RAND_add(3) OpenSSL RAND_add(3)
RAND_add, RAND_seed, RAND_status, RAND_event, RAND_screen - add entropy to the PRNG
void RAND_seed(const void *buf, int num);
void RAND_add(const void *buf, int num, double entropy);
int RAND_event(UINT iMsg, WPARAM wParam, LPARAM lParam);
RAND_add() mixes the num bytes at buf into the PRNG state. Thus, if the data at buf are
unpredictable to an adversary, this increases the uncertainty about the state and makes
the PRNG output less predictable. Suitable input comes from user interaction (random key
presses, mouse movements) and certain hardware events. The entropy argument is (the lower
bound of) an estimate of how much randomness is contained in buf, measured in bytes.
Details about sources of randomness and how to estimate their entropy can be found in the
literature, e.g. RFC 1750.
RAND_add() may be called with sensitive data such as user entered passwords. The seed val-
ues cannot be recovered from the PRNG output.
OpenSSL makes sure that the PRNG state is unique for each thread. On systems that provide
"/dev/urandom", the randomness device is used to seed the PRNG transparently. However, on
all other systems, the application is responsible for seeding the PRNG by calling
RAND_add(), RAND_egd(3) or RAND_load_file(3).
RAND_seed() is equivalent to RAND_add() when num == entropy.
RAND_event() collects the entropy from Windows events such as mouse movements and other
user interaction. It should be called with the iMsg, wParam and lParam arguments of all
messages sent to the window procedure. It will estimate the entropy contained in the event
message (if any), and add it to the PRNG. The program can then process the messages as
The RAND_screen() function is available for the convenience of Windows programmers. It
adds the current contents of the screen to the PRNG. For applications that can catch Win-
dows events, seeding the PRNG by calling RAND_event() is a significantly better source of
randomness. It should be noted that both methods cannot be used on servers that run with-
out user interaction.
RAND_status() and RAND_event() return 1 if the PRNG has been seeded with enough data, 0
The other functions do not return values.
rand(3), RAND_egd(3), RAND_load_file(3), RAND_cleanup(3)
RAND_seed() and RAND_screen() are available in all versions of SSLeay and OpenSSL.
RAND_add() and RAND_status() have been added in OpenSSL 0.9.5, RAND_event() in OpenSSL
0.9.7a 2000-03-22 RAND_add(3)