New 128-bit GSM Encryption Cracked in Just 2 Hours

Posted 01/15/2010 at 3:47pm | by Bart Salisbury

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Talking away on a GSM wireless network? Might as well be talking through tin cans tied to a string. You’d be getting about the same assurance of privacy. It’s not bad enough the A5/1 security algorithm, which serves as the security backbone for the GSM network, was recently cracked. Now the another security algorithm, the KASUMI system, has also been cracked. And it took scientists at Weizmann Institute of Science in Israel two hours to do it.

The KASUMI is a 128bit A5/3 algorithm, which is implemented across 3G networks. Using a “related-key sandwich attack”, the team of Orr Dunkelman, Nathan Keller, and Adi Shamir tore it apart using a simple PC and two hours time. From their description, it was child’s play. They started with by using one key for encryption of a message, and then changed it to a different key. They write: “By using this distinguisher and analyzing the single remaining round, we can derive the complete 128 bit key of the full Kasumi by using only 4 related keys, 226 data, 230 bytes of memory, and 232 time. These complexities are so small that we have actually simulated the attack in less than two hours on a single PC, and experimentally verified its correctness and complexity.” See, child’s play.

Karsten Nohl, who was the wunderkind behind the cracking of the A5/1 algorithm, says this new research shows that it’s time to reconsider KASUMI and move away from A5/1. “The attack should stand as a reminder that A5/3 and any other cipher will need to be replaced eventually.  Hopefully this fact is considered when upgrading GSM,” said Nohl.

The GSM industry is taking an ostrich approach to these announcements: bury its head and the threat no longer exists. Which means, for the time being, you’d better watch what you say when using your GSM phone.

Image Credit: gsherman/Flickr