Cryptography and SSL/TLS Toolkit



BN_rand, BN_priv_rand, BN_pseudo_rand, BN_rand_range, BN_priv_rand_range, BN_pseudo_rand_range - generate pseudo-random number


 #include <openssl/bn.h>

 int BN_rand(BIGNUM *rnd, int bits, int top, int bottom);

 int BN_priv_rand(BIGNUM *rnd, int bits, int top, int bottom);

 int BN_pseudo_rand(BIGNUM *rnd, int bits, int top, int bottom);

 int BN_rand_range(BIGNUM *rnd, BIGNUM *range);

 int BN_priv_rand_range(BIGNUM *rnd, BIGNUM *range);

 int BN_pseudo_rand_range(BIGNUM *rnd, BIGNUM *range);


BN_rand() generates a cryptographically strong pseudo-random number of bits in length and stores it in rnd. If bits is less than zero, or too small to accommodate the requirements specified by the top and bottom parameters, an error is returned. The top parameters specifies requirements on the most significant bit of the generated number. If it is BN_RAND_TOP_ANY, there is no constraint. If it is BN_RAND_TOP_ONE, the top bit must be one. If it is BN_RAND_TOP_TWO, the two most significant bits of the number will be set to 1, so that the product of two such random numbers will always have 2*bits length. If bottom is BN_RAND_BOTTOM_ODD, the number will be odd; if it is BN_RAND_BOTTOM_ANY it can be odd or even. If bits is 1 then top cannot also be BN_RAND_TOP_TWO.

BN_rand_range() generates a cryptographically strong pseudo-random number rnd in the range 0 <= rnd < range.

BN_priv_rand() and BN_priv_rand_range() have the same semantics as BN_rand() and BN_rand_range() respectively. They are intended to be used for generating values that should remain private, and mirror the same difference between RAND_bytes(3) and RAND_priv_bytes(3).


Always check the error return value of these functions and do not take randomness for granted: an error occurs if the CSPRNG has not been seeded with enough randomness to ensure an unpredictable byte sequence.


The functions return 1 on success, 0 on error. The error codes can be obtained by ERR_get_error(3).


ERR_get_error(3), RAND_add(3), RAND_bytes(3), RAND_priv_bytes(3), RAND(7), RAND_DRBG(7)


  • Starting with OpenSSL release 1.1.0, BN_pseudo_rand() has been identical to BN_rand() and BN_pseudo_rand_range() has been identical to BN_rand_range(). The "pseudo" functions should not be used and may be deprecated in a future release.

  • The BN_priv_rand() and BN_priv_rand_range() functions were added in OpenSSL 1.1.1.

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