PKIX Working Group A. Arsenault
INTERNET DRAFT DOD
S. Turner
IECA
Expires in six months from September 8, 1999
Internet X.509 Public Key Infrastructure
PKIX Roadmap
<draft-ietf-pkix-roadmap-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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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 PKI. 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,
in hopes of making it easier to build implementations that will
actually interoperate.
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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
references. Section 7 discusses security considerations, and Section
8 provides contact information for the editors.
2 Terminology
There are a number of terms used and misused throughout PKI-related
literature. To limit confusion caused by some of those terms,
throughout this document, we will use the following terms in the
following ways:
- Certification Authority (CA) - an authority trusted by one or
more users to create and assign certificates. Optionally the
certification authority may create the user's keys. It is
important to note that the CA is responsible for the
certificates during their whole lifetime, not just for issuing
them.
- Certificate Policy (CP) - a named set of rules that indicates
the applicability of a certificate to a particular community
and/or class of application with common security requirements.
For example, a particular certificate policy might indicate
applicability of a type of 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 certification authority employs in issuing
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certificates.
- Root CA - a CA that is directly trusted by an end entity; 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.
- Top CA - a CA that is at the top of a PKI hierarchy.
Note: This is often also called a "root CA," from 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 user. Readers new to
PKIX should be aware that these terms are not used consistently
throughout the PKIX documents, as [RFC2459] 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."
- Subordinate CA - A "subordinate CA" is one that is not a root CA
for the end entity in question. Often, a subordinate CA will not
be a Root CA for any entity but this is not mandatory.
- 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 attributes requested in a certificate; and verifying
that the subject has possession of the private key associated
with the public key requested for a certificate.
- End-entity - a subject of a certificate who is not a CA.
- Relying party - a user or agent (e.g., a client or server) who
relies on the data in a certificate in making decisions.
- Subject - a subject is the entity (CA or end-entity) named in a
certificate. Subjects can be human users, computers (as
represented by DNS names or IP addresses), or even software
agents.
3 PKIX Theory
3.1 Certificate-using Systems and PKIs
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At the heart of recent efforts to improve Internet security are a
group of security protocols such as S/MIME, TLS, and IPSec. All of
these protocols rely on public-key cryptography to provide services
such as confidentiality, data integrity, data origin authentication,
and non-repudiation. The purpose of a PKI is to provide trusted and
efficient key and 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 public key certificates, 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 certificate.
A certificate has a limited valid lifetime which is indicated in its
signed contents. Because a certificate's signature and timeliness can
be independently checked by a certificate-using client, certificates
can be distributed via untrusted communications and server systems,
and can be cached in unsecured storage in certificate-using systems.
Certificates are used in the process of validating signed data.
Specifics vary according to which algorithm is used, but the general
process works as follows:
Note: there is no specific order in which the checks listed below
must be made; implementers 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 certificate;
- The recipient validates that no certificate 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 certificates
are within their validity periods at the time the data was
signed;
- The recipient verifies that the data are not claimed to
have any attributes for which the certificate indicates
that the signer is not authorized;
- The recipient verifies that the data have not been altered
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since signing, by using the public key in the certificate.
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].
A collection of certificates, with their issuing CA's, subjects,
relying parties, RA's, and repositories, is referred to as a Public
Key Infrastructure, or PKI.
3.2 PKIX History
[This still needs more work.]
In the beginning there was ITU-T Recommendation X.509. It defines a
widely accepted basis for a public-key infrastructure, including data
formats and procedures related to distribution of public keys via
certificates digitally signed by CAs. X.509 does not however include
a profile to specify the support requirements for many of the
certificate 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
certificates 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
groups that developed these profiles as they are seen as
complimentary.
The development of the management protocols has not been so
straightforward. [CMP] was developed to define a message protocol
that is used between entities in a PKI. The demand for an enrollment
protocol and the desire to use PKCS-10 message format as the
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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 PKCS10 [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 PKCS10
as the certificate request message format. Then, [CMMF] was developed
to define an extended set of management messages that flow between
the components of the Internet PKI. CMMF 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 [CMP]. It also included [PKCS10] as the
certificate request syntax, which caused work on [CRS] to stop.
Information from [CMMF] has been moved into [CMP] and [CMC] so [CMMF]
is being discontinued.
Development of the operational protocols has been slightly more
straightforward. Two documents for LDAPv2 have been developed one for
defining LDAPv2 as an access protocol to repositories [PKI-LDAPv2];
and one for storing PKI information in an LDAP directory [SCHEMA].
Using FTP and HTTP to retrieve certificates and CRL from PKI
repositories was documented without a fight in [FTPHTTP]. Likewise,
methods, headers, and content-types ancillary to HTTP/1.1 to publish
and retrieve X.509 certificates and CRLs was documented in [WEB]
without much argument.
