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Source wikipedia
http://en.wikipedia.org/wiki/Cipher
What Cipher means
In cryptography, a cipher (or cypher) is an algorithm for performing encryption and decryption â a series of well-defined steps that can be followed as a procedure. An alternative term is encipherment. In non-technical usage, a âcipherâ is the same thing as a âcodeâ; however, the concepts are distinct in cryptography. In classical cryptography, ciphers were distinguished from codes. Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase. For example, âUQJHSEâ could be the code for âProceed to the following coordinatesâ. When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it; it should resemble random gibberish to those not intended to read it.
The operation of a cipher usually depends on a piece of auxiliary information, called a key or, in traditional NSA parlance, a cryptovariable. The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be difficult, if not nearly impossible, to decrypt the resulting cipher into readable plaintext.
Most modern ciphers can be categorized in several ways:
* By whether they work on blocks of symbols usually of a fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).
* By whether the same key is used for both encryption and decryption (symmetric key algorithms), or if a different key is used for each (asymmetric key algorithms). If the algorithm is symmetric, the key must be known to the recipient and to no one else. If the algorithm is an asymmetric one, the encyphering key is different from, but closely related to, the decyphering key. If one key cannot be deduced from the other, the asymmetric key algorithm has the public/private key property and one of the keys may be made public without loss of confidentiality. The Feistel cipher uses a combination of substitution and transposition techniques. Most block cipher algorithms are based on this structure.
Contents
[hide]
* 1 Etymology of âCipherâ
* 2 Ciphers versus codes
* 3 Types of Cipher
o 3.1 Historical ciphers
o 3.2 Modern ciphers
* 4 Key Size and Vulnerability
* 5 References
* 6 See also
* 7 External links
[edit] Etymology of âCipherâ
âCipherâ (Middle French as cifre and Medieval Latin as cifra, from the Arabic sifr = zero) is alternatively spelled âcypherâ (however, this variant is now uncommon); similarly âciphertextâ and âcyphertextâ, and so forth.
The word âcipherâ in former times meant âzeroâ and had the same origin (see Zero â Etymology), and later was used for any decimal digit, even any number. There are these theories about how the word âcipherâ may have come to mean encoding:
* Encoding often involved numbers.
* The Roman number system was very cumbersome because there was no concept of zero (or empty space). The concept of zero (which was also called âcipherâ), which we all now think of as natural, was very alien in medieval Europe, so confusing and ambiguous to common Europeans that in arguments people would say âtalk clearly and not so far fetched as a cipherâ. Cipher came to mean concealment of clear messages or encryption.
o The French formed the word âchiffreâ and adopted the Italian word âzeroâ.
o The English used âzeroâ for â0â, and âcipherâ from the word âcipheringâ as a means of computing.
o The Germans used the words âZifferâ (digit, âZahlâ) and âChiffreâ.
Dr. Al-Kadi (ref-3) concluded that the Arabic word sifr, for the digit zero, developed into the European technical term for encryption.
[edit] Ciphers versus codes
Main article: Code (cryptography)
In non-technical usage, a â(secret) codeâ typically means a âcipherâ. Within technical discussions, however, the words âcodeâ and âcipherâ refer to two different concepts. Codes work at the level of meaning â that is, words or phrases are converted into something else and this chunking generally shortens the message.
An example of this is the Telegraph Code which were used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of Telegrams.
Ciphers, on the other hand, work at a lower level: the level of individual letters, small groups of letters, or, in modern schemes, individual bits. Some systems used both codes and ciphers in one system, using superencipherment to increase the security. In some cases the terms codes and ciphers are also used synonymously to substitution and transposition.
Historically, cryptography was split into a dichotomy of codes and ciphers; and coding had its own terminology, analogous to that for ciphers: âencoding, codetext, decodingâ and so on.
However, codes have a variety of drawbacks, including susceptibility to cryptanalysis and the difficulty of managing a cumbersome codebook. Because of this, codes have fallen into disuse in modern cryptography, and ciphers are the dominant technique.
