Computer Security and Cryptography

17.3 SECRECY, DIGITAL SIGNATURES, AND AUTHENTICATION

Our focus until now has been on secrecy systems for hiding information from a surreptitious but passive wiretapper who only monitors communications. Cryptography provided a possible solution by altering the form of the message so that only the authorized parties might be able to read the message. Enciphering the electronic fund's transfer message of Figure 17.2 might hide its contents (Fig. 17.3). However, it does not provide proof of the origin of the message. Webster's dictionary defines authentication as a “process by which each party to a communication verifies the identity of the other.”

Authentication occurs in many day-to-day activities, including using

• A photo-ID when cashing a check,
• A driver's license when making a credit card purchase, and
• A passport when crossing national boundaries.

Figure 17.2 Plaintext transaction data.

Figure 17.3 Secrecy by means of enciphering plaintext transaction data.

Figure 17.4 Authentication appended to plaintext transaction data.

Figure 17.5 A digital signature appended to plaintext transaction ...