Secure Programming Cookbook for C and C++

4.11. Algorithmically Generating Symmetric Keys from One Base Secret

Problem

You want to generate a key to use for a short time from a long-term secret (generally a key, but perhaps a password). If a short-term key is compromised, it should be impossible to recover the base secret. Multiple entities in the system should be able to compute the same derived key if they have the right base secret.

For example, you might want to have a single long-term key and use it to create daily encryption keys or session-specific keys.

Solution

Mix a base secret and any unique information you have available, passing them through a pseudo-random function (PRF), as discussed in the following section.

Discussion

The basic idea behind secure key derivation is to take a base secret and a unique identifier that distinguishes the key to be derived (called a distinguisher ) and pass those two items through a pseudo-random function. The PRF acts very much like a cryptographic one-way hash from a theoretical security point of view, and indeed, such a one-way hash is often good as a PRF.

There are many different ad hoc solutions for doing key derivation, ranging from the simple to the complex. On the simple side of the spectrum, you can concatenate a base key with unique data and pass the string through SHA1. On the complex side is the PBKDF2 function from PKCS #5 (described in Recipe 4.10).

The simple SHA1 approach is perhaps too simple for general-purpose requirements. In particular, there are cases where you ...