.\" $OpenBSD: BN_new.3,v 1.22 2022/11/21 22:04:04 schwarze Exp $
.\" full merge up to: OpenSSL man3/BN_new 2457c19d Mar 6 08:43:36 2004 +0000
.\" selective merge up to: man3/BN_new 681acb31 Sep 29 13:10:34 2017 +0200
.\" full merge up to: OpenSSL man7/bn 05ea606a May 20 20:52:46 2016 -0400
.\"
.\" This file was written by Ulf Moeller <ulf@openssl.org>.
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.Dd $Mdocdate: November 21 2022 $
.Dt BN_NEW 3
.Os
.Sh NAME
.Nm BN_new ,
.Nm BN_init ,
.Nm BN_clear ,
.Nm BN_free ,
.Nm BN_clear_free
.Nd allocate and free BIGNUMs
.Sh SYNOPSIS
.In openssl/bn.h
.Ft BIGNUM *
.Fo BN_new
.Fa void
.Fc
.Ft void
.Fo BN_init
.Fa "BIGNUM *"
.Fc
.Ft void
.Fo BN_clear
.Fa "BIGNUM *a"
.Fc
.Ft void
.Fo BN_free
.Fa "BIGNUM *a"
.Fc
.Ft void
.Fo BN_clear_free
.Fa "BIGNUM *a"
.Fc
.Sh DESCRIPTION
The BN library performs arithmetic operations on integers of arbitrary
size.
It was written for use in public key cryptography, such as RSA and
Diffie-Hellman.
.Pp
It uses dynamic memory allocation for storing its data structures.
That means that there is no limit on the size of the numbers manipulated
by these functions, but return values must always be checked in case a
memory allocation error has occurred.
.Pp
The basic object in this library is a
.Vt BIGNUM .
It is used to hold a single large integer.
This type should be considered opaque and fields should not be modified
or accessed directly.
.Pp
.Fn BN_new
allocates and initializes a
.Vt BIGNUM
structure, in particular setting the value to zero and the flags to
.Dv BN_FLG_MALLOCED .
The security-relevant flag
.Dv BN_FLG_CONSTTIME
is not set by default.
.Pp
.Fn BN_init
initializes an existing uninitialized
.Vt BIGNUM .
It is deprecated and dangerous: see
.Sx CAVEATS .
.Pp
.Fn BN_clear
is used to destroy sensitive data such as keys when they are no longer
needed.
It erases the memory used by
.Fa a
and sets it to the value 0.
.Pp
.Fn BN_free
frees the components of the
.Vt BIGNUM
and, if it was created by
.Fn BN_new ,
also the structure itself.
.Fn BN_clear_free
additionally overwrites the data before the memory is returned to the
system.
If
.Fa a
is a
.Dv NULL
pointer, no action occurs.
.Sh RETURN VALUES
.Fn BN_new
returns a pointer to the
.Vt BIGNUM .
If the allocation fails, it returns
.Dv NULL
and sets an error code that can be obtained by
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr ASN1_bn_print 3 ,
.Xr BN_add 3 ,
.Xr BN_add_word 3 ,
.Xr BN_BLINDING_new 3 ,
.Xr BN_bn2bin 3 ,
.Xr BN_cmp 3 ,
.Xr BN_copy 3 ,
.Xr BN_CTX_new 3 ,
.Xr BN_CTX_start 3 ,
.Xr BN_generate_prime 3 ,
.Xr BN_get0_nist_prime_521 3 ,
.Xr BN_GF2m_add 3 ,
.Xr BN_kronecker 3 ,
.Xr BN_mod_inverse 3 ,
.Xr BN_mod_mul_montgomery 3 ,
.Xr BN_mod_mul_reciprocal 3 ,
.Xr BN_mod_sqrt 3 ,
.Xr BN_nist_mod_521 3 ,
.Xr BN_num_bytes 3 ,
.Xr BN_rand 3 ,
.Xr BN_security_bits 3 ,
.Xr BN_set_bit 3 ,
.Xr BN_set_flags 3 ,
.Xr BN_set_negative 3 ,
.Xr BN_swap 3 ,
.Xr BN_zero 3 ,
.Xr crypto 3 ,
.Xr get_rfc3526_prime_8192 3
.Sh HISTORY
.Fn BN_new ,
.Fn BN_clear ,
.Fn BN_free ,
and
.Fn BN_clear_free
first appeared in SSLeay 0.5.1 and have been available since
.Ox 2.4 .
.Pp
.Fn BN_init
first appeared in SSLeay 0.9.1 and has been available since
.Ox 2.6 .
.Sh CAVEATS
.Fn BN_init
must not be called on a
.Vt BIGNUM
that was used and contains an actual number, or the memory
used for storing the number is leaked immediately.
Besides, it must not be called on a number allocated with
.Fn BN_new ,
or the
.Vt BIGNUM
structure itself will likely be leaked later on.
It can only be used on static
.Vt BIGNUM
structures, on
.Vt BIGNUM
structures on the stack, or on
.Vt BIGNUM
structures
.Xr malloc 3 Ap ed
manually, but all of these options are discouraged because they
will no longer work once the
.Vt BIGNUM
data type is made opaque.