A5/2 is a stream cipher used to provide voice privacy in the

GSM The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...

cellular telephone protocol. It was designed in 1992-1993 (finished March 1993) as a replacement for the relatively stronger (but still weak) A5/1, to allow the GSM standard to be exported to countries "with restrictions on the import of products with cryptographic security features". The cipher is based on a combination of four

linear-feedback shift register In computing, a linear-feedback shift register (LFSR) is a shift register whose input bit is a linear function of its previous state. The most commonly used linear function of single bits is exclusive-or (XOR). Thus, an LFSR is most often a ...

s with irregular clocking and a non-linear combiner. In 1999,

Ian Goldberg Ian Avrum Goldberg (born March 31, 1973) is a cryptographer and cypherpunk. He is best known for breaking Netscape's implementation of SSL (with David Wagner), and for his role as chief scientist of Radialpoint (formerly Zero Knowledge Syste ...

and David A. Wagner cryptanalyzed A5/2 in the same month it was reverse engineered, and showed that it was extremely weak – so much so that low end equipment can probably break it in real time. In 2003, Elad Barkan,

Eli Biham Eli Biham ( he, אלי ביהם) is an Israeli cryptographer and cryptanalyst, currently a professor at the Technion - Israel Institute of Technology Computer Science department. Starting from October 2008 and till 2013, Biham was the dean of t ...

and Nathan Keller presented a ciphertext-only attack based on the

error correcting codes In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...

used in GSM communication. They also demonstrated a vulnerability in the GSM protocols that allows a

man-in-the-middle attack In cryptography and computer security, a man-in-the-middle, monster-in-the-middle, machine-in-the-middle, monkey-in-the-middle, meddler-in-the-middle, manipulator-in-the-middle (MITM), person-in-the-middle (PITM) or adversary-in-the-middle (AiTM) ...

to work whenever the mobile phone supports A5/2, regardless of whether it was actually being used. Since July 1, 2006, the

GSMA The GSM Association (commonly referred to as 'the GSMA' or ''Global System for Mobile Communications'', originally ''Groupe Spécial Mobile'') is an industry organisation that represents the interests of mobile network operators worldwide. More ...

(GSM Association) mandated that GSM Mobile Phones will not support the A5/2 Cipher any longer, due to its weakness, and the fact that A5/1 is deemed mandatory by the 3GPP association. In July 2007, the 3GPP has approved a change request to prohibit the implementation of A5/2 in any new mobile phones, stating: "It is mandatory for A5/1 and non encrypted mode to be implemented in mobile stations. It is prohibited to implement A5/2 in mobile stations." If the network does not support A5/1 then an unencrypted connection can be used.

References

External links

A5/2
at CryptoDox
A5/2 withdrawal
at security.osmocom.org * Ian Goldberg, David Wagner, Lucky Green. The (Real-Time) Cryptanalysis of A5/2. Rump session of Crypto'99, 1999. * * Tool for cracking the GSM A5/2 cipher, written by Nicolas Paglieri and Olivier Benjamin
A52HackTool
(with full source code – C language – GNU GPL) {{DEFAULTSORT:A5 2 Stream ciphers Broken stream ciphers

See also

References

External links