X.509 V3 certificate extension configuration
Several of the OpenSSL utilities can add extensions to a certificate or
certificate request based on the contents of a configuration file. The file
format is based on the
Typically the application will contain an option to point to an extension
section. Each line of the extension section takes the form:
is present, then the extension will be
The format of extension_options
depends on the
value of extension_name
There are four main types of extension: string extensions, multi-valued
extensions, raw extensions, and arbitrary extensions.
String extensions simply have a string which contains either the value itself or
how it is obtained. For example:
nsComment="This is a
Multi-valued extensions have a short form and a long form. The short form is a
list of names and values:
The long form allows the values to be placed in a separate section:
Both forms are equivalent.
The syntax of raw extensions is governed by the extension code: it can for
example contain data in multiple sections. The correct syntax to use is
defined by the extension code itself: check out the certificate policies
extension for an example.
If an extension type is unsupported, then the arbitrary extension syntax must be
used; see the ARBITRARY
section for more details.
The following sections describe each supported extension in detail.
This is a multi-valued extension which indicates whether a certificate is a CA
certificate. The first (mandatory) name is CA
followed by TRUE
. If CA
, then an optional
name followed by an non-negative value
can be included. For example:
A CA certificate must include the basicConstraints
value with the CA
field set to
. An end user certificate must either set
exclude the extension entirely. Some software may require the inclusion of
set to FALSE
end entity certificates.
parameter indicates the maximum number
of CAs that can appear below this one in a chain. So if you have a CA with a
of zero it can only be used to sign end
user certificates and not further CAs.
Key usage is a multi-valued extension consisting of a list of names of the
permitted key usages.
The supported names are: digitalSignature
This extensions consists of a list of usages indicating purposes for which the
certificate public key can be used for.
These can either be object short names or the dotted numerical form of OIDs.
While any OID can be used, only certain values make sense. In particular the
following PKIX, NS and MS values are meaningful:
This is really a string extension and can take two possible values. Either the
which will automatically follow the
guidelines in RFC 3280 or a hex string giving the extension value to include.
The use of the hex string is strongly discouraged. Example:
The authority key identifier extension permits two options,
both can take the optional value always
If the keyid
option is present, an attempt is made
to copy the subject key identifier from the parent certificate. If the value
is present, then an error is returned if
the option fails.
option copies the issuer and serial
number from the issuer certificate. This will only be done if the
option fails or is not included unless the
flag will always include the value.
The subject alternative name extension allows various literal values to be
included in the configuration file. These include
(an email address),
(a uniform resource indicator),
(a DNS domain name),
(a registered ID: OBJECT IDENTIFIER),
(an IP address),
(a distinguished name), and
option can include a special
value. This will automatically include any
email addresses contained in the certificate subject name in the extension.
The IP address used in the IP
options can be in
either IPv4 or IPv6 format.
The value of dirName
should point to a section
containing the distinguished name to use as a set of name value pairs. Multi
values AVAs can be formed by prefacing the name with a
can include arbitrary data associated
with an OID: the value should be the OID followed by a semicolon and the
content in standard
subjectAltName=otherName:188.8.131.52;UTF8:some other identifier
The issuer alternative name option supports all the literal options of subject
alternative name. It does not support the
because that would not make sense. It does support an additional
that will copy all the subject alternative name values from the issuer
certificate (if possible). Example:
issuerAltName = issuer:copy
The authority information access extension gives details about how to access
certain information relating to the CA. Its syntax is
has the same syntax as subject
alternative name (except that
can be any valid OID
but only certain values are meaningful, for example
authorityInfoAccess = OCSP;URI:http://ocsp.my.host/
authorityInfoAccess = caIssuers;URI:http://my.ca/ca.html
This is a multi-valued extension whose options can be either in
pair form using the same form as subject alternative name or a single value
representing a section name containing all the distribution point fields.
pair a new DistributionPoint with the fullName field set to the given value,
both the cRLissuer and reasons fields are omitted in this case.
In the single option case, the section indicated contains values for each field.
In this section:
If the name is fullname
, the value field should
contain the full name of the distribution point in the same format as subject
If the name is relativename
, then the value field
should contain a section name whose contents represent a DN fragment to be
placed in this field.
