Symmetric-key algorithms are a class of algorithm

Algorithm

In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...

s for cryptography

Cryptography

Cryptography is the practice and study of techniques for secure communication in the presence of third parties...

 that use trivially related, often identical, cryptographic keys for both encryption of plaintext and decryption of ciphertext. The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transformation to go between the two keys. The keys, in practice, represent a shared secret

Shared secret

In cryptography, a shared secret is a piece of data, known only to the parties involved, in a secure communication. The shared secret can be a password, a passphrase, a big number or an array of randomly chosen bytes....

 between two or more parties that can be used to maintain a private information link. Other terms for symmetric-key encryption are secret-key, single-key, shared-key, one-key, and private-key encryption. Use of the last and first terms can create ambiguity with similar terminology used in public-key cryptography

Public-key cryptography

Public-key cryptography refers to a cryptographic system requiring two separate keys, one to lock or encrypt the plaintext, and one to unlock or decrypt the cyphertext. Neither key will do both functions. One of these keys is published or public and the other is kept private...

. Symmetric-key cryptography is to be contrasted with asymmetric-key cryptography.

Types of symmetric-key algorithms

Symmetric-key encryption can be divided into stream cipher

Stream cipher

In cryptography, a stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream . In a stream cipher the plaintext digits are encrypted one at a time, and the transformation of successive digits varies during the encryption...

s and block cipher

Block cipher

In cryptography, a block cipher is a symmetric key cipher operating on fixed-length groups of bits, called blocks, with an unvarying transformation. A block cipher encryption algorithm might take a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext...

s. 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  (AES) algorithm approved by NIST in December 2001 uses 128-bit blocks.

Some examples of popular and well-respected symmetric algorithms include Twofish

Twofish

In cryptography, Twofish is a symmetric key block cipher with a block size of 128 bits and key sizes up to 256 bits. It was one of the five finalists of the Advanced Encryption Standard contest, but was not selected for standardisation...

, Serpent

Serpent (cipher)

Serpent is a symmetric key block cipher which was a finalist in the Advanced Encryption Standard contest, where it came second to Rijndael. Serpent was designed by Ross Anderson, Eli Biham, and Lars Knudsen....

, AES

Advanced Encryption Standard

Advanced Encryption Standard is a specification for the encryption of electronic data. It has been adopted by the U.S. government and is now used worldwide. It supersedes DES...

 (Rijndael), Blowfish

Blowfish (cipher)

Blowfish is a keyed, symmetric block cipher, designed in 1993 by Bruce Schneier and included in a large number of cipher suites and encryption products. Blowfish provides a good encryption rate in software and no effective cryptanalysis of it has been found to date...

, CAST5, RC4

RC4

In cryptography, RC4 is the most widely used software stream cipher and is used in popular protocols such as Secure Sockets Layer and WEP...

, 3DES

Triple DES

In cryptography, Triple DES is the common name for the Triple Data Encryption Algorithm block cipher, which applies the Data Encryption Standard cipher algorithm three times to each data block....

, and IDEA

International Data Encryption Algorithm

In cryptography, the International Data Encryption Algorithm is a block cipher designed by James Massey of ETH Zurich and Xuejia Lai and was first described in 1991. As a block cipher, it is also symmetric. The algorithm was intended as a replacement for the Data Encryption Standard[DES]...

.

Cryptographic primitives based on symmetric ciphers

Symmetric ciphers are often used to achieve other cryptographic primitive

Cryptographic primitive

Cryptographic primitives are well-established, low-level cryptographic algorithms that are frequently used to build computer security systems. These routines include, but are not limited to, one-way hash functions and encryption functions.- Rationale :...

s than just encryption.

Encrypting a message does not guarantee that this message is not changed while encrypted. Hence often a message authentication code

Message authentication code

In cryptography, a message authentication code is a short piece of information used to authenticate a message.A MAC algorithm, sometimes called a keyed hash function, accepts as input a secret key and an arbitrary-length message to be authenticated, and outputs a MAC...

 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

CBC-MAC

In cryptography, a cipher block chaining message authentication code , is a technique for constructing a message authentication code from a block cipher. The message is encrypted with some block cipher algorithm in CBC mode to create a chain of blocks such that each block depends on the proper...

).

However, symmetric ciphers also can be used for non-repudiation

Non-repudiation

Non-repudiation refers to a state of affairs where the purported maker of a statement will not be able to successfully challenge the validity of the statement or contract. The term is often seen in a legal setting wherein the authenticity of a signature is being challenged...

 purposes by ISO 13888-2 standard.

Another application is to build hash functions 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

Known-plaintext attack

The known-plaintext attack is an attack model for cryptanalysis where the attacker has samples of both the plaintext , and its encrypted version . These can be used to reveal further secret information such as secret keys and code books...

s, chosen plaintext attacks, differential cryptanalysis

Differential cryptanalysis

Differential cryptanalysis is a general form of cryptanalysis applicable primarily to block ciphers, but also to stream ciphers and cryptographic hash functions. In the broadest sense, it is the study of how differences in an input can affect the resultant difference at the output...

 and linear cryptanalysis

Linear cryptanalysis

In cryptography, linear cryptanalysis is a general form of cryptanalysis based on finding affine approximations to the action of a cipher. Attacks have been developed for block ciphers and stream ciphers...

. Careful construction of the functions for each round can greatly reduce the chances of a successful attack.

Key generation

When used with asymmetric ciphers for key transfer, pseudorandom key generator

Cryptographically secure pseudorandom number generator

A cryptographically secure pseudo-random number generator is a pseudo-random number generator with properties that make it suitable for use in cryptography.Many aspects of cryptography require random numbers, for example:...

s are nearly always used to generate the symmetric cipher session keys. However, lack of randomness in those generators or in their initialization vector

Initialization vector

In cryptography, an initialization vector is a fixed-size input to a cryptographic primitive that is typically required to be random or pseudorandom...

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 for its initialization.