BN_NUM_BYTES(3) OpenSSL BN_NUM_BYTES(3)
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 (some-
thing like 512, 1024, 2048, ...). This is because generat-
ing 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).
bn(3), DH_size(3), DSA_size(3), RSA_size(3)
BN_num_bytes(), BN_num_bits() and BN_num_bits_word() are
available in all versions of SSLeay and OpenSSL.
MirOS BSD #10-current 2005-04-29 1
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