PKIX Working Group A. Arsenault
INTERNET DRAFT DOD
S. Turner
IECA
Expires 10 September, 2000 March 10, 2000
Internet X.509 Public Key Infrastructure
PKIX Roadmap
<draft-ietf-pkix-roadmap-05.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
This document is an Internet-Draft. Internet-Drafts are working
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Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document provides an overview or "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 Public Key Infrastructure and Privilege Management
Infrastructure (PMI). It identifies each document developed by the
PKIX working group, and describes the relationships among the various
documents. It also provides advice to would-be PKIX implementors
about some of the issues discussed at length during PKIX development,
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in hopes of making it easier to build implementations that will
actually interoperate.
1 Introduction
1.1 This Document
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 misinterpretations of the documents, and help developers build
interoperable implementations. Section 6 contains a list of
contributors we wish to thank. Section 7 provides a list references.
Section 8 discusses security considerations, and Section 9 provides
contact information for the editors. Finally, Section 10 provides a
disclaimer.
1.2 Changes Since Last Version
Updated references to current drafts.
Add references to 2459bis (son-of-rfc 2459), 2510bis, Technical
Requirements for a Non-Repudiation Service, A String Represenation of
General Name.
Revised 5th paragraph in clause 3.2 as per comments.
Updated clause 5.2.2 as per comments.
Rewrote clause 5.3 Key Usage to addres more issues with respect to
the DS and NR bits. Also to change its focus from when to set the
bits to driving the definition of non-repudiation service
requirements.
Added a new clause 5.4 to talk about non-repudiation requirements.
1.3 To Do
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Add in paragraph to talk about PMI functions.
Add in paragraph to talk about the whole QC naming issues
(dnQualifier vs serialNumber, serialNumber specification, etc.).
Rewrite trust models section.
2 Terminology
There are a number of terms used and misused throughout PKI-related
and PMI-related literature. To limit confusion caused by some of
those terms, throughout this document, we will use the following
terms in the following ways:
- Attribute Authority (AA) - An authority trusted by one or more
users to create and sign attribute certificates. It is important
to note that the AA is responsible for the attribute certificates
during their whole lifetime, not just for issuing them.
- Attribute Certificate (AC) - A data structure containing a set of
attributes for an end-entity and some other information, which is
digitally signed with the private key of the AA which issued it.
- Certificate - Can refer to either an AC or a public key
certificate. Where there is no distinction made the context
should be assumed to apply to both an AC and a public key
certificate.
- Certification Authority (CA) - An authority trusted by one or
more users to create and assign public key certificates.
Optionally the CA may create the user's keys. It is important to
note that the CA is responsible for the public key certificates
during their whole lifetime, not just for issuing them.
- Certificate Policy (CP) - A named set of rules that indicates the
applicability of a public key certificate to a particular
community or class of application with common security
requirements. For example, a particular certificate policy might
indicate applicability of a type of public key certificate to the
authentication of electronic data interchange transactions for
the trading of goods within a given price range.
- Certification Practice Statement (CPS) - A statement of the
practices which a CA employs in issuing public key certificates.
- End-entity - A subject of a certificate who is not a CA in the
PKIC or an AA in the PMI. (An EE from the PKI can be an AA in the
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PMI.)
- Public Key Certificate (PKC) - A data structure containing the
public key of an end-entity and some other information, which is
digitally signed with the private key of the CA which issued it.
- Public Key Infrastructure (PKI) - The set of hardware, software,
people, policies and procedures needed to create, manage, store,
distribute, and revoke PKCs based on public-key cryptography.
- Privilege Management Infrastructure (PMI) - A collection of ACs,
with their issuing AA's, subjects, relying parties, and
repositories, is referred to as a Privilege Management
Infrastructure.
- Registration Authority (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 values requested in a PKC; and verifying that the
subject has possession of the private key associated with the
public key requested for a PKC.
- Relying party - A user or agent (e.g., a client or server) who
relies on the data in a certificate in making decisions.
- Root CA - A CA that is directly trusted by an EE; that is,
securely acquiring the value of a Root CA public key requires
some out-of-band step(s). This term is not meant to imply that a
Root CA is necessarily at the top of any hierarchy, simply that
the CA in question is trusted directly.
- Subordinate CA - A "subordinate CA" is one that is not a Root CA
for the EE in question. Often, a subordinate CA will not be a
Root CA for any entity but this is not mandatory.
- Subject - A subject is the entity (AA, CA, or EE) named in a
certificate. Subjects can be human users, computers (as
represented by Domain Name Service (DNS) names or Internet
Protocol (IP) addresses), or even software agents.
- Top CA - A CA that is at the top of a PKI hierarchy.
Note: This is often also called a "Root CA," since in data
structures terms and in graph theory, the node at the top of a tree
is the "root." However, to minimize confusion in this document, we
elect to call this node a "Top CA," and reserve "Root CA" for the
CA directly trusted by the EE. Readers new to PKIX should be aware
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that these terms are not used consistently throughout the PKIX
documents, as [FORMAT] uses "Root CA" to refer to what this and
other documents call a "Top CA," and "most-trusted CA" to refer to
what this and other documents call a "Root CA."
3 PKIX Theory
3.1 Certificate-using Systems
3.1.1 Certificate-using Systems and PKIs
At the heart of recent efforts to improve Internet security are a
group of security protocols such as Secure Multipurpose Internet Mail
Extensions (S/MIME), Transport Layer Security (TLS), and Internet
Protocol Security (IPSec). All of these protocols rely on public-key
cryptography to provide services such as confidentiality, data
integrity, data origin authentication, and non-repudiation. The
purpose of a PKI is to provide trusted and efficient key and public
key certificate management, thus enabling the use of authentication,
non-repudiation, and confidentiality.
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 PKCs, which are data structures that
bind public key values to subjects. The binding is achieved by having
a trusted CA verify the subject's identity and digitally sign each
PKC.
A PKC has a limited valid lifetime, which is indicated in its signed
contents. Because a PKC's signature and timeliness can be
independently checked by a certificate-using client, PKCs can be
distributed via untrusted communications and server systems, and can
be cached in unsecured storage in certificate-using systems.
PKCs 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 verifies that the claimed identity
of the user is in accordance with the identity contained in the
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PKC;
- The recipient validates that no PKC in the path is revoked (e.g.,
by retrieving a suitably-current Certificate Revocation List
(CRL) or querying an on-line certificate status responder), and
that all PKCs are within their validity periods at the time the
data was signed;
- The recipient verifies that the data are not claimed to have any
values for which the PKC 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 PKC.
If all of these checks pass, the recipient can accept that the data
was signed by the purported signer. The process for keys used for
encryption is similar.
Note: It is of course possible that the data was 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 [POLPROC].
3.1.2 Certificate-using Systems and PMIs
Many systems use the PKC to perform identity based access control
decisions (i.e., the identity may be used to support identity-based
access control decisions after the client proves that it has access
to the private key that corresponds to the public key contained in
the PKC). For many systems this is sufficient, but increasingly
systems are beginning to find that rule-based, role-based, and rank-
based access control is required. These forms of access control
decisions require additional information that is normally not
included in a PKC, because the lifetime of the information is much
shorter than the lifetime of the public-private key pair. To support
binding this information to a PKC the Attribute Certificate (AC) was
defined in ANSI and later incorporated into ITU-T Recommendation
X.509. The AC format allows any additional information to be bound to
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a PKC by including, in a digitally signed data structure, a reference
back to one specific PKC or to multiple PKCs, useful when the subject
has the same identity in multiple PKCs. Additionally, the AC can be
constructed in such a way that it is only useful at one or more
particular targets (e.g., web server, mail host).
Users of a PMI must be confident that the identity purporting to
posess an attribute has the right to possess that attribute. This
confidence may be obtained through the use of PKCs or it may be
configured in the AC-using system. If PKCs are used the party making
the access control decision can determine "if the AC issuer is
trusted to issue ACs containing this attribute."
3.2 PKIX History
In the beginning there was ITU-T Recommendation X.509. It defines a
widely accepted basis for a PKI, including data formats and
procedures related to distribution of public keys via PKCs digitally
signed by CAs. X.509 does not however include a profile to specify
the support requirements for many of the PKC data structure's sub-
fields, for any of the extensions, nor for certain data values. PKIX
was formed in October 1995 to deliver a profile for the Internet PKI
of X.509 version 3 PKCs and version 2 CRLs. The Internet PKI profile
[FORMAT] went through eleven draft versions before becoming an RFC.
Other profiles have been developed in PKIX for particular algorithms
to make use of [FORMAT]. There has been no sense of conflict between
the authors that developed these profiles as they are seen as
complimentary.
