Secure Programming Cookbook for C and C++

11.9. Using the OpenSSL Random Number API

Problem

Many functions in the OpenSSL library require the use of the OpenSSL pseudo-random number generator. Even if you use something like /dev/urandom yourself, OpenSSL will use its own API under the hood and thus must be seeded properly.

Unfortunately, some platforms and some older versions of OpenSSL require the user to provide a secure seed. Even modern implementations of OpenSSL merely read a seed from /dev/urandom when it is available; a paranoid user may wish to do better.

When using OpenSSL, you may want to use the provided PRNG for other needs, just for the sake of consistency.

Solution

OpenSSL exports its own API for manipulating random numbers, which we discuss in the next section. It has its own cryptographic PRNG, which must be securely seeded.

To use the OpenSSL randomness API, you must include openssl/rand.h in your code and link against the OpenSSL crypto library.

Discussion

Warning

Be sure to check all return values for the functions below; they may return errors.

With OpenSSL, you get a cryptographic PRNG but no entropy gateway. Recent versions of OpenSSL try to seed its PRNG using /dev/random, /dev/urandom, and EGD, trying several well-known EGD socket locations. However, OpenSSL does not try to estimate how much entropy its PRNG has. It is up to you to ensure that it has enough before the PRNG is used.

On Windows systems, a variety of sources are used to attempt to gather entropy, although none of them actually provides much ...