A5/4 spec

Hi all,

recently I've been mentioning A5/4 in a couple of meetings. I've found the (already approved) change requests that introduce A5/4 by coincidence today:

http://www.3gpp.org/ftp/tsg_sa/TSG_SA/TSGS_45/docs/SP-090635.zip http://www.3gpp.org/ftp/tsg_sa/TSG_SA/TSGS_46/docs/SP-090821.zip

So basically A5/4 (and GEA4) use a 128bit Kc, and this mandates the use of an USIM in USIM application mode. Old GSM SIM cards are not sufficient.

So A5/4 will be the first algorithm that uses a different Kc and will thus not suffer from semi-active downgrading attacks (like you can do with old phones and A5/2 to break A5/1 or A5/3, or you can do today with A5/3 if you later use the same RAND on an A5/1 encrypted session).

Regards, Harald