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376 Chapter 17
Ki (128 bit), RAND (128 bit)
A8
Kc (64 bit)
Figure 17.4 : GSM K c generation
any algorithm that it wishes to generate SRES from K i and RAND. The GSM specification
just uses the name A3 to reference such an algorithm. Similarly, the service provider is also
free to use any algorithm that it wishes to generate K c from K i and the name A8 is just used
by the specification to reference this algorithm. Most GSM implementations combine the A3
and A8 functionality and use a single algorithm to serve both the purposes. The COMP128
algorithm, which is the reference algorithm specified in the GSM specification, takes as input
the 128-bit K i and the 128-bit RAND and generates the 32-bit SRES and a 54-bit number.
The 54-bit number is appended with 10-zeros to form the 64-bit session key: K c . We will see
in Section 17.2.3 how this session key is used for providing confidentiality.
GSM allows the service provider to choose an algorithm for A3 and A8 implementation
while still ensuring seamless roaming among networks of different service providers. This
is an important accomplishment and is achieved because even though the authentication
process is carried out between the ME and the servicing MSC, the servicing MSC utilizes
the HLR of the ME to authenticate the network. Indirectly therefore, it is the home
network of the ME which authenticates the ME into another service provider’s network.
Since the A3 and A8 algorithms need to execute only at the HLR and the SIM 9 (both of
which “ belong to ” the service provider), they can be proprietary algorithms.
One of the finer details of the authentication process in GSM is the use of five sets of
security triplets that the MSC gets from the HLR. Even though only one set of triplets is
required for authenticating a subscriber into the network, five sets are requested so as to
improve roaming performance. Realize that a ME needs to authenticate with a MSC each
time it enters its network from another service provider’s network. Instead of contacting
the HLR for security triplets each time a ME roams into its coverage area, the MSC gets
five sets of triplets: one for the current authentication process and four for future use. This
reduces the roaming/handover time and improves system performance.
9 The A3 and A8 algorithms are implemented in the SIM.
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