BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
 #include <openssl/bn.h>
 int BN_num_bytes(const BIGNUM *a);
 int BN_num_bits(const BIGNUM *a);
 int BN_num_bits_word(BN_ULONG w);
BN_num_bytes() returns the size of a BIGNUM in bytes.
BN_num_bits_word() returns the number of significant bits
    in a word. If we take 0x00000432 as an example, it returns 11, not 16, not
    32. Basically, except for a zero, it returns floor(log2(w))+1.
BN_num_bits() returns the number of significant bits in a
    BIGNUM, following the same principle as
  BN_num_bits_word().
BN_num_bytes() is a macro.
Some have tried using BN_num_bits() on individual numbers in RSA keys, DH
  keys and DSA keys, and found that they don't always come up with the number of
  bits they expected (something like 512, 1024, 2048, ...). This is because
  generating a number with some specific number of bits doesn't always set the
  highest bits, thereby making the number of significant bits a little
  lower. If you want to know the "key size" of such a key, either use
  functions like RSA_size(), DH_size() and DSA_size(), or
  use BN_num_bytes() and multiply with 8 (although there's no real
  guarantee that will match the "key size", just a lot more
  probability).
DH_size(3), DSA_size(3), RSA_size(3)
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