Secure Programming Cookbook for C and C++

4.2. Generating Random Symmetric Keys

Problem

You want to generate a secure symmetric key. You already have some mechanism for securely transporting the key to anyone who needs it. You need the key to be as strong as the cipher you’re using, and you want the key to be absolutely independent of any other data in your system.

Solution

Use one of the recipes in Chapter 11 to collect a byte array of the necessary length filled with entropy.

When you’re done using a key, you should delete it securely to prevent local attackers from recovering it from memory. This is discussed in Recipe 13.2.

Discussion

In Recipe 11.2, we present APIs for getting random data, including key material. We recommend using the spc_keygen( ) function from that API. See that recipe for considerations on which function to use.

To actually implement spc_keygen( ), use one of the techniques from Chapter 11. For example, you may want to use the randomness infrastructure that is built into the operating system (see Recipe 11.3 and Recipe 11.4), or you may want to collect your own entropy, particularly on an embedded platform where the operating system provides no such services (see Recipe 11.19 through Recipe 11.23).

In many cases, you may want to derive short-term keys from a single “master” key. See Recipe 4.11 for a discussion of how to do so.

Be conservative when choosing a symmetric key length. We recommend 128-bit symmetric keys. (See Recipe 5.3.)

See Also

Recipe 4.11, Recipe 5.3, Recipe 11.2, Recipe 11.3, Recipe ...