PKIX Working Group R. Housley (SPYRUS)
Internet Draft W. Ford (Nortel)
D. Solo (BBN)
expires in six months June 1996
Internet Public Key Infrastructure
Part I: X.509 Certificate and CRL Profile
<draft-ietf-pkix-ipki-part1-02.txt>
Status of this Memo
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Abstract
This is the second draft of the Internet Public Key Infrastructure
X.509 Certificate and CRL Profile. Since the first version was
distributed, ISO has completed work on X.509 Version 3 Certificates
and X.509 Version 2 Certificate Revocation Lists (CRLs). Many of the
Internet community requirements that were in the previous version of
this document have been included in the final ISO document. As a
result, this document has gotten simpler. Please send comments on
this document to the ietf-pkix@tandem.com mail list.
1 Executive Summary
<< Write this last. >>
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2 Requirements and Assumptions
Goal is to develop a profile and associated management structure to
facilitate the adoption/use of X.509 certificates within Internet
applications for those communities wishing to make use of X.509
technology. Such applications may include WWW, electronic mail, user
authentication, electronic payment systems, IPSEC, as well as others.
In order to relieve some of the obstacles to using X.509
certificates, this document defines a profile to promote the
development of certificate management systems; development of
application tools; and interoperability determined by policy, as
opposed to syntax.
Some communities will need to supplement, or possibly replace, this
profile in order to meet the requirements of specialized application
domains or environments with additional authorization, assurance, or
operational requirements. However, for basic applications, common
representations of frequently used attributes are defined so that
application developers can obtain necessary information without
regard to the issuer of a particular certificate or certificate
revocation list (CRL).
As supplemental authorization and attribute management tools emerge,
such as attribute certificates, it may be appropriate to limit the
authenticated attributes that are included in a certificate. These
other management tools may be more appropriate method of conveying
many authenticated attributes.
2.1 Communication and Topology
The users of certificates will operate in a wide range of
environments with respect to their communication topology, especially
users of secure electronic mail. This profile supports users without
high bandwidth, real-time IP connectivity, or high connection
availablity. In addition, the profile allows for the presence of
firewall or other filtered communication.
2.2 Acceptability Criteria
The goal of the Internet Public Key Infrstructure (PKI) is to meet
the needs of deterministic, automated identification, authentication,
access control, and authorization functions. Support for these
services determines the attributes contained in the certificate as
well as the ancillary control information in the certificate such as
policy data and certification path constraints.
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2.3 User Expectations
Users of the Internet PKI are people and processes who use client
software and are the subjects named in certificates. These uses
include readers and writers of electronic mail, the clients for WWW
browsers, and the key manager for IPSEC within a router. This
profile recognizes the limitations of the platforms these users
employ and the sophistication/attentiveness of the users themselves.
This manifests itself in minimal user configuration responsibility
(e.g., root keys, rules), explicit platform usage constraints within
the certificate, certification path constraints which shield the user
from many malicious actions, and applications which sensibly automate
validation functions.
2.4 Administrator Expectations
As with users, the Internet PKI profile is structured to support the
individuals who generally operate Certification Authorities (CAs).
Providing administrators with unbounded choices increases the chances
that a subtle CA administrator mistake will result in broad
compromise. Also, unbounded choices greatly complicates the software
that must process and validate the certificates created by the CA.
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3 Overview of Approach
Following is a simplified view of the architectural model assumed by
the PKIX specifications.
+---+
| C | +------------+
| e | <-------------------->| End entity |
| r | Operational +------------+
| t | transactions ^
| | and management | Management
| / | transactions | transactions
| | |
| C | PKI users v
| R | -------+-------+--------+------
| L | PKI management ^ ^
| | entities | |
| | v |
| R | +------+ |
| e | <-------------- | RA | <-----+ |
| p | certificate | | | |
| o | publish +------+ | |
| s | | |
| i | v v
| t | +------------+
| o | <--------------------------| CA |
| r | certificate publish +------------+
| y | CRL publish ^
| | |
+---+ | Management
| transactions
v
+------+
| CA |
+------+
Figure 1 - PKI Entities
The components in this model are:
end entity: user of PKI certificates and/or end user system that
the PKI certifies;
CA: certification authority;
RA: registration authority, i.e., an optional system to
which a CA delegates certain manaagement functions;
repository: a system or collection of distributed systems that
store certificates and CRLs and serves as a means of
distributing these certificates and CRLs to end
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entities.
3.1 X.509 Version 3 Certificate
Application of public key technology requires the user of a public
key to be confident that the public key belongs to the correct remote
subject (person or system) with which an encryption or digital
signature mechanism will be used. This confidence is obtained
through the use of public key certificates, which are data structures
that bind public key values to subject identities. The binding is
achieved by having a trusted certification authority (CA) digitally
sign each certificate. A certificate has a limited valid lifetime
which is indicated in its signed contents. Because a certificate's
signature and timeliness can be independently checked by a
certificate-using client, certificates can be distributed via
untrusted communications and server systems, and can be cached in
unsecured storage in certificate-using systems.
The standard known as ITU-T X.509 (formerly CCITT X.509) or ISO/IEC
9594-8, which was first published in 1988 as part of the X.500
Directory recommendations, defines a standard certificate format. 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 are used to
support directory access control.
The Internet Privacy Enhanced Mail (PEM) proposals, published in
1993, include specifications for a public key infrastructure based on
X.509 v1 certificates [RFC 1422]. The experience gained in attempts
to deploy RFC 1422 made it clear that the v1 and v2 certificate
formats are deficient in several respects. Most importantly, more
fields were needed to carry information which PEM design and
implementation experience has proven necessary. In response to these
new requirements, ISO/IEC 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-AM].
ISO/IEC and ANSI X9 have also developed a set of standard extensions
for use in the v3 extensions field [X.509-AM][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 and ANSI standard extensions are very broad in
their applicability. In order to develop interoperable
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implementations of X.509 v3 systems for Internet use, it is necessary
to specify a profile for use of the X.509 v3 extensions tailored for
the Internet. It is one goal of this document to specify a profile
for Internet WWW, electronic mail, and IPSEC applications.