[Need to add text about OpenCDP vs DistributionPoints, Why DCP was
started, information on TSP, and OCSP, and caching OCSP.]
3.3 Overview of the PKIX Approach
PKIX is an effort to develop specifications for a Public Key
Infrastructure for the Internet using X.509 certificates. The PKIX
working group was initially chartered in 1995. A Public Key
Infrastructure, or PKI, is defined as:
The set of hardware, software, people, policies and procedures needed
to create, manage, store, distribute, and revoke certificates based
on public-key cryptography.
A PKI consists of five types of components [MISPC]:
- Certification Authorities (CAs) that issue and revoke
certificates;
- Organizational Registration Authorities (ORAs) that vouch for
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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
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3.4 X.509 certificates
ITU-T X.509 (formerly CCITT X.509) or ISO/IEC/ITU 9594-8, which was
first published in 1988 as part of the X.500 Directory
recommendations, defines a standard certificate format [X.509]. The
certificate format in the 1988 standard is called the version 1 (v1)
format.
When X.500 was revised in 1993, two more fields were added, resulting
in the version 2 (v2) format. These two fields may be used to support
directory access control.
The Internet Privacy Enhanced Mail (PEM) RFCs, published in 1993,
include specifications for a public key infrastructure based on
X.509v1 certificates [PEM]. The experience gained in attempts to
deploy [PEM] made it clear that the v1 and v2 certificate formats are
deficient in several respects. Most importantly, more fields were
needed to carry information which PEM design and implementation
experience has proven necessary. In response to these new
requirements, ISO/IEC/ITU and ANSI X9 developed the X.509 version 3
(v3) certificate format. The v3 format extends the v2 format by
adding provision for additional extension fields. Particular
extension field types may be specified in standards or may be defined
and registered by any organization or community. In June 1996,
standardization of the basic v3 format was completed [X.509].
ISO/IEC/ITU and ANSI X9 have also developed standard extensions for
use in the v3 extensions field [X.509][X9.55]. These extensions can
convey such data as additional subject identification information,
key attribute information, policy information, and certification path
constraints. However, the ISO/IEC/ITU and ANSI X9 standard extensions
are very broad in their applicability. In order to develop
interoperable implementations of X.509 v3 systems for Internet use,
it is necessary to specify a profile for use of the X.509 v3
extensions tailored for the Internet. It is one goal of PKIX to
specify a profile for Internet, electronic mail, and IPsec
applications. Environments with additional requirements may build on
this profile or may 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
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certificate or certificates for that subject. Registration involves
the subject providing its name (e.g., common name, fully-qualified
domain name, IP address), and other attributes to be put in the
certificate, followed by the CA (possibly with help from the RA)
verifying in accordance with its Certfication 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 and/or certificate 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 certificate for a
subject's public key, and returns that certificate to the subject
and/or posts that certificate 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
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.
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3.5.6 Key Update
All key pairs need to be updated regularly (i.e., replaced with a new
key pair) and new certificates issued. This will happen in two cases:
normally, when a key has passed its maximum usable lifetime; and
exceptionally, when a key has been compromised and must be replaced.
3.5.6.1 Key Expiry
In the normal case, a PKI needs to provide a facility to gracefully
transition from a certificate with an existing key to a new
certificate with a new key. This is particularly true when the key to
be updated is that of a CA. Users will know in advance that the key
will expire on a certain date; the PKI, working together with
certificate-using applications, should allow for appropriate keys to
work before and after the transition. There are a number of ways to
do this; see [insert appropriate reference here] for an example of
one.
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 certificates
and keys; users will not have known in advance what was about to
happen. Still, the PKI must support the ability to declare that the
previous certificate is now invalid and shall not be used, and to
announce the validity and availability of the new certificate.
Note: 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 certificate must be revoked
and all certificates subordinate to it must also be revoked. Until
such time as the Root CA has been issued a new certificate and the
Root CA issues certificates 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.
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Additionally, once the Root CA is brought back up with a new key, it
will likely be necessary to re-issue certificates, signed with the
new key, to all subordinate users, since their current certificate
would be signed with a now-revoked key.
3.5.7 Cross-certification
A cross-certificate is a certificate issued by one CA to another CA
which contains a public CA key associated with the private CA
signature key used for issuing certificates. Typically, a cross-
certificate is used to allow client systems/end entities in one
administrative domain to communicate security with client systems/end
users in another administrative domain. Use of a cross-certificate
issued from CA_1 to CA_2 allows user Alice, who trusts CA_1, to
accept a certificate used by Bob, which was issued by CA_2. 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 certificate is issued, it is expected to be in use for its
entire validity period. However, various circumstances may cause a
certificate to become invalid prior to the expiration of the validity
period. Such circumstances include change of name, change of
association between subject and CA (e.g., an employee terminates
employment with an organization), and compromise or suspected
compromise of the corresponding private key. Under such
circumstances, the CA needs to revoke the certificate.
