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.IX Title "BN_num_bytes 3"
.TH BN_num_bytes 3 2019-12-20 1.0.2u OpenSSL
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.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"
\&\fBBN_num_bytes()\fR returns the size of a \fBBIGNUM\fR in bytes.
.PP
\&\fBBN_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
\&\fBBN_num_bits()\fR returns the number of significant bits in a \fBBIGNUM\fR,
following the same principle as \fBBN_num_bits_word()\fR.
.PP
\&\fBBN_num_bytes()\fR is a macro.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The size.
.SH NOTES
.IX Header "NOTES"
Some have tried using \fBBN_num_bits()\fR 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 \fIsignificant\fR bits a little lower.  If you want to know the "key
size" of such a key, either use functions like \fBRSA_size()\fR, \fBDH_size()\fR
and \fBDSA_size()\fR, or use \fBBN_num_bytes()\fR and multiply with 8 (although
there's no real guarantee that will match the "key size", just a lot
more probability).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBbn\fR\|(3), \fBDH_size\fR\|(3), \fBDSA_size\fR\|(3),
\&\fBRSA_size\fR\|(3)
.SH HISTORY
.IX Header "HISTORY"
\&\fBBN_num_bytes()\fR, \fBBN_num_bits()\fR and \fBBN_num_bits_word()\fR are available in
all versions of SSLeay and OpenSSL.