Environments with additional requirements may build on this profile
or may replace it.
3.2 Certification Paths and Trust
A user of a security service requiring knowledge of a public key
generally needs to obtain and validate a certificate containing the
required public key. If the public-key user does not already hold an
assured copy of the public key of the CA that signed the certificate,
then it might need an additional certificate to obtain that public
key. In general, a chain of multiple certificates may be needed,
comprising a certificate of the public key owner (the end entity)
signed by one CA, and zero or more additional certificates of CAs
signed by other CAs. Such chains, called certification paths, are
required because a public key user is only initialized with a limited
number (often one) of assured CA public keys.
There are different ways in which CAs might be configured in order
for public key users to be able to find certification paths. For
PEM, RFC 1422 defined a rigid hierarchical structure of CAs. There
are three types of PEM certification authority:
(a) Internet Policy Registration Authority (IPRA): This authority,
operated under the auspices of the Internet Society, acts as the root
of the PEM certification hierarchy at level 1. It issues
certificates only for the next level of authorities, PCAs. All
certification paths start with the IPRA.
(b) Policy Certification Authorities (PCAs): PCAs are at level 2 of
the hierarchy, each PCA being certified by the IPRA. A PCA must
establish and publish a statement of its policy with respect to
certifying users or subordinate certification authorities. Distinct
PCAs aim to satisfy different user needs. For example, one PCA (an
organizational PCA) might support the general electronic mail needs
of commercial organizations, and another PCA (a high-assurance PCA)
might have a more stringent policy designed for satisfying legally
binding signature requirements.
(c) Certification Authorities (CAs): CAs are at level 3 of the
hierarchy and can also be at lower levels. Those at level 3 are
certified by PCAs. CAs represent, for example, particular
organizations, particular organizational units (e.g., departments,
groups, sections), or particular geographical areas.
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RFC 1422 furthermore has a name subordination rule which requires
that a CA can only issue certificates for entities whose names are
subordinate (in the X.500 naming tree) to the name of the CA itself.
The trust associated with a PEM certification path is implied by the
PCA name. The name subordination rule ensures that CAs below the PCA
are sensibly constrained as to the set of subordinate entities they
can certify (e.g., a CA for an organization can only certify entities
in that organization's name tree). Certificate user systems are able
to mechanically check that the name subordination rule has been
followed.
The RFC 1422 CA hierarchical model has been found to have several
deficiencies, including:
(a) The pure top-down hierarchy, with all ertification paths
starting from the root, is too restrictive for many purposes. For
some applications, verification of certification paths should start
with a public key of a CA in a user's own domain, rather than
mandating that verification commence at the top of a hierarchy. In
many environments, the local domain is often the most trusted. Also,
initialization and key-pair-update operations can be more effectively
conducted between an end entity and a local management system.
(b) The name subordination rule introduces undesirable constraints
upon the X.500 naming system an organization may use.
(c) Use of the PCA concept requires knowledge of individual PCAs to
be built into certificate chain verification logic. In the
particular case of Internet mail, this is not a major problem -- the
PCA name can always be displayed to the human user who can make a
decision as to what trust to imply from a particular chain. However,
in many commercial applications, such as electronic commerce or EDI,
operator intervention to make policy decisions is impractical. The
process needs to be automated to a much higher degree. In fact, the
full process of certificate chain processing needs to be
implementable in trusted software.
Because of the above shortcomings, it is proposed that more flexible
CA structures than the RFC 1422 hierarchy be supported by the PKIX
specifications. In fact, the main reason for the structural
restrictions imposed by RFC 1422 was the restricted certificate
format provided with X.509 v1. With X.509 v3, most of the
requirements addressed by RFC 1422 can be addressed using certificate
extensions, without a need to restrict the CA structures used. In
particular, the certificate extensions relating to certificate
policies obviate the need for PCAs and the constraint extensions
obviate the need for the name subordination rule.
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3.3 Revocation
When a certificate is issued, it is expected to be in use for its
entire validity period. However, various circumstances may cause a
certificate to become invalid prior to the expiration of the validity
period. Such circumstances might include change of name, change of
association between subject and CA (e.g., an employee terminates
employment with an organization), and compromise or suspected
compromise of the corresponding private key. Under such
circumstances, the CA needs to revoke the certificate.
X.509 defines one method of certificate revocation. This method
involves each CA periodically issuing a signed data structure called
a certificate revocation list (CRL). A CRL is a time stamped list
identifying revoked certificates which is signed by a CA and made
freely available in a public repository. Each revoked certificate is
identified in a CRL by its certificate serial number. When a
certificate-using system uses a certificate (e.g., for verifying a
remote user's digital signature), that system not only checks the
certificate signature and validity but also acquires a suitably-
recent CRL and checks that the certificate serial number is not on
that CRL. The meaning of "suitably-recent" may vary with local
policy, but it usually means the most recently-issued CRL. A CA
issues a new CRL on a regular periodic basis (e.g., hourly, daily, or
weekly). Entries are added to CRLs as revocations occur, and an
entry may be removed when the certificate expiration date is reached.
An advantage of this 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 periodic CRL is
issued -- this may be up to one hour, one day, or one week depending
on the frequency that the CA issues CRLs.
Another potential problem with CRLs is the risk of a CRL growing to
an entirely unacceptable size. In the 1988 and 1993 versions of
X.509, the CRL for the end-user certificates needed to cover the
entire population of end-users for one CA. It is desirable to allow
such populations to be in the range of thousands, tens of thousands,
or possibly even hundreds of thousands of users. The end-user CRL is
therefore at risk of growing to such sizes, which present major
communication and storage overhead problems. With the version 2 CRL
format, introduced along with the v3 certificate format, it becomes
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possible to arbitrarily divide the population of certificates for one
CA into a number of partitions, each partition being associated with
one CRL distribution point (e.g., directory entry or URL) from which
CRLs are distributed. Therefore, the maximum CRL size can be
controlled by a CA. Separate CRL distribution points can also exist
for different revocation reasons. For example, routine revocations
(e.g., name change) may be placed on a different CRL to revocations
resulting from suspected key compromises, and policy may specify that
the latter CRL be updated and issued more frequently than the former.
