[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

PKIX Roadmap

To: PKIX <ietf-pkix@xxxxxxx>
Subject: PKIX Roadmap
From: Sean Turner <turners@xxxxxxxx>
Date: Wed, 26 Aug 1998 16:10:58 -0400
Organization: IECA, Inc.
Sender: owner-ietf-pkix@xxxxxxx

Here is the PKIX Roadmap document Al provided me.  I didn't both
assigning the "draft-..." name format because I know Al has some
additional text his planning on including before he submits it to the
RFC editor.

Cheers,

spt

PKIX Working Group A. Arsenault
Internet Draft DOD
Expires in six months from August 7, 1998

Internet X.509 Public Key Infrastructure
PKIX Roadmap
<draft-ietf-pkix-roadmap-00.txt>

Status of this Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of 6 months
and may be updated, replaced, or may become obsolete by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as work in progress.

To view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

Abstract

This document provides a roadmap of the work done by the IETF PKIX
working group. It describes some of the terminology used in the
working group's documents, and the theory behind an X.509-based PKI.
It identifies each document developed by the PKIX working group, and
describes the relationships among the various document. It also
provides advice to would-be PKIX implementors about some of the issues
discussed at length during PKIX development, in hopes of making it
easier to build implementations that will actually interoperate.

1 INTRODUCTION

This document is an informational Internet draft that provides a
roadmap to the documents produced by the PKIX working group. It is
intended to provide information; there are no requirements or
specifications in this document.

Section 2 of this document defines key terms used in this document.
Section 3 covers PKIX theory; it explains what the PKIX working
group's basic assumptions were. Section 4 provides an overview of the
various PKIX documents. It identifies which documents address which
areas, and describes the relationships among the various documents.
Section 5 contains Advice to implementors. Its primary purpose is to
capture some of the major issues discussed by the PKIX working group,
as a way of explaining WHY some of the requirements and specifications
say what they say. This should cut down on the number of
ARSENAULT PAGE [1]
INTERNET DRAFT August 1998

misinterpretations of the documents, and help developers build
interoperable implementations. Section 6 contains a list of
references. Section 7 discusses security considerations, and Section 8
provides contact information for the editors.

2 Terminology

There are a number of terms used and misused throughout PKI-related
literature. To limit confusion caused by some of those terms,
throughout this document, we will use the following terms in the
following ways:

Certification Authority (CA): an entity that can issue and sign
certificates for a set of subjects
Root CA: a CA whose certificate is self-issued; that is, the
issuer and subject are the same entity.
Registration Agent (RA): an optional entity given responsibility
for performing some of the administrative tasks necessary in the
registration of subjects, such as: confirming the subject's identity;
validating that the subject is entitled to have the attributes
requested in a certificate; and verifying that the subject has
possession of the private key associated with the public key requested
for a certificate.
End-entity: a subject of a certificate who is not a CA.
Relying party: a user or agent (e.g., a client or server) who
relies on the data in a certificate in making decisions.
Subject: a subject is the entity (CA or end-entity) named in a
certificate. Subjects can be human users, computers (as represented by
DNS names or IP addresses), or even software agents.

3 PKIX Theory

3.1 Certificate-using Systems and PKIs

At the heart of recent efforts to improve Internet security are a group
of security protocols such as S/MIME, TLS, and IPSec. All of these
protocols rely on public-key cryptography to provide services such as
confidentiality, data integrity, data origin authentication, and non-
repudiation.

Users of public key-based systems must be confident that, any time they
rely on a public key, the associated private key is owned by the
subject with which they are communicating. (This applies whether an
encryption or digital signature mechanism is 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
achieved by having a trusted CA digitally sign each certificate.
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

ARSENAULT PAGE [2]
INTERNET DRAFT August 1998

can be distributed via untrusted communications and server systems, and
can be cached in unsecured storage in certificate-using systems.
Certificates are used in the process of validating signed data.
Specifics vary according to which algorithm is used, but the general
process works as follows: (note: there is no specific order in which
the checks listed below must be made; implementors are free to
implement them in the most efficient way for their systems)

- the recipient of signed data determines the identity of the
purported signer, and retrieves a certificate for that signer;
and retrieves certificates for any CA's necessary to build a
certificate path or chain of trust from the sender to the
recipient;

- the recipient validates that no certificate in the path has
been revoked (e.g., by retrieving a suitably-current
Certificate Revocation List (CRL) or querying an on-line
certificate status responder), and that all certificates were
within their validity periods at the time the data were
signed;

- the recipient verifies that the data are not claimed to have
any attributes for which the certificate indicates that the
signer is not authorized;

- the recipient verifies that the data have not been altered
since signing, by using the public key in the certificate.

If all of these checks pass, the recipient can accept that the data
were signed by the purported signer. The process for keys used for encryption is similar.

(Note: it is of course possible that data were signed by someone very
different from the signer, if for example the purported signer's
private key was compromised. Security depends on all parts of the
certificate-using SYSTEM, including but not limited to: physical
security of the place the computer resides; personnel security (i.e.,
the trustworthiness of the people who actually develop, install, run,
and maintain the system); the security provided by the operating system
on which the private key is used; and the security provided the CA. A
failure in any one of these areas can cause the entire system security
to fail. PKIX is limited in scope, however, and only directly
addresses issues related to the operation of the PKI subsystem. For
guidance in many of the other areas, see [PKIX-4].)

A collection of certificates, with their issuing CA's, subjects,
relying parties, RA's, and repositories, is referred to as a Public Key
Infrastructure, or PKI.

