SMIME Certificates

1
S/MIME Certificates

• Cullen Jennings
• fluffy_at_cisco.com

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 Elses Problem
• If there is no viable work to make certificates
  available in typical SIP deployments, we cant
  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
b.com
a.com
Caller Alice_at_a.com
Callee Bob_at_b.com

• Callee with address Bob_at_b.com publishes public
  certificate at b.com
• Does with HTTPS PUT with Digest authentication
• Caller wants to call bob_at_b.com 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

5
Analysis
2 (HTTPS)
1 (HTTPSDigest)
3 (S/MIMESIP)

• 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 bob_at_b.com 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 bob_at_b.com.
• A similar scheme can be done in reverse so the
  caller can sign

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.
• Its 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?