As with the X.509 v3 certificate format, in order to facilitate
interoperable implementations from multiple vendors, the X.509 v2 CRL
format needs to be profiled for Internet use. It is one goal of this
document to specify such profiles.
Furthermore, it is recognized that on-line methods of revocation
notification may be applicable in some environments as an alternative
to the X.509 CRL. On-line revocation checking eliminates the latency
between a revocation report and CRL the next issue. Once the
revocation is reported, any query to the on- line service will
correctly reflect the certificate validation impacts of the
revocation. Therefore, this document will also consider standard
approaches to on-line revocation notification.
3.4 Operational Protocols
Operational protocols are required to deliver certificates and CRLs
to certificate using client systems. Provision is needed for a
variety of different means of certificate and CRL delivery, including
request/delivery procedures based on E-mail, http, X.500, and
WHOIS++. These specifications include definitions of, and/or
references to, message formats and procedures for supporting all of
the above operational environments, including definitions of or
references to appropriate MIME content types.
3.5 Management Protocols
Management protocols are required to support on-line interactions
between Public Key Infrastructure (PKI) components. For example,
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. The set of functions which potentially need to
be supported by management protocols include:
(a) registration: This is the process whereby a user first makes
itself known to a CA, prior to that CA issuing a certificate or
certificates for that user.
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(b) initialization: Before a client system can operate securely it
is necessary to install in it necessary key materials which have the
appropriate relationship with keys stored elsewhere in the
infrastructure. For example, the client needs to be securely
initialized with the public key of a CA, to be used in validating
certificate paths. Furthermore, a client typically needs to be
initialized with its own key pair(s).
(c) certification: This is the process in which a CA issues a
certificate for a user's public key, and returns that certificate to
the user's client system and/or posts that certificate in a public
repository.
(d) key pair recovery: As an option, user client key materials
(e.g., a user's private key used for encryption purposes) may be
backed up by a CA or a key backup system associated with a CA. If a
user needs to recover these backed up key materials (e.g., as a
result of a forgotten password or a lost key chain file), an on-line
protocol exchange may be needed to support such recovery.
(e) key pair update: All key pairs need to be updated regularly,
i.e., replaced with a new key pair, and new certificates issued.
(f) revocation request: An authorized person advises a CA of an
abnormal situation requiring certificate revocation.
(g) cross-certification: Two CAs exchange the information necessary
to establish cross-certificates between those CAs.
Note that on-line protocols are not the only way of implementing the
above functions. For all functions there are off-line methods of
achieving the same result, and this specification does not mandate
use of on- line protocols. For example, when hardware tokens are
used, many of the functions may be achieved through as part of the
physical token delivery. Furthermore, some of the above functions
may be combined into one protocol exchange. In particular, two or
more of the registration, initialization, and certification functions
can be combined into one protocol exchange.
Part 3 of the PKIX series of specifications defines a set of standard
message formats supporting the above functions. The protocols for
conveying these messages in different environments (on-line, e-mail,
and WWW) are also specified.
4 Certificate and Certificate Extensions Profile
The goal of this document is to create a profile for X.509 v3
certificates that will foster interoperability and a reusable public
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key infrastructure. Since the publication of earlier versions of
this draft, substantial changes have been made to the Amendment
[X.509-AM] to X.509 defining version 3 and certificate extensions.
Those changes have brought the base document into close alignment
with the recommendations of earlier versions of this draft. As a
result, this document provides a concise profile rather than
attempting to recreate the Amendment as a standalone document.
Certificates may be used in a wide range of applications and
environments covering a broad spectrum of interoperability goals and
a broader spectrum of operational and assurance requirements. The
goal of this document is to establish a common baseline for generic
applications requiring broad interoperability and limited special
purpose requirements. In particular, the emphasis will be on
supporting the use of X.509 v3 certificates for informal internet
electronic mail, IPSEC, and WWW applications. Other efforts are
looking at certificate profiles for payment systems.
4.1 Basic Certificate Fields
The X.509 v3 certificate basic syntax is as follows. For signature
calculation, the certificate is encoded using the ASN.1 distinguished
encoding rules (DER) [X.208]. ASN.1 DER encoding is a tag, length,
value encoding system for each element.
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING }
TBSCertificate ::= SEQUENCE {
version [0] Version DEFAULT v1,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueID [1] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version must be v2 or v3
subjectUniqueID [2] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version must be v2 or v3
extensions [3] Extensions OPTIONAL
-- If present, version must be v3
}
Version ::= INTEGER { v1(0), v2(1), v3(2) }
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CertificateSerialNumber ::= INTEGER
Validity ::= SEQUENCE {
notBefore UTCTime,
notAfter UTCTime }
UniqueIdentifier ::= BIT STRING
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
Extensions ::= SEQUENCE OF Extension
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING }
The following items describe a proposed use of the X.509 v3
certificate for the Internet.
4.1.1 Version
This field describes the version of the encoded certificate. When
extensions are used, as expected in this profile, use X.509 version 3
(value is 2). If no extensions are present, but a UniqueIdentifier
is present, use version 2 (value is 1). If only basic fields are
present, use version 1 (the value is omitted from the certificate as
the default value).
Implementations should be prepared to accept any version certificate.
In particular, at a minimum, implementations should recognize version
3 certificates; determine whether any critical extensions are
present; and accept certificates without critical extensions even if
they don't recognize any extensions. A certificate with an
unrecognized critical extension must always be rejected.
Generation of version 2 certificates is not expected by
implementations based on this profile.
4.1.2 Serial number
The serial number is an integer assigned by the certification
authority to each certificate. It must be unique for each
certificate issued by a given CA (i.e., the issuer name and serial
number identify a unique certificate).
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4.1.3 Signature
This field contains the algorithm identifier for the algorithm used
by the CA to sign the certificate. Section 7.2 lists the supported
signature algorithms.
