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RFC 5280 - Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
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RFC5280 - Internet X.509 Public Key Infrastructure Certificate
Network Working Group D. Cooper
Request for Comments: 5280 NIST
Obsoletes: 3280, 4325, 4630 S. Santesson
Category: Standards Track Microsoft
S. Farrell
Trinity College Dublin
S. Boeyen
Entrust
R. Housley
Vigil Security
W. Polk
NIST
May 2008
Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo profiles the X.509 v3 certificate and X.509 v2 certificate
revocation list (CRL) for use in the Internet. An overview of this
approach and model is provided as an introduction. The X.509 v3
certificate format is described in detail, with additional
information regarding the format and semantics of Internet name
forms. Standard certificate extensions are described and two
Internet-specific extensions are defined. A set of required
certificate extensions is specified. The X.509 v2 CRL format is
described in detail along with standard and Internet-specific
extensions. An algorithm for X.509 certification path validation is
described. An ASN.1 module and examples are provided in the
appendices.
Table of Contents
1. Introduction ....................................................4
2. Requirements and Assumptions ....................................6
2.1. Communication and Topology .................................7
2.2. Acceptability Criteria .....................................7
2.3. User Expectations ..........................................7
2.4. Administrator Expectations .................................8
3. Overview of Approach ............................................8
3.1. X.509 Version 3 Certificate ................................9
3.2. Certification Paths and Trust .............................10
3.3. Revocation ................................................13
3.4. Operational Protocols .....................................14
3.5. Management Protocols ......................................14
4. Certificate and Certificate Extensions Profile .................16
4.1. Basic Certificate Fields ..................................16
4.1.1. Certificate Fields .................................17
4.1.1.1. tbsCertificate ............................18
4.1.1.2. signatureAlgorithm ........................18
4.1.1.3. signatureValue ............................18
4.1.2. TBSCertificate .....................................18
4.1.2.1. Version ...................................19
4.1.2.2. Serial Number .............................19
4.1.2.3. Signature .................................19
4.1.2.4. Issuer ....................................20
4.1.2.5. Validity ..................................22
4.1.2.5.1. UTCTime ........................23
4.1.2.5.2. GeneralizedTime ................23
4.1.2.6. Subject ...................................23
4.1.2.7. Subject Public Key Info ...................25
4.1.2.8. Unique Identifiers ........................25
4.1.2.9. Extensions ................................26
4.2. Certificate Extensions ....................................26
4.2.1. Standard Extensions ................................27
4.2.1.1. Authority Key Identifier ..................27
4.2.1.2. Subject Key Identifier ....................28
4.2.1.3. Key Usage .................................29
4.2.1.4. Certificate Policies ......................32
4.2.1.5. Policy Mappings ...........................35
4.2.1.6. Subject Alternative Name ..................35
4.2.1.7. Issuer Alternative Name ...................38
4.2.1.8. Subject Directory Attributes ..............39
4.2.1.9. Basic Constraints .........................39
4.2.1.10. Name Constraints .........................40
4.2.1.11. Policy Constraints .......................43
4.2.1.12. Extended Key Usage .......................44
4.2.1.13. CRL Distribution Points ..................45
4.2.1.14. Inhibit anyPolicy ........................48
4.2.1.15. Freshest CRL (a.k.a. Delta CRL
Distribution Point) ......................48
4.2.2. Private Internet Extensions ........................49
4.2.2.1. Authority Information Access ..............49
4.2.2.2. Subject Information Access ................51
5. CRL and CRL Extensions Profile .................................54
5.1. CRL Fields ................................................55
5.1.1. CertificateList Fields .............................56
5.1.1.1. tbsCertList ...............................56
5.1.1.2. signatureAlgorithm ........................57
5.1.1.3. signatureValue ............................57
5.1.2. Certificate List "To Be Signed" ....................58
5.1.2.1. Version ...................................58
5.1.2.2. Signature .................................58
5.1.2.3. Issuer Name ...............................58
5.1.2.4. This Update ...............................58
5.1.2.5. Next Update ...............................59
5.1.2.6. Revoked Certificates ......................59
5.1.2.7. Extensions ................................60
5.2. CRL Extensions ............................................60
5.2.1. Authority Key Identifier ...........................60
5.2.2. Issuer Alternative Name ............................60
5.2.3. CRL Number .........................................61
5.2.4. Delta CRL Indicator ................................62
5.2.5. Issuing Distribution Point .........................65
5.2.6. Freshest CRL (a.k.a. Delta CRL Distribution
Point) .............................................67
5.2.7. Authority Information Access .......................67
5.3. CRL Entry Extensions ......................................69
5.3.1. Reason Code ........................................69
5.3.2. Invalidity Date ....................................70
5.3.3. Certificate Issuer .................................70
6. Certification Path Validation ..................................71
6.1. Basic Path Validation .....................................72
6.1.1. Inputs .............................................75
6.1.2. Initialization .....................................77
6.1.3. Basic Certificate Processing .......................80
6.1.4. Preparation for Certificate i+1 ....................84
6.1.5. Wrap-Up Procedure ..................................87
6.1.6. Outputs ............................................89
6.2. Using the Path Validation Algorithm .......................89
6.3. CRL Validation ............................................90
6.3.1. Revocation Inputs ..................................91
6.3.2. Initialization and Revocation State Variables ......91
6.3.3. CRL Processing .....................................92
7. Processing Rules for Internationalized Names ...................95
7.1. Internationalized Names in Distinguished Names ............96
7.2. Internationalized Domain Names in GeneralName .............97
7.3. Internationalized Domain Names in Distinguished Names .....98
7.4. Internationalized Resource Identifiers ....................98
7.5. Internationalized Electronic Mail Addresses ..............100
8. Security Considerations .......................................100
9. IANA Considerations ...........................................105
10. Acknowledgments ..............................................105
11. References ...................................................105
11.1. Normative References ....................................105
11.2. Informative References ..................................107
Appendix A. Pseudo-ASN.1 Structures and OIDs ....................110
A.1. Explicitly Tagged Module, 1988 Syntax ....................110
A.2. Implicitly Tagged Module, 1988 Syntax ....................125
Appendix B. ASN.1 Notes ..........................................133
Appendix C. Examples .............................................136
C.1. RSA Self-Signed Certificate ..............................137
C.2. End Entity Certificate Using RSA .........................140
C.3. End Entity Certificate Using DSA .........................143
C.4. Certificate Revocation List ..............................147
1. Introduction
This specification is one part of a family of standards for the X.509
Public Key Infrastructure (PKI) for the Internet.
This specification profiles the format and semantics of certificates
and certificate revocation lists (CRLs) for the Internet PKI.
Procedures are described for processing of certification paths in the
Internet environment. Finally, ASN.1 modules are provided in the
appendices for all data structures defined or referenced.
Section 2 describes Internet PKI requirements and the assumptions
that affect the scope of this document. Section 3 presents an
architectural model and describes its relationship to previous IETF
and ISO/IEC/ITU-T standards. In particular, this document's
relationship with the IETF PEM specifications and the ISO/IEC/ITU-T
X.509 documents is described.
Section 4 profiles the X.509 version 3 certificate, and Section 5
profiles the X.509 version 2 CRL. The profiles include the
identification of ISO/IEC/ITU-T and ANSI extensions that may be
useful in the Internet PKI. The profiles are presented in the 1988
Abstract Syntax Notation One (ASN.1) rather than the 1997 ASN.1
syntax used in the most recent ISO/IEC/ITU-T standards.
Section 6 includes certification path validation procedures. These
procedures are based upon the ISO/IEC/ITU-T definition.
Implementations are REQUIRED to derive the same results but are not
required to use the specified procedures.
Procedures for identification and encoding of public key materials
and digital signatures are defined in [RFC3279], [RFC4055], and
[RFC4491]. Implementations of this specification are not required to
use any particular cryptographic algorithms. However, conforming
implementations that use the algorithms identified in [RFC3279],
[RFC4055], and [RFC4491] MUST identify and encode the public key
materials and digital signatures as described in those
specifications.
Finally, three appendices are provided to aid implementers. Appendix
A contains all ASN.1 structures defined or referenced within this
specification. As above, the material is presented in the 1988
ASN.1. Appendix B contains notes on less familiar features of the
ASN.1 notation used within this specification. Appendix C contains
examples of conforming certificates and a conforming CRL.
This specification obsoletes [RFC3280]. Differences from RFC 3280
are summarized below:
* Enhanced support for internationalized names is specified in
Section 7, with rules for encoding and comparing
Internationalized Domain Names, Internationalized Resource
Identifiers (IRIs), and distinguished names. These rules are
aligned with comparison rules established in current RFCs,
including [RFC3490], [RFC3987], and [RFC4518].
* Sections 4.1.2.4 and 4.1.2.6 incorporate the conditions for
continued use of legacy text encoding schemes that were
specified in [RFC4630]. Where in use by an established PKI,
transition to UTF8String could cause denial of service based on
name chaining failures or incorrect processing of name
constraints.
* Section 4.2.1.4 in RFC 3280, which specified the
privateKeyUsagePeriod certificate extension but deprecated its
use, was removed. Use of this ISO standard extension is neither
deprecated nor recommended for use in the Internet PKI.
* Section 4.2.1.5 recommends marking the policy mappings extension
as critical. RFC 3280 required that the policy mappings
extension be marked as non-critical.
* Section 4.2.1.11 requires marking the policy constraints
extension as critical. RFC 3280 permitted the policy
constraints extension to be marked as critical or non-critical.
* The Authority Information Access (AIA) CRL extension, as
specified in [RFC4325], was added as Section 5.2.7.
* Sections 5.2 and 5.3 clarify the rules for handling unrecognized
CRL extensions and CRL entry extensions, respectively.
* Section 5.3.2 in RFC 3280, which specified the
holdInstructionCode CRL entry extension, was removed.
* The path validation algorithm specified in Section 6 no longer
tracks the criticality of the certificate policies extensions in
a chain of certificates. In RFC 3280, this information was
returned to a relying party.
* The Security Considerations section addresses the risk of
circular dependencies arising from the use of https or similar
schemes in the CRL distribution points, authority information
access, or subject information access extensions.
* The Security Considerations section addresses risks associated
with name ambiguity.
* The Security Considerations section references RFC 4210 for
procedures to signal changes in CA operations.
The ASN.1 modules in Appendix A are unchanged from RFC 3280, except
that ub-emailaddress-length was changed from 128 to 255 in order to
align with PKCS #9 [RFC2985].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Requirements and Assumptions
The goal of this specification is to develop a profile to facilitate
the use of X.509 certificates within Internet applications for those
communities wishing to make use of X.509 technology. Such
applications may include WWW, electronic mail, user authentication,
and IPsec. In order to relieve some of the obstacles to using X.509
certificates, this document defines a profile to promote the
development of certificate management systems, development of
application tools, and interoperability determined by policy.
Some communities will need to supplement, or possibly replace, this
profile in order to meet the requirements of specialized application
domains or environments with additional authorization, assurance, or
operational requirements. However, for basic applications, common
representations of frequently used attributes are defined so that
application developers can obtain necessary information without
regard to the issuer of a particular certificate or certificate
revocation list (CRL).
A certificate user should review the certificate policy generated by
the certification authority (CA) before relying on the authentication
or non-repudiation services associated with the public key in a
particular certificate. To this end, this standard does not
prescribe legally binding rules or duties.
As supplemental authorization and attribute management tools emerge,
such as attribute certificates, it may be appropriate to limit the
authenticated attributes that are included in a certificate. These
other management tools may provide more appropriate methods of
conveying many authenticated attributes.
2.1. Communication and Topology
The users of certificates will operate in a wide range of
environments with respect to their communication topology, especially
users of secure electronic mail. This profile supports users without
high bandwidth, real-time IP connectivity, or high connection
availability. In addition, the profile allows for the presence of
firewall or other filtered communication.
