Secure Programming Cookbook for C and C++

10.1. Understanding Public Key Infrastructure (PKI)

Problem

You want a fundamental understanding of PKI.

Solution

Read the following discussion for an overview of basic PKI concepts. For a more detailed treatment, we recommend the book Planning for PKI: Best Practices Guide for Deploying Public Key Infrastructure by Russ Housley and Tim Polk (John Wiley & Sons).

Discussion

One of the big motivators behind public key cryptography is that there is some hope for securely exchanging encryption keys in an insecure medium. However, that is not as easy as it sounds. If used in a naïve manner, the basic public key methods for communication are susceptible to a man-in-the-middle attack, in which the two parties end up talking to an attacker who relays messages, instead of to each other (we discuss this attack in Recipe 7.1).

Man-in-the-middle attacks are possible because public key cryptography in and of itself provides no means of establishing trust. PKI provides the means to establish trust by binding public keys and identities together in a way that gives reasonable assurance that you are communicating securely with the expected entity.

Using public key cryptography,^[1] you can be sure that if you encrypt data with a public key, only someone with the corresponding private key can decrypt it. If you simply exchange public keys over an insecure medium, there is no easy way to be sure that the public keys you receive belong to the people you think they do. In other words, traditional public key ...