3.2 Overview of the PKIX Approach

PKIX is an effort to develop specifications for a Public Key
Infrastructure for the Internet using X.509 certificates. The PKIX

ARSENAULT PAGE [3]
INTERNET DRAFT August 1998

working group was initially chartered in 1995. A Public Key
Infrastructure, or PKI, is formally defined as

(insert definition of PKI - from MISPC or someplace else - here)

Figure 1 is a simplified view of the architectural model assumed by the
PKIX Working Group.

+---+
| C | +------------+
| e | <-------------------->| End entity |
| r | Operational +------------+
| t | transactions ^
| | and management | Management
| / | transactions | transactions
| | | PKI users
| C | v
| R | -------------------+--+-----------+----------------
| L | ^ ^
| | | | PKI management
| | v | entities
| R | +------+ |
| e | <---------------------| RA | <---+ |
| p | Publish certificate +------+ | |
| o | | |
| s | | |
| I | v v
| t | +------------+
| o | <------------------------------| CA |
| r | Publish certificate +------------+
| y | Publish CRL ^
| | |
+---+ Management |
transactions |
v
+------+
| CA |
+------+
Figure 1 - PKI Entities

3.3 X.509 certificates

ITU-T X.509 (formerly CCITT X.509) or ISO/IEC/ITU 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. These two fields may be used to support
directory access control.

The Internet Privacy Enhanced Mail (PEM) RFCs, published in 1993,

ARSENAULT PAGE [4]
INTERNET DRAFT August 1998

include specifications for a public key infrastructure based on X.509v1
certificates [RFC 1422]. The experience gained in attempts to deploy
RFC 1422 made it clear that the v1 and v2 certificate formats are
deficient in several respects. Most importantly, more fields were
needed to carry information which PEM design and implementation
experience has proven necessary. In response to these new
requirements, ISO/IEC/ITU 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 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 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 PKIX 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.

3.4 Functions of a PKI

This section describes the major functions of a PKI. In some cases,
PKIs may provide extra functions.

3.4.1 Registration

This is the process whereby a subject first makes itself known to a CA
(directly, or through an RA), prior to that CA issuing a certificate or
certificates for that subject. Registration involves the subject
providing its name (e.g., common name, fully-qualified domain name, IP
address), and other attributes to be put in the certificate, followed
by the CA (possibly with help from the RA) verifying in accordance with
its CPS that the name and other attributes are correct.

3.4.2 Initialization

Initialization is when the subject - e.g., the user or client system -
gets the values needed to begin communicating with the PKI. For
example, initialization can involve providing the client system with
the public key and/or certificate of a CA, or generating the client
system's own public/private key pair.

3.4.3 Certification

This is the process in which a CA issues a certificate for a subject's
public key, and returns that certificate to the subject and/or posts
that certificate in a repository.
ARSENAULT PAGE [5]
INTERNET DRAFT August 1998

3.4.4 Key Pair Recovery

In some implementations, key exchange or encryption keys will be
required by local policy to be "backed up", or recoverable in case the
key is lost and access to previously-encrypted information is needed.
Such implementations can include those where the private key exchange
key is stored on a hardware token which can be lost or broken, or when
a private key file is protected by a password which can be forgotten.
Often, a company is concerned about being able to read mail encrypted
by or for a particular employee when that employee is no longer
available because she is ill or no longer works for the company.
In these cases, the user's private key can be backed up by a CA or by a
separate key backup system. If a user or her employer needs to recover
these backed up key materials, the PKI must provide a system that
permits the recovery WITHOUT providing an unacceptable risk of
compromise of the private key.

3.4.5 Key Update

All key pairs need to be updated regularly, i.e., replaced with a new
key pair, and new certificates issued. This will happen in two cases:
normally, when a key has passed its maximum usable lifetime; and
exceptionally, when a key has been compromised and must be replaced.
In the normal case, a PKI needs to provide a facility to gracefully
transition from a certificate with an existing key to a new certificate
with a new key. This is particularly true when the key to be updated
is that of a CA. Users will know in advance that the key will expire
on a certain date; the PKI, working together with certificate-using
applications, should allow for appropriate keys to work before and
after the transition. There are a number of ways to do this; see
[insert appropriate reference here] for an example of one.

In the case of a key compromise, the transition will not be "graceful"
in that there will be an unplanned switch of certificates and keys;
users will not have known in advance what was about to happen. Still,
the PKI must support the ability to declare that the previous
certificate is now invalid and shall not be used, and to announce the
validity and availability of the new certificate.

Note, however, that the compromise of a private key associated with a
self-signed rootCA certificate is always catastrophic. That is, once
the rootCA's private signature key has been compromised, there is no
way to reliably convince users and subordinate CA's to accept a new key
for the rootCA. If the key is compromised, any "update" message
telling subordinates to switch to a new key could have come from an
attacker in possession of the old key, and could point to a new public
key for which the attacker already has the private key.

When a rootCA's private signature key is compromised, the only option
is dismantling the entire infrastructure subordinate to that rootCA and
starting over again from scratch. It is possible to have anticipated
this event, and "pre-placed" replacement rootCA keys with all relying
parties, but some secure, out-of-band mechanism will have to be used to

ARSENAULT PAGE [6]
INTERNET DRAFT August 1998

tell users to make the switch, and this will only help if the
replacement key has not been compromised.

