In cryptography, a block cipher is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation. Cryptography (or cryptology; from Greek grc κρυπτός kryptos, "hidden secret" and grc γράφω gráphō, "I write" Symmetric-key algorithms are a class of Algorithms for Cryptography that use trivially related often identical Cryptographic keys for both decryption In Cryptography, a cipher (or cypher) is an Algorithm for performing Encryption and Decryption &mdash a series of well-defined steps A bit is a binary digit, taking a value of either 0 or 1 Binary digits are a basic unit of Information storage and communication When encrypting, a block cipher might take a (for example) 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext. In Cryptography, plaintext is the information which the sender wishes to transmit to the receiver(s The exact transformation is controlled using a second input — the secret key. In Cryptography, a key is a piece of information (a Parameter) that determines the functional output of a cryptographic algorithm Decryption is similar: the decryption algorithm takes, in this example, a 128-bit block of ciphertext together with the secret key, and yields the original 128-bit block of plaintext.
To encrypt messages longer than the block size (128 bits in the above example), a mode of operation is used. In modern Cryptography, symmetric key Ciphers are generally divided into Stream ciphers and Block ciphers Block ciphers operate on a
Block ciphers can be contrasted with stream ciphers; a stream cipher operates on individual digits one at a time, and the transformation varies during the encryption. In Cryptography, a stream cipher is a symmetric key Cipher where plaintext bits are combined with a Pseudorandom cipher bit stream ( Keystream The distinction between the two types is not always clear-cut: a block cipher, when used in certain modes of operation, acts effectively as a stream cipher.
An early and highly influential block cipher design was the Data Encryption Standard (DES), developed at IBM and published as a standard in 1977. The Data Encryption Standard ( DES) is a Cipher (a method for Encrypting information selected by NBS as an official Federal Information International Business Machines Corporation abbreviated IBM and nicknamed "Big Blue", is a multinational Computer Technology A successor to DES, the Advanced Encryption Standard (AES), was adopted in 2001. In Cryptography, the Advanced Encryption Standard ( AES) also known as Rijndael, is a Block cipher adopted as an Encryption
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Generalities
A block cipher consists of two paired algorithms, one for encryption, E, and another for decryption, E-1. Both algorithms accept two inputs: an input block of size n bits and a key of size k bits, yielding an n-bit output block. In Cryptography, a key is a piece of information (a Parameter) that determines the functional output of a cryptographic algorithm For any one fixed key, decryption is the inverse function of encryption, so that
for any block M and key K. In Mathematics, if &fnof is a function from A to B then an inverse function for &fnof is a function in the opposite direction from B
For each key K, EK is a permutation (a bijective mapping) over the set of input blocks. In several fields of Mathematics the term permutation is used with different but closely related meanings In Mathematics, a bijection, or a bijective function is a function f from a set X to a set Y with the property Each key selects one permutation from the possible set of 2n!.
The block size, n, is typically 64 or 128 bits, although some ciphers have a variable block size. In modern Cryptography, symmetric key Ciphers are generally divided into Stream ciphers and Block ciphers Block ciphers operate on a 64 bits was the most common length until the mid-1990s, when new designs began to switch to the longer 128-bit length. One of several modes of operation is generally used along with a padding scheme to allow plaintexts of arbitrary lengths to be encrypted. In Cryptography, padding refers to a number of distinct practices Each mode has different characteristics in regard to error propagation, ease of random access and vulnerability to certain types of attack. Typical key sizes (k) include 40, 56, 64, 80, 128, 192 and 256 bits. In Cryptography, key size or key length is the size (usually measured in bits or bytes of the key used in a cryptographic algorithm (such as a Cipher As of 2006, 80 bits is normally taken as the minimum key length needed to prevent brute force attacks. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. In Cryptanalysis, a brute force attack is a method of defeating a Cryptographic scheme by trying a large number of possibilities for example possible keys
Iterated block ciphers
Most block ciphers are constructed by repeatedly applying a simpler function. This approach is known as iterated block cipher (see also product cipher). In Cryptography, a product cipher is a popular type of Block cipher that works by executing in sequence a number of simple transformations such as substitution Each iteration is termed a round, and the repeated function is termed the round function; anywhere between 4 to 32 rounds are typical.
