O 7-zip usa encriptação AES-256 para arquivos 7z / zip.
link diz:
7-Zip also supports encryption with AES-256 algorithm. This algorithm uses cipher key with length of 256 bits. To create that key 7-Zip uses derivation function based on SHA-256 hash algorithm. A key derivation function produces a derived key from text password defined by user. For increasing the cost of exhaustive search for passwords 7-Zip uses big number of iterations to produce cipher key from text password.
Os arquivos de ajuda do 7-zip dizem isso sobre a segurança de um arquivo 7z com senha: