PKIX Working Group R. Housley (SPYRUS)
Internet Draft W. Ford (NorTel)
S. Farrell (SSE)
D. Solo (BBN)
expires in six months November 1995
Internet Public Key Infrastructure
draft-ietf-pkix-ipki-00.txt
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Abstract
This is the first draft of the Internet Public Key Infrastructure.
It is provided as a strawman for the first meeting of the PKIX
Working Group. The intent of this strawman is to generate productive
discussions at the first meeting.
1 Executive Summary
<< Write this last. >>
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
Housley, Ford, Farrell, Solo [Page 1]
INTERNET DRAFT November 1995
technology. Such applications may include HTTP, electronic mail,
IPSP, user authentication, electronic payment systems, as well as
others. In order to relieve some of the obstacles to using X.509
certificates, this draft will define profiles, rules, and management
protocols that should serve to promote the development of reusable
certificate management systems; development of reusable application
tools; and interoperabilty determined by policy, not syntax.
Many communities will need to supplement, or possibly replace, this
profile in order to meet the requirements of specialized domains or
environments with additional authorization, assurance, or operational
requirements. However, for basic applications, it is essential that
a core of features be defined and that common means of representing
common information be agreed to so that application developers can
obtain necessary information without regard to the issuer of a
particular certificate.
As supplemental authorization and attribute management tools emerge,
such as attribute certificates, it may be appropriate to limit what
the certificate is used for in terms of conveying authenticated
attributes as opposed to other means of conveying information.
<< Note, this section needs to be expanded >>
2.1 Communication and Topology
The users of certificates will operate in a wide range of
environments with respect to their communication topology, especially
for secure electronic mail users. This profile will allow for users
without high bandwidth, real-time IP connectivity, or high
availablity of a connection. In addition, the profile must allow for
the presence of firewall or other filtered communication.
2.2 Access Control and Acceptability Decisions
The goal of the Public Key Infrstructure (PKI) is to meet the needs
of deterministic, automated access control and authorization
functions. This will drive the types of attributes and the nature of
the identity contained in the certificate as well as the ancillary
control information in the certificate such as policy data and
certification path constraints.
2.3 User Expectations
In this context, user refers to the users of the client software and
the subjects of the certificates. These are the readers and writers
of electronic mail, the clients for WWW browsers, etc. A goal of
this profile is to recognize the limitations of both the platforms
Housley, Ford, Farrell, Solo [Page 2]
INTERNET DRAFT November 1995
these users will employ and the sophistication/attentiveness of the
users. This manifests itself in requirements to simplify the
configuration responsibility of the user (e.g., root keys, rules),
make platform usage constraints explicit in the certificate, to
construct certification path constraints which shield the user from
malicious action, and to construct applications which sensibly
automate checking functions.
2.4 Administration Expectations
As with users, the certificate profile should also be structured to
be consistent with the types of individuals who must administer the
CA space. Providing such an administrator with unbounded choices
complicates not only the software that must process these
certificates but also increases the chances that a subtle mistake by
the CA administrator will result in broader compromise.
3 Overview of Approach
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, and are not applicable to public
key infrastructures.
The Internet Privacy Enhanced Mail (PEM) proposals, published in
1993, included specifications for a public key infrastructure based
on X.509 version 1 certificates [RFC 1422]. The experience gained in
attempts to deploy RFC 1422 made it clear that the v1 and v2
Housley, Ford, Farrell, Solo [Page 3]
INTERNET DRAFT November 1995
certificate formats were deficient in several respects. Most
importantly, more fields were needed to carry information which PEM
design and implementation experience had 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 having a
need. In August, 1995, standardization of the basic v3 format was
completed [ISO TC].
ISO/IEC and ANSI X9 have also developed a set of standard extensions
for use in the v3 extensions field [ISO DAM]. 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
implementations of X.509 v3 systems for Internet use, it is necessary
to specify profiles of use of the X.509 v3 extensions tailored for
the Internet. It is one goal of this document to specify such
profiles.
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.
Housley, Ford, Farrell, Solo [Page 4]
INTERNET DRAFT November 1995
(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.
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,
Housley, Ford, Farrell, Solo [Page 5]
INTERNET DRAFT November 1995
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.
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.
Housley, Ford, Farrell, Solo [Page 6]
INTERNET DRAFT November 1995
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 a 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
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 elimiates 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 Supporting 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
Housley, Ford, Farrell, Solo [Page 7]
INTERNET DRAFT November 1995
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.
