int BN_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_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 top is -1, the most significant bit of the random number can be zero. If top is 0, it is set to 1, and if top is 1, 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 true, the number will be odd. The value of bits must be zero or greater. If bits is 1 then top cannot also be 1.
BN_pseudo_rand() does the same, but pseudo-random numbers generated by this function are not necessarily unpredictable. They can be used for non-cryptographic purposes and for certain purposes in cryptographic protocols, but usually not for key generation etc.
BN_rand_range() generates a cryptographically strong pseudo-random number rnd in the range 0 <= rnd < range. BN_pseudo_rand_range() does the same, but is based on BN_pseudo_rand(), and hence numbers generated by it are not necessarily unpredictable.
The PRNG must be seeded prior to calling BN_rand() or BN_rand_range().
The functions return 1 on success, 0 on error. The error codes can be obtained by ERR_get_error(3).
BN_rand() is available in all versions of SSLeay and OpenSSL. BN_pseudo_rand() was added in OpenSSL 0.9.5. The top == -1 case and the function BN_rand_range() were added in OpenSSL 0.9.6a. BN_pseudo_rand_range() was added in OpenSSL 0.9.6c.