International Association
for Cryptologic Research

The stream cipher ChaCha is an ARX type algorithm developed by Daniel Bernstein in 2008. Since its development, ChaCha has received a lot of attention and is currently being used in several systems. The most powerful cryptanalysis of reduced versions of this cipher was presented by Choudhuri and Maitra on FSE 2017 by using differential-linear cryptanalysis. In their work they show that is possible to obtain linear relations between bits from different rounds with high probability and use the proposed equations to create multi-bit differentials and improve previous attacks. In this work, we provide new linear approximations that can be used in a similar fashion but with increased efficiency. Therefore, we show that using these new equations is possible to improve the attacks against 6 and 7 rounds of ChaCha.