Subject: [Sip] TLS post connection verification

From: "Eron Stein" <[email-masked]>
Date: Wed, 19 Mar 2003 09:13:46 -0500

Hi,

When sending a request (ie REGISTER) to a server I can compare the request 
URI to the common name (or the alt dns name) in the certificate. If the 
names match, I can conclude that the certificate is OK.
(I'm using OpenSSL, and they recommend this post connection assertion).

I have two questions thou:

1 - What name should I use for comparison when accepting a connection?
Usually only the UAC will demand certificate, I am concerned with te case of 
two proxies trying to connect using TLS and the UAS proxy asking for client 
certificates. (what uri will the UAS proxy has, there is no message yet).

2 - how should broken connection be handled? lets say UAC1 sent a request 
over TLS to UAS1. the handshake went well and the request sent. than for 
some reason, the connection was broken and UAS1 now needs to reestablish the 
connection. What should UAS1 do? use TLS w/out certificates?

Regards,
Eron Stein

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---

From: "Cullen Jennings" <[email-masked]>
Date: Mon, 24 Mar 2003 21:17:59 -0800

If element A wants to send a SIP message over TLS to a next hop called
foo.com, A needs to check that the SubjectAltName in the certificate
provided over the TLS connection to A matches foo.com. Openssl can not do
this for you automatically, A needs to call the SSL_get_peer_certificate
after A has connected. foo.com may request, and look at, the certificate of
A or may not depending on if mutual TLS is being done or not.

There is very little point in using TLS if you don't check that the server
you connected to is the server you meant to connect to. If all you know is
the IP address of the server and the server presents a certificate with a
FQDN, you are pretty much out of luck.

Cullen

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of Eron
> Stein
> Sent: Wednesday, March 19, 2003 6:14 AM
> To: [email-masked]
> Subject: [Sip] TLS post connection verification
>
>
> Hi,
>

---

From: Eric Rescorla <[email-masked]>
Date: 24 Mar 2003 22:30:08 -0800

> A or may not depending on if mutual TLS is being done or not.
Exactly.

> FQDN, you are pretty much out of luck.
Just to be clear, it depends on your threat model. If you believe
that only passive attacks will be mounted, then TLS is worth something
even without checking the certificates. However, active attacks
are getting easier to mount...

-Ekr