As of April 2016, in newly proposed TLS1.3, as encryption methods of symmetric key encryption scheme, AES in the block encryption and ChaCha20 in the stream encryption are adopted.
However,ChaCha20 doesn't have sufficient safey evaluation like AES.
ChaCha20 was proposed as a variant of Salsa 20, and the analysis of Salsa 20 is important as the first step in understanding the improvement to ChaCha20 and the design of stream cipher.
As a method of evaluating the safety of Salsa 20/ChaCha 20, there is a method using diffusion, but the method has not been established.
In this research, we evaluate the safety by newly designing diffusion evaluation criteria and conditions for Salsa 20/ChaCha 20.