(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.
Section 9 defines a set of standard protocols supporting the above
functions. The protocols for conveying these exchanges in different
environments (on-line, E-mail, and WWW) are specified in Section 10.
Housley, Ford, Farrell, Solo [Page 8]
INTERNET DRAFT November 1995
4 Certificate and Certificate Extensions Profile
As described above, one goal of this draft is to create a profile for
X.509 v3 certificates that will foster interoperability and a
reusable public key infrastructure. To achieve this goal, some
assumptions need to be made about the nature of information to be
included along with guidelines for how extensibility will be
employed.
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 draft 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. The draft will define
a baseline set of information along with common locations within a
certificate and common representations for common information.
Environments with additional requirements may build on this profile
or may replace it.
4.1 Basic Certificate Fields
The X.509 v3 certificate Basic syntax is as follows. For signature
calculation, the certificate is ASN.1 DER encoded (reference). ASN.1
DER encoding is a tag, length, value encoding system for each
element.
Certificate ::= SIGNED { 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) }
CertificateSerialNumber ::= INTEGER
Housley, Ford, Farrell, Solo [Page 9]
INTERNET DRAFT November 1995
Validity ::= SEQUENCE {
notBefore UTCTime,
notAfter UTCTime }
UniqueIdentifier ::= BIT STRING
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT 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 UniqueID 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).
<< All capabilites available in X.509 v2 certificates are available
in X.509 v3 certificates. Since there are so few X.509 v2
certificate implementations, should the profile prohibit the use of
v2? >>
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).
<< Do we want to define a maximum value for the serial number? >>
4.1.3 Signature
This field contains the algorithm identifier for the algorithm used
to sign the certificate.
4.1.4 Issuer Name
The issuer name provides a globally unique identifier of the
authority signing the certificate. The syntax of the issuer name is
an X.500 distinguished name. A name in the certificate may provide
semantic information, may provide a reference to an external
information store or service, provides a unique identifier, may
Housley, Ford, Farrell, Solo [Page 10]
INTERNET DRAFT November 1995
provide authorization information, or may provide a basis for
managing the CA relationships and certificate paths (other purposes
are also possible). This strawman suggests that the issuer (and
subject) name fields must provide a globally unique identifier. In
addition, they should contain semantic information identifying the
issuer/subject (e.g. a full name, organization name, etc.). Access
information will be provided in a separate extension (when other than
via X.500 directory) and internet specific identities (electronic
mail address, DNS name, and URLs) will be carried in alternative name
extensions.
<< Further discussion of naming guidelines for internet use is
needed. >>
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).
4.1.6 Subject Name
The purpose of the subject name is to provide a unique identifier of
the subject of the certificate. The syntax of the subject name is an
X.500 distinguished name. The discussion in section 4.1.4 on issuer
names applies to subject names as well.
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. Based on the approach to naming, names will
not be reused and internet certificates will not make use of these
unique identifiers.
4.2 Certificate Extensions
The extensions already defined by ANSI X9 and ISO for X.509 v3
certificates provide methods for associating additional attributes
with users or public keys and for managing the certification
hierarchy. 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
Housley, Ford, Farrell, Solo [Page 11]
INTERNET DRAFT November 1995
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.
4.2.1 Subject Alternative Name
The altNames 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, and a URL. Each of these
are IA5 strings. Multiple instances may be included. Whenever such
identities are to be bound in a certificate, the subject alternative
name (or issuer alternative name) field shall be used.
<< This implies that encoding of such identities within the subject
or issuer distinguished name is discouraged. >>
<< Note definition is based on a recommended change to the DAM. >>
AltNames ::= SEQUENCE OF GeneralName
GeneralName ::= CHOICE {
otherName [0] INSTANCE OF OTHER-NAME,
rfc822Name [1] IA5String,
dNSName [2] IA5String,
x400Address [3] ORAddress,
directoryName [4] Name,
ediPartyName [5] IA5String,
url [6] IA5String }
<< Should we permit an IP address? With the current list of choices,
IPSec would use dnsName. This leads to trusted resolution of DNS
Names to IP Addresses which is not done today. Maybe IP address is
too specific and LAN address should be allowed too. >>
4.2.2 Issuer Alternative Name
As with 4.2.1, this extension is used to bind Internet style
identities to the issuer name.
