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 critical is present then the extension will be critical.
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 and 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 arbitrary extension syntax
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
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
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
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 decipherOnly.
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 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
Subject Key Identifier.
This is really a string extension and can take two possible values.
Either the word hash which will automatically follow the guidelines in
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.
Subject Alternative Name.
The subject alternative name extension allows various literal values to
be included in the configuration file. These include email (an email
address) URI a uniform resource indicator, DNS (a DNS domain name), RID
(a registered ID: OBJECT IDENTIFIER), IP (an IP address), dirName (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 extension.
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 preceding the name with a + 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:22.214.171.124;UTF8:some other identifier
Issuer Alternative Name.
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
example OCSP and caIssuers.
authorityInfoAccess = OCSP;URI:http://ocsp.my.host/
authorityInfoAccess = caIssuers;URI:http://my.ca/ca.html
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
If the name is "relativename" then the value field should contain a
section name whose contents represent a DN fragment to be placed in
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
Full distribution point example:
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, for example:
certificatePolicies= 126.96.36.199, 188.8.131.52
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
The ia5org option changes the type of the organization field. In
RFC2459 it can only be of type DisplayText. In RFC3280 IA5Strring is
also permissible. Some software (for example some versions of MSIE)
may require ia5org.
This is a multi-valued extension which consisting of the names
requireExplicitPolicy or inhibitPolicyMapping and a non negative intger
value. At least one component must be present.
policyConstraints = requireExplicitPolicy:3
Inhibit Any Policy
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 or excluded 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 form should consist of an
IP addresses and subnet mask separated by a /.
issuingDistributionPoint = idp_section
OCSP No Check
The OCSP No Check extension is a string extension but its value is
noCheck = ignored
The following extensions are non standard, Netscape specific and
largely obsolete. Their use in new applications is discouraged.
Netscape String extensions.
Netscape Comment (nsComment) is a string extension containing a comment
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, server, email, objsign,
reserved, sslCA, emailCA, objCA.
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
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 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:
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
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
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 X509v3 extension code was first added to OpenSSL 0.9.2.
Policy mappings, inhibit any policy and name constraints support was
added in OpenSSL 0.9.8
The directoryName and otherName option as well as the ASN1 option for
arbitrary extensions was added in OpenSSL 0.9.8
req(1), ca(1), x509(1), ASN1_generate_nconf(3)
1.0.1f 2018-12-04 X509V3_CONFIG(5SSL)
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