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S/MIME Certificates

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

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S/MIME Certificates

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  1. S/MIME Certificates Cullen Jennings fluffy@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 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. 1 2 4 3 Mechanism a.com b.com • Callee with address Bob@b.com publishes public certificate at b.com • Does with HTTPS PUT with Digest authentication • Caller wants to call bob@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 Caller Alice@a.com Callee Bob@b.com

  5. b.com Caller Alice@a.com Callee Bob@b.com Analysis 2 (HTTPS) 1 (HTTPS+Digest) • 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@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@b.com. A similar scheme can be done in reverse so the caller can sign 3 (S/MIME+SIP)

  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?

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