Cipher
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Basic Information

A cipher is a tool of cryptography - an algorithm for the encryption and decryption of information.

The best cyphers are known as 'pad cyphers' - the encryption is based on a random sequence key that is known at both ends of the message for that method only and then destroyed. The name comes from the tear-off pads that were originally used for this technique, although modern methods are fully electronic.

Since the encryption key is entirely random and used for one message only it is virtually unbreakable unless the interceptor has access to one of the 'pads' and knowledge of the method by which the correct 'pad page' is chosen for a given message. Technically any key can be 'brute forced' by trying thousands of numbers until the decode makes sense but this takes a great deal of time and computing power and can be made prohibitive by using long enough keys.

US government agencies used to base their cypher keys on atmospheric noise recorded at a variety of locations in the midwest making it chaotic if not entirely random and has tended to be unbreakable.

Sources

Game and Story Use

  • Many organizations use ciphers. Unless the players enjoy analyzing encrypted documents by themselves, getting a hold of the right cipher algorithm for decoding a vital message can be a useful MacGuffin for the PCs to chase after.
    • This is particularly fun with pad cyphers. Back in the Cold War era this can be as simple as finding actual pad pages the cypher clerk was too lazy to burn - or as hard as stealing a pad, duplicating it and getting it back before anyone notices that it has been compromised.
      • In a modern or futuristic game, a completely legal adventure might involve getting a one-time pad from Alice to Bob without Eve getting hold of it.
  • In a modern system, governments tend to demand key access to civilian pad cypher systems - using encryption that an agency can't crack is a good way to attract their attention. More subtle methods are advised.
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