Provided by: openssl_3.4.1-1ubuntu3_amd64 
NAME
x509v3_config - X509 V3 certificate extension configuration format
DESCRIPTION
Several OpenSSL commands can add extensions to a certificate or certificate request based on the contents
of a configuration file and CLI options such as -addext. The syntax of configuration files is described
in config(5). The commands typically have an option to specify the name of the configuration file, and a
section within that file; see the documentation of the individual command for details.
This page uses extensions as the name of the section, when needed in examples.
Each entry in the extension section takes the form:
name = [critical, ]value(s)
If critical is present then the extension will be marked as critical.
If multiple entries are processed for the same extension name, later entries override earlier ones with
the same name.
The format of values depends on the value of name, many have a type-value pairing where the type and
value are separated by a colon. There are four main types of extension:
string
multi-valued
raw
arbitrary
Each is described in the following paragraphs.
String extensions simply have a string which contains either the value itself or how it is obtained.
Multi-valued extensions have a short form and a long form. The short form is a comma-separated list of
names and values:
basicConstraints = critical, CA:true, pathlen:1
The long form allows the values to be placed in a separate section:
[extensions]
basicConstraints = critical, @basic_constraints
[basic_constraints]
CA = true
pathlen = 1
Both forms are equivalent.
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:
subjectAltName = URI:ldap://somehost.com/CN=foo,OU=bar
will produce an error but the equivalent form:
[extensions]
subjectAltName = @subject_alt_section
[subject_alt_section]
subjectAltName = URI:ldap://somehost.com/CN=foo,OU=bar
is valid.
OpenSSL does not support multiple occurrences of the same field within a section. In this example:
[extensions]
subjectAltName = @alt_section
[alt_section]
email = steve@example.com
email = steve@example.org
will only recognize the last value. To specify multiple values append a numeric identifier, as shown
here:
[extensions]
subjectAltName = @alt_section
[alt_section]
email.1 = steve@example.com
email.2 = steve@example.org
The syntax of raw extensions is defined by the source code that parses the extension but should be
documented. See "Certificate Policies" for an example of a raw extension.
If an extension type is unsupported, then the arbitrary extension syntax must be used, see the "ARBITRARY
EXTENSIONS" section for more details.
STANDARD EXTENSIONS
The following sections describe the syntax of each supported extension. They do not define the semantics
of the extension.
Basic Constraints
This is a multi-valued extension which indicates whether a certificate is a CA certificate. The first
value is CA followed by TRUE or FALSE. If CA is TRUE then an optional pathlen name followed by a
nonnegative value can be included.
For example:
basicConstraints = CA:TRUE
basicConstraints = CA:FALSE
basicConstraints = critical, CA:TRUE, pathlen:1
A CA certificate must include the basicConstraints name with the CA parameter set to TRUE. An end-user
certificate must either have CA:FALSE or omit the extension entirely. The pathlen parameter specifies
the maximum number of CAs that can appear below this one in a chain. A pathlen of zero means the CA
cannot sign any sub-CA's, and can only sign end-entity certificates.
Key Usage
Key usage is a multi-valued extension consisting of a list of names of the permitted key usages. The
defined values are: "digitalSignature", "nonRepudiation", "keyEncipherment", "dataEncipherment",
"keyAgreement", "keyCertSign", "cRLSign", "encipherOnly", and "decipherOnly".
Examples:
keyUsage = digitalSignature, nonRepudiation
keyUsage = critical, keyCertSign
Extended Key Usage
This extension consists of a list of values indicating purposes for which the certificate public key can
be used. Each value can be either a short text name or an OID. The following text names, and their
intended meaning, are known:
Value Meaning according to RFC 5280 etc.
----- ----------------------------------
serverAuth SSL/TLS WWW Server Authentication
clientAuth SSL/TLS WWW 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
While IETF RFC 5280 says that id-kp-serverAuth and id-kp-clientAuth are only for WWW use, in practice
they are used for all kinds of TLS clients and servers, and this is what OpenSSL assumes as well.
Examples:
extendedKeyUsage = critical, codeSigning, 1.2.3.4
extendedKeyUsage = serverAuth, clientAuth
Subject Key Identifier
The SKID extension specification has a value with three choices.
none
No SKID extension will be included.
hash
The process specified in RFC 5280 section 4.2.1.2. (1) is followed: The keyIdentifier is composed of
the 160-bit SHA-1 hash of the value of the BIT STRING subjectPublicKey (excluding the tag, length,
and number of unused bits).