Housley, Ford, Farrell, Solo [Page 12]
INTERNET DRAFT November 1995
4.2.3 Certificate Policies
The certificatePolicies extension contains an object identifier (OID)
which indicates the policy under which the certificate has been
issued. Use of policies is discussed elsewhere in this draft.
4.2.4 Key Attributes
The keyAttributes extension contains information about the key itself
including a unique key identifier, a key usage period (lifetime of
the key as opposed to the lifetime of the certificate), and key
usage. The Internet certificate should use the keyAttributes
extension and contain a key identifier and private key validity to
aid in system management. The key usage field in this extension is
intended to be advisory (as contrasted with the key usage restriction
extension which imposes mandatory restrictions). The key usage field
in this extension should not be used.
KeyAttributes ::= SEQUENCE {
keyIdentifier KeyIdentifier OPTIONAL,
intendedKeyUsage KeyUsage OPTIONAL,
privateKeyUsagePeriod PrivateKeyValidity OPTIONAL }
KeyIdentifier ::= OCTET STRING
PrivateKeyValidity ::= SEQUENCE {
notBefore [0] GeneralizedTime OPTIONAL,
notAfter [1] GeneralizedTime OPTIONAL }
KeyUsage ::= BIT STRING {
digitalSignature (0),
nonRepudiation (1),
keyEncipherment (2),
dataEncipherment (3),
keyAgreement (4),
keyCertSign (5),
offLineCRLSign (6) }
4.2.5 Key Usage Restriction
The keyUsageRestriction extension defines mandatory restrictions on
the use of the key contained in the certificate based on policy
and/or usage (e.g., signature, encryption). This field should be
used whenever the use of the key is to be restricted based on either
usage or policy (see discussion in policies). The usage restriction
would 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).
Housley, Ford, Farrell, Solo [Page 13]
INTERNET DRAFT November 1995
keyUsageRestriction ::= SEQUENCE {
certPolicySet SEQUENCE OF CertPolicyId OPTIONAL,
restrictedKeyUsage KeyUsage OPTIONAL }
4.2.6 Basic Constraints
The basicConstraints extension identifies whether the subject of the
certificate is a CA or an end user. In addition, this field can
limit the authority of the CA in terms of the certificates it can
issue. Discussion of certification path restriction is covered
elsewhere in this draft. The subject type field should be present in
all Internet certificates.
basicConstraints ::= SEQUENCE {
subjectType SubjectType,
pathLenConstraint INTEGER OPTIONAL,
permittedSubtrees SEQUENCE OF GeneralName OPTIONAL,
excludedSubtrees SEQUENCE OF GeneralName OPTIONAL }
SubjectType ::= BIT STRING {
cA (0),
endEntity (1) }
4.2.7 CRL Distribution Points
The cRLDistributionPoints extension identifies the CRL distribution
point or points to which a certificate user should refer to acertain
if the certificate has been revoked. This extenstion provides a
mechanism to divide the CRL inot manageable pieces if the CA has a
large constituency.
<< Need a section which discusses the alternatives. Should permit
URLs as one method to name the location for the most recent CRL. >>
4.2.8 Information Access
The informationAccess field is proposed as a private extension to
tell how information about a subject or CA (or ancillary CA services)
may be accessed. For example, this field might provide a pointer to
information about a user (e.g., a URL) or might tell how to access CA
information such as certificate status or on-line validation
services. The structure of this extension is TBD.
<< Suggestions on the ASN.1 syntax are welcome. >>
Housley, Ford, Farrell, Solo [Page 14]
INTERNET DRAFT November 1995
4.2.9 Other extensions
The DAM defines additional extensions; however, this draft does not
include them as there use is not part of the basic Internet profile.
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 draft is to create a profile for
X.509 v2 CRLs that will foster interoperability and a reusable public
key infrastructure. To achieve this goal, some assumptions need to
be made about the nature of information to be included along with
guidelines for how extensibility will be employed.
