xref: /openssl/doc/man3/BN_num_bytes.pod (revision 4746f25a)
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