The development of the management protocols has not been so
straightforward. The Certificate Management Protocol (CMP) was
developed to define a message protocol that is used between entities
in a PKI [CMP]. The demand for an enrollment protocol and the desire
to use PKCS-10 message format as the certificate request syntax lead
to the development of two different documents in two different
groups. The Certificate Request Syntax (CRS) draft was developed in
the SMIME WG which used PKCS-10 [PKCS10] as the certification request
message format. Certificate Request Message Format [CRMF] draft was
also developed but in the PKIX WG. It was to define a simple
enrollment protocol that would subsume both the CMP and CRS
enrollment protocols, but it did not use PKCS-10 as the certificate
request message format. Then the certificate management message
format document, was developed to define an extended set of
management messages that flow between the components of the Internet
PKI. Certificate Management Messages over CMS (CMC) was developed to
allow the use of an existing protocol (S/MIME) as a PKI management
protocol, without requiring the development of an entirely new
protocol such as CMP [CMC]. It also included [PKCS10] as the
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certificate request syntax, which caused work on the CRS draft to
stop. Information from the certificate management message format
document was moved into [CMP] and [CMC] so work on the certificate
management message format document was discontinued.
Another long debated topic in the WG dealt with certificate
revocation. Numerous drafts have been developed to address different
issues related certificate revocations. CMP supports revocation
request, response, revocation announcement, and requests for CRL
messages. CMC defines revocation request, revocation response, and
requests for CRL messages, but uses CMS as the encapsulating
protocol. [OCSP] was developed to address concerns that not all
relying parties want to go through the process checking CRLs from
every CA in the certification path. It defines an on-line mechanism
to determine the status of a given certificate, which may provide
more timely revocation information than is possible with CRLs. The
Simple Certification Verification Protocol (SCVP) was produced to
allow relying parties to off-load all of their certification
verification to another entity [SCVP]. The WG was arguable split over
whether such a function should be supported and whether it should be
its own protocol or included in OCSP. In response, a draft defining
OCSP Extensions [OCSPX] was produced to include the functions of
SCVP. One other draft called Open CRL Distribution Point (OCDP) was
produced which documented two extensions: one to support an
alternative CRL partitioning mechanism to the CRL Distribution Point
mechanism documented in [FORMAT] and one to support identifying other
revocation sources available to certificate-users. The work from
this draft was subsumed by an ITU-T | ISO/IEC Amendment to X.509,
hence work on this draft was halted.
Development of the operational protocols has been slightly more
straightforward. Three documents for the Light Weight Directory
Access Protocol (LDAP) have been developed one for defining LDAPv2 as
an access protocol to repositories [PKI-LDAPv2]; one for storing PKI
information in an LDAP directory [SCHEMA]; and one for LDAPv3
requirements for PKI [PKI-LDAPv3]. Using the File Transfer Protocol
(FTP) and the Hyper Text Transmission Protocol (HTTP) to retrieve
PKCs and CRLs from PKI repositories was documented in [FTPHTTP].
In late 1998 the PKIX charter was revised to include protocols for
time stamping and data certification services. [TSP] was developed to
define protocols required to interact with a Time Stamp Authority
(TSA) who asserts that a datum existed at a given time. [DVCS] allows
to verify and assert the validity of all signatures attached to the
signed document using all appropriate status information and PKCs or
to verify and assert the validity of one or more PKCs at the
specified time. Both [DVCS] and [TSP] use [CMS] as an encapsulating
(though in [TSP] request for a time stamp are not required to use
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[CMS]). [ETNPT] was developed to use [DCVS] to maintain the trust in
a token issued by a non-repudiation Trusted Third Party (NR TTP)
after the key initially used to establish trust in the token expires.
Around the same time, a work item for ACs, defined in [X.509], was
added. ACs are similar to PKCs, but they do not bind public keys to
identities rather they bind attributes to identities. The attributes
bound to the identity can represent anything, but are mostly used to
support rule-based, role-based, and rank-based access control
decisions. Two drafts have since been developed: the Internet
Attribute Certificates Profile for Authorizations [AC] and the
Limited AttributeCertificate Acquisition Protocol [LAAP]. The first
profiles the fields and extensions of the AC and the second provides
a deliberately limited protocol to access a repository when LDAP is
not appropriate.
Other drafts have been produced to address specific issues. [DHPOP]
was developed to define two mechanisms by which a signature can
produced using a Diffie-Hellman pair. This draft provides a
mechanism to Diffie-Hellam key pairs to issue a PKCS-10 certification
request. After some operational experience with [CMP], two drafts,
one for using HTTP as the transport protocol [CMP-HTTP] and one for
Transmission Control Protocol (TCP) [CMP-TCP], were written to solve
problems encountered by implementors.
From the alphabet soup above, it is clear why this roadmap is
required.
Note that [FORMAT] and [CMP] are currently being updated based on
implementation experience, see [2459bis] and [2510bis].
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3.3 Overview of the PKIX Approach
3.3.1 PKI
PKIX is an effort to develop specifications for a PKI for the
Internet using PKCs. A PKI is defined as:
The set of hardware, software, people, policies and procedures needed
to create, manage, store, distribute, and revoke PKCs based on
public-key cryptography.
A PKI consists of five types of components [MISPC]:
- Certification Authorities (CAs) that issue and revoke PKCs;
- Organizational Registration Authorities (ORAs) that vouch for the
binding between public keys and certificate holder identities and
other attributes;
- Certificate holders that are issued certificates and can sign
digital documents and encrypt documents;
- Clients that validate digital signatures and their certification
paths from a known public key of a trusted CA;
- Repositories that store and make available certificates and
Certificate Revocation Lists (CRLs).
Figure 1 is a simplified view of the architectural model assumed by
the PKIX Working Group.
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+---+
| 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
3.3.2 PMI
PKIX is also an effort to develop specifications for a Privilege
Management Infrastructure for the Internet using ACs. A Privilege
Management Infrastructure, or PMI, is defined as:
The set of hardware, software, people, policies and procedures needed
to create, manage, store, distribute, and revoke ACs.
A PMI consists of five types of components [AC]:
- Attribute Authorities (AAs), or Attribute Certificate Issuer,
that issue and revoke ACs;
Note: AAs may implicitly revoke ACs by using very short validity
periods.
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- Attribute Certificate Users that parses or processes an AC;
- Attribute Certificate Verifiers that check the validity of an AC
and then makes use of the result;
- Clients that request an action for which authorization checks are
to be made;
- Repositories that store and make available certificates and
Certificate Revocation Lists (CRLs).
Figure 2 is an example of the exchanges that may involve ACs.
+--------------+
| | Server Acquisition
| AC Issuer +----------------------------+
| | |
+--+-----------+ |
| |
| Client |
| Acquisition |
| |
+--+-----------+ +--+------------+
| | AC "push" | |
| Client +-------------------------+ Server |
| | (part of app. protocol) | |
+--+-----------+ +--+------------+
| |
| Client | Server
| Lookup +--------------+ | Lookup
| | | |
+---------------+ Repository +---------+
| |
+--------------+
Figure 2: AC Exchanges
3.4 Certificates
3.4.1 Public Key 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 public key certificate format
[X.509]. The public key certificate format in the 1988 standard is
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called the version 1 (v1) format.
When X.500 was revised in 1993, two more fields,
subjectUniqueIdentifier and issuerUniqueIdentifier 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,
include specifications for a public key infrastructure based on X.509
v1 public key certificates [PEM]. The experience gained in attempts
to deploy [PEM] made it clear that the v1 and v2 public key
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) public key 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, electronic mail, and IPSec
applications, etc. Environments with additional requirements may
build on this profile or may replace it.
3.4.2 Attribute Certificates
ANSI X.9 first published the Attribute Certificate format in [put
date in here] as part of [put reference in here]. It defined the
standard version 1 (v1) AC format. They later created a version 2
(v2) AC by modifying the owner field to point to either an identity
or a specific PKC and including an extension mechanism. In 1997 ITU-T
included it in [X.509].
ANSI, ITU-T, and IETF have developed standard extensions and
attributes for use in the v2 ACs. Extensions can convey such
informatoin as an audit identity that can be used to create an audit
trail, identity specific servers and services where the AC owner can
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use their AC, point to a specific issuer's key, and indicate where to
get revocation information. The AC is generic enough to allow any
attribute to be conveyed in the data structure. Without limiting the
attributes and extensions that can be included in an AC it is very
difficult to develop interoperable implementations for Internet use.
It is the goal of PKIX to specify a profile for the Internet,
electronic mail, IPSec applications, etc. Environments with
additional requirements may build on this profile or replace it.
3.5 Functions of a PKI
This section describes the major functions of a PKI. In some cases,
PKIs may provide extra functions.
3.5.1 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 PKC or
PKCs 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 PKC, followed by the
CA (possibly with help from the RA) verifying in accordance with its
Certification Practice Statement (CPS) that the name and other
attributes are correct.
