1. Cullen Jennings fluffy@cisco.com
S/MIME Certificates
Cullen Jennings

2. E2E SIP Security Requires S/MIME
In order to use S/MIME you need to discover certificates for your peers
This is not Somebody Else’s Problem
If there is no viable work to make certificates available in typical SIP deployments, we can’t base our security on it
Strong, ubiquitous, identity is one of the best tools in dealing with SPAM

3. What the certs draft provides
No extra work on the part of the human using a UA
No extra expense for end user certificates
Enterprise only need to run a web commerce style web server
A revocation mechanism that works

4. Mechanism
a.com
b.com 4 2 1
Caller
Callee 3
Callee with address publishes public certificate at b.com
Does with HTTPS PUT with Digest authentication
Caller wants to call and gets the certificate from b.com
Done with HTTPS GET.
Caller encrypts stuff for Callee
Uses S/MIME in SIP
Callee fetches caller certificate (from a.com) to verify Caller certificate
Uses HTTPS GET
This is easier than - it is an online problem with no long term storage aspect

5. Analysis
Callee
Caller
b.com
2 (HTTPS)
1 (HTTPS+Digest)
3 (S/MIME+SIP)
Callee trusts it is talking to b.com because of the HTTPS certificate. B.com trusts it is bob because of the digest authentication. Transaction is privacy and integrity protected by HTTPS
Caller trusts that it is talking to b.com because of HTTPS certificate and trusts the certificate for is really the right one for bob because it came from b.com
S/MIME is used to encrypt data for Bob using the public key from the certificate for
A similar scheme can be done in reverse so the caller can sign
This is easier than - it is an online problem with no long term storage aspect

6. Relationship with Identity
Identity provides a mechanism to leverage the domains certificate for asserting identity
Certs leverages the domains certificate to provide encryption and signing
The key thing in Identity is that it describes how to describe certain assertions and put them in messages.
It’s not as worried about getting the crypto credentials to do this other than it needs them.
The key thing in Certs is getting the crypto credentials for S/MIME.

7. Relationship to PKIX & Sacred
This work uses the PKIX and SACRED frameworks and security
Using SACRED to move private keys off the UA and onto the server could be done
Generally poor form to have private keys floating around
Will not work for FIPS compliant phones that need to keep the private key in tamper resistant hardware
Certs has good security including revocation.

8. Next Steps - Pick one of below:
Security folks agree this will work from a security point of view. The SIPish folks need to decide if it is deployable.
Move forward with this work
Define transports, HTTP, XCAP, SIP, …
Find an alternative way to use S/MIME
Pursue some web of trust model?
Abandon S/MIME
Find an alternative way to meet the needs.
Kerberos?

9. Questions for the WG Can we deprecate S/MIME?
Is it OK just saying every end user needs to buy a certificate and securely install it in all their devices?
Do we have any other alternatives?
What do we need to fix to move forward with this work?