X.509 defines one method of certificate revocation. This method
involves each CA periodically issuing a signed data structure called
a certificate revocation list (CRL). A CRL is a time stamped list
identifying revoked certificates which is signed by a CA and made
freely available in a public repository. Each revoked certificate is
identified in a CRL by its certificate serial number. When a
certificate-using system uses a certificate, that system not only
checks the certificate signature and validity but also acquires a
suitably-recent CRL and checks that the certificate serial number is
not on that CRL. The meaning of "suitably-recent" may vary with local
policy, but it usually means the most recently-issued CRL. A CA
issues a new CRL on a regular periodic basis (e.g., hourly, daily, or
weekly). CA's may also issue CRLs aperiodically; e.g., if an
important key is deemed compromised, the CA may issue a new CRL to
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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 certificate's validity period. [Say why here]
An advantage of the CRL revocation method is that CRLs may be
distributed by exactly the same means as certificates themselves,
namely, via untrusted communications and server systems.
One limitation of the CRL revocation method, using untrusted
communications and servers, is that the time granularity of
revocation is limited to the CRL issue period. For example, if a
revocation is reported now, that revocation will not be reliably
notified to certificate-using systems until the next CRL is issued -
this may be up to one hour, one day, or one week depending on the
frequency that the CA issues CRLs.
As with the X.509 v3 certificate format, in order to facilitate
interoperable implementations from multiple vendors, the X.509 v2 CRL
format 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 [OCSP] to facilitate on-line checking of the
status of certificates.
On-line revocation checking may significantly reduce the latency
between a revocation report and the distribution of the information
to relying parties. Once the CA accepts the report as authentic and
valid, any query to the on-line service will correctly reflect the
certificate validation impacts of the revocation. However, these
methods impose new security requirements; the certificate validator
must trust the on-line validation service while the repository does
not need to be trusted.
3.5.9 Certificate and Revocation Notice Distribution/Publication
As alluded to in sections 3.1 and 3.5.8 above, the PKI is responsible
for the distribution of certificates and certificate revocation
notices (whether in CRL form or in some other form) in the system.
"Distribution" of certificates includes transmission of the
certificate to its owner, and may also include publication of the
certificate in a repository. "Distribution" of revocation notices may
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involve posting CRLs in a repository, transmitting them to end-
entities, and/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 certificate standards and the X.509v2 CRL
standards for the Internet. The second area involves operational
protocols, in which relying parties can obtain information such as
certificates or certificate status. The third area covers management
protocols, in which different entities in the system exchange
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 Profile
An X.509v3 certificate is a very complex data structure. It consists
of basic information fields, plus a number of optional certificate
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.509v3 certificate format to be used
with a wide range of applications in a wide range of environments.
Unfortunately, this same flexibility makes it extremely difficult to
produce independent implementations that will actually interoperate
with one another. In order to build an Internet PKI based on X.509v3
certificates, the PKIX working group had to develop a profile of the
X.509v3 specification.
A profile of the X.509v3 specification is a description of the
contents of the certificate and which certificate extensions must be
supported, which extensions may be supported, and which extensions
may not be supported. [FORMAT] provides such a profile of X.509v3 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 certificates, have a number of optional
extensions. In order to promote interoperability, it is necessary to
constrain the choices an implementor supports.
In addition to profiling the certificate and CRL formats, it is
necessary to define particular Object Identifiers (OIDs) for certain
encryption algorithms, because there are a variety of OIDs registered
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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.
Certain 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 certificates, often termed Qualified Certificates,
supporting these types of "legal" signatures. Partly as a result of
this there is a need for a specific certificate 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 certificates into qualified certificates. This work is
reflected in [QC].
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], and [PKI-LDAPv2].
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 [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 [CMMF] were inserted into both [CMP] and [CMC], and thus
[CMMF] has been dropped as a PKIX document.
3.6.4 Policy Outline
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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
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 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.
At the Minneapolis IETF meeting, it was disclosed that the materials
covered in the Timestamp Internet draft may be covered by patent(s).
Use of the material covered by the patent(s) in question has not be
granted by the patentholder. Thus, anyone interested in implementing
the PKIX Timestamp draft must be aware of this intellectual property
issue.
The second new effort is the definition of a Data Certification
Server, or DCS, protocol [DCS]. A DCS is a Trusted Third Party that
verifies the correctness of specific data submitted to it.
