Efficient Implementation of Linearisation Attacks on F-FCSR-16 Type Key-stream Generators

Feedback with Carry Shift Registers (FCSRs) are being explored as a potential replacement of Linear Feedback Shift Registers (LFSRs) in the design of stream ciphers. Many cryptographic schemes based on FCSRs have been proposed in the open literature. The most efficient attack against ciphers based on FCSRs are `linearisation attacks', which consists of three variants, namely `Conventional Linearisation Attacks (CLAs)', `Fast Linearisation Attacks (FLAs)' and `Improved Linearisation Attacks (ILAs)'. This paper describes applications of linearisation attacks on F-FCSR-16 key-stream generator. For this purpose, the paper presents State Recovery Algorithms (SRAs) based on CLAs, FLAs and ILAs for F-FCSR-16. The paper also presents efficient techniques of implementation for these attacks on F-FCSR-16 type key-stream generators. Further more, the paper also presents a comparative analysis of the results of CLAs, FLAs and ILAs on F-FCSR-16 key-stream generator. This analysis shows the trade-offs between CLAs, FLAs and ILAs in terms of data time and memory complexities, when compared with each other and determines the relative efficiency of these attacks on F-FCSR-16.