Short Message Service (sms) security solution for mobile devices

3. GSM 
Authentication 
 
The GSM network uses a challenge-response mechanism for 
authentication [12]. Figure 3 shows the authentication process.
Figure 3. GSM Authentication Mechanism 
A 128-bit random number (RAND) is generated by the HLR and sent to 
the Mobile Station (MS). The MS encrypts the RAND by using the authentication 
algorithm (A3) and the individual subscriber authentication key (Ki). The output is 
a 32-bit signed response (SRES) that is sent back to the network. Upon 
receiving the signed response (SRES) from the subscriber, the GSM network 
repeats the same computation to produce SRES’. If SRES and SRES’ are the 
same, the identity of the subscriber is authenticated. If SRES and SRES’ do not 
match, the connection is terminated and an authentication failure message is 
sent to the MS. 

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Throughout the entire authentication process, the individual subscriber 
authentication key (Ki) is never transmitted over the radio channel. Ki is only 
present in the SIM, AuC, HLR, and VLR. The calculation of the signed response 
is processed within the SIM to protect confidential subscriber information such as 
the IMSI or Ki.
4.
Data Confidentiality 
Data confidentiality is achieved through the use of the key generation 
algorithm (A8) and the encrypting algorithm A5. Figure 4 illustrates the encryption 
process.
Figure 4. GSM Encryption Mechanism 
The SIM uses the random number (RAND) used in the authentication 
process and the individual subscriber key (Ki) to generate the 64-bit cipher key 
(Kc) based on the key generating algorithm (A8). Once generated, the cipher 
key is used as the key to the A5 algorithm for subsequent encryption of data 

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between the Mobile Equipment (ME) and the Base Transceiver Station (BTS). 
On the network end, the Kc is generated in the same manner by the HLR and 
passed to the BTS. After the authentication process, a cipher mode request is 
sent to the ME to decide on the cipher to use. Once the cipher is agreed upon, all 
subsequent radio traffic between the ME and the BTS is encrypted using Kc. The 
same Kc is used for the entire session of communication. The GSM standard 
allows for regular key change through re-authentication of the ME for added 
security. However, this is not implemented for many systems. As a result, the 
same Kc may be used for days. Similar to the authentication process, the 
computation of the ciphering key (Kc) takes place within the SIM. Therefore, the 
individual subscriber authentication key (Ki) does not leave the SIM.