is to rotate keys frequently. Really, big
ISPs like Google and Yahoo and Apple and Microsoft ought to be putting
out keys with one week of use and two weeks of life. That doesn't
solve the whole problem because there's no reason why The Internet
Archive or other entities can't just start archiving the DKIM public
keys. Moreover, even if you rotate the keys, that merely downgrades
the DKIM information from being a cryptographically authenticated
super cookie to an unauthenticated super cookie (like the ones Verizon
was using).

The comments I made that started this -- cryptographically weak DKIM
-- is also a partial solution at best. But I want to swing back to my
point, which is that we crypto people have a tendency to look at
crypto parameters and just blithely start using them everywhere,
looking at the surface structure of what we're doing and not the deep
structure.

Part of the deep structure is that DKIM had cryptographic
authentication post-delivery as something between an non-goal and an
anti-goal. We didn't want to have to force MUAs to have to play with
them, and didn't even really *want* MUAs to do it, certainly not long
term. Yahoo had plans to show DKIM status in their web client, but
only for a short term. Yes, your email came from your bank. Moreover,
this is why DKIM is really important for banks, and not at all
important for domains like callas.org.

Back in pre-Snowden days, it was hard to argue this sort of privacy /
metadata / traffic analysis stuff without looking like a looney. I
also realize that DKIM is not a *mere* tracking cookie, it serves and
succeeds at improving the public health of the Internet. My wry points
about weak-keyed DKIM are there in part to look at the whole issue.
The good that DKIM provides is not improved by 8K keys. It's not
improved by 4K keys, or even 2K keys (at the time of this writing).
That's because a DKIM key needs to be alive for no more than a week,
and in the vast majority of cases not even an entire minute! The
threat model of DKIM is also such that if someone managed to
compromise a DKIM key, they are only able to authenticate a spam message.

Yes, yes, I know that that "only" could empty someone's bank account,
but the defense against that adversary is a defense-in-depth strategy
which has a series of filters and checkpoints and the whole point of
defense in depth is that each screen plays a part in the filter as a
whole. My point is that DKIM is designed to be effective with lower
levels of cryptographic security and that's part of why it's so cool!
Increasing the crypto in just about *any* system is strengthening the
strongest link in the chain. The DKIM link in the anti-fraud,
anti-spam email defense does not benefit with even 2K keys. It
benefits by having operations management rotate those keys and having
operations management look at the broader problem of cruft in our
basement of email -- like the way headers in general add to the
surveillance-friendly problems of email stores and list archives.

Like many security issues, crypto is only part of the complete
breakfast. DKIM headers are an issue because they provide long-term
cryptographic surveillance. But the surveillance tags are there in
non-cryptographic places, too, and it is wrong to say that we don't
need to fix them because well, the park is still dirty after we clean
up our litter. We must clean up our litter and get others to clean
theirs up, too.

    Jon

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