A hex string (possibly with ":" separating bytes)
The provided value is output directly. This choice is strongly discouraged.
By default the x509, req, and ca apps behave as if hash was given.
Example:
subjectKeyIdentifier = hash
Authority Key Identifier
The AKID extension specification may have the value none indicating that no AKID shall be included.
Otherwise it may have the value keyid or issuer or both of them, separated by ",". Either or both can
have the option always, indicated by putting a colon ":" between the value and this option. For self-
signed certificates the AKID is suppressed unless always is present.
By default the x509, req, and ca apps behave as if none was given for self-signed certificates and
keyid"," issuer otherwise.
If keyid is present, an attempt is made to copy the subject key identifier (SKID) from the issuer
certificate except if the issuer certificate is the same as the current one and it is not self-signed.
The hash of the public key related to the signing key is taken as fallback if the issuer certificate is
the same as the current certificate. If always is present but no value can be obtained, an error is
returned.
If issuer is present, and in addition it has the option always specified or keyid is not present, then
the issuer DN and serial number are copied from the issuer certificate. If this fails, an error is
returned.
Examples:
authorityKeyIdentifier = keyid, issuer
authorityKeyIdentifier = keyid, issuer:always
Subject Alternative Name
This is a multi-valued extension that supports several types of name identifier, including 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 syntax of each is
described in the following paragraphs.
The email option has two special values. "copy" will automatically include any email addresses contained
in the certificate subject name in the extension. "move" will automatically move any email addresses
from the certificate subject name to the extension.
The IP address used in the IP option can be in either IPv4 or IPv6 format.
The value of dirName is specifies the configuration section containing the distinguished name to use, as
a set of name-value pairs. Multi-valued AVAs can be formed by prefacing the name with a + character.
The value of otherName can include arbitrary data associated with an OID; the value should be the OID
followed by a semicolon and the content in specified using the syntax in ASN1_generate_nconf(3).
Examples:
subjectAltName = email:copy, email:my@example.com, URI:http://my.example.com/
subjectAltName = IP:192.168.7.1
subjectAltName = IP:13::17
subjectAltName = email:my@example.com, RID:1.2.3.4
subjectAltName = otherName:1.2.3.4;UTF8:some other identifier
[extensions]
subjectAltName = dirName:dir_sect
[dir_sect]
C = UK
O = My Organization
OU = My Unit
CN = My Name
Non-ASCII Email Address conforming the syntax defined in Section 3.3 of RFC 6531 are provided as
otherName.SmtpUTF8Mailbox. According to RFC 8398, the email address should be provided as UTF8String. To
enforce the valid representation in the certificate, the SmtpUTF8Mailbox should be provided as follows
subjectAltName=@alts
[alts]
otherName = 1.3.6.1.5.5.7.8.9;FORMAT:UTF8,UTF8String:nonasciiname.example.com
Issuer Alternative Name
This extension supports most of the options of subject alternative name; it does not support email:copy.
It also adds issuer:copy as an allowed value, which copies any subject alternative names from the issuer
certificate, if possible.
Example:
issuerAltName = issuer:copy
Authority Info Access
This extension gives details about how to retrieve information that related to the certificate that the
CA makes available. The syntax is access_id;location, where access_id is an object identifier (although
only a few values are well-known) and location has the same syntax as subject alternative name (except
that email:copy is not supported).
Possible values for access_id include OCSP (OCSP responder), caIssuers (CA Issuers), ad_timestamping (AD
Time Stamping), AD_DVCS (ad dvcs), caRepository (CA Repository).
Examples:
authorityInfoAccess = OCSP;URI:http://ocsp.example.com/,caIssuers;URI:http://myca.example.com/ca.cer
authorityInfoAccess = OCSP;URI:http://ocsp.example.com/
CRL distribution points
This is a multi-valued extension whose values can be either a name-value pair using the same form as
subject alternative name or a single value specifying the section name containing all the distribution
point values.
When a name-value pair is used, a DistributionPoint extension will be set with the given value as the
fullName field as the distributionPoint value, and the reasons and cRLIssuer fields will be omitted.
When a single option is used, the value specifies the section, and that section can have the following
items:
fullname
The full name of the distribution point, in the same format as the subject alternative name.
relativename
The value is taken as a distinguished name fragment that is set as the value of the
nameRelativeToCRLIssuer field.
