Codes and Ciphers :: Running Key Cipher
To encrypt a plaintext message using the Vigenère Cipher, one locates the row with the first letter to be encrypted, and the column with the first letter of the keyword. The ciphertext letter is located at the intersection of the row and column. This continues for the entire length of the message.
In a Running Key cipher, the keyword is the text of a predetermined book or passage. For example, if the chossed book was "A Tale of Two Cities" by Charles Dickens, then the keyword would be
It was the best of times, it was the worst of times...
Enciphering and deciphering the message is performed using the exact same method as the Vigenère Cipher.
If the predetermined passage is a string of random letters that is only used once and then discarded, this is similar to a One-time Pad.
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