Encryption is the process of obscuring information to make it unreadable without special knowledge, sometimes referred to as scrambling. Encryption has been used to protect communications for centuries, but only organizations and individuals with an extraordinary need for secrecy had made use of it. Now used in protecting widely-used systems, such as Internet e-commerce, mobile telephone networks and bank automatic teller machines. Encryption can be used to ensure secrecy, but other techniques are still needed to make communications secure, particularly to verify the integrity and authenticity of a message; for example, a message authentication code (MAC) or digital signatures.
A Cipher 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 procedure 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“
The original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it. The operation of a cipher usually depends on a piece of auxiliary information, called a key. The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext
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. In rotor machines, several rotor disks provided polyalphabetic substitution, while plug boards provided transposition. 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, 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, 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.