4.1.4 Issuer Name
The issuer name identifies the entity who has signed (and issued the
certificate). The issuer identity may be carried in the issuer name
field and/or the issuerAltName extension. If identity information is
present only in the issuerAltName extension, then the issuer name may
be an empty sequence and the issuerAltName extension must be
critical.
<< Should we say anything about name constraints here? >>
4.1.5 Validity
This field indicates the dates on which the certificate becomes valid
(notBefore) and on which the certificate ceases to be valid
(notAfter). It is strongly recommended that UTCTime values be
expressed Greenwich Mean Time (Zulu) and not use seconds (i.e., times
are YYMMDDHHMMZ). If seconds are used, a value of 00 seconds should
never be encoded.
4.1.6 Subject Name
The subject name identifies the entity associated with the public key
stored in the subject public key field. The subject identity may be
carried in the subject field and/or the subjectAltName extension. If
identity information is present only in the subjectAltName extension
(e.g., a key bound only to an email address or URI), then the subject
name may be an empty sequence and the subjectAltName extension must
be critical.
<< Should we say anything about name constraints here? >>
4.1.7 Subject Public Key Info
This field is used to carry the public key and identify the algorithm
with which the key is used.
4.1.8 Unique Identifiers
The subject and issuer unique identifier are present in the
certificate to handle the possibility of reuse of subject and/or
issuer names over time. This profile recommends that names not be
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reused and that Internet certificates not make use of unique
identifiers. CA's conforming to this profile should not generate
certificates with unique identifiers. Applications conforming to
this profile should be capable of parsing unique identifiers and
making comparisons.
4.2 Certificate Extensions
The extensions defined for X.509 v3 certificates provide methods for
associating additional attributes with users or public keys, for
managing the certification hierarchy, and for managing CRL
distribution. The X.509 v3 certificate format also allows
communities to define private extensions to carry information unique
to those communities. Each extension in a certificate may be
designated as critical or non-critical. A certificate using system
(an application validating a certificate) must reject the certificate
if it encounters a critical extension it does not recognize. A non-
critical extension may be ignored if it is not recognized. The
following presents recommended extensions used within Internet
certificates and standard locations for information. Communities may
elect to use additional extensions; however, caution should be
exercised in adopting any critical extensions in certificates which
might be used in a general context.
The extensions referenced below are defined in detail in the X.509
Amendment along with the specification of syntax and object
identifiers. Because the intent is to align exactly with those
definitions, the material is not reproduced here. The following
sections describe Internet profiling decisions and Internet private
extensions.
<< Once finalized, the certificate ASN.1 definition will be included
in an appendix to this document. >>
4.2.1 Standard Extensions
4.2.1.1 Authority Key Identifier
The authority key identifier extension provides a means of
identifying the particular public key used to sign a certificate.
The identification can be based on either the key identifier (the
subject key identifier in the issuer's certificate) or on the issuer
name and serial number. The key identifier method is recommended in
this profile. This extension would be used where an issuer has
multiple signing keys (either due to multiple concurrent key pairs or
due to changeover). In general, this extension should be included in
certificates.
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4.2.1.2 Subject Key Identifier
The subject key identifier extension provides a means of identifying
the particular public key used in an application. Where a reference
to a public key identifier is needed (as with an Authority Key
Identifier) and one is not included in the associated certificate, a
SHA-1 hash of the subject public key shall be used. The hash shall
be calculated over the value (excluding tag and length) of the
subject public key field in the certificate. This extension should
be marked non-critical.
4.2.1.3 Key Usage
The key usage extension defines the purpose (e.g., encipherment,
signature, certificate signing) of the key contained in the
certificate. The usage restriction might be employed when a
multipurpose key is to be restricted (e.g., when an RSA key should be
used only for signing or only for key encipherment). The profile
recommends that when used, this be marked as a critical extension.
4.2.1.4 Private Key Usage Period
This profile recommends against the use of this extension. CA's
conforming to this profile should not generate certificates with
private key usage period extensions.
4.2.1.5 Certificate Policies
The certificate policies extension contains a sequence of policy
information terms, each of which consists of an object identifier
(OID) and optional qualifiers. These policy information terms
indicate the policy under which the certificate has been issued and
the purposes for which the certificate may be used. This profile
strongly recommends that a simple OID be present in this field.
Optional qualifiers which may be present are expected to provide
information about obtaining CA rules, not change the definition of
the policy. Applications are expected to have a list of those
policies which they will accept and to compare the policy OIDs in the
certificate to that list.
<< Do we want to say anything about criticality? >>
4.2.1.6 Policy Mappings
This extension may be supported by CAs and/or applications, and it is
always non-critical.
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4.2.1.7 Subject Alternative Name
The subject alternative names extension allows additional identities
to be bound to the subject of the certificate. Defined options
include an rfc822 name (electronic mail address), a DNS name, an IP
address, and a URI. Other options exist, including completely local
definitions. Multiple instances of a name and multiple name forms
may be included. Whenever such identities are to be bound into a
certificate, the subject alternative name (or issuer alternative
name) extension shall be used. (Note: a form of such an identifier
may also be present in the subject distinguished name; however, the
alternative name extension is the preferred location for finding such
information.) Further, if the only subject identity included in the
certificate is an alternative name form (e.g., an electronic mail
address), then the subject distinguished name should be empty (an
empty sequence), the subjectAltName extension should be used, and the
subjectAltName extension must be marked critical.
<< In the previous version we said: The use of otherName should not
be used in conjunction with this profile. Should we put this
restriction back? >>
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.
4.2.1.8 Issuer Alternative Name
As with 4.2.1.7, this extension is used to associate Internet style
identities with the certificate issuer. If the only issuer identity
included in the certificate is an alternative name form (e.g., an
electronic mail address), then the issuer distinguished name should
be empty (an empty sequence), the issuerAltName extension should be
used, and the issuerAltName extension must be marked critical.
4.2.1.9 Subject Directory Attributes
The subject directory attributes extension is not recommended as an
essential part of this profile, but it may be used in local
environments. This extension is always non-critical.
