Symmetric-key algorithm

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Symmetric-key encryption: the same key is used for both encryption and decryption Simple symmetric encryption.png
Symmetric-key encryption: the same key is used for both encryption and decryption

Symmetric-key algorithms [lower-alpha 1] 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. [1] The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link. [2] 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). [3] [4] However, symmetric-key encryption algorithms are usually better for bulk encryption. With exception of the one-time pad 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. [5] [6] [7]

Contents

Types

Symmetric-key encryption can use either stream ciphers or block ciphers. [8]

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. [9]

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 in December 2001, uses 128-bit blocks.

Implementations

Examples of popular symmetric-key algorithms include Twofish, Serpent, AES (Rijndael), Camellia, Salsa20, ChaCha20, Blowfish, CAST5, Kuznyechik, RC4, DES, 3DES, Skipjack, Safer, and IDEA. [10]

Use as a cryptographic primitive

Symmetric ciphers are commonly used to achieve other cryptographic primitives than just encryption.[ citation needed ]

Encrypting a message does not guarantee that it will remain unchanged 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 an AEAD cipher (e.g. AES-GCM).

However, symmetric ciphers cannot be used for non-repudiation purposes except by involving additional parties. [11] See the 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.[ citation needed ]

Security of symmetric ciphers

Symmetric ciphers have historically 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.[ citation needed ] 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. [12]

Most modern symmetric-key algorithms appear to be resistant to the threat of post-quantum cryptography. [13] 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. [14] 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. [15] For this reason, AES-256 is believed to be "quantum resistant". [16] [17]

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 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. [18] [19] [20]

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. [21] [22]

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. [23]

Examples of reciprocal ciphers include:

The majority of all modern ciphers can be classified as either a stream cipher, most of which use a reciprocal XOR cipher combiner, or a block cipher, most of which use a Feistel cipher or Lai–Massey scheme with a reciprocal transformation in each round. [29]

Notes

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

Related Research Articles

Blowfish is a symmetric-key block cipher, designed in 1993 by Bruce Schneier and included in many cipher suites and encryption products. Blowfish provides a good encryption rate in software, and no effective cryptanalysis of it has been found to date. However, the Advanced Encryption Standard (AES) now receives more attention, and Schneier recommends Twofish for modern applications.

In cryptography, a block cipher is a deterministic algorithm that operates on fixed-length groups of bits, called blocks. Block ciphers are the elementary building blocks of many cryptographic protocols. They are ubiquitous in the storage and exchange of data, where such data is secured and authenticated via encryption.

<span class="mw-page-title-main">Cipher</span> Algorithm for encrypting and decrypting information

In cryptography, a cipher is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "code", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.

<span class="mw-page-title-main">Cryptanalysis</span> Study of analyzing information systems in order to discover their hidden aspects

Cryptanalysis refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic security systems and gain access to the contents of encrypted messages, even if the cryptographic key is unknown.

<span class="mw-page-title-main">Encryption</span> Process of converting plaintext to ciphertext

In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decipher a ciphertext back to plaintext and access the original information. Encryption does not itself prevent interference but denies the intelligible content to a would-be interceptor.

<span class="mw-page-title-main">One-time pad</span> Encryption technique

In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is larger than or equal to the size of the message being sent. In this technique, a plaintext is paired with a random secret key. Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition.

A chosen-plaintext attack (CPA) is an attack model for cryptanalysis which presumes that the attacker can obtain the ciphertexts for arbitrary plaintexts. The goal of the attack is to gain information that reduces the security of the encryption scheme.

In cryptography, an initialization vector (IV) or starting variable is an input to a cryptographic primitive being used to provide the initial state. The IV is typically required to be random or pseudorandom, but sometimes an IV only needs to be unpredictable or unique. Randomization is crucial for some encryption schemes to achieve semantic security, a property whereby repeated usage of the scheme under the same key does not allow an attacker to infer relationships between segments of the encrypted message. For block ciphers, the use of an IV is described by the modes of operation.