The name CRLIssuer
, if present, should contain a
value for this field in subject alternative name format.
If the name is reasons
, the value field should
consist of a comma separated field containing the reasons. Valid reasons are:
Full distribution point example:
This extension should only appear in CRLs. It is a multi-valued extension whose
syntax is similar to the "section" pointed to by the CRL
distribution points extension with a few differences.
The names reasons
are not recognized.
The name onlysomereasons
is accepted, which sets
this field. The value is in the same format as the CRL distribution point
The names onlyuser
are also accepted. The values should
be a boolean values (TRUE
) to indicate the value of the corresponding
This is a raw extension. All the fields of this extension can be set by using
the appropriate syntax.
If you follow the PKIX recommendations and just use one OID, then you just
include the value of that OID. Multiple OIDs can be set separated by commas,
If you wish to include qualifiers, then the policy OID and qualifiers need to be
specified in a separate section: this is done by using the
syntax instead of a literal OID
The section referred to must include the policy OID using the name
qualifiers can be included using the
qualifiers can be set using the syntax:
The value of the userNotice
qualifier is specified
in the relevant section. This section can include
are text strings, and
is a comma separated list of
numbers. The organization
options (if included) must
be present. If you use the
option with IE5 then you need the
option at the top level to modify the
encoding: otherwise it will not be interpreted properly. Example:
policyIdentifier = 184.108.40.206
explicitText="Explicit Text Here"
option changes the type of the
field. In RFC 2459, it can only be
of type DisplayText
. In RFC 3280,
is also permissible. Some software
(for example some versions of MSIE) may require
This is a multi-valued extension which consists of the names
and a non-negative integer
value. At least one component must be present. Example:
This is a string extension whose value must be a non-negative integer. Example:
inhibitAnyPolicy = 2
The name constraints extension is a multi-valued extension. The name should
begin with the word permitted
, followed by a semicolon. The rest of
the name and the value follows the syntax of subjectAltName except
supported and the IP
form should consist of an IP
addresses and subnet mask separated by a slash. Examples:
The OCSP no check extension is a string extension, but its value is ignored.
noCheck = ignored
This is a multi-valued extension consisting of a list of TLS extension
identifiers. Each identifier may be a number in the range from 0 to 65535 or a
supported name. When a TLS client sends a listed extension, the TLS server is
expected to include that extension in its reply.
The supported names are: status_request
tlsfeature = status_request
The following extensions are non-standard, Netscape specific and largely
obsolete. Their use in new applications is discouraged.
Netscape comment (nsComment
) is a string extension
containing a comment which will be displayed when the certificate is viewed in
some browsers. Example:
nsComment = Some Random Comment
Other supported extensions in this category are:
This is a multi-valued extensions which consists of a list of flags to be
included. It was used to indicate the purposes for which a certificate could
be used. The basicConstraints
, and extended key usage extensions are
now used instead.
Acceptable values for nsCertType
If an extension is not supported by the OpenSSL code, then it must be encoded
using the arbitrary extension format. It is also possible to use the arbitrary
format for supported extensions. Extreme care should be taken to ensure that
the data is formatted correctly for the given extension type.
There are two ways to encode arbitrary extensions.
The first way is to use the word ASN1
the extension content using the same syntax as
220.127.116.11=critical,ASN1:UTF8String:Some random data
field1 = UTF8:field1
field2 = UTF8:field2
It is also possible to use the word DER
the raw encoded data in any extension.
The value following DER
is a hex dump of the DER
encoding of the extension. Any extension can be placed in this form to
override the default behaviour. For example:
- standard configuration file
X509v3 extension code was first added to OpenSSL 0.9.2.
There is no guarantee that a specific implementation will process a given
extension. It may therefore sometimes be possible to use certificates for
purposes prohibited by their extensions because a specific application does
not recognize or honour the values of the relevant extensions.
options should be used with caution. It is possible to create totally invalid
extensions if they are not used carefully.
If an extension is multi-value and a field value must contain a comma, the long
form must be used. Otherwise the comma would be misinterpreted as a field
separator. For example,
will produce an error, but the following form is valid:
Due to the behaviour of the OpenSSL CONF library, the same field name can only
occur once in a section. That means that
will only use the last value. This can be worked around by using the form: