Copyright Goodheart-Willcox Co., Inc. 455 Chapter 15 Network Security Cryptography Th ere are two basic types of cryptography in use today: symmetric-key encryption and asymmetric-key encryption. Symmetric means the same key is used to encrypt and decrypt the message. With asymmetric cryptography, a diff erent key is used to en- crypt the message than is used to decrypt the message. A key is a software code used to encode or decode data. Symmetric-Key Encryption Symmetric-key encryption, or secret-key cryptography, uses a key that only the sender and the receiver know. Both parties use the same key to perform encryption and decryption, as illustrated in Figure 15-8. Th e term symmetric is used because the same key is used for both coding and decoding the message. Figure 15-8 The symmetric-key encryption system uses the same key to encrypt and decrypt data. Same keys Station1 Message: Hi Tracey Encrypted message: kjhhoiuhsjnsmlicijwknwlcij kjhhoiuhsjnsmlicijwknwlcij Hi Tracey Station2 Symmetric-Key Encryption (workstation icons) RedlineVector/Shutterstock.com Goodheart-Willcox Publisher Symmetric keys are typically used when large amounts of data are to be encrypt- ed. Th e symmetric key encodes and decodes faster than more complex methods and can be just as secure. Symmetric algorithms are usually much faster than asymmetric and tend to use smaller keys. However, there is the issue of key exchange. How do you get the key to the other party? A few common symmetric ciphers are described here. DES DES is the oldest modern symmetric cipher, fi rst published in 1976. It uses a 56-bit key to encrypt 64-bit blocks of code. It is a robust algorithm, but the key length is con- sidered too short for modern usage. Th ere is a variation called 3DES that uses three diff erent DES keys to encrypt the message. AES Advanced Encryption Standard (AES) was the algorithm eventually chosen to replace DES. It is a block cipher that works on 128-bit blocks. It can have one of three key sizes of 128, 192, or 256 bits. Th is was selected by the United States government to be the replacement for DES and is now the most widely used symmetric-key algorithm. According to the US government, AES with a 256-bit key is secure enough for top- secret documents. Blowfi sh Blowfi sh is a symmetric block cipher. It uses a variable-length key ranging from 32 to 448 bits. Blowfi sh was designed in 1993 by Bruce Schneier. Th ere is no copyright or patent on this algorithm, so it is very popular for use in open-source products.