BN_num_bytes(3) OpenSSL BN_num_bytes(3)
BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
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
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.
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).
openssl_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.
1.0.1i 2009-07-20 BN_num_bytes(3)