Table of Contents### Types of Cryptography

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In cryptography, an ordinary message (the plaintext) is processed by an encryption algorithm to produce a scrambled message (the ciphertext). The receiver uses a matching decryption algorithm to recover the plaintext from the ciphertext. If these algorithms were known to everyone, there would be no security, because anyone could decrypt the ciphertext. Therefore, in addition to the algorithm, there is an additional piece of input data called a key. The key is secret, even though many people may know the algorithm. The principle is the same as that of a combination lock. Many people may use locks with the same design, but each one chooses a different combination. The combination of the lock is equivalent to the key of the encryption algorithm.

There are two basic types of cryptographic algorithms: secret-key systems (sometimes called symmetric) and public-key systems (sometimes called asymmetric). In secret-key cryptography, both encryption and decryption operations use the same key (that is, the key is used symmetrically). Systems for public-key cryptography use related but different keys for the operations of encryption and decryption operations (in other words, the keys are asymmetric). As we shall see, having two keys means that one can be published—that is, made public—which gives public-key cryptography its name. The other key is kept secret and is sometimes called a private key. Secret-key systems have been around for many hundreds of years; public-key systems are a recent invention, dating from the mid-1970s. Both types of systems allow for secret communications, but public-key systems can more easily grow to worldwide scale and more easily permit unaffiliated persons to communicate securely. Public-key systems can also be used to provide digital signatures, which are analogous to handwritten signatures on letters, contracts, and other documents. We shall have more to say about digital signatures later in this chapter.