Symmetric Encryption
Symmetric encryption uses a single shared key for encryption and decryption. These are known as dual-use keys, as they can be used to lock and unlock data. Symmetric encryption is the oldest form of encryption. Historical systems such as scytale and Caesar’s cipher are types of symmetric encryption. Symmetric encryption offers users privacy by keeping individuals who do not have the key from having access to the true contents of the message. Figure 1.2 shows the symmetric encryption process.
Notice how the plain text is encrypted with the single shared key and is then transmitted to the recipient of the message, who goes through the same process to decrypt the message. The dual use of keys is what makes this system so simple, but it also introduces weakness. Symmetric encryption is fast, and with a small key it can be used to encrypt bulk data very quickly. It is also strong and difficult to break if the key is of...