NAME
RAND_add, RAND_seed, RAND_status, RAND_event, RAND_screen — add entropy to the PRNG
Synopsis
#include <openssl/rand.h>
void
RAND_seed(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(void);
DESCRIPTION
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 values 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 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.
RETURN VALUES
RAND_status() and RAND_event() return 1 if the PRNG has been
seeded with enough data, 0 otherwise.
The other functions do not return values.
SEE ALSO
rand(3), RAND_egd(3), RAND_load_file(3), RAND_cleanup(3)
HISTORY
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 0.9.5a.
