RAND_screen (3)

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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 state. Thus, if the data at buf are unpredictable to an adversary, this increases the uncertainty about the state and makes the 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.

RAND_add() may be called with sensitive data such as user entered passwords. The seed values cannot be recovered from the

output.

OpenSSL makes sure that the

state is unique for each thread. On systems that provide "/dev/urandom", the randomness device is used to seed the transparently. However, on all other systems, the application is responsible for seeding the 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

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

For applications that can catch Windows events, seeding the 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 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.