Sharky's Vigenere Cipher
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Vigenere coding is one of the most ancient forms of cryptography. In a Vigenere cipher, messages are encoded by mapping letters to numeric values and adding the plaintext values to the keytext values, modulo the number of letters in the alphabet. In this implementation, A=0, Z=25, and the modulus is 26. Despite its simplicity, the Vigenere cipher is capable of producing 100% crack-proof ciphers if used correctly. To produce an unbreakable cipher, you must:
Unfortunately, these steps take all the fun out of the Vigenere cipher. Most people use English words or phrases for the key (violating step 2) that are significantly shorter than the message being coded (violating step 1). Then they use the same key to encrypt additional messages (violating step 3) because they don't want to remember a new key for each message. Some people use quotations from their favorite book for the key or invent one and write it down (violating step 5). Lastly, it's no fun to handle text that doesn't look like text, so they don't remove the decorations (violating step 4). The moral of the story is that security is free, but it requires effort.
Vigenere ciphers with moderate keys are appropriate for situations that do not require failproof security. For example, Sharky uses the Vigenere cipher to encrypt sensitive passages in his personal diary. (It's good enough to deter most of the people most of the time!)
Copyright © 2001 Sharkysoft. All rights reserved. Last updated 2003.05.27.