This is different from the TSP service in that a TSA will not attempt
to parse and/or verify a message sent to it for certification;
instead, it will merely append a reliable indication of the current
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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 correctness or relevance of
that string of bits. By contrast, the DCS certifies possession of
data or the validity of another entity's signature. As part of this,
the DCS 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 DCS's CA,
or the Root CA in a hierarchy).
The DCS 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.
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.509v3 certificates 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 certificates.
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The procedures are provided as examples only. Implementations are not
required to use the procedures provided; they may implement whichever
procedures are efficient for their situation. However,
implementations are required to derive the same results as the
example procedures.
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-01.txt>
DESCRIPTION: This document provides Object Identifiers (OIDs) and
other guidance for IPKI users who use the Elliptic Curve Digital
Signature Algorithm (ECDSA). It profiles the format and semantics of
the subjectPublicKeyInfo field and the keyUsage extension in X.509 V3
certificates containing ECDSA keys. This document should be used by
anyone wishing to support ECDSA; others who do not support ECDSA are
not required to comply with it.
STATUS: 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
certificates containing KEA keys. This document should be used by
anyone wishing to support KEA; others who do not support ECDSA are
not required to comply with it.
STATUS: Informational RFC.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Enhanced CRL
Distribution Options (OpenCDP) <draft-ietf-pkix-ocdp-01.txt>
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: Under WG review.
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DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Qualified
Certificates <draft-ietf-pkix-qc-00.txt>
DESCRIPTION: This document profiles the format for and defines
requirements on information content in a specific type of
certificates called Qualified Certificates. A "Qualified Certificate"
is a certificate 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: Under WG review.
DOCUMENT TITLE: An Internet AttributeCertificate Profile for
Authorizations <draft-ietf-pkix-acx509prof-00.txt>
DESCRIPTION: This document profiles the format for an defines
requirements on X.509 Attribute Certificates to support authorization
services required by various Internet protocols (TLS, CMS, and the
consumers of CMS, etc.). Two profiles are defined on that supports
basic authorizations and on the supports proxiable services.
STATUS: Under WG review.
DOCUMENT TITLE: Diffie-Hellman Proof-of-Possesion Algorithms <draft-
ietf-pkix-dhpop-00.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/or 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.
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
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certificate 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 certificate 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 <draft-ietf-pkix-ocsp-08.txt>
DESCRIPTION: This document specifies a protocol useful in determining
the current status of a certificate without the use of CRLs. A major
complaint about certificate-based systems is the need for a relying
party to retrieve a current CRL as part of the certificate validation
process. Depending on the size of the CRL, this can cause severe
problems for bandwidth-challenged devices. Depending on the frequency
of CRL issuance, this can also cause timeliness problems. (E.g., if
CRLs are only published weekly, with no interim releases, a
certificate could actually have been revoked for just short of one
week without it being on the current CRL, and thus improper use of
that certificate could still be occurring.)
OCSP attempts to address those problems. It provides a mechanism
whereby a relying party identifies one or more certificates to an
approved OCSP "responder", and the responder sends back the current
status of the certificate(s) - e.g., "revoked", "notRevoked",
"unknown". This can dramatically reduce the bandwidth required to
transmit revocation status - a relying party does not have to
retrieve a CRL of many entries to check the status of one
certificate. It can (although it is not guaranteed to) improve the
timeliness of revocation notification, and thus reduce the window of
opportunity for someone trying to use a revoked certificate.
STATUS: Approved as Proposed Standard.
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
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relying party's end system. This document describes how to support
OCSP caching at intermediary servers.
STATUS: Has been discontinued.
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: 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: Has been discontinued.
4.3 Management Protocols
DOCUMENT TITLE: Certificate Management Messages over CMS <draft-
ietf-pkix-cmc-04.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: Under WG review.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
Management Message Formats
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DESCRIPTION: This document contains the formats for a series of
messages important in certificate/PKI management. These messages let
CA's, RA's, and relying parties communicate with each other. Note
that this document only specifies message formats; it does not
specify a protocol for transferring messages. That protocol can be
either CMP or CMC, or perhaps another custom protocol.
STATUS: Has been discontinued, as all useful information from it has
been moved into [CMP] and [CMC].
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.
4.4 Policy Outline
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Certificate
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
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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 and Data Certification Services
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Time Stamp
Protocols <draft-ietf-pkix-time-stamp-01.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: Under WG review.
DOCUMENT TITLE: Internet X.509 Public Key Infrastructure Data
Certification Server Protocols <draft-ietf-pkix-dcs-00.txt>
DESCRIPTION: This document defines a data certification service, or
DCS, 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 DCS certifies possession of data or the validity
of another entity's signature. As part of this, the DCS 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 DCS's CA, or the Root CA
in a hierarchy).
