Cryptography
Single Key Cryptography
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Polyalphabetic Cipher --------------------- Blaise de Vigenere made his own cipher system, but through a historical accident, he has also been linked to the following, weaker system. Nonetheless, it is a great example to look at. Plain : abcdefghi... Cipher 1 : bcdefghij... Cipher 2 : cdefghijk... Cipher 3 : defghijkl... Cipher 4 : efghijklm... . . . Cipher 26 : zabcdefgh... To encrypt, find the letter in plaintext. Then encrypt it with cipher 1. Then find the next plaintext letter and encrypt it with cipher 2 and so on. Plaintext : 'head' Ciphertext : 'igdh' 'i' comes from cipher 1, 'g' from cipher 2... You continue this until Cipher 26 and then go back to Cipher 1. This way, the frequency table will be thrown off a lot more since the letter 'E' will be encrypted with different letters. Even words with repeating letters like "book" will not be encrypted with the same two letters in the middle: "book" --> "cqrn" The polyalphabetic cipher can be further strengthened by using a key/password in the plaintext and/or randomizing each of the ciphers like so: Plain : VOXFANbcdeghi... Cipher 1 : asdflkjhqeriy... Cipher 2 : zxmbnalfkerit... . . . As you can see, this is much more secure than plain old regular single letter substitutions and rotations. But with a sufficently long message, this can also fall victim to a frequency table attack. Although it would have to be much, much longer than for the ciphers listed in page 2. You can use a key/password in various ways here. Be imaginative. -=============- | Transposition | --------------- This type of cryptosystem relies less on creating a cipher alphabet. Instead, it just encrypts the plaintext itself. Railfence --------- This cipher takes the plaintext message and staggers it in two rows. For example, take the plaintext "baby vox is the best" which then becomes: b b v x s h b s a y o i t e e t Take this and your new ciphertext will be "bbvxshbsayoiteet". This is pretty jumbled up and can't be read easily. You can use a key to generate a value and use that value to create more than 2 rows. You can also use the value to pick a number so that the cipher system doesn't break it up every other letter, but at different values. Lots of different ways to encrypt your data. Lots of different uses for keys and passwords. Be creative. -nexusb0mb
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