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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 ...
The KL-7, introduced in the mid-1950s, was the first U.S. cipher machine that was considered safe against known-plaintext attack. [8]: p.37 Classical ciphers are typically vulnerable to known-plaintext attack. For example, a Caesar cipher can be solved
Stream ciphers are defined as using plain text digits that are combined with a pseudorandom cipher digit stream. Stream ciphers are typically faster than block ciphers and may have lower hardware complexity, but may be more susceptible to attacks.
An autokey cipher (also known as the autoclave cipher) is a cipher that incorporates the message (the plaintext) into the key. The key is generated from the message in some automated fashion, sometimes by selecting certain letters from the text or, more commonly, by adding a short primer key to the front of the message.
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 .
Stream ciphers, where plaintext bits are combined with a cipher bit stream by an exclusive-or operation (), can be very secure if used properly. [citation needed] However, they are vulnerable to attacks if certain precautions are not followed:
Decryption is similar: the decryption algorithm takes, in this example, a 128-bit block of ciphertext together with the secret key, and yields the original 128-bit block of plain text. [ 4 ] For each key K , E K is a permutation (a bijective mapping) over the set of input blocks.
The Goppa code can correct up to errors, and the word is at distance at most from . Therefore, the correct code word m ^ = m S {\displaystyle {\hat {m}}=mS} is obtained. Multiplying with the inverse of S {\displaystyle S} gives m = m ^ S − 1 = m S S − 1 {\displaystyle m={\hat {m}}S^{-1}=mSS^{-1}} , which is the plain text message.