3.5.2 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 or PKC of a CA, or generating the client system's own
public-private key pair.
3.5.3 Certification
This is the process in which a CA issues a PKC for a subject's public
key, and returns that PKC to the subject or posts that PKC in a
repository.
3.5.4 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
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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.
3.5.5 Key Generation
Depending on the CA's policy, the private-public key pair can either
be generated by the user in his local environment, or generated by
the CA. In the latter case, the key material may be distributed to
the user in an encrypted file or on a physical token (e.g., a smart
card or PCMCIA card).
3.5.6 Key Update
All key pairs need to be updated regularly (i.e., replaced with a new
key pair) and new PKCs 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.
3.5.6.1 Key Expiry
In the normal case, a PKI needs to provide a facility to gracefully
transition from a PKC with an existing key to a new PKC 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 PKC-using applications,
should allow for appropriate keys to work before and after the
transition. There are a number of ways to do this; see [CMP] for an
example of one.
3.5.6.2 Key Compromise
In the case of a key compromise, the transition will not be
"graceful" in that there will be an unplanned switch of PKCs 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
PKC is now invalid and shall not be used, and to announce the
validity and availability of the new PKC.
Note: compromise of a private key associated with a Root CA is
catastrophic for users relying on that Root CA. If a Root CA's
private key is compromised, that CA's PKC must be revoked and all
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PKCs subordinate to it must also be revoked. Until such time as the
Root CA has been issued a new PKC and the Root CA issues PKCs to
users relying upon it, users relying on that Root CA are cut off
from the rest of the system, as there is no way to develop a valid
certification path back to a trusted node.
Further, users will likely have to be notified by out-of-band
mechanisms about the change in CA keys. If the old 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. It is possible to have anticipated this
event, and "pre-placed" replacement Root CA keys with all relying
parties, but some secure, out-of-band mechanism will have to be used
to tell users to make the switch, and this will only help if the
replacement key has not been compromised.
Additionally, once the Root CA is brought back up with a new key, it
will likely be necessary to re-issue PKCs, signed with the new key,
to all subordinate users, since their current PKC would be signed
with a now-revoked key.
3.5.7 Cross-certification
A cross-certificate is a PKC issued by one CA to another CA which
contains a public CA key associated with the private CA signature key
used for issuing PKCs. Typically, a cross-certificate is used to
allow client systems orend entities in one administrative domain to
communicate securely with client systems or 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 PKC used by Bob,
which was issued by CA_2. 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, CA_2 does not have to issue a cross-
certificate for CA_1.
3.5.8 Revocation
When a PKC is issued, it is expected to be in use for its entire
validity period. However, various circumstances may cause a PKC 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
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revoke the PKC.
X.509 defines one method of PKC 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 PKCs which is signed by a CA and made freely available in a
public repository. Each revoked PKC is identified in a CRL by its PKC
serial number. When a certificate-using system uses a PKC, that
system not only checks the PKC signature and validity but also
acquires a suitably-recent CRL and checks that the PKC 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. For example, 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 appearing on one regularly scheduled CRL issued beyond the
revoked PKC's validity period. Leaving the revoked PKC on the CRL for
this extra period allows for PKCs that are revoked prior to issuing a
new CRL and whose invalidity date falls before the CRL issuing time
to be accounted for. If the revoked PKC is not retained on the CRL
for this extra period then the possibility arises that a revoked PKC
may never appear on a CRL.
An advantage of the CRL revocation method is that CRLs may be
distributed by exactly the same means as PKCs 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 PKC format, in order to facilitate interoperable
implementations from multiple vendors, the X.509 v2 CRL format needed
to be profiled for Internet use. This was done as part of [FORMAT].
However, PKIX does not require CAs to issue CRLs. On-line methods of
revocation notification may be applicable in some environments as an
alternative to the X.509 CRL. PKIX defines a protocol known as OCSP
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to facilitate on-line checking of the status of PKCs [OCSP].
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
PKC validation impacts of the revocation. However, these methods
impose new security requirements; the PKC validator must trust the
on-line validation service while the repository does not need to be
trusted.
3.5.9 Certificate and Revocation Notice Distribution and Publication
As alluded to in sections 3.1 and 3.5.8 above, the PKI is responsible
for the distribution of PKCs and PKC revocation notices (whether in
CRL form or in some other form) in the system. "Distribution" of PKCs
includes transmission of the PKC to its owner, and may also include
publication of the PKC in a repository. "Distribution" of revocation
notices may involve posting CRLs in a repository, transmitting them
to end-entities, or forwarding them to on-line responders.
3.6 Parts of PKIX
This section identifies the five different areas in which the PKIX
working group has developed documents. The first area involves
profiles of the X.509 v3 PKC standards and the X.509 v2 CRL standards
for the Internet. The second area involves operational protocols, in
which relying parties can obtain information such as PKCs or PKC
status. The third area covers management protocols, in which
different entities in the system exchange information needed for
proper management of the PKI. The fourth area provides information
about certificate policies and certificate practice statements,
covering the areas of PKI security not directly addressed in the rest
of PKIX. The fifth area deals with providing time stamping and data
certification services, which can be used to build such services as
non-repudiation.
3.6.1 Profiles
An X.509 v3 PKC is a very complex data structure. It consists of
basic information fields, plus a number of optional extensions. Many
of the fields and numerous extensions can take on a wide range of
options. This provides an enormous degree of flexibility, which
allows the X.509 v3 PKC 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.509 v3 PKCs, the PKIX
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working group had to develop a profile of the X.509 v3 PKC
specification.
A profile of the X.509 v3 PKC specification is a description of the
contents of the PKC and which extensions must be supported, which
extensions may be supported, and which extensions may not be
supported. [FORMAT] provides such a profile of X.509 v3 PKC for the
Internet PKI. In addition, [FORMAT] suggests ranges of values for
many of the extensions.
[FORMAT] also provides a profile for Version 2 CRLs for use in the
Internet PKI. CRLs, like PKCs, 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 PKC and CRL formats, it is necessary to
define particular Object Identifiers (OIDs) for certain encryption
algorithms, because there are a variety of OIDs registered for some
algorithm suites. Many of the OIDs are defined in [FORMAT] to promote
interoperability. Also, PKIX has produced two documents ([ECDSA] and
[KEA]) which provide guidance on the proper implementation of
specific algorithms.
Some countries are in a process of updating their legal frameworks in
order to regulate and incorporate recognition of signatures in
electronic form. Many of these frameworks introduce certain basic
requirements on PKCs, often termed Qualified Certificates, supporting
these types of "legal" signatures. Partly as a result of this there
is a need for a specific PKC profile providing standardized support
for certain related issues such as a common structure for expressing
unambiguous identities of certified subjects (unmistakable identity).
In December 1998, PKIX adopted as a work item the development of a
refinement of [RFC2459] that further profiles PKIX PKC into qualified
certificates. This work is reflected in [QC].
Like the X.509 v3 PKC, the AC also a very complex data structure
consisting of basic information fields, a number of optional
extensions, and a virtually unlimited number of attributes. Again,
many of the fields, extensions, and attributes can take on a wide
range of options allowing an enormous degree of flexibility. In order
to build an Internet PMI based on ACs, the PKIX working group had to
develop a profile of the AC.
The AC profile is description of the contents of the AC, the allowed
and required extensions, and applicable attributes. [AC] provides
such a profile of the X.509 v2 AC.
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3.6.2 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 [FTPHTTP], [OCSP], [PKI-LDAPv2], and [PKI-
LDAPv3]. A limited protocol to support AC retrieval has also been
document in [LAAP].
[DHPOP] defines a procedure for producing signatures withg the
Diffie-Hellman key agreement algorithm. This signature mechanism was
developed to support PKCS-10 certificate requests.
3.6.3 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
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.
Originally, the PKIX working group developed two documents, [CRMF]
and certificate management message format (CMMF), that together
described the necessary set of message formats, and two other
documents, [CMP] and [CMC], that described protocols for exchanging
those messages. However, the message formats defined in the CMMF
draft were inserted into both [CMP] and [CMC], and thus the (CMMF)
draft has been dropped as a PKIX document.
[CMP-HTTP] and [CMP-TCP] were developed, after some implementation
experience, to update the procedure documented in [CMP] for using CMP
with HTTP and TCP and the transport protocols [CMP].
3.6.4 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 (CP) and
certification practice statement (CPS), and then following those
documents. The CP and CPS should address physical and personnel
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security, subject identification requirements, revocation policy, and
a number of other topics. [POLPROC] provides a framework for
certification practice statements.