[edit] Types of Cipher
There are a variety of different types of encryption. Algorithms used earlier in the history of cryptography are substantially different from modern methods, and modern ciphers can be classified according to how they operate and whether they use one or two keys.
[edit] Historical ciphers
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include simple substitution ciphers and transposition ciphers. For example âGOOD DOGâ can be encrypted as âPLLX XLPâ where âLâ substitutes for âOâ, âPâ for âGâ, and âXâ for âDâ in the message. Transposition of the letters âGOOD DOGâ can result in âDGOGDOOâ. These simple ciphers and examples are easy to crack, even without plaintext-ciphertext pairs.
Simple ciphers were replaced by polyalphabetic substitution ciphers which changed the substitution alphabet for every letter. For example âGOOD DOGâ can be encrypted as âPLSX TWFâ where âLâ, âSâ, and âWâ substitute for âOâ. With even a small amount of known or estimated plaintext, simple polyalphabetic substitution ciphers and letter transposition ciphers designed for pen and paper encryption are easy to crack.
During the early twentieth century, electro-mechanical machines were invented to do encryption and decryption using transposition, polyalphabetic substitution, and a kind of âadditiveâ substitution. In rotor machines, several rotor disks provided polyalphabetic substitution, while plug boards provided another substitution. Keys were easily changed by changing the rotor disks and the plugboard wires. Although these encryption methods were more complex than previous schemes and required machines to encrypt and decrypt, other machines such as the British Bombe were invented to crack these encryption methods.
[edit] Modern ciphers
Modern encryption methods can be divided by two criteria: by type of key used, and by type of input data.
By type of key used ciphers are divided into:
* symmetric key algorithms (Private-key cryptography), where the same key is used for encryption and decryption, and
* asymmetric key algorithms (Public-key cryptography), where two different keys are used for encryption and decryption.
In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption.
Type of input ciphers data can be distinguished into two types:
* block ciphers, which encrypt block of data of fixed size, and
* stream ciphers, which encrypt continuous streams of data
Registered at Network Solutions
Talking offers above $250
Source wikipedia
http://en.wikipedia.org/wiki/Cipher
What Cipher means
In cryptography, a cipher (or cypher) is an algorithm for performing encryption and decryption â a series of well-defined steps that can be followed as a procedure. An alternative term is encipherment. In non-technical usage, a âcipherâ is the same thing as a âcodeâ; however, the concepts are distinct in cryptography. In classical cryptography, ciphers were distinguished from codes. Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase. For example, âUQJHSEâ could be the code for âProceed to the following coordinatesâ. When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it; it should resemble random gibberish to those not intended to read it.
The operation of a cipher usually depends on a piece of auxiliary information, called a key or, in traditional NSA parlance, a cryptovariable. The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be difficult, if not nearly impossible, to decrypt the resulting cipher into readable plaintext.
Most modern ciphers can be categorized in several ways:
* By whether they work on blocks of symbols usually of a fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).
* By whether the same key is used for both encryption and decryption (symmetric key algorithms), or if a different key is used for each (asymmetric key algorithms). If the algorithm is symmetric, the key must be known to the recipient and to no one else. If the algorithm is an asymmetric one, the encyphering key is different from, but closely related to, the decyphering key. If one key cannot be deduced from the other, the asymmetric key algorithm has the public/private key property and one of the keys may be made public without loss of confidentiality. The Feistel cipher uses a combination of substitution and transposition techniques. Most block cipher algorithms are based on this structure.
Contents
[hide]
* 1 Etymology of âCipherâ
* 2 Ciphers versus codes
* 3 Types of Cipher
o 3.1 Historical ciphers
o 3.2 Modern ciphers
* 4 Key Size and Vulnerability
* 5 References
* 6 See also
* 7 External links
[edit] Etymology of âCipherâ
âCipherâ (Middle French as cifre and Medieval Latin as cifra, from the Arabic sifr = zero) is alternatively spelled âcypherâ (however, this variant is now uncommon); similarly âciphertextâ and âcyphertextâ, and so forth.
