BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
#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);
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.
The size.
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).
DH_size(3),
DSA_size(3),
RSA_size(3)
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