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The inverse process, of extracting the original information, is called decryption and can only be accomplished using auxilary information, called a key.
Most encryption methods can be divided into symmetric key algorithms and asymmetric key algorithms. 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 him to perform decryption.