Many block ciphers can be categorised as Feistel networks, or, as more general substitution-permutation networks. In Cryptography, a Feistel cipher is a symmetric structure used in the construction of Block ciphers named after the German IBM cryptographer Horst In Cryptography, an SP-network, or substitution- Permutation network ( SPN) is a series of linked mathematical operations used in Block cipher Arithmetic operations, logical operations (especially XOR), S-boxes and various permutations are all frequently used as components. Arithmetic or arithmetics (from the Greek word αριθμός = number is the oldest and most elementary branch of mathematics used by almost everyone A logic gate performs a logical operation on one or more logic inputs and produces a single logic output In Cryptography, a substitution box (or S-box) is a basic component of Symmetric key algorithms In Block ciphers they are typically used to obscure In several fields of Mathematics the term permutation is used with different but closely related meanings
History
Lucifer is generally considered to be the first civilian block cipher, developed at IBM in the 1970s based on work done by Horst Feistel. In Cryptography, Lucifer was the name given to several of the earliest civilian Block ciphers developed by Horst Feistel and his colleagues at IBM International Business Machines Corporation abbreviated IBM and nicknamed "Big Blue", is a multinational Computer Technology Horst Feistel ( 30 January 1915 (1 &ndash 14 November 1990) was a Cryptographer who worked on the design of A revised version of the algorithm was adopted as a US government FIPS standard, the Data Encryption Standard (DES). The United States of America —commonly referred to as the Federal Information Processing Standards ( FIPS) are publicly announced standards developed by the United States Federal government for use by all non-military The Data Encryption Standard ( DES) is a Cipher (a method for Encrypting information selected by NBS as an official Federal Information It was chosen by the US National Bureau of Standards (NBS) after a public invitation for submissions and some internal changes by NBS (and, potentially, the NSA). The National Security Agency/ Central Security Service ( NSA/CSS) is a cryptologic intelligence agency of the United States government DES was publicly released in 1976 and has been widely used.
DES was designed, among other things, to resist a certain cryptanalytic attack known to the NSA and rediscovered by IBM, though unknown publicly until rediscovered again and published by Eli Biham and Adi Shamir in the late 1980s. Eli Biham (אלי ביהם is an Israeli Cryptographer and cryptanalyst, currently a professor at the Technion Israeli Institute of Technology Adi Shamir (עדי שמיר born 1952 is an Israeli cryptographer. The technique is called differential cryptanalysis and remains one of the few general attacks against block ciphers; linear cryptanalysis is another, but may have been unknown even to NSA, prior to its publication by Mitsuru Matsui. Differential cryptanalysis is a general form of Cryptanalysis applicable primarily to Block ciphers but also to Stream ciphers and Cryptographic hash In Cryptography, linear cryptanalysis is a general form of Cryptanalysis based on finding affine approximations to the action of a Cipher. is a Japanese Cryptographer and senior researcher for Mitsubishi Electric Company DES prompted a large amount of other work and publications in cryptography and cryptanalysis in the open community and it inspired many new cipher designs. Cryptography (or cryptology; from Greek grc κρυπτός kryptos, "hidden secret" and grc γράφω gráphō, "I write" Cryptanalysis (from the Greek kryptós, "hidden" and analýein, "to loosen" or "to untie" is the study of methods for
DES has a block size of 64 bits and a key size of 56 bits. In Cryptography, key size or key length is the size (usually measured in bits or bytes of the key used in a cryptographic algorithm (such as a Cipher 64-bit blocks became common in block cipher designs after DES. Key length depended on several factors, including government regulation. In Cryptography, a key is a piece of information (a Parameter) that determines the functional output of a cryptographic algorithm Many observers in the 1970s commented that the 56-bit key length used for DES was too short. As time went on, its inadequacy became apparent, especially after a special purpose machine designed to break DES was demonstrated in 1998 by the Electronic Frontier Foundation. The Electronic Frontier Foundation ( EFF) is an international non-profit advocacy and legal organization based in the United States with the stated purpose of being dedicated A variant of DES, Triple DES, triple-encrypts blocks with (usually) two different keys (2TDES), resulting in a 112-bit keys and 80-bit security. In Cryptography, Triple DES is a Block cipher formed from the Data Encryption Standard (DES Cipher by using it three times It was widely adopted as a replacement and is still (2004) considered secure.