3.6.5 Time-Stamp, Data Verification, and Data Certification Services
In late 1998, the PKIX working group began two efforts that were not
in the original working group charter, but were deemed to be
appropriate because they described infrastructure services that could
be used to provide desired security services. The first of these is
time stamping, described in [TSP]. Time stamping is a service in
which a trusted third party - a Time Stamp Authority, or TSA - signs
a message, in order to provide evidence that it existed prior to a
given time. Time stamping provides some support for non-repudiation,
in that a user cannot claim that a transaction was later forged after
compromise of a private key, because the existence of the signed time
stamp indicates that the transaction in question could not have been
created after the indicated time.
[TSP] also defines the role of a Temporal Data Authority, or TDA. A
TDA is a Trusted Third Party (TTP) that creates a temporal data
token. This temporal data token associates a message with a
particular event and provides supplementary evidence for the time
included in the time stamp token. For example, a TDA could associate
the message with the most recent closing value of the Dow Jones
Average. The temporal data with which the message is associated
should be unpredictable in order to prevent forward dating of tokens.
The third iteration of the draft removed support for TDAs as no one
in the WG expressed a requirement for the role.
At the Minneapolis IETF meeting, it was disclosed that the materials
covered in [TSP] draft may be covered by patent(s). Use of the
material covered by the patent(s) in question has not be granted by
the patent holder. Thus, anyone interested in implementing the PKIX
[TSP] draft must be aware of this intellectual property issue.
The second new effort is the definition of a Data Validation and
Certification Server, or DVCS, protocol [DVCS]. A DVCS is a Trusted
Third Party that verifies the correctness of specific data submitted
to it. It also allows the delegation of trustworthy servers and
allows for chaining of verifications.
This services offered by DVCS are different from the TSP service in
that a TSA will not attempt to parse or verify a message sent to it
for certification; instead, it will merely append a reliable
indication of the current time, and sign the resulting string-of-
bits. This offers an indication that the given string-of-bits existed
at a specified time; it does not offer any indication of the
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correctness or relevance of that string of bits. By contrast, the
DVCS certifies possession of data or the validity of another entity's
signature. As part of this, the DVCS verifies the mathematical
correctness of the actual signature value contained in the request
and also checks the full certification path from the signing entity
to a trusted point (e.g., the DVCS's CA, or the Root CA in a
hierarchy).
The DVCS supports non-repudiation in two ways. First, it provides
evidence that a signature or PKC was valid at the time indicated in
the token. The token can be used even after the corresponding PKC
expires and its revocation information is no longer available on CRLs
(for example). Second, the production of a data certification token
in response to a signed request for certification of another
signature or PKC also provides evidence that due diligence was
performed by the requester in validating the signature or PKC.
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 (RFC 2459)
DESCRIPTION: This document describes the profiles to be used for
X.509 v3 PKCs and version 2 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 [FORMAT] to ensure that their systems will be able to
interoperate with other users of the PKI.
[FORMAT] 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 PKCs. 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
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example procedures.
STATUS: Proposed Standard.
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-02.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
PKCs 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: Finished WG Last Call.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure
Representation of Key Exchange Algorithm (KEA) Keys in Internet X.509
Public Key Infrastructure Certificates (RFC 2528)
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
PKCs 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: Informational RFC.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Qualified
Certificates <draft-ietf-pkix-qc-03.txt>
DESCRIPTION: This document profiles the format for and defines
requirements on information content in a specific type of PKCs called
Qualified Certificates. A "Qualified Certificate" is a PKC that is
issued to a natural person (i.e., a living human being); contains an
unmistakable identity based on a real name or a pseudonym of the
subject; exclusively indicates non-repudiation as the key usage for
the certificate's public key; and meets a number of requirements.
STATUS: WG Last Call.
DOCUMENT TITLE: An Internet AttributeCertificate Profile for
Authorizations <draft-ietf-pkix-acx509prof-02.txt>
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DESCRIPTION: This document profiles the format for an defines
requirements on X.509 v2 ACs to support authorization services
required by various Internet protocols (TLS, CMS, and the consumers
of CMS, etc.). Two profiles are defined in support of basic
authorizations and in support of services that can operate via proxy.
STATUS: Under WG review.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
and CRL Profile <draft-ietf-pkix-new-part1-00.txt>
DESCRIPTION: This document is an update of [FORMAT].
STATUS: Under WG review.
DOCUMENT TITLE: A String Representation of General Name <draft-ietf-
pkix-generalname-00.txt>
DESCRIPTION: This document specifies a string format for the ASN.1
construct GeneralName.
STATUS: Under WG review.
4.2 Operational Protocols
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Operational
Protocols - LDAPv2 (RFC 2559)
DESCRIPTION: This document describes the use of LDAPv2 as a protocol
for PKI elements to publish and retrieve certificates and CRLs from a
repository. [LDAPv2] is a protocol that allows publishing and
retrieving of information.
STATUS: Proposed Standard.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure LDAPv2
Schema (RFC 2587)
DESCRIPTION: This document defines a minimal schema necessary to
support the use of LDAPv2 for PKC and CRL retrieval and related
functions for PKIX. This document supplements [LDAPv2] by identifying
the PKIX-related attributes that must be present.
STATUS: Proposed Standard.
DOCUMENT TITLE: X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP (RFC 2560)
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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: Proposed Standard.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Operational
Protocols: FTP and HTTP (RFC 2585)
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.
STATUS: Proposed Standard.
DOCUMENT TITLE: Diffie-Hellman Proof-of-Possession Algorithms <draft-
ietf-pkix-dhpop-02.txt>
DESCRIPTION: This documents describes two signing algorithms using
the Diffie-Hellman key agreement process to provide a shared secret
as the basis of the signature. It allows Diffie-Hellman, a key
agreement algorithm, to be used instead of requiring that the public
key being requested for certification correspond to an algorithm that
is capable of signing and encrypting. The first algorithm generates a
signature for a specific verifier where the signer and recipient have
the same public key parameters. The second algorithm generates a
signature for arbitrary verifiers where the signer and recipient do
not have the same public key parameters.
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STATUS: IETF Last Call.
DOCUMENT TITLE: Limited Attribute Certificate Acquisition Protocol
<draft-ietf-pkix-laap-00.txt>
DESCRIPTION: This document specifies a deliberately limited protocol
for requesting ACs from a server. It is intended to be complementary
to the use of LDAP for AC retrieval, covering those cases where use
of an LDAP server is not suitable due to the type of authorization
model being employed.
STATUS: Under WG review.
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4.3 Management Protocols
DOCUMENT TITLE: Certificate Management Messages over CMS <draft-ietf-
pkix-cmc-05.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 the
certificate request message body specified in the Certificate Request
Message Format [CRMF] documents, as well as a variety of other
certificate management messages. The primary purpose of this
specification is to allow the use of an existing protocol (S/MIME) as
a PKI management protocol, without requiring the development of an
entirely new protocol such as CMP. A secondary purpose is to codify
in IETF standards the current industry practice of using PKCS-10
messages [PKCS10] for certificate requests.
STATUS: IETF Last Call.
DOCUMENT TITLE: Internet X.509 Certificate Request Message Format
(RFC 2511)
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. 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. This document only
specifies the format of a message. Specification of a protocol to
transport that message is beyond the scope of CRMF.
STATUS: Proposed Standard.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
Management Protocols (RFC 2510)
DESCRIPTION: This document specifies a new protocol specifically
developed for the purpose of transporting messages like those
specified in CRMF among PKI elements. In general, CMP will be used in
conjunction with CRMF, and will then be run over a transfer service
(e.g., S/MIME, HTTP) to provide a complete PKI management service.
STATUS: Proposed Standard.
DOCUMENT TITLE: Simple Certificate Validation Protocol (SCVP) <draft-
ietf-pkix-scvp-01.txt>
DESCRIPTION: The SCVP protocol allows a client to offload certificate
handling to a server. The server can give a variety of valuable
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information about the certificate, such as whether or not the
certificate is valid, a chain to a trusted root, and so on.
STATUS: Under WG review.
DOCUMENT TITLE: Using HTTP as a Transport Protocol for CMP <draft-
ietf-pkix-cmp-http-00.txt>
DESCRIPTION: This document describes how to layer [CMP] over [HTTP].
A simple method for doing so is described in [CMP], but that method
does not accommodate a polling mechanism, which may be required in
some environments. This document specifies an alternative method
which uses the polling protocol defined in [CMP]. A new Content-Type
for messages is also defined.
STATUS: Under WG review.
DOCUMENT TITLE: Using TCP as a Transport Protocol for CMP <draft-
ietf-pkix-cmp-tcp-00.txt>
DESCRIPTION: This document describes how to layer Certificate
Management Protocols [CMP] over [TCP]. A method for doing so is
described in [CMP], but that method does not solve problems
encountered by implementors. This document specifies an enhanced
method which extends the protocol.
STATUS: Under WG review.
