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Polygraphic substitution cipher: the unit of substitution is a sequence of two or more letters rather than just one (e.g., Playfair cipher) Transposition cipher: the ciphertext is a permutation of the plaintext (e.g., rail fence cipher) Historical ciphers are not generally used as a standalone encryption technique because they are quite easy to ...
Plaintext is used as input to an encryption algorithm; the output is usually termed ciphertext, particularly when the algorithm is a cipher. Codetext is less often used, and almost always only when the algorithm involved is actually a code .
The adversary receives the encryption of m b, and attempts to "guess" which plaintext it received, and outputs a bit b'. A cipher has indistinguishable encryptions under a chosen-plaintext attack if after running the above experiment the adversary can't guess correctly (b=b') with probability non-negligibly better than 1/2. [3]
Classical ciphers are typically vulnerable to known-plaintext attack. For example, a Caesar cipher can be solved using a single letter of corresponding plaintext and ciphertext to decrypt entirely. A general monoalphabetic substitution cipher needs several character pairs and some guessing if there are fewer than 26 distinct pairs.
A cipher whose key space is too small is subject to brute force attack with access to nothing but ciphertext by simply trying all possible keys. All that is needed is some way to distinguish valid plaintext from random noise, which is easily done for natural languages when the ciphertext is longer than the unicity distance .
Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher (or cypher) is a pair of algorithms that carry out the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and, in each instance, by a "key".
One of the most famous military encryption developments was the Caesar cipher, in which a plaintext letter is shifted a fixed number of positions along the alphabet to get the encoded letter. A message encoded with this type of encryption could be decoded with a fixed number on the Caesar cipher.
A secret knowledge is required to apply the inverse cipher to the ciphertext. This secret knowledge is usually a short number or string called a key. In a cryptographic attack a third party cryptanalyst analyzes the ciphertext to try to "break" the cipher, to read the plaintext and obtain the key so that future enciphered messages can be read ...