3.4.6 Cross-certification

A cross-certificate is a certificate issued by one CA to another CA
which contains a CA signature key used for issuing certificates.
Typically, a cross-certificate is used to allow client systems/end
entities in one administrative domain to communicate security with
client systems/end users in another administrative domain. Use of a
cross-certificate issued from CA_1 to CA_2 allows user Alice, who
trusts CA_1, to accept a certificate used by Bob, which was issued by
CA_2. (Note: cross-certificates can also be issued from one CA to
another CA in the same administrative domain, if required.)

Cross-certificates can be issued in only one direction, or in both
directions, between two CA's. That is, just because CA_1 issues a
cross-certificate for CA_2 does not require CA_2 to issue a cross-
certificate for CA_1.

3.4.7 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 which is signed by a CA 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 CA issues a new
CRL on a regular periodic basis (e.g., hourly, daily, or weekly). CA's
may also issue CRLs aperiodically; e.g., if an important key is deemed
compromised, the CA may issue a new CRL to expedite notification of
that fact, even if the next CRL does not have to be issued for some
time. (A problem of aperiodic CRL issuance is that end-entities may
not know that a new CRL has been issued, and thus may not retrieve it
from a repository.)

An entry is added to the CRL as part of the next update following
notification of revocation. An entry may be removed from the CRL after

ARSENAULT PAGE [7]
INTERNET DRAFT August 1998

appearing on one regularly scheduled CRL issued beyond the revoked
certificate's validity period.

An advantage of the CRL revocation method is that CRLs may be
distributed by exactly the same means as certificates themselves,
namely, via untrusted communications and server systems.
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 the next CRL is issued -- this may be
up to one hour, one day, or one week depending on the frequency that
the CA issues CRLs.

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 PKIX
to specify that profile. However, PKIX does not require CAs to issue
CRLs. Message formats and protocols supporting on-line revocation
notification may be 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 the 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
must trust the on-line validation service while the repository does
not need to be trusted.

3.4.8 Certificate and Revocation Notice Distribution/Publication

As alluded to in sections 3.4.3 and 3.4.7 above, the PKI is responsible
for the distribution of certificates and certificate revocation notices
(whether in CRL form or in some other form) in the system.

"Distribution" of certificates includes transmission of the certificate
to its owner, and may also include publication of the certificate in a
repository. Distribution of revocation notices may involve posting
CRLs in a repository, transmitting them to end-entities, and/or
forwarding them to on-line responders.

3.5 Parts of PKIX

This section identifies the four different areas in which the PKIX
working group has developed documents. The first area involves
profiles of the X.509 v3 certificate standards and the X.509v2 CRL
standards for the Internet. The second area involves operational
protocols, in which relying parties can obtain information such as
certificates or certificate status. The third area covers management
protocols, in which different entities in the system exchange

ARSENAULT PAGE [8]
INTERNET DRAFT August 1998

information needed for proper management of the PKI. The last area
provides information about certificate policies and certificate
practice statements, covering the areas of PKI security not directly
addressed in the rest of PKIX.

3.5.1 Profile

An X.509v3 certificate is a very complex data structure. It consists
of a basic certificate, plus a number of optional certificate
extensions. Many of the fields of both the base certificate and
numerous extensions can take on a wide range of options. This provides
an enormous degree of flexibility, which allows the X.509v3 certificate
format to be used with a wide range of applications in a wide range of
environments. Unfortunately, this same flexibility makes it extremely
difficult to produce independent implementations that will actually
interoperate with one another. In order to build an Internet PKI based
on X.509v3 certificates, the PKIX working group had to develop a
profile of the X.509v3 specification.

A profile of the X.509v3 specification is a description of which
certificate extensions must be supported, which extensions may be
supported, and which extensions may not be supported. [PKIX-1]
provides such a profile of X.509v3 for the Internet PKI. In addition,
[PKIX-1] suggests ranges of values for many of the extensions.

[PKIX-1] also provides a profile for Version 2 CRLs for use in the
Internet PKI. CRLs, like certificates, have a number of optional
extensions. In order to promote interoperability, it is necessary to
constrain the choices an implementor supports.

In addition to profiling the certificate and CRL formats, it is
necessary to specify particular Object Identifiers (OIDs) for certain
encryption algorithms, because there are a variety of OIDs registered
for some algorithm suites. Thus, PKIX has produced at least two
documents ([ECDSA] and [KEA]) which provide guidance on the proper
implementation of specific algorithms.

3.5.2 Operational Protocols

Operational protocols are required to deliver certificates and CRLs
(or other certificate status information) to certificate using systems.
Provision is 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 [FTP], [OCSP],[LDAP], and [WEB].

3.5.3 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 which cross-certify each
other. A management protocol can be used to carry user or client

ARSENAULT PAGE [9]
INTERNET DRAFT August 1998

system registration information, or a request for revocation of a
certificate.

There are two parts to a management protocol. The first is the
format of the messages that will be sent, and the second is the actual
protocol that governs the transmission of those messages. The PKIX
working group has developed two documents ([CRMF] and [CMMF]) that
together describe the necessary set of message, and two other documents
([CMP] and [CMC]) that describe protocols for exchanging those
messages.

3.5.4 Policy Outline

As mentioned before, profiling certificates and specifying operational
and management protocols only addresses a part of the problem of
actually developing and implementing a secure PKI. What is also needed
is the development of a certificate policy and certification practice
statement, and then following those documents. The CP and CPS should
address physical and personnel security, subject identification
requirements, revocation policy, and a number of other topics. [PKIX-
4] provides a framework for certification practice statements.

4 PKIX Documents

This section describes each of the documents written by the PKIX
working group. As PKIX progresses, this section will need to be
continually updated to reflect the status of each document (e.g.,
Proposed Standard, Draft Standard, Standard, Informational Draft,
Informational RFC, something-that-was-just-thrown-out-for-
consideration, etc.)

