BN_num_bytes(3)

     NAME
          BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM
          size

     SYNOPSIS
           #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);


     DESCRIPTION
          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.

     RETURN VALUES
          The size.

     NOTES
          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).

     SEE ALSO
          bn(3), DH_size(3), DSA_size(3), RSA_size(3)

     HISTORY
          BN_num_bytes(), BN_num_bits() and BN_num_bits_word() are
          available in all versions of SSLeay and OpenSSL.