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.\"     $NetBSD: BN_num_bytes.3,v 1.14 2015/06/12 17:01:13 christos Exp $
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.\" ========================================================================
.\"
.IX Title "BN_num_bytes 3"
.TH BN_num_bytes 3 "2009-07-19" "1.0.1n" "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 "LIBRARY"
libcrypto, -lcrypto
.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.
.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"
\&\fIopenssl_bn\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.