Chuck wrote:aye amazing what we did without computers eh! might be worth bringing back if Cameron goes ahead and starts his uberspyonus society!
Unfortunately one time pads are too much of a pain in the arse to use for anything except on a limited basis as generation of really good random keys is very hard, and you MUST use a different key for each message (otherwise it's not a one-time pad any more - the clue is in the name!) On a related note, if you've not already written to your MP about the communications bill, I strongly urge you to do so.
Good random keys can be generated a number of ways; however very common during the war was to have a spinning wire cage with lots of wooden balls with letters engraved on them. The cage would be spun a predefined number of times before a ball was withdrawn and the letter on the ball would be written down as the next character of the key. Two copies would be made; one for the person doing the encryption and one person doing the decryption.
Flaws with this method are:
1) The old dears doing the letter picking would occasionally decide that they'd had too many of the same letter in a row and would then pick another letter instead, breaking the randomness and making it easier to crack. (This is related to one of the major flaws in the Engima machine, where it would never use the same letter twice in a row in the key, making it substantially easier to crack.)
2) You need to get one copy of the key to someone in the field to use it, giving opportunities for it to be copied and then used to try and decrypt anything intercepted and suspected of being a one-time pad cypher.
3) Even if the old dear doesn't give in to temptation 1, this is not really a random process and in theory at least if the same wire cage and balls are used, a pattern can be detected which may be good enough to crack the message.
Having a really good source of random numbers is very important for all sorts of things, so there are a number of modern methods used, which include cameras aimed at lava lamps (really) and cosmic ray detectors.
Gaz