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  • From: Simon Spero <ses AT unc.edu>
  • To: "Internetworkers: http://www.ibiblio.org/internetworkers/" <internetworkers AT lists.ibiblio.org>
  • Subject: Re: [internetworkers] crypto
  • Date: Wed, 28 Jul 2004 12:52:27 -0400

The most useful test to decide whether your security system is secure "enough" is:
Is the value of this information to my most capable potential enemy less than the cost to them of breaking the weakest part of my system?

The most useful test for deciding whether to increase the security of your system is:
is the value of a particular increase greater than the cost of making that increase?


A good example are the One Time Pads mentioned by Alan earlier. One time pads when used correctly are absolutely unbreakable even in theory; however the cost of key management is mind boggling. The key is the same length as the data; Two copies of this enormous key must be generated absolutely randomly, with no leakage. Every time a byte of key is used, it must be absolutely destroyed before the message is sent. If one key is possibly compromised, you have to start over from scratch. And if the cypher clerk is bribed, blackmailed, you're hosed anyway (or even if they're just lazy or incompetent- that's how we know that Hiss and the Rosenbergs really were guilty).

Nevertheless, OTPS are still sometimes used; for low volume messages, which have to remain secret forever, and whose value to an enemy is overwhelming.

In the end, there's no way you can be absolutely certain of anythin. The NKVD could have implanted devices in your brain to make you think that this is the 21st century, and are trying to guide your illusions to make you reveal where the King of France is hiding the last breeding pair of unicorns. Or not. Whatever. Incidentally, where are those unicorns?





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