This program demonstrates how to encipher and decipher Vigenère text.
A Vigenère Cipher
is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. A Caesar cipher replaces every letter by another. For a Caesar shift of +1, ‘a’ becomes ‘b’, ‘b’ becomes ‘c’ and so forth for all 26 letters. For a Caesar shift of +2, ‘a’ becomes ‘c’, ‘b’ becomes ‘d’ and so forth. A Vigenère cipher goes one step further so that every letter in a message is shifted according to a key or password. Say 'cat' was the password. That means the first letter of the message to be encrypted (the plaintext) is shifted by ‘c’ (+2 Caesar shift), the second by ‘a’ (+0 or no shift), and the third by ‘t’ (+19 Caesar shift). This is repeated throughout the plaintext, so the fourth letter is shifted by a +2 Caesar shift (‘c’) and the process repeats. Whitespaces between letters do not count. The longer the key, the harder it is to decipher the message.
To use the program, enter text and click on the buttons shown. Hyperlinks to the left can be used to move to various sections. The program can encipher and decipher text composed of English letters from between two sentences to about one page, using any password up to six letters, usually in a few seconds. But the program is not infallible. Long passwords and short text are the hardest to decipher. See the link labeled Program Details on the next page for more details on program operation and limitations.