DES has been superseded as a United States Federal Standard by the Advanced Encryption Standard (AES), adopted by National Institute of Standards and Technology (NIST) in 2001 after a 5-year public competition. The United States of America —commonly referred to as the In Cryptography, the Advanced Encryption Standard ( AES) also known as Rijndael, is a Block cipher adopted as an Encryption The Advanced Encryption Standard (AES the Block cipher ratified as a standard by National Institute of Standards and Technology of the United States (NIST was The cipher was developed by two Belgian cryptographers, Joan Daemen and Vincent Rijmen, and submitted under the name Rijndael. The Kingdom of Belgium is a Country in northwest Europe. It is a founding member of the European Union and hosts its headquarters as well as those Joan Daemen (born 1965 in Achel, Limburg, Belgium) is a Belgian Cryptographer and one of the designers of Rijndael, the Advanced Vincent Rijmen (born 16 October 1970, in Leuven, near Brussels, Belgium) is a Belgian Cryptographer and one of the designers (See AES page for pronunciation. In Cryptography, the Advanced Encryption Standard ( AES) also known as Rijndael, is a Block cipher adopted as an Encryption ) AES has a block size of 128 bits and three possible key sizes, 128, 192 and 256 bits. In Cryptography, key size or key length is the size (usually measured in bits or bytes of the key used in a cryptographic algorithm (such as a Cipher The US Government permits the use of AES to protect classified information in systems approved by NSA. The National Security Agency/ Central Security Service ( NSA/CSS) is a cryptologic intelligence agency of the United States government
Cryptanalysis
In addition to linear and differential cryptanalysis, there is a growing catalog of attacks: truncated differential cryptanalysis, partial differential cryptanalysis, integral cryptanalysis, which encompasses square and integral attacks, slide attacks, boomerang attacks, the XSL attack, impossible differential cryptanalysis and algebraic attacks. Differential cryptanalysis is a general form of Cryptanalysis applicable primarily to Block ciphers but also to Stream ciphers and Cryptographic hash In Cryptography, truncated differential cryptanalysis is a generalization of Differential cryptanalysis, an attack against Block ciphers Lars Knudsen In Cryptography, integral cryptanalysis is a cryptanalytic attack that is particularly applicable to Block ciphers based on Substitution-permutation The slide attack is a form of Cryptanalysis designed to deal with the prevailing idea that even weak Ciphers can become very strong by increasing the number of rounds In Cryptography, the boomerang attack is a method for the Cryptanalysis of Block ciphers based on Differential cryptanalysis. In Cryptography, the XSL attack is a method of Cryptanalysis for Block ciphers The attack was first In Cryptography, impossible differential cryptanalysis is a form of Differential cryptanalysis for Block ciphers. For a new block cipher design to have any credibility, it must demonstrate evidence of security against known attacks.
Tweakable block ciphers
M. Liskov, R. Rivest, and D. Wagner have described a generalized version of block ciphers called "tweakable" block ciphers. A tweakable block cipher accepts a second input called the tweak along with its usual plaintext or ciphertext input. The tweak, along with the key, selects the permutation computed by the cipher. If changing tweaks is sufficiently lightweight (compared with a usually-fairly-expensive key setup operation), then some interesting new operation modes become possible. The disk encryption theory article describes some of these modes. Disk encryption is a special case of data at rest protection when the storage media is a sector-addressable device (e
Block ciphers and other cryptographic primitives
Block ciphers can be used to build other cryptographic primitives. For these other primitives to be cryptographically secure care has to be taken to build them the right way.
