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Created page with "'''Symmetric-key algorithms''' are algorithms for cryptography that use the same cryptographic keys for both encryption of plaintext and dec..."

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'''Symmetric-key algorithms''' are [[algorithm]]s for [[cryptography]] that use the same [[Key (cryptography)|cryptographic keys]] for both encryption of [[plaintext]] and decryption of [[ciphertext]]. The keys may be identical or there may be a simple transformation 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. This requirement that both parties have access to the secret key is one of the main drawbacks of symmetric key encryption, in comparison to [[public-key encryption]] (also known as [[public-key encryption|asymmetric key encryption]]).

== Types of symmetric-key algorithms ==
Symmetric-key encryption can use either [[stream cipher]]s or [[block cipher]]s.

* Stream ciphers encrypt the digits (typically bytes) of a message one at a time.
* Block ciphers take a number of bits and encrypt them as a single unit, padding the plaintext so that it is a multiple of the block size. Blocks of 64 bits were commonly used. The [[Advanced Encryption Standard]] (AES) algorithm approved by [[NIST]] in December 2001, and the [[Galois/Counter Mode|GCM]] block cipher mode of operation use 128-bit blocks.

== Implementations ==
Examples of popular symmetric-key algorithms include [[Twofish]], [[Serpent (cipher)|Serpent]], [[Advanced Encryption Standard|AES]] (Rijndael), [[Blowfish (cipher)|Blowfish]], [[CAST5]], [[Kuznyechik]], [[RC4]], [[Triple DES|3DES]], [[Skipjack (cipher)|Skipjack]], Safer+/++ (Bluetooth), and [[International Data Encryption Algorithm|IDEA]].

== Cryptographic primitives based on symmetric ciphers ==
Symmetric ciphers are commonly used to achieve other [[cryptographic primitive]]s than just encryption.

Encrypting a message does not guarantee that this message is not changed while encrypted. Hence often a [[message authentication code]] is added to a ciphertext to ensure that changes to the ciphertext will be noted by the receiver. Message authentication codes can be constructed from symmetric ciphers (e.g. [[CBC-MAC]]).

However, symmetric ciphers cannot be used for [[non-repudiation]] purposes except by involving additional parties. See the [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=44736 ISO/IEC 13888-2 standard].

Another application is to build [[hash function]]s from block ciphers. See [[one-way compression function]] for descriptions of several such methods.

== Construction of symmetric ciphers ==

Many modern block ciphers are based on a construction proposed by [[Horst Feistel]]. Feistel's construction makes it possible to build invertible functions from other functions that are themselves not invertible.

== Security of symmetric ciphers ==
Symmetric ciphers have historically been susceptible to [[known-plaintext attack]]s, [[chosen-plaintext attack]]s, [[differential cryptanalysis]] and [[linear cryptanalysis]]. Careful construction of the functions for each round can greatly reduce the chances of a successful attack.

== Key management ==

== Key establishment ==

Symmetric-key algorithms require both the sender and the recipient of a message to have the same secret key.
All early cryptographic systems required one of those people to somehow receive a copy of that secret key over a physically secure channel.

Nearly all modern cryptographic systems still use symmetric-key algorithms internally to encrypt the bulk of the messages, but they eliminate the need for a physically secure channel by using [[Diffie–Hellman key exchange]] or some other [[public-key cryptography|public-key protocol]] to securely come to agreement on a fresh new secret key for each message ([[forward secrecy]]).

== Key generation ==

When used with asymmetric ciphers for key transfer, [[Cryptographically secure pseudorandom number generator|pseudorandom key generator]]s are nearly always used to generate the symmetric cipher session keys. However, lack of randomness in those generators or in their [[initialization vector]]s is disastrous and has led to cryptanalytic breaks in the past. Therefore, it is essential that an implementation uses a source of high [[Entropy (information theory)|entropy]] for its initialization.

== Reciprocal cipher ==

A reciprocal cipher is a cipher where, just as one enters the [[plaintext]] into the [[cryptography]] system to get the [[ciphertext]], one could enter the ciphertext into the same place in the system to get the plaintext. A reciprocal cipher is also sometimes referred as self-reciprocal cipher. Examples of reciprocal ciphers include:
* [[Beaufort cipher]]

==Source==

[http://wikipedia.org/ http://wikipedia.org/]

[[Category:Cryptographic algorithms]]
==See Also on BitcoinWiki==
* [[Attrace]]
* [[Asura Coin]]
* [[Cloudbric]]
* [[Bitbose]]
* [[ZeroBank]]

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