.\"	$OpenBSD: BN_set_flags.3,v 1.1 2017/01/30 01:29:31 schwarze Exp $
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.\" Copyright (c) 2017 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: January 30 2017 $
.Dt BN_SET_FLAGS 3
.Os
.Sh NAME
.Nm BN_set_flags ,
.Nm BN_get_flags
.Nd enable and inspect flags on BIGNUM objects
.Sh SYNOPSIS
.In openssl/bn.h
.Ft void
.Fo BN_set_flags
.Fa "BIGNUM *b"
.Fa "int flags"
.Fc
.Ft int
.Fo BN_get_flags
.Fa "const BIGNUM *b"
.Fa "int flags"
.Fc
.Sh DESCRIPTION
.Fn BN_set_flags
enables the given
.Fa flags
on
.Fa b .
The
.Fa flags
argument can contain zero or more of the following constants OR'ed
together:
.Bl -tag -width Ds
.It Dv BN_FLG_CONSTTIME
If this flag is set on the divident
.Fa a
in
.Xr BN_div 3 ,
on the exponent
.Fa p
in
.Xr BN_mod_exp 3 ,
or on the divisor
.Fa a
or the modulus
.Fa n
in
.Xr BN_mod_inverse 3 ,
these functions prefer algorithms with an execution time independent
of the respective numbers, to avoid exposing sensitive information
to timing attacks.
.Pp
If this flag is set on the exponent
.Fa p
in
.Xr BN_exp 3
or if the modulus
.Fa m
is even for
.Xr BN_mod_exp 3 ,
an error occurs.
.Pp
Various functions automatically set this flag on sensitive data.
For example, the default implementations of
.Xr DH_generate_key 3 ,
.Xr DSA_generate_key 3 ,
and
.Xr RSA_generate_key_ex 3
set it on the generated private key.
.It Dv BN_FLG_MALLOCED
If this flag is set,
.Xr BN_free 3
and
.Xr BN_clear_free 3
will not only clear and free the components of
.Fa b ,
but also
.Fa b
itself.
This flag is set internally by
.Xr BN_new 3 .
Setting it manually on an existing
.Vt BIGNUM
object is usually a bad idea and can cause calls to
.Xr free 3
with bogus arguments.
.It Dv BN_FLG_STATIC_DATA
If this flag is set,
.Xr BN_clear_free 3
will neither clear nor free the memory used for storing the number.
Consequently, setting it manually on an existing
.Vt BIGNUM
object is usually a terrible idea that can cause both disclosure
of secret data and memory leaks.
This flag is automatically set on the constant
.Vt BIGNUM
objects returned by
.Xr BN_value_one 3
and by the functions documented in
.Xr BN_get0_nist_prime_521 3 .
.El
.Pp
.Fn BN_get_flags
interpretes
.Fa flags
as a bitmask and returns those of the given flags that are set in
.Fa b ,
OR'ed together, or 0 if none of the given
.Fa flags
is set.
The
.Fa flags
argument has the same syntax as for
.Fn BN_set_flags .
.Pp
These functions are currently implemented as macros, but they are
likely to become real functions in the future when the
.Vt BIGNUM
data type will be made opaque.
.Sh RETURN VALUES
.Fn BN_get_flags
returns zero or more of the above constants, OR'ed together.
.Sh SEE ALSO
.Xr BN_mod_exp 3 ,
.Xr BN_mod_inverse 3 ,
.Xr BN_new 3 ,
.Xr BN_with_flags 3
.Sh CAVEATS
No public interface exists to clear a flag once it is set.
So think twice before using
.Fn BN_set_flags .