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.\" Copyright (c) 2017, 2022, 2023 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: August 9 2023 $
.Dt D2I_ASN1_OBJECT 3
.Os
.Sh NAME
.Nm d2i_ASN1_OBJECT ,
.Nm i2d_ASN1_OBJECT ,
.Nm OBJ_get0_data ,
.Nm OBJ_length
.Nd decode and encode ASN.1 object identifiers
.Sh SYNOPSIS
.In openssl/asn1.h
.Ft ASN1_OBJECT *
.Fo d2i_ASN1_OBJECT
.Fa "ASN1_OBJECT **val_out"
.Fa "unsigned char **der_in"
.Fa "long length"
.Fc
.Ft int
.Fo i2d_ASN1_OBJECT
.Fa "const ASN1_OBJECT *val_in"
.Fa "unsigned char **der_out"
.Fc
.In openssl/objects.h
.Ft const unsigned char *
.Fn OBJ_get0_data "const ASN1_OBJECT *val_in"
.Ft size_t
.Fn OBJ_length "const ASN1_OBJECT *val_in"
.Sh DESCRIPTION
These functions decode and encode ASN.1 object identifiers.
For details about the semantics, examples, caveats, and bugs, see
.Xr ASN1_item_d2i 3 .
.Pp
The LibreSSL implementation of
.Fn d2i_ASN1_OBJECT
always calls
.Xr ASN1_OBJECT_free 3
if an existing object is passed in via
.Fa val_out
and it always creates a new object from scratch.
Other implementations may attempt to reuse an existing object,
which is fragile and prone to bugs.
Consequently, always passing
.Dv NULL
for the
.Fa val_out
argument is recommended.
.Pp
The objects returned from
.Fn d2i_ASN1_OBJECT
and the data contained in them are always marked as dynamically
allocated, so when they are no longer needed,
.Xr ASN1_OBJECT_free 3
can be called on them.
.Pp
.Fn i2d_ASN1_OBJECT
encodes the object identifier pointed to by
.Fa val_in
into DER format.
.Fn OBJ_get0_data
and
.Fn OBJ_length
only deal with the content octets of that DER encoding,
without taking the identifier and length octets into account.
.Sh RETURN VALUES
.Fn d2i_ASN1_OBJECT
returns a pointer to the new
.Vt ASN1_OBJECT
object or
.Dv NULL
if an error occurs.
With other implementations, it might return a pointer to the reused
.Vt ASN1_OBJECT .
.Pp
.Fn i2d_ASN1_OBJECT
returns the number of octets successfully encoded
or a value <= 0 if an error occurs.
.Pp
.Fn OBJ_get0_data
returns an internal pointer to the first content octet of the DER
encoding of
.Fa val_in .
The other content octets follow the returned pointer contiguously.
.Fn OBJ_length
returns the number of content octets contained in the DER encoding of
.Fa val_in .
This number is always smaller than the total length of the encoding
returned by
.Xr ASN1_object_size 3 .
.Pp
If
.Fa val_in
is a
.Dv NULL
pointer or points to an empty object, for example one freshly created with
.Xr ASN1_OBJECT_new 3 ,
.Fn OBJ_get0_data
returns
.Dv NULL
and
.Fn OBJ_length
returns zero.
.Sh SEE ALSO
.Xr a2d_ASN1_OBJECT 3 ,
.Xr ASN1_item_d2i 3 ,
.Xr ASN1_OBJECT_new 3 ,
.Xr ASN1_put_object 3 ,
.Xr OBJ_nid2obj 3
.Sh STANDARDS
ITU-T Recommendation X.690, also known as ISO/IEC 8825-1:
Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical Encoding
Rules (CER) and Distinguished Encoding Rules (DER),
section 8.19: Encoding of an object identifier value
.Sh HISTORY
.Fn d2i_ASN1_OBJECT
and
.Fn i2d_ASN1_OBJECT
first appeared in SSLeay 0.5.1 and have been available since
.Ox 2.4 .
.Pp
.Fn OBJ_get0_data
and
.Fn OBJ_length
first appeared in OpenSSL 1.1.0 and have been available since
.Ox 7.1 .
.Sh CAVEATS
.Fn d2i_ASN1_OBJECT
never sets the long and short names of the object, not even if the
object identifier matches one that is built into the library.
To find the names of an object identifier parsed from DER or BER
input, call
.Xr OBJ_obj2nid 3
on the returned object, and then
.Xr OBJ_nid2sn 3
and
.Xr OBJ_nid2ln 3
on the result.
.Pp
Calling
.Fn OBJ_get0_data
and then accessing memory in front of the returned pointer
results in undefined behaviour.
In particular, it is not possible to find the identifier or
length octets in that way; use
.Xr ASN1_put_object 3
or
.Fn i2d_ASN1_OBJECT
instead.