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Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both the encryption of plaintext and the 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. The 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 asymmetric-key encryption). However, symmetric-key encryption algorithms are usually better for bulk encryption. They have a smaller key size, which means less storage space and faster transmission. Due to this, asymmetric-key encryption is often used to exchange the secret key for symmetric-key encryption.


Types

Symmetric-key encryption can use either
stream cipher stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream ...
s or
block cipher In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called ''blocks''. Block ciphers are specified cryptographic primitive, elementary components in the design of many cryptographic protocols and ...
s. * Stream ciphers encrypt the digits (typically bytes), or letters (in substitution ciphers) of a message one at a time. An example is ChaCha20. ** Substitution ciphers are well-known ciphers, but can be easily decrypted using a frequency table. * Block ciphers take a number of bits and encrypt them in a single unit, padding the plaintext to achieve a multiple of the block size. The Advanced Encryption Standard (AES) algorithm, approved by
NIST The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
in December 2001, uses 128-bit blocks.


Implementations

Examples of popular symmetric-key algorithms include Twofish, Serpent,
AES AES may refer to: Businesses and organizations Companies * AES Corporation, an American electricity company * AES Data, former owner of Daisy Systems Holland * AES Eletropaulo, a former Brazilian electricity company * AES Andes, formerly AES Gener ...
(Rijndael), Camellia, Salsa20, ChaCha20, Blowfish,
CAST5 In cryptography, CAST-128 (alternatively CAST5) is a symmetric-key block cipher used in a number of products, notably as the default cipher in some versions of GPG and PGP. It has also been approved for Government of Canada use by the Communic ...
, Kuznyechik, RC4, DES,
3DES In cryptography, Triple DES (3DES or TDES), officially the Triple Data Encryption Algorithm (TDEA or Triple DEA), is a Symmetric-key algorithm, symmetric-key block cipher, which applies the Data Encryption Standard, DES cipher algorithm three ti ...
, Skipjack, Safer, and IDEA.


Use as a cryptographic primitive

Symmetric ciphers are commonly used to achieve other
cryptographic primitive Cryptographic primitives are well-established, low-level cryptographic algorithms that are frequently used to build cryptographic protocols for computer security systems. These routines include, but are not limited to, one-way hash functions and ...
s than just encryption. Encrypting a message does not guarantee that it will remain unchanged while encrypted. Hence, often a
message authentication code In cryptography, a message authentication code (MAC), sometimes known as a ''tag'', is a short piece of information used for authenticating a message. In other words, to confirm that the message came from the stated sender (its authenticity) and ...
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 an AEAD cipher (e.g. AES-GCM). However, symmetric ciphers cannot be used for non-repudiation purposes except by involving additional parties. See th
ISO/IEC 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 The known-plaintext attack (KPA) is an attack model for cryptanalysis where the attacker has access to both the plaintext (called a crib), and its encrypted version (ciphertext). These can be used to reveal further secret information such as secr ...
s, chosen-plaintext attacks,
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 information input can aff ...
and linear cryptanalysis. Careful construction of the functions for each round can greatly reduce the chances of a successful attack. It is also possible to increase the key length or the rounds in the encryption process to better protect against attack. This, however, tends to increase the processing power and decrease the speed at which the process runs due to the amount of operations the system needs to do. Most modern symmetric-key algorithms appear to be resistant to the threat of post-quantum cryptography. Quantum computers would exponentially increase the speed at which these ciphers can be decoded; notably, Grover's algorithm would take the square-root of the time traditionally required for a brute-force attack, although these vulnerabilities can be compensated for by doubling key length. For example, a 128 bit AES cipher would not be secure against such an attack as it would reduce the time required to test all possible iterations from over 10 quintillion years to about six months. By contrast, it would still take a quantum computer the same amount of time to decode a 256 bit AES cipher as it would a conventional computer to decode a 128 bit AES cipher. For this reason, AES-256 is believed to be "quantum resistant".


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 either the sender or the recipient 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 Diffie–Hellman key exchangeSynonyms of Diffie–Hellman key exchange include: * Diffie–Hellman–Merkle key exchange * Diffie–Hellman key agreement * Diffie–Hellman key establishment * Diffie–Hellman key negotiation * Exponential key exc ...
or some other public-key protocol to securely come to agreement on a fresh new secret key for each session/conversation (forward secrecy).


Key generation

When used with asymmetric ciphers for key transfer, pseudorandom key generators are nearly always used to generate the symmetric cipher session keys. However, lack of randomness in those generators or in their initialization vectors is disastrous and has led to cryptanalytic breaks in the past. Therefore, it is essential that an implementation use a source of high 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. Practically all mechanical cipher machines implement a reciprocal cipher, a mathematical involution on each typed-in letter. Instead of designing two kinds of machines, one for encrypting and one for decrypting, all the machines can be identical and can be set up (keyed) the same way. Examples of reciprocal ciphers include: *
Atbash Atbash ( he, אתבש; also transliterated Atbaš) is a monoalphabetic substitution cipher originally used to encrypt the Hebrew alphabet. It can be modified for use with any known writing system with a standard collating order. Encryption Th ...
*
Beaufort cipher The Beaufort cipher, created by Sir Francis Beaufort, is a substitution cipher similar to the Vigenère cipher, with a slightly modified enciphering mechanism and tableau. Its most famous application was in a rotor-based cipher machine, the H ...
* Enigma machine * Marie Antoinette and Axel von Fersen communicated with a self-reciprocal cipher. * the Porta polyalphabetic cipher is self-reciprocal. *
Purple cipher In the history of cryptography, the "System 97 Typewriter for European Characters" (九七式欧文印字機) or "Type B Cipher Machine", codenamed Purple by the United States, was an encryption machine used by the Japanese Foreign Office fr ...
* RC4 * ROT13 * XOR cipher * Vatsyayana cipher The majority of all modern ciphers can be classified as either a
stream cipher stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream ...
, most of which use a reciprocal XOR cipher combiner, or a
block cipher In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called ''blocks''. Block ciphers are specified cryptographic primitive, elementary components in the design of many cryptographic protocols and ...
, most of which use a Feistel cipher or Lai–Massey scheme with a reciprocal transformation in each round.


Notes


References

{{DEFAULTSORT:Symmetric-Key Algorithm Cryptographic algorithms