Commit 70696f45 authored by Richard Levitte's avatar Richard Levitte
Browse files

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

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

PR: 907
parent 80bbc9ce

doc/crypto/BN_num_bytes.pod

+23 −3
Original line number Diff line number Diff line
@@ -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