NAME
x509v3.cnf
—
X.509 V3 certificate extension
configuration format
DESCRIPTION
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
,]
extension_optionsIf 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:
basicConstraints=critical,CA:true,pathlen:1
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.
STANDARD EXTENSIONS
The following sections describe each supported extension in detail.
Basic constraints
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 a 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 CA
to
FALSE
or exclude the extension entirely. Some
software may require the inclusion of
basicConstraints
with CA
set
to FALSE
for end entity certificates.
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
Key usage is a multi-valued extension consisting of a list of names of the permitted key usages.
The supported names 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 purposes for which the certificate public key can be used.
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:
value | meaning |
serverAuth |
TLS server authentication |
clientAuth |
TLS 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 |
Examples:
extendedKeyUsage=critical,codeSigning,1.2.3.4 extendedKeyUsage=serverAuth,clientAuth
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
discouraged. Example:
subjectKeyIdentifier=hash
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.
Example:
authorityKeyIdentifier=keyid,issuer
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 can include a special
copy
value. This will automatically include any
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 prefacing 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(3) format. Examples:
subjectAltName=email:copy,email:my@other.address,URI:http://my.url.here/ subjectAltName=IP:192.168.7.1 subjectAltName=IP:13::17 subjectAltName=email:my@other.address,RID:1.2.3.4 subjectAltName=otherName:1.2.3.4;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
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). Example:
issuerAltName =
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
. 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.
The name 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
reasons are: keyCompromise
,
CACompromise
,
affiliationChanged
,
superseded
,
cessationOfOperation
,
certificateHold
,
privilegeWithdrawn
, and
AACompromise
.
Simple examples:
crlDistributionPoints=URI:http://myhost.com/myca.crl crlDistributionPoints=URI:http://my.com/my.crl,URI:http://oth.com/my.crl
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.
The names reasons
and
CRLissuer
are not recognized.
The name onlysomereasons
is accepted,
which sets this field. The value is in the same format as the CRL
distribution point reasons
field.
The names onlyuser
,
onlyCA
, onlyAA
, and
indirectCRL
are also accepted. The values should be
a boolean values (TRUE
or
FALSE
) to indicate the value of the corresponding
field. Example:
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
Certificate policies
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= 1.2.4.5,
1.1.3.4
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:
CPS
.nnn=valueuserNotice
qualifiers can be set using the
syntax:
userNotice
.nnn=@noticeThe 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, and
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. Example:
certificatePolicies=ia5org,1.2.3.4,1.5.6.7.8,@polsect [polsect] policyIdentifier = 1.3.5.8 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
The 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
ia5org
.
Policy constraints
This is a multi-valued extension which consists 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
The name constraints extension is a multi-valued extension. The
name should begin with the word permitted
or
excluded
, followed by a semicolon. 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 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: 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. Example:
nsComment = Some Random
Comment
Other supported extensions in this category 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:
1.2.3.4=critical,ASN1:UTF8String:Some random data 1.2.3.4=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=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
FILES
- /etc/ssl/x509v3.cnf
- standard configuration file
SEE ALSO
openssl(1), ASN1_generate_nconf(3), OPENSSL_config(3), openssl.cnf(5)
HISTORY
X509v3 extension code was first added to OpenSSL 0.9.2.
CAVEATS
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.
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,
subjectAltName=URI:ldap://somehost.com/CN=foo,OU=bar
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