BN_NUM_BYTES(3) BSD Library Functions Manual 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);
DESCRIPTIONBN_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. Basi‐
cally, except for a zero, it returns
floor(log2(w)) + 1.
BN_num_bits() returns the number of significant bits in a BIGNUM, follow‐
ing 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 num‐
ber 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(3), DH_size(3), and DSA_size(3), or
use BN_num_bytes() and multiply with 8 (although there's no real guaran‐
tee that will match the "key size", just a lot more probability).
SEE ALSObn(3), DH_size(3), DSA_size(3), RSA_size(3)HISTORYBN_num_bytes(), BN_num_bits(), and BN_num_bits_word() are available in
all versions of SSLeay and OpenSSL.
BSD April 18, 2024 BSD
