Symmetric key cryptography

The study of symmetric-key ciphers involves the study of block ciphers and stream ciphers and to their applications. A block cipher is the modern embodiment of Alberti's polyalphabetic cipher: block ciphers take as input a block of plaintext and a key, and output a block of ciphertext of the same size. Block ciphers are used in a mode of operation to implement a cryptosystem. DES and AES are block ciphers which have been designated cryptography standards by the US government (though DES's designation was eventually withdrawn after the AES was adopted). Despite its delisting as an official standard, DES (especially its still-approved and much more secure triple-DES variant) remains quite popular; it is used across a wide range of applications, from ATM encryption to e-mail privacy and secure remote access. Many other block ciphers have been designed and released, with considerable variation in quality. .

Stream ciphers, in contrast to the 'block' type, create an arbitrarily long stream of key material, which is combined with the plaintext bit by bit or character by character, somewhat like the one-time pad. In a stream cipher, the output stream is created based on an internal state which changes as the cipher operates. That state's change is controlled by the key, and, in some stream ciphers, by the plaintext stream as well. RC4 is an example of a well-known stream cipher <ref name= schneierbook>