CRLs 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
draft is to establish a common baseline for generic applications
requiring broad interoperability and limited special purpose
requirements. Emphasis will be on support for X.509 v2 CRLs. The
draft will define a baseline set of information along with common
locations within a CRL and common representations for common
information. Environments with additional 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 ::= SIGNED SEQUENCE {
version Version DEFAULT v1,
signature AlgorithmIdentifier,
issuer Name,
lastUpdate UTCTime,
nextUpdate UTCTime,
revokedCertificates SIGNED SEQUENCE OF SEQUENCE {
signature AlgorithmIdentifier,
issuer Name,
userCertificate SerialNumber,
revocationDate UTCTime,
crlEntryExtensions Extensions OPTIONAL } OPTIONAL,
crlExtensions [0] Extensions OPTIONAL } }
Housley, Ford, Farrell, Solo [Page 15]
INTERNET DRAFT November 1995
Version ::= INTEGER { v1(0), v2(1) }
SerialNumber ::= INTEGER
The following items describe a proposed use of the X.509 v2 CRL for
the Internet.
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 (value is 1).
If neither CRL extensions nor CRL entry extensions are present, use
version 1 (the value is omitted).
5.1.2 Signature
This field contains the algorithm identifier for the algorithm used
to sign the CRL.
5.1.3 Issuer Name
The issuer name provides a globally unique identifier of the
certification authority signing the CRL. The syntax of the issuer
name is an X.500 distinguished name. This strawman suggests that the
issuer name must provide a globally unique identifier. In addition,
it should contain semantic information identifying the certification
authority.
<< Any changes to 4.1.4 must be reflected here too. >>
5.1.4 Last Update
This field indicates the date on which this CRL was issued.
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.
5.1.6 Revoked Certificates
Revoked certificates are listed. The certificates are named by the
combination of the issuer name and the user certificate serial
number. The date on which the revocation occured is specified. Each
revocation entry is individually signed. This profile mandates the
use of same signature algorithm to sign each CRL entry and the whole
CRL. CRL entry extensions are discussed in section 5.3.
Housley, Ford, Farrell, Solo [Page 16]
INTERNET DRAFT November 1995
5.2 CRL Extensions
The extensions already defined by ANSI X9 and ISO for X.509 v2 CRLs
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 extension.
However, an unrecognized non-critical extension may be ignored. The
following presents recommended extensions used within Internet CRLs
and standard locations for information. 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 authorityKeyIdentifier is a non-critical CRL extension that
allows the CA to include an identifier of the key used to sign the
CRL. This extension is useful when a CA uses more than one key. See
section 7 for a discussion key changeover.
AuthorityKeyId ::= SEQUENCE {
keyIdentifier [0] KeyIdentifier OPTIONAL,
certIssuer [1] Name OPTIONAL,
certSerialNumber [2] CertificateSerialNumber OPTIONAL }
( CONSTRAINED BY {
-- certIssuer and certSerialNumber constitute a logical pair,
-- and if either is present both must be present. Either this
-- pair or the keyIdentifier field or all shall be present. -- } )
5.2.2 Issuer Alternative Name
The issuerAltName is a non-critical CRL extension that provides a CA
name, in a form other than an X.500 distinguished name. The syntax
for the issuerAltName is the same as described in section 4.2.1. Each
of the alternate names is an IA5 string. Multiple instances may be
included. Whenever such alternative names are included in a CRL, the
issuer alternative name field shall be used.
5.2.3 CRL Number
The cRLNumber is a non-critical CRL extension which conveys a
monotonically increacing sequence number for each CRL issued by a
given CA through a given CA X.500 Directory entry or CRL distribution
point. This extension allows users to easily determine is a
particular CRL superceeds another CRL. Use of this CRL extension is
strongly encouraged.
Housley, Ford, Farrell, Solo [Page 17]
INTERNET DRAFT November 1995
CRLNumber ::= INTEGER
5.2.4 Issuing Distribution Point
The issuingDistributionPoint is a critical CRL extension that
identifiers the CRL distribution point for this particular CRL, and
it indicates whether the CRL covers revocation for end entities
certificate only, CA certificates only, or a limitied set of reason
codes. Support for CRL distribution points is strongly encouraged.
However, the use of certificateHold is strongly discouraged.
DistributionPoint ::= SEQUENCE {
distributionPoint DistributionPointName,
reasons ReasonFlags OPTIONAL }
DistributionPointName ::= CHOICE {
fullName [0] Name,
nameRelativeToCA [1] RelativeDistinguishedName }
ReasonFlags ::= BIT STRING {
unused (0),
keyCompromise (1),
caCompromise (2),
affiliationChanged (3),
superseded (4),
cessationOfOperation (5),
certificateHold (6) }
5.2.5 Delta CRL Indicator
The deltaCRLIndicator is a critical CRL extension that identifies a
delta-CRL. The use of delta-CRLs is strongly discouraged. Rather,
CAs are encouraged to always issue complete CRLs.
