In cryptography, encryption is the process of obscuring information to make it unreadable without special knowledge. While encryption has been used to protect communications for centuries, only organisations and individuals with an extraordinary need for secrecy have made use of it.
In the mid-1970s, strong encryption emerged from the sole preserve of secretive government agencies into the public domain, and is now employed in protecting widely-used systems, such as Internet e-commerce, mobile telephone networks and bank automatic teller machines.
Encryption can be used to ensure secrecy, but other techniques are still needed to make communications secure, particularly to verify the integrity and authenticity of a message; for example, a message authentication code (MAC) or digital signatures. Another consideration is protection against traffic analysis.
Ciphers
A cipher 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.
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 the same key, it should be difficult, if not impossible, to
decrypt the resulting ciphertext into readable plaintext.
"Cipher" is alternatively spelled "cypher"; similarly "ciphertext" and "cyphertext",
and so forth. The word descends from the Arabic word for zero:
like (the Italian) zero (which remained in use for 0, the crucial innovation in
positional Arabic versus Roman numerals) but soon was used for any decimal
digit, even any number. While it may have come to mean encoding because that
often involved numbers, a theory says conservative Catholic opponents of the
Arabic (heathen) numerals equated it with any 'dark secret'.
Ciphers versus codes
Main article: Code (cryptography)
In non-technical usage, a "(secret) code" is the same thing as a cipher. Within
technical discussions, however, they are distinguished into two concepts. Codes
work at the level of meaning — that is, words or phrases are converted into
something else. 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.
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 ciphers. They include substitution ciphers and transposition ciphers.
During the early 20th century, more sophisticated machines for encryption were
used, rotor machines, which were more complex than previous schemes.
Encryption methods can be divided into symmetric key algorithms and asymmetric
key algorithms. 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 him
to perform decryption.
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).
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