The DCS 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
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by the requester in validating the signature or public key
certificate.
STATUS: Under WG review.
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.
STATUS: Under WG review.
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)
(DCS/TSA/TDA) after the key initially used to establish trust in the
token expires. The document describes a general format for storage of
DCS/TS/TDA tokens for this purpose, which establishes a chain of
custody for the data.
STATUS: Under WG review.
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
certificates associate public keys with names of entities. Each
certificate contains at least one name for the owner of a particular
public key. The name can be an X.500 distinguished name, contained in
the subjectDN field of the certificate. There can also be names such
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as RFC822 e-mail addresses, DNS domain names, and URIs associated
with the key; these attributes are kept in the subjectAltName
extension of the certificate. A certificate must contain at least one
of these name forms, it may contain multiple forms if deemed
appropriate by the CA based on the intended usage of the certificate.
5.1.1 Name Forms
There are two possible places to put a name in an X.509v3
certificate. One is the subject field in the base certificate (often
called the "Distinguished Name" or "DN" field), and the other is in
the subjectAltName extension.
5.1.1.1 Distinguished Names
According to [FORMAT], a PKIX certificate must have a non-null value
in the Subject field, except for an end-entity certificate, which is
permitted to have an empty subject field. Furthermore, if a
certificate 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 certificate may have one or more values
in the subjectAltName extension.
The subjectAltName extension allows additional identities to be bound
to the subject of the certificate - e.g., it allows "umbc.edu" and
"130.85.1.3" to be associated with a particular subject, as well as
"C=US, O=University of Maryland, L=Baltimore, CN=UMBC". X.509-defined
options for this extension include: Internet electronic mail
addresses; DNS names; IP addresses; and uniform resource indentifiers
(URIs). Other options can exist, including locally-defined name
forms.
A single subjectAltName extension can include multiple name forms,
and multiple instances of each name form.
Whenever such Alternate Name forms are to be bound into a
certificate, the subject alternative name (or issuer alternative
name) extension must be used. It is technically possible to embed an
Alternate Name Form in the subject field. For example, one could make
a DN contain an IP address via a kludge such as "C=US, L=Baltimore,
CN=130.85.1.3". However, this usage is deprecated; the alternative
name extension is the preferred location for finding such
information. 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
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implementations generating new certificates with electronic mail
addresses MUST use the rfc822Name in the subject alternative name
field to describe such entities. 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
certificate is an alternative name form, then the subject
distinguished name must be empty (technically, an empty sequence),
and the subjectAltName extension must be present. If the subject
field contains an empty sequence, the subjectAltName extension must
be marked critical.
If the subjectAltName extension is present, the sequence must contain
at least one entry. Unlike the subject field, conforming CAs shall
not issue certificates with subjectAltNames containing empty
GeneralName fields. For example, an rfc822Name is represented as an
IA5String. While an empty string is a valid IA5String, such an
rfc822Name is not permitted by PKIX. The behavior of clients that
encounter such a certificate when processing a certification path is
not defined by this working group.
Because the subject alternative name is considered to be definitively
bound to the public key, all parts of the subject alternative name
must be verified by the CA.
5.1.1.2.1 Internet e-mail addresses
When the subjectAltName extension contains an Internet mail address,
the adress is included as an rfc822Name. The format of an rfc822Name
is an "addr-spec" as defined in [RFC-822]. 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 signifigance is attached to the case. In
addition, while the string " " is a legal domain name, subjectAltName
extensions with a dNSName " " are not permitted. Finally, the use of
the DNS representation for Internet mail addresses (wpolk.nist.gov
instead of wpolk@nist.gov) is not permitted; such identities are to
be encoded as rfc822Name.
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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 [RFC1883].
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
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 certificates for users
subordinate to it. The only requirement - and this can be enforced
procedurally - is that no two distinct entities beneath this Top CA
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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.ieft.org, is irrelevant and causes no interoperability
problems - those are different domains. However, if there were to be
another node on the Internet with domain name www.ietf.org, then
there would be a problem due to name duplication.
The same applies for IP addresses. As long as only one node on the
Internet responds to the IP address 130.85.1.3, there is no problem,
despite the fact that there are 100 different corporate Intranets,
each using that same IP address. However, if two different nodes on
the Internet each responded to the IP address 130.85.1.3, there would
be a problem.
