Encryption and decryption are processes that ensure confidentiality that only authorized persons can access the information. Encryption is the process of translating the plain text data (plaintext) into random and mangled data (called cipher-text).
Decryption is the reverse process of converting the cipher-text back to plaintext. Encryption and decryption are done by cryptography. In cryptography a key is a piece of information (parameter) that determines the functional output of a cryptographic algorithm.
