Cryptography is divided into three categories: public key, hash functions, and secret key. Secret key encryption has two types: stream and block ciphers, with four main modes of operation. It is used in pay-TV transmission and was used in the Cold War-era hotline between the White House and Kremlin.
The art of cryptography, or coded communication, can be divided into three broad categories: public key cryptography, which is code that uses one key for encryption and a separate key for decryption; hash functions, which rely on mathematical conversions to permanently encrypt information; and secret key cryptography, which is a code that uses the same key for both encrypting and decrypting transmitted data. The last category derives its name from the fact that both the sender and the recipient must keep their key secret to prevent messages from being successfully intercepted by third parties.
Secret key encryption, also known as symmetric encryption, can be divided into two main types, based on the type of encryption 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. Also, self-synchronizing stream ciphers feed from the previous data volume, as opposed to synchronous stream ciphers, which operate on a key independent of message volume and progression.
There are four main modes of secret key cryptography block cipher operation. Electronic Codebook (ECB) mode is the lowest level of encryption; Cipher Block Chaining (CBC) incorporates a level of sender-receiver feedback into the ECB equation; Cipher Feedback (CFB) allows you to encrypt data at a much smaller character level; and Output Feedback (OFB) uses an even more complex independent encoding algorithm to prevent two blocks of data from being encoded in the exact same way.
All in all, secret key cryptography is a mathematician’s paradise, capable of being made more complex both by the intricacies of the governing algorithm and by how often that algorithm, or key, is changed. An everyday application that makes use of secret key encryption is the continuous transmission of pay-TV content to a cable or satellite subscriber. As piracy of these signals has increased, so have cable and satellite companies’ efforts to constantly update and download new smart card decryption keys into each receiver.
A complex form of secret key encryption was used to secure the Cold War-era telephone line that directly connected the White House and the Kremlin. Known as One-Time Pad (OTP), it generated a very large set of random numbers to be used only once as a decryption key. This type of encryption is said to be impossible to break if used correctly.
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