This profile does not assume the deployment of an X.500 directory
system [X.500] or a Lightweight Directory Access Protocol (LDAP)
directory system [RFC4510]. The profile does not prohibit the use of
an X.500 directory or an LDAP directory; however, any means of
distributing certificates and certificate revocation lists (CRLs) may
be used.
2.2. Acceptability Criteria
The goal of the Internet Public Key Infrastructure (PKI) is to meet
the needs of deterministic, automated identification, authentication,
access control, and authorization functions. Support for these
services determines the attributes contained in the certificate as
well as the ancillary control information in the certificate such as
policy data and certification path constraints.
2.3. User Expectations
Users of the Internet PKI are people and processes who use client
software and are the subjects named in certificates. These uses
include readers and writers of electronic mail, the clients for WWW
browsers, WWW servers, and the key manager for IPsec within a router.
This profile recognizes the limitations of the platforms these users
employ and the limitations in sophistication and attentiveness of the
users themselves. This manifests itself in minimal user
configuration responsibility (e.g., trusted CA keys, rules), explicit
platform usage constraints within the certificate, certification path
constraints that shield the user from many malicious actions, and
applications that sensibly automate validation functions.
2.4. Administrator Expectations
As with user expectations, the Internet PKI profile is structured to
support the individuals who generally operate CAs. Providing
administrators with unbounded choices increases the chances that a
subtle CA administrator mistake will result in broad compromise.
Also, unbounded choices greatly complicate the software that process
and validate the certificates created by the CA.
3. Overview of Approach
Following is a simplified view of the architectural model assumed by
the Public-Key Infrastructure using X.509 (PKIX) specifications.
The components in this model are:
end entity: user of PKI certificates and/or end user system that is
the subject of a certificate;
CA: certification authority;
RA: registration authority, i.e., an optional system to which
a CA delegates certain management functions;
CRL issuer: a system that generates and signs CRLs; and
repository: a system or collection of distributed systems that stores
certificates and CRLs and serves as a means of
distributing these certificates and CRLs to end entities.
CAs are responsible for indicating the revocation status of the
certificates that they issue. Revocation status information may be
provided using the Online Certificate Status Protocol (OCSP)
[RFC2560], certificate revocation lists (CRLs), or some other
mechanism. In general, when revocation status information is
provided using CRLs, the CA is also the CRL issuer. However, a CA
may delegate the responsibility for issuing CRLs to a different
entity.
Note that an Attribute Authority (AA) might also choose to delegate
the publication of CRLs to a CRL issuer.
+---+
| C | +------------+
| e | <-------------------->| End entity |
| r | Operational +------------+
| t | transactions ^
| i | and management | Management
| f | transactions | transactions PKI
| i | | users
| c | v
| a | ======================= +--+------------+ ==============
| t | ^ ^
| e | | | PKI
| | v | management
| & | +------+ | entities
| | <---------------------| RA |<----+ |
| C | Publish certificate +------+ | |
| R | | |
| L | | |
| | v v
| R | +------------+
| e | <------------------------------| CA |
| p | Publish certificate +------------+
| o | Publish CRL ^ ^
| s | | | Management
| i | +------------+ | | transactions
| t | <--------------| CRL Issuer |<----+ |
| o | Publish CRL +------------+ v
| r | +------+
| y | | CA |
+---+ +------+
Figure 1. PKI Entities
3.1. X.509 Version 3 Certificate
Users of a public key require confidence that the associated private
key is owned by the correct remote subject (person or system) with
which an encryption or digital signature mechanism will be used.
This confidence is obtained through the use of public key
certificates, which are data structures that bind public key values
to subjects. The binding is asserted by having a trusted CA
digitally sign each certificate. The CA may base this assertion upon
technical means (a.k.a., proof of possession through a challenge-
response protocol), presentation of the private key, or on an
assertion by the subject. A certificate has a limited valid
lifetime, which is indicated in its signed contents. Because a
certificate's signature and timeliness can be independently checked
by a certificate-using client, certificates can be distributed via
untrusted communications and server systems, and can be cached in
unsecured storage in certificate-using systems.
ITU-T X.509 (formerly CCITT X.509) or ISO/IEC 9594-8, which was first
published in 1988 as part of the X.500 directory recommendations,
defines a standard certificate format [X.509]. The certificate
format in the 1988 standard is called the version 1 (v1) format.
When X.500 was revised in 1993, two more fields were added, resulting
in the version 2 (v2) format.
The Internet Privacy Enhanced Mail (PEM) RFCs, published in 1993,
include specifications for a public key infrastructure based on X.509
v1 certificates [RFC1422]. The experience gained in attempts to
deploy RFC 1422 made it clear that the v1 and v2 certificate formats
were deficient in several respects. Most importantly, more fields
were needed to carry information that PEM design and implementation
experience had proven necessary. In response to these new
requirements, the ISO/IEC, ITU-T, and ANSI X9 developed the X.509
version 3 (v3) certificate format. The v3 format extends the v2
format by adding provision for additional extension fields.
Particular extension field types may be specified in standards or may
be defined and registered by any organization or community. In June
1996, standardization of the basic v3 format was completed [X.509].
ISO/IEC, ITU-T, and ANSI X9 have also developed standard extensions
for use in the v3 extensions field [X.509][X9.55]. These extensions
can convey such data as additional subject identification
information, key attribute information, policy information, and
certification path constraints.
However, the ISO/IEC, ITU-T, and ANSI X9 standard extensions are very
broad in their applicability. In order to develop interoperable
implementations of X.509 v3 systems for Internet use, it is necessary
to specify a profile for use of the X.509 v3 extensions tailored for
the Internet. It is one goal of this document to specify a profile
for Internet WWW, electronic mail, and IPsec applications.
Environments with additional requirements may build on this profile
or may replace it.
3.2. Certification Paths and Trust
A user of a security service requiring knowledge of a public key
generally needs to obtain and validate a certificate containing the
required public key. If the public key user does not already hold an
assured copy of the public key of the CA that signed the certificate,
the CA's name, and related information (such as the validity period
or name constraints), then it might need an additional certificate to
obtain that public key. In general, a chain of multiple certificates
may be needed, comprising a certificate of the public key owner (the
end entity) signed by one CA, and zero or more additional
certificates of CAs signed by other CAs. Such chains, called
certification paths, are required because a public key user is only
initialized with a limited number of assured CA public keys.
There are different ways in which CAs might be configured in order
for public key users to be able to find certification paths. For
PEM, RFC 1422 defined a rigid hierarchical structure of CAs. There
are three types of PEM certification authority:
(a) Internet Policy Registration Authority (IPRA): This
authority, operated under the auspices of the Internet
Society, acts as the root of the PEM certification hierarchy
at level 1. It issues certificates only for the next level
of authorities, PCAs. All certification paths start with the
IPRA.
(b) Policy Certification Authorities (PCAs): PCAs are at level 2
of the hierarchy, each PCA being certified by the IPRA. A
PCA shall establish and publish a statement of its policy
with respect to certifying users or subordinate certification
authorities. Distinct PCAs aim to satisfy different user
needs. For example, one PCA (an organizational PCA) might
support the general electronic mail needs of commercial
organizations, and another PCA (a high-assurance PCA) might
have a more stringent policy designed for satisfying legally
binding digital signature requirements.
(c) Certification Authorities (CAs): CAs are at level 3 of the
hierarchy and can also be at lower levels. Those at level 3
are certified by PCAs. CAs represent, for example,
particular organizations, particular organizational units
(e.g., departments, groups, sections), or particular
geographical areas.
RFC 1422 furthermore has a name subordination rule, which requires
that a CA can only issue certificates for entities whose names are
subordinate (in the X.500 naming tree) to the name of the CA itself.
The trust associated with a PEM certification path is implied by the
PCA name. The name subordination rule ensures that CAs below the PCA
are sensibly constrained as to the set of subordinate entities they
can certify (e.g., a CA for an organization can only certify entities
in that organization's name tree). Certificate user systems are able
to mechanically check that the name subordination rule has been
followed.
RFC 1422 uses the X.509 v1 certificate format. The limitations of
X.509 v1 required imposition of several structural restrictions to
clearly associate policy information or restrict the utility of
certificates. These restrictions included:
(a) a pure top-down hierarchy, with all certification paths
starting from IPRA;
(b) a naming subordination rule restricting the names of a CA's
subjects; and
(c) use of the PCA concept, which requires knowledge of
individual PCAs to be built into certificate chain
verification logic. Knowledge of individual PCAs was
required to determine if a chain could be accepted.
With X.509 v3, most of the requirements addressed by RFC 1422 can be
addressed using certificate extensions, without a need to restrict
the CA structures used. In particular, the certificate extensions
relating to certificate policies obviate the need for PCAs and the
constraint extensions obviate the need for the name subordination
rule. As a result, this document supports a more flexible
architecture, including:
(a) Certification paths start with a public key of a CA in a
user's own domain, or with the public key of the top of a
hierarchy. Starting with the public key of a CA in a user's
own domain has certain advantages. In some environments, the
local domain is the most trusted.
(b) Name constraints may be imposed through explicit inclusion of
a name constraints extension in a certificate, but are not
required.
(c) Policy extensions and policy mappings replace the PCA
concept, which permits a greater degree of automation. The
application can determine if the certification path is
acceptable based on the contents of the certificates instead
of a priori knowledge of PCAs. This permits automation of
certification path processing.
X.509 v3 also includes an extension that identifies the subject of a
certificate as being either a CA or an end entity, reducing the
reliance on out-of-band information demanded in PEM.
This specification covers two classes of certificates: CA
certificates and end entity certificates. CA certificates may be
further divided into three classes: cross-certificates, self-issued
certificates, and self-signed certificates. Cross-certificates are
CA certificates in which the issuer and subject are different
entities. Cross-certificates describe a trust relationship between
the two CAs. Self-issued certificates are CA certificates in which
the issuer and subject are the same entity. Self-issued certificates
are generated to support changes in policy or operations. Self-
signed certificates are self-issued certificates where the digital
signature may be verified by the public key bound into the
certificate. Self-signed certificates are used to convey a public
key for use to begin certification paths. End entity certificates
are issued to subjects that are not authorized to issue certificates.
3.3. Revocation
When a certificate is issued, it is expected to be in use for its
entire validity period. However, various circumstances may cause a
certificate to become invalid prior to the expiration of the validity
period. Such circumstances include change of name, change of
association between subject and CA (e.g., an employee terminates
employment with an organization), and compromise or suspected
compromise of the corresponding private key. Under such
circumstances, the CA needs to revoke the certificate.
X.509 defines one method of certificate revocation. This method
involves each CA periodically issuing a signed data structure called
a certificate revocation list (CRL). A CRL is a time-stamped list
identifying revoked certificates that is signed by a CA or CRL issuer
and made freely available in a public repository. Each revoked
certificate is identified in a CRL by its certificate serial number.
When a certificate-using system uses a certificate (e.g., for
verifying a remote user's digital signature), that system not only
checks the certificate signature and validity but also acquires a
suitably recent CRL and checks that the certificate serial number is
not on that CRL. The meaning of "suitably recent" may vary with
local policy, but it usually means the most recently issued CRL. A
new CRL is issued on a regular periodic basis (e.g., hourly, daily,
or weekly). An entry is added to the CRL as part of the next update
following notification of revocation. An entry MUST NOT be removed
from the CRL until it appears on one regularly scheduled CRL issued
beyond the revoked certificate's validity period.
An advantage of this revocation method is that CRLs may be
distributed by exactly the same means as certificates themselves,
namely, via untrusted servers and untrusted communications.
One limitation of the CRL revocation method, using untrusted
communications and servers, is that the time granularity of
revocation is limited to the CRL issue period. For example, if a
revocation is reported now, that revocation will not be reliably
notified to certificate-using systems until all currently issued CRLs
are scheduled to be updated -- this may be up to one hour, one day,
or one week depending on the frequency that CRLs are issued.
