.\"	$OpenBSD: BN_set_flags.3,v 1.5 2021/12/06 19:45:27 schwarze Exp $
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.\" Copyright (c) 2017 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: December 6 2021 $
.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
interprets
.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 .
.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.