a528d4f0a9
If something was "present in all versions" of SSLeay, or if it was added to a version of SSLeay (and therefore predates OpenSSL), remove mention of it. Documentation history now starts with OpenSSL. Remove mention of all history before OpenSSL 0.9.8, inclusive. Remove all AUTHOR sections. Reviewed-by: Tim Hudson <tjh@openssl.org>
52 lines
1.4 KiB
Text
52 lines
1.4 KiB
Text
=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)>, L<DH_size(3)>, L<DSA_size(3)>,
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L<RSA_size(3)>
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=cut
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