As with the X.509 v3 certificate format, in order to facilitate
interoperable implementations from multiple vendors, the X.509 v2 CRL
format needs to be profiled for Internet use. It is one goal of this
document to specify that profile. However, this profile does not
require the issuance of CRLs. Message formats and protocols
supporting on-line revocation notification are defined in other PKIX
specifications. On-line methods of revocation notification may be
applicable in some environments as an alternative to the X.509 CRL.
On-line revocation checking may significantly reduce the latency
between a revocation report and the distribution of the information
to relying parties. Once the CA accepts a revocation report as
authentic and valid, any query to the on-line service will correctly
reflect the certificate validation impacts of the revocation.
However, these methods impose new security requirements: the
certificate validator needs to trust the on-line validation service
while the repository does not need to be trusted.
3.4. Operational Protocols
Operational protocols are required to deliver certificates and CRLs
(or status information) to certificate-using client systems.
Provisions are needed for a variety of different means of certificate
and CRL delivery, including distribution procedures based on LDAP,
HTTP, FTP, and X.500. Operational protocols supporting these
functions are defined in other PKIX specifications. These
specifications may include definitions of message formats and
procedures for supporting all of the above operational environments,
including definitions of or references to appropriate MIME content
types.
3.5. Management Protocols
Management protocols are required to support on-line interactions
between PKI user and management entities. For example, a management
protocol might be used between a CA and a client system with which a
key pair is associated, or between two CAs that cross-certify each
other. The set of functions that potentially need to be supported by
management protocols include:
(a) registration: This is the process whereby a user first makes
itself known to a CA (directly, or through an RA), prior to
that CA issuing a certificate or certificates for that user.
(b) initialization: Before a client system can operate securely,
it is necessary to install key materials that have the
appropriate relationship with keys stored elsewhere in the
infrastructure. For example, the client needs to be securely
initialized with the public key and other assured information
of the trusted CA(s), to be used in validating certificate
paths.
Furthermore, a client typically needs to be initialized with
its own key pair(s).
(c) certification: This is the process in which a CA issues a
certificate for a user's public key, and returns that
certificate to the user's client system and/or posts that
certificate in a repository.
(d) key pair recovery: As an option, user client key materials
(e.g., a user's private key used for encryption purposes) may
be backed up by a CA or a key backup system. If a user needs
to recover these backed-up key materials (e.g., as a result
of a forgotten password or a lost key chain file), an on-line
protocol exchange may be needed to support such recovery.
(e) key pair update: All key pairs need to be updated regularly,
i.e., replaced with a new key pair, and new certificates
issued.
(f) revocation request: An authorized person advises a CA of an
abnormal situation requiring certificate revocation.
(g) cross-certification: Two CAs exchange information used in
establishing a cross-certificate. A cross-certificate is a
certificate issued by one CA to another CA that contains a CA
signature key used for issuing certificates.
Note that on-line protocols are not the only way of implementing the
above functions. For all functions, there are off-line methods of
achieving the same result, and this specification does not mandate
use of on-line protocols. For example, when hardware tokens are
used, many of the functions may be achieved as part of the physical
token delivery. Furthermore, some of the above functions may be
combined into one protocol exchange. In particular, two or more of
the registration, initialization, and certification functions can be
combined into one protocol exchange.
The PKIX series of specifications defines a set of standard message
formats supporting the above functions. The protocols for conveying
these messages in different environments (e.g., email, file transfer,
and WWW) are described in those specifications.
4. Certificate and Certificate Extensions Profile
This section presents a profile for public key certificates that will
foster interoperability and a reusable PKI. This section is based
upon the X.509 v3 certificate format and the standard certificate
extensions defined in [X.509]. The ISO/IEC and ITU-T documents use
the 1997 version of ASN.1; while this document uses the 1988 ASN.1
syntax, the encoded certificate and standard extensions are
equivalent. This section also defines private extensions required to
support a PKI for the Internet community.
Certificates may be used in a wide range of applications and
environments covering a broad spectrum of interoperability goals and
a broader spectrum of operational and assurance requirements. The
goal of this document is to establish a common baseline for generic
applications requiring broad interoperability and limited special
purpose requirements. In particular, the emphasis will be on
supporting the use of X.509 v3 certificates for informal Internet
electronic mail, IPsec, and WWW applications.
4.1. Basic Certificate Fields
The X.509 v3 certificate basic syntax is as follows. For signature
calculation, the data that is to be signed is encoded using the ASN.1
distinguished encoding rules (DER) [X.690]. ASN.1 DER encoding is a
tag, length, value encoding system for each element.
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING }
TBSCertificate ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueID [1] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version MUST be v2 or v3
subjectUniqueID [2] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version MUST be v2 or v3
extensions [3] EXPLICIT Extensions OPTIONAL
-- If present, version MUST be v3
}
Version ::= INTEGER { v1(0), v2(1), v3(2) }
CertificateSerialNumber ::= INTEGER
Validity ::= SEQUENCE {
notBefore Time,
notAfter Time }
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
UniqueIdentifier ::= BIT STRING
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
-- contains the DER encoding of an ASN.1 value
-- corresponding to the extension type identified
-- by extnID
}
The following items describe the X.509 v3 certificate for use in the
Internet.
4.1.1. Certificate Fields
The Certificate is a SEQUENCE of three required fields. The fields
are described in detail in the following subsections.
4.1.1.1. tbsCertificate
The field contains the names of the subject and issuer, a public key
associated with the subject, a validity period, and other associated
information. The fields are described in detail in Section 4.1.2;
the tbsCertificate usually includes extensions, which are described
in Section 4.2.
4.1.1.2. signatureAlgorithm
The signatureAlgorithm field contains the identifier for the
cryptographic algorithm used by the CA to sign this certificate.
[RFC3279], [RFC4055], and [RFC4491] list supported signature
algorithms, but other signature algorithms MAY also be supported.
An algorithm identifier is defined by the following ASN.1 structure:
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
The algorithm identifier is used to identify a cryptographic
algorithm. The OBJECT IDENTIFIER component identifies the algorithm
(such as DSA with SHA-1). The contents of the optional parameters
field will vary according to the algorithm identified.
This field MUST contain the same algorithm identifier as the
signature field in the sequence tbsCertificate (Section 4.1.2.3).
4.1.1.3. signatureValue
The signatureValue field contains a digital signature computed upon
the ASN.1 DER encoded tbsCertificate. The ASN.1 DER encoded
tbsCertificate is used as the input to the signature function. This
signature value is encoded as a BIT STRING and included in the
signature field. The details of this process are specified for each
of the algorithms listed in [RFC3279], [RFC4055], and [RFC4491].
By generating this signature, a CA certifies the validity of the
information in the tbsCertificate field. In particular, the CA
certifies the binding between the public key material and the subject
of the certificate.
4.1.2. TBSCertificate
The sequence TBSCertificate contains information associated with the
subject of the certificate and the CA that issued it. Every
TBSCertificate contains the names of the subject and issuer, a public
key associated with the subject, a validity period, a version number,
and a serial number; some MAY contain optional unique identifier
fields. The remainder of this section describes the syntax and
semantics of these fields. A TBSCertificate usually includes
extensions. Extensions for the Internet PKI are described in Section
4.2.
4.1.2.1. Version
This field describes the version of the encoded certificate. When
extensions are used, as expected in this profile, version MUST be 3
(value is 2). If no extensions are present, but a UniqueIdentifier
is present, the version SHOULD be 2 (value is 1); however, the
version MAY be 3. If only basic fields are present, the version
SHOULD be 1 (the value is omitted from the certificate as the default
value); however, the version MAY be 2 or 3.
Implementations SHOULD be prepared to accept any version certificate.
At a minimum, conforming implementations MUST recognize version 3
certificates.
Generation of version 2 certificates is not expected by
implementations based on this profile.
4.1.2.2. Serial Number
The serial number MUST be a positive integer assigned by the CA to
each certificate. It MUST be unique for each certificate issued by a
given CA (i.e., the issuer name and serial number identify a unique
certificate). CAs MUST force the serialNumber to be a non-negative
integer.
Given the uniqueness requirements above, serial numbers can be
expected to contain long integers. Certificate users MUST be able to
handle serialNumber values up to 20 octets. Conforming CAs MUST NOT
use serialNumber values longer than 20 octets.
Note: Non-conforming CAs may issue certificates with serial numbers
that are negative or zero. Certificate users SHOULD be prepared to
gracefully handle such certificates.
4.1.2.3. Signature
This field contains the algorithm identifier for the algorithm used
by the CA to sign the certificate.
This field MUST contain the same algorithm identifier as the
signatureAlgorithm field in the sequence Certificate (Section
4.1.1.2). The contents of the optional parameters field will vary
according to the algorithm identified. [RFC3279], [RFC4055], and
[RFC4491] list supported signature algorithms, but other signature
algorithms MAY also be supported.
4.1.2.4. Issuer
The issuer field identifies the entity that has signed and issued the
certificate. The issuer field MUST contain a non-empty distinguished
name (DN). The issuer field is defined as the X.501 type Name
[X.501]. Name is defined by the following ASN.1 structures:
Name ::= CHOICE { -- only one possibility for now --
rdnSequence RDNSequence }
RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
RelativeDistinguishedName ::=
SET SIZE (1..MAX) OF AttributeTypeAndValue
AttributeTypeAndValue ::= SEQUENCE {
type AttributeType,
value AttributeValue }
AttributeType ::= OBJECT IDENTIFIER
AttributeValue ::= ANY -- DEFINED BY AttributeType
DirectoryString ::= CHOICE {
teletexString TeletexString (SIZE (1..MAX)),
printableString PrintableString (SIZE (1..MAX)),
universalString UniversalString (SIZE (1..MAX)),
utf8String UTF8String (SIZE (1..MAX)),
bmpString BMPString (SIZE (1..MAX)) }
The Name describes a hierarchical name composed of attributes, such
as country name, and corresponding values, such as US. The type of
the component AttributeValue is determined by the AttributeType; in
general it will be a DirectoryString.
The DirectoryString type is defined as a choice of PrintableString,
TeletexString, BMPString, UTF8String, and UniversalString. CAs
conforming to this profile MUST use either the PrintableString or
UTF8String encoding of DirectoryString, with two exceptions. When
CAs have previously issued certificates with issuer fields with
attributes encoded using TeletexString, BMPString, or
UniversalString, then the CA MAY continue to use these encodings of
the DirectoryString to preserve backward compatibility. Also, new
CAs that are added to a domain where existing CAs issue certificates
with issuer fields with attributes encoded using TeletexString,
BMPString, or UniversalString MAY encode attributes that they share
with the existing CAs using the same encodings as the existing CAs
use.
As noted above, distinguished names are composed of attributes. This
specification does not restrict the set of attribute types that may
appear in names. However, conforming implementations MUST be
prepared to receive certificates with issuer names containing the set
of attribute types defined below. This specification RECOMMENDS
support for additional attribute types.
Standard sets of attributes have been defined in the X.500 series of
specifications [X.520]. Implementations of this specification MUST
be prepared to receive the following standard attribute types in
issuer and subject (Section 4.1.2.6) names:
* country,
* organization,
* organizational unit,
* distinguished name qualifier,
* state or province name,
* common name (e.g., "Susan Housley"), and
* serial number.
In addition, implementations of this specification SHOULD be prepared
to receive the following standard attribute types in issuer and
subject names:
* locality,
* title,
* surname,
* given name,
* initials,
* pseudonym, and
* generation qualifier (e.g., "Jr.", "3rd", or "IV").
The syntax and associated object identifiers (OIDs) for these
attribute types are provided in the ASN.1 modules in Appendix A.
