Why does cellular communication protocols use A5 or KASUMI and not AES or 3DES or RSA?

I have read that A5 is not that secure, is this the case?

Are A5 or KASUMI used anywhere else? Why only in cellular?

  • Note that KASUMI is A5/3. A5 is the name of keystream generator protocols, not the cipher itself. At least originally (with A5/1), it was chosen in part because all existing ciphers were too heavy. A newer, lighter one was developed using three LFSRs (which are very light on the hardware). – forest Apr 5 '18 at 5:42
  • So are these implemented on SIM ? cant an applet on SIM perform heavy ones as well , since I have seen all the algorithms being implemented on a smart card applet. – stng Apr 5 '18 at 5:45
  • They are implemented in the mobile device's baseband processor. The SIM does not actively perform any action activity for the device (i.e. it is not a coprocessor). It just functions for authenticating to the network and, for some older or more limited devices, storing SMS messages. – forest Apr 5 '18 at 5:46
  • So where does the key that encrypts the voice call exists ? in the baseband processor? – stng Apr 5 '18 at 5:53
  • 1) limited power 2) compatible hardware – again Apr 5 '18 at 6:06

It seems to me this is for historical and technical reasons. A5/1 and A5/2 seem to be rather old designs from late 1980s. They do not appear to be in use any more. There was no AES at that time and 3DES is block cipher and GSM networks vendors apparently wanted stream ciphers back then. KASUMI seems to have been accepted also around the the time or before AES was announced, so they couldn't have made AES the standard cipher either. Also, AES is USA standard while these GSM network standards are international, and politics is a thing.

  • A5/1 is still very widely in use, used by many billions of people. Either way, this seems to be more of an opinion (though it seems reasonable). Do you have any authoritative sources to back it up? – forest Apr 5 '18 at 5:53
  • Only Wikipedia, but I do not have a reason to disbelieve the dates they mention. – wilx Apr 5 '18 at 5:55
  • @forest: Block to stream cipher requires more memory. If you look at A5/1, it seems rather trivial to implement with few registers and logic circuits. Also, I think you can resynchronize with stream cipher encrypted data stream if bytes get lost but with block ciphers you are lost. I think that is what you might want if you are implement reliable communication technology. But you are right, it is unauthoritative opinion of mine. – wilx Apr 5 '18 at 6:09
  • Yeah that's true, and A5/1 was particularly trivial to implement (just a few LFSRs). – forest Apr 5 '18 at 6:11
  • 1
    Some possible additional information: aftenposten.no/verden/i/Olkl/… – forest May 6 '18 at 2:13

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