DOCUMENT TITLE: OCSP Extensions <draft-ietf-pkix-ocspx-00.txt>
DESCRIPTION: This document defines Internet-standard extensions to
OCSP that enable a client to delegate processing of certificate
acceptance functions to a trusted server. The client may control the
degree to which delegation takes place. In addition limited support
is provided for delegating authorization decisions.
STATUS: Under WG review.
DOCUMENT TITLE: Certificate Management Protocols <draft-ietf-pkix-
rfc2510bis-00.txt>
DESCRIPTION: This document is an update of [CMP].
STATUS: Under WG review.
4.4 Policy Outline
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
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Policy and Certification Practices Framework (RFC 2527)
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: Informational RFC.
4.5 Time-Stamp, Data Validation, and Data Certification Services
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Time Stamp
Protocols <draft-ietf-pkix-time-stamp-06.txt>
DESCRIPTION: This document defines the specification for a time stamp
service. It defines a Time Stamp Authority, or TSA, a trusted third
party who maintains a reliable time service. When the TSA receives a
time stamp request, it appends the current time to the request and
signs it into a token to certify that the original request existed
prior to the indicated time. This helps provide non-repudiation by
preventing someone (either a legitimate user or an attacker who has
successfully compromised a key) from "back-dating" a transaction. It
also makes it more difficult to challenge a transaction by asserting
that it has been back-dated. Note that the TSA does not provide any
data parsing service; that is, the TSA does not attempt to validate
that which it signs; it merely regards it as a string of bits whose
meaning is unimportant, but existence is vital.
STATUS: In WG Last.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Data
Certification Server Protocols <draft-ietf-pkix-dcs-04.txt>
DESCRIPTION: This document defines a data validation and
certification service, or DVCS, which can be used to certify both the
existence and correctness of a message or signature. In contrast to
the time stamp service described above, the DVCS certifies possession
of data or the validity of another entity's signature. As part of
this, the DVCS verifies the mathematical correctness of the actual
signature value contained in the request and also checks the full
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certification path from the signing entity to a trusted point (e.g.,
the DVCS's CA, or the Root CA in a hierarchy).
The DVCS supports non-repudiation in two ways. First, it provides
evidence that a signature or public key certificate was valid at the
time indicated in the token. The token can be used even after the
corresponding public key certificate expires and its revocation
information is no longer available on CRLs (for example). Second, the
production of a data certification token in response to a signed
request for certification of another signature or public key
certificate also provides evidence that due diligence was performed
by the requester in validating the signature or public key
certificate.
STATUS: Under WG review.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Technical
Requirements for a non-Repudiation Service <draft-ietf-pkix-
technr-01.txt>
DESCRIPTION: This document describes those features of a service
which processes signed documents which must be present in order for
that service to constitute a "technical non-repudiation" service. A
technical non-repudiation service must permit an independent verifier
to determine whether a given signature was applied to a given data
object by the private key associated with a given valid certificate,
at a time later than the signature. The features of a technical non-
repudiation service are expected to be necessary for a full non-
repudiation service, although they may not be sufficient.
This document is intended to clarify the definition of the "non-
repudiation" service in RFC 2459. It should thus serve as a guide to
when the nonRepudiation bit of the keyUsage extension should be set
and to when a Certificate Authority is required to archive CRL's.
STATUS: Under WG Review.
4.6 Expired Documents
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
Management Message Formats
DESCRIPTION: This document contains the formats for a series of
messages important in certificate and 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.
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STATUS: Work has been discontinued. All useful information from it
has been moved into [CMP] and [CMC].
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: Expired.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Enhanced CRL
Distribution Options (OpenCDP)
DESCRIPTION: This document proposes an alternative to the CRL
Distribution Point (CDP) approach documented in [FORMAT]. OCDP
separates the CRL location function from the process of certificate
and CRL validation, and thus claims some benefits over the CDP
approach.
STATUS: Work has been discontinued, as all useful information has
been incorporated into [X.509]. An updated [FORMAT] RFC should
profile the use of the CDP approach.
DOCUMENT TITLE: Internet Public Key Infrastructure: Caching the
Online Certificate Status Protocol
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: Work has been discontinued.
DOCUMENT TITLE: Basic Event Representation Token <draft-ietf-pkix-
bert1-01.txt>
DESCRIPTION: This document defines a finite method of representing a
discrete instant in time as a referable event. The Basic Event
Representation Token (BERT) is a light-weight binary token designed
for use in large numbers over short periods of time. The tokens
contain only a single instance of an event stamp and the trust
process is referenced against an external reference.
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STATUS: Expired.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Extending
trust in non repudiation tokens in time <draft-ietf-pkix-extend-
trust-non-repudiation-token-00.txt>
DESCRIPTION: This document describes a method to maintain the trust
in a token issued by a non-repudiation Trusted Third Party (NR TTP)
(DVCS/TSA/TDA) after the key initially used to establish trust in the
token expires. The document describes a general format for storage of
DVCS/TS/TDA tokens for this purpose, which establishes a chain of
custody for the data.
STATUS: Expired.
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
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 PKCs
associate public keys with names of entities. Each PKC 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 PKC. There can also be names such as RFC822 e-mail addresses, DNS
domain names, and uniform resource identifiers (URIs) associated with
the key; these attributes are kept in the subjectAltName extension of
the PKC. A PKC 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 PKC.
5.1.1 Name Forms
There are two possible places to put a name in an X.509 v3 PKC. One
is the subject field in the base PKC (often called the "Distinguished
Name" or "DN" field), and the other is in the subjectAltName
extension.
5.1.1.1 Distinguished Names
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According to [FORMAT], a CA's PKC must have a non-null value in the
subject field, while EE's PKCs are permitted to have an empty subject
field. If a PKC has a non-null subject field, it MUST contain an
X.500 Distinguished Name.
5.1.1.2 SubjectAltName Forms
In addition to the DN, a PKIX PKC may have one or more values in the
subjectAltName extension.
The subjectAltName extension allows additional identities to be bound
to the subject of the PKC (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 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.
Whenever such alternate name forms are to be bound into a PKC, the
subjectAltName (or issuerAltName) 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. As another example, some
legacy implementations exist where an RFC822 name is embedded in the
subject distinguished name as an EmailAddress attribute. Per
[FORMAT], PKIX-compliant implementations generating new PKCs with
electronic mail addresses MUST use the rfc822Name in the
subjectAltName extension to describe such EEs. Simultaneous inclusion
of the EmailAddress attribute in the subject distinguished name to
support legacy implementation is deprecated but permitted.
In line with this, if the only subject identity included in a PKC 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 PKCs 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 PKC
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when processing a certification path is not defined by this working
group.
Because the subject's alternative name is considered to be
definitively bound to the public key, all parts of the subject's
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 address is included as an rfc822Name. The format of an rfc822Name
is an "addr-spec" as defined in [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 [DNS]. Note that while upper and lower case 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 " " 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 [IP]. 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 [IP], the octet string must contain
exactly four octets. For IP Version 6, as specified in [IPv6], the
octet string must contain exactly sixteen octets.
5.1.1.2.4 URIs
[FORMAT] notes "When the subjectAltName extension contains a URI, the
name MUST be stored in the uniformResourceIdentifier (an IA5String).
The name MUST be a non-relative URL, and MUST follow the URL syntax
and encoding rules specified in [RFC 1738]. The name must include
both a scheme (e.g., "http" or "ftp") and a scheme-specific-part. The
scheme-specific-part must include a fully qualified domain name or IP
address as the host. As specified in [RFC 1738], the scheme name is
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not case-sensitive (e.g., "http" is equivalent to "HTTP"). The host
part is also not case-sensitive, but other components of the scheme-
specific-part may be case-sensitive. When comparing URIs, conforming
implementations MUST compare the scheme and host without regard to
case, but assume the remainder of the scheme-specific-part is case
sensitive."
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 Top CA is established with DN "O=IETF,
OU=PKIX, CN=PKIX_CA". That CA will then issue PKCs for subjects
subordinate to it. The only requirement, which can be enforced
procedurally, is that no two distinct entities beneath this Top CA
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 Top CA,
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
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be a problem.
5.1.3 Certificate Path Construction
Certificate path construction has been the topic of many discussions
in the WG. The issue centered around how best to get a certificate
when the connection between the subject's name and the name of their
CA, as well as the CA's repository (or directory), may not be
obvious. Many proposals were put forth, including implementing a
global X.500 Directory Service, using DNS SRV records, and using an
extension to point to the directory provider. At the end of the
discussion the group decided to use the authority information access
extension defined in [FORMAT], which allows the CA to indicate the
access method and location of CA information and services. The
extension would be included in subject's certificates and could be
used to associate an Internet style identity for the location of
repository to retrieve the issuer's certificate in cases where such a
location is not related to the issuer's name.
