The information ciphered on the Um
interface involves signaling, speech, and data. The
implementation of ciphering guarantees
the information security and prevents user information or conversation contents
from unauthorized access.
The ciphering procedure is initiated on
the network side. The BTS and MS cipher and decipher
the information by using the A5
algorithm and the ciphering key (Kc) generated by the A8
algorithm on the basis of the
capability of the MS and BTS. Thus, the security of the information on the Um
interface is ensured. The Kc is generated by the GSM authentication center
(AuC) and stored in the MSC/VLR. The Kc is sent to the BTS before the ciphering
procedure begins.
The MS and the network adopt the A8
algorithm to generate the Kc by using the same Ki and random number (RAND).
A ciphering or deciphering sequence is
generated through the A5 algorithm on the basis of the Kc stored in the MS and the network and
the frame number from the current pulse stream. The network uses the same
ciphering sequence in the uplink and downlink. For each burst, the data is
ciphered or deciphered as follows:
·
One
sequence is used for the MS ciphering and BTS deciphering.
·
The
other sequence is used for the BTS ciphering and MS deciphering.