It is notable that the only historic cryptography that has lasted more than 100 years has relied on security-by-obscurity. There are still a lot of archeological secret messages nobody has been able to decode, sometimes despite pretty big efforts.
That assumes that those messages are decodable, which is not a given. For example it's not at all sure if the Voynich manuscript even can be decoded (it might be a hoax, glossolalia or similar).