Another discussion related to certificate path construction was where
to store the CA and EE PKCs in the directory (specifically LDAPv2
directories). Two camps emerged with different views on where to
store CA and cross-certificates. In the CA's directory entry, one
camp wanted self-issued PKCs stored in the cACertificate attribute,
PKCs issued to this CA stored in the forward element of the
crossCertificatePair, and PKCs issued from this CA for other CAs in
the reverse element of the crossCertificatePair attribute. The other
camp wanted all CA PKCs stored in the cACertificate attribute, and
PKCs issued to and from another domain stored in the
crossCertificatePair attribute. There was a short discussion that the
second was more efficient than first, and that one solution or the
other was more widely deployed. The end result was to indicate that
self-issued PKCs and PKCs issued to the CA by CAs in the same domain
as the CA are stored in the cACertificate attribute. The
crossCertificatePair attribute's forward element will include all but
self-issued PKCs issued to the CA. The reverse element may include a
subset of PKCs issued by the CA to other CAs. With this resolution
both camp's implementations are supported and are free to chose the
location of CA PKCs to best support their implementation.
5.1.4 Name Constraints
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 PKC?"
That is, [FORMAT] states that:
Subject's alternative names may be constrained in the same manner
as subject distinguished names using the name constraints extension
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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 PKC
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 PKCs - is the
name of the EE PKC considered to be properly subordinate to the name
of the CA?
The answer is "yes". The general rules for deciding whether a PKC
meets name constraints are:
- If a PKC complies with name constraints in any one of its name
forms, then the PKC is deemed to comply with name constraints.
- If a PKC contains a name form that its issuer does not, the PKC
is deemed to comply with name constraints for that name form.
In deciding whether a name form meets name constraints, the following
rules apply (in all cases Name B is the name in the name constraints
extension):
5.1.4.1 rfc822Names
Three variations are allowed:
- If the name constraint is specified as "larry@mail.mit.edu", then
Name A is subordinate to Name B (in B's subtree) if Name A
contains all of Name B's name (specifies a particular mailbox).
For example, larry@mail.mit.edu is subordinate, but
larry_sanders@mail.mit.edu is not.
- If the name constraint is specified as "mail.mit.edu", then Name
A is subordinate to Name B (in B's subtree) if Name A contains
all of Name B's name (specified all mailboxes on host
mail.mit.edu). For example, curly@mail.mit.edu and
mo@mail.mit.edu are subordinate, but mo@mail6.mit.edu and
curly@smtp.mail.mit.edu are not.
- If the name constraint is specified as ".mit.edu", then Name A is
subordinate to Name B (in B's subtree) if Name A contains all of
Name B's name, with the addition of zero or more additional host
or domain names to the left of Name B's name. That is,
smtp.mit.edu is subordinate to .mit.edu, as is pop.mit.edu.
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However, mit.edu is not subordinate to .mit.edu. When the
constraint begins with a "." it specifies any address within a
domain. In the previous example, "mit.edu" is not in the domain
of ".mit.edu".
Note: If rfc822 names are constrained, but the PKC does not contain a
subjectAltName extension, the EmailAddress attribute will be used to
constrain the name in the subject distinguished name. For example (c
is country, o is organization, ou is organizational unit, and em is
EmailAddress), Name A ("c=US, o=MIT, ou=CS, em=curly@mail.mit.edu")
is subordinate to Name B ("c=US, o=MIT, ou=CS") (in B's subtree) if
Name A contains all of Name B's names.
5.1.4.2 dNSNames
Name A is subordinate to Name B (in B's subtree) if Name A contains
all of Name B's name, with the addition of zero or more additional
domain names to the left of Name B's name. That is, www.mit.edu is
subordinate to mit.edu, as is larry.cs.mit.edu. However, www.mit.edu
is not subordinate to web.mit.edu.
5.1.4.3 x.400 addresses
Name A is subordinate to Name B (in B's subtree) if Name A contains
all of Name B's name. For example (c is country-name, admd is
administrative-domain-name, and o is organization-name, ou is
organizational-unit-name, pn is personal-name, sn=surname, and gn is
given-name in both Name A and B), the mnemonic X.400 address (using
PrintableString choices for c and admd) "c=US, admd=AT&T, o=MIT,
ou=cs, pn='sn=Doe,gn=John'" is subordinate to "c=US, admd=AT&T,
o=MIT, ou=cs" and "c=US, admd=AT&T, o=MIT, pn='sn=DOE,gn=JOHN'" (pn
is a SET that includes, among other things, sn and gn).
5.1.4.5 DNs
Name A is subordinate to Name B (in B's subtree), if Name A contains
all of Name B's name, treating attribute values encoded in different
types as different strings, ignoring case in PrintableString (in all
other types case is not ignored), removing leading and trailing white
space in PrintableString, and converting internal substrings of one
or more consecutive white space characters to a single space. For
example, (c is country, o is organization, ou is organizational unit,
and cn is common name):
- Assuming PrintableString types for all attribute values in Name A
and B, "c=US, o=MIT, ou=CS" is subordinate to "c=us, o=MIT,
ou=cs", as is "c=US, o=MIT, ou=CS, ou=administration". Another
example, "c=US, o=MIT, ou=CS, cn= John Doe" (note the white
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spaces) is subordinate to "c=US, o=MIT, ou=CS, cn=John Doe".
- Assuming UTF8String types for all attribute values in Name A and
B, "c=US, o=MIT, ou=CS, ou=administration" is subordinate to
"c=US, o=MIT, ou=CS", but "c=US, o=MIT, ou=cs,
ou=Administration". "c=US, o=MIT, ou=CS, cn= John Smith" (note
the white spaces) is not subordinate to "c=US, o=MIT, ou=CS, cn=
John Smith".
- Assuming UTF8String types for all attribute values in Name A and
PrintableString types for all attribute values in Name B, "c=US,
o=MIT, ou=cs" is subordinate to "c=US, o=MIT, ou=CS" if the
verification software supports the full comparison algorithm in
the X.500 series. "c=US, o=MIT, ou=cs" is NOT subordinate to
"c=US, o=MIT, ou=CS" if the verification software supports the
comparison algorithm in [FORMAT].
5.1.4.6 URIs
The constraints apply only to the host part of the name. Two
variations are allowed:
- If the name constraint is specified as ".mit.edu", then Name A is
subordinate to Name B (in B's subtree) if Name A contains all of
Name B's name, with the addition of zero or more additional host
or domain names to the left of Name B's name. That is,
www.mit.edu is subordinate to .mit.edu, as is curly.cs.mit.edu.
However, mit.edu is not subordinate to .mit.edu. When the
constraint begins with a "." it specifies a host.
- If the name constraint is specified as "abc.mit.edu", then Name A
is subordinate to Name B (in B's subtree) if Name A contains all
of Name B's name. No leftward expansion of the host or domain
name is allowed.
5.1.4.7 iPaddresses
Two variations are allowed depending on the IP version:
- For IPv4 addresses: Name A (128.32.1.1 encoded as 80 20 01 01) is
subordinate to Name B (128.32.1.0/255.255.255.0 encoded as 80 20
00 00 FF FF FF 00) (in B's subtree) if Name A falls within the
net denoted in Name B's CIDR notation.
- For IPv6 addresses: Name A (4224.0.0.0.8.2048.8204.16762 encoded
as 10 80 00 00 00 00 00 00 00 08 08 00 20 0C 41 7A) is
subordinate to Name B (4224.0.0.0.8.2048.8204.0/
65535.65535.65535.65535.65535.65535.65535.0 encoded as 10 80 00
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00 00 00 00 00 00 08 08 00 20 0C 00 00 FF FF FF FF FF FF FF FF FF
FF FF FF FF FF 00 00) (in B's subtree) if Name A falls within the
net denoted in Name B's CIDR notation.
5.1.4.8 Others
As [FORMAT] does not define requirements for the name forms
otherName, ediPartyName, or registeredID there are no corresponding
name constraints requirements.
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 PKCs
would be a useful thing to do, because it would allow a single PKC 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 PKC with a wildcard for the group
would simplify management.
After much discussion, the PKIX working group decided to permit the
use of wildcards in PKCs. That is, it is permissible for a PKIX-
conformant CA to issue a PKC with a wildcard. However, the semantics
of subjectAltName extension that include wildcard characters are not
addressed by PKIX. Applications with specific requirements may use
such names but must define the semantics.