Stream ciphers can be built using block ciphers. In Cryptography, a stream cipher is a symmetric key Cipher where plaintext bits are combined with a Pseudorandom cipher bit stream ( Keystream OFB-mode and CTR mode are block modes that turn a block cipher into a stream cipher.
Cryptographic hash functions can be built using block ciphers. A cryptographic Hash function is a transformation that takes an input (or 'message' and returns a fixed-size string which is called the hash value (sometimes See one-way compression function for descriptions of several such methods. In Cryptography, a one-way compression function is a function that transforms two fixed length inputs to an output of the same size as one of the inputs The methods resemble the block cipher modes of operation usually used for encryption.
Just as block ciphers can be used to build hash functions, hash functions can be used to build block ciphers. A cryptographic Hash function is a transformation that takes an input (or 'message' and returns a fixed-size string which is called the hash value (sometimes Examples of such block ciphers are SHACAL, BEAR and LION. In Cryptography, SHACAL-1 and SHACAL-2 are Block ciphers based on Cryptographic hash functions from the SHA family.
Cryptographically secure pseudorandom number generators (CSPRNGs) can be built using block ciphers. A cryptographically secure pseudo-random number generator ( CSPRNG) is a Pseudo-random number generator (PRNG with properties that make it suitable for use in
Message authentication codes (MACs) are often built from block ciphers. A cryptographic message authentication code (MAC is a short piece of information used to Authenticate a message CBC-MAC, OMAC and PMAC are such MACs. In Cryptography, a Cipher Block Chaining Message Authentication Code, abbreviated CBC-MAC, is a technique for constructing a Message authentication code OMAC (One-key MAC is a Message authentication code constructed from a Block cipher much like the PMAC algorithm PMAC, which stands for Parallelizable MAC, is a Message authentication code algorithm
Authenticated encryption is also built from block ciphers. Authenticated Encryption (AE is a term used to describe Encryption systems which simultaneously protect Confidentiality and Authenticity (integrity of It means to both encrypt and MAC at the same time. That is to both provide confidentiality and authentication. Authentication (from Greek αυθεντικός real or genuine from authentes author is the act of establishing or confirming something (or someone as CCM, EAX, GCM and OCB are such authenticated encryption modes. CCM mode (Counter with CBC-MAC) is a mode of operation for cryptographic Block ciphers It is an Authenticated encryption algorithm designed EAX mode is a mode of operation for cryptographic block ciphers GCM mode (Galois/Counter Mode is a mode of operation for symmetric key cryptographic Block ciphers It is an Authenticated encryption algorithm designed OCB mode (Offset Codebook Mode is a mode of operation for cryptographic Block ciphers Encryption and authentication It was designed to provide
See also
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Cryptography Portal |
- Cryptography
- Block cipher modes of operation
- Confusion and diffusion
- Pseudorandom permutation
- Advanced Encryption Standard process
- Topics in cryptography
- Disk encryption
References
- M. Cryptography (or cryptology; from Greek grc κρυπτός kryptos, "hidden secret" and grc γράφω gráphō, "I write" In Cryptography, confusion and diffusion are two properties of the operation of a secure Cipher which were identified by Claude Shannon in his In Cryptography, a pseudorandom permutation, abbreviated PRP, is an idealized Block cipher. The Advanced Encryption Standard (AES the Block cipher ratified as a standard by National Institute of Standards and Technology of the United States (NIST was This article is intended to be an 'analytic glossary' or alternatively an organized collection of annotated pointers Liskov, R. Rivest, and D. Wagner, "Tweakable Block Ciphers", Crypto 2002 PDF.
External links
- A list of many symmetric algorithms, the majority of which are block ciphers.
- The block cipher lounge
- What is a block cipher? from RSA FAQ
- Block Ciphers and Cryptanalysis (PDF). FAQ is an Acronym for " Frequently Asked Question(s " Technical Report: an introduction to mathematics of block ciphers and methods for cryptanalysis.
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