In the case of a cipher, if keys are suspected to be words, the same technique can be used to break messages encrypted with it.
An example of a dictionary attack occurred in the Second World War, when British codebreakers working on German Enigma-ciphered messages used the German word eins as part of a dictionary attack; eins, the word for the number one, appeared in 90% of all Engima messages, as the Enigma machine's keyboard had no numerals.
Clifford Stoll's book, The Cuckoo's Egg, contains an interesting, and unusually readable, account of a dictionary attack against the encrypted passwords kept in the passwd[?] file in Unix systems, and of the reaction to the successful attack of the man (Robert Morris Sr) who invented the encryption system used for those passwords.
See also:
Well known examples of dictionary attack software tools include John the Ripper (http://www.openwall.com/john/) and L0phtCrack (http://www.atstake.com/research/lc3/)
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