Cryptology ePrint Archive: Report 2006/424

Security Analysis of Voice-over-IP Protocols

Prateek Gupta and Vitaly Shmatikov

Abstract: The transmission of voice communications as datagram packets over IP networks, commonly known as Voice-over-IP (VoIP) telephony, is rapidly gaining wide acceptance. With private phone conversations being conducted on insecure public networks, security of VoIP communications is increasingly important. We present a structured security analysis of the VoIP protocol stack, which consists of signaling (SIP), session description (SDP), key establishment (SDES, MIKEY, and ZRTP) and secure media transport (SRTP) protocols. Using a combination of manual and tool-supported formal analysis, we uncover several design flaws and attacks, most of which are caused by subtle inconsistencies between the assumptions that protocols at different layers of the VoIP stack make about each other.

The most serious attack is a replay attack on SDES, which causes SRTP to repeat the keystream used for media encryption, thus completely breaking transport-layer security. We also demonstrate a man-in-the-middle attack on ZRTP, which allows the attacker to convince the communicating parties that they have lost their shared secret. If they are using VoIP devices without displays and thus cannot execute the ``human authentication'' procedure, they are forced to communicate insecurely, or not communicate at all, i.e., this becomes a denial of service attack. Finally, we show that the key derivation process used in MIKEY cannot be used to prove security of the derived key in the standard cryptographic model for secure key exchange.

Category / Keywords: cryptographic protocols, voice-over-ip

Publication Info: 20th IEEE Computer Security Foundations Symposium (CSF)

Date: received 16 Nov 2006, last revised 30 Apr 2007

Contact author: shmat at cs utexas edu

Available format(s): PDF | BibTeX Citation

Version: 20070430:133916 (All versions of this report)

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