.\"	$OpenBSD: BN_set_flags.3,v 1.3 2018/04/29 15:58:21 schwarze Exp $
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.\" Copyright (c) 2017 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: April 29 2018 $
.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
or the divisor
.Fa d
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 select algorithms with an execution time independent
of the respective numbers, to avoid exposing sensitive information
to timing side-channel attacks.
.Pp
This flag is off by default for
.Vt BIGNUM
objects created with
.Xr BN_new 3 .
.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 HISTORY
.Fn BN_set_flags
and
.Fn BN_get_flags
first appeared in SSLeay 0.9.1 and have been available since
.Ox 2.6 .
.Sh CAVEATS
No public interface exists to clear a flag once it is set.
So think twice before using
.Fn BN_set_flags .
.Sh BUGS
Even if the
.Dv BN_FLG_CONSTTIME
flag is set on
.Fa a
or
.Fa b ,
.Fn BN_gcd
neither fails nor operates in constant time, potentially allowing
timing side-channel attacks.
.Pp
Even if the
.Dv BN_FLG_CONSTTIME
flag is set on
.Fa p ,
if the modulus
.Fa m
is even,
.Xr BN_mod_exp 3
does not operate in constant time, potentially allowing
timing side-channel attacks.
.Pp
If
.Dv BN_FLG_CONSTTIME
is set on
.Fa p ,
.Fn BN_exp
fails instead of operating in constant time.