4.2.1.10 Basic Constraints
The basic constraints extension identifies whether the subject of the
certificate is a CA and how deep a certification path may exist
through that CA. This profile requires the use of this extension,
and it shall critical for all certificates issued to CAs.
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4.2.1.11 Name Constraints
The name constraints extension provides permitted and excluded
subtrees that place restrictions on names that may be included within
a certificate issued by a given CA. Restrictions may apply to the
subject distringuished name or subject alternative names. Any name
matching a restriction in the excluded subtrees field is invalid
regardless of information appearing in the permitted subtrees.
Restrictions for the rfc822, dNSName, and uri name forms are all
expressed in terms of strings with wild card matching. An "*" is the
wildcard character. The minimum and maximum fields in general
subtree are not used for these name forms. For uris and rfc822 names,
the restriction applies to the host part of the name. Examples would
be foo.bar.com; www*.bar.com; *.xyz.com.
<< X.208 (1988) defines is IA5String as PrintableString plus
NumericString plus SPACE and DELETE. Thus, it does not allow the for
the * character. What should we use instead? >>
4.2.1.12 Policy Constraints
The policy constraints extension may be used by CAs.
4.2.1.13 CRL Distribution Points
The CRL distribution points extension identifies how CRL information
is obtained. The profile recommends support for this extension by
CAs and applications. Further discussion of CRL management is
contained in section 5.
4.2.2 Private Internet Extensions
4.2.2.1 Subject Information Access
The name information in the certificate identifies the entity to
which the public key is bound. In some instances, it may also be
necessary to know where to find additional information about the
named entity. In the case of X.500 names, this relationship is
automatic. The subject information access extension provides a means
of identifying where and how to find information about the subject.
The extension specifies a method of obtaining information and a
general name form indicating where. This extension should always be
non-critical.
subjectInfoAccess EXTENSION ::= {
SYNTAX SubjectInfoAccessSyntax
IDENTIFIED BY { TBD-OID-1 } }
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SubjectInfoAccessSyntax ::= SEQUENCE SIZE (1..MAX) OF AccessDescription
AccessDescription ::= SEQUENCE {
accessMethod OBJECT IDENTIFIER,
accessLocation GeneralName }
<< What upper bound should be assigned for MAX? >>
<< Where is the list of access method OIDs going to be specified? >>
4.2.2.2 Authority Information Access
The authority information access extension indicates how to access CA
information and services for the issuer of the certificate in which
the extension appears. Information and services include certificate
status or on-line validation services, certificate retrieval, CA
policy data, and CA certificates (certificates certifying the target
CA to aid in certification path navigation). This extension may be
included in subject or CA certificates and may be critical or non-
critical.
<< What does it mean to mark this extension critical? >>
authorityInfoAccess EXTENSION ::= {
SYNTAX AuthorityInfoAccessSyntax
IDENTIFIED BY { TBD-OID-2 } }
AuthorityInfoAccessSyntax ::= SEQUENCE {
certStatus [0] SEQUENCE OF AccessDescription,
certRetrieval [1] SEQUENCE OF AccessDescription,
caPolicy [2] SEQUENCE OF AccessDescription,
caCerts [3] SEQUENCE OF AccessDescription }
4.2.2.3 CA Information Access
The CA information access extension indicates how to access CA
information and services for the subject of the certificate in which
the extension appears. Information and services include certificate
status or on-line validation services, certificate retrieval, CA
policy data, and CA certificates (certificates certifying the target
CA to aid in cert path navigation). This extension may be included
only in CA certificates and may be critical or non-critical. CA
certificates may include both an authority and a caInfoAccess
extension to describe access methods for both the CA and its issuer.
caInfoAccess EXTENSION ::= {
SYNTAX CAInfoAccessSyntax
IDENTIFIED BY { TBD-OID-2 } }
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CAInfoAccessSyntax ::= SEQUENCE {
certStatus [0] SEQUENCE OF AccessDescription,
certRetrieval [1] SEQUENCE OF AccessDescription,
caPolicy [2] SEQUENCE OF AccessDescription,
caCerts [3] SEQUENCE OF AccessDescription }
4.3 Examples
<< Certificate samples including descriptive text and ASN.1 encoded
blobs will be inserted. >>
5 CRL and CRL Extensions Profile
As described above, one goal of this X.509 v2 CRL profile is to
foster the creation of an interoperable and reusable Internet PKI.
To achieve this goal, guidelines for the use of extensions are
specified, and some assumptions are made about the nature of
information included in the CRL.
CRLs may be used in a wide range of applications and environments
covering a broad spectrum of interoperability goals and an even
broader spectrum of operational and assurance requirements. This
profile establishes a common baseline for generic applications
requiring broad interoperability. Emphasis is placed on support for
X.509 v2 CRLs. The profile defines a baseline set of information
that can be expected in every CRL. Also, the profile defines common
locations within the CRL for frequently used attributes, and common
representations for these attributes.
This profile does not define any private Internet CRL extensions or
CRL entry extensions.
Environments with additional or special purpose requirements may
build on this profile or may replace it.
5.1 CRL Fields
The X.509 v2 CRL syntax is as follows. For signature calculation,
the data that is to be signed is ASN.1 DER encoded. ASN.1 DER
encoding is a tag, length, value encoding system for each element.
CertificateList ::= SEQUENCE {
tbsCertList TBSCertList,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING }
TBSCertList ::= SEQUENCE {
version Version OPTIONAL,
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-- if present, must be v2
signature AlgorithmIdentifier,
issuer Name,
thisUpdate UTCTime,
nextUpdate UTCTime,
revokedCertificates SEQUENCE OF SEQUENCE {
userCertificate CertificateSerialNumber,
revocationDate UTCTime,
crlEntryExtensions Extensions OPTIONAL } OPTIONAL,
crlExtensions [0] Extensions OPTIONAL }
Version ::= INTEGER { v1(0), v2(1), v3(2) }
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
-- contains a value of the type
-- registered for use with the
-- algorithm object identifier value
CertificateSerialNumber ::= INTEGER
Extensions ::= SEQUENCE OF Extension
Extension ::= SEQUENCE {
extnId OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING }
-- contains a DER encoding of a value
-- of the type registered for use with
-- the extnId object identifier value
The following items describe the proposed use of the X.509 v2 CRL for
in Internet PKI.
