src/lib/libcrypto/man/BN_num_bytes.3

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.Dd $Mdocdate: November 22 2022 $
.Dt BN_NUM_BYTES 3
.Os
.Sh NAME
.Nm BN_num_bits_word ,
.Nm BN_num_bits ,
.Nm BN_num_bytes
.Nd get BIGNUM size
.Sh SYNOPSIS
.In openssl/bn.h
.Ft int
.Fo BN_num_bits_word
.Fa "BN_ULONG w"
.Fc
.Ft int
.Fo BN_num_bits
.Fa "const BIGNUM *a"
.Fc
.Ft int
.Fo BN_num_bytes
.Fa "const BIGNUM *a"
.Fc
.Sh DESCRIPTION
.Fn BN_num_bits_word
returns the number of significant bits in
.Fa w ,
that is, the minimum number of digits needed to write
.Fa w
as a binary number.
Except for an argument of 0, this is
.Pp
.D1 floor(log2( Ns Fa w ) ) No + 1 .
.Pp
.Vt BN_ULONG
is a macro that expands to
.Vt unsigned long Pq = Vt uint64_t
on
.Dv _LP64
platforms and
.Vt unsigned int Pq = Vt uint32_t
elsewhere.
.Pp
.Fn BN_num_bits
returns the number of significant bits in the value of the
.Fa "BIGNUM *a" ,
following the same principle as
.Fn BN_num_bits_word .
.Pp
.Fn BN_num_bytes
is a macro that returns the number of significant bytes in
.Fa a ,
i.e. the minimum number of bytes needed to store the value of
.Fa a ,
that is,
.Fn BN_num_bits a
divided by eight and rounded up to the next integer number.
.Sh RETURN VALUES
.Fn BN_num_bits_word
returns the number of significant bits in
.Fa w
or 0 if
.Fa w
is 0.
The maximum return value that can occur is
.Dv BN_BITS2 ,
which is 64 on
.Dv _LP64
platforms and 32 elsewhere.
.Pp
.Fn BN_num_bits
returns the number of significant bits and
.Fn BN_num_bytes
the number of significant bytes in
.Fa a ,
or 0 if the value of
.Fa a
is 0.
.Sh SEE ALSO
.Xr BN_new 3 ,
.Xr BN_security_bits 3 ,
.Xr DH_size 3 ,
.Xr DSA_size 3 ,
.Xr RSA_size 3
.Sh HISTORY
.Fn BN_num_bytes
and
.Fn BN_num_bits
first appeared in SSLeay 0.5.1.
.Fn BN_num_bits_word
first appeared in SSLeay 0.5.2.
These functions have been available since
.Ox 2.4 .
.Sh CAVEATS
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 smaller.
If you want to know the "key size" of such a key, use functions like
.Xr RSA_size 3 ,
.Xr DH_size 3 ,
and
.Xr DSA_size 3 .