Cryptography Reference
In-Depth Information
SIM chip in the handset
computer at the base station
SRAND
Ki
Ki
algorithm A8
algorithm A3
algorithm A8
algorithm A3
SRES
SRES
SI M
Basis
compare
authenticate
session key
session key
ciphertext
voice
algorithm A5
algorithm A5
voice
(encryption unit
in handset)
(encryption unit in
receiver handset,
or at interface to
wireline network)
Figure 6.2:
Authentication and creating session keys in GSM networks.
has the phone's home network send it sets of
SRAND, SRES
, and
Kc
for
authentication and encryption, referred to as
triplets
, in advance. This explains
why roaming contracts have to be stipulated among GSM providers in several
countries before you can use their networks. Roaming contracts are signed for
billing reasons, but also for cryptological purposes: if a subscriber switches on
his German cell phone in Italy, the Italian network must be able to see that
it needs to fetch the
SRAND, SRES
, and
Kc
triplet from Germany (triplets are
normally kept in stock after the first call).
You may have noticed that the first logon abroad takes much longer than the
connections you established afterwards. The reason is clear now: for the first call,
the network abroad has to procure triplets, while it will have some in stock later
on. As a sideline, in all these activities, the secret
Ki
never flows over a network;
it remains in the network provider's database computer and in the handset.
