Cryptography and SSL/TLS Toolkit



RAND_add, RAND_poll, RAND_poll_ex, RAND_poll_cb, RAND_seed, RAND_status, RAND_event, RAND_screen - add randomness to the PRNG or get its status


 #include <openssl/rand.h>

 int RAND_status(void);

 typedef void (*RAND_poll_cb)(void *arg,
                              const void *buf, int num, double randomness);
 int RAND_poll_ex(RAND_poll_cb cb, void *arg);
 int RAND_poll();

 void RAND_add(const void *buf, int num, double randomness);
 void RAND_seed(const void *buf, int num);


 #if OPENSSL_API_COMPAT < 0x10100000L
 int RAND_event(UINT iMsg, WPARAM wParam, LPARAM lParam);
 void RAND_screen(void);


Random numbers are a vital part of cryptography, including key generation, creating salts, etc., and software-based generators must be "seeded" with external randomness before they can be used as a cryptographically-secure pseudo-random number generator (CSPRNG). The availability of common hardware with special instructions and modern operating systems, which may use items such as interrupt jitter and network packet timings, can be reasonable sources of seeding material.

RAND_status() indicates whether or not the CSPRNG has been sufficiently seeded. If not, functions such as RAND_bytes(3) will fail.

RAND_poll_ex() uses the system's capabilities to obtain a buffer containing random bits which can then be used to seed a CSPRNG. The exact features used depends on how OpenSSL was configured, and a summary can be displayed with the OpenSSL version(1) command. This function is normally called as needed by the CSPRNG. The arg parameter is an arbitrary pointer which will be passed as an argument to the callback. The cb function is called each time there is data to add.

RAND_poll() invokes RAND_poll_ex() with cb and arg set so that it will call RAND_add(), to add the randomness to the global CSPRNG.

RAND_add() mixes the num bytes at buf into the PRNG state. The randomness argument is an estimate of how much randomness is contained in buf, in bytes, and should be a number between zero and num. Details about sources of randomness and how to estimate their randomness can be found in the literature; for example NIST SP 800-90B. The content of buf cannot be recovered from subsequent CSPRNG output. This function will not normally be needed, as RAND_poll() should have been configured to do the appropriate seeding for the local platform. Applications that need to keep random state in an external file should use RAND_load_file(3).

RAND_seed() is equivalent to RAND_add() with randomness set to num.

RAND_event() and RAND_screen() are equivalent to RAND_poll().


RAND_status() returns 1 if the CSPRNG has been seeded with enough data, 0 otherwise.

RAND_poll() returns 1 if it generated seed data, 0 otherwise.

RAND_event() returns RAND_status().

The other functions do not return values.


RAND_event() and RAND_screen() were deprecated in OpenSSL 1.1.0 and should not be used.


RAND_bytes(3), RAND_egd(3), RAND_load_file(3)

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Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at