Copyright Goodheart-Willcox Co., Inc. 456 Networking Fundamentals Asymmetric-Key Encryption Asymmetric-key encryption, or public-key cryptography, uses two keys: a private key and a public key, shown in Figure 15-9. Typically, the originator of the encryption system owns the private key. For example, a teacher who wishes to communicate with his or her students across the Internet while ensuring privacy would retain a private key. A public key is then issued to all students. A message is encoded using the private key and can only be decoded using the public key. Th e owner of the public key can send a message in return that can only be decoded by the private key. Figure 15-9 The asymmetric key encryption system uses two different keys—one key for encrypting the data and the other for decrypting the data. Station1 Message: Hi Tracey Encrypted message: kjhhoiuhsjnsmlicijwknwlcij kjhhoiuhsjnsmlicijwknwlcij Hi Tracey Station2 Different keys Asymmetric Key Encryption (workstation icon) RedlineVector/Shutterstock.com Goodheart-Willcox Publisher In this method, a message created with a private key can only be decoded with a public key, and a message encrypted with a public key can only be decoded with a private key. A message created with a public key cannot be decoded with another public key. Th is means that all the encryption and decryption is centered on the owner of the private key. It takes both the public and private keys to code and de- code an encrypted message. Examples of asymmetric algorithms include RSA and Diffi e-Hellman. RSA RSA is probably the most widely used asymmetric cipher today. Th is public key method was developed in 1977 by three mathematicians: Ron Rivest, Adi Shamir, and Len Adlema. Th e name RSA is derived from the fi rst letter of each mathematician’s last name. Th e secret to the math behind RSA is that it is very diffi cult to factor large numbers into their prime factors. RSA depends on this for security. Diffi e-Hellman Diffi e-Hellman was the fi rst publicly described asymmetric algorithm. Th is is a cryp- tographic protocol that allows two parties to establish a shared key over an insecure channel. In other words, Diffi e-Hellman is often used to allow parties to exchange a symmetric key through some unsecure medium, such as the Internet. It was devel- oped by Whitfi eld Diffi e and Martin Hellman in 1976. Certifi cate Authority (CA) Typically, a service referred to as the certifi cate authority (CA) contains the securi- ty list of users authorized to access the private key owner’s messages using a public key, as demonstrated in Figure 15-10. Th e private key owner consults the CA for this information before sending a message. Th e CA sends the requested information in the form of a digital certifi cate. A digital certifi cate is a fi le that commonly contains 4.2 NET