The key is an algorithm, a detailed mathematical procedure generating a number sequence within a specific amount of bits. The sequence allows access to the message. It is an encryption algorithm because only the correct key will unlock the encryption. The message may have more than one layer of encryption thereby requiring a sequence of keys. Encryption systems are either symmetrical or asymmetrical.
Symmetrical systems use the same key to encrypt and decrypt the data.
Cryptologists, refer to the Caesar cipher as a classic representative of symmetrical encryption systems. It is the simplest encryption system. Another name for symmetrical systems is Private Key Encryption. A characteristic of symmetrical systems is they invite penetration since messages remain secure as long as the channels in which they travel are secure. Asymmetrical systems, on the other hand, are more difficult to penetrate because they use different keys to encrypt and decrypt data.