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dd8dec69 UM |
1 | =pod |
2 | ||
3 | =head1 NAME | |
4 | ||
4d524e10 | 5 | BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size |
dd8dec69 UM |
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 | ||
70696f45 RL |
19 | BN_num_bytes() returns the size of a B<BIGNUM> in bytes. |
20 | ||
21 | BN_num_bits_word() returns the number of significant bits in a word. | |
22 | If we take 0x00000432 as an example, it returns 11, not 16, not 32. | |
23 | Basically, except for a zero, it returns I<floor(log2(w))+1>. | |
24 | ||
25 | BN_num_bits() returns the number of significant bits in a B<BIGNUM>, | |
26 | following the same principle as BN_num_bits_word(). | |
dd8dec69 UM |
27 | |
28 | BN_num_bytes() is a macro. | |
29 | ||
30 | =head1 RETURN VALUES | |
31 | ||
32 | The size. | |
33 | ||
70696f45 RL |
34 | =head1 NOTES |
35 | ||
36 | Some have tried using BN_num_bits() on individual numbers in RSA keys, | |
37 | DH keys and DSA keys, and found that they don't always come up with | |
38 | the number of bits they expected (something like 512, 1024, 2048, | |
39 | ...). This is because generating a number with some specific number | |
40 | of bits doesn't always set the highest bits, thereby making the number | |
41 | of I<significant> bits a little lower. If you want to know the "key | |
42 | size" of such a key, either use functions like RSA_size(), DH_size() | |
43 | and DSA_size(), or use BN_num_bytes() and multiply with 8 (although | |
44 | there's no real guarantee that will match the "key size", just a lot | |
45 | more probability). | |
46 | ||
dd8dec69 UM |
47 | =head1 SEE ALSO |
48 | ||
9e183d22 | 49 | L<DH_size(3)>, L<DSA_size(3)>, |
9b86974e | 50 | L<RSA_size(3)> |
dd8dec69 | 51 | |
e2f92610 RS |
52 | =head1 COPYRIGHT |
53 | ||
9e183d22 | 54 | Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved. |
e2f92610 | 55 | |
4746f25a | 56 | Licensed under the Apache License 2.0 (the "License"). You may not use |
e2f92610 RS |
57 | this file except in compliance with the License. You can obtain a copy |
58 | in the file LICENSE in the source distribution or at | |
59 | L<https://www.openssl.org/source/license.html>. | |
60 | ||
61 | =cut |