5.1.3 Certificate Path Construction
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
attribute 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 end-entity certificates 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 certificates stored in
the cACertificate attribute, certificates issued to this CA stored in
the forward element of the crossCertificatePair, and certificates
issued from this CA for other CAs in the reverse element of the
crossCertificatePair attribute. The other camp wanted all CA
certificates stored in the cACertificate attribute, and certificates
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issued to/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
certificates and certificates 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 certificates issued to the CA. The reverse element may
include a subset of certificates 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 certificates 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
certificate?" That is, [FORMAT] states that:
Subject alternative names may be constrained in the same manner as
subject distinguished names using the name constraints extension
as described in section 4.2.1.11.
What does this mean? Suppose that there is a CA with DN "O=IETF,
OU=PKIX, CN=PKIX_CA", with the subjectAltName extension having
dNSName "PKIX_CA.IETF.ORG". Suppose that that CA has issued a
certificate with an empty DN, with subjectAltName extension having
dNSName set to "PKIX_CA.IETF.ORG", and rfc822Name set to
Steve@PKIX_CA.IETF.ORG. The question is, are name constraints
enforced on these two certificates - is the name of the end-entity
certificate considered to be properly subordinate to the name of the
CA?
The answer is "yes". The general rules for deciding whether a
certificate meets name constraints are:
- If a certificate complies with name constraints in any one of
its name forms, then the certificate is deemed to comply with
name constraints.
- If a certificate contains a name form that its issuer does
not, the certificate is deemed to comply with name constraints
for that name form.
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
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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. 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 certificate does not
contain a subject alternative name, 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
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all of Name B's name. For example (c is country-name, admd is
administrative-domain-name, and o is orgnaization-name, ou is
organizational-unit-name, pn is personal-name, sn=surname, and gn is
given-name in both Name A and B), the mneumonic 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 PrinatString 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=adminstration". Another
example, "c=US, o=MIT, ou=CS, cn= John Doe" (note the white
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=Adminstration". "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
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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
conatins 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 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
certificates would be a useful thing to do, because it would allow a
single certificate to be used by all members of a group. These
members would presumably have attributes in common; e.g., access
rights to some set of resources, and so issuance of a certificate
with a wildcard for the group would simplify management.
After much discussion, the PKIX working group decided to permit the
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use of wildcards in certificates. That is, it is permissible for a
PKIX-conformant CA to issue a certificate with a wildcard. However,
the semantics of subject alternative names that include wildcard
characters are not addressed by PKIX. Applications with specific
requirements may use such names but must define the semantics.
5.1.6 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.
5.2 POP
Proof of Possession, or POP, means that the CA is adequately
convinced that the entity requesting a certificate containing a
public key Y has access to the private key X corresponding to that
public key.
POP is important because it provides an appropriate level of
assurance in the correct operation of the PKI as a whole. At its
lowest level, POP counters the "self-inflicted denial of service";
that is, an entity voluntarily getting a certificate that cannot be
used to sign or encrypt/decrypt information. However, as the
following two examples demonstrate, POP also counters less direct,
but more severe, threats.
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 certificate from Charlie, a CA, containing
Y. Alice uses X to sign a transaction T. Without POP, Mal could also
get a certificate from Charlie containing the same public key, Y.
Now, there are two possible threats: Mal could claim to have been the
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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.)
One level of protection can be gained by having Alice, as the true
signer of the transaction, include in the signed information her
certificate or an identifier of her certificate (e.g., a hash of her
certificate). This might make it more difficult for Mal to claim
authorship - he would have to assert that he incorrectly included
Alice's certificate, rather than his own. However, it would not stop
Alice from falsely repudiating her actions. Since the certificate
itself is a public item, Mal indeed could have inserted Alice's
certificate into the signed transaction, and thus its presence does
not indicate that Alice was the one who participated in the now-
repudiated transaction. The only reliable way to stop this attack is
to require that Mal prove he possesses X before his certificate is
issued.
For signing keys used only for authentication, and not for non-
repudiation, the threat is lower because users may not care about
Alice's after-the-fact repudiation, and thus POP becomes less
important. However, POP SHOULD still be done wherever feasible in
this environment, by either off-line or on-line means.
5.2.2 POP for Key Management Keys
Similarly, POP for key management keys (that is, keys used for either
key agreement or key exchange) can help to prevent undermining
confidence in the PKI. 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 certificate
repository (e.g., search the repository for all records with
subjectPublicKey=Dorothy's-value) turns up the fact that several
students have certificates 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 certificates 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). Thus, the service to be
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provided by the PKI - allowing users to communicate with one another,
with confidence in who they are communicating with - has been totally
defeated. If the CA is providing POP, then either no students will
have such certificates, or Al can know with certainty that the
students do indeed know Dorothy's private key, and act accordingly.
