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_FLG_TOPTWO.
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.
Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.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
    <https://www.openssl.org/source/license.html>.