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.\" $OpenBSD: BN_num_bytes.3,v 1.5 2016/12/10 21:13:25 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>.
.\" Copyright (c) 2000, 2004 The OpenSSL Project. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in
.\" the documentation and/or other materials provided with the
.\" distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\" software must display the following acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\" endorse or promote products derived from this software without
.\" prior written permission. For written permission, please contact
.\" openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\" nor may "OpenSSL" appear in their names without prior written
.\" permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\" acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: December 10 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_new 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.
|
