Provided by: openssl_3.0.5-2ubuntu1_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 documened. 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. If the value is the word
none then no SKID extension will be included. If the value is the word hash, or by
default for the x509, req, and ca apps, 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).
Otherwise, the value must be a hex string (possibly with ":" separating bytes) to output
directly, however, this is strongly discouraged.
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.
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 2004-2021 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>.