SIP SecuritySIP Security

Henning SchulzrinneColumbia University

04/19/23 52nd IETF SLC

Priority security Priority security requirementsrequirements REGISTER protection

authentication and integrity confidentiality (harder)

DOS prevention for non-authenticated requests authenticated requests already prevent DOS and

amplification, but not realistic for INVITE End-to-end authentication

for random clients (very hard) for repeat visitors

End-to-end message confidentiality

04/19/23 52nd IETF SLC

Re-using existing Re-using existing technologytechnology Options include:

Enhanced C/R (digest) authentication IP DOS prevention S/MIME Shared secret via common infrastructure Transport-layer security

Pointless to argue about which we don’t need – all have different strengths and weaknesses

Does not preclude new mechanisms

04/19/23 52nd IETF SLC

Enhanced digestEnhanced digest Protect selectable subset of headers Minimal extension to Digest Ease of implementation – trivial

addition to existing Digest No infrastructure No privacy REGISTER

04/19/23 52nd IETF SLC

IP-reachability securityIP-reachability security DOSA prevention: Simply ensure that

INVITE comes from valid IP address Inherent in Digest, but not likely to be

common for INVITE Require guessing of large random number Must be stateless in server Options:

NULL authentication Special Digest qop value

Does not prevent use as reflector

04/19/23 52nd IETF SLC

S/MIMES/MIME Existing solution, existing code Treat SIP message like email

attachment: Content-Type: message/sip ??? Requires client certs?

What if ssh-style security is sufficient (same host as last time, but can’t prevent MiM for first attempt)

04/19/23 52nd IETF SLC

Shared secretShared secret Avoid SIP-PGP mistakes:

canonical form header ordering special headers

SIP part is easy once infrastructure is assumed (CMS?)

04/19/23 52nd IETF SLC

Automating future trustAutomating future trust Authentication not very helpful for

random callers as long as identities are cheap – yes, it’s indeed slimebag@coolmail.com

Want to ensure subsequent call is from same person

D-H works except for active MiM – ssh has the same problem!

04/19/23 52nd IETF SLC

Transport-layer securityTransport-layer security TLS works for server authentication

Is this indeed sip.example.com? Works well iff

number of peers small (some evidence in DNS measurements – Zipf distribution)

setup delay for new peers reasonable (need measurements!)