In addition, implementations of this specification MUST be prepared
to receive the domainComponent attribute, as defined in [RFC4519].
The Domain Name System (DNS) provides a hierarchical resource
labeling system. This attribute provides a convenient mechanism for
organizations that wish to use DNs that parallel their DNS names.
This is not a replacement for the dNSName component of the
alternative name extensions. Implementations are not required to
convert such names into DNS names. The syntax and associated OID for
this attribute type are provided in the ASN.1 modules in Appendix A.
Rules for encoding internationalized domain names for use with the
domainComponent attribute type are specified in Section 7.3.
Certificate users MUST be prepared to process the issuer
distinguished name and subject distinguished name (Section 4.1.2.6)
fields to perform name chaining for certification path validation
(Section 6). Name chaining is performed by matching the issuer
distinguished name in one certificate with the subject name in a CA
certificate. Rules for comparing distinguished names are specified
in Section 7.1. If the names in the issuer and subject field in a
certificate match according to the rules specified in Section 7.1,
then the certificate is self-issued.
4.1.2.5. Validity
The certificate validity period is the time interval during which the
CA warrants that it will maintain information about the status of the
certificate. The field is represented as a SEQUENCE of two dates:
the date on which the certificate validity period begins (notBefore)
and the date on which the certificate validity period ends
(notAfter). Both notBefore and notAfter may be encoded as UTCTime or
GeneralizedTime.
CAs conforming to this profile MUST always encode certificate
validity dates through the year 2049 as UTCTime; certificate validity
dates in 2050 or later MUST be encoded as GeneralizedTime.
Conforming applications MUST be able to process validity dates that
are encoded in either UTCTime or GeneralizedTime.
The validity period for a certificate is the period of time from
notBefore through notAfter, inclusive.
In some situations, devices are given certificates for which no good
expiration date can be assigned. For example, a device could be
issued a certificate that binds its model and serial number to its
public key; such a certificate is intended to be used for the entire
lifetime of the device.
To indicate that a certificate has no well-defined expiration date,
the notAfter SHOULD be assigned the GeneralizedTime value of
99991231235959Z.
When the issuer will not be able to maintain status information until
the notAfter date (including when the notAfter date is
99991231235959Z), the issuer MUST ensure that no valid certification
path exists for the certificate after maintenance of status
information is terminated. This may be accomplished by expiration or
revocation of all CA certificates containing the public key used to
verify the signature on the certificate and discontinuing use of the
public key used to verify the signature on the certificate as a trust
anchor.
4.1.2.5.1. UTCTime
The universal time type, UTCTime, is a standard ASN.1 type intended
for representation of dates and time. UTCTime specifies the year
through the two low-order digits and time is specified to the
precision of one minute or one second. UTCTime includes either Z
(for Zulu, or Greenwich Mean Time) or a time differential.
For the purposes of this profile, UTCTime values MUST be expressed in
Greenwich Mean Time (Zulu) and MUST include seconds (i.e., times are
YYMMDDHHMMSSZ), even where the number of seconds is zero. Conforming
systems MUST interpret the year field (YY) as follows:
Where YY is greater than or equal to 50, the year SHALL be
interpreted as 19YY; and
Where YY is less than 50, the year SHALL be interpreted as 20YY.
4.1.2.5.2. GeneralizedTime
The generalized time type, GeneralizedTime, is a standard ASN.1 type
for variable precision representation of time. Optionally, the
GeneralizedTime field can include a representation of the time
differential between local and Greenwich Mean Time.
For the purposes of this profile, GeneralizedTime values MUST be
expressed in Greenwich Mean Time (Zulu) and MUST include seconds
(i.e., times are YYYYMMDDHHMMSSZ), even where the number of seconds
is zero. GeneralizedTime values MUST NOT include fractional seconds.
4.1.2.6. Subject
The subject field identifies the entity associated with the public
key stored in the subject public key field. The subject name MAY be
carried in the subject field and/or the subjectAltName extension. If
the subject is a CA (e.g., the basic constraints extension, as
discussed in Section 4.2.1.9, is present and the value of cA is
TRUE), then the subject field MUST be populated with a non-empty
distinguished name matching the contents of the issuer field (Section
4.1.2.4) in all certificates issued by the subject CA. If the
subject is a CRL issuer (e.g., the key usage extension, as discussed
in Section 4.2.1.3, is present and the value of cRLSign is TRUE),
then the subject field MUST be populated with a non-empty
distinguished name matching the contents of the issuer field (Section
5.1.2.3) in all CRLs issued by the subject CRL issuer. If subject
naming information is present only in the subjectAltName extension
(e.g., a key bound only to an email address or URI), then the subject
name MUST be an empty sequence and the subjectAltName extension MUST
be critical.
Where it is non-empty, the subject field MUST contain an X.500
distinguished name (DN). The DN MUST be unique for each subject
entity certified by the one CA as defined by the issuer field. A CA
MAY issue more than one certificate with the same DN to the same
subject entity.
The subject field is defined as the X.501 type Name. Implementation
requirements for this field are those defined for the issuer field
(Section 4.1.2.4). Implementations of this specification MUST be
prepared to receive subject names containing the attribute types
required for the issuer field. Implementations of this specification
SHOULD be prepared to receive subject names containing the
recommended attribute types for the issuer field. The syntax and
associated object identifiers (OIDs) for these attribute types are
provided in the ASN.1 modules in Appendix A. Implementations of this
specification MAY use the comparison rules in Section 7.1 to process
unfamiliar attribute types (i.e., for name chaining) whose attribute
values use one of the encoding options from DirectoryString. Binary
comparison should be used when unfamiliar attribute types include
attribute values with encoding options other than those found in
DirectoryString. This allows implementations to process certificates
with unfamiliar attributes in the subject name.
When encoding attribute values of type DirectoryString, conforming
CAs MUST use PrintableString or UTF8String encoding, with the
following exceptions:
(a) When the subject of the certificate is a CA, the subject
field MUST be encoded in the same way as it is encoded in the
issuer field (Section 4.1.2.4) in all certificates issued by
the subject CA. Thus, if the subject CA encodes attributes
in the issuer fields of certificates that it issues using the
TeletexString, BMPString, or UniversalString encodings, then
the subject field of certificates issued to that CA MUST use
the same encoding.
(b) When the subject of the certificate is a CRL issuer, the
subject field MUST be encoded in the same way as it is
encoded in the issuer field (Section 5.1.2.3) in all CRLs
issued by the subject CRL issuer.
(c) TeletexString, BMPString, and UniversalString are included
for backward compatibility, and SHOULD NOT be used for
certificates for new subjects. However, these types MAY be
used in certificates where the name was previously
established, including cases in which a new certificate is
being issued to an existing subject or a certificate is being
issued to a new subject where the attributes being encoded
have been previously established in certificates issued to
other subjects. Certificate users SHOULD be prepared to
receive certificates with these types.
Legacy implementations exist where an electronic mail address is
embedded in the subject distinguished name as an emailAddress
attribute [RFC2985]. The attribute value for emailAddress is of type
IA5String to permit inclusion of the character '@', which is not part
of the PrintableString character set. emailAddress attribute values
are not case-sensitive (e.g., "subscriber@example.com" is the same as
"SUBSCRIBER@EXAMPLE.COM").
Conforming implementations generating new certificates with
electronic mail addresses MUST use the rfc822Name in the subject
alternative name extension (Section 4.2.1.6) to describe such
identities. Simultaneous inclusion of the emailAddress attribute in
the subject distinguished name to support legacy implementations is
deprecated but permitted.
4.1.2.7. Subject Public Key Info
This field is used to carry the public key and identify the algorithm
with which the key is used (e.g., RSA, DSA, or Diffie-Hellman). The
algorithm is identified using the AlgorithmIdentifier structure
specified in Section 4.1.1.2. The object identifiers for the
supported algorithms and the methods for encoding the public key
materials (public key and parameters) are specified in [RFC3279],
[RFC4055], and [RFC4491].
4.1.2.8. Unique Identifiers
These fields MUST only appear if the version is 2 or 3 (Section
4.1.2.1). These fields MUST NOT appear if the version is 1. The
subject and issuer unique identifiers are present in the certificate
to handle the possibility of reuse of subject and/or issuer names
over time. This profile RECOMMENDS that names not be reused for
different entities and that Internet certificates not make use of
unique identifiers. CAs conforming to this profile MUST NOT generate
certificates with unique identifiers. Applications conforming to
this profile SHOULD be capable of parsing certificates that include
unique identifiers, but there are no processing requirements
associated with the unique identifiers.
4.1.2.9. Extensions
This field MUST only appear if the version is 3 (Section 4.1.2.1).
If present, this field is a SEQUENCE of one or more certificate
extensions. The format and content of certificate extensions in the
Internet PKI are defined in Section 4.2.
4.2. Certificate Extensions
The extensions defined for X.509 v3 certificates provide methods for
associating additional attributes with users or public keys and for
managing relationships between CAs. The X.509 v3 certificate format
also allows communities to define private extensions to carry
information unique to those communities. Each extension in a
certificate is designated as either critical or non-critical. A
certificate-using system MUST reject the certificate if it encounters
a critical extension it does not recognize or a critical extension
that contains information that it cannot process. A non-critical
extension MAY be ignored if it is not recognized, but MUST be
processed if it is recognized. The following sections present
recommended extensions used within Internet certificates and standard
locations for information. Communities may elect to use additional
extensions; however, caution ought to be exercised in adopting any
critical extensions in certificates that might prevent use in a
general context.
Each extension includes an OID and an ASN.1 structure. When an
extension appears in a certificate, the OID appears as the field
extnID and the corresponding ASN.1 DER encoded structure is the value
of the octet string extnValue. A certificate MUST NOT include more
than one instance of a particular extension. For example, a
certificate may contain only one authority key identifier extension
(Section 4.2.1.1). An extension includes the boolean critical, with
a default value of FALSE. The text for each extension specifies the
acceptable values for the critical field for CAs conforming to this
profile.
Conforming CAs MUST support key identifiers (Sections 4.2.1.1 and
4.2.1.2), basic constraints (Section 4.2.1.9), key usage (Section
4.2.1.3), and certificate policies (Section 4.2.1.4) extensions. If
the CA issues certificates with an empty sequence for the subject
field, the CA MUST support the subject alternative name extension
(Section 4.2.1.6). Support for the remaining extensions is OPTIONAL.
Conforming CAs MAY support extensions that are not identified within
this specification; certificate issuers are cautioned that marking
such extensions as critical may inhibit interoperability.
At a minimum, applications conforming to this profile MUST recognize
the following extensions: key usage (Section 4.2.1.3), certificate
policies (Section 4.2.1.4), subject alternative name (Section
4.2.1.6), basic constraints (Section 4.2.1.9), name constraints
(Section 4.2.1.10), policy constraints (Section 4.2.1.11), extended
key usage (Section 4.2.1.12), and inhibit anyPolicy (Section
4.2.1.14).
In addition, applications conforming to this profile SHOULD recognize
the authority and subject key identifier (Sections 4.2.1.1 and
4.2.1.2) and policy mappings (Section 4.2.1.5) extensions.
4.2.1. Standard Extensions
This section identifies standard certificate extensions defined in
[X.509] for use in the Internet PKI. Each extension is associated
with an OID defined in [X.509]. These OIDs are members of the id-ce
arc, which is defined by the following:
id-ce OBJECT IDENTIFIER ::= { joint-iso-ccitt(2) ds(5) 29 }
4.2.1.1. Authority Key Identifier
The authority key identifier extension provides a means of
identifying the public key corresponding to the private key used to
sign a certificate. This extension is used where an issuer has
multiple signing keys (either due to multiple concurrent key pairs or
due to changeover). The identification MAY be based on either the
key identifier (the subject key identifier in the issuer's
certificate) or the issuer name and serial number.