5.1.6 Name Encoding
A very important topic that consumed much of the WG's time was the
implementation of the directory string choices. While the long term
goal of the IETF was clear, use UTF8String, the short term goals were
not so clear. Many implementations only use PrintableString, others
use BMPString, and still others use Latin1String (ISO 8859-1) and tag
it as TeletexString (there are others still). To ensure that there is
consistency with encodings [FORMAT] defines a set of rules for the
string choices. PrintableString was kept as the first choice because
of it's widespread support by vendors. BMPString was the second
choice, also for it's widespread vendor support. Failing support by
PrintableString and BMPString, UTF8String must be used. In keeping
with the IETF mandate, UTF8String can be used at any time if the CA
supports it. Also, processing implementations that wish to support
TeletexString should handle the entire ISO 8859-1 character set and
not just the Latin1String subset.
5.1.7 Uniqueness
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[Add text in here about the QC uniqueness issues.]
5.2 POP
Proof of Possession, or POP, or also CA POP, means that the CA is
adequately convinced that the entity requesting a PKC containing a
public key Y has access to the private key X corresponding to that
public key.
There has been some debate whether POP was or not needed.
This question needs to be addressed separately considering the
context of use of the key, in particular whether a key is used for an
authentication or non repudiation service, or in a confidentiality
service. Note that this does not map to the key usage bit directly,
since it is possible to use a confidentiality key to perform an
authentication service, e.g. by asking to decrypt an encrypted
challenge.
5.2.1 POP for Signing Keys
It is important to provide POP for keys used to sign material, in
order to provide non-repudiation of transactions. For example,
suppose Alice legitimately has private key X and its corresponding
public key Y. Alice has a PKC from Charlie, a CA, containing Y. Alice
uses X to sign a transaction T. Without POP, Mal could also get a PKC
from Charlie containing the same public key, Y. Now without POP,
there are two possible threats: Mal could claim to have been the real
signer of T; or Alice can falsely deny signing T, claiming that it
was instead Mal. Since no one can reliably prove that Mal did or did
not ever possess X, neither of these claims can be refuted, and thus
the service provided by and the confidence in the PKI has been
defeated. (Of course, if Mal really did possess X, Alice's private
key, then no POP mechanism in the world will help, but that is a
different problem.)
Protection can be gained by having Alice, as the true signer of the
transaction, include in the signed information her PKC or an
identifier of her PKC (e.g., a hash of her PKC). This makes
impossible for Mal to claim authorship because he does not know the
private key from Alice and thus is unable to include his certificate
under the signature.
The adequate protection may be obtained in the general case, by
mandating the inclusion of a reference of the certificate every time
a signature (or non repudiation) key is being used in a protocol.
However, because all the protocols were not doing so, a conservative
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measure has been taken by requesting POP to be performed by CAs. It
should thus be understood, that this measure is not strictly
necessary and is only a temporary measure to make old protocols
secure. New protocols or data formats are being developed. If the key
being used is always used in a context where the identifier of the
certificate is included in the protocol, then POP by the CA is not
necessary. The inclusion of the identifier of the certificate in the
protocol or data format may be understood as POP being performed at
the transaction time rather than by the CA, at the registration time
of the user in the PKI.
5.2.2 POP for Key Management Keys
Suppose that Al is a new instructor in the Computer Science
Department of a local University. Al has created a draft final exam
for the Introduction to Networking course he is teaching. He wants to
send a copy of the draft final to Dorothy, the Department Head, for
her review prior to giving the exam. This exam will of course be
encrypted, as several students have access to the computer system.
However, a quick search of the PKC repository (e.g., search the
repository for all records with subjectPublicKey=Dorothy's-value)
turns up the fact that several students have PKCs containing the same
public key management key as Dorothy. At this point, if no POP has
been done by the CA, Al has no way of knowing whether all of the
students have simply created these PKCs without knowing the
corresponding private key (and thus it is safe to send the encrypted
exam to Dorothy), or whether the students have somehow acquired
Dorothy's private key (and thus it is certainly not safe to send the
exam).
The later case may seem unsafe. However, if the other students have
acquired the key, they do not even need to publish their certificates
and can simply decrypt in parallel.
The end story is that, if the key only known to Dorothy, there is no
problem. Thus POP by the CA is not needed.
If the key, like a Diffie-Hellman key, is used for an authentication
service the answer depends from the protocol being used. In the
former example, the decryption was done locally and no data was sent
back on the wire. In an authentication protocol, the story is
different because either some encrypted or decrypted data is sent
back. If the data sent back contains the identifier of the
certificate in a way that it cannot be modified without that
modification being detected, then there is no need for POP. On the
contrary, POP by the CA is needed.
As a conservative measure, POP for encryption keys is recommended,
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but it should be realized that it is not always needed.
In general it should be noticed that POP at the time of the
transaction is much superior than POP made by the CA, since it is
possible in real time to be sure that everything is correct, rather
than rely on that verification to be done at the time of registration
by the CA. Should the CA fail in that verification, then there is a
security breach. On the contrary, doing POP at the time of the
transaction, eliminates that problem.
CMP requires that POP be provided for all keys, either through on-
line or out-of-band means. There are any number of ways to provide
POP, and the choice of which to use is a local matter. Additionally,
a PKC requester can provide POP to either a CA or to an RA, if the RA
can adequately assure the CA that POP has been performed. Some of the
acceptable ways to provide POP include:
- Out-of-band means:
- For keys generated by the CA or an RA (e.g., on a token such as
a smart card or PCMCIA card), possession of the token can
provide adequate proof of possession of the private key.
- For user-generated keys, another approach can be used in
environments where "key recovery" requirements force the
requester to provide a copy of the private key to the CA or an
RA. In this case, the CA will not issue the requested PKC until
such time as the requester has provided the private key. This
approach is in general not recommended, as it is extremely
risky (e.g., the system designers need to figure out a way to
protect the private keys from compromise while they are being
sent to the CA/RA/other authority, and how to protect them
there), but it can be used.
- On-line means:
- For signature keys that are generated by an EE, the requester
of a PKC can be required to sign some piece of data (typically,
the PKC request itself) using the private key. The CA will then
use the requested public key to verify the signature. If the
signature verification works, the CA can safely conclude that
the requester had access to the private key. If the signature
verification process fails, the CA can conclude that the
requester did not have access to the correct private key, and
reject the request.
- For key management keys that are generated by the requester,
the CA can send the user the requested public key, along with
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the CA's own public key, to encrypt some piece of data, and
send it to the requester to be decrypted. For example, the CA
can generate some random challenge, and require some action to
be taken after decryption (e.g., "decrypt this encrypted random
number and send it back to me"). If the requester does not take
the requested action, the CA concludes that the requester did
not possess the private key, and the PKC is not issued.
Another method of providing POP for key management keys is for the CA
to generate the requested PKC, and then send it to the requester in
encrypted form. If the requester does not have access to the
appropriate private key, the requester cannot decrypt the PKC, and
thus cannot use it. After some period of time in which the PKC is not
used, the CA will revoke the PKC. (This only works if the PKC is not
made available to any untrusted entities until after the requester
has successfully decrypted it.)
5.3 Key Usage Bits
The key usage extension defines the purpose (e.g., encipherment,
signature, certificate signing) of the key contained in the PKC. This
extension is used when a key that could be used for more than one
operation is to be restricted. For example, if a CA's RSA key should
be used only for signing CRLS, the cRLSign bit 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.
[FORMAT] includes some text for how the bits in the KeyUsage type are
used. Developing the text for some of the bits was easy; however,
many discussions were needed to arrive at a common agreement on the
meaning of the digitalSignature (DS bit) and nonRepudiation (NR bit)
bits and when they should be set. The group quickly realized that key
usage extension mixes services and mechanisms. The DS bit indicates a
mechanism - a public key used to verify a digital signature. The NR
bit indicates a service - a public key used to verify a digital
signature and to provide a non-repudiation service. When trying to
indicate when each bit should be indicated arguments were based on:
- The lifetime of the object being singed. Some felt that the DS
bit should be set when the certificate is used to sign ephemeral
objects (e.g., bind tokens) while the NR bit should be set for
things that are survive longer (e.g., documents). Of course, the
problem with this distinction to determine how long is the time
period for ephemeral?
- A concious act taken by the signer. Many felt that the NR bit
should be set only when the subject has expressly acknowledged
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that they want to use the private key to sign an object. Signing
a document say where there is a concious decision by the subject
would be appropriate for the key usage extension to contain NR,
but when the key is used for authentication purposes, which can
occur automically and more frequently, the DS bit is more
appropriate. The discussion also concluded that since some
authentication schemes occur automatically, that the DS bit and
NR bit should never be set together in the same certificate. Some
agreed to the differentiation of the bits based on the time, but
did not agree that the two could not be in the same key usage
extension.
- The procedures followed by the CA. Some felt that NR bit was kind
of 'quality mark' indicating to the verifier that the verifier
could be assured that the CA is implementing appropriate
procedures for checking the subject's identity, performing
certificate archival, etc. Other felt that it was not entirely
the CAs job and that some other entity must be involved.
