Cipher

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.

See Also

• Mathematics
• One Time Pad
• Public Key Encryption
• Deniable Encryption

Sources

Bibliography

1. Analysis of Criminal Codes and Ciphers

2. Wikipedia entry

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.
• Some agencies practice sending the occasional gibberish message under cipher as an additional precaution - if their cipher system is broken, they then hope for whoever is reading their mail to intercept the spoof message, assume that the cipher has changed and abandon it.
• Knowledge of which ciphers have been broken is important.
  • Hiding that you've broken a cipher can keep an enemy from retracting it, allowing you to prepare for their actions. However, this also requires you to only act on information you could plausibly have obtained through other means if you don't want to give the game away.
  • Sending a message under a cipher that you know the enemy has broken is a great way of luring them into a trap, so long as a) the enemy doesn't know you know, and b) you have some way of letting the recipient know to disregard it.
  • Taunting an enemy with their own broken ciphers can stoke their paranoia if they were already prone to it, especially if you can pass it off as the work of your spies rather than your mathematicians.
• A character is prone to quoting from the Bible, usually using chapter and verse. He might be particularly devout, or he might be giving an ally the key to a book code.
• (may be better moved somewhere else) how about "encoding" a computer file by disguising it as another type? Could be as simple as changing the extension, creating an apparently corrupt and unreadable file until the intended recipient, who knows what format it is meant to be changes the extension back. Equally misidentification of the nature of encoded data could be a huge barrier to decryption - analysing something on the basis that it is encrypted text or graphics might not help if it is actually an audio file.