CRLIssuer
The value must in the same format as the subject alternative name.
reasons
A multi-value field that contains the reasons for revocation. The recognized values are:
"keyCompromise", "CACompromise", "affiliationChanged", "superseded", "cessationOfOperation",
"certificateHold", "privilegeWithdrawn", and "AACompromise".
Only one of fullname or relativename should be specified.
Simple examples:
crlDistributionPoints = URI:http://example.com/myca.crl
crlDistributionPoints = URI:http://example.com/myca.crl, URI:http://example.org/my.crl
Full distribution point example:
[extensions]
crlDistributionPoints = crldp1_section
[crldp1_section]
fullname = URI:http://example.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. The following names have meaning:
fullname
The full name of the distribution point, in the same format as the subject alternative name.
relativename
The value is taken as a distinguished name fragment that is set as the value of the
nameRelativeToCRLIssuer field.
onlysomereasons
A multi-value field that contains the reasons for revocation. The recognized values are:
"keyCompromise", "CACompromise", "affiliationChanged", "superseded", "cessationOfOperation",
"certificateHold", "privilegeWithdrawn", and "AACompromise".
onlyuser, onlyCA, onlyAA, indirectCRL
The value for each of these names is a boolean.
Example:
[extensions]
issuingDistributionPoint = critical, @idp_section
[idp_section]
fullname = URI:http://example.com/myca.crl
indirectCRL = TRUE
onlysomereasons = keyCompromise, CACompromise
Certificate Policies
This is a raw extension that supports all of the defined fields of the certificate extension.
Policies without qualifiers are specified by giving the OID. Multiple policies are comma-separated. For
example:
certificatePolicies = 1.2.4.5, 1.1.3.4
To include policy qualifiers, use the "@section" syntax to point to a section that specifies all the
information.
The section referred to must include the policy OID using the name policyIdentifier. cPSuri qualifiers
can be included using the syntax:
CPS.nnn = value
where "nnn" is a number.
userNotice qualifiers can be set using the syntax:
userNotice.nnn = @notice
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. Some software might require the ia5org option at the top level; this
changes the encoding from Displaytext to IA5String.
Example:
[extensions]
certificatePolicies = ia5org, 1.2.3.4, 1.5.6.7.8, @polsect
[polsect]
policyIdentifier = 1.3.5.8
CPS.1 = "http://my.host.example.com/"
CPS.2 = "http://my.your.example.com/"
userNotice.1 = @notice
[notice]
explicitText = "Explicit Text Here"
organization = "Organisation Name"
noticeNumbers = 1, 2, 3, 4
The character encoding of explicitText can be specified by prefixing the value with UTF8, BMP, or VISIBLE
followed by colon. For example:
[notice]
explicitText = "UTF8:Explicit Text Here"
Policy Constraints
This is a multi-valued extension which consisting of the names requireExplicitPolicy or
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
Name Constraints
This 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 /.
Examples:
nameConstraints = permitted;IP:192.168.0.0/255.255.0.0
nameConstraints = permitted;email:.example.com
nameConstraints = excluded;email:.com
OCSP No Check
This is a string extension. It is parsed, but 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 (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: status_request and status_request_v2.
Example:
tlsfeature = status_request
DEPRECATED EXTENSIONS
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. Other extensions of this type are: nsBaseUrl, nsRevocationUrl,
nsCaRevocationUrl, nsRenewalUrl, nsCaPolicyUrl and nsSslServerName.
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.
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
ASN1_generate_nconf(3). For example:
[extensions]
1.2.3.4 = critical, ASN1:UTF8String:Some random data
1.2.3.4.1 = ASN1:SEQUENCE:seq_sect
[seq_sect]
field1 = UTF8:field1
field2 = UTF8:field2
It is also possible to use the word DER to include the raw encoded data in any extension.
1.2.3.4 = critical, DER:01:02:03:04
1.2.3.4.1 = DER:01020304
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:
basicConstraints = critical, DER:00:01:02:03
WARNINGS
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 invalid extensions if they
are not used carefully.
SEE ALSO
openssl-req(1), openssl-ca(1), openssl-x509(1), ASN1_generate_nconf(3)
COPYRIGHT
Copyright 2004-2023 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance
with the License. You can obtain a copy in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
3.4.1 2025-04-03 X509V3_CONFIG(5SSL)