This specification of the A5/4 encryption algorithms for GSM and ECSD, and of the GEA4 encryption algorithm for
GPRS has been derived from TS 55.216
. The only essential change is the change of external key length input from
64 bits to 128 bits.
This document should be read in conjunction with the entire specification of the A5/3 and GEA3 algorithms
The normative part of the specification of the block cipher (KASUMI) on which the A5/3, A5/4, GEA3 and GEA4
algorithms are based can be found in TS 35.202