The keyIdentifier field of the authorityKeyIdentifier extension MUST
be included in all certificates generated by conforming CAs to
facilitate certification path construction. There is one exception;
where a CA distributes its public key in the form of a "self-signed"
certificate, the authority key identifier MAY be omitted. The
signature on a self-signed certificate is generated with the private
key associated with the certificate's subject public key. (This
proves that the issuer possesses both the public and private keys.)
In this case, the subject and authority key identifiers would be
identical, but only the subject key identifier is needed for
certification path building.
The value of the keyIdentifier field SHOULD be derived from the
public key used to verify the certificate's signature or a method
that generates unique values. Two common methods for generating key
identifiers from the public key are described in Section 4.2.1.2.
Where a key identifier has not been previously established, this
specification RECOMMENDS use of one of these methods for generating
keyIdentifiers or use of a similar method that uses a different hash
algorithm. Where a key identifier has been previously established,
the CA SHOULD use the previously established identifier.
This profile RECOMMENDS support for the key identifier method by all
certificate users.
Conforming CAs MUST mark this extension as non-critical.
id-ce-authorityKeyIdentifier OBJECT IDENTIFIER ::= { id-ce 35 }
AuthorityKeyIdentifier ::= SEQUENCE {
keyIdentifier [0] KeyIdentifier OPTIONAL,
authorityCertIssuer [1] GeneralNames OPTIONAL,
authorityCertSerialNumber [2] CertificateSerialNumber OPTIONAL }
KeyIdentifier ::= OCTET STRING
4.2.1.2. Subject Key Identifier
The subject key identifier extension provides a means of identifying
certificates that contain a particular public key.
To facilitate certification path construction, this extension MUST
appear in all conforming CA certificates, that is, all certificates
including the basic constraints extension (Section 4.2.1.9) where the
value of cA is TRUE. In conforming CA certificates, the value of the
subject key identifier MUST be the value placed in the key identifier
field of the authority key identifier extension (Section 4.2.1.1) of
certificates issued by the subject of this certificate. Applications
are not required to verify that key identifiers match when performing
certification path validation.
For CA certificates, subject key identifiers SHOULD be derived from
the public key or a method that generates unique values. Two common
methods for generating key identifiers from the public key are:
(1) 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).
(2) The keyIdentifier is composed of a four-bit type field with
the value 0100 followed by the least significant 60 bits of
the SHA-1 hash of the value of the BIT STRING
subjectPublicKey (excluding the tag, length, and number of
unused bits).
Other methods of generating unique numbers are also acceptable.
For end entity certificates, the subject key identifier extension
provides a means for identifying certificates containing the
particular public key used in an application. Where an end entity
has obtained multiple certificates, especially from multiple CAs, the
subject key identifier provides a means to quickly identify the set
of certificates containing a particular public key. To assist
applications in identifying the appropriate end entity certificate,
this extension SHOULD be included in all end entity certificates.
For end entity certificates, subject key identifiers SHOULD be
derived from the public key. Two common methods for generating key
identifiers from the public key are identified above.
Where a key identifier has not been previously established, this
specification RECOMMENDS use of one of these methods for generating
keyIdentifiers or use of a similar method that uses a different hash
algorithm. Where a key identifier has been previously established,
the CA SHOULD use the previously established identifier.
Conforming CAs MUST mark this extension as non-critical.
id-ce-subjectKeyIdentifier OBJECT IDENTIFIER ::= { id-ce 14 }
SubjectKeyIdentifier ::= KeyIdentifier
4.2.1.3. Key Usage
The key usage extension defines the purpose (e.g., encipherment,
signature, certificate signing) of the key contained in the
certificate. The usage restriction might be employed when a key that
could be used for more than one operation is to be restricted. For
example, when an RSA key should be used only to verify signatures on
objects other than public key certificates and CRLs, the
digitalSignature and/or nonRepudiation bits would be asserted.
Likewise, when an RSA key should be used only for key management, the
keyEncipherment bit would be asserted.
Conforming CAs MUST include this extension in certificates that
contain public keys that are used to validate digital signatures on
other public key certificates or CRLs. When present, conforming CAs
SHOULD mark this extension as critical.
id-ce-keyUsage OBJECT IDENTIFIER ::= { id-ce 15 }
KeyUsage ::= BIT STRING {
digitalSignature (0),
nonRepudiation (1), -- recent editions of X.509 have
-- renamed this bit to contentCommitment
keyEncipherment (2),
dataEncipherment (3),
keyAgreement (4),
keyCertSign (5),
cRLSign (6),
encipherOnly (7),
decipherOnly (8) }
Bits in the KeyUsage type are used as follows:
The digitalSignature bit is asserted when the subject public key
is used for verifying digital signatures, other than signatures on
certificates (bit 5) and CRLs (bit 6), such as those used in an
entity authentication service, a data origin authentication
service, and/or an integrity service.
The nonRepudiation bit is asserted when the subject public key is
used to verify digital signatures, other than signatures on
certificates (bit 5) and CRLs (bit 6), used to provide a non-
repudiation service that protects against the signing entity
falsely denying some action. In the case of later conflict, a
reliable third party may determine the authenticity of the signed
data. (Note that recent editions of X.509 have renamed the
nonRepudiation bit to contentCommitment.)
The keyEncipherment bit is asserted when the subject public key is
used for enciphering private or secret keys, i.e., for key
transport. For example, this bit shall be set when an RSA public
key is to be used for encrypting a symmetric content-decryption
key or an asymmetric private key.
The dataEncipherment bit is asserted when the subject public key
is used for directly enciphering raw user data without the use of
an intermediate symmetric cipher. Note that the use of this bit
is extremely uncommon; almost all applications use key transport
or key agreement to establish a symmetric key.
The keyAgreement bit is asserted when the subject public key is
used for key agreement. For example, when a Diffie-Hellman key is
to be used for key management, then this bit is set.
The keyCertSign bit is asserted when the subject public key is
used for verifying signatures on public key certificates. If the
keyCertSign bit is asserted, then the cA bit in the basic
constraints extension (Section 4.2.1.9) MUST also be asserted.
The cRLSign bit is asserted when the subject public key is used
for verifying signatures on certificate revocation lists (e.g.,
CRLs, delta CRLs, or ARLs).
The meaning of the encipherOnly bit is undefined in the absence of
the keyAgreement bit. When the encipherOnly bit is asserted and
the keyAgreement bit is also set, the subject public key may be
used only for enciphering data while performing key agreement.
The meaning of the decipherOnly bit is undefined in the absence of
the keyAgreement bit. When the decipherOnly bit is asserted and
the keyAgreement bit is also set, the subject public key may be
used only for deciphering data while performing key agreement.
If the keyUsage extension is present, then the subject public key
MUST NOT be used to verify signatures on certificates or CRLs unless
the corresponding keyCertSign or cRLSign bit is set. If the subject
public key is only to be used for verifying signatures on
certificates and/or CRLs, then the digitalSignature and
nonRepudiation bits SHOULD NOT be set. However, the digitalSignature
and/or nonRepudiation bits MAY be set in addition to the keyCertSign
and/or cRLSign bits if the subject public key is to be used to verify
signatures on certificates and/or CRLs as well as other objects.
Combining the nonRepudiation bit in the keyUsage certificate
extension with other keyUsage bits may have security implications
depending on the context in which the certificate is to be used.
Further distinctions between the digitalSignature and nonRepudiation
bits may be provided in specific certificate policies.
This profile does not restrict the combinations of bits that may be
set in an instantiation of the keyUsage extension. However,
appropriate values for keyUsage extensions for particular algorithms
are specified in [RFC3279], [RFC4055], and [RFC4491]. When the
keyUsage extension appears in a certificate, at least one of the bits
MUST be set to 1.
4.2.1.4. Certificate Policies
The certificate policies extension contains a sequence of one or more
policy information terms, each of which consists of an object
identifier (OID) and optional qualifiers. Optional qualifiers, which
MAY be present, are not expected to change the definition of the
policy. A certificate policy OID MUST NOT appear more than once in a
certificate policies extension.
In an end entity certificate, these policy information terms indicate
the policy under which the certificate has been issued and the
purposes for which the certificate may be used. In a CA certificate,
these policy information terms limit the set of policies for
certification paths that include this certificate. When a CA does
not wish to limit the set of policies for certification paths that
include this certificate, it MAY assert the special policy anyPolicy,
with a value of { 2 5 29 32 0 }.
Applications with specific policy requirements are expected to have a
list of those policies that they will accept and to compare the
policy OIDs in the certificate to that list. If this extension is
critical, the path validation software MUST be able to interpret this
extension (including the optional qualifier), or MUST reject the
certificate.
To promote interoperability, this profile RECOMMENDS that policy
information terms consist of only an OID. Where an OID alone is
insufficient, this profile strongly recommends that the use of
qualifiers be limited to those identified in this section. When
qualifiers are used with the special policy anyPolicy, they MUST be
limited to the qualifiers identified in this section. Only those
qualifiers returned as a result of path validation are considered.
This specification defines two policy qualifier types for use by
certificate policy writers and certificate issuers. The qualifier
types are the CPS Pointer and User Notice qualifiers.
The CPS Pointer qualifier contains a pointer to a Certification
Practice Statement (CPS) published by the CA. The pointer is in the
form of a URI. Processing requirements for this qualifier are a
local matter. No action is mandated by this specification regardless
of the criticality value asserted for the extension.
User notice is intended for display to a relying party when a
certificate is used. Only user notices returned as a result of path
validation are intended for display to the user. If a notice is
duplicated, only one copy need be displayed. To prevent such
duplication, this qualifier SHOULD only be present in end entity
certificates and CA certificates issued to other organizations.
The user notice has two optional fields: the noticeRef field and the
explicitText field. Conforming CAs SHOULD NOT use the noticeRef
option.
The noticeRef field, if used, names an organization and
identifies, by number, a particular textual statement prepared by
that organization. For example, it might identify the
organization "CertsRUs" and notice number 1. In a typical
implementation, the application software will have a notice file
containing the current set of notices for CertsRUs; the
application will extract the notice text from the file and display
it. Messages MAY be multilingual, allowing the software to select
the particular language message for its own environment.
An explicitText field includes the textual statement directly in
the certificate. The explicitText field is a string with a
maximum size of 200 characters. Conforming CAs SHOULD use the
UTF8String encoding for explicitText, but MAY use IA5String.
Conforming CAs MUST NOT encode explicitText as VisibleString or
BMPString. The explicitText string SHOULD NOT include any control
characters (e.g., U+0000 to U+001F and U+007F to U+009F). When
the UTF8String encoding is used, all character sequences SHOULD be
normalized according to Unicode normalization form C (NFC) [NFC].
If both the noticeRef and explicitText options are included in the
one qualifier and if the application software can locate the notice
text indicated by the noticeRef option, then that text SHOULD be
displayed; otherwise, the explicitText string SHOULD be displayed.
