Definition of Ciphers & types of Ciphers?

In cryptography, a cipher (or cypher) is an algorithm for performing encryption and decryption — a series of well-defined steps that can be followed as a procedure. An alternative term is encipherment. In most cases, that process is varied depending on a key which changes the detailed operation of the algorithm. In non-technical usage, a "cipher" is the same thing as a "code"; however, the concepts are distinct in cryptography. In classical cryptography, ciphers were distinguished from codes. Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase. For example, UQJHSE could be the code for "Proceed to the following coordinates".










Types of Ciphers:

There are a variety of different types of encryption. Algorithms used earlier in the history of cryptography are substantially different from modern methods, and modern ciphers can be classified according to how they operate and whether they use one or two keys.
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include simple substitution ciphers and transposition ciphers. For example GOOD DOG can be encrypted as PLLX XLP where L substitutes for O, P for G, and X for D in the message. Transposition of the letters GOOD DOG can result in DGOGDOO. These simple ciphers are easy to crack, even without plaintext-ciphertext pairs.
Simple ciphers were replaced by polyalphabetic substitution ciphers which changed the substitution alphabet for every letter. For example GOOD DOG can be encrypted as PLSX TWF where L, S, and W substitute for O. With even a small amount of known plaintext, polyalphabetic substitution ciphers and letter transposition ciphers designed for pen and paper encryption are easy to crack.
During the early twentieth century, electro-mechanical machines were invented to do encryption and decryption using a combination of transposition, polyalphabetic substitution, and "additive" substitution. In rotor machines, several rotor disks provided polyalphabetic substitution, while plug boards provided transposition. Keys were easily changed by changing the rotor disks and the plugboard wires. Although these encryption methods were more complex than previous schemes and required machines to encrypt and decrypt, other machines such as the British Bombe were invented to crack these encryption methods.
Modern encryption methods can be divided into symmetric key algorithms (Private-key cryptography) and asymmetric key algorithms (Public-key cryptography). In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform decryption.
Symmetric key ciphers can be distinguished into two types, depending on whether they work on blocks of symbols of fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).