In cryptography, a block cipher mode of operation is an algorithm that uses a block cipher to provide information security such as confidentiality or authenticity. A block cipher by itself is only suitable for the secure cryptographic transformation of one fixed-length group of bits called a block. A mode of operation describes how to repeatedly apply a cipher's single-block operation to securely transform amounts of data larger than a block.

<span class="mw-page-title-main">Ciphertext</span> Encrypted information

In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher to decrypt it. This process prevents the loss of sensitive information via hacking. Decryption, the inverse of encryption, is the process of turning ciphertext into readable plaintext. Ciphertext is not to be confused with codetext because the latter is a result of a code, not a cipher.

Articles related to cryptography include:

In cryptography, Khufu and Khafre are two block ciphers designed by Ralph Merkle in 1989 while working at Xerox's Palo Alto Research Center. Along with Snefru, a cryptographic hash function, the ciphers were named after the Egyptian Pharaohs Khufu, Khafre and Sneferu.

Probabilistic encryption is the use of randomness in an encryption algorithm, so that when encrypting the same message several times it will, in general, yield different ciphertexts. The term "probabilistic encryption" is typically used in reference to public key encryption algorithms; however various symmetric key encryption algorithms achieve a similar property, and stream ciphers such as Freestyle which are inherently random. To be semantically secure, that is, to hide even partial information about the plaintext, an encryption algorithm must be probabilistic.

Multiple encryption is the process of encrypting an already encrypted message one or more times, either using the same or a different algorithm. It is also known as cascade encryption, cascade ciphering, multiple encryption, and superencipherment. Superencryption refers to the outer-level encryption of a multiple encryption.

Encryption software is software that uses cryptography to prevent unauthorized access to digital information. Cryptography is used to protect digital information on computers as well as the digital information that is sent to other computers over the Internet.

In cryptanalysis, attack models or attack types are a classification of cryptographic attacks specifying the kind of access a cryptanalyst has to a system under attack when attempting to "break" an encrypted message generated by the system. The greater the access the cryptanalyst has to the system, the more useful information they can get to utilize for breaking the cypher.

In cryptography, a distinguishing attack is any form of cryptanalysis on data encrypted by a cipher that allows an attacker to distinguish the encrypted data from random data. Modern symmetric-key ciphers are specifically designed to be immune to such an attack. In other words, modern encryption schemes are pseudorandom permutations and are designed to have ciphertext indistinguishability. If an algorithm is found that can distinguish the output from random faster than a brute force search, then that is considered a break of the cipher.

<span class="mw-page-title-main">Cryptography</span> Practice and study of secure communication techniques

Cryptography, or cryptology, is the practice and study of techniques for secure communication in the presence of adversarial behavior. More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, information security, electrical engineering, digital signal processing, physics, and others. Core concepts related to information security are also central to cryptography. Practical applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, and military communications.

In cryptography, format-preserving encryption (FPE), refers to encrypting in such a way that the output is in the same format as the input. The meaning of "format" varies. Typically only finite sets of characters are used; numeric, alphabetic or alphanumeric. For example:

The following outline is provided as an overview of and topical guide to cryptography:

