"Cryptography is the science of writing messages that no one except the intended receiver can read. Cryptanalysis is the science of reading them anyway." (Missler, Cosmic Codes pg.5)
Vigenere's polyalphabetic substitution cipher, attributed to him around 1553, was considered unbreakable for centuries. It's based on work devised by Giovan Battista & Leon Battista Alberti. (Alberti is the understated father of modern Cryptology.) And while this type of cipher is easily cracked by the NSA or FBI, your average message snooper will be confounded. Test it out with your friends!