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

       DH_size(3), DSA_size(3), RSA_size(3)

COPYRIGHT

       Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use this file except in compliance with
       the License.  You can obtain a copy in the file LICENSE in the source distribution or at
       <https://www.openssl.org/source/license.html>.