  Mathematical problems and their solutions of the Fourth International
Students' Olympiad in cryptography NSUCRYPTO'2017 are presented. We consider
problems related to attacks on ciphers and hash functions, cryptographic
Boolean functions, the linear branch number, addition chains, error correction
codes, etc. We discuss several open problems on algebraic structure of
cryptographic functions, useful proof-of-work algorithms, the Boolean hidden
shift problem and quantum computings.
