BN_num_bytes(3ssl)
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.