Page 60 - From GMS to LTE

46 From GSM to LTE-Advanced Pro and 5G

calculated in the AuC and on the SIM card by using a random number (RAND) and the
secret key (Ki) as input parameters for the A8 algorithm. Together with the GSM frame
number, which is increased for every air interface frame, Kc is then used as input
parameter for the A5 ciphering algorithm. The A5 algorithm computes a 114‐bit
sequence which is XOR combined with the bits of the original data stream. As the frame
number is different for every burst, it is ensured that the 114‐bit ciphering sequence
also changes for every burst, which further enhances security.
To be as flexible as possible, a number of different ciphering algorithms have been
specified for GSM. These are called A5/1, A5/2, A5/3 and so on. The intent of allowing
several ciphering algorithms was to enable export of GSM network equipment to coun-
tries where export restrictions prevent the sale of some ciphering algorithms and
technologies. Furthermore, it is possible to introduce new ciphering algorithms into
already existing networks to react to security issues if a flaw is detected in one of the
currently used algorithms. The selection of the ciphering algorithm also depends on
the capabilities of the mobile device. During the establishment of a connection, the
mobile device informs the network about the ciphering algorithms that it supports.
The network can then choose an algorithm that is supported by the network and the
mobile device.
When the mobile device establishes a new connection with the network, its identity
is verified before it is allowed to proceed with the call setup. This procedure has already
been described in Section 1.6.4. Once the mobile device and subscriber have been
authenticated, the MSC usually starts encryption by sending a ciphering command
to the mobile device. The ciphering command message contains, among other information
elements, the ciphering key, Kc, which is used by the base station for the ciphering of
the connection on the air interface. Before the BSC forwards the message to the mobile
device, however, the ciphering key is removed from the message because this informa-
tion must not be sent over the air interface. The mobile device does not need to receive
the ciphering key from the network as the SIM card calculates the Kc on its own and
forwards the key to the mobile device together with the SRES during the authentication

Ki
A8 Kc
RAND Current frame number

A5/x

114 Bit ciphering frame
Burst to be encrypted Bitwise XOR
delivered by the cipherer
114 Bit original burst
To the modulator
114 Bit ciphered burst
Figure 1.38 Ciphering of an air interface burst.