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.

Secure communications

See also: Information security

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Cryptography is commonly used for securing communications. Desired properties can include:

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• Confidentiality, also known as secrecy: only an authorised recipient should be able to extract the contents of the message from its encrypted form. Otherwise, it should not be possible to obtain any significant information about the message contents.
• Integrity: the recipient should be able to determine if the message has been altered during transmission.
• Authentication: the recipient should be able to identify the sender, and verify that the purported sender actually did send the message.
• Non-repudiation: the sender should not be able to deny sending the message.
• Anti-replay: the message should not be allowed to be sent multiple times to the recipient without the sender knowing.
• Proof of delivery: the sender should be able to prove the recipient received the message.

Cryptography can provide mechanisms to help achieve all of the above. However, some goals are not always necessary, practical or even desirable in some contexts. For example, the sender of a message may wish to remain anonymous; clearly non-repudiation would be inappropriate in that case.

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