fa5cea169a
Add a note as to how these functions do not always return the key size, and how one can deal with that. PR: 907
58 lines
1.6 KiB
Plaintext
58 lines
1.6 KiB
Plaintext
=pod
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=head1 NAME
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BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
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=head1 SYNOPSIS
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#include <openssl/bn.h>
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int BN_num_bytes(const BIGNUM *a);
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int BN_num_bits(const BIGNUM *a);
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int BN_num_bits_word(BN_ULONG w);
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=head1 DESCRIPTION
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BN_num_bytes() returns the size of a B<BIGNUM> in bytes.
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BN_num_bits_word() returns the number of significant bits in a word.
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If we take 0x00000432 as an example, it returns 11, not 16, not 32.
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Basically, except for a zero, it returns I<floor(log2(w))+1>.
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BN_num_bits() returns the number of significant bits in a B<BIGNUM>,
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following the same principle as BN_num_bits_word().
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BN_num_bytes() is a macro.
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=head1 RETURN VALUES
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The size.
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=head1 NOTES
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Some have tried using BN_num_bits() on individual numbers in RSA keys,
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DH keys and DSA keys, and found that they don't always come up with
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the number of bits they expected (something like 512, 1024, 2048,
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...). This is because generating a number with some specific number
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of bits doesn't always set the highest bits, thereby making the number
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of I<significant> bits a little lower. If you want to know the "key
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size" of such a key, either use functions like RSA_size(), DH_size()
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and DSA_size(), or use BN_num_bytes() and multiply with 8 (although
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there's no real guarantee that will match the "key size", just a lot
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more probability).
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=head1 SEE ALSO
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L<bn(3)|bn(3)>, L<DH_size(3)|DH_size(3)>, L<DSA_size(3)|DSA_size(3)>,
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L<RSA_size(3)|RSA_size(3)>
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=head1 HISTORY
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BN_num_bytes(), BN_num_bits() and BN_num_bits_word() are available in
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all versions of SSLeay and OpenSSL.
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=cut
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