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Encyclopedia > Symmetric key algorithm

Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related cryptographic keys for both decryption and encryption. Flowcharts are often used to represent algorithms. ... Cryptography has had a long and colourful history. ... A key is a piece of information that controls the operation of a cryptography algorithm. ...

The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transform to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.

Other terms for symmetric-key encryption are single-key and private-key encryption. Use of the latter term can sometimes conflict with the term private key in public key cryptography. Public key cryptography is a form of cryptography which generally allows users to communicate securely without having prior access to a shared secret key, by using a pair of cryptographic keys, designated as public key and private key, which are related mathematically. ...

## Types of symmetric-key algorithms GA_googleFillSlot("encyclopedia_square");

Symmetric-key algorithms can be divided into stream ciphers and block ciphers. Stream ciphers encrypt the bits of the message one at a time, and block ciphers take a number of bits and encrypt them as a single unit. Blocks of 64 bits have been commonly used; the Advanced Encryption Standard algorithm approved by NIST in December 2001 uses 128-bit blocks. The operation of A5/1, a LFSR-based stream cipher used to encrypt mobile phone conversations. ... In cryptography, a block cipher is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation. ... General Designer(s) Vincent Rijmen and Joan Daemen First published 1998 Derived from Square (cipher) Cipher(s) based on this design Crypton (cypher), Anubis (cipher), GRAND CRU Algorithm detail Block size(s) 128 bits note Key size(s) 128, 192 or 256 bits note Structure Substitution-permutation network Number of... As a non-regulatory agency of the United States Department of Commerce’s Technology Administration, the National Institute of Standards (NIST) develops and promotes measurement, standards, and technology to enhance productivity, facilitate trade, and improve the quality of life. ... 2001: A Space Odyssey. ...

Symmetric-key algorithms are not always used alone. In modern cryptosystem designs, both asymmetric and symmetric algorithms are used to take advantage of the virtues of both. Such systems include SSL, PGP and GPG, etc. Asymmetric key algorithms make key distribution for faster symmetric key algorithms. In modern times symmetric key algorithms have been largely from one of two classes: block cyphers and stream cyphers. A cryptosystem (or cryptographic system) is the package of all procedures, protocols, cryptographic algorithms and instructions used for encoding and decoding messages using cryptography. ... SSL can mean more than one thing: Secure Sockets Layer, a communications protocol. ... Pgp is an acronym for: Pretty Good Privacy, a computer program for the encryption and decryption of data; P-glycoprotein, a type of protein Party for the Government of the People (Partido por el Gobierno del Pueblo} Pearl of Great Price the ICAO code for Perm Airlines This page concerning... The GNU Privacy Guard (GnuPG or GPG) is a free software replacement for the PGP suite of cryptographic software, released under the GNU General Public License. ...

Some examples of popular and well-respected symmetric algorithms include Twofish, Serpent, AES (aka Rijndael), Blowfish, CAST5, RC4, 3DES, and IDEA. In cryptography, Twofish is a symmetric key block cipher with a block size of 128 bits and key sizes up to 256 bits. ... This article is about the block cipher. ... The following article is about the block cipher. ... In cryptography, RC4 (or ARCFOUR) is the most widely-used software stream cipher and is used in popular protocols such as Secure Sockets Layer (SSL) (to protect Internet traffic) and WEP (to secure wireless networks). ... In cryptography, Triple DES (also 3DES) is a block cipher formed from the Data Encryption Standard (DES) cipher. ...

## Speed

Symmetric-key algorithms are generally much less computationally intensive than asymmetric key algorithms. In practice, this means that a quality asymmetric key algorithm is hundreds or thousands of times slower than a quality symmetric key algorithm. The disadvantage of symmetric-key algorithms is the requirement of a shared secret key, with one copy at each end. Since keys are subject to potential discovery by a cryptographic adversary, they need to be changed often and kept secure during distribution and in service. The consequent requirement to choose, distribute and store keys without error and without loss is difficult to reliably achieve. In cryptography, an asymmetric key algorithm uses a pair of cryptographic keys to encrypt and decrypt. ... In Policy Debate, a disadvantage (abbreviated as DA) is an argument that a team brings up against a policy action that is being considered. ... In cryptography, and particularly in analytic or academic discussions of it, a secure channel is a method or technique assumed to provide means by which data can be transferred from one place or user to another without risk of interception or tampering. ... In cryptography, key management includes all of the provisions made in a cryptosystem design, in cryptographic protocols in that design, in user procedures, and so on, which are related to generation, exchange, storage, safeguarding, use, vetting, and replacement of keys. ...

