When talking about cryptography, we refer to senders and receivers wishing to exchange messages or plaintext by exchanging ciphertext. It is assumed that an eavesdropper reading ciphertext should not be able to extract corresponding plaintext. This characteristic is called confidentiality. The process performed by a sender to hide plaintext is called encryption, the reverse operation is called decryption. These processes are often expressed as mathematic functions or com- puting algorithms. The encryption and decryption algorithms together constitute a cipher. Cipher algorithms intended for general use cannot be secret. So cannot the eavesdropper just invoke the decryption process to extract plaintext? Ciphers use keys to solve this problem. The key.
