Sankha Subhra Dey SSD

1
SIP Authentication

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

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 users current location, match with
  their capabilities and preferencesin order to
  deliver invitation
• Carry opaque session descriptions
• Modification of sessions
• Termination of sessions

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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

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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

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SIP Digest Authentication

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

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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

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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

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Digest Authentication Calculation

• Request-digest ltgt ltKD (H (A1), unq
  (nonce-value)?
• nc-value
• unq (cnonce-value)?
• unq (qop-value)?
• H (A2)?
• ) ltgt
• 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

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SIP REGISTER with DIGEST Authentication
Proxy Server
User Agent
REGISTER ltltAoRgtgt (with out credentials)?
407 Proxy Authentication Required
REGISTER ltltAoRgtgt (password encrypted with key)?
200 OK
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SIP INVITE with DIGEST Authentication
UA
UA
Proxy Server
INVITE ltltuser1_at_domain1gtgt (without credentials)?
407 Proxy Authentication Required
ACK
INVITE ltltuser1_at_domain1gtgt (with encrypted
password)?
100 Trying
INVITE ltltuser1_at_domain1gtgt (password removed)?
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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

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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 - ltsips alice_at_atlanta.comgt

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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)?

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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

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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

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Information Sources

• RFC 3261(2002)?
• RFC 2671 (1999)?
• Jonathan Rosenberg, Dynamicsoft, at VoN
  Developer's? Conference, 2001 (www.jdrosen.net/pap
  ers/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/DigestAuthe
  nticationReport.pdf)?