4.1 Profile

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
and CRL Profile <draft-ietf-pkix-ipki-part1-08.txt>

DESCRIPTION: This document describes the profiles to be used for
X.509v3 certificates and version2 CRLs by Internet PKI participants.
The profiles include the identification of ISO/IEC/ITU and ANSI
extensions which may be useful in the Internet PKI. The profiles are
presented in the 1988 Abstract Syntax Notation One (ASN.1) rather than
the 1994 syntax used in the ISO/IEC/ITU standards. Would-be PKIX
implementors and developers of certificate-using applications should
start with [PKIX-1] to ensure that their systems will be able
interoperate with other users of the IPKI.

[PKIX-1] also includes path validation procedures. The procedures
presented are based upon the ISO/IEC/ITU definition, but the
presentation assumes one or more self-signed trusted CA certificates.
The procedures are provided as examples only. Implementations are not
required to use the procedures provided; they may implement whichever
procedures are efficient for their situation. However, implementations
are required to derive the same results as the example procedures.

ARSENAULT PAGE [10]
INTERNET DRAFT August 1998

STATUS:

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure:
Representation of Elliptic Curve Digital Signature Algorithm (ECDSA)
Keys and Signatures in Internet X.509 Public Key Infrastructure
Certificates <draft-ietf-pkix-ipki-ecdsa-01.txt>

DESCRIPTION: This document provides Object Identifiers (OIDs) and
other guidance for IPKI users who use the Elliptic Curve Digital
Signature Algorithm (ECDSA). It profiles the format and semantics of
the subjectPublicKeyInfo field and the keyUsage extension in X.509 V3
certificates containing ECDSA keys. This document should be used by
anyone wishing to support ECDSA; others who do not support ECDSA are
not required to comply with it.

STATUS:

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Representation
of Key Exchange Algorithm (KEA) Keys in Internet X.509 Public Key
Infrastructure Certificates

DESCRIPTION: This document provides Object Identifiers (OIDs) and other
guidance for IPKI users who use the Key Exchange Algorithm (KEA). It
profiles the format and semantics of the subjectPublicKeyInfo field and
the keyUsage extension in X.509 V3 certificates containing KEA keys.
This document should be used by anyone wishing to support KEA; others
who do not support ECDSA are not required to comply with it.

STATUS:

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure OPEN CRL
DISTRIBUTION PROCESS (OpenCDP) <draft-ietf-pkix-ocdp-00.txt>

DESCRIPTION: This document proposes an alternative to the CRL
Distribution Point (CDP) approach documented in [PKIX-1]. OCDP
separates the CRL location function from the process of certificate and
CRL validation, and thus claims some benefits over the CDP approach.

STATUS:

4.2 Operational Protocols

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Operational
Protocols - LDAPv2 <draft-ietf-pkix-ipki2opp-07.txt>

DESCRIPTION: This document describes the use of LDAPv2 as a protocol
for PKI elements to publish and retrieve certificates and CRLs from a
certificate repository. LDAPv2 [RFC abcd] is a protocol that allows
publishing and retrieving of information.

STATUS:

ARSENAULT PAGE [11]
INTERNET DRAFT August 1998

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure LDAPv2 Schema
<draft-ietf-pkix-ldapv2-schema-00.txt>

DESCRIPTION: This document defines a minimal schema necessary to
support the use of LDAPv2 for certificate and CRL retrieval and related
functions for PKIX. This document supplements [LDAP] by identifying
the PKIX-related attributes that must be present.

STATUS:

DOCUMENT TITLE: X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol (OCSP) <draft-ietf-pkix-ocsp-04.txt>

DESCRIPTION: This document specifies a protocol useful in determining
the current status of a certificate without the use of CRLs. A major
complaint about certificate-based systems is the need for a relying
party to retrieve a current CRL as part of the certificate validation
process. Depending on the size of the CRL, this can cause severe
problems for bandwidth-challenged devices. Depending on the frequency
of CRL issuance, this can also cause timeliness problems. (E.g., if
CRLs are only published weekly, with no interim releases, a certificate
could actually have been revoked for just short of one week without it
being on the current CRL, and thus improper use of that certificate
could still be occurring.)

OCSP attempts to address those problems. It provides a mechanism
whereby a relying party identifies one or more certificates to an
approved OCSP responder, and the responder sends back the current
status of the certificate(s) - e.g., revoked, notRevoked, unknown.
This can dramatically reduce the bandwidth required to
transmit revocation status a relying party does not have to retrieve
a CRL of many entries to check the status of one certificate. It can
(although it is not guaranteed to) improve the timeliness of revocation
notification, and thus reduce the window of opportunity for someone
trying to use a revoked certificate.

STATUS:

DOCUMENT TITLE: Internet Public Key Infrastructure: Caching the Online
Certificate Status Protocol <draft-ietf-pkix-ocsp-caching-00.txt>

DESCRIPTION: To improve the degree to which it can scale, OCSP allows
caching of responses; e.g., at intermediary servers, or even at the
relying party's end system. This document describes how to support
OCSP caching at intermediary servers.

