RAND_ADD(3) OpenSSL RAND_ADD(3)
RAND_add, RAND_seed, RAND_status, RAND_event, RAND_screen -
add entropy to the PRNG
#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);
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 cer-
tain 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 random-
ness 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
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RAND_ADD(3) OpenSSL RAND_ADD(3)
events, seeding the PRNG by calling RAND_event() is a signi-
ficantly better source of randomness. It should be noted
that both methods cannot be used on servers that run without
user interaction.
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.
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 0.9.5a.
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