1=pod 2 3=head1 NAME 4 5BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size 6 7=head1 SYNOPSIS 8 9 #include <openssl/bn.h> 10 11 int BN_num_bytes(const BIGNUM *a); 12 13 int BN_num_bits(const BIGNUM *a); 14 15 int BN_num_bits_word(BN_ULONG w); 16 17=head1 DESCRIPTION 18 19BN_num_bytes() returns the size of a B<BIGNUM> in bytes. 20 21BN_num_bits_word() returns the number of significant bits in a word. 22If we take 0x00000432 as an example, it returns 11, not 16, not 32. 23Basically, except for a zero, it returns I<floor(log2(w))+1>. 24 25BN_num_bits() returns the number of significant bits in a B<BIGNUM>, 26following the same principle as BN_num_bits_word(). 27 28BN_num_bytes() is a macro. 29 30=head1 RETURN VALUES 31 32The size. 33 34=head1 NOTES 35 36Some have tried using BN_num_bits() on individual numbers in RSA keys, 37DH keys and DSA keys, and found that they don't always come up with 38the number of bits they expected (something like 512, 1024, 2048, 39...). This is because generating a number with some specific number 40of bits doesn't always set the highest bits, thereby making the number 41of I<significant> bits a little lower. If you want to know the "key 42size" of such a key, either use functions like RSA_size(), DH_size() 43and DSA_size(), or use BN_num_bytes() and multiply with 8 (although 44there's no real guarantee that will match the "key size", just a lot 45more probability). 46 47=head1 SEE ALSO 48 49L<DH_size(3)>, L<DSA_size(3)>, 50L<RSA_size(3)> 51 52=head1 COPYRIGHT 53 54Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved. 55 56Licensed under the Apache License 2.0 (the "License"). You may not use 57this file except in compliance with the License. You can obtain a copy 58in the file LICENSE in the source distribution or at 59L<https://www.openssl.org/source/license.html>. 60 61=cut 62
