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19-463: The Service central de la sécurité des systèmes d'informations ( SCSSI ) (English: Central Service for Information System Security) was responsible for regulation of the use of cryptosystems by the government of France . Users for authentication must make a declaration to the SCSSI that is then verified, and users for privacy purposes are required to have an authorization from SCSSI. SCSSI superseded

38-567: A symmetric-key or public-key type of cryptosystem. A classical example of a cryptosystem is the Caesar cipher . A more contemporary example is the RSA cryptosystem. Another example of a cryptosystem is the Advanced Encryption Standard (AES). AES is a widely used symmetric encryption algorithm that has become the standard for securing data in various applications. Paillier cryptosystem

57-543: A cryptosystem consists of three algorithms: one for key generation , one for encryption, and one for decryption. The term cipher (sometimes cypher ) is often used to refer to a pair of algorithms, one for encryption and one for decryption. Therefore, the term cryptosystem is most often used when the key generation algorithm is important. For this reason, the term cryptosystem is commonly used to refer to public key techniques; however both "cipher" and "cryptosystem" are used for symmetric key techniques. Mathematically,

76-413: A cryptosystem or encryption scheme can be defined as a tuple ( P , C , K , E , D ) {\displaystyle ({\mathcal {P}},{\mathcal {C}},{\mathcal {K}},{\mathcal {E}},{\mathcal {D}})} with the following properties. For each e ∈ K {\displaystyle e\in {\mathcal {K}}} , there

95-427: A fresh new secret key for each session/conversation (forward secrecy). 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

114-425: 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. See

133-486: 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

152-418: 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: The majority of all modern ciphers can be classified as either a stream cipher , most of which use a reciprocal XOR cipher combiner, or

171-405: A source of high entropy for its initialization. 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

190-772: 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 in December 2001, uses 128-bit blocks. Examples of popular symmetric-key algorithms include Twofish , Serpent , AES (Rijndael), Camellia , Salsa20 , ChaCha20 , Blowfish , CAST5 , Kuznyechik , RC4 , DES , 3DES , Skipjack , Safer , and IDEA . Symmetric ciphers are commonly used to achieve other cryptographic primitives than just encryption. Encrypting

209-437: Is d ∈ K {\displaystyle d\in {\mathcal {K}}} such that D d ( E e ( p ) ) = p {\displaystyle D_{d}(E_{e}(p))=p} for all p ∈ P {\displaystyle p\in {\mathcal {P}}} . Note; typically this definition is modified in order to distinguish an encryption scheme as being either

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228-406: Is another example used to preserve and maintain privacy and sensitive information. It is featured in electronic voting, electronic lotteries and electronic auctions. This cryptography-related article is a stub . You can help Misplaced Pages by expanding it . Symmetric-key algorithm Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both

247-636: 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. 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. Symmetric ciphers have historically been susceptible to known-plaintext attacks , chosen-plaintext attacks , differential cryptanalysis and linear cryptanalysis . Careful construction of

266-412: 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. Symmetric-key encryption can use either stream ciphers or block ciphers . Stream ciphers encrypt the digits (typically bytes ), or letters (in substitution ciphers) of a message one at

285-917: The Interministerial Office for Information Systems Security Service (DCSSI) in February 1996, and French law on ciphering changed in July 1996. It is somewhat similar to the British Communications-Electronics Security Group ( CESG ), which is part of GCHQ (the equivalent French organisation is DGSE ). SCSSI was one of the founding organisations of the Common Criteria , together with GCHQ , BSI , and NLNCSA SCSSI became DCSSI in 2001 (cf. décret no 2001-693 du 31 juillet 2001 ) which became ANSSI in 2009 (cf. décret n° 2009-834 du 7 juillet 2009 ). Cryptosystem Typically,

304-611: 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. With exception of

323-432: 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

342-437: 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

361-415: 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". Symmetric-key algorithms require both the sender and the recipient of

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