Cryptography

Cryptography is an interdisciplinary subject, drawing from several fields. Older forms of cryptography were chiefly concerned with patterns in language. More recently, the emphasis has shifted, and cryptography makes extensive use of mathematics, particularly discrete mathematics, including topics from number theory, information theory, computational complexity, statistics and combinatorics. Cryptography is also considered a branch of engineering, but it is considered to be an unusual one as it deals with active, intelligent and malevolent opposition (see cryptographic engineering and security engineering). Cryptography is a tool used within computer and network security.

Symmetric key cryptography

Main article: Symmetric key algorithm

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Symmetric key ciphers either use the same key for encryption and decryption, or the key used for decryption is easily calculated from the key used for encryption. Other terms include secret-key, private-key, one-key and single-key cryptography.

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Symmetric key ciphers can be broadly grouped into block ciphers and stream ciphers. Stream ciphers encrypt one bit at a time, in contrast to a block cipher, which operates on a group of bits (a "block") of a certain length all in one go. Depending on the mode of operation, block ciphers can be implemented as self-synchronizing stream ciphers (CFB mode). Likewise, stream ciphers can be made to work on individual blocks of plaintext at a time. Thus, there is some duality between the two. The block ciphers DES, IDEA and AES, and the stream cipher RC4, are among the most well-known symmetric key ciphers.

Related Topics:
Block cipher - Stream cipher - Mode of operation - DES - IDEA - AES - RC4

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Other cryptographic primitives are sometimes classified as symmetric cryptography:

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• Cryptographic hash functions produce a hash of a message. While it should be easy to compute, it must be very difficult to invert (one-way), though other properties are usually needed as well. MD5 and SHA-1 are well-known hash functions.
• Message authentication codes (MACs), also known as keyed-hash functions, are similar to hash functions, except that a key is needed to compute the hash. As the name suggests, they are commonly used for message authentication. They are often constructed from other primitives, such as block ciphers, unkeyed-hash functions or stream ciphers.