.\"	$OpenBSD: BN_num_bytes.3,v 1.4 2016/11/20 17:58:44 schwarze Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
.\"
.\" This file was written by Ulf Moeller <ulf@openssl.org>
.\" and Richard Levitte <levitte@openssl.org>.
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.Dd $Mdocdate: November 20 2016 $
.Dt BN_NUM_BYTES 3
.Os
.Sh NAME
.Nm BN_num_bytes ,
.Nm BN_num_bits ,
.Nm BN_num_bits_word
.Nd get BIGNUM size
.Sh SYNOPSIS
.In openssl/bn.h
.Ft int
.Fo BN_num_bytes
.Fa "const BIGNUM *a"
.Fc
.Ft int
.Fo BN_num_bits
.Fa "const BIGNUM *a"
.Fc
.Ft int
.Fo BN_num_bits_word
.Fa "BN_ULONG w"
.Fc
.Sh DESCRIPTION
.Fn BN_num_bytes
returns the size of a
.Vt BIGNUM
in bytes.
.Pp
.Fn BN_num_bits_word
returns the number of significant bits in a word.
As an example, 0x00000432 returns 11, not 16 or 32.
Basically, except for a zero, it returns
.Pp
.D1 floor(log2( Ns Fa w ) ) No + 1 .
.Pp
.Fn BN_num_bits
returns the number of significant bits in a
.Sy BIGNUM ,
following the same principle as
.Fn BN_num_bits_word .
.Pp
.Fn BN_num_bytes
is a macro.
.Pp
Some have tried using
.Fn 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
.Em significant
bits a little lower.
If you want to know the "key size" of such a key, either use functions
like
.Xr RSA_size 3 ,
.Xr DH_size 3 ,
and
.Xr DSA_size 3 ,
or use
.Fn BN_num_bytes
and multiply with 8 (although there's no real guarantee that will match
the "key size", just a lot more probability).
.Sh RETURN VALUES
The size.
.Sh SEE ALSO
.Xr bn 3 ,
.Xr DH_size 3 ,
.Xr DSA_size 3 ,
.Xr RSA_size 3
.Sh HISTORY
.Fn BN_num_bytes ,
.Fn BN_num_bits ,
and
.Fn BN_num_bits_word
are available in all versions of SSLeay and OpenSSL.