[cryptography] server-signed client certs (Re: SSL is not "broken by design")

ianG iang at iang.org
Mon Sep 26 08:17:41 EDT 2011

On 26/09/11 16:49 PM, Adam Back wrote:
> What about introducing the concept of server signed client certs.  A 
> server
> could recognize its own server key pair signature on the client cert, 
> even
> though the server cert is not a proper CA cert.

Hmmm... interesting idea!

Typically, the server application will store the entire cert in its db 
as account info.  So analysing the signature as own-CA would save that 
database lookup.  This might make sense if we are relying on say Apache 
to do all the processing, because it is automatable, and Apache doesn't 
have much in the way of database processing capabilities.

Although typically, I would suggest that Apache should be put in 
pass-thru mode and let the application do *all* the login processing.  
By this I mean, analyse the crypto results from the certs, and put it 
all in the PHP variables.  Never ever drop the connections on its own 
decisions, in my experience, letting Apache make security decisions like 
that always results in lousy user performance, which reduces overall 
security (user does it another way).

(It's an interesting idea ... )

> Then the password request
> on the client goes to the browser/os key store.  So long as you had CA
> pinning that would help the phishing situation.


> Of course there is still the UI problem to somehow have the user 
> detect the
> difference between the key store password dialog and a fake dialog put 
> up by
> a hostile web page.  There are some things that can help like user 
> cutomized
> dialogs where the hostile site cant know the customization.



> Adam
> On Mon, Sep 26, 2011 at 07:52:20AM +1000, ianG wrote:
>> On 25/09/11 10:09 AM, James A. Donald wrote:
>>> On 2011-09-25 4:30 AM, Ben Laurie wrote:
>>>> I'm just saying I think its hard to detect when a password is being
>>>> asked for as part of the risk assessment.
>>> http and https do not know there are such things as logons.  Logons 
>>> need to be built into the protocol, rather than added on top.  Your 
>>> browser should know you are logged on.
>> When using client certs, it works: the browser, the server and https 
>> do know if you're logged on [0].
>> The problem with HTTPS login is that it was sacrificed at the alter 
>> of some unworkable commercial dream, thus forcing developers to rely 
>> on passwords.  Any client cert is better than the current best saved 
>> password situation, because the technical security of a public key 
>> pair always exceeds a password [1].  But while vendors will slave to 
>> make saving passwords easier (so as to cope with the explosion of 
>> sites & contexts) ... they won't work to make client certs better.
>> All of this (again) aligns well with key continuity / pinning / and 
>> various other buzzwords.  But, really, you have to try it. There's no 
>> point in talking about it.
>> iang
>> [0]  Where, logged in means, is using an appropriate client cert.  
>> This involves an amount of code in the application to figure out, but 
>> it seems about the same amount of code as doing the login the other 
>> way, via passwords and so forth.  There are some additional 
>> complications such as new certs, but this is just coding and matching 
>> on the names.
>> [1] this deliberately ignores 1980s advice to remember your password...
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