[cryptography] Let's go back to the beginning on this

Marsh Ray marsh at extendedsubset.com
Fri Sep 16 12:21:55 EDT 2011


On 09/16/2011 03:58 AM, Ben Laurie wrote:
> On Fri, Sep 16, 2011 at 8:57 AM, Peter Gutmann
> <pgut001 at cs.auckland.ac.nz>  wrote:
>> Marsh Ray<marsh at extendedsubset.com>  writes:
>>
>>> The CAs can each fail on you independently. Each one is a potential weakest
>>> link in the chain that the Relying Party's security hangs from. So their
>>> reliability statistics multiply:
>>>
>>> one CA:   0.99      = 99% reliability
>>> two CAs:  0.99*0.99 = 98% reliability
>>> 100 CAs:  0.99**100 = 37% reliability
>>
>> I realise that this is playing with numbers to some extent (i.e. we don't know
>> what the true reliability figure actually is), but once you take it out to what
>> we currently have in browsers:
>
> We could have a stab at it. A = Integral of number of CAs in trusted
> root/number of years CAs have been around = ? (I'd guess 100?).

This data could probably be collected quite accurately with some code 
archeology.

> B = Total failures/number of years = ? (1, maybe?)

Difficult to know quantitatively, even about the present.

Iran may have the dubious distinction of the first CA-issued MitM to 
have failed. I don't believe it was the first to have occurred, but up 
until very recently, some asserted that it had never happened.

> So failure rate = A/B = 1% p.a.
>
> giving reliability of 99% p.a.. What do you know?

It's got to be worse than that because we know that for several months 
in 2011 there was an attacker with the who could mint himself whatever 
certs he wanted from Comodo and DigiNotar (he claims to still have this 
capability from several others). As far as I can tell, my Android phone 
is still vulnerable to his *.*.com certificate. Given that SSL has only 
been around about 16 years, the SSL-PKI cannot be more than 97% reliable.

> Anyone got better numbers?

There are some complicating factors that make it difficult for this 
reliability analysis to get better than a back-of-the-envelope upper bound.

* The actual number of organizations knowing private keys with the 
ability to sign something the user's browser will trust is unknown. Yes, 
that's right, CAs refuse to disclose how many and to whom sub-CAs have 
been issued. We only get a lower bound from the SSL Observatory.

* A 'failure' of a CA is probably often perceived by a small set, or 
only one, user. Presumably some of these sub-CAs have been loaded into 
SSL intercepting firewalls (e.g., BlueCoat devices) by corporations, 
this is one of the arguments given for issuance. Whether or not you 
believe that that is legitimate, it certainly seems plausible that a 
guest on the network could be intercepted without prior informed consent.

* The degree of independence between the trusted root CAs and their 
sub-CAs is not well understood (the 150 vs 600 vs 1500 debate).

* The issuance of a cert, a user's reliance on a cert, successful and 
failed attacks on users, attacks on CAs, etc. are discrete events and 
may be better modeled stochastically. Bayesian methods may be better at 
dealing with the presence of the large unknowns. (Alas my own Bayes-fu 
and Markov-fu are not presently up to the challenge).

Whether you arrive at 37% or 99% reliable, any honest analysis will show 
the current system is ineffective against a significant set of adversaries.

- Marsh



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