1. SIP Security
Matt Hsu

2. Agenda SIP Security Overview SIP Security Mechanisms SIP Threat Models
Summary
Reference

3. SIP Security Overview How to insure security for SIP call setup
Register protection, DoS…..
NAT, Firewall Traversal of RTP Media packets

4. SIP Security Mechanisms
End-to-end mechanisms
Basic authentication
Digest authentication (similar to HTTP digest)
Message body encryption using S/MIME
Hop-by-hop mechanisms
Transport Layer Security (TLS)
IP Security (IPSec)
The SIPS URI schema
Security Mechanism Agreement for the Session Initiation Protocol (SIP) RFC 3329

5. Basic authentication Horribly Vulnerable to Replay Attack
Client
Server
Horribly Vulnerable to Replay Attack
Cleartext Password
Deprecated in New RFC
INVITE
401 Authorize Yourself
WWW-Authenticate:
Basic realm=“mufasa”
INVITE
Authorization: Basic
QWxhZGRpbjpvcGVuI==
200 OK
Base 64 encoded

6. SIP Digest authentication
SIP Server
SIP Client
REQUEST
Generate the
Nonce value
CHALLENGE
Nonce, realm
Compute response = F(nonce,
Username, password, realm)
REALM: A string to be displayed to users so they know which username and password to use. This string should contain at least the name of the host performing the authentication and might additionally indicate the collection of users who might have access. An example might be
The challenge includes a realm parameter
and a domain list, which together define the protection space of the requested
resource; a nonce value which is calculated into the credentials to protect against replay
attacks; an opaque value, which is returned unchanged by the client; a stale flag,
which indicates whether the previous request from the client was rejected because of
a stale nonce value; an algorithm parameter, which identifies the algorithm to be used
in the calculation of the credentials; qop option, which specifies the depth of integrity
protection.
F= MD5
REQUEST
Nonce, realm,
Username, response
Authenticate: compute
F(nonce, username, password, realm)
And compare with response

7. SIP Digest authentication
This mechanism is borrowed from HTTP Authentication: RFC 2617 but modified slightly
Client Authentication
No message integrity protection
No confidentiality

8. S/MIME A IETF standard for email security Mutual authentication
INVITE SIP/2.0
From:
To:
Content-Type: multipart
A IETF standard for security
Mutual authentication
Payload integrity and confidentiality
Big overhead
SDP
INVITE SIP/2.0
From:
To:
Content-Type: SDP
SDP text
signature
certificate

9. IPSec Authentication and integrity Replay protection
Supports TCP and UDP
IKE barely supported
Not usually integrated with SIP application
Policy managed at the OS level

10. TLS Authentication, integrity, confidentiality Replay protection
Supports TCP only
Resides in application layer
Firewall and NAT Traversal

11. SIPS URI Schema
New URI schema

12. Security Mechanism Agreement for the Session Initiation Protocol (SIP)
Client List
Client
Server
Server List
Turn on security
Server List
Ok or Error
Security Agreement Message Flow

13. SIP Threats Model Registration Hijacking Impersonating a server
The server could be impersonated by an attacker
Tampering with message bodies
Tearing down sessions
Insert a BYE message
Denial of Service attacks

14. Summary
CPL-SL (in master thesis) could solve some SIP security threats

15. Reference SIP Security Agreement RFC 3329
SIP Security Mechanisms Update, Ben Campbell
An overview of SIP Security, Samir Chatterjee