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The operation of a cipher usually depends on a piece of auxiliary information, called a key (or, in traditional NSA parlance, a cryptovariable). The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message.
To use the Vigenère square to encrypt a message, a coder first chooses a keyword to use and then repeats it until it is the same length as the message to be encoded. If LEMON is the keyword, each letter of the repeated keyword will tell what cipher (what row) to use for each letter of the message to be coded. The cipher alphabet on the second ...
The Vigenère cipher uses a Caesar cipher with a different shift at each position in the text; the value of the shift is defined using a repeating keyword. [14] If the keyword is as long as the message, is chosen at random , never becomes known to anyone else, and is never reused, this is the one-time pad cipher, proven unbreakable.
A stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream . In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream.
A book cipher is a cipher in which each word or letter in the plaintext of a message is replaced by some code that locates it in another text, the key. A simple version of such a cipher would use a specific book as the key, and would replace each word of the plaintext by a number that gives the position where that word occurs in that book.
Leon Battista Alberti conceived of the first polyalphabetic cipher system using rotating disks, described in his work De Cifris around 1467 [5]. This system, known as the Alberti cipher, allowed for encrypted messages using a more complex method of substitution compared to the earlier monoalphabetic ciphers.
A scytale. In cryptography, a scytale (/ ˈ s k ɪ t əl iː /; also transliterated skytale, Ancient Greek: σκυτάλη skutálē "baton, cylinder", also σκύταλον skútalon) is a tool used to perform a transposition cipher, consisting of a cylinder with a strip of parchment wound around it on which is written a message.
The cipher, known today as the Wheel Cipher or the Jefferson Disk, although never actually built, was theorized as a spool that could jumble an English message up to 36 characters. The message could be decrypted by plugging in the jumbled message to a receiver with an identical cipher. [5]