[reSIProcate] Security class and client-side authentication (longish)

[Bob Bramwell]

Random responses to random thoughts inline:

Cullen Jennings wrote:
> Random thoughts inline
> 
> I think we should consider a design meeting to sort some of this out.

Sounds good.  How should we go about this?

> 
> A TLS server certainly can act act as a client and is a superset of that.
> 
> The S/MIME certs are different from the TLS certs
> 
> The Identity header uses the TLS cert but performs S/MIME like functions
> 
> The system needs to support CA certs and self signed certs
> 
> The system needs to allow a user to install new root certs

See next inline comment...

> 
> Some servers support a bunch of domains and need to be able to deal with
> multiple TLS certs (one for each domain)

Right.  This was part of the intent behind some changes I made to the various 
addTlsTransport functions a couple of months back.  One can now pass in a 
pre-configured Security object that has all the "right" certificates loaded.

> 
> Clients needs to be able to store fingerprints of non CA signed servers
> certs they are willing to trust

Agreed, but this may be outside the purview of reSIProcate.  I'd hate to see a 
lot of x509 and TLS clutter in the midst of all the other stuff.

> 
> 
> On 10/22/04 5:51 PM, "Bob Bramwell" <bob at jasomi.com> wrote:
> 
> 
>>This note is part reality check and part change proposal.  Your constructive
>>feedback will be most welcome.
>>
>>I am trying to set things up to allow TLS connections to do client
>>authentication.  There are a number of things, notably in the Security class,
>>that will need to change to accomodate this.
>>
>>At present a Security object has two public certificates associated with it:
>>its 
>>publicCert and its publicIdentityCert.  The former is used as the certificate
>>presented to a TLS client when it connects to a TlsConnection created in
>>server 
>>mode.  The latter only appears to be used for signing stuff (computeIdentity
>>and 
>>checkIdentity functions).  No provision is made for a client side certificate
>>per se. 
> 
> 
> uhmm the PublicCert would be used for this

Well, maybe not.  At present the PublicCert is used as the server certificate. 
If we have a situation where the transport is being used as a TLS client and the 
server requires mutual authentication, we *may* want a separate client side 
certificate, mightn't we?  I would have thought that situation might arise in 
practice.

> 
> 
>>However, if the Security object is not actually going to be used to
>>checkIdentity or computeIdentity (these functions do not actually appear to be
>>called anywhere by code in the reSIProcate source tree) the publicIdentityCert
>>*could* be overloaded to act as a client certificate.
>>
>>Question:  *should* the publicIdentityCert be overloaded, or should a
>>            different certificate be attached to the Security object for
>>            this purpose?
> 
> 
> No the identity is claims is wrong. It is going to assert something like
> "fluffy at cisco.com" not "host22.cisco.com"

Fair enough, although it is unclear to me that a Security object will ever be 
used *both* for TlsTransport certificates *and* for S/MIME signing.

> 
> 
>>When a Security object is created it is either a server or a client; this is
>>recorded in the mTlsServer member variable.  When a Security object is used
>>(e.g. by TlsConnection) to obtain an SSL context (using getTlsCtx) the
>>argument 
>>isServer is passed in the call.  Why does this need to be specified in two
>>different places?  What problem is it designed to solve?  The mTlsServer
>>member 
>>variable is only used in getTlsCtx, and it is far from clear what is supposed
>>to 
>>happen if mTlsServer and isServer have different values.
> 
> 
> One can be a server, but not a tlsServer meaning you can act as a server but
> you can not accept incoming TLS connections.

OK... I'll have to digest that one.
> 
> 
>>Considering the situation when a TlsTransport is used both as a server and as
>>a 
>>client, it would be convenient to have a Security object that returned a
>>different CTX depending on the role.
> 
> 
> How would the TLS context be different for the two roles?

Viz. my comment following "uhmm the PublicCert would be used for this" earlier, 
I think (and my understanding may be deeply flawed here) that one *might* want a 
"client context" to have different certificates from a "server context".

> 
> 
>>It therefore seems sensible to remove
>>the 
>>Security constructor tlsServer argument and the mTlsServer member variable
>>altogether.
>>
>>Question: *should* the Security::mTlsServer variable be removed (and all that
>>           that implies)?
>>
> 
> 
> You might be right that this is exactly the right thing to do but I have
> certainly not got my head wrapped around what you are talking about yet

Did my earlier inline comments help?  We can bop this around offline if you 
think it might be useful.

>  
> 
>>The Security object only ever creates one SSL_CTX which it stashes in the
>>ctxTls 
>>member variable.  At present, therefore, if a Security::getTlsCtx call was
>>made 
>>in "client" mode before a later call in "server" mode, the SSL_CTX provided to
>>the server would have no certificate information.  This seems entirely broken,
>>and probably reinforces the case for maintaining at least two SSL_CTX's - one
>>for each role.
>>
> 
> 
> I think there is confusion about server and client mode - and I may be
> confused on how I remember it being and how it really is - server means you
> have a certificate. client mode means you don't. Perhaps these are named
> terribly wrong
> 

At present you are mostly correct, give or take the fact that there are still 
two places you have to specify whether or not this is a server side Security 
object.  My original commentary is not quite correct.  Whether or not a 
certificate is installed is determined by the setting of mTlsServer, effectively 
when the object is created.  However, the scenario I described is still possible:
1. a Security object is created with tlsServer == false
2. getTlsCtx is called with isServer == false;
    a client side SSL_CTX is created and cached with no certificates attached
3. getTlsCtx is called with isServer == true;
    the cached SSL_CTX is returned (with no certificate)

I'm not saying I can think of a sensible way to create these conditions, but I 
simply don't understand why the design even makes it possible.  Any TlsTransport 
may end up being used in either mode, depending on how things get routed.

However, when you say "...client mode means you don't", that goes to the heart 
of the matter.  If I am going to support mutual (client side) authentication I 
*must* have a certificate (or several) and at present I haven't.

> 
>>Finally (I hope) the TLS standard requires that a client certificate is sent
>>only if it is signed (ultimately) by one of the acceptable CAs provided in the
>>certificate request.
> 
> 
> No self signed certs are fine to send - they just must be received carefully

Ummm... not according to the OpenSSL client_cert_cb man page, and I assume they 
base their statement on an RFC of some kind (obviously I haven't looked this up)

> 
>>The Security class could allow the certificate selection
>>to be performed by a caller-provided mechanism by using the
>>SSL_CTX_set_client_cert_cb mechanism.
>>
>>Question: *should* a user of the Security class be allowed to determine how
>>           certificate selection is performed, or should this be nailed down
>>           beyond user interference in the Security implementation?
>>
> 
> 
> Well, let's look at what they would want to be able to control and find a
> way to allow that
>  
> 
>>I think that covers my immediate concerns; at least regarding reSIProcate, as
>>opposed to those about world hunger, AIDS, global warming, etc..  If anyone
>>feels like providing me with a better sense of direction I would be greatly
>>obliged.  In particular, if there are M's I should be RTFing please let me
>>know.
> 
> 
> Glad to see some work on this and happy to help (though likely after IETF)
> 

Thanks.  I may have to make my changes locally and see how things go.  I've been 
stalling this for too long already, and at this point I simply have to make 
something work!  However, it kinda sounds like this is not a widely interesting 
problem, so if I check in some backward compatible mods that are later deemed to 
be a Bad Idea, I assume no one will get too upset.  NOW is the time to tell me 
if I'm wrong about that!


-- 
Bob Bramwell            Jasomi Networks (Canada) | This space
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