Note: While the explicitText has a maximum size of 200 characters,
some non-conforming CAs exceed this limit. Therefore, certificate
users SHOULD gracefully handle explicitText with more than 200
characters.
id-ce-certificatePolicies OBJECT IDENTIFIER ::= { id-ce 32 }
anyPolicy OBJECT IDENTIFIER ::= { id-ce-certificatePolicies 0 }
certificatePolicies ::= SEQUENCE SIZE (1..MAX) OF PolicyInformation
PolicyInformation ::= SEQUENCE {
policyIdentifier CertPolicyId,
policyQualifiers SEQUENCE SIZE (1..MAX) OF
PolicyQualifierInfo OPTIONAL }
CertPolicyId ::= OBJECT IDENTIFIER
PolicyQualifierInfo ::= SEQUENCE {
policyQualifierId PolicyQualifierId,
qualifier ANY DEFINED BY policyQualifierId }
-- policyQualifierIds for Internet policy qualifiers
id-qt OBJECT IDENTIFIER ::= { id-pkix 2 }
id-qt-cps OBJECT IDENTIFIER ::= { id-qt 1 }
id-qt-unotice OBJECT IDENTIFIER ::= { id-qt 2 }
PolicyQualifierId ::= OBJECT IDENTIFIER ( id-qt-cps | id-qt-unotice )
Qualifier ::= CHOICE {
cPSuri CPSuri,
userNotice UserNotice }
CPSuri ::= IA5String
UserNotice ::= SEQUENCE {
noticeRef NoticeReference OPTIONAL,
explicitText DisplayText OPTIONAL }
NoticeReference ::= SEQUENCE {
organization DisplayText,
noticeNumbers SEQUENCE OF INTEGER }
DisplayText ::= CHOICE {
ia5String IA5String (SIZE (1..200)),
visibleString VisibleString (SIZE (1..200)),
bmpString BMPString (SIZE (1..200)),
utf8String UTF8String (SIZE (1..200)) }
4.2.1.5. Policy Mappings
This extension is used in CA certificates. It lists one or more
pairs of OIDs; each pair includes an issuerDomainPolicy and a
subjectDomainPolicy. The pairing indicates the issuing CA considers
its issuerDomainPolicy equivalent to the subject CA's
subjectDomainPolicy.
The issuing CA's users might accept an issuerDomainPolicy for certain
applications. The policy mapping defines the list of policies
associated with the subject CA that may be accepted as comparable to
the issuerDomainPolicy.
Each issuerDomainPolicy named in the policy mappings extension SHOULD
also be asserted in a certificate policies extension in the same
certificate. Policies MUST NOT be mapped either to or from the
special value anyPolicy (Section 4.2.1.4).
In general, certificate policies that appear in the
issuerDomainPolicy field of the policy mappings extension are not
considered acceptable policies for inclusion in subsequent
certificates in the certification path. In some circumstances, a CA
may wish to map from one policy (p1) to another (p2), but still wants
the issuerDomainPolicy (p1) to be considered acceptable for inclusion
in subsequent certificates. This may occur, for example, if the CA
is in the process of transitioning from the use of policy p1 to the
use of policy p2 and has valid certificates that were issued under
each of the policies. A CA may indicate this by including two policy
mappings in the CA certificates that it issues. Each policy mapping
would have an issuerDomainPolicy of p1; one policy mapping would have
a subjectDomainPolicy of p1 and the other would have a
subjectDomainPolicy of p2.
This extension MAY be supported by CAs and/or applications.
Conforming CAs SHOULD mark this extension as critical.
id-ce-policyMappings OBJECT IDENTIFIER ::= { id-ce 33 }
PolicyMappings ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
issuerDomainPolicy CertPolicyId,
subjectDomainPolicy CertPolicyId }
4.2.1.6. Subject Alternative Name
The subject alternative name extension allows identities to be bound
to the subject of the certificate. These identities may be included
in addition to or in place of the identity in the subject field of
the certificate. Defined options include an Internet electronic mail
address, a DNS name, an IP address, and a Uniform Resource Identifier
(URI). Other options exist, including completely local definitions.
Multiple name forms, and multiple instances of each name form, MAY be
included. Whenever such identities are to be bound into a
certificate, the subject alternative name (or issuer alternative
name) extension MUST be used; however, a DNS name MAY also be
represented in the subject field using the domainComponent attribute
as described in Section 4.1.2.4. Note that where such names are
represented in the subject field implementations are not required to
convert them into DNS names.
Because the subject alternative name is considered to be definitively
bound to the public key, all parts of the subject alternative name
MUST be verified by the CA.
Further, if the only subject identity included in the certificate is
an alternative name form (e.g., an electronic mail address), then the
subject distinguished name MUST be empty (an empty sequence), and the
subjectAltName extension MUST be present. If the subject field
contains an empty sequence, then the issuing CA MUST include a
subjectAltName extension that is marked as critical. When including
the subjectAltName extension in a certificate that has a non-empty
subject distinguished name, conforming CAs SHOULD mark the
subjectAltName extension as non-critical.
When the subjectAltName extension contains an Internet mail address,
the address MUST be stored in the rfc822Name. The format of an
rfc822Name is a "Mailbox" as defined in Section 4.1.2 of [RFC2821].
A Mailbox has the form "Local-part@Domain". Note that a Mailbox has
no phrase (such as a common name) before it, has no comment (text
surrounded in parentheses) after it, and is not surrounded by "<" and
">". Rules for encoding Internet mail addresses that include
internationalized domain names are specified in Section 7.5.
When the subjectAltName extension contains an iPAddress, the address
MUST be stored in the octet string in "network byte order", as
specified in [RFC791]. The least significant bit (LSB) of each octet
is the LSB of the corresponding byte in the network address. For IP
version 4, as specified in [RFC791], the octet string MUST contain
exactly four octets. For IP version 6, as specified in
[RFC2460], the octet string MUST contain exactly sixteen octets.
When the subjectAltName extension contains a domain name system
label, the domain name MUST be stored in the dNSName (an IA5String).
The name MUST be in the "preferred name syntax", as specified by
Section 3.5 of [RFC1034] and as modified by Section 2.1 of
[RFC1123]. Note that while uppercase and lowercase letters are
allowed in domain names, no significance is attached to the case. In
addition, while the string " " is a legal domain name, subjectAltName
extensions with a dNSName of " " MUST NOT be used. Finally, the use
of the DNS representation for Internet mail addresses
(subscriber.example.com instead of subscriber@example.com) MUST NOT
be used; such identities are to be encoded as rfc822Name. Rules for
encoding internationalized domain names are specified in Section 7.2.
When the subjectAltName extension contains a URI, the name MUST be
stored in the uniformResourceIdentifier (an IA5String). The name
MUST NOT be a relative URI, and it MUST follow the URI syntax and
encoding rules specified in [RFC3986]. The name MUST include both a
scheme (e.g., "http" or "ftp") and a scheme-specific-part. URIs that
include an authority ([RFC3986], Section 3.2) MUST include a fully
qualified domain name or IP address as the host. Rules for encoding
Internationalized Resource Identifiers (IRIs) are specified in
Section 7.4.
As specified in [RFC3986], the scheme name is not case-sensitive
(e.g., "http" is equivalent to "HTTP"). The host part, if present,
is also not case-sensitive, but other components of the scheme-
specific-part may be case-sensitive. Rules for comparing URIs are
specified in Section 7.4.
When the subjectAltName extension contains a DN in the directoryName,
the encoding rules are the same as those specified for the issuer
field in Section 4.1.2.4. The DN MUST be unique for each subject
entity certified by the one CA as defined by the issuer field. A CA
MAY issue more than one certificate with the same DN to the same
subject entity.
The subjectAltName MAY carry additional name types through the use of
the otherName field. The format and semantics of the name are
indicated through the OBJECT IDENTIFIER in the type-id field. The
name itself is conveyed as value field in otherName. For example,
Kerberos [RFC4120] format names can be encoded into the otherName,
using a Kerberos 5 principal name OID and a SEQUENCE of the Realm and
the PrincipalName.
Subject alternative names MAY be constrained in the same manner as
subject distinguished names using the name constraints extension as
described in Section 4.2.1.10.
If the subjectAltName extension is present, the sequence MUST contain
at least one entry. Unlike the subject field, conforming CAs MUST
NOT issue certificates with subjectAltNames containing empty
GeneralName fields. For example, an rfc822Name is represented as an
IA5String. While an empty string is a valid IA5String, such an
rfc822Name is not permitted by this profile. The behavior of clients
that encounter such a certificate when processing a certification
path is not defined by this profile.
Finally, the semantics of subject alternative names that include
wildcard characters (e.g., as a placeholder for a set of names) are
not addressed by this specification. Applications with specific
requirements MAY use such names, but they must define the semantics.
id-ce-subjectAltName OBJECT IDENTIFIER ::= { id-ce 17 }
SubjectAltName ::= GeneralNames
GeneralNames ::= SEQUENCE SIZE (1..MAX) OF GeneralName
GeneralName ::= CHOICE {
otherName [0] OtherName,
rfc822Name [1] IA5String,
dNSName [2] IA5String,
x400Address [3] ORAddress,
directoryName [4] Name,
ediPartyName [5] EDIPartyName,
uniformResourceIdentifier [6] IA5String,
iPAddress [7] OCTET STRING,
registeredID [8] OBJECT IDENTIFIER }
OtherName ::= SEQUENCE {
type-id OBJECT IDENTIFIER,
value [0] EXPLICIT ANY DEFINED BY type-id }
EDIPartyName ::= SEQUENCE {
nameAssigner [0] DirectoryString OPTIONAL,
partyName [1] DirectoryString }
4.2.1.7. Issuer Alternative Name
As with Section 4.2.1.6, this extension is used to associate Internet
style identities with the certificate issuer. Issuer alternative
name MUST be encoded as in 4.2.1.6. Issuer alternative names are not
processed as part of the certification path validation algorithm in
Section 6. (That is, issuer alternative names are not used in name
chaining and name constraints are not enforced.)
Where present, conforming CAs SHOULD mark this extension as non-
critical.
id-ce-issuerAltName OBJECT IDENTIFIER ::= { id-ce 18 }
IssuerAltName ::= GeneralNames
4.2.1.8. Subject Directory Attributes
The subject directory attributes extension is used to convey
identification attributes (e.g., nationality) of the subject. The
extension is defined as a sequence of one or more attributes.
Conforming CAs MUST mark this extension as non-critical.
id-ce-subjectDirectoryAttributes OBJECT IDENTIFIER ::= { id-ce 9 }
SubjectDirectoryAttributes ::= SEQUENCE SIZE (1..MAX) OF Attribute
4.2.1.9. Basic Constraints
The basic constraints extension identifies whether the subject of the
certificate is a CA and the maximum depth of valid certification
paths that include this certificate.
The cA boolean indicates whether the certified public key may be used
to verify certificate signatures. If the cA boolean is not asserted,
then the keyCertSign bit in the key usage extension MUST NOT be
asserted. If the basic constraints extension is not present in a
version 3 certificate, or the extension is present but the cA boolean
is not asserted, then the certified public key MUST NOT be used to
verify certificate signatures.
The pathLenConstraint field is meaningful only if the cA boolean is
asserted and the key usage extension, if present, asserts the
keyCertSign bit (Section 4.2.1.3). In this case, it gives the
maximum number of non-self-issued intermediate certificates that may
follow this certificate in a valid certification path. (Note: The
last certificate in the certification path is not an intermediate
certificate, and is not included in this limit. Usually, the last
certificate is an end entity certificate, but it can be a CA
certificate.) A pathLenConstraint of zero indicates that no non-
self-issued intermediate CA certificates may follow in a valid
certification path. Where it appears, the pathLenConstraint field
MUST be greater than or equal to zero. Where pathLenConstraint does
not appear, no limit is imposed.
Conforming CAs MUST include this extension in all CA certificates
that contain public keys used to validate digital signatures on
certificates and MUST mark the extension as critical in such
certificates. This extension MAY appear as a critical or non-
critical extension in CA certificates that contain public keys used
exclusively for purposes other than validating digital signatures on
certificates. Such CA certificates include ones that contain public
keys used exclusively for validating digital signatures on CRLs and
ones that contain key management public keys used with certificate
enrollment protocols. This extension MAY appear as a critical or
non-critical extension in end entity certificates.
CAs MUST NOT include the pathLenConstraint field unless the cA
boolean is asserted and the key usage extension asserts the
keyCertSign bit.
id-ce-basicConstraints OBJECT IDENTIFIER ::= { id-ce 19 }
BasicConstraints ::= SEQUENCE {
cA BOOLEAN DEFAULT FALSE,
pathLenConstraint INTEGER (0..MAX) OPTIONAL }
4.2.1.10. Name Constraints
The name constraints extension, which MUST be used only in a CA
certificate, indicates a name space within which all subject names in
subsequent certificates in a certification path MUST be located.