STATUS:

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Operational
Protocols: FTP and HTTP <draft-ietf-pkix-opp-ftp-http-04.txt>

DESCRIPTION: This document describes the use of the File Transfer
Protocol (FTP) and the Hyper-text Transfer Protocol (HTTP) to obtain
certificates and CRLs from PKI repositories.
ARSENAULT PAGE [12]
INTERNET DRAFT August 1998

STATUS:

DOCUMENT TITLE: WEB based Certificate Access Protocol-- WebCAP/1.0

DESCRIPTION: This document specifies a set of methods, headers, and
content-types ancillary to HTTP/1.1 to publish, retrieve X.509
certificates and Certificate Revocation Lists. This protocol also
facilitates determining current status of a digital certificate without
the use of CRLs. This protocol defines new methods, request and
response bodies, error codes to HTTP/1.1 protocol for securely
publishing, retrieving, and validating certificates across a firewall.

STATUS:

4.3 Management Protocols

DOCUMENT TITLE: Certificate Management Messages over CMS <draft-ietf-
pkix-cmc-00.txt>

DESCRIPTION: This document defines the means by which PKI clients and
servers may exchange PKI messages when using S/MIME's Cryptographic
Message Syntax [CMS]as a transaction envelope. CMC supports message
bodies specified in the Certificate Management Message Formats [CMMF]
and Certificate Request Message Format [CRMF] documents. The purpose of
this specification is to allow the use of an existing protocol (S/MIME)
as a PKI management protocol, rather than requiring the development of
a new, custom protocol for the task.

STATUS:

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
Management Message Formats <draft-ietf-pkix-cmmf-02.txt>

DESCRIPTION: This document contains the formats for a series of
messages important in certificate/PKI management. These messages let
CA's, RA's, and relying parties communicate with each other. Note that
this document only specifies message formats; it does not specify a
protocol for transferring messages. That protocol can be either CMP or
CMC, or perhaps another custom protocol.

STATUS:

DOCUMENT TITLE: Internet X.509 Certificate Request Message Format
<draft-ietf-pkix-crmf-01.txt>

DESCRIPTION: CRMF specifies a format recommended for use whenever a
relying party is requesting a certificate from a CA or requesting that
an RA help it get a certificate. This document is distinct from CMMF
for historical reasons (the request message format was needed before
many of the other message formats had to be finalized, and so it was
put into a separate document). Like CMMF, this document only specifies
the format of a message. Specification of a protocol to transport that
message is beyond the scope of CRMF.
ARSENAULT PAGE [13]
INTERNET DRAFT August 1998

STATUS:

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
Management Protocols <draft-ietf-pkix-ipki3cmp-08.txt>
DESCRIPTION: This document specifies a new protocol specifically
developed for the purpose of transporting messages like those specified
in CMMF and CRMF among PKI elements. In general, CMP will be used in
conjunction with CMMF and CRMF, and will then be run over a transfer
service (e.g., S/MIME, HTTP) to provide a complete PKI management
service.

STATUS:

4.4 Policy Outline

DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
Policy and Certification Practices Framework <draft-ietf-pkix-ipki-
part4-03.txt>

DESCRIPTION: As noted before, the specification and implementation of
certificate profiles, operational protocols, and management protocols
is only part of building a PKI. Equally as important - if not more
important - is the development and enforcement of a certificate
security policy, and a Certification Practice Statement (CPS). The
purpose of this document (PKIX-4) is to establish a clear relationship
between certificate policies and(CPSs), and to present a framework to
assist the writers of certificate policies or CPSs with their tasks.
In particular, the framework identifies the elements that may need to
be considered in formulating a certificate policy or a CPS. The
purpose is not to define particular certificate policies or CPSs, per
se.

STATUS:

4.5 DOCUMENT RELATIONSHIPS

Figure 2 shows graphically the relationships among the PKIX documents.

CERT and CRL PROFILES
Certificate and CRL Profile
|
------ Representation of Elliptic Curve Digital Signature
| Algorithm (ECDSA)Keys and Signatures in Internet X.509
| Public Key Infrastructure Certificates
|
------ Representation of Key Exchange Algorithm (KEA) Keys in
| Internet X.509 Public Key Infrastructure Certificates
|
------- OPEN CRL DISTRIBUTION PROCESS (OpenCDP)

ARSENAULT PAGE [14]
INTERNET DRAFT August 1998

Operational Protocols
|
|
----------- Internet X.509 Public Key Infrastructure Operational
| Protocols - LDAPv2 <draft-ietf-pkix-ipki2opp-07.txt>
| |
| |
| ------ Internet X.509 Public Key Infrastructure LDAPv2
| Schema <draft-ietf-pkix-ldapv2-schema-00.txt>
|
|-----X.509 Internet Public Key Infrastructure Online Certificate
| Status Protocol - OCSP <draft-ietf-pkix-ocsp-04.txt>
| |
| |
| -----Internet Public Key Infrastructure: Caching the Online
| Certificate Status Protocol
| <draft-ietf-pkix-ocsp-caching-00.txt>
|
|--------Internet X.509 Public Key Infrastructure Operational
| Protocols: FTP and HTTP <draft-ietf-pkix-opp-ftp-http-04.txt>
|
|--------WEB based Certificate Access Protocol-- WebCAP/1.0

Management Protocols

Message Formats

Internet X.509 Public Key Infrastructure Certificate Management Message
Formats

Internet X.509 Certificate Request Message Format <draft-ietf-pkix-
crmf-01.txt>

Protocols

Internet X.509 Public Key Infrastructure Certificate Management
Protocols

Certificate Management Messages over CMS <draft-ietf-pkix-cmc-00.txt>

Policy Outline

Internet X.509 Public Key Infrastructure Certificate Policy and
Certification Practices Framework

Figure 2: Document Relationships

5 Advice to Implementors

This section provides guidance to those who would implement various
parts of the PKIX suite of documents. The topics discussed in this
section engendered significant discussion in the working group, and
there was at times either widespread disagreement or widespread

ARSENAULT PAGE [15]
INTERNET DRAFT August 1998

misunderstanding of them. Thus, this discussion is provided to help
readers of the PKIX document set understand these issues, in the hope
of fostering greater interoperability among eventual PKIX
implementations.

