- Cryptography : A field at the intersection of mathematics and computer science that is concerned with the security of information, typically the confidentiality, integrity and authenticity of some message.
- Information theory : Theory of the probability of transmission of messages with specified accuracy when the bits of information constituting the messages are subject, with certain probabilities, to transmission failure, distortion, and accidental additions.
- Block cipher : A symmetric cipher that operates on fixed-size blocks of plaintext, giving a block of ciphertext for each
- Stream cipher : A cipher that encrypts data by mixing it with the output of a pseudorandom number generator controlled by a key; to decrypt, run the same generator with the same key to get the same pseudorandom data, then reverse the mixing step.
- Caesar cipher : One of the first ciphers, developed by Julius Caesar
- Feistel cipher : Add brief definition or description
- One-time pad : A cipher system in which the cryptographic key, i.e. the secret used to encrypt and decrypt messages, is a sequence of random values, each one of which is only ever used once, and only to encrypt one particular letter or word.
- Playfair cipher : Add brief definition or description
- Polyalphabetic substitution : Add brief definition or description
- Data Encryption Standard : A block cipher specification issued by the U.S. government in 1976, intended for sensitive but unclassified data. It is now obsolescent, succeeded by the Advanced Encryption Standard, but still used in commercial systems.
- Advanced Encryption Standard : A US government standard issued in 2002 for a stronger block cipher to succeed the earlier Data Encryption Standard.
- Enigma machine : The primary high-security cryptographic communications security machine of Nazi Germany. Unknown to the Germans, it had been substantially cryptanalyzed by the British Government Code and Cipher School, with French, Polish, and U.S. help.
- PURPLE machine : WWII Japanese cipher machine used for diplomatic communications; system broken by U.S. and made available to U.K. in exchange for ULTRA communications intelligence on Germany
- Symmetrical cryptosystem : Add brief definition or description
- Asymmetrical cryptosystem : Add brief definition or description
- Key management (cryptography) : Add brief definition or description
- Code (cryptography) : A means of substituting, for the linguistically meaningful symbols of plaintext composed of words or other symbols meaningful to humans, into inherently meaningless numbers, letters, or words that make no sense to a recipient who is not in possession of a codebook or other means of reversing the substitution of symbols
- Communications intelligence : The subset of SIGINT concerned with signals intended to be intelligible to human beings, in the form of voice, messages, or images.
- Communications security : Policies, procedures and technologies used to protect information while it is being transmitted
- Communications Security Establishment : The Canadian government organization responsible for communications security and signals intelligence
- : The Russian organization now responsible for signals intelligence and communications security, but with some authority to do work under commercial contracts
- Hash (computing) : Add brief definition or description
- Pretty Good Privacy : E-mail encryption package created by Phillip Zimmerman.
- Government Communications Headquarters : The British government agency responsible for signals intelligence and information assurance
- Government Communications Security Bureau : The organization, in the government of New Zealand, which has responsibility for information security and signals intelligence
- National Security Agency : An organization within the United States Department of Defense, with the dual roles of the principal signals intelligence agency in the United States intelligence community, but also having the responsibility for information assurance of military, diplomatic, and other critical communications.
- ULTRA : ULTRA was the main code word, in the Second World War, for British signals intelligence directed at Nazi Germany.