5.3 CRL Entry Extensions
The CRL entry extensions already defined by ANSI X9 and ISO for X.509
v2 CRLs 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. However, an unrecognized
non-critical CRL entry extension may be ignored. The following
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
Housley, Ford, Farrell, Solo [Page 18]
INTERNET DRAFT November 1995
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. The inclusion of reason
codes is encouraged. The reasonCode extension permits certificates
to placed on hold or suspended. The processing associated with
suspended certificates greatly complicates certificate validation.
The use of this feature is strongly discouraged.
CRLReason ::= ENUMERATED {
unspecified (0),
keyCompromise (1),
caCompromise (2),
affiliationChanged (3),
superseded (4),
cessationOfOperation (5),
certificateHold (6),
certHoldRelease (7),
removeFromCRL (8) }
5.3.2 Expiration Date
The expirationDate is a non-critical CRL entry extension that
indicates the expiration of a hold entry in a CRL. The use of this
extension is strongly discouraged.
5.3.3 Instruction Code
The instructionCode 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 use of this extension is strongly discouraged.
5.3.4 Invalidity Date
The invalidityDate 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
(which is the date that the CA revoked the certificate). The use of
this extension is encouraged.
InvalidityDate ::= GeneralizedTime
Housley, Ford, Farrell, Solo [Page 19]
INTERNET DRAFT November 1995
5.4 Examples
<< CRL samples including descriptive text and ASN.1 encoded blobs
will be inserted. >>
6 Certificate and CRL Distribution
6.1 Distribution via X.500
Within an X.500 Directory, the certificate for an end entity can be
found in the userCertificate attribute. This attribute is normally
associated with the strongAuthenticationUser object class.
Within an X.500 Directory, the certificate for a certification
authority can be found in the cACertificate attribute, and the most
recent CRL can be found in the certificateRevocationList attribute.
These attributes are normally associated with the
certificationAuthority object class.
6.2 Distribution via Electronic Mail
RFC 1424 specifies methods for key certification, certificate
revocation list (CRL) storage, and CRL retrieval. These services are
required of an RFC 1422 certification authority. Each service
involves an electronic mail request and an electronic mail reply.
<< Need to define a format for one user to send his certificate to
another. This format could be used to obtain arbitrary certificates
from a certificate server or to solicit certificates from the user
themselves. >>
6.3 Distribution via HTTP
<< Need to define a convention for using HTTP to obtain certificates
from a server. >>
As discussed in section 4.2.7, the user certificate may contain a URL
that specifies the location where the most recent CRL which could
contain an entry revoking the certificate can be found. HTTP can be
used to fetch the most recent CRL from this location.
6.4 On-line Certificate Validation
As discussed above, consumers of certificates must be able to
determine the validity of a certificate when using the certificate.
There are many possible approaches to informing consumers on the
status of the certificate and these approaches have different
operational characteristics. One alternative is to provide an on-
Housley, Ford, Farrell, Solo [Page 20]
INTERNET DRAFT November 1995
line validation service. Such a service reduces the complexity of
the client applications (by moving it to the on-line service), and it
provides the most timely status possible.
In addition, on-line validation servers can also help to resolve the
root key management a distribution problem by providing a single
trusted agent for asserting root key status where the agent is
independent of the certification hierarchy itself.
The on-line validation could be performed by either the CA who issued
the certificate (directly or via a delegatee) or as a general service
by a "trusted" third party. Note, this service could also be
extended to the validation of any certificate like item (e.g., PGP
credential, DNS record, STT credential) and could facilitate
application interaction between users using different certificate
formats.
The general model involves a request/response format which might be
transferred using a number of alternative transport protocols. In
general, the requestor sends the certificate (or a user reference)
along with an indication of the service to be provided. This service
might be coupled with the general certificate distribution service by
adding service flags to that request as well.
The request should contain: << this section is still in progress >>
Certificate (or cert path)
Service parameters
Parse cert for me
Check CRLs
Result format (ASN, Text, HTML, ....)