5.1 Names

PKIX has been referred to as a "name-centric" PKI because the
certificates associate public keys with names of entities. Each
certificate contains at least one name for the owner of a particular
public key. The name can be an X.500 distinguished name, contained in
the subjectDN field of the certificate. There can also be names such
as RFC822 e-mail addresses, DNS domain names, and URIs associated with
the key; these attributes are kept in the subjectAltName extension of
the certificate. A certificate must contain at least one of these name
forms, it may contain multiple forms if deemed appropriate by the CA
based on the intended usage of the certificate.

5.1.1 Name Forms

There are two possible places to put a name in an X.509v3 certificate.
One is the subject field in the base certificate (often called the
Distinguished Name or DN field), and the other is in the
subjectAltName extension.

5.1.1.1 Distinguished Names

According to [PKIX-1], a PKIX certificate must have a non-null value in
the Distinguished Name field, except for an end-entity certificate,
which is permitted to have an empty DN field.

5.1.1.2 SubjectAltName Forms

In addition to the DN, a PKIX certificate may have one or more values
in the subjectAltName extension.

The subjectAltName extension allows additional identities to be bound
to the subject of the certificate; e.g., it allows "umbc.edu" and
"130.85.1.3" to be associated with a particular subject, as well as
"C=US, O=University of Maryland, L=Baltimore, CN=UMBC". X.509-defined
options for this extension include: Internet electronic mail
addresses; DNS names; IP addresses; and uniform resource indentifiers
(URIs). Other options can exist, including locally-defined name forms.
A single subjectAltName extension can include multiple name forms, and
multiple instances of each name form.

Note: whenever such Alternate Name forms are to be bound into a
certificate, the subject alternative name (or issuer alternative name)
extension must be used. It is technically possible to embed an
Alternate Name Form in the subject field. For example, one could make
a DN contain an IP address via a kludge such as "C=US, L=Baltimore,
CN=130.85.1.3". However, this usage is deprecated; the alternative
name extension is the preferred location for finding such information.)

ARSENAULT PAGE [16]
INTERNET DRAFT August 1998

In line with this, if the only subject identity included in a
certificate is an alternative name form, then the subject distinguished
name must be empty (technically, an empty sequence), and the
subjectAltName extension must be present. If the subject field contains
an empty sequence, the subjectAltName extension must be marked
critical.

If the subjectAltName extension is present, the sequence must contain
at least one entry. Unlike the subject field, conforming CAs shall 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 PKIX. The behavior of clients that encounter such a
certificate when processing a certification path is not defined by this
working group.

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.

5.1.1.2.1 Internet e-mail addresses

When the subjectAltName extension contains an Internet mail address,
the adress is included as an rfc822Name. The format of an rfc822Name is
an "addr-spec" as defined in RFC 822 [RFC 822]. An addr-spec has the
form local-part@domain; it does not have a phrase (such as a common
name) before it, or a comment (text surrounded in parentheses) after
it, and it is not surrounded by "<" and ">".

5.1.1.2.2 DNS Names

When the subjectAltName extension contains a domain name service label,
the domain name is stored in the dNSName attribute(an IA5String). The
string shall be in the "preferred name syntax," as specified by RFC
1034 [RFC 1034]. Note that while upper and lower case letters are
allowed in domain names, no signifigance is attached to the case. In
addition, while the string " " is a legal domain name, subjectAltName
extensions with a dNSName " " are not permitted. Finally, the use of
the DNS representation for Internet mail addresses (wpolk.nist.gov
instead of wpolk@nist.gov) is not permitted; such identities are to be
encoded as rfc822Name.

5.1.1.2.3 IP addresses

When the subjectAltName extension contains an iPAddress, the address
shall be stored in the octet string in "network byte order," as
specified in RFC 791 [RFC 791]. 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 RFC 791, the octet string must contain
exactly four octets. For IP Version 6, as specified in RFC 1883, the
octet string must contain exactly sixteen octets [RFC1883].

ARSENAULT PAGE [17]
INTERNET DRAFT August 1998
5.1.1.2.4 URIs

(is there any guidance about URIs as name forms?)

5.1.2 Scope of Names

The original X.500 work presumed that every subject in the world would
have a globally-unique distinguished name. Thus, every subject could
be easily located, and there would never be a conflict. All that would
be needed is a sufficiently-large name space, and rules for
constructing names based on subordination and location.

Obviously, that is not practical in the real world, for a variety of
reasons. There is no single entity in the world trusted by everybody
to reside at the top of the name space, and there is no way to enforce
uniqueness on names for all entities. (These were among the reasons
for the failure of PEM to be widely implemented.)

This does not mean, however, that a name-based PKI cannot work. It is
important to recognize that the scope of names in PKIX is local; they
need to be defined and unique only within their own domain.
Suppose for example that a rootCA is established with DN "O=IETF,
OU=PKIX,CN=PKIX_CA". That CA will then issue certificates for names
subordinate to it. The only requirement - and this can be enforced
procedurally - is that no two distinct entities beneath this rootCA
have the same name. We can't prevent somebody else in the world from
creating her own CA, called "O=IETF, OU=PKIX, CN=PKIX_CA", and it is
not necessary to do so. Within the domain of the original rootCA,
there will be no conflict, and the fact that there is another CA of the
same name in some other domain is irrelevant.

