Secure Programming Cookbook for C and C++

7.1. Determining When to Use Public Key Cryptography

Problem

You want to know when to use public key cryptography as opposed to symmetric cryptography.

Solution

Use public key cryptography only for key exchange or digital signatures. Otherwise, there are a lot of disadvantages and things that can go wrong (particularly when using it for general-purpose encryption). Because public key operations are computationally expensive, limit digital signatures to authentication at connection time and when you need non-repudiation.

Tip

Whenever you use public key encryption, be sure to remember also to perform proper authentication and message integrity checking.

Discussion

Public key cryptography allows parties to communicate securely without having to establish a key through a secure channel in advance of communication, as long as a trusted third party is involved. Therein lies the first rub. Generally, if you use public key cryptography, you need to determine explicitly with whom you’re communicating, and you need to check with a trusted third party in a secure manner. To do that, you will need to have identification data that is bound to your trusted third party, which you’ll probably need to authenticate over some secure channel.

Figure 7-1 (A) illustrates why public key cryptography on its own does not provide secure communication. Suppose the server has a {public key, private key} pair, and the client wishes to communicate with the server. If the client hasn’t already securely obtained the ...