OneTimePads are the simplest of all algorithms, and were invented in the early 20th century. The basic idea is that you have a pad of key letters. You then add one key letter to one plain text letter, and never repeat the key letters (eg. T(20) to L(12) in order to get F(6) - 20 + 12 = 6 mod 26). The system works with any given alphabet, including the binary one. It is probably the only provably secure algorithm available.

It is also quite useless. The key has to be as long as the message. Mordern CryptographY encrypts large things (eg. Digital Movies, Internet Connections, Telephone Conversations, etc.) - dealing with OneTimePads of such a size is simply impracticable.

OneTimePads have been used in specialised circumstances. KGB spies used pencil and paper OneTimePads to communicate. The NSA broke the system because the spies used the OneTimePads more than once[sic!].