Wednesday, August 18, 2010

Attacking Mobile Communications

Today our (Stephen Williams and I) favourite paper at Crypto'10 is "A
Practical-Time Related-Key Attack on the KASUMI Cryptosystem in GSM and
3G Telephony" by O. Dunkelman, N. Keller and A. Shamir. It isn't an easy
task to describe differential attacks in a blog entry in the first place
and their new sandwich attack uses differentials in a very artful manner
so we're not going to elaborate on the details of the attack except to
notice that they spread the previously known boomerang attacks over
additional rounds somewhat similar to super s-box attacks. (Except of
course, there are no s-boxes involved, neither in boomerang nor in
sandwich attacks.)

However we would like to point out a couple of things:
  • The authors do not claim to have broken A5/3. They explicitly point out that due to the operation mode of KASUMI in A5/3 they haven't found a way to apply their attack on the entire protocol. We think that this is an important reminder of how much depends on the mode of operation.
  • The attack also reminds us how important it is to take proper care of differentials in the design of ciphers: KASUMI is a simplified version of MISTY and while KASUMI has suffered a serious blow today, MISTY stands untainted.
  • After listening to the talk today, our confidence in A5/3 hasn't grown. We don't know whether ETSI/SAGE (who designed KASUMI) wanted it to be less secure than MISTY but we consider it to be precariously close to being a failure equal to A5/1 and A5/2. NIST has generated a lot more trust in AES by its open design process than ETSI/SAGE managed to generate for A5/3.
  • We greatly appreciate that development of A5/4 is on its way already; at the rump session ETSI/SAGE called for participation in the evaluation of the proposed new algorithms which are based on ZUC. The relevant links are and the GSM webpage.

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