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 certificate 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
certificate 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 end-entity, the
requester of a certificate can be required to sign some
piece of data (typically, the certificate 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 the CA's own private key, to encrypt some piece of
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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 certificate is not issued.
Another method of providing POP for key management keys is for the CA
to generate the requested certificate, 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
certificate, and thus cannot use it. After some period of time in
which the certificate is not used, the CA will revoke the
certificate. (This only works if the certificate 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
certificate. This extension is used when a key that could be used for
more than one operation is to be restricted. For example, when an RSA
key should be used only for signing, the digitalSignature and/or
nonRepudiation bits would be asserted. Likewise, when an RSA key
should be used only for key management, the keyEncipherment bit would
be asserted. When used, this extension should be marked critical.
The eight bits defined for this extension identify seven mechanisms
and one service, namely:
- digitalSignature
- nonRepudiation
- keyEncipherment
- dataEncipherment
- keyAgreement
- keyCertSign
- cRLSign
- encipherOnly
- decipherOnly
According to [FORMAT], bits in the KeyUsage type are used as follows:
- The digitalSignature bit is asserted when the subject public key is
used to verify digital signatures that have purposes other than
non-repudiation, certificate signature, and CRL signature. For
example, the digitalSignature bit is asserted when the subject
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public key is used to provide authentication via the signing of
ephemeral data.
- The nonRepudiation bit is asserted when the subject public key is
used to verify digital signatures used to provide a non-repudiation
service which protects against the signing entity falsely denying
some action, excluding certificate or CRL signing.
- The keyEncipherment bit is asserted when the subject public key is
used for key transport. For example, when an RSA key is to be used
for key management, this bit must asserted.
- The dataEncipherment bit is asserted when the subject public key is
used for enciphering user data, other than cryptographic keys.
- The keyAgreement bit is asserted when the subject public key is
used for key agreement. For example, when a Diffie-Hellman key is
to be used for key management, this bit must asserted.
- The keyCertSign bit is asserted when the subject public key is used
for verifying a signature on certificates. This bit may only be
asserted in CA certificates.
- The cRLSign bit is asserted when the subject public key is used for
verifying a signature on revocation information (e.g., a CRL).
- The meaning of the encipherOnly bit is undefined in the absence of
the keyAgreement bit. When the encipherOnly bit is asserted and the
keyAgreement bit is also set, the subject public key may be used
only for enciphering data while performing key agreement.
- The meaning of the decipherOnly bit is undefined in the absence of
the keyAgreement bit. When the decipherOnly bit is asserted and the
keyAgreement bit is also set, the subject public key may be used
only for deciphering data while performing key agreement.
PKIX does not restrict the combinations of bits that may be set in an
instantiation of the keyUsage extension.
The discussion on the PKIX mailing list has centered on the
digitalSignature bit and the nonRepudiation bit. The question has
come down to something like: When support for the service of non-
repudiation is desired, should both the digitalSignature and
nonRepudiation bits be set, or just the nonRepudiation bit?
(It is noted that provision of the service of non-repudiation
requires more than a single bit set in a certificate. It requires an
entire infrastructure of components to preserve for some period of
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time the keys, certificates, 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 should not be used as a component of a system providing a non-
repudiation service.)
According to [SIMONETTI], the intent is that the digitalSignature bit
should be set when what is desired is the ability to sign ephemeral
transactions; e.g., for a single session authentication. These
transactions are "ephemeral" in the sense that they are important
only while they are in existence; after the session is terminated,
there is no long-term record of the digital signature and its
properties kept. When something is intended to be kept for some
period of time, the nonRepudiation bit should be set. This generally
implies that an application will digitally sign something; thus, some
implementors turn on the digitalSignature bit as well. Other
implementors, however, keep the two bits mutually exclusive, to
prevent a single key from being used for both ephemeral and long-term
signing.
While [FORMAT] is silent on this specific issue, the working group's
general conclusion is that a certificate may have either or both bits
set. If only the nonRepudiation bit is set, the key should not be
used for ephemeral transactions. If only the digitalSignature bit is
set, the key should not be used for long-term signing. If both bits
are set, the key may be used for either purpose.
To actually enforce this requires that an application understands
whether it is signing ephemeral transactions or for the long-term.
The application developers will have to understand the difference,
and set up their checks on the key usage bits field accordingly. For
example, TLS implementors should check only the digitalSignature bit,
and ignore the nonRepudiation bit. S/MIME implementors, though, will
have a difficult choice to make, since their application could be
used for either purpose, and they will generally accept signing using
keys associated with certificates having either or both bits being
turned on. Certification Authorities should know what applications
they are providing certificates for, and provide certificates
according to the requirements of those applications. If CA's are tied
into non-repudiation systems, they may treat certificates differently
when the nonRepudiation bit is turned on (e.g., store information
associated with the certificate - like the user's identification
provided during certificate registration, or certificate generation
date/time stamps - for longer periods of time, in more secure
environments).