This is analogous to the current DNS or IP address situations. On the
Internet, there is only one node called www.ietf.org. The fact that
there might be 10 different intranets, each with a host given the DNS
name www.ietf.org, is irrelevant and causes no interoperability
problems - those are different domains. However, if there were to be
another node on the Internet with domain name www.ietf.org, then there
would be a problem due to name duplication.

The same applies for IP addresses. As long as only one node on the
Internet responds to the IP address 130.85.1.3, there is no problem,
despite the fact that there are 100 different corporate Intranets, each
using that same IP address. However, if two different nodes on the
Internet each responded to the IP address 130.85.1.3, there would be a
problem.

5.1.3 Certificate Path Construction

Path construction - make point that there is no single best way to
construct a path. Implementors can pick the way that is most efficient
for them. Discuss some of the issues being hashed out in the "ldap"
discussion on the mail list. If there is ever a resolution, include it
in this section.

ARSENAULT PAGE [18]
INTERNET DRAFT August 1998

5.1.4 Name Constraints

(Note: this section still needs a lot of work.)

A question that has arisen a number of times is "how does one enforce
Name constraints when there is more than one name form in a
certificate?" That is, [PKIX-1] states that:

Subject alternative names may be constrained in the same manner as
subject distinguished names using the name constraints extension as
described in section 4.2.1.11.

What does this mean? Suppose that there is a CA with DN "O=IETF,
OU=PKIX, CN=PKIX_CA", with the subjectAltName extension having dNSName
"PKIX_CA.IETF.ORG". Suppose that that CA has issued a certificate with
an empty DN, with subjectAltName extension having dNSName set to
"PKIX_CA.IETF.ORG", and rfc822Name set to Steve@PKIX_CA.IETF.ORG. The
question is, are name constraints enforced on these two certificates -
is the name of the end-entity certificate considered to be properly
subordinate to the name of the CA?

The answer is "yes". In deciding whether a name form meets name
constraints, the following rules apply:

- for DNs:
- for rfc822Names:
- for dNSNames:
- for URIs:
- for iPaddresses

The general rules are:

If a certificate complies with name constraints in any one of its
name forms, then the certificate is deemed to comply with name
constraints.

If a certificate contains a name form that its issuer does not,
the certificate is deemed to comply with name constraints for
that name form.

5.1.5 Wildcards in Name Forms

A "wildcard" in a name form is a placeholder for a set of names; e.g.
"*.mit.edu" meaning "any domain name ending in .mit.edu", and "*@aol.com"
meaning "email address that uses aol.com". There are many people who
believe that allowing wildcards in name forms in PKIX certificates
would be a useful thing to do, because it would allow a single
certificate to be used by all members of a group. These members would
presumably have attributes in common; e.g., access rights to some set
of resources, and so issuance of a certificate with a wildcard for the
group would simplify management.

ARSENAULT PAGE [19]
INTERNET DRAFT August 1998

After much discussion, the PKIX working group decided to permit the use
of wildcards in certificates. That is, it is permissible for a PKIX-
conformant CA to issue a certificate with a wildcard. However, the
semantics of subject alternative names that include wildcard characters
are not addressed by PKIX. Applications with specific requirements may
use such names but must define the semantics.

5.2 POP

- The importance of PoP
- PoP for signature keys vs. PoP for key-management keys
- What the CA/RA has to do
- Different ways of accomplishing this

5.3 Key Usage Bits

The key usage extension defines the purpose (e.g., encipherment,
signature, certificate signing) of the key contained in the
certificate. This extension is used 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 for signing, 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. When used, this extension should be marked critical.
The eight bits defined for this extension identify seven mechanisms and
one service, namely:
digitalSignature
nonRepudiation
keyEncipherment
dataEncipherment
keyAgreement
keyCertSign
cRLSign
encipherOnly
decipherOnly

Bits in the KeyUsage type are used as follows:

The digitalSignature bit is asserted when the subject public key is
used to verify digital signatures that have purposes other than non-
repudiation, certificate signature, and CRL signature. For example,
the digitalSignature bit is asserted when the subject public key is
used to provide authentication via the signing of ephemeral data.
(NOTE TO ME: STEAL SIMONETTI's WORDS FOR USE IN THIS SECTION)

The nonRepudiation bit is asserted when the subject public key is used
to verify digital signatures used to provide a non- repudiation service
which protects against the signing entity falsely denying some action,
excluding certificate or CRL signing.

The keyEncipherment bit is asserted when the subject public key is used
for key transport. For example, when an RSA key is to be used for key
management, this bit must asserted.

ARSENAULT PAGE [20]
INTERNET DRAFT August 1998

The dataEncipherment bit is asserted when the subject public key is
used for enciphering user data, other than cryptographic keys.
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, this bit must asserted.

The keyCertSign bit is asserted when the subject public key is used for
verifying a signature on certificates. This bit may only be asserted
in CA certificates.

The cRLSign bit is asserted when the subject public key is used for
verifying a signature on revocation information (e.g., a CRL).
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.

PKIX does not restrict the combinations of bits that may be set in an
instantiation of the keyUsage extension.

