.\" $OpenBSD: X509V3_EXT_get_nid.3,v 1.5 2024/10/03 03:31:47 tb Exp $
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.\" Copyright (c) 2024 Theo Buehler <tb@openbsd.org>
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.Dd $Mdocdate: October 3 2024 $
.Dt X509V3_EXT_GET_NID 3
.Os
.Sh NAME
.Nm X509V3_EXT_get_nid ,
.Nm X509V3_EXT_get
.Nd retrieve X.509v3 certificate extension methods
.Sh SYNOPSIS
.In openssl/x509v3.h
.Ft const X509V3_EXT_METHOD *
.Fo X509V3_EXT_get_nid
.Fa "int nid"
.Fc
.Ft const X509V3_EXT_METHOD *
.Fo X509V3_EXT_get
.Fa "X509_EXTENSION *ext"
.Fc
.Sh DESCRIPTION
An X.509v3 certificate extension contains an Object Identifier (OID),
a boolean criticality indicator, and an opaque extension value
.Po
an
.Vt ASN1_OCTET_STRING
.Pc
whose meaning is determined by the OID.
The library's
.Vt X509V3_EXT_METHOD
type,
which is not yet documented in detail,
contains a numeric identifier (NID) to represent the OID and various
handlers for encoding, decoding, printing, and configuring the
extension's value.
Criticality is handled separately, for example as an argument to
.Xr X509V3_add1_i2d 3 .
.Sh RETURN VALUES
.Fn X509V3_EXT_get_nid
returns the
.Vt X509V3_EXT_METHOD
corresponding to the numeric identifier
.Fa nid ,
or
.Dv NULL
if there is none.
.Pp
.Fn X509V3_EXT_get
returns the
.Vt X509V3_EXT_METHOD
associated with the extension type of
.Fa ext ,
or
.Dv NULL
if there is none.
.Sh SEE ALSO
.Xr i2s_ASN1_ENUMERATED_TABLE 3 ,
.Xr OBJ_create 3 ,
.Xr X509_EXTENSION_get_object 3 ,
.Xr X509V3_get_d2i 3
.Sh STANDARDS
RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile
.Bl -dash -compact
.It
section 4.2: Certificate Extensions
.El
.Sh HISTORY
These functions first appeared in OpenSSL 0.9.2b and
have been available since
.Ox 2.6 .
.Sh CAVEATS
LibreSSL only supports built-in extension methods.
Other implementations have incomplete support for custom extension methods,
whose API is not threadsafe, does not affect the behavior of
.Xr X509_verify_cert 3 ,
and has various other surprising quirks.
Both functions prefer built-in methods over custom methods with the same OID.