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.\" ========================================================================
.\"
.IX Title "BN_num_bytes 3openssl"
.TH BN_num_bytes 3openssl "2016-05-03" "1.0.1t" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
BN_num_bits, BN_num_bytes, BN_num_bits_word \- get BIGNUM size
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/bn.h>
\&
\& int BN_num_bytes(const BIGNUM *a);
\&
\& int BN_num_bits(const BIGNUM *a);
\&
\& int BN_num_bits_word(BN_ULONG w);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fIBN_num_bytes()\fR returns the size of a \fB\s-1BIGNUM\s0\fR in bytes.
.PP
\&\fIBN_num_bits_word()\fR 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 \fIfloor(log2(w))+1\fR.
.PP
\&\fIBN_num_bits()\fR returns the number of significant bits in a \fB\s-1BIGNUM\s0\fR,
following the same principle as \fIBN_num_bits_word()\fR.
.PP
\&\fIBN_num_bytes()\fR is a macro.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The size.

.\" Oracle has added the ARC stability level to this manual page
.SH ATTRIBUTES
See
.BR attributes (5)
for descriptions of the following attributes:
.sp
.TS
box;
cbp-1 | cbp-1
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE 
=
Availability	library/security/openssl
=
Stability	Uncommitted
.TE 
.PP
.SH "NOTES"
.IX Header "NOTES"
Some have tried using \fIBN_num_bits()\fR on individual numbers in \s-1RSA\s0 keys,
\&\s-1DH\s0 keys and \s-1DSA\s0 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 \fIsignificant\fR bits a little lower.  If you want to know the \*(L"key
size\*(R" of such a key, either use functions like \fIRSA_size()\fR, \fIDH_size()\fR
and \fIDSA_size()\fR, or use \fIBN_num_bytes()\fR and multiply with 8 (although
there's no real guarantee that will match the \*(L"key size\*(R", just a lot
more probability).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIbn\fR\|(3), \fIDH_size\fR\|(3), \fIDSA_size\fR\|(3),
\&\fIRSA_size\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIBN_num_bytes()\fR, \fIBN_num_bits()\fR and \fIBN_num_bits_word()\fR are available in
all versions of SSLeay and OpenSSL.


.\" Oracle has added source availability information to this manual page
This software was built from source available at https://java.net/projects/solaris-userland.  The original community source was downloaded from  http://www.openssl.org/source/openssl-1.0.1t.tar.gz

Further information about this software can be found on the open source community website at http://www.openssl.org/.