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Doctor Who is a long running, award winning British science fiction television programme produced by the BBC. The programme depicts the adventures of a mysterious time traveller known as "the Doctor" who travels in his space and time ship, the TARDIS, which appears from the exterior to be a blue police phone box. With his companions, he explores time and space, solving problems and righting wrongs. The programme is listed in the Guinness World Records as the longest running science fiction television show in the world and is also a significant part of British popular culture.
A Substitution cipher is one where each letter of the alphabet is substituted with another letter or symbol. For example, in a piece of text the word AND may be replaced by the word HJP, where H represents A, and J represents N, and P represents D.
Substitution ciphers can be broken using a method known as frequency analysis. This method involves finding the letter that appears most frequently in the ciphertext and replacing it with the most frequently used letter in English which happens to be E, T, then A.
Click here http://www.simonsingh.net/The_Black_Chamber/substitution.html in the Black Chamber to find out more about substitution ciphers. You can also find out how to crack the substitution cipher http://www.simonsingh.net/The_Black_Chamber/crackingsubstitution.html