RAND_add, RAND_seed, RAND_status, RAND_event, RAND_screen - Add entropy
to the PRNG
const void *buf, int num ); void RAND_add(
const void *buf, int num, double entropy ); int RAND_status(
void ); int RAND_event(
UINT iMsg, WPARAM wParam, LPARAM lParam ); void RAND_screen(
The RAND_add() function 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 random‐
ness and how to estimate their entropy can be found in the literature,
e.g. RFC 1750.
The RAND_add() function may be called with sensitive data such as user
entered passwords. The seed values cannot be recovered from the PRNG
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 appli‐
cation is responsible for seeding the PRNG by calling the RAND_add(),
RAND_egd() or RAND_load_file() functions.
The RAND_seed() function is equivalent to the RAND_add() function when
num == entropy.
The RAND_event() function 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 win‐
dow procedure. It will estimate the entropy contained in the event mes‐
sage (if any), and add it to the PRNG. The program can then process the
messages as usual.
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 Windows 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
without user interaction.
The RAND_status() and RAND_event() functions return 1 if the PRNG has
been seeded with enough data, 0 otherwise.
The other functions do not return values.
The RAND_seed() and RAND_screen() functions are available in all ver‐
sions of SSLeay and OpenSSL. The RAND_add() and RAND_status() functions
have been added in OpenSSL 0.9.5, and RAND_event() in OpenSSL 0.9.5a.
Functions: rand_ssl(3), RAND_egd(3), RAND_load_file(3), RAND_cleanup(3)RAND_add(3)