/usr/man/cat.3/BN_num_bytes.3

BN_num_bytes(3)              OpenSSL              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.

1.0.2t               Last change: 2019-09-10                    1

See also BN_num_bits(3)
See also BN_num_bits_word(3)