Editing Ciphertext (section)

== Types of ciphers ==
{{Main|Cipher}}
The [[history of cryptography]] began thousands of years ago. Cryptography uses a variety of different types of encryption. Earlier algorithms were performed by hand and are substantially different from modern [[algorithm]]s, which are generally executed by a machine.

=== Historical ciphers ===
Historical pen and paper ciphers used in the past are sometimes known as [[classical cipher]]s. They include:

* [[Substitution cipher]]: the units of plaintext are replaced with ciphertext (e.g., [[Caesar cipher]] and [[one-time pad]])
** [[Polyalphabetic substitution|Polyalphabetic substitution cipher]]: a substitution cipher using multiple substitution alphabets (e.g., [[Vigenère cipher]] and [[Enigma machine]])
** [[Polygraphic substitution]] cipher: the unit of substitution is a sequence of two or more letters rather than just one (e.g., [[Playfair cipher]])
* [[Transposition cipher]]: the ciphertext is a [[permutation]] of the plaintext (e.g., [[Rail fence|rail fence cipher]])

Historical ciphers are not generally used as a standalone encryption technique because they are quite easy to crack. Many of the classical ciphers, with the exception of the one-time pad, can be cracked using [[Brute force attack|brute force]].

=== Modern ciphers ===
Modern ciphers are more secure than classical ciphers and are designed to withstand a wide range of attacks. An attacker should not be able to find the key used in a modern cipher, even if they know any specifics about the plaintext and its corresponding ciphertext. Modern encryption methods can be divided into the following categories:

* [[Private-key cryptography]] ([[symmetric key algorithm]]): one shared key is used for encryption and decryption
* [[Public-key cryptography]] ([[asymmetric key algorithm]]): two different keys are used for encryption and decryption

In a symmetric key algorithm (e.g., [[Data Encryption Standard|DES]], [[Advanced Encryption Standard|AES]]), the sender and receiver have a shared key established in advance: the sender uses the shared key to perform encryption; the receiver uses the shared key to perform decryption. Symmetric key algorithms can either be [[block cipher]]s or [[stream cipher]]s. Block ciphers operate on fixed-length groups of bits, called blocks, with an unvarying transformation. Stream ciphers encrypt plaintext digits one at a time on a continuous stream of data, with the transformation of successive digits varying during the encryption process.

In an asymmetric key algorithm (e.g., [[RSA (algorithm)|RSA]]), there are two different keys: a ''public key'' and a ''private key.'' The ''public key'' is published, thereby allowing any sender to perform encryption. The ''private key'' is kept secret by the receiver, thereby allowing only the receiver to correctly perform decryption.