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cory doctorow
Which is OK, because now we have computers — big, bulky,
primitive mechanical computers, but computers still. Computers
are machines for rearranging numbers, and so scientists on both
sides engage in a fiendish competition to invent the most clever-
est method they can for rearranging numerically represented text
so that the other side can’t unscramble it. The existence of the
message isn’t a secret anymore, but the cipher is.
But this is still too many secrets. If Bobby intercepts one of
Adolf’s Enigma machines, he can give Churchill all kinds of intel-
ligence. I mean, this was good news for Churchill and us, but bad
news for Adolf. And at the end of the day, it’s bad news for anyone
who wants to keep a secret.
Enter keys: a cipher that uses a key is still more secure. Even if
the cipher is disclosed, even if the ciphertext is intercepted, with-
out the key (or a break), the message is secret. Post-war, this is
doubly important as we begin to realize what I think of as Schnei-
er’s Law: “Any person can invent a security system so clever that
she or he can’t think of how to break it.” This means that the only
experimental methodology for discovering if you’ve made mis-
takes in your cipher is to tell all the smart people you can about
it and ask them to think of ways to break it. Without this criti-
cal step, you’ll eventually end up living in a fool’s paradise, where
your attacker has broken your cipher ages ago and is quietly de-
crypting all her intercepts of your messages, snickering at you.
Best of all, there’s only one secret: the key. And with dual-key
crypto it becomes a lot easier for Alice and Bob to keep their keys
secret from Carol, even if they’ve never met. So long as Alice and
Bob can keep their keys secret, they can assume that Carol won’t
gain access to their cleartext messages, even though she has
access to the cipher and the ciphertext. Conveniently enough, the
keys are the shortest and simplest of the secrets, too: hence even
easier to keep away from Carol. Hooray for Bob and Alice.
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