Miyaji Laboratory

Abstract

In recent years, information and communication technology has developed rapidly and has become a fundamental part of social systems. Cryptography is an essential element to maintain confidentiality, integrity, and availability of data in information communication. Currently, TLS1.3, an encrypted communication protocol standardized by the IETF (The Internet Engineering Task Force), uses only ChaCha20 as a stream cipher among the encryption methods of the symmetric key cryptography.In ChaCha20, a random number is generated by repeatedly computing a round function (RD), which is an operation using 32-bit addition, exclusive disjunction, and cyclic shift. The closer the random number is to the true random number, the more secure the cipher is, and if there is a bias in each bit of the generated random number, the less secure it is. The purpose of this study is to investigate the propagation of the effect of changing multiple bits in ChaCha20 and to analyze the security.