The International Olympiad in Cryptography NSUCRYPTO is the unique Olympiad
containing scientific mathematical problems for professionals, school and
university students from any country. Its aim is to involve young researchers
in solving curious and tough scientific problems of modern cryptography. In
2020, it was held for the seventh time. Prizes and diplomas were awarded to 84
participants in the first round and 49 teams in the second round from 32
countries. In this paper, problems and their solutions of NSUCRYPTO'2020 are
presented. We consider problems related to attacks on ciphers and hash
functions, protocols, permutations, primality tests, etc. We discuss several
open problems on JPEG encoding, Miller -- Rabin primality test, special bases
in the vector space, AES-GCM. The problem of a modified Miller -- Rabin
primality test was solved during the Olympiad. The problem for finding special
bases was partially solved