References

  1. ↑ Kartit, Zaid (February 2016). "Applying Encryption Algorithms for Data Security in Cloud Storage, Kartit, et al". Advances in Ubiquitous Networking: Proceedings of UNet15: 147. ISBN   9789812879905.
  2. ↑ Delfs, Hans; Knebl, Helmut (2007). "Symmetric-key encryption". Introduction to cryptography: principles and applications. Springer. ISBN   9783540492436.
  3. ↑ Mullen, Gary; Mummert, Carl (2007). Finite fields and applications. American Mathematical Society. p. 112. ISBN   9780821844182.
  4. ↑ "Demystifying symmetric and asymmetric methods of encryption". Geeks for Geeks. 2017-09-28.
  5. ↑ Johnson, Leighton (2016), "Security Component Fundamentals for Assessment", Security Controls Evaluation, Testing, and Assessment Handbook, Elsevier, pp. 531–627, doi:10.1016/b978-0-12-802324-2.00011-7, ISBN   9780128023242, S2CID   63087943 , retrieved 2021-12-06
  6. ↑ Alvarez, Rafael; Caballero-Gil, Cándido; Santonja, Juan; Zamora, Antonio (2017-06-27). "Algorithms for Lightweight Key Exchange". Sensors. 17 (7): 1517. doi: 10.3390/s17071517 . ISSN   1424-8220. PMC   5551094 . PMID   28654006.
  7. ↑ Bernstein, Daniel J.; Lange, Tanja (2017-09-14). "Post-quantum cryptography". Nature. 549 (7671): 188–194. Bibcode:2017Natur.549..188B. doi:10.1038/nature23461. ISSN   0028-0836. PMID   28905891. S2CID   4446249.
  8. ↑ Pelzl & Paar (2010). Understanding Cryptography . Berlin: Springer-Verlag. p.  30. Bibcode:2010uncr.book.....P.
  9. ↑ Bellare, Mihir; Rogaway, Phillip (2005). Introduction to Modern Cryptography (PDF).
  10. ↑ Roeder, Tom. "Symmetric-Key Cryptography". www.cs.cornell.edu. Retrieved 2017-02-05.
  11. ↑ "ISO/IEC 13888-2:2010". ISO. Retrieved 2020-02-04.
  12. ↑ David R. Mirza Ahmad; Ryan Russell (2002). Hack proofing your network (2nd ed.). Rockland, MA: Syngress. pp. 165–203. ISBN   1-932266-18-6. OCLC   51564102.
  13. ↑ Daniel J. Bernstein (2009). "Introduction to post-quantum cryptography" (PDF). Post-Quantum Cryptography.
  14. ↑ Daniel J. Bernstein (2010-03-03). "Grover vs. McEliece" (PDF).{{cite journal}}: Cite journal requires |journal= (help)
  15. ↑ Wood, Lamont (2011-03-21). "The Clock Is Ticking for Encryption". Computerworld. Retrieved 2022-12-05.
  16. ↑ O'Shea, Dan (2022-04-29). "AES-256 joins the quantum resistance". Fierce Electronics. Retrieved 2022-12-05.
  17. ↑ Weissbaum, François; Lugrin, Thomas (2023), Mulder, Valentin; Mermoud, Alain; Lenders, Vincent; Tellenbach, Bernhard (eds.), "Symmetric Cryptography", Trends in Data Protection and Encryption Technologies, Cham: Springer Nature Switzerland, pp. 7–10, doi: 10.1007/978-3-031-33386-6_2 , ISBN   978-3-031-33386-6 , retrieved 2023-09-12
  18. ↑ Ian Goldberg and David Wagner. "Randomness and the Netscape Browser". January 1996 Dr. Dobb's Journal. quote: "it is vital that the secret keys be generated from an unpredictable random-number source."
  19. ↑ Ristenpart, Thomas; Yilek, Scott (2010). "When Good Randomness Goes Bad: Virtual Machine Reset Vulnerabilities and Hedging Deployed Cryptography" (PDF). NDSS Symposium 2010. Random number generators (RNGs) are consistently a weak link in the secure use of cryptography.
  20. ↑ "Symmetric Cryptography". James. 2006-03-11.
  21. ↑ Paul Reuvers and Marc Simons. Crypto Museum. "Enigma Uhr". 2009.
  22. ↑ Chris Christensen. "Simple Substitution Ciphers". 2006.
  23. ↑ Greg Goebel. "The Mechanization of Ciphers". 2018.
  24. ↑ "... the true Beaufort cipher. Notice that we have reciprocal encipherment; encipherment and decipherment are identically the same thing." -- Helen F. Gaines. "Cryptanalysis: A Study of Ciphers and Their Solution". 2014. p. 121.
  25. ↑ Greg Goebel. "The Mechanization of Ciphers". 2018.
  26. ↑ Friedrich L. Bauer. "Decrypted Secrets: Methods and Maxims of Cryptology". 2006. p. 144
  27. ↑ David Salomon. "Coding for Data and Computer Communications". 2006. p. 245
  28. ↑ Greg Goebel. "US Codebreakers In The Shadow Of War". 2018.
  29. ↑ says, J. H. (2021-01-14). "Block Cipher vs Stream Cipher: What They Are & How They Work". Hashed Out by The SSL Store™. Retrieved 2021-09-05.
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