Security and Usability Aspects of Man-in-the-Middle Attacks on ZRTP

Abstract: ZRTP is a protocol designed to set up a shared secret between two communication parties which is subsequently used to secure the media stream (i.e. the audio data) of a VoIP connection. It uses Diffie-Hellman (DH) key exchange to agree upon a session key, which is inherently vulnerable to active Man-in-the-Middle (MitM) attacks. Therefore ZRTP introduces some proven methods to detect such attacks. The most important measure is a so called Short Authentication String (SAS). This is a set of characters that is derived essentially from the public values of the Diffie-Hellman key exchange and displayed to the end users for reading out and comparing over the phone. If the SAS on the caller's and the callee's side match, there is a high probability that no MitM attack is going on. Furthermore, ZRTP offers a form of key continuity by caching key material from previous sessions for use in the next call. In order to prevent that a MitM can manipulate the Diffie-Hellman key e...

• Type: Journal
• Year: 2008
• Where: JUCS
• Authors: Martin Petraschek, Thomas Hoeher, Oliver Jung, Helmut Hlavacs, Wilfried N. Gansterer
• Keywords: Diffie-Hellman Key Exchange, JUCS 2008, Key Exchange, ZRTP