Abstract: Pyjamask algorithm is one of the campaign algorithms shortlisted in the second round of LWC competition. This algorithm has the advantages of simple structure, light weight and high efficiency, and good parallel computing ability of nonlinear components, which has attracted the attention of many cryptographers. At present, there are relatively few researches on the security of this algorithm, and a new round of security evaluation is urgently needed. Based on the characteristics of Pyjamasks structure and adjustment parameters, this paper proposes a method for forging plaintexts, which can accurately forge authentication tags. Theoretical analysis shows that the success probability is 1 with negligible data complexity and time complexity when selecting a set of plaintexts, and the success probability is also 1 with high selected data requirements when selecting s+1 sets of plaintexts.

Key words: Pyjamask algorithm, forgery attack, authenticated encryption