The word âcipherâ in former times meant âzeroâ and had the same origin (see Zero â Etymology), and later was used for any decimal digit, even any number. There are these theories about how the word âcipherâ may have come to mean encoding:
* Encoding often involved numbers.
* The Roman number system was very cumbersome because there was no concept of zero (or empty space). The concept of zero (which was also called âcipherâ), which we all now think of as natural, was very alien in medieval Europe, so confusing and ambiguous to common Europeans that in arguments people would say âtalk clearly and not so far fetched as a cipherâ. Cipher came to mean concealment of clear messages or encryption.
o The French formed the word âchiffreâ and adopted the Italian word âzeroâ.
o The English used âzeroâ for â0â, and âcipherâ from the word âcipheringâ as a means of computing.
o The Germans used the words âZifferâ (digit, âZahlâ) and âChiffreâ.
Dr. Al-Kadi (ref-3) concluded that the Arabic word sifr, for the digit zero, developed into the European technical term for encryption.
[edit] Ciphers versus codes
Main article: Code (cryptography)
In non-technical usage, a â(secret) codeâ typically means a âcipherâ. Within technical discussions, however, the words âcodeâ and âcipherâ refer to two different concepts. Codes work at the level of meaning â that is, words or phrases are converted into something else and this chunking generally shortens the message.
An example of this is the Telegraph Code which were used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of Telegrams.
Ciphers, on the other hand, work at a lower level: the level of individual letters, small groups of letters, or, in modern schemes, individual bits. Some systems used both codes and ciphers in one system, using superencipherment to increase the security. In some cases the terms codes and ciphers are also used synonymously to substitution and transposition.
Historically, cryptography was split into a dichotomy of codes and ciphers; and coding had its own terminology, analogous to that for ciphers: âencoding, codetext, decodingâ and so on.
However, codes have a variety of drawbacks, including susceptibility to cryptanalysis and the difficulty of managing a cumbersome codebook. Because of this, codes have fallen into disuse in modern cryptography, and ciphers are the dominant technique.
[edit] Types of Cipher
There are a variety of different types of encryption. Algorithms used earlier in the history of cryptography are substantially different from modern methods, and modern ciphers can be classified according to how they operate and whether they use one or two keys.
[edit] Historical ciphers
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include simple substitution ciphers and transposition ciphers. For example âGOOD DOGâ can be encrypted as âPLLX XLPâ where âLâ substitutes for âOâ, âPâ for âGâ, and âXâ for âDâ in the message. Transposition of the letters âGOOD DOGâ can result in âDGOGDOOâ. These simple ciphers and examples are easy to crack, even without plaintext-ciphertext pairs.
Simple ciphers were replaced by polyalphabetic substitution ciphers which changed the substitution alphabet for every letter. For example âGOOD DOGâ can be encrypted as âPLSX TWFâ where âLâ, âSâ, and âWâ substitute for âOâ. With even a small amount of known or estimated plaintext, simple polyalphabetic substitution ciphers and letter transposition ciphers designed for pen and paper encryption are easy to crack.
During the early twentieth century, electro-mechanical machines were invented to do encryption and decryption using transposition, polyalphabetic substitution, and a kind of âadditiveâ substitution. In rotor machines, several rotor disks provided polyalphabetic substitution, while plug boards provided another substitution. Keys were easily changed by changing the rotor disks and the plugboard wires. Although these encryption methods were more complex than previous schemes and required machines to encrypt and decrypt, other machines such as the British Bombe were invented to crack these encryption methods.
[edit] Modern ciphers
Modern encryption methods can be divided by two criteria: by type of key used, and by type of input data.
By type of key used ciphers are divided into:
* symmetric key algorithms (Private-key cryptography), where the same key is used for encryption and decryption, and
* asymmetric key algorithms (Public-key cryptography), where two different keys are used for encryption and decryption.
In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption.
Type of input ciphers data can be distinguished into two types:
* block ciphers, which encrypt block of data of fixed size, and
* stream ciphers, which encrypt continuous streams of data
