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