5.1.1 Version
This field describes the version of the encoded CRL. When extensions
are used, as expected in this profile, use version 2 (the integer
value is 1). If neither CRL extensions nor CRL entry extensions are
present, use version 1 (the integer value must be omitted).
5.1.2 Signature
This field contains the algorithm identifier for the algorithm used
to sign the CRL. Section 7.2 lists the signature algorithms used in
the Internet PKI.
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5.1.3 Issuer Name
The issuer name identifies the entity who has signed (and issued the
CRL). The issuer identity may be carried in the issuer name field
and/or the issuerAltName extension. If identity information is
present only in the issuerAltName extension, then the issuer name may
be an empty sequence and the issuerAltName extension must be
critical.
5.1.4 This Update
This field indicates the issue date of this CRL.. It is strongly
recommended that UTCTime values be expressed Greenwich Mean Time
(Zulu) and not use seconds (i.e., times are YYMMDDHHMMZ). If seconds
are used, a value of 00 seconds should never be encoded.
5.1.5 Next Update
This field indicates the date by which the next CRL will be issued.
The next CRL could be issued before the indicated date, but it will
not be issued any later than the indicated date.
As described in the previous section, the UTCTime for Next Update
should be expressed Greenwich Mean Time (Zulu) and not use seconds.
5.1.6 Revoked Certificates
Revoked certificates are listed. The revoked certificates are named
by their serial numbers. Certificates are uniquely identified by the
combination of the issuer name or issuer alternative name along with
the user certificate serial number. The date on which the revocation
occured is specified. As described in section 5.1.4, the UTCTime for
revocationDate should be expressed Greenwich Mean Time (Zulu) and not
use seconds.. CRL entry extensions are discussed in section 5.3.
When a CA wishes to revoke a certificate that it issued to another
CA, the revocation shall appear on the CRL. The revocation should
also appear on the authority revocation list (ARL). The CA is
revoking a certificate that it issued.
5.2 CRL Extensions
The extensions defined by ANSI X9 and ISO for X.509 v2 CRLs [X.509-
AM] [X9.55] provide methods for associating additional attributes
with CRLs. The X.509 v2 CRL format also allows communities to define
private extensions to carry information unique to those communities.
Each extension in a CRL may be designated as critical or non-
critical. A CRL validation must fail if it encounters an critical
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extension which it does not know how to process. However, an
unrecognized non-critical extension may be ignored. The following
presents those extensions used within Internet CRLs. Communities may
elect to use additional extensions; however, caution should be
exercised in adopting any critical extensions in CRLs which might be
used in a general context.
5.2.1 Authority Key Identifier
The authority key identifier extension provides a means of
identifying the particular public key used to sign a CRL. The
identification can be based on either the key identifier (the subject
key identifier in the CRL signer's certificate) or on the issuer name
and serial number. The key identifier method is recommended in this
profile. This extension would be used where an issuer has multiple
signing keys, either due to multiple concurrent key pairs or due to
changeover. In general, this non-critical extension should be
included in certificates.
5.2.2 Issuer Alternative Name
The issuer alternative names extension allows additional identities
to be associated with the issuer of the CRL. Defined options include
an rfc822 name (electronic mail address), a DNS name, an IP address,
and a URI. Multiple instances of a name and multiple name forms may
be included. Whenever such identities are used, the issuere
alternative name extension shall be used. Further, if the only
issuer identity included in the CRL is an alternative name form
(e.g., an electronic mail address), then the issuer distinguished
name should be empty (an empty sequence), the issuerAltName extension
should be used, and the issuerAltName extension must be marked
critical.
5.2.3 CRL Number
The CRL number is a non-critical CRL extension which conveys a
monotonically increacing sequence number for each CRL issued by a
given CA through a specific CA X.500 Directory entry or CRL
distribution point. This extension allows users to easily determine
when a particular CRL superceeds another CRL. CAs conforming to this
profile shall include this CRL.
5.2.4 Issuing Distribution Point
The issuing distribution point is a critical CRL extension that
identifies the CRL distribution point for a particular CRL, and it
indicates whether the CRL covers revocation for end entity
certificates only, CA certificates only, or a limitied set of reason
codes. Since this extension is critical, all certificate users must
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be prepared to receive CRLs with this extension.
The CRL is signed using the CA's private key. CRL Distribution
Points do not have their own key pairs. If the CRL is stored in the
X.500 Directory, it is stored in the Directory entry corresponding to
the CRL distribution point, which may be different that the Directory
entry of the CA.
CRL distribution points, if used by a CA, should be partition the CRL
on the basis of compromise and routine revocation. That is, the
revocations with reason code keyCompromise (1) shall appear in one
distribution point, and the revocations with other reason codes shall
appear in another distribution point.
5.2.5 Delta CRL Indicator
The delta CRL indicator is a critical CRL extension that identifies a
delta-CRL. The use of delta-CRLs can significantly improve
processing time for applications which store revocation information
in a format other than the CRL structure. This allows changes to be
added to the local database while ignoring unchanged information that
is already in the local databse.
When a delta-CRL is issued, the CAs shall also issue a complete CRL.
The value of BaseCRLNumber identifies the CRL number of the base CRL
that was used as the starting point in the generation of this delta-
CRL. The delta-CRL contains the changes between the base CRL and the
current CRL issued along with the delta-CRL. It is the decision of a
CA as to whether to provide delta-CRLs. Again, a delta-CRL shall not
be issued without a corresponding CRL. The value of CRLNumber for
both the delta-CRL and the corresponding CRL shall be identical.
A CRL user constructing a locally held CRL from delta-CRLs shall
consider the constructed CRL incomplete and unusable if the CRLNumber
of the received delta-CRL is more that one greater that the CRLnumber
of the delta-CRL last processed.
