Encryption and Decryption
The term encryption is used to describe the transformation of plain text into a form that makes the original text incomprehensible to an unauthorized recipient that does not have a matching key to decode or decrypt the encrypted message. Decryption is the reverse of encryption, which means that decryption is the transformation of encrypted data back into plain text. A cipher is a cryptographic algorithm that is used for data encryption and decryption. One function is used for encryption and other function is used for decryption. You must use a key in addition to a cipher for encryption and decryption.
Encryption is used for data confidentiality, which is one of the most important requirements for any VPN implementation. An encryption/decryption algorithm built into IPsec makes IPsec a desirable protocol for IPsec implementation.
Cryptographic algorithms can be categorized as follows: