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:
If B<critical> is present then the extension will be critical.
The format of B<extension_options> depends on the value of B<extension_name>.
There are four main types of extension: I<string> extensions, I<multi-valued>
extensions, I<raw> and I<arbitrary> extensions.
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 I<arbitrary> extension syntax
must be used, see the L<ARBITRARY EXTENSIONS|/"ARBITRARY EXTENSIONS"> section for more details.
=head1 STANDARD EXTENSIONS
The following sections describe each supported extension in detail.
=head2 Basic Constraints.
This is a multi valued extension which indicates whether a certificate is
a CA certificate. The first (mandatory) name is B<CA> followed by B<TRUE> or
B<FALSE>. If B<CA> is B<TRUE> then an optional B<pathlen> name followed by an
non-negative value can be included.
A CA certificate B<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.
=head2 Key Usage.
Key usage is a multi valued extension consisting of a list of names of the
permitted key usages.
The supported names are: digitalSignature, nonRepudiation, keyEncipherment,
dataEncipherment, keyAgreement, keyCertSign, cRLSign, encipherOnly
=head2 Extended Key Usage.
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:
serverAuth SSL/TLS Web Server Authentication.
clientAuth SSL/TLS Web Client Authentication.
codeSigning Code signing.
emailProtection E-mail Protection (S/MIME).
timeStamping Trusted Timestamping
OCSPSigning OCSP Signing
ipsecIKE ipsec Internet Key Exchange
msCodeInd Microsoft Individual Code Signing (authenticode)
msCodeCom Microsoft Commercial Code Signing (authenticode)
msCTLSign Microsoft Trust List Signing
msEFS Microsoft Encrypted File System
=head2 Subject Key Identifier.
This is really a string extension and can take two possible values. Either
the word B<hash> 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.
=head2 Authority Key Identifier.
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.
=head2 Subject Alternative Name.
The subject alternative name extension allows various literal values to be
included in the configuration file. These include B<email> (an email address)
B<URI> a uniform resource indicator, B<DNS> (a DNS domain name), B<RID> (a
registered ID: OBJECT IDENTIFIER), B<IP> (an IP address), B<dirName>
(a distinguished name) and otherName.
The email option include a special 'copy' value. This will automatically
include any email addresses contained in the certificate subject name in
The IP address used in the B<IP> options can be in either IPv4 or IPv6 format.
The value of B<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 B<+> character.
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:184.108.40.206;UTF8:some other identifier
=head2 Issuer Alternative Name.
The issuer alternative name option supports all the literal options of
subject alternative name. It does B<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).
issuerAltName = issuer:copy
=head2 Authority Info Access.
The authority information access extension gives details about how to access
certain information relating to the CA. Its syntax is accessOID;location
where I<location> 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
=head2 CRL distribution points
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 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: "keyCompromise",
"CACompromise", "affiliationChanged", "superseded", "cessationOfOperation",
"certificateHold", "privilegeWithdrawn" and "AACompromise".
Full distribution point example:
=head2 Issuing Distribution Point
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" field.
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.
=head2 Certificate Policies.
This is a I<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= 220.127.116.11, 18.104.22.168
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 = 22.214.171.124
explicitText="Explicit Text Here"
The B<ia5org> option changes the type of the I<organization> field. In RFC2459
it can only be of type DisplayText. In RFC3280 IA5String is also permissible.
Some software (for example some versions of MSIE) may require ia5org.
ASN1 type of explicitText can be specified by prepending B<UTF8>,
B<BMP> or B<VISIBLE> prefix followed by colon. For example:
explicitText="UTF8:Explicit Text Here"
=head2 Policy Constraints
This is a multi-valued extension which consisting of the names
B<requireExplicitPolicy> or B<inhibitPolicyMapping> and a non negative integer
value. At least one component must be present.
policyConstraints = requireExplicitPolicy:3
=head2 Inhibit Any Policy
This is a string extension whose value must be a non negative integer.
inhibitAnyPolicy = 2
=head2 Name Constraints
The name constraints extension is a multi-valued extension. The name should
begin with the word B<permitted> or B<excluded> followed by a B<;>. The rest of
the name and the value follows the syntax of subjectAltName except email:copy
is not supported and the B<IP> form should consist of an IP addresses and
subnet mask separated by a B</>.
=head2 OCSP No Check
The OCSP No Check extension is a string extension but its value is ignored.
noCheck = ignored
=head2 TLS Feature (aka Must Staple)
This is a multi-valued extension consisting of a list of TLS extension
identifiers. Each identifier may be a number (0..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: B<status_request> and B<status_request_v2>.
tlsfeature = status_request
=head1 DEPRECATED EXTENSIONS
The following extensions are non standard, Netscape specific and largely
obsolete. Their use in new applications is discouraged.
=head2 Netscape String extensions.
Netscape Comment (B<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: B<nsBaseUrl>,
B<nsRevocationUrl>, B<nsCaRevocationUrl>, B<nsRenewalUrl>, B<nsCaPolicyUrl>
=head2 Netscape Certificate Type
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: B<client>, B<server>, B<email>,
B<objsign>, B<reserved>, B<sslCA>, B<emailCA>, B<objCA>.
=head1 ARBITRARY EXTENSIONS
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 L<ASN1_generate_nconf(3)>.
126.96.36.199=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.
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 B<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:
=head1 SEE ALSO
L<req(1)>, L<ca(1)>, L<x509(1)>,
Copyright 2004-2019 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
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