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Explain a little better what BN_num_bits() and BN_num_bits_word() do.

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Add a note as to how these functions do not always return the key size, and
how one can deal with that.

PR: 907
This commit is contained in:
Richard Levitte 2004-07-01 12:33:42 +00:00
parent 466d27b994
commit 4b7693a75e
1 changed files with 23 additions and 3 deletions
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26
doc/crypto/BN_num_bytes.pod
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@ -16,8 +16,14 @@ BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
=head1 DESCRIPTION
These functions return the size of a B<BIGNUM> in bytes or bits,
and the size of an unsigned integer in bits.
BN_num_bytes() returns the size of a B<BIGNUM> in bytes.
BN_num_bits_word() returns the number of significant bits in a word.
If we take 0x00000432 as an example, it returns 11, not 16, not 32.
Basically, except for a zero, it returns I<floor(log2(w))+1>.
BN_num_bits() returns the number of significant bits in a B<BIGNUM>,
following the same principle as BN_num_bits_word().
BN_num_bytes() is a macro.
@ -25,9 +31,23 @@ BN_num_bytes() is a macro.
The size.
=head1 NOTES
Some have tried using BN_num_bits() on individual numbers in RSA keys,
DH keys and DSA keys, and found that they don't always come up with
the number of bits they expected (something like 512, 1024, 2048,
...). This is because generating a number with some specific number
of bits doesn't always set the highest bits, thereby making the number
of I<significant> bits a little lower. If you want to know the "key
size" of such a key, either use functions like RSA_size(), DH_size()
and DSA_size(), or use BN_num_bytes() and multiply with 8 (although
there's no real guarantee that will match the "key size", just a lot
more probability).
=head1 SEE ALSO
L<bn(3)|bn(3)>
L<bn(3)|bn(3)>, L<DH_size(3)|DH_size(3)>, L<DSA_size(3)|DSA_size(3)>,
L<RSA_size(3)|RSA_size(3)>
=head1 HISTORY