In order to ensure secure communications between everyone in a population of n people a total of

keys are needed.[1] Very often these days, the much slower asymmetric algorithms are used to distribute symmetric-keys at the start of a session, then the higher speed symmetric-key algorithms take over. The same problems of reliable key distribution still exists at the asymmetric level, but they are somewhat more tractable. However, the symmetric key is nearly always generated in realtime.

## Reversibility

Cryptographic functions must, by definition, be reversible, since you need to be able to both encrypt and (provided you have the right key) decrypt messages.

Various methods have been used historically to manage this. There have been book ciphers, in which the shared key is related to some content in a book, auto-key ciphers in which the key is partially derived from the plaintext, grill ciphers (supposedly first invented by the Italian mathematician Gerolamo Cardano), etc. In modern times, after computers became available, most symmetric ciphers have been based on 'rounds'. Usually a rather simple scheme is used repeatedly as in the following generic example. This general method is usually ascribed to Horst Feistel. For a more indepth description of this method (with diagrams) see Feistel cipher. The plain text term has a different meaning. ... Gerolamo Cardano or Jerome Cardan or Girolamo Cardan (September 24, 1501 - September 21, 1576) was a celebrated Italian Renaissance mathematician, physician, astrologer, and gambler. ... Horst Feistel (30 January 1915(1) - 14 November 1990) was one of the first non-government researchers in modern cryptography. ... Feistel cipher - Wikipedia /**/ @import /skins-1. ...

The bits to be encoded are split into two parts P1 and P2. P1 is unchanged, P2 is added (or exclusive-or'd) to a one-way hashed function f (varied by a key or 'salt') of P1. The two results are then swapped over. This is called 'a round'. In cryptography, a cryptographic hash function is a hash function with certain additional security properties to make it suitable for use as a primitive in various information security applications, such as authentication and message integrity. ...

i.e. where p1, p2, key are bit vectors; ',' is a concatenation operator and f is a function

p1, p2 -> p2', p1 such that:

p2' = p2 + f(p1, key)

Since the output of the round still has access to the value P1, and the addition is a reversible operation, then this operation may be undone, for any one way function f.

Whilst a single round is very insecure, as p1 is unchanged, repeating this operation more than once, often with different functions and keys, greatly improves the strength.

To decrypt multiple rounds, each round is undone in reverse order, hence for decryption the keys are applied in reverse order.

After sufficient rounds (typically between 8 and 64) have been performed, the output is very scrambled until, ideally, brute force attacks are the quickest way to crack the code.

## Attacks on symmetric cyphers

Symmetric ciphers have historically often been susceptible to known-plaintext attacks, chosen plaintext attacks, differential cryptanalysis and linear cryptanalysis. Careful construction of the functions for each round can greatly reduce the chances of a successful attack The known-plaintext attack (KPA) is an attack model for cryptanalytic where the attacker has samples of both the plaintext and its encrypted version (ciphertext) and is at liberty to make use of them to reveal further secret information; typically this is the secret key. ... A chosen plaintext attack is any form of cryptanalysis which presumes that the attacker has the capability to choose arbitrary plaintexts to be encrypted and obtain the corresponding ciphertexts. ... Differential cryptanalysis is a general form of cryptanalysis applicable primarily to block ciphers, but also to stream ciphers and cryptographic hash functions. ... In cryptography, linear cryptanalysis is a general form of cryptanalysis based on finding affine approximations to the action of a cipher. ...

When used with asymmetric ciphers for key transfer, pseudorandom key generators are nearly always used to generate the keys; however lack of randomness in initialisation of the pseudorandom generator has very commonly lead to cryptanalytic breaks in the past. Very careful implementation based on sufficient sources of any available randomness is required.

## Notes

• ^  Beutelspacher, Albrecht (1994). "The Future Has Already Started or Public Key Cryptography" Cryptology, translation from German by J. Chris Fisher, 102. ISBN 0-88385-504-6.

Results from FactBites:

 Symmetric-key algorithm - Wikipedia, the free encyclopedia (810 words) Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related cryptographic keys for both decryption and encryption. Since keys are subject to potential discovery by a cryptographic adversary, they need to be changed often and kept secure during distribution and in service. Symmetric ciphers have historically often been susceptible to known-plaintext attacks, chosen plaintext attacks, differential cryptanalysis and linear cryptanalysis.
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