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.
0.9.7l 2004-07-01 BN_num_bytes(3)
|