Restrictions apply to the subject distinguished name and apply to
subject alternative names. Restrictions apply only when the
specified name form is present. If no name of the type is in the
certificate, the certificate is acceptable.
Name constraints are not applied to self-issued certificates (unless
the certificate is the final certificate in the path). (This could
prevent CAs that use name constraints from employing self-issued
certificates to implement key rollover.)
Restrictions are defined in terms of permitted or excluded name
subtrees. Any name matching a restriction in the excludedSubtrees
field is invalid regardless of information appearing in the
permittedSubtrees. Conforming CAs MUST mark this extension as
critical and SHOULD NOT impose name constraints on the x400Address,
ediPartyName, or registeredID name forms. Conforming CAs MUST NOT
issue certificates where name constraints is an empty sequence. That
is, either the permittedSubtrees field or the excludedSubtrees MUST
be present.
Applications conforming to this profile MUST be able to process name
constraints that are imposed on the directoryName name form and
SHOULD be able to process name constraints that are imposed on the
rfc822Name, uniformResourceIdentifier, dNSName, and iPAddress name
forms. If a name constraints extension that is marked as critical
imposes constraints on a particular name form, and an instance of
that name form appears in the subject field or subjectAltName
extension of a subsequent certificate, then the application MUST
either process the constraint or reject the certificate.
Within this profile, the minimum and maximum fields are not used with
any name forms, thus, the minimum MUST be zero, and maximum MUST be
absent. However, if an application encounters a critical name
constraints extension that specifies other values for minimum or
maximum for a name form that appears in a subsequent certificate, the
application MUST either process these fields or reject the
certificate.
For URIs, the constraint applies to the host part of the name. The
constraint MUST be specified as a fully qualified domain name and MAY
specify a host or a domain. Examples would be "host.example.com" and
".example.com". When the constraint begins with a period, it MAY be
expanded with one or more labels. That is, the constraint
".example.com" is satisfied by both host.example.com and
my.host.example.com. However, the constraint ".example.com" is not
satisfied by "example.com". When the constraint does not begin with
a period, it specifies a host. If a constraint is applied to the
uniformResourceIdentifier name form and a subsequent certificate
includes a subjectAltName extension with a uniformResourceIdentifier
that does not include an authority component with a host name
specified as a fully qualified domain name (e.g., if the URI either
does not include an authority component or includes an authority
component in which the host name is specified as an IP address), then
the application MUST reject the certificate.
A name constraint for Internet mail addresses MAY specify a
particular mailbox, all addresses at a particular host, or all
mailboxes in a domain. To indicate a particular mailbox, the
constraint is the complete mail address. For example,
"root@example.com" indicates the root mailbox on the host
"example.com". To indicate all Internet mail addresses on a
particular host, the constraint is specified as the host name. For
example, the constraint "example.com" is satisfied by any mail
address at the host "example.com". To specify any address within a
domain, the constraint is specified with a leading period (as with
URIs). For example, ".example.com" indicates all the Internet mail
addresses in the domain "example.com", but not Internet mail
addresses on the host "example.com".
DNS name restrictions are expressed as host.example.com. Any DNS
name that can be constructed by simply adding zero or more labels to
the left-hand side of the name satisfies the name constraint. For
example, www.host.example.com would satisfy the constraint but
host1.example.com would not.
Legacy implementations exist where an electronic mail address is
embedded in the subject distinguished name in an attribute of type
emailAddress (Section 4.1.2.6). When constraints are imposed on the
rfc822Name name form, but the certificate does not include a subject
alternative name, the rfc822Name constraint MUST be applied to the
attribute of type emailAddress in the subject distinguished name.
The ASN.1 syntax for emailAddress and the corresponding OID are
supplied in Appendix A.
Restrictions of the form directoryName MUST be applied to the subject
field in the certificate (when the certificate includes a non-empty
subject field) and to any names of type directoryName in the
subjectAltName extension. Restrictions of the form x400Address MUST
be applied to any names of type x400Address in the subjectAltName
extension.
When applying restrictions of the form directoryName, an
implementation MUST compare DN attributes. At a minimum,
implementations MUST perform the DN comparison rules specified in
Section 7.1. CAs issuing certificates with a restriction of the form
directoryName SHOULD NOT rely on implementation of the full ISO DN
name comparison algorithm. This implies name restrictions MUST be
stated identically to the encoding used in the subject field or
subjectAltName extension.
The syntax of iPAddress MUST be as described in Section 4.2.1.6 with
the following additions specifically for name constraints. For IPv4
addresses, the iPAddress field of GeneralName MUST contain eight (8)
octets, encoded in the style of RFC 4632 (CIDR) to represent an
address range [RFC4632]. For IPv6 addresses, the iPAddress field
MUST contain 32 octets similarly encoded. For example, a name
constraint for "class C" subnet 192.0.2.0 is represented as the
octets C0 00 02 00 FF FF FF 00, representing the CIDR notation
192.0.2.0/24 (mask 255.255.255.0).
Additional rules for encoding and processing name constraints are
specified in Section 7.
The syntax and semantics for name constraints for otherName,
ediPartyName, and registeredID are not defined by this specification,
however, syntax and semantics for name constraints for other name
forms may be specified in other documents.
id-ce-nameConstraints OBJECT IDENTIFIER ::= { id-ce 30 }
NameConstraints ::= SEQUENCE {
permittedSubtrees [0] GeneralSubtrees OPTIONAL,
excludedSubtrees [1] GeneralSubtrees OPTIONAL }
GeneralSubtrees ::= SEQUENCE SIZE (1..MAX) OF GeneralSubtree
GeneralSubtree ::= SEQUENCE {
base GeneralName,
minimum [0] BaseDistance DEFAULT 0,
maximum [1] BaseDistance OPTIONAL }
BaseDistance ::= INTEGER (0..MAX)
4.2.1.11. Policy Constraints
The policy constraints extension can be used in certificates issued
to CAs. The policy constraints extension constrains path validation
in two ways. It can be used to prohibit policy mapping or require
that each certificate in a path contain an acceptable policy
identifier.
If the inhibitPolicyMapping field is present, the value indicates the
number of additional certificates that may appear in the path before
policy mapping is no longer permitted. For example, a value of one
indicates that policy mapping may be processed in certificates issued
by the subject of this certificate, but not in additional
certificates in the path.
If the requireExplicitPolicy field is present, the value of
requireExplicitPolicy indicates the number of additional certificates
that may appear in the path before an explicit policy is required for
the entire path. When an explicit policy is required, it is
necessary for all certificates in the path to contain an acceptable
policy identifier in the certificate policies extension. An
acceptable policy identifier is the identifier of a policy required
by the user of the certification path or the identifier of a policy
that has been declared equivalent through policy mapping.
Conforming applications MUST be able to process the
requireExplicitPolicy field and SHOULD be able to process the
inhibitPolicyMapping field. Applications that support the
inhibitPolicyMapping field MUST also implement support for the
policyMappings extension. If the policyConstraints extension is
marked as critical and the inhibitPolicyMapping field is present,
applications that do not implement support for the
inhibitPolicyMapping field MUST reject the certificate.
Conforming CAs MUST NOT issue certificates where policy constraints
is an empty sequence. That is, either the inhibitPolicyMapping field
or the requireExplicitPolicy field MUST be present. The behavior of
clients that encounter an empty policy constraints field is not
addressed in this profile.
Conforming CAs MUST mark this extension as critical.
id-ce-policyConstraints OBJECT IDENTIFIER ::= { id-ce 36 }
PolicyConstraints ::= SEQUENCE {
requireExplicitPolicy [0] SkipCerts OPTIONAL,
inhibitPolicyMapping [1] SkipCerts OPTIONAL }
SkipCerts ::= INTEGER (0..MAX)
4.2.1.12. Extended Key Usage
This extension indicates one or more purposes for which the certified
public key may be used, in addition to or in place of the basic
purposes indicated in the key usage extension. In general, this
extension will appear only in end entity certificates. This
extension is defined as follows:
id-ce-extKeyUsage OBJECT IDENTIFIER ::= { id-ce 37 }
ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId
KeyPurposeId ::= OBJECT IDENTIFIER
Key purposes may be defined by any organization with a need. Object
identifiers used to identify key purposes MUST be assigned in
accordance with IANA or ITU-T Recommendation X.660 [X.660].
This extension MAY, at the option of the certificate issuer, be
either critical or non-critical.
If the extension is present, then the certificate MUST only be used
for one of the purposes indicated. If multiple purposes are
indicated the application need not recognize all purposes indicated,
as long as the intended purpose is present. Certificate using
applications MAY require that the extended key usage extension be
present and that a particular purpose be indicated in order for the
certificate to be acceptable to that application.
If a CA includes extended key usages to satisfy such applications,
but does not wish to restrict usages of the key, the CA can include
the special KeyPurposeId anyExtendedKeyUsage in addition to the
particular key purposes required by the applications. Conforming CAs
SHOULD NOT mark this extension as critical if the anyExtendedKeyUsage
KeyPurposeId is present. Applications that require the presence of a
particular purpose MAY reject certificates that include the
anyExtendedKeyUsage OID but not the particular OID expected for the
application.
If a certificate contains both a key usage extension and an extended
key usage extension, then both extensions MUST be processed
independently and the certificate MUST only be used for a purpose
consistent with both extensions. If there is no purpose consistent
with both extensions, then the certificate MUST NOT be used for any
purpose.
The following key usage purposes are defined:
anyExtendedKeyUsage OBJECT IDENTIFIER ::= { id-ce-extKeyUsage 0 }
id-kp OBJECT IDENTIFIER ::= { id-pkix 3 }
id-kp-serverAuth OBJECT IDENTIFIER ::= { id-kp 1 }
-- TLS WWW server authentication
-- Key usage bits that may be consistent: digitalSignature,
-- keyEncipherment or keyAgreement
id-kp-clientAuth OBJECT IDENTIFIER ::= { id-kp 2 }
-- TLS WWW client authentication
-- Key usage bits that may be consistent: digitalSignature
-- and/or keyAgreement
id-kp-codeSigning OBJECT IDENTIFIER ::= { id-kp 3 }
-- Signing of downloadable executable code
-- Key usage bits that may be consistent: digitalSignature
id-kp-emailProtection OBJECT IDENTIFIER ::= { id-kp 4 }
-- Email protection
-- Key usage bits that may be consistent: digitalSignature,
-- nonRepudiation, and/or (keyEncipherment or keyAgreement)
id-kp-timeStamping OBJECT IDENTIFIER ::= { id-kp 8 }
-- Binding the hash of an object to a time
-- Key usage bits that may be consistent: digitalSignature
-- and/or nonRepudiation
id-kp-OCSPSigning OBJECT IDENTIFIER ::= { id-kp 9 }
-- Signing OCSP responses
-- Key usage bits that may be consistent: digitalSignature
-- and/or nonRepudiation
4.2.1.13. CRL Distribution Points
The CRL distribution points extension identifies how CRL information
is obtained. The extension SHOULD be non-critical, but this profile
RECOMMENDS support for this extension by CAs and applications.
Further discussion of CRL management is contained in Section 5.
The cRLDistributionPoints extension is a SEQUENCE of
DistributionPoint. A DistributionPoint consists of three fields,
each of which is optional: distributionPoint, reasons, and cRLIssuer.
While each of these fields is optional, a DistributionPoint MUST NOT
consist of only the reasons field; either distributionPoint or
cRLIssuer MUST be present. If the certificate issuer is not the CRL
issuer, then the cRLIssuer field MUST be present and contain the Name
of the CRL issuer. If the certificate issuer is also the CRL issuer,
then conforming CAs MUST omit the cRLIssuer field and MUST include
the distributionPoint field.
