x509v3_config - X509 V3 certificate extension configuration format
Several of the OpenSSL utilities can add extensions to a certificate or
certificate request based on the contents of a configuration file.
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 critical.
The format of extension_options
depends on the value of
There are four main types of extension: string
String extensions simply have a string which contains either the value itself or
how it is obtained.
nsComment="This is a Comment"
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
must be used, see the ARBITRARY EXTENSIONS 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 pathlen
name followed by an non-negative value can be included.
A CA certificate must
include the basicConstraints value with the CA
field set to TRUE. An end user certificate must either set CA to FALSE or
exclude the extension entirely. Some software may require the inclusion of
basicConstraints with CA set to FALSE for end entity certificates.
The pathlen parameter indicates the maximum number of CAs that can appear below
this one in a chain. So if you have a CA with a pathlen 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 supporte names are: digitalSignature, nonRepudiation, keyEncipherment,
dataEncipherment, keyAgreement, keyCertSign, cRLSign, encipherOnly and
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 of 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:
serverAuth SSL/TLS Web Server Authentication.
clientAuth SSL/TLS Web Client Authentication.
codeSigning Code signing.
emailProtection E-mail Protection (S/MIME).
timeStamping Trusted Timestamping
msCodeInd Microsoft Individual Code Signing (authenticode)
msCodeCom Microsoft Commercial Code Signing (authenticode)
msCTLSign Microsoft Trust List Signing
msSGC Microsoft Server Gated Crypto
msEFS Microsoft Encrypted File System
nsSGC Netscape Server Gated Crypto
This is really a string extension and can take two possible values. Either the
which will automatically follow the guidelines in RFC3280 or
a hex string giving the extension value to include. The use of the hex string
is strongly discouraged.
The authority key identifier extension permits two options. keyid and issuer:
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 "always" is
present then an error is returned if the option fails.
The issuer option copies the issuer and serial number from the issuer
certificate. This will only be done if the keyid option fails or is not
included unless the "always" flag will always include the value.
The subject alternative name extension allows various literal values to be
included in the configuration file. These include email
a uniform resource indicator, DNS
(a DNS domain
(a registered ID: OBJECT IDENTIFIER), IP
(a distinguished name) and otherName.
The email option include a special 'copy' value. This will automatically include
and email addresses contained in the certificate subject name in the
The IP address used in the IP
options can be in either IPv4 or IPv6
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 preceeding the name with a +
otherName 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:18.104.22.168;UTF8:some other identifier
The issuer alternative name option supports all the literal options of subject
alternative name. It does not
support the email:copy option because
that would not make sense. It does support an additional issuer:copy option
that will copy all the subject alternative name values from the issuer
certificate (if possible).
issuserAltName = issuer:copy
The authority information access extension gives details about how to access
certain information relating to the CA. Its syntax is accessOID;location where
has the same syntax as subject alternative name (except that
email:copy is not supported). accessOID can be any valid OID but only certain
values are meaningful, for example OCSP and caIssuers.
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 name:value pair
using the same form as subject alternative name or a single value representing
a section name containing all the distribution point fields.
For a name:value 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 alternative name.
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
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:
"privilegeWithdrawn" and "AACompromise".
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" and "CRLissuer" are not recognized.
The name "onlysomereasons" is accepted which sets this field. The
value is in the same format as the CRL distribution point "reasons"
The names "onlyuser", "onlyCA", "onlyAA" and
"indirectCRL" are also accepted the values should be a boolean value
(TRUE or FALSE) to indicate the value of the corresponding field.
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 using one OID then you just
include the value of that OID. Multiple OIDs can be set separated by commas,
certificatePolicies= 22.214.171.124, 126.96.36.199
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 @section syntax
instead of a literal OID value.
The section referred to must include the policy OID using the name
policyIdentifier, cPSuri qualifiers can be included using the syntax:
userNotice qualifiers can be set using the syntax:
The value of the userNotice qualifier is specified in the relevant section. This
section can include explicitText, organization and noticeNumbers options.
explicitText and organization are text strings, noticeNumbers is a comma
separated list of numbers. The organization and noticeNumbers options (if
included) must BOTH be present. If you use the userNotice option with IE5 then
you need the 'ia5org' option at the top level to modify the encoding:
otherwise it will not be interpreted properly.
policyIdentifier = 188.8.131.52
explicitText="Explicit Text Here"
option changes the type of the organization
RFC2459 it can only be of type DisplayText. In RFC3280 IA5Strring is also
permissible. Some software (for example some versions of MSIE) may require
This is a multi-valued extension which consisting of the names
and a non negative
intger value. At least one component must be present.
policyConstraints = requireExplicitPolicy:3
This is a string extension whose value must be a non negative integer.
inhibitAnyPolicy = 2
The name constraints extension is a multi-valued extension. The name should
begin with the word permitted
followed by a
. The rest of the name and the value follows the syntax of
subjectAltName except email:copy is not supported and the IP
should consist of an IP addresses and subnet mask separated by a /
issuingDistributionPoint = idp_section
The OCSP No Check extension is a string extension but its value is ignored.
noCheck = ignored
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.
nsComment = "Some Random Comment"
Other supported extensions in this category are: nsBaseUrl
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, keyUsage and extended key usage extensions are
now used instead.
Acceptable values for nsCertType are: client
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 followed by the extension content using
the same syntax as ASN1_generate_nconf
(3). For example:
184.108.40.206=critical,ASN1:UTF8String:Some random data
field1 = UTF8:field1
field2 = UTF8:field2
It is also possible to use the word DER to include the raw encoded data in any
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
There is no guarantee that a specific implementation will process a given
extension. It may therefore be sometimes 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.
The DER and ASN1 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 equivalent form:
Due to the behaviour of the OpenSSL conf
library the same field name can
only occur once in a section. This means that:
will only recognize the last value. This can be worked around by using the form:
The X509v3 extension code was first added to OpenSSL 0.9.2.
Policy mappings, inhibit any policy and name constraints support was added in
option as well as the ASN1
option for arbitrary extensions was added in OpenSSL 0.9.8