"The FBI today challenged anyone in the online community to break a cipher code on its site.  The code was created by FBI cryptanalysts. The bureau invited hackers to a similar code-cracking challenge last year  and got tens of thousands of responses it said.

A number of sites host such cipher challenges, including this one at the University of South Hampton.

The FBI offers a few primers on the subject including:

A relatively basic form of substitution cipher is the Caesar Cipher, named for its Roman origins. The Caesar Cipher involves writing two alphabets, one above the other. The lower alphabet is shifted by one or more characters to the right or left and is used as the cipher text to represent the plain text letter in the alphabet above it."

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