Sign result (with a specified algorithm)
Other qualifiers
Desired domain/policy OID (does this validate to a specific Root)
A possible Syntax:
ValidationRequest ::= SEQUENCE {
CertPathType OBJECT IDENTIFIER,
CertPath SEQUENCE OF OCTET STRING,
TargetRootID Name OPTIONAL,
ServiceParams SEQUENCE OF ServiceParam OPTIONAL }
ServiceParam ::= INTEGER {
ASNresult (1),
Textresult (2),
HTMLresult (3),
.... }
Housley, Ford, Farrell, Solo [Page 21]
INTERNET DRAFT November 1995
The response should contain:
Status: current/valid, expired (date), revoked (date/reason), suspended
Cert path problem: what failed, where, and why
Policy/attribute/constraints from validated cert path
Parsed data: name, key, attributes
Could be signed by validator or rely on secure channel
A possible syntax:
ValidationResponse ::= OPTIONALLY SIGNED SEQUENCE {
validator Name OPTIONAL,
certInfo CHOICE {
cert OCTET STRING,
reference IssuerSerial,
certdata T61 STRING },
-- text including name, key, and attributes
status StatusCode,
detail ANY Defined By StatusCode OPTIONAL,
validationData ???? }
-- problems with cert path, policy attributes, etc.
StatusCode ::= INTEGER {
valid (1),
revoked (2),
expired (3),
suspended (4) }
7 Key Pair Updating Procedures
A fundamental principle of the PKI is that it must be possible to
update all of the cryptographic keys used, both by end entity's and
by PKI components (e.g., CAs). Furthermore, for the PKI to be
usable, the update of one key pair must not force the update of any
other key pair or Certificate. In this section, we deal with the
update of CA key pairs. Key updating for end entities is dealt with
in section 9.4.
For CA key pair updating we will fulfil the following requirements:
(a) All certificates valid before the update must remain valid.
(b) A subject whose certificate is verifiable using the new CA
public key must also be able to verify certificates verifiable using
the old public key.
(c) End entities who directly trust the old CA key pair must be able
to verify certificates signed using the new key CA private key. This
Housley, Ford, Farrell, Solo [Page 22]
INTERNET DRAFT November 1995
is required for situations where the old CA public key is "hardwired"
into the end entity's cryptographic equipment (e.g., smartcard
memory).
(d) All entities (not just those certified by that CA) must have
both the new and old CA public keys available from the time of the
change (whether or not they trust it is a local matter).
The basis of the scheme described below is that the CA protects its
new public key using its previous private key and vice-versa. Thus
when a CA updates its key pair it must generate two new cACertificate
attribute values if certificates are made available using an X.500
directory.
Note that the scheme below does not make use of any of the X.509 v3
certificate extensions as it must be able to work for X.509 v1
certificates. However, the presence of the KeyIdentifier extension
permits efficiency improvements.
Note that the change of a CA key affects both certificate
verification and CRL checking.
It is worth noting that the operation involved here is key update,
only the key pair (and related attributes) of the CA are changed.
Thus, this operation cannot be used in the event of a CA key
compromise.
While the scheme could be generalised to cover cases where the CA
updates its key pair more than once during the validity period of one
of its end entity's certificates, this generalisation seems of
dubious value. Therefore, the validity period of a CA key must be
greater than the validity period of any certificate issued by that
CA.
We first present the data structures required then specify the steps
involved in changing the CA key and the various possibilities for
certificate verification. Note that the description below assumes
that X.500 is used for publishing certificates. This assumption is
simply for clarity of presentation, if the same data structures are
published some other way, the scheme still works.
7.1 ASN.1 Data Types
-- existing CA cert from X.509
-- this contains the current and old CA certificate(s)
-- all entities under this CA need a local copy of
-- one of these
CACertificate ::= ATTRIBUTE
Housley, Ford, Farrell, Solo [Page 23]
INTERNET DRAFT November 1995
WITH ATTRIBUTE-SYNTAX Certificate
-- Securing the old CA public key with the new private key and
-- vice-versa. Securing the new CA public key with the old private
-- key is needed to avoid having to issue the new CA public key
-- using out-of-band means to entities certified using the old CA
-- key; with this they can verify certificates signed using the new
-- CA private key.
-- The data structures can be stored in this X.500 attribute
CALinkages ::= ATTRIBUTE
WITH ATTRIBUTE-SYNTAX CALinkage
CALinkage ::= SIGNED SEQUENCE {
protectedCACertSerial INTEGER,
-- the serial number in the CACertificate
-- value which we wish to link to
protectingCACertSerial INTEGER,
-- the serial number in the CACertificate
-- value which contains the public key
-- corresponding to the private key used
-- to sign this
caName Name,
link HASH Certificate }
7.2 CA Operator Actions
To change the key of the CA, the CA operator does the following:
(1) Generates a new key pair.
