A team of Israeli scientists have uncovered a possible means of cracking the GSM mobile phone network encryption code, opening the door to attacks that could enable eavesdroppers to listen into calls.
The GSM Association, a trade group for suppliers and mobile network operators, is downplaying the problem. It admits a potential vulnerability exists but argues that this would be very difficult to exploit in practice.
Researchers Professor Eli Biham and doctoral student Elad Barkan, and Nathan Keller, all of the Technion Institute in Haifa, discovered basic weaknesses in the encryption scheme used in GSM networks.
According to Biham, the attack allows an eavesdropper to tap into a conversation while a call is been set up and a phone at the receiver's end is still ringing. After this, a conversation can be overheard.
"Using a special device it's possible to steal calls and impersonate callers in the middle of a call as it's happening," Biham told Reuters.
The security loophole arises because of a fundamental mistake made by GSM developers in creating a system which corrected for interference of the line prior to encrypting a conversation, he explained.
The researchers have produced a paper, Instant Ciphertext-Only Cryptanalysis of GSM Encrypted Communication, documenting their findings. The paper was presented at last month's Crypto Conference in Santa Barbara, California but news of this alarming discovery only broke yesterday.
The GSM Association admits the Israeli researchers are onto something but say the attack requires the use of complex technology, which few phone phreakers have access to, and would need to be targeted at a specific caller.
According to Reuters, the Association claims an attacker would have to "transmit distinctive data over the air to masquerade as a GSM base station". An attacker would also have to be placed between a caller and a base station to intercept a call, it adds.
"This (technique) goes further than previous academic papers, (but) it is nothing new or surprising to the GSM community. The GSM Association believes that the practical implications of the paper are limited," it said in a statement.
The Association said an upgrade to the A5/2 encryption algorithm, available since July 2002, addresses the security weaknesses highlighted by the Israelis.
Biham disputes this and says the new encryption system, which was put in place in response to previous attacks, is itself vulnerable.
Both parties agree that the issue does not affect 3G phones, which use different protocols and security mechanisms than legacy GSM handsets. ®