5.3 CRL Entry Extensions
The CRL entry extensions already defined by ANSI X9 and ISO for X.509
v2 CRLs [X.509-AM] [X9.55] provide methods for associating additional
attributes with CRL entries. The X.509 v2 CRL format also allows
communities to define private CRL entry extensions to carry
information unique to those communities. Each extension in a CRL
entry may be designated as critical or non-critical. A CRL
validation must fail if it encounters an critical CRL entry extension
which it does not know how to process. However, an unrecognized
non-critical CRL entry extension may be ignored. The following
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presents recommended extensions used within Internet CRL entries and
standard locations for information. Communities may elect to use
additional CRL entry extensions; however, caution should be exercised
in adopting any critical extensions in CRL entries which might be
used in a general context.
5.3.1 Reason Code
The reasonCode is a non-critical CRL entry extension that identifies
the reason for the certificate revocation. As are strongly encouraged
to include reason codes in CRL entries; however, the reason code CRL
entry extension should be absent instead of using the unspecified (0)
reasonCode value.
5.3.2 Hold Instruction Code
The hold instruction code is a non-critical CRL entry extension that
provides a registered instruction identifier which indicates the
action to be taken after encountering a certificate that has been
placed on hold.
<< The Directory Specification does not define any standard hold
instruction codes. Where will they be defined? Are any specific to
the Internet environment? >>
5.3.3 Invalidity Date
The invalidity date is a non-critical CRL entry extension that
provides the date on which it is known or suspected that the private
key was compromised or that the certificate otherwise became invalid.
This date may be earlier than the revocation date in the CRL entry,
but it must be later than the issue date of the previously issued
CRL. Remember that the revocation date in the CRL entry specifies
the date that the CA revoked the certificate. Whenever this
information is available, CAs are strongly encouraged to share it
with CRL users.
The GeneralizedTime values included in this field shall be expressed
in Greenwich Mean Time (Zulu) and omit trailing zeros in fractional
seconds. Normally, GeneralizedTime will be expressed as
YYYYMMDDHHMMSSZ.
5.4 Examples
<< CRL samples including descriptive text and ASN.1 encoded blobs
will be inserted. >>
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6 Certificate Path Validation
<< There used to be long section here that contained the
certification path validation procedures. The major deviations from
the procedures outlined in [X.509-AM] were due to certificate hold
processing. These procedures are no longer onerous, so there is no
reason to prohibit the use of the hold facility. Does anyone see a
reason to reatin this section? >>
7 Algorithm Support
7.1 One-way Hash Functions
One-way hash functions are also called message digest algorithms.
SHA-1 is be the most popular one-way hash function used in the
Internet PKI. However, PEM uses MD2 for certificates [RFC 1422] [RFC
1423]. For this reason, MD2 is included in this profile.
7.1.1 MD2 One-way Hash Function
MD2 was also developed by Ron Rivest, but RSA Data Security has not
placed the MD2 algorithm in the public domain. Rather, RSA Data
Security has granted license to use MD2 for non-commerical Internet
Privacy-Enhanced Mail. For this reason, MD2 may continue to be used
with PEM certificates, but SHA-1 is preferred. MD2 is fully
described in RFC 1319 [RFC 1319].
At the Selected Areas in Cryptography '95 conference in May 1995,
Rogier and Chauvaud presented an attack on MD2 that can nearly find
collisions [RC95]. Collisions occur when two different messages
generate the same message digest. A checksum operation in MD2 is the
only remaining obstacle to the success of the attack. For this
reason, the use of MD2 for new applications is discouraged. It is
still reasonable to use MD2 to verify existing signatures, as the
ability to find collisions in MD2 does not enable an attacker to find
new messages having a previously computed hash value.
<< More information on the attack and its implications can be
obtained from a RSA Laboratories security bulletin. These bulletins
are available from <http://www.rsa.com/>. >>
7.1.2 SHA-1 One-way Hash Function
SHA-1 was developed by the U.S. Government. SHA-1 is fully described
in FIPS 180-1 [FIPS 180-1].
SHA-1 is the one-way hash function of choice for use with both RSA
the DSA signature algorithms.
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7.2 Signature Algorithms
RSA and DSA are the most popular signature algorithms used in the
Internet.
There is some ambiguity in 1988 X.509 document with respect to the
definition of the SIGNED macro and the representation of a signature
in a certificate or a CRL. The interpretation selected for the
Internet requires that the data to be signed (e.g., the one-way hash
function output value) is first ASN.1 encoded as an OCTET STRING and
the result is encrypted (e.g., using RSA Encryption) to form the
signed quantity. Asd part of the SIGNED macro, this signature value
is then ASN.1 encoded as a BIT STRING.
7.2.1 RSA Signature Algorithm
A patent statement regarding the RSA algorithm can be found at the
end of this profile.
The RSA algorithm is named for it's inventors: Rivest, Shamir, and
Adleman. The RSA signature algorithm is defined in PKCS #1 [PKCS#1].
It combines either the MD2 or the SHA-1 one-way hash function with
the RSA asymmetric encryption algorithm. As defined in PKCS #1, the
ASN.1 object identifiers used to identify these signature algorithms
are:
md2WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 2 }
sha-1WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithm(2) 29 }
When either of these object identifiers is used within the ASN.1 type
AlgorithmIdentifier, the parameters component of that type shall the
ASN.1 type NULL.
7.2.2 DSA Signature Algorithm
A patent statement regarding the DSA can be found at the end of this
profile.