(2) Calculate the certificate for the new key pair.
(3) Create a CALinkage (based on the old CA certificate) using the
new private key.
(4) Create a CALinkage (based on the new CA certificate) using the
old private key.
(5) Publish these new data structures.
7.3 Verifying Certificates
Normally when verifying a signature, the verifier simply verifies the
certificate containing the public key of the signer. However, once a
CA is allowed to update it's key there are a range of new
possibilities. These are shown in the table below.
The term PSE (personal security environment) is used to denote
Housley, Ford, Farrell, Solo [Page 24]
INTERNET DRAFT November 1995
locally held and trusted information. This can only be assumed to
include a single CA public key.
CACertificate contains CACertificate contains only
NEW public key OLD public key
PSE Contains PSE Contains PSE Contains PSE Contains
NEW public OLD public NEW public OLD public
key key key key
Signer's Case 1: Case 3: Case 5: Case 7:
cert is This is the In this case Although the In this case
protected standard the verifier CA operator the CA
using NEW case where must access has not operator has
public the the updated the not updated
key verifier directory in directory the the directory
can order to get verifier can attributes
directly the value of verify the and so the
verify the the NEW certificate verification
certificate public key directly - will FAIL
without this is thus
using the the same as
directory case 1.
Signer's Case 2: Case 4: Case 6: Case 8:
cert is In this In this case The verifier Although the
protected case the the verifier thinks this CA operator
using OLD verifier can directly is the has not
public must access verify the situation of updated the
key the certificate case 2 and directory, the
directory without will access verifier can
in order to using the the verify the
get the directory directory, certificate
value of however the directly --
the OLD verification this is thus
public key will FAIL the same as
case 4.
7.3.1 Verification in cases 1, 4, 5 and 8
In these cases the verifier has a local copy of the CA public key
which can be used to verify the certificate directly. This is the
same as the situation where no key change has ever occurred.
Note that case 8 may arise between the time when the CA operator has
generated the new key pair and the time when the CA operator stores
the updated attributes in the Directory. Case 5 can only arise if
the CA operator has issued both the signer's and verifier's
Housley, Ford, Farrell, Solo [Page 25]
INTERNET DRAFT November 1995
certificates during this "gap" (the CA operator should avoid this as
it leads to the failure cases described below).
7.3.2 Verification in case 2
In case 2 the verifier must get access to the old public key of the
CA. The verifier does the following:
(1) Lookup the CACertificate attribute in the directory and pick the
appropriate value.
(2) Lookup the associated CALinkages attribute value.
(3) Verify that these are correct using the new CA key (which the
verifier has locally).
(4) If correct then check the signer's certificate using the old CA
key.
Case 2 will arise when the CA operator has issued the signer's
certificate, then changed key and then issued the verifier's
certificate, so it is quite a typical case.
7.3.3 Verification in case 3
In case 3 the verifier must get access to the new public key of the
CA. The verifier does the following:
(1) Lookup the CACertificate attribute in the directory and pick the
appropriate value.
(2) Lookup the associated CALinkages attribute value.
(3) Verify that these are correct using the old CA key (which the
verifier has stored locally).
(4) If correct then check the signer's certificate using the new CA
key.
Case 3 will arise when the CA operator has issued the verifier's
certificate, then changed key and then issued the signer's
certificate, so it is also quite a typical case.
7.3.4 Failure of verification in case 6
In this case, the CA has issued the verifier's PSE containing the new
key without updating the directory attributes. This means that the
verifier has no means to get a trustworthy version of the CA's old
Housley, Ford, Farrell, Solo [Page 26]
INTERNET DRAFT November 1995
key and so verification fails.
Note that the failure is the CA operator's fault.
7.3.5 Failure of verification in case 7
In this case the CA has issued the signer's certificate protected
with the new key without updating the directory attributes. This
means that the verifier has no means to get a trustworthy version of
the CA's new key and so verification fails.
Note that the failure is the CA operator's fault.
