X509V3_CONFIG(5) OpenSSL X509V3_CONFIG(5)
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 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 ARBITRART EXTENSIONS section for more
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 or FALSE. If CA is TRUE 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
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 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
RFC3280 or a hex string giving the extension value to include. The use of the hex string is strongly discouraged.
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.
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 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 preceeding 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 ASN1_generate_nconf() format.
subjectAltName=otherName:188.8.131.52;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 values
from the issuer certificate (if possible).
issuserAltName = issuer:copy
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 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
CRL distribution points.
This is a multi-valued extension that supports all the literal options of subject alternative name. Of the few software packages that
currently interpret this extension most only interpret the URI option.
Currently each option will set a new DistributionPoint with the fullName field set to the given value.
Other fields like cRLissuer and reasons cannot currently be set or displayed: at this time no examples were available that used these
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= 184.108.40.206, 220.127.116.11
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 = 18.104.22.168
explicitText="Explicit Text Here"
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 /.
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 which will be displayed when the certificate is viewed in some
nsComment = "Some Random Comment"
Other supported extensions in this category are: nsBaseUrl, nsRevocationUrl, nsCaRevocationUrl, nsRenewalUrl, nsCaPolicyUrl and
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: 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 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(). For example:
22.214.171.124=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 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 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)
50 2013-03-05 X509V3_CONFIG(5)