The Digital Signature Algorithm (DSA) is also called the Digital
Signature Standard (DSS). DSA was developed by the U.S. Government,
and DSA is used in conjunction with the the SHA-1 one-way hash
function. DSA is fully described in FIPS 186 [FIPS 186]. The ASN.1
object identifiers used to identify this signature algorithm is:
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dsaWithSHA-1 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithm(2) 27 }
The DSA algorithm syntax includes optional parameters. These
parameters are commonly referred to as p, q, and g. The
AlgorithmIdentifier within subjectPublicKeyInfo is the only place
within a certificate where these parameters shall be present. If the
DSA algorithm parameters are absent from the subjectPublicKeyInfo
AlgorithmIdentifier and the CA signed the subject certificate using
DSA, then the certificate issuer's DSA parameters apply to the
subject's DSA key. If the DSA algorithm parameters are absent from
the subjectPublicKeyInfo AlgorithmIdentifier and the CA signed the
subject certificate using a signature algorithm other than DSA, then
the subject's DSA parameters are distributed by other means. The
parameters are included using the following ASN.1 structure:
Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }
When signing, DSA algorithm generates two values. These values are
commonly referred to as r and s. To easily transfer these two values
as one siganture, they are ASN.1 encoded using the following ASN.1
structure:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }
7.3 Subject Public Key Algorithms
<< Add a section that lists the public key algorithms that are
supported by this profile. Obviously, RSA, DSA, Diffie-Hellman, and
KEA will be included. Are there others? >>
<< Should a different algorithm identifier be assigned to RSA
signature keys and RSA key management keys? If so, there will be one
subsection for each within this section.>>
References
[FIPS 180-1] Federal Information Processing Standards Publication
(FIPS PUB) 180-1, Secure Hash Standard, 17 April 1995.
[Supersedes FIPS PUB 180 dated 11 May 1993.]
[FIPS 186] Federal Information Processing Standards Publication
(FIPS PUB) 186, Digital Signature Standard, 18 May 1994.
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[PKCS#1] PKCS #1: RSA Encryption Standard, Version 1.4, RSA Data
Security, Inc., 3 June 1991.
[RC95] Rogier, N. and Chauvaud, P., "The compression function of
MD2 is not collision free," Presented at Selected Areas in
Cryptography '95, Carleton University, Ottawa, Canada,
18-19 May 1995.
[RFC 1319] Kaliski, B., "The MD2 Message-Digest Algorithm," RFC 1319,
RSA Laboratories, April 1992.
[RFC 1422] Kent, S., "Privacy Enhancement for Internet Electronic
Mail: Part II: Certificate-Based Key Management," RFC
1422,
BBN Communications, February 1993.
[RFC 1423] Balenson, D., "Privacy Enhancement for Internet Electronic
Mail: Part III: Algorithms, Modes, and Identifiers,"
RFC 1423, Trusted Information Systems, February 1993.
[X.208] << Do we want to reference the 1988 or 1993 version? >>
[X.509-AM] << Need final reference >>
[X9.55] << Need final reference >>
Patent Statements
The Internet PKI relies on the use of patented public key technology.
The Internet Standards Process as defined in RFC 1310 requires a
written statement from the Patent holder that a license will be made
available to applicants under reasonable terms and conditions prior
to approving a specification as a Proposed, Draft or Internet
Standard.
Patent statements for DSA, RSA, and Diffie-Hellman follow. These
statements have been supplied by the patent holders, not the authors
of this profile.
Digital Signature Algorithm (DSA)
The U.S. Government holds patent 5,231,668 on the Digital
Signature Algorithm (DSA), which has been incorporated into
Federal Information Processing Standard (FIPS) 186. The patent
was issued on July 27, 1993.
The National Institute of Standards and Technology (NIST) has a
long tradition of supplying U.S. Government-developed techniques
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to committees and working groups for inclusion into standards on a
royalty-free basis. NIST has made the DSA patent available
royalty-free to users worldwide.
Regarding patent infringement, FIPS 186 summarizes our position;
the Department of Commerce is not aware of any patents that would
be infringed by the DSA. Questions regarding this matter may be
directed to the Deputy Chief Counsel for NIST.
RSA Signature and Encryption
<< Now that PKP has dissolved, a revised patent statement for RSA
from RSADSI is needed. >>
Diffie-Hellman Key Agreement
<< Now that PKP has dissolved, a revised patent statement for
Diffie-Hellman from Cylink is needed. >>
Obsolete PKP Patent Statement
<< This statement is included here until a replacement from RSADSI
and Cylink can be obtained. >>
The Massachusetts Institute of Technology and the Board of
Trustees of the Leland Stanford Junior University have granted
Public Key Partners (PKP) exclusive sub-licensing rights to the
following patents issued in the United States, and all of their
corresponding foreign patents:
Cryptographic Apparatus and Method
("Diffie-Hellman")......................... No. 4,200,770
Public Key Cryptographic Apparatus
and Method ("Hellman-Merkle").............. No. 4,218,582
Cryptographic Communications System and
Method ("RSA")............................. No. 4,405,829
Exponential Cryptographic Apparatus
and Method ("Hellman-Pohlig").............. No. 4,424,414
These patents are stated by PKP to cover all known methods of
practicing the art of Public Key encryption, including the
variations collectively known as El Gamal.
Public Key Partners has provided written assurance to the Internet
Society that parties will be able to obtain, under reasonable,
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nondiscriminatory terms, the right to use the technology covered
by these patents. This assurance is documented in RFC 1170 titled
"Public Key Standards and Licenses". A copy of the written
assurance dated April 20, 1990, may be obtained from the Internet
Assigned Number Authority (IANA).
The Internet Society, Internet Architecture Board, Internet
Engineering Steering Group and the Corporation for National
Research Initiatives take no position on the validity or scope of
the patents and patent applications, nor on the appropriateness of
the terms of the assurance. The Internet Society and other groups
mentioned above have not made any determination as to any other
intellectual property rights which may apply to the practice of
this standard. Any further consideration of these matters is the
user's own responsibility.
Security Considerations
This entire memo is about security mechanisms.
Author Addresses:
Russell Housley
SPYRUS
PO Box 1198
Herndon, VA 22070
USA
housley@spyrus.com
Warwick Ford
Nortel Secure Networks
PO Box 3511, Station C
Ottawa, Ontario
Canada KY 4H7
wford@bnr.ca
David Solo
BBN
150 CambridgePark Drive
Cambridge, MA 02140
USA
solo@bbn.com
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