When the distributionPoint field is present, it contains either a
SEQUENCE of general names or a single value, nameRelativeToCRLIssuer.
If the DistributionPointName contains multiple values, each name
describes a different mechanism to obtain the same CRL. For example,
the same CRL could be available for retrieval through both LDAP and
HTTP.
If the distributionPoint field contains a directoryName, the entry
for that directoryName contains the current CRL for the associated
reasons and the CRL is issued by the associated cRLIssuer. The CRL
may be stored in either the certificateRevocationList or
authorityRevocationList attribute. The CRL is to be obtained by the
application from whatever directory server is locally configured.
The protocol the application uses to access the directory (e.g., DAP
or LDAP) is a local matter.
If the DistributionPointName contains a general name of type URI, the
following semantics MUST be assumed: the URI is a pointer to the
current CRL for the associated reasons and will be issued by the
associated cRLIssuer. When the HTTP or FTP URI scheme is used, the
URI MUST point to a single DER encoded CRL as specified in
[RFC2585]. HTTP server implementations accessed via the URI SHOULD
specify the media type application/pkix-crl in the content-type
header field of the response. When the LDAP URI scheme [RFC4516] is
used, the URI MUST include a <dn> field containing the distinguished
name of the entry holding the CRL, MUST include a single <attrdesc>
that contains an appropriate attribute description for the attribute
that holds the CRL [RFC4523], and SHOULD include a <host>
(e.g., <ldap://ldap.example.com/cn=example%20CA,dc=example,dc=com?
certificateRevocationList;binary>). Omitting the <host> (e.g.,
<ldap:///cn=CA,dc=example,dc=com?authorityRevocationList;binary>) has
the effect of relying on whatever a priori knowledge the client might
have to contact an appropriate server. When present,
DistributionPointName SHOULD include at least one LDAP or HTTP URI.
If the DistributionPointName contains the single value
nameRelativeToCRLIssuer, the value provides a distinguished name
fragment. The fragment is appended to the X.500 distinguished name
of the CRL issuer to obtain the distribution point name. If the
cRLIssuer field in the DistributionPoint is present, then the name
fragment is appended to the distinguished name that it contains;
otherwise, the name fragment is appended to the certificate issuer
distinguished name. Conforming CAs SHOULD NOT use
nameRelativeToCRLIssuer to specify distribution point names. The
DistributionPointName MUST NOT use the nameRelativeToCRLIssuer
alternative when cRLIssuer contains more than one distinguished name.
If the DistributionPoint omits the reasons field, the CRL MUST
include revocation information for all reasons. This profile
RECOMMENDS against segmenting CRLs by reason code. When a conforming
CA includes a cRLDistributionPoints extension in a certificate, it
MUST include at least one DistributionPoint that points to a CRL that
covers the certificate for all reasons.
The cRLIssuer identifies the entity that signs and issues the CRL.
If present, the cRLIssuer MUST only contain the distinguished name
(DN) from the issuer field of the CRL to which the DistributionPoint
is pointing. The encoding of the name in the cRLIssuer field MUST be
exactly the same as the encoding in issuer field of the CRL. If the
cRLIssuer field is included and the DN in that field does not
correspond to an X.500 or LDAP directory entry where CRL is located,
then conforming CAs MUST include the distributionPoint field.
id-ce-cRLDistributionPoints OBJECT IDENTIFIER ::= { id-ce 31 }
CRLDistributionPoints ::= SEQUENCE SIZE (1..MAX) OF DistributionPoint
DistributionPoint ::= SEQUENCE {
distributionPoint [0] DistributionPointName OPTIONAL,
reasons [1] ReasonFlags OPTIONAL,
cRLIssuer [2] GeneralNames OPTIONAL }
DistributionPointName ::= CHOICE {
fullName [0] GeneralNames,
nameRelativeToCRLIssuer [1] RelativeDistinguishedName }
ReasonFlags ::= BIT STRING {
unused (0),
keyCompromise (1),
cACompromise (2),
affiliationChanged (3),
superseded (4),
cessationOfOperation (5),
certificateHold (6),
privilegeWithdrawn (7),
aACompromise (8) }
4.2.1.14. Inhibit anyPolicy
The inhibit anyPolicy extension can be used in certificates issued to
CAs. The inhibit anyPolicy extension indicates that the special
anyPolicy OID, with the value { 2 5 29 32 0 }, is not considered an
explicit match for other certificate policies except when it appears
in an intermediate self-issued CA certificate. The value indicates
the number of additional non-self-issued certificates that may appear
in the path before anyPolicy is no longer permitted. For example, a
value of one indicates that anyPolicy may be processed in
certificates issued by the subject of this certificate, but not in
additional certificates in the path.
Conforming CAs MUST mark this extension as critical.
id-ce-inhibitAnyPolicy OBJECT IDENTIFIER ::= { id-ce 54 }
InhibitAnyPolicy ::= SkipCerts
SkipCerts ::= INTEGER (0..MAX)
4.2.1.15. Freshest CRL (a.k.a. Delta CRL Distribution Point)
The freshest CRL extension identifies how delta CRL information is
obtained. The extension MUST be marked as non-critical by conforming
CAs. Further discussion of CRL management is contained in Section 5.
The same syntax is used for this extension and the
cRLDistributionPoints extension, and is described in Section
4.2.1.13. The same conventions apply to both extensions.
id-ce-freshestCRL OBJECT IDENTIFIER ::= { id-ce 46 }
FreshestCRL ::= CRLDistributionPoints
4.2.2. Private Internet Extensions
This section defines two extensions for use in the Internet Public
Key Infrastructure. These extensions may be used to direct
applications to on-line information about the issuer or the subject.
Each extension contains a sequence of access methods and access
locations. The access method is an object identifier that indicates
the type of information that is available. The access location is a
GeneralName that implicitly specifies the location and format of the
information and the method for obtaining the information.
Object identifiers are defined for the private extensions. The
object identifiers associated with the private extensions are defined
under the arc id-pe within the arc id-pkix. Any future extensions
defined for the Internet PKI are also expected to be defined under
the arc id-pe.
id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) }
id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }
4.2.2.1. Authority Information Access
The authority information access extension indicates how to access
information and services for the issuer of the certificate in which
the extension appears. Information and services may include on-line
validation services and CA policy data. (The location of CRLs is not
specified in this extension; that information is provided by the
cRLDistributionPoints extension.) This extension may be included in
end entity or CA certificates. Conforming CAs MUST mark this
extension as non-critical.
id-pe-authorityInfoAccess OBJECT IDENTIFIER ::= { id-pe 1 }
AuthorityInfoAccessSyntax ::=
SEQUENCE SIZE (1..MAX) OF AccessDescription
AccessDescription ::= SEQUENCE {
accessMethod OBJECT IDENTIFIER,
accessLocation GeneralName }
id-ad OBJECT IDENTIFIER ::= { id-pkix 48 }
id-ad-caIssuers OBJECT IDENTIFIER ::= { id-ad 2 }
id-ad-ocsp OBJECT IDENTIFIER ::= { id-ad 1 }
Each entry in the sequence AuthorityInfoAccessSyntax describes the
format and location of additional information provided by the issuer
of the certificate in which this extension appears. The type and
format of the information are specified by the accessMethod field;
the accessLocation field specifies the location of the information.
The retrieval mechanism may be implied by the accessMethod or
specified by accessLocation.
This profile defines two accessMethod OIDs: id-ad-caIssuers and
id-ad-ocsp.
In a public key certificate, the id-ad-caIssuers OID is used when the
additional information lists certificates that were issued to the CA
that issued the certificate containing this extension. The
referenced CA issuers description is intended to aid certificate
users in the selection of a certification path that terminates at a
point trusted by the certificate user.
When id-ad-caIssuers appears as accessMethod, the accessLocation
field describes the referenced description server and the access
protocol to obtain the referenced description. The accessLocation
field is defined as a GeneralName, which can take several forms.
When the accessLocation is a directoryName, the information is to be
obtained by the application from whatever directory server is locally
configured. The entry for the directoryName contains CA certificates
in the crossCertificatePair and/or cACertificate attributes as
specified in [RFC4523]. The protocol that application uses to access
the directory (e.g., DAP or LDAP) is a local matter.
Where the information is available via LDAP, the accessLocation
SHOULD be a uniformResourceIdentifier. The LDAP URI [RFC4516] MUST
include a <dn> field containing the distinguished name of the entry
holding the certificates, MUST include an <attributes> field that
lists appropriate attribute descriptions for the attributes that hold
the DER encoded certificates or cross-certificate pairs [RFC4523],
and SHOULD include a <host> (e.g., <ldap://ldap.example.com/cn=CA,
dc=example,dc=com?cACertificate;binary,crossCertificatePair;binary>).
Omitting the <host> (e.g., <ldap:///cn=exampleCA,dc=example,dc=com?
cACertificate;binary>) has the effect of relying on whatever a priori
knowledge the client might have to contact an appropriate server.
Where the information is available via HTTP or FTP, accessLocation
MUST be a uniformResourceIdentifier and the URI MUST point to either
a single DER encoded certificate as specified in [RFC2585] or a
collection of certificates in a BER or DER encoded "certs-only" CMS
message as specified in [RFC2797].
Conforming applications that support HTTP or FTP for accessing
certificates MUST be able to accept individual DER encoded
certificates and SHOULD be able to accept "certs-only" CMS messages.
HTTP server implementations accessed via the URI SHOULD specify the
media type application/pkix-cert [RFC2585] in the content-type header
field of the response for a single DER encoded certificate and SHOULD
specify the media type application/pkcs7-mime [RFC2797] in the
content-type header field of the response for "certs-only" CMS
messages. For FTP, the name of a file that contains a single DER
encoded certificate SHOULD have a suffix of ".cer" [RFC2585] and the
name of a file that contains a "certs-only" CMS message SHOULD have a
suffix of ".p7c" [RFC2797]. Consuming clients may use the media type
or file extension as a hint to the content, but should not depend
solely on the presence of the correct media type or file extension in
the server response.
The semantics of other id-ad-caIssuers accessLocation name forms are
not defined.
An authorityInfoAccess extension may include multiple instances of
the id-ad-caIssuers accessMethod. The different instances may
specify different methods for accessing the same information or may
point to different information. When the id-ad-caIssuers
accessMethod is used, at least one instance SHOULD specify an
accessLocation that is an HTTP [RFC2616] or LDAP [RFC4516] URI.
The id-ad-ocsp OID is used when revocation information for the
certificate containing this extension is available using the Online
Certificate Status Protocol (OCSP) [RFC2560].
When id-ad-ocsp appears as accessMethod, the accessLocation field is
the location of the OCSP responder, using the conventions defined in
[RFC2560].
Additional access descriptors may be defined in other PKIX
specifications.
4.2.2.2. Subject Information Access
The subject information access extension indicates how to access
information and services for the subject of the certificate in which
the extension appears. When the subject is a CA, information and
services may include certificate validation services and CA policy
data. When the subject is an end entity, the information describes
the type of services offered and how to access them. In this case,
the contents of this extension are defined in the protocol
specifications for the supported services. This extension may be
included in end entity or CA certificates. Conforming CAs MUST mark
this extension as non-critical.
id-pe-subjectInfoAccess OBJECT IDENTIFIER ::= { id-pe 11 }
SubjectInfoAccessSyntax ::=
SEQUENCE SIZE (1..MAX) OF AccessDescription
AccessDescription ::= SEQUENCE {
accessMethod OBJECT IDENTIFIER,
accessLocation GeneralName }
Each entry in the sequence SubjectInfoAccessSyntax describes the
format and location of additional information provided by the subject
of the certificate in which this extension appears. The type and
format of the information are specified by the accessMethod field;
the accessLocation field specifies the location of the information.
The retrieval mechanism may be implied by the accessMethod or
specified by accessLocation.
This profile de