[Swordmaker]

If two parties were to agree in advance that target1=tartet3 and target3=target2 and that every message starting with the letters ‘a’, ‘I’ or ‘m’ should be ignored then all the decryption in the world would not help.

There are several ways to make completely unbreakable ciphers. The best way is the public code book method. You and the target agree to communicate with a preset variable code which uses a common public book with a large number of pages that has in it all letters and numbers symbols, etc. ideally, every one of these should occur on multiple pages throughout the book. In advance you and your partner setup a schedule of directions. I.e. Odd days start from the bottom of the beginning page. Even from the top. Monday's, Wednesday's, Friday's count pages from the starting page toward the front, Tuesday's, Thursday's, and Saturday, toward the back. Sunday's messages stay on the start page and ignore first set of numbers. Set similar rules for line count direction and letter count directions.

Sender sends start page, move to page, line number, word number, letter count for each character in his message. If the letter count takes one to a space or punctuation mark, that's it.

This can all be automated in computers that both have a digital copy of the book downloaded. . . or it can be done tediously by hand. Both encryption and decryption can be computerized and since you NEVER use the same code sequence for the same letter, without knowing the book, the rules, and the days, it's unbreakable.

[UCANSEE2]

Both encryption and decryption can be computerized and since you NEVER use the same code sequence for the same letter, without knowing the book, the rules, and the days, it's unbreakable.

Not if you've got...

[The Duke]

Sounds like a variation of the one-time pad. One affect all this does have is bringing back the days of having to use human agents to transport keys/code books around.