RAND_DRBG_generate.3ssl - Man Page

generate random bytes using the given drbg instance


 #include <openssl/rand_drbg.h>

 int RAND_DRBG_generate(RAND_DRBG *drbg,
                        unsigned char *out, size_t outlen,
                        int prediction_resistance,
                        const unsigned char *adin, size_t adinlen);

 int RAND_DRBG_bytes(RAND_DRBG *drbg,
                     unsigned char *out, size_t outlen);


RAND_DRBG_generate() generates outlen random bytes using the given DRBG instance drbg and stores them in the buffer at out.

Before generating the output, the DRBG instance checks whether the maximum number of generate requests (reseed interval) or the maximum timespan (reseed time interval) since its last seeding have been reached. If this is the case, the DRBG reseeds automatically. Additionally, an immediate reseeding can be requested by setting the prediction_resistance flag to 1. See Notes section for more details.

The caller can optionally provide additional data to be used for reseeding by passing a pointer adin to a buffer of length adinlen. This additional data is mixed into the internal state of the random generator but does not contribute to the entropy count. The additional data can be omitted by setting adin to NULL and adinlen to 0;

RAND_DRBG_bytes() generates outlen random bytes using the given DRBG instance drbg and stores them in the buffer at out. This function is a wrapper around the RAND_DRBG_generate() call, which collects some additional data from low entropy sources (e.g., a high resolution timer) and calls RAND_DRBG_generate(drbg, out, outlen, 0, adin, adinlen).

Return Values

RAND_DRBG_generate() and RAND_DRBG_bytes() return 1 on success, and 0 on failure.


The reseed interval and reseed time interval of the drbg are set to reasonable default values, which in general do not have to be adjusted. If necessary, they can be changed using RAND_DRBG_set_reseed_interval(3) and RAND_DRBG_set_reseed_time_interval(3), respectively.

A request for prediction resistance can only be satisfied by pulling fresh entropy from one of the approved entropy sources listed in section 5.5.2 of [NIST SP 800-90C]. Since the default DRBG implementation does not have access to such an approved entropy source, a request for prediction resistance will always fail. In other words, prediction resistance is currently not supported yet by the DRBG.

See Also

RAND_bytes(3), RAND_DRBG_set_reseed_interval(3), RAND_DRBG_set_reseed_time_interval(3), RAND_DRBG(7)


The RAND_DRBG functions were added in OpenSSL 1.1.1.

Referenced By

RAND_DRBG_get0_master.3ssl(3), RAND_DRBG_new.3ssl(3), RAND_DRBG_reseed.3ssl(3).

The man page RAND_DRBG_bytes.3ssl(3) is an alias of RAND_DRBG_generate.3ssl(3).

2023-02-06 1.1.1q OpenSSL