5.4 Trust Models

6 Acknowledgements

A lot of the information in this document was taken from the PKIX
source documents; the authors of those deserve the credit for their own
words. Other good material was taken from mail posted to the PKIX
working group mail list (ietf-pkix@imc.org). Among those with good
things to say were (in alphabetical order, with apologies to anybody
I've missed): Sharon Boeyen, Santosh Chokhani, Warwick Ford, Russ
Housley, Steve Kent, Tim Polk, Stefan Santesson, Dave Simonetti,

7 References

[CACHE] "Internet Public Key Infrastructure: Caching the Online
Certificate Status Protocol," <draft-ieft-pkix-ocsp-caching-00.txt>

[CMC] Myers, M., Liu, X., Fox, B., and Weinstein, J., "Certificate
Management Messages over CMS," <draft-ieft-pkix-cmc-00.txt>, March 1998

[CMMF] Adams, C., and Myers, M., "Internet X.509 Public Key
Infrastructure Certificate Management Message Formats," <draft-ietf-
pkixx-cmmf-02.txt>, July 1998

[CRMF] Myers, M., Adams, C., Solo, D., and Kemp, D., "Internet X.509
Certificate Request Message Format," <draft-ieft-pkix-crmf-01.txt>, May
1998

ARSENAULT PAGE [21]
INTERNET DRAFT August 1998

[CMP] Adams, C., and Farrell, S., "Internet X.509 Public Key
Infrastructure Certificate Management Protocols," <draft-ietf-pkix-
ipki3cmp-08.txt>, May 1998

[ECDSA] Bassham, L., Johnson, D., and Polk, W., "Internet x.509 Public
Key Infrastructure: Representation of Elliptic Curve Digital Signature
Algorithm (ECDSA) Keys and Signatures in Internet X.509 Public Key
Infrastructure Certificates," <draft-ietf-pkix-ipki-ecdsa-01.txt>,
November 1997

[FTP] Housley, R., and Hoffman, P., "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP," <draft-ietf-pkix-
opp-ftp-http-04.txt>, July 1998

[KEA] Housley, R., and Polk, W., "Internet X.509 Public Key
Infrastructure Representation of Key Exchange Algorithm (KEA) Keys in
Internet X.509 Public Key Infrastructure Certificates," <draft-ietf-
pkix-ipki-kea-02.txt>, 5 August 1998.

[LDAP] Boeyen, S., Howes, T., and Richard, P., "Internet X.509 Public
Key Infrastructure Operational Protocols - LDAPv2," <draft-ietf-pkix-
ipki2opp-07.txt>, March 1998.

[OCDP] Hallam-Baker, P., and Ford, W., "Internet X.509 Public Key
Infrastructure Open CRL Distribution Process (OpenCDP)," <draft-ietf-
pkix-ocdp-00.txt>, April 1998

[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and Adams, C.,
"X.509 Internet Public Key Infrastructure Online Certificate Status
Protocol - OCSP," <draft-ietf-pkix-ocsp-05.txt>, June 1998.

[PKIX-1] Housley, R., Ford, W., Polk, W., and Solo, D., "Internet X.509
Public Key Infrastructure Certificate and CRL Profile," <draft-ietf-
pkix-ipki-part1-09.txt>, July 28, 1998.

[PKIX-4] Chokhani, S., and Ford, W., "Internet X.509 Public Key
Infrastructure Certificate Policy and Certification Practices
Framework," <draft-ietf-pkix-ipki-part4-03.txt>; 25 April 1998.

[RFC 1422] Kent, S., "Privacy Enhancement for Internet Electronic
Mail: Part II: Certificate-Based Key Management," February 1993.

[RFC 791] Postel, J., "Internet Protocol", September 1981.

[RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", August 1982.

[RFC 1034] Mockapetris, P.V., "Domain names - concepts and facilities",
November 1987.

[RFC 1777] Yeong, Y., Howes, T., and Kille, S., "Lightweight Directory
Access Protocol", March 1995

ARSENAULT PAGE [22]
INTERNET DRAFT August 1998

[RFC 1883] Deering, S., and Hinden, R., "Internet Protocol, Version 6
[IPv6] Specification", December 1995.

[SCHEMA] Boeyen, S., Howes, T., and Richard, P., "Internet X.509 Public
Key Infrastructure LDAPv2 Schema," <draft-ietf-pkix-ldapv2-schema-
00.txt>, March 1998

[WEB] Reddy, S., "WEB based Certificate Access Protocol-- WebCAP/1.0,"
<draft-ietf-pkix-webcap-00.txt>, April 19, 1998

[X.509] ITU-T Recommendation X.509 (1997 E): Information Technology -
Open Systems Interconnection - The Directory: Authentication Framework,
June 1997.

[X9.42] ANSI X9.42-199x, Public Key Cryptography for The Financial
Services Industry: Agreement of Symmetric Algorithm Keys Using Diffie-
Hellman (Working Draft), December 1997.

[X9.55] ANSI X9.55-1995, Public Key Cryptography For The Financial
Services Industry: Extensions To Public Key Certificates And
Certificate Revocation Lists, 8 December, 1995.

[X9.57] ANSI X9.57-199x, Public Key Cryptography For The Financial
Services Industry: Certificate Management (Working Draft), 21 June, 1996.

8 Security Considerations

9 Editor's Address

Alfred Arsenault
U. S. Department of Defense
9800 Savage Road Suite 6734
Fort George G. Meade, MD 20755-6734
(410) 684-7114
awarsen@missi.ncsc.mil

10 Disclaimer

This work constitutes the opinion of the editor only, and may not
reflect the opinions or policies of his employer.

ARSENAULT PAGE [23]

Prev by Date: Re: Authentication vs. binding signature, and ephemeral vs.permanent key usage
Next by Date: RE title change : directory enabled certificate status draft
Previous by thread: directory enabled certificate status draft
Next by thread: Re: PKIX Roadmap
Index(es):
- Date
- Thread