In the end the WG agreed to a few things:
- Provision of the service of non-repudiation requires more than a
single bit set in a PKC. It requires an entire infrastructure of
components to preserve for some period of time the keys, PKCs,
revocation status, signed material, etc., as well as a trusted
source of time. However, the nonRepudiation key usage bit is
provided as an indicator that such keys could be used as a
component of a system providing a non-repudiation service.
- The certificate policy is the appropriate place to indicate the
permitted combinations of key usages. That is, a policy may
indicate that the DS and NR bits can not be set in the same
certificate while another may say that the DS and NR bits can be
set in the same certificate.
[2459bis] includes new text indicating the above agreements.
5.4 Non-Repudiation
The major benefit of the whole DS bit vs NR bit discussion is
development of the Technical Requirements for Non-Repudiation
[TECHNR] draft. To fill this void [TECHNR] was developed to "describe
those features of a service which processes signed documents which
must be present in order for that service to constitute a 'technical
non-repudiation' service." The basic understanding of nonon-
repudiation is that it requires that a digital signature be preserved
in such a manner that it can convince a neutral third party that it
was actually created by someone with access to the private key of a
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certified key pair. Whether this definition of non-repudiation is
enough to form a legally bind agreement is still being debated.
[Add text to describe the different modes.]
5.5 Trust Models
An important design decision is where in the PKI the trust point for
a particular EE should be located (i.e., where is the EE's Root CA) .
There are two extremes: the Top CA or the CA who issues the EE's
certificate. Of course, the trust point for a particular EE can be
anywhere in the PKI, but the following presents the advanatages and
disadvantages of locating the the trust point at these two places.
[Rewrite section]
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
we've missed): Sharon Boeyen, Santosh Chokhani, Warwick Ford, Russ
Housley, Steve Kent, Ambarish Malpani, Matt Fite, Michael Myers, Tim
Polk, Stefan Santesson, Dave Simonetti, Paul Hoffman, Denis Pinkas,
Ed Greck, Tom Gindin, Parag Namjoshi, Peter Sylvester, and Michael
Zolotarev.
7 References
[2459bis] Housley, R., Ford, W., Polk, W., and Solo, D., "Internet
X.509 Public Key Infrastructure Certificate and CRL Profile," <draft-
ietf-pkix-new-part1-00.txt>, 22 October 1999.
[2510bis] Adams, C., Farrell, S., "Internet X.509 Public Key
Infrastructure Certificate Management Protocols", <draft-ietf-pkix-
rfc2510bis-00.txt>, March 2000.
[AC] S. Farrell, R. HousleyMcNeil, M., and Glassey, T., "An Internet
Attribute Certificate Profile for Authorization," <draft-ietf-pkix-
ac509prof-02.txt>, 08 March 2000.
[CMC] Myers, M., Liu, X., Fox, B., and Weinstein, J., "Certificate
Management Messages over CMS," <draft-ieft-pkix-cmc-05.txt>, 14 July
1999.
[CMP] Adams, C., Farrell, S., "Internet X.509 Public Key
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INTERNET DRAFT PKIX Roadmap 22 October 1999
Infrastructure Certificate Management Protocols", RFC 2510, March
1999.
[CMP-HTTP] Tschal"ar, R., Kapoor, A., and Adams, C., "Using HTTP as a
Transport Protocol for CMP", <draft-ietf-pkix-cmp-http-00.txt>,
August 1999.
[CMP-TCP] Tschal"ar, R., Kapoor, A., and Adams, C., "Using TCP as a
Transport Protocol for CMP", <draft-ietf-pkix-cmp-tcp-00.txt>, August
1999.
[INTEROP] Moskowitz, R., "CMP Interoperability Testing: Results and
Agreements", <draft-moskowitz-cmpinterop-00.txt>, June 1999.
[CMS] R. Housley, "Cryptographic Message Syntax," RFC 2630, July
1999.
[CRMF] Myers, M., Adams, C., Solo, D., and Kemp, D., "Internet X.509
Certificate Request Message Format," RFC 2511, March 1999.
[DHPOP] Prafullchandra, H., and Schaad, J., "Diffie-Hellman Proof-of-
Possession Algorithms," <draft-ietf-pkix-dhpop-02.txt>, 1 October
1999.
[DNS] Mockapetris, P.V., "Domain names - concepts and facilities,"
RFC 1034, November 1987.
[DVCS] Adams, C., Sylvester, P., Zolotarev, M., Zuccherato, R.,
"Internet X.509 Public Key Infrastructure Data Certification Server
Protocols", <draft-ietf-pkix-dcs-04.txt>, 08 March 2000.
[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-02.txt>, 22 October 1999.
[ETNPT] Namjoshi, P., "Internet X.509 Public Key Infrastructure
Extending trust in non repudiation tokens in time," <draft-ietf-pkix-
extend-trust-non-repudiation-token-00.txt>, 28 May 1999.
[IP] Postel, J., "Internet Protocol," RFC 791, September 1981.
[IPv6] Deering, S., and Hinden, R., "Internet Protocol, Version 6
[IPv6] Specification," RFC 1883, December 1995.
[FORMAT] Housley, R., Ford, W., Polk, W., and Solo, D., "Internet
X.509 Public Key Infrastructure Certificate and CRL Profile," RFC
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INTERNET DRAFT PKIX Roadmap 22 October 1999
2459, January 1999.
[FTPHTTP] Housley, R., and Hoffman, P., "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP," RFC 2585, 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," RFC 2528,
March 1999.
[LAAP] Farrell, S., Chadwick, C.W., "Limited AttributeCertificate
Acquisition Protocol", <draft-ietf-pkix-laap-00.txt>, 12 Octoboer
1999.
[LDAPv2] Yeong, Y., Howes, T., and Kille, S., "Lightweight Directory
Access Protocol", RFC 1777, March 1995.
[MISPC] Burr, W., Dodson, D., Nazario, N., and Polk, W., "MISPC
Minimum Interoperability Specification for PKI Components, Version
1", <http://csrc.nist.gov/pki/mispc/welcome.html>, 3 September 1997.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and Adams,
C., "X.509 Internet Public Key Infrastructure Online Certificate
Status Protocol - OCSP," RFC 2560, June 1999.
[OCSPX] Myers, M., Ankney, R., Malpani, A., Galperin, S., and Adams,
C., "OCSP Extensions," <draft-ietf-pkix-ocspx-00.txt>, 3 September
1999.
[PEM] Kent, S., "Privacy Enhancement for Internet Electronic Mail:
Part II: Certificate-Based Key Management," RFC 1422, February 1993.
[PKCS10] RSA Laboratories, "The Public-Key Cryptography
Standards(PKCS)," RSA Data Security Inc., Redwood City, California,
November 1993 Release.
[PKI-LDAPv2] Boeyen, S., Howes, T., and Richard, P., "Internet X.509
Public Key Infrastructure Operational Protocols - LDAPv2," RFC 2559,
April 1999.
[PKI-LDAPv3] Chadwick, D.W., "Internet X.509 Public Key
Infrastructure Operational Protocols - LDAPv3," <draft-ietf-pkix-
ldap-v3-01.txt>, August 1999.
[POLPRAC] Chokhani, S., and Ford, W., "Internet X.509 Public Key
Infrastructure Certificate Policy and Certification Practices
Framework," RFC 2527, March 1999.
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[QC] Santesson, S., Polk, W., Barzin, P., and Nystrom, M., "Internet
X.509 Public Key Infrastructure Qualified Certificates", <draft-ietf-
pkix-qc-03.txt>, February 2000.
[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages," RFC 822, August 1982.
[SCHEMA] Boeyen, S., Howes, T., and Richard, P., "Internet X.509
Public Key Infrastructure LDAPv2 Schema," RFC 2587, June 1999.
[SCVP] Malpani, A., Hoffman, P., "Simple Certificate Validation
Protocol (SCVP)," <draft-ietf-pkix-scvp-01.txt>, 9 August 1999.
[SIMONETTI] Simonetti, D., "Re: German Key Usage", posting to ietf-
pkix@imc.org mailing list, 12 August 1998.
[TECHNR] Internet X.509 Public Key Infrastructure Technical
Requirements for a non-Repudiation Service <draft-ietf-pkix-
technr-01.txt>
[TSP] Adams, C., Cain, P., Pinkas, D., and Zuccherato, R., "Internet
X.509 Public Key Infrastructure Time Stamp Protocols", <draft-ietf-
pkix-time-stamp-06.txt>, 02 March 2000.
[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
TBSL
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
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awarsen@missi.ncsc.mil
Sean Turner IECA, Inc. 9010 Edgepark Road Vienna, VA 22182 (703)
628-3180 turners@ieca.com
10 Disclaimer
This work constitutes the opinion of the editors only, and may not
reflect the opinions or policies of their employers.
Expires 10 September, 2000
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