jim@randvax.UUCP (Jim Gillogly) (01/12/88)
In article <1499@osiris.UUCP> mjr@osiris.UUCP (Marcus J. Ranum) writes: > Another problem with Gywn's suggestion that the only way to tell if it >is encrypted (decrypt it) - how can you tell that it is really "meaningful" ? >Anyone who has listened to first year latin students trying to "decrypt" >latin will realize that garbage in does not always imply the same garbage out. With the kinds of ciphers Doug Gwyn was referring to, you can tell. There can be ambiguity if the message is short enough, but the usual case is that you figure out the system and the keyword, use it to break part of the message, and the rest of the message falls out nicely. For example, we could apply Mr. Ranum's argument to DES with a key search: we've tried our 7th New Collegiate on a 64-bit block, and discover that the key "methionine" (suitably folded) results in legal 7-bit ascii plaintext looking like "he purpo" for that 64-bit block. Proceeding further, we find that "rhinencephalon" (also folded) results in "73ux&aUc", which is also legal ascii and could easily be part of a uuencoded piece of source code. In order to see whether either of these is the correct keyword, we try them on the next 64-bit block... There can be ambiguity in decrypting some kinds of cipher, such as a "book code". A lot of people have "decrypted" the Beale Cyphers without being able to prove it -- and unless they come up with a key text that supports their own view (or they dig up the treasure, of course) nobody else will believe them. For interesting reading on times when people have had to prove (i.e. demonstrate to a court) that they got the right reading, try "The Codebreakers" (David Kahn), "The Zimmermann Telegram" (Barbara Tuchman), and "The Shakespearean Ciphers Examined" (William and Elizebeth Friedman). [Elizebeth Friedman specialized in helping convict drug runners by breaking their ciphers; she would testify in court about the validity of her decryption.] -- Jim Gillogly {hplabs, ihnp4}!sdcrdcf!randvax!jim jim@rand-unix.arpa [HASA: U (Spam) division]