The bottom line is that this is an area where PKI implementors are
somewhat limited in what they can do. The onus is on developers of
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certificate-using systems to understand their requirements and
request certificates with the appropriate bits set.
5.4 Trust Models
(This section will describe the various trust models that PKIX can
support. It is important to note that PKIX is bound to neither a pure
hierarchical model a la PEM, nor a web of trust model a la PGP. PKIX
can support either of those models, or any flavor in between. The
implications of different trust models should be described:
- efficiency of revocation - certification path building - etc.)
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, and.
7 References
[BERT1] McNeil, M., and Glassey, T., "Basic Event Representation
Token," <draft-ietf-pkix-bert1-01.txt>, May 1999.
[CACHE] "Internet Public Key Infrastructure: Caching the Online
Certificate Status Protocol," <draft-ieft-pkix-ocsp-caching-00.txt>,
April 1998.
[CMC] Myers, M., Liu, X., Fox, B., and Weinstein, J., "Certificate
Management Messages over CMS," <draft-ieft-pkix-cmc-04.txt>, May
1999.
[CMP] Adams, C., Farrell, S., "Internet X.509 Public Key
Infrastructure Certificate Management Protocols", RFC 2510, March
1999.
[CMS] R. Housley, "Cryptographic Message Syntax," <draft-ietf-smime-
cms-13.txt>, April 1999.
[CRMF] Myers, M., Adams, C., Solo, D., and Kemp, D., "Internet X.509
Certificate Request Message Format," RFC 2511, March 1999.
[DCS] Adams, C., and Zuccherato, R., "Internet X.509 Public Key
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Infrastructure Data Certification Server Protocols", <draft-ietf-
pkix-dcs-00.txt>, 23 September 1998.
[DHPOP] Prafullchandra, H., and Schaad, J., "Diffie-Hellman Proof-
of-Possession Algorithms," <draft-ietf-pkix-dhpop-00.txt>, February
1999.
[DNS] Mockapetris, P.V., "Domain names - concepts and facilities,"
RFC 1034, November 1987.
[ECDSA] Bassham, L., Johnson, D., and Polk, W., "Internet x.509
Public Key Infrastructure: Representation of Elliptic Curve Digital
Signature Algorithm (ECDSA) Keys and Signatures in Internet X.509
Public Key Infrastructure Certificates," <draft-ietf-pkix-ipki-
ecdsa-01.txt>, November 1997.
[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>, 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
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.
[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> September 3, 1997.
[OCDP] Hallam-Baker, P., and Ford, W., "Internet X.509 Public Key
Infrastructure Enhanced CRL Distribution Options (OpenCDP)," <draft-
ietf-pkix-ocdp-01.txt>, August 7, 1998.
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[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and Adams,
C., "X.509 Internet Public Key Infrastructure Online Certificate
Status Protocol - OCSP," <draft-ietf-pkix-ocsp-08.txt>, March 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.
[POLPRAC] Chokhani, S., and Ford, W., "Internet X.509 Public Key
Infrastructure Certificate Policy and Certification Practices
Framework," RFC 2527, March 1999.
[QC] Santesson, S., Polk, W., and Gloeckner, P., "Internet X.509
Public Key Infrastructure Qualified Certificates", <draft-ietf-pkix-
qc-00.txt>, 3 February 1999.
[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.
[SIMONETTI] Simonetti, D., "Re: German Key Usage", posting to ietf-
pkix@imc.org mailing list, 12 August 1998.
[TSP] Adams, C., Cain, P., Pinkas, D., and Zuccherato, R., "Internet
X.509 Public Key Infrastructure Time Stamp Protocols", <draft-ietf-
pkix-time-stamp-02.txt>, May 1999.
[WEB] Reddy, S., "WEB based Certificate Access Protocol--
WebCAP/1.0," <draft-ietf-pkix-webcap-00.txt>, April 19, 1998.
[X.509] ITU-T Recommendation X.509 (1997 E): Information Technology -
Open Systems Interconnection - The Directory: Authentication
Framework, June 1997.
[X9.42] ANSI X9.42-199x, Public Key Cryptography for The Financial
Services Industry: Agreement of Symmetric Algorithm Keys Using
Diffie-Hellman (Working Draft), December 1997.
[X9.55] ANSI X9.55-1995, Public Key Cryptography For The Financial
Arsenault & Turner [Page 41]
INTERNET DRAFT PKIX Roadmap 08 September 1999
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
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.
Arsenault & Turner [Page 42]
