[messaging] The Simple Thing

[Joseph Bonneau]

Let me try to summarize this thread (as I understand it) since I've been
lurking and I think there may be some connections between ideas missing.
Here's an attempt at outlining how MITM detection would work in two
discussed cases as I understand it:

CT-style (I think we should call it CT-style to avoid confusion with
Certificate Transparency proper for TLS certificates)
*Alice looks up Bob's key.
*The Evil Log inserts a spurious key for Bob. We're assuming (I think
almost all of us are willing to assume this) that log-consistency auditors
ensure the log has to actually put the spurious key into a globally
consistent log forever. Trying to locally fork Alice's view is too risky if
some non-zero proportion of users gossip out of band.
*Later on (after up to the MMD) Bob gets a ping from his monitor that "a
new key for Bob has been logged." Bob concludes that the Evil Log is evil.
Alice learns nothing.

The Simple Thing
*Alice looks up Bob's key. Two versions seem to have been discussed at
different points:
     Version (a)-Alice gets it directly from Bob over an untrusted channel.
     Version (b)-Alice gets it from a semi-trusted key directory/service
provider for Bob's address.
*In Version (a), a MITM simply changes Bob's transmitted key. In Version
(b), the Evil Directory signs a spurious key for Bob and returns it to
Alice.
*Ideally, Alice asks Bob out-of-band if this new key is correct before
sending anything. If so, Bob detects the attack and warns Alice not to
send. In Version (b) Bob furthermore concludes that the Evil Directory is
evil.

The assessment is that CT-style allows only the recipient to detect the
attack, after the fact, and The Simple Thing allows the sender to detect
the attack before sending. To me this wasn't the most intuitive summary-in
both cases it's only the intended recipient (Bob) who can be certain an
attack took place and that the Evil Log or Evil Directory has been evil.

The difference is whom you need to be "paranoid" (or just perceptive). The
Simple Thing detects attacks if the sender is paranoid and actually insists
on preemptive fingerprint checks and CT-style detects attacks if the
recipient is paranoid and has monitoring alerts set up and actually checks
them.

"Being paranoid" means slightly different things of course: setting up
monitoring vs. doing fingerprint checks. Without hard data we can't really
be sure, though intuitively it seems to me that setting up monitoring and
checking against your own recent activity is probably easier. For one
thing, in a CT-style system each key change should only require one check
(by Bob) whereas with The Simple Thing each key change of Bob's requires
all of his paranoid contacts to initiate a fingerprint check.

The costs also seem more naturally aligned in CT-style systems: if Bob
changes keys more often he's the one that has to do more checking of
reports from monitors, whereas in The Simple Thing frequent changes by Bob
impose a burden on others.

So CT probably has some usability advantages, at the cost of complexity and
extra parties (auditors, monitors) needing to operate.

A seemingly-obvious point I haven't seen yet: it's perfectly natural to
have both systems in place. Nothing prevents layering The Simple Thing on
top of a CT-style log. Paranoid Alice can certainly check out of band if
she looks up a new key for Bob in the log and it's different from what
she's used previously. Paranoid Bob can set up monitoring. Now you get
detection if either sender or receiver is paranoid.

On Fri, Oct 3, 2014 at 7:54 PM, Tao Effect <contact at taoeffect.com> wrote:

> Dear elijah,
>
> On Oct 3, 2014, at 11:43 AM, elijah <elijah at riseup.net> wrote:
>
> In the auditing-infrastructure thing, the hope is that user agents will
> be written to smartly and automatically perform the auditing. Yes, it is
> detection after the fact. The prediction is that the number of people
> running an auditing user agent will be greater than the number of
> senders doing fingerprint verification, and that this greater number
> will provider greater deterrent against bogus key endorsements.
>
>
> In the CT world, auditing and monitoring are two very different things,
> and they must not be confused.
>
> Auditing does not detect mis-issued certificates/keys/whatever before the
> fact, during the fact, or after the fact [1].
>
> Kind regards,
> Greg Slepak
>
> [1]
> https://blog.okturtles.com/2014/09/the-trouble-with-certificate-transparency/
>
> --
> Please do not email me anything that you are not comfortable also sharing with
> the NSA.
>
>
>
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>
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