- NAME
- DESCRIPTION
- STANDARD EXTENSIONS
  - Basic Constraints
  - Key Usage
  - Extended Key Usage
  - Subject Key Identifier
  - Authority Key Identifier
  - Subject Alternative Name
  - Issuer Alternative Name
  - Authority Info Access
  - CRL distribution points
  - Issuing Distribution Point
  - Certificate Policies
  - Policy Constraints
  - Inhibit Any Policy
  - Name Constraints
  - OCSP No Check
  - TLS Feature (aka Must Staple)
- DEPRECATED EXTENSIONS
  - Netscape String extensions
  - Netscape Certificate Type
- ARBITRARY EXTENSIONS
- WARNINGS
- SEE ALSO
- HISTORY
- COPYRIGHT

# 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` (with an alternative name `contentCommitment`), `keyEncipherment`, `dataEncipherment`, `keyAgreement`, `keyCertSign`, `cRLSign`, `encipherOnly`, and `decipherOnly`.

Examples:

    keyUsage = digitalSignature, nonRepudiation

    keyUsage = digitalSignature, contentCommitment

    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 Subject Key Identifier (SKID) extension takes three possible forms:

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 specified value is used directly. This choice is strongly discouraged.

See openssl-x509(1), openssl-req(1), and openssl-ca(1) documentation for further details relevant to each of the command-line utilities.

Example:

    subjectKeyIdentifier = hash

## Authority Key Identifier

The Authority Key Identifier (SKID) extension takes two different forms.

The value none indicates that no AKID shall be included.

Otherwise, the value syntax is that of a comma-separated list of either or both of the keyid or issuer keywords.

The keyid keyword asks that the AKID include the issuer's subject key identifier.

The issuer keyword asks that the AKID include the serial number and issuer distinguished name (grandparent name of subject certificate) of the issuer certificate. Unless qualified with `always` (as described below), these are added only as a fallback if the keyid is not requested, or no SKID was available in the issuer certificate.

Either or both keywords can be suffixed with an optional qualifier, which can be either `always` or `nonss`. If the qualifier is present, it is separated from the keyword by a colon (`:`).

The `always` qualifier makes that AKID element mandatory, an error is raised if that element cannot be included. The issuer certificate's issuer name and serial number are no longer a fallback when the always qualifier is used. The `nonss` qualifier makes that AKID element further conditional on the certificate not being self-signed.

When creating a self-signed certificate, be sure to specify a SKID extension if you want to have a mandatory (`always` qualified) keyid in the AKID extension.

See openssl-x509(1), openssl-req(1), and openssl-ca(1) for further details relevant to that specific command-line utility.

Examples:

    # Keyid preferred, otherwise issuer name & serial
    authorityKeyIdentifier = keyid, issuer

    # As above, but skip the extension entirely if self-signed
    authorityKeyIdentifier = keyid:nonss, issuer:nonss

    # Keyid when possible, issuer name & serial always
    authorityKeyIdentifier = keyid, issuer:always

    # Keyid when possible, issuer as fallback when not self-signed
    authorityKeyIdentifier = keyid, issuer:nonss

## 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)

# HISTORY

OpenSSL 4.0 updated the syntax of the subjectKeyIdentifier (SKID) and authorityKeyIdentifier (AKID) extensions, described above, to introduce the `nonss` qualifier for the keyid and issuer keywords. The command-line utilities no longer have specialised built-in logic to add these extensions, they are handled through configuration files and command-line options just like any other extension.

# COPYRIGHT

Copyright 2004-2026 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.