| • Science | • People | • Locations | • Timeline |
In cryptography, encryption is the process of obscuring information to make it unreadable without special knowledge. This is usually done for secrecy, and typically for confidential communications. Encryption can also be used for authentication. Even when encrypted, messages can still be subject to traffic analysis although this cannot typically be used to reveal the actual contents of the message.
A cipher (sometimes spelt cypher) is an algorithm for performing encryption (and the reverse, decryption) — a series of well-defined steps that can be followed as a procedure. An alternative term is encipherment. 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.
Ciphers are usually parameterised by a piece of auxillary information, called a key. The encrypting procedure is varied depending on the key which changes the detailed operation of the algorithm. Without the key, the cipher cannot be used to encrypt, or more importantly, to decrypt.
In non-technical usage, a "cipher" is the same thing as a "(secret) code"; however, in technical discussions they are distinguished into two concepts: codes work at the level of meaning; that is, words or phrases are converted into something else, while ciphers work at a lower level: the level of individual letters, or small groups of letters — or in modern ciphers, individual bits. Some systems used both codes and ciphers in one system, using superencipherment to increase the security.
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 susceptibilty 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 paradigm.
There are a variety of different types of encryption. Algorithms used earlier in the history of cryptography are substantially different to 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 ciphers. They include substitution ciphers and transposition ciphers. During the early 1900s, more sophisticated machines for encryption were used, rotor machineIn cryptography, a rotor machine is a electro-mechanical device used for encrypting and decrypting secret messages. Rotor machines were the cryptographic state-of-the-art for a brief but prominent period of history; they were in widespread use in the 1930s, which were more complex than previous schemes.
Encryption methods can be divided into symmetric key algorithmA symmetric-key algorithm is an algorithm for cryptography that uses the same cryptographic key to encrypt and decrypt the message. Actually, it is sufficient for it to be easy to compute the decryption key from the encryption key and vice versa. Other tes and asymmetric key algorithmIn cryptography, an asymmetric key algorithm uses a pair of different, though related, cryptographic keys to encrypt and decrypt. The two keys are related mathematically; a message encrypted by the algorithm using one key can be decrypted by the same algos. In a symmetric key algorithmA symmetric-key algorithm is an algorithm for cryptography that uses the same cryptographic key to encrypt and decrypt the message. Actually, it is sufficient for it to be easy to compute the decryption key from the encryption key and vice versa. Other te (e.g., DESThis article is about the DES encryption algorithm. For other uses, see DES (disambiguation). The Data Encryption Standard DES is a cipher (a method for encrypting information) selected as an official Federal Information Processing Standard (FIPS) for the and AESThis article is about the block cipher. For the society AES, see Audio Engineering Society. In cryptography, the Advanced Encryption Standard AES , also known as Rijndael is a block cipher adopted as an encryption standard by the US government, and is exp), 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 algorithmIn cryptography, an asymmetric key algorithm uses a pair of different, though related, cryptographic keys to encrypt and decrypt. The two keys are related mathematically; a message encrypted by the algorithm using one key can be decrypted by the same algo (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 him to perform decryption. Common asymmetric encryption algorithms available today are all based on the Diffie-Hellman key agreement algorithm.
Symmetric key ciphers can be distinguished into two types, depending on whether they work on blocks of symbols usually of a fixed size ( block ciphers), or on a continuous stream of symbols ( stream ciphers).