X.509 V3 certificate extension
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 openssl.cnf(5) format.
Typically the application will contain an option to point to an extension section. Each line of the extension section takes the form:
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 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 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:
basicConstraints=critical,@bs_section [bs_section] CA=true pathlen=1
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, then an optional
pathlen name followed by an non-negative value can
be included. For example:
basicConstraints=CA:TRUE basicConstraints=CA:FALSE basicConstraints=critical,CA:TRUE, pathlen:0
A CA certificate must include the
basicConstraints value with the
CA field set to
TRUE. An end
user certificate must either set
FALSE or exclude the extension entirely. Some
software may require the inclusion of
FALSE for end entity certificates.
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 supported names are:
keyUsage=digitalSignature, nonRepudiation keyUsage=critical, keyCertSign
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:
||SSL/TLS web server authentication|
||SSL/TLS web client authentication|
||E-mail protection (S/MIME)|
||IPsec internet key exchange|
||Microsoft individual code signing (authenticode)|
||Microsoft commercial code signing (authenticode)|
||Microsoft trust list signing|
||Microsoft encrypted file system|
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 RFC 3280 or a hex string giving the
extension value to include. The use of the hex string is strongly
Authority key identifier
The authority key identifier extension permits two options,
issuer: both can
take the optional value
keyid option is present, an attempt
is made to copy the subject key identifier from the parent certificate. If
always is present, then an error is
returned if the option fails.
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
URI (a uniform resource indicator),
DNS (a DNS domain name),
(a registered ID: OBJECT IDENTIFIER),
IP (an IP
dirName (a distinguished name), and
copy value. This will automatically include any
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 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
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(3) format. Examples:
subjectAltName=email:copy,email:firstname.lastname@example.org,URI:http://my.url.here/ subjectAltName=IP:192.168.7.1 subjectAltName=IP:13::17 subjectAltName=email:email@example.com,RID:126.96.36.199 subjectAltName=otherName:188.8.131.52;UTF8:some other identifier subjectAltName=dirName:dir_sect [dir_sect] C=UK O=My Organization OU=My Unit CN=My Name
Issuer alternative name
The issuer alternative name option supports all the literal
options of subject alternative name. It does not support the
copy option because
that would not make sense. It does support an additional
copy option that will
copy all the subject alternative name values from the issuer certificate (if
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
is not supported). accessOID 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
CRL distribution points
This is a multi-valued extension whose options can be either in name:value pair form 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.
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
Full distribution point example:
crlDistributionPoints=crldp1_section [crldp1_section] fullname=URI:http://myhost.com/myca.crl CRLissuer=dirName:issuer_sect reasons=keyCompromise, CACompromise [issuer_sect] C=UK O=Organisation CN=Some Name
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.
CRLissuer are not recognized.
onlysomereasons is accepted,
which sets this field. The value is in the same format as the CRL
indirectCRL are also accepted. The values should be
a boolean values (
FALSE) to indicate the value of the corresponding
issuingDistributionPoint=critical, @idp_section [idp_section] fullname=URI:http://myhost.com/myca.crl indirectCRL=TRUE onlysomereasons=keyCompromise, CACompromise [issuer_sect] C=UK O=Organisation CN=Some Name
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, 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
qualifiers can be included using the syntax:
userNotice qualifiers can be set using the
The value of the
qualifier is specified in the relevant section. This section can include
organization are text strings, and
noticeNumbers is a comma separated list of numbers.
noticeNumbers options (if included) must
both be present. If
you use the
userNotice option with IE5 then you need
ia5org option at the top level to modify the
encoding: otherwise it will not be interpreted properly. Example:
certificatePolicies=ia5org,18.104.22.168,22.214.171.124.8,@polsect [polsect] policyIdentifier = 126.96.36.199 CPS.1="http://my.host.name/" CPS.2="http://my.your.name/" userNotice.1=@notice [notice] explicitText="Explicit Text Here" organization="Organisation Name" noticeNumbers=1,2,3,4
ia5org option changes the type of the
organization field. In RFC 2459, it can only be of
type DisplayText. In RFC 3280,
IA5String is also permissible. Some software (for
example some versions of MSIE) may require
This is a multi-valued extension which consists of the names
inhibitPolicyMapping and a non-negative integer
value. At least one component must be present. Example:
policyConstraints = requireExplicitPolicy:3
Inhibit any policy
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
excluded, followed by a semicolon. The rest of the
name and the value follows the syntax of subjectAltName except
copy is not supported
IP form should consist of an IP addresses
and subnet mask separated by a slash. Examples:
nameConstraints=permitted;IP:192.168.0.0/255.255.0.0 nameConstraints=permitted;email:.somedomain.com nameConstraints=excluded;email:.com
OCSP no check
The OCSP no check extension is a string extension, but its value is ignored. Example:
noCheck = ignored
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 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:
tlsfeature = status_request
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 browsers. Example:
nsComment = Some Random Comment
Other supported extensions in this category are:
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
keyUsage, and extended key usage extensions are now
Acceptable values for
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
followed by the extension content using the same syntax as
ASN1_generate_nconf(3). For example:
188.8.131.52=critical,ASN1:UTF8String:Some random data 184.108.40.206=ASN1:SEQUENCE:seq_sect [seq_sect] field1 = UTF8:field1 field2 = UTF8:field2
It is also possible to use the word
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:
- standard configuration file
openssl(1), ASN1_generate_nconf(3), openssl.cnf(5)
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:
subjectAltName=@subject_alt_section [subject_alt_section] subjectAltName=URI:ldap://somehost.com/CN=foo,OU=bar
Due to the behaviour of the OpenSSL CONF library, the same field name can only occur once in a section. That means that
subjectAltName=@alt_section [alt_section] email=steve@here email=steve@there
will only use the last value. This can be worked around by using the form:
[alt_section] email.1=steve@here email.2=steve@there