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The art of cryptography, or communicating in code, can be divided into three broad categories: public key cryptography, which is a code that uses one key for encryption and a separate key for decryption; hash functions, which rely on mathematical conversions to permanently encrypt the information; and secret key cryptography, which is a code that uses the same key for both the encryption and decryption of the transmitted data. The last category derives its name from the fact that both the sender and receiver must keep their key a secret in order to prevent messages from being successfully intercepted by a third party.
Secret key cryptography, also known as symmetric encryption, can be separated into two main types, based on the type of coding scheme used. Stream ciphers, for example, allow the sender and receiver to constantly update and change the secret key; block ciphers, on the other hand, consistently encode one block of data at a time. Furthermore, self-synchronizing stream ciphers feed off the previous volume of data, as opposed to synchronous stream ciphers, which work off a key that is independent of the volume and progression of the message.
There are four major modes of secret key cryptography block cipher operation. Electronic Codebook mode (ECB) corresponds to the basest level of encryption; Cipher Block Chaining (CBC) incorporates a sender-receiver feedback layer into the ECB equation; Cipher Feedback (CFB) allows data to be encrypted at a much smaller character level; and Output Feedback (OFB) employs an even more complex, independent coding algorithm to prevent two blocks of data from being coded in the same, identical way.
All in all, secret key cryptography is a mathematician's paradise, able to be made more complex by both the intricacies of the governing algorithm and the frequency with which that algorithm, or key, is changed. One everyday application that makes use of secret key cryptography is the ongoing transmission of paid television content to a cable or satellite subscriber. As the piracy of these signals has increased, so too have the efforts of cable and satellite companies to constantly update and download new de-scrambling keys to the smart cards inside each receiver.
A complex form of secret key cryptography was used to protect the Cold War Era phone line that directly linked the White House and the Kremlin. Known as a One-Time Pad (OTP), it generated a very large set of random numbers to be used only once as the decoding key. This type of encryption is said to be impossible to break when used properly.