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In Cryptography , encryption is the process of transforming Information (referred to as Plaintext ) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a Key . The result of the process is '''encrypted''' information (in cryptography, referred to as ciphertext). In many contexts, the word encryption also implicitly refers to the reverse process, '''decryption''' (e.g. “ Software For Encryption ” can typically also perform decryption), to make the encrypted information readable again (i.e. to make it unencrypted). Encryption has long been used by militaries and governments to facilitate secret communication. Encryption is now used in protecting information within many kinds of civilian systems, such as Computer s, Network s (e.g. the Internet E-commerce ), Mobile Telephone s, and bank Automatic Teller Machine s. Encryption is also used in Digital Rights Management to restrict the use of copyrighted material and in software Copy Protection to protect against Reverse Engineering and software Piracy . Encryption, by itself, can protect the confidentiality of messages, but other techniques are still needed to verify the integrity and authenticity of a message; for example, a Message Authentication Code (MAC) or Digital Signature s. Standards and Cryptographic Software and Hardware to perform encryption are widely available, but successfully using encryption to ensure security is a challenging problem. A single slip-up in system design or execution can allow successful attacks. Sometimes an adversary can obtain unencrypted information without directly undoing the encryption. See Traffic Analysis , TEMPEST . HISTORY See Also: History of cryptography Encryption has been used to protect communications since ancient times, but only organizations and individuals with extraordinary need for confidentiality had bothered to exert the effort required to implement it. Encryption, and successful attacks on it, played a vital role in World War II . Many of the encryption techniques developed then were closely-guarded secrets (Kahn). In the mid-1970s, with the introduction of the U.S. Data Encryption Standard and Public Key Cryptography , strong encryption emerged from the preserve of secretive government agencies into the public domain. CIPHERS In Cryptography , a cipher (or '''cypher''') is an Algorithm for performing encryption and Decryption — a series of well-defined steps that can be followed as a procedure. An alternative term is '''encipherment'''. In non-technical usage, a “cipher” is the same thing as a “ Code ”; 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, “UQJHSE” could be the code for “Proceed to the following coordinates”. 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 impossible, to decrypt the resulting cipher into readable plaintext. Most modern ciphers can be categorized in several ways:
Etymology of “Cipher” “Cipher” is alternatively spelled “cypher” (however, this variant is now uncommon and therefore often incorrectly considered an error by native speakers); similarly “ Ciphertext ” and “ Cyphertext ”, and so forth. It also got into Middle French as ''cifre'' and Medieval Latin as ''cifra,'' from the Arabic ''sifr'' (zero). The word “cipher” in former times meant “zero” and had the same origin (see Zero — Etymology ), and later was used for any decimal digit, even any number. There are these theories about how the word “cipher” may have come to mean encoding:
Dr. Al-Kadi (ref-3) concluded that the Arabic word ''sifr'', for the digit zero, developed into the European technical term for encryption. CIPHERS VERSUS CODES In non-technical usage, a “(secret) to increase the security. In some cases the terms codes and ciphers are also used synonym 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: “''encoding'', ''codetext'', ''decoding''” 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. 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. Historical pen and paper ciphers used in the past are sometimes known as Classical Cipher s. They include simple Substitution Cipher s and Transposition Cipher s. For example “GOOD DOG” can be encrypted as “PLLX XLP” where “L” substitutes for “O”, “P” for “G”, and “X” for “D” in the message. Transposition of the letters “GOOD DOG” can result in “DGOGDOO”. 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 “GOOD DOG” can be encrypted as “PLSX TWF” where “L”, “S”, and “W” substitute for “O”. 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 “additive” substitution. In Rotor Machine s, 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. Modern encryption methods can be divided into Symmetric Key Algorithm s ( Private-key Cryptography ) and Asymmetric Key Algorithm s ( Public-key Cryptography ). 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. Symmetric key ciphers can be distinguished into two types, depending on whether they work on blocks of symbols of fixed size ( Block Cipher s), or on a continuous stream of symbols ( Stream Cipher s). KEY SIZE AND VULNERABILITY In a pure mathematical attack (i.e., lacking any other information to help break a cypher), three factors above all, count:
Since the desired effect is computational difficulty, in theory one would choose an algorithm and desired difficulty level, thus decide the key length accordingly. An example of this process can be found at Key Length which uses multiple reports to suggest that a symmetric cypher with 128 Bit s, an asymmetric cypher with 3072 bit keys, and an elliptic curve cypher with 512 bits, all have similar difficulty at present. . REFERENCES
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