Information security enhancement by increased randomness of stream ciphers in GSM
Ram Prakash Prajapat, Rajesh Bhadada, Giriraj Sharma · 2023
Information security is a crucial issue and needs to be addressed efficiently. Lots of researches are being carried out in this field now a day. Encryption of the original information is used to ensure privacy during exchange of information. In Global System for Mobile (GSM) standard, when any new or existing user requests to get access to the resources of the network, an authentication process takes place, for each location update. Once the voice traffic initiates after signaling, encryption comes into the picture to ensure privacy during the call. Here in this process, the plaintext is encrypted in to cipher-text using stream ciphers. Ciphers are basically pseudo-random number (bits) generators. For stronger security, strong ciphers with strong randomness are required. Linear Feedback Shift Registers (LFSRs) based AS algorithm family is used for encryption in GSM. The A5/1 cipher is stronger and most widely used. While A3 is used for authentication and A8 supports both processes by generating K cipher key. A triplet is generated under challenge-response mechanism for authentication. The configuration of AS was never shared by the developer of GSM, ETSI. However, it got reverse engineered & became public. Many crypto attacks took place like plain-text attacks, Rainbow Table attacks, Bias Birthday attacks and Randomsubgraph attacks. Some ways are proposed in this paper to ensure better security by enhancing the randomness of the generated bit stream being used for encryption. These are incorporation of user’s current location i.e. the CGI, reuse of already generated 32 bits of Signed Response during authentication process and conversion of linear FSRs into nonlinear FSRs. Statistical Test Suite is used to test the various properties of random bit stream and an attempt has been made to achieve better randomness, hence more security.