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Sankha Subhra Dey (SSD) ‏ Computer Science, Columbia University VoIP Security Prof. Henning Schulzrinne November 5, 2008

Sankha Subhra Dey (SSD) ‏ Computer Science, Columbia University VoIP Security Prof. Henning Schulzrinne November 5, 2008. SIP Authentication. Discussion Topics. Overviews SIP Authentication Authentication mechanisms TLS Overview HTTP digest, S/MIME

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Sankha Subhra Dey (SSD) ‏ Computer Science, Columbia University VoIP Security Prof. Henning Schulzrinne November 5, 2008

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  1. Sankha Subhra Dey (SSD)‏ Computer Science, Columbia University VoIP Security Prof. Henning Schulzrinne November 5, 2008 SIP Authentication

  2. Discussion Topics • Overviews • SIP • Authentication • Authentication mechanisms • TLS Overview • HTTP digest, S/MIME • Issues with SIP authentication mechanisms

  3. SIP: Overview • SIP is an application layer protocol • Text-based • Based on a request-response model • End users negotiate stream details using SDP • All clients register with a registrar server for a domain • Main Functions • Invite users to sessions • Find the user’s current location, match with their capabilities and preferencesin order to deliver invitation • Carry opaque session descriptions • Modification of sessions • Termination of sessions

  4. SIP: Authentication • How do we define authentication? • Identifying an object • Knowing that the identity of an object is same as what the object claims • Used to identify the following cases: • REGISTER • INVITE • Re-INVITE • BYE • Mechanisms • HTTP digest authentication (RFC 3261)‏ • Basic authentication (deprecated, will not be discussed)‏ • Transport layer mechanism • Using S/MIME for authentication

  5. Some possible attacks in the absence of SIP authentication • Replay Attacks • Simple • Replay a timestamped message (can be detected)‏ • Replay a message that did not reach destination (cannot be detected)‏ • Registration Hijacking • Request Spoofing • INVITE • BYE • CANCEL

  6. SIP Digest Authentication • Both ends use the same shared secret key. • The key is used to encrypt certain information such as the user’s password. • Originated from HTTP, and is often called HTTP digest. • RFC 3261 describes how digest authentication is applied to SIP.

  7. Digest Authentication Mechanism • Server receives INVITE request • Server sends a PAR response containing a realm, algorithm and a nonce value (among other fields)‏ • Realm is the associated domain • Nonce is a random number (prevents replay)‏ • Algorithm to be used for digest calculation (say, MD5)‏ • Client computes a response using the nonce and the username and a secret password • The password is assumed to be stored by the server as well • Client sends back original request with computed response

  8. Digest Authentication Mechanism (contd.)‏ • Other important header components and motivation behind them: • Nonce: could be a digest of client IP address and a time-stamp • Prevents replay attacks (attacker must use correct IP address before the time stamp expires)‏ • Cnonce (optional): generated, stored and sent to the server by client • Client can vary the input to this hash rather than let the server choose it • Server must include it in response • Nonce-count: server maintains its own copy of this count • Protects against replay attacks • Qop (optional): Quality of protection (auth: authentication, auth-int: authentication and integrity)‏ • Username and password: unique information known only by the client and the server

  9. Request-digest = <”> <KD (H (A1), unq (nonce-value)‏ “:” nc-value “:” unq (cnonce-value)‏ “:” unq (qop-value)‏ “:” H (A2)‏ ) <”> where A1 = unq (username-value) “:” unq (realm-value) “:” passwd A2 = Method “:” digest-uri-value KD (secret, data) = string obtained by applying the digest algorithm on data with secret H (data) = string obtained by applying the checksum algorithm to the data unq (X) = value of quoted string X, without the surrounding quotes Digest Authentication Calculation

  10. SIP REGISTER with DIGEST Authentication Proxy Server User Agent REGISTER <<AoR>> (with out credentials)‏ 407 Proxy Authentication Required REGISTER <<AoR>> (password encrypted with key)‏ 200 OK

  11. SIP INVITE with DIGEST Authentication UA UA Proxy Server INVITE <<user1@domain1>> (without credentials)‏ 407 Proxy Authentication Required ACK INVITE <<user1@domain1>> (with encrypted password)‏ 100 Trying INVITE <<user1@domain1>> (password removed)‏

  12. TLS: Overview • Transport Layer Security (TLS) is a version of Secure Sockets Layer • TLS/SSL is the basis for web security • HTTPS = HTTP over TLS/SSL • Functions • Server to client (optionally, vice versa) authentication using public keys • Negotiation of shared private session key • Encryption of all messages once the connection has been established

  13. Secure SIP using sips • SIPS is a low cost means of encryption. • It specifies TLS (transport layer security) over TCP • Not subject to tear down attacks • Same technology used for SSL. A SIPS call will fail rather than complete insecurely. • Example - <sips: alice@atlanta.com>

  14. Using S/MIME for authentication • Carry replicates of SIP header fields inside a MIME body. • Enables authentication by signing the replicated header fields • Helps to verify the identity of the sender • RFC 3261 • Replication of all header fields inside a MIME part. • Problems with this proposal • SIP header fields might get altered by intermediate SIP entities.

  15. Using S/MIME • Problems with RFC 3261 S/MIME proposal (contd.)‏ • Makes it difficult for recipients to identify the legal or malicious changes. • SIP messages can be large in size, • causes overhead for processing and transporting of messages • Solution: RFC 3893 • Proposed to overcome these problems (discussed in my previous presentation on SIP Identity)‏

  16. Issues with authentication mechanisms • Digest authentication • Lack of securing all headers and parameters • Only protects Request URI and method fields • Requires preexisting user configurations on server • SIP over TLS • Only allows hop-by-hop authentication • S/MIME • Public keys difficult to distribute and maintain

  17. Summary • Overview of SIP and Authentication • HTTP digest authentication • Overview of TLS • Secure SIP using sips • Using S/MIME for authentication • Issues with SIP authentication mechanisms

  18. Information Sources • RFC 3261(2002)‏ • RFC 2671 (1999)‏ • Jonathan Rosenberg, Dynamicsoft, at VoN Developer's‏ Conference, 2001 (www.jdrosen.net/papers/devconfw2001_proxies.ppt)‏ • Stephen Kingham, SIP workshop, Tokyo, 2005 (www.apan.net/meetings/tokyo2006/presentation/sip-DNS-Authentication-Peering-SRK1.ppt)‏ • Qi Qiu,Study of Digest Authentication for SIP (www.site.uottawa.ca/~bob/gradstudents/DigestAuthenticationReport.pdf)‏

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