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blosxom


Contact/Impressum

       
Fri, 12 Nov 2010
A brief history on the withdrawal of the A5/2 ciphering algorithm in GSM

Recently, I wanted to investigate when and how A5/2 has been withdrawn from both GSM networks and GSM phones alike. Unfortunately there was no existing article discussing this history online, so I went through dozens of meeting reports and other documents that I could find online to recover what had happened.

If you don't know what this is all about: It is about the A5/2 air-interface encryption algorithm that was used in certain GSM networks until about 2005-2007.

A5/2 was specified as a security by obscurity algorithm behind closed doors in the late 1980ies. It was intentionally made weaker than it's (already weak) brother A5/1. The idea was to sell only equipment with A5/2 to the countries of the eastern block, while the less-weak A5/1 encryption was to be used by the western European countries.

A5/2 had been reverse engineered and disclosed in the late 1990ies, and has undergone a lot of attention from cryptographers such as Ian Goldberg and David A. Wagner. In a 1999 paper, they already expect that it can be broken in real-time.

It took several more papers until in August 2003, finally, the proponents of the GSM systems (ETSI/3GPP/GSMA) have realized that there is a problem. And the problem was worse than they thought: Since they key generation for A5/1 and A5/2 is the same, a semi-active downgrade attack can be used to retroactively break previously-recorded, encrypted A5/1 calls. The only solution to this problem is to remove A5/2 from all equipment, to make sure the downgrade is not possible anymore.

Starting from 2004 the security related working groups of 3GPP and GSMA thought about removing A5/2, and in the following years they convinced their respective bodies (3GPP, GSMA), and members thereof (operators, equipment makers) to fix this problem.

Ever since that time, it is known that using the same key generation for different algorithms enables down-grade attacks. However, the key generation for the then-new A5/3 algorithm was unmodified. So now that A5/1 has been broken in recent years, even if the operators deploy A5/3, the same model of down-grading attacks to A5/1 can be done again.

I have put down a time-line at the still mostly-empty security.osmocom.org website. Some of the goodies from it:

  • It took from 1999-2007 until this gaping security hole was fixed. Call that incident response!
  • Unnamed Northern American Operators (and the PTCRB) were the biggest blockers to remove A5/2 support from their networks. This is particularly strange since US operators should always have had A5/1 access.
  • As a breaking of the more secure A5/1 was already anticipated even back then, in 2002 A5/3 was first specified. Five years later (2007) there was virtually no support for A5/3 among manufacturers of GSM network equipment
  • It took until January 2009 until the GSMA discussed A5/3 testing with mobile phone makers
  • It took until November 2009 until there was a plug-fest testing interoperability between A5/3 enabled GSM network equipment and A5/3 enabled phones.

And what do we learn from all this?

  • GSM equipment manufacturers and mobile operators have shown no interest in fixing gaping holes in their security system
  • Prior to that first A5/2 attack, they have never thought of procedures for upgrading the entire system with new ciphering systems (i.e. proactive plans for incident response)
  • Even after the A5/2 disaster, they have not learned the slightest bit. The same problem that was happening with A5/1 - A5/2 downgrade attacks can today be done with A5/3 - A5/1 downgrade attacks. And this before the majority of the operators has even started to use the already-7-year-old A5/3 in their production networks.
  • The security work group of 3GPP has had a lot of insight into the actual threats to GSM security even 10 years ago. You can see that e.g. in the Technical Recommendation 33.801. But nobody wanted to hear them!
  • Similar problems exist with the authentication algorithms. It took 12 years from first practical attacks on COMP128v1 until the GSMA is looking at withdrawing it.

[ /gsm | permanent link ]