Encryption is the process of transforming information (called plaintext) into an unreadable form (called ciphertext) using an encryption algorithm combined with a parameter (called an encryption key). Only those who possess the decryption key can reverse the process and recover the original plaintext. Conceptual questions about encryption may get better answers on crypto.stackexchange.com.
In cryptography, encryption is the process of transforming information (called plaintext) into an unreadable form (called ciphertext) using an encryption algorithm combined with a parameter called an encryption key. Reversing the process, that is transforming the ciphertext into plaintext, is called decryption. Only those who possess the decryption key can reverse the process and recover the original plaintext. In symmetric encryption the encryption and decryption keys are the one and the same; this single key is sometimes called the shared secret key. Rijndael(AES), TwoFish, and Cha-Cha are examples of symmetric encryption algorithms. In asymmetric encryption, also known as public key encryption, the encryption and decryption keys are different. The encryption key is called the public key and the decryption key is called the private key. RSA, Diffie-Hellman (DH), El-Gamal (ElG), and elliptic curve variants of DH and ElG are examples of public key encryption algorithms.
The term "encryption" should be reserved for transformations intended to be reversed or "decrypted". In particular, cryptographic transformations intended to be one way, such as cryptographic hash algorithms (e.g. SHA1, SHA2, SHA3) and password hashing algorithms (e.g. bcrypt, scrypt) should not be referred to as "encryption".
Encryption has long been used by militaries and governments to facilitate secret communication. Encryption is now commonly used in protecting information within many kinds of civilian systems. It can be used to protect data "at rest", such as files on computers and storage devices (e.g. USB flash drives). In recent years there have been numerous reports of confidential data such as customers' personal records being exposed through loss or theft of laptops or backup drives. Encrypting such files at rest helps protect them should physical security measures fail. Digital rights management systems which prevent unauthorized use or reproduction of copyrighted material and protect software against reverse engineering (see also copy protection) are another somewhat different example of using encryption on data at rest.
Encryption is also used to protect data in transit, for example data being transferred via networks (e.g. the Internet, e-commerce), mobile telephones, wireless microphones, wireless intercom systems, Bluetooth devices and bank automatic teller machines. There have been numerous reports of data in transit being intercepted in recent years. Encrypting data in transit also helps to secure it as it is often difficult to physically secure all access to networks.
Encryption, by itself, can protect the confidentiality of messages, but other techniques are still needed to protect the integrity and authenticity of a message; for example, verification of a message authentication code (MAC) or a digital signature. Standards and cryptographic software and hardware to perform encryption are widely available, but successfully using encryption to ensure security may be a challenging problem. A single slip-up in system design or execution can allow successful attacks. Sometimes an adversary can obtain unencrypted information without directly undoing the encryption. See, e.g., traffic analysis, TEMPEST, or Trojan horse.
One of the earliest public key encryption applications was called Pretty Good Privacy (PGP). It was written in 1991 by Phil Zimmermann and was purchased by Network Associates (now PGP Corporation) in 1997.
Source: Wikipedia.
See also: cryptography
Note: conceptual questions about encryption might get more attention and better answers from the Crypto stack exchange.
