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.

Plain Text

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Cipher Text

B C D E F G H I J K L M N O P Q R S T U V W X Y Z A

In this example, the plain text K is enciphered with the cipher text L. The phrase 'Lucky Dog' would be enciphered as follows:

Plain Text: L U C K Y D O G

Cipher Text: M V D L Z E P H

Ciphers can be made more secure by using a keyword to scramble one of the alphabets. Keywords can be placed in the plain text, the cipher text, or both, and any word can be used as a key if repeated letters are dropped. Here the word SECRETLY (minus the second E) is used as the plain text keyword.

Plain Text

S E C R T L Y A B D F G H I J K M N O P Q U V W X Z

Cipher Text

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

The FBI of course doesn't always invite folks to break code on its site. In fact last spring a consultant managed to access the bureau's National Crime Information Center database.