7.4 Revocation - Change of CA Key
As we saw above, the verification of a certificate becomes more
complex once the CA is allowed to change its key. This is also true
for revocation checks as the CA may have signed the CRL using a newer
private key than the one within the user's PSE. The analysis of the
alternatives is exactly as for certificate verification.
8 Guidelines for Certificate Policy Definition
<< To Be Decided >>
9 Supporting Management Protocols
The certificate management protocol exchanges defined in this section
support management communications between client systems, each of
which supports one or more users, and CAs. In addition, one
management protocol exchange is defined for use between two CAs, for
the purpose of establishing cross-certificates. Each exchange is
defined in terms of a sequence of messages between the two systems
concerned. This section defines the contents of the messages
exchanged.
The protocols for conveying these exchanges in different environments
(on-line, E-mail, and WWW) are specified in Section 10.
The protocol exchanges defined in this document are:
- One-Step Registration/Certification
- User Registration
- User Initialization/Certification with Client-Generated
Encryption Key Pair
- User Initialization/Certification with Centrally-Generated
Encryption Key Pair
- Encryption Key Pair Recovery
Housley, Ford, Farrell, Solo [Page 27]
INTERNET DRAFT November 1995
- Key Pair Update for Client-Generated Key Pair
- Key Pair Update for Centrally-Generated Key Pair
- Key Pair Update (Centrally-Initiated)
- Revocation Request
- Cross-Certification
The following notes apply to the protocol exchange descriptions:
- In exchanges between a client system and a CA, the protocol exchange
is initiated by the client system. The one exception to this is the
Key Pair Update (Centrally-Initiated) exchange.
- To provide an upgrade path, a protocol version indicator is always
included in the first message of an exchange.
- A message type indicator is included in the protected part of all
messages.
- All messages include an optional transaction identifier which is used
to assist correlation of request and response messages for one
transaction. This identifier is generated by the initiator of the
exchange and will typically include the initiator's name plus a
transaction sequence number.
- The initial message from the client to the CA may optionally contain
the client system time. This is used to facilitate the correction
of client time problems by central administrators.
- Responses from CA to client include the CA system time. The client
can use this time to check that its own system time is within a
reasonable range.
- Random numbers are used in some of the protocols to prevent replay
of the exchanges.
- Responses can be aborted at any time. An enumerated error code is
sent from the aborting end and can be decoded into a user readable
error string at the other end. Error codes are not specified in
this version of this document.
- Items in square brackets [] are optional.
- In every instance in which a public key is transferred, it is
transferred in the form of X.509 subjectPublicKeyInfo, including
algorithm identifier and (optional) parameters.
- When a new key pair is generated by a client, a key identifier may
optionally be sent to the CA along with the public key for inclusion
in the certificate. However, the CA may override this value with a
key identifier of its own. If the client is concerned about the key
identifier value used, it should check the new certificate.
- Where this description refers to an encryption key pair, this could
be a key pair for RSA key transport or could be key pair for key
establishment using, for example, a Diffie-Hellman based algorithm.
Note that in this version of this document, the message contents are
defined at an outline level only. A future version of this document
will fill out the full details of message syntax in ASN.1.
Housley, Ford, Farrell, Solo [Page 28]
INTERNET DRAFT November 1995
9.1 One-Step Registration/Certification
9.1.1 Overview of Exchange
This protocol exchange is used to support registration of a user,
together with request and issue of certificate(s), for use in
environments in which client systems generate their own key pair or
pairs. It is a simple exchange, designed
for easy implementation, but lacks some of the features and
protective measures inherent in the exchanges defined subsequently.
The user must have a pre-established digital signature key pair.
Furthermore, the user must have a preestablished reliably-known copy
of the public key of the CA concerned (this generally requires some
form of off-line data exchange to ensure that the correct public key
is known).
If the request is accepted by the CA, it results in the generation of
certificate(s) for client-generated digital signature and/or
encryption public keys.
9.1.2 Detailed Description
A single message is used for a user to register with a CA and request
certificate issuance.
RegCertRequest:: client-to-CA
{
protocol version
message type
[transaction identifier]
[client system time]
user unique name (DN)
[user signature public key]
[user signature key identifier]
[client-generated encryption public key]
[client-generated encryption key identifier]
user attributes
[certificate policy]
} Signature (signed with user signature private key)
No specific message is defined to return the generated
certificate(s). It is assumed that the client will obtain a copy of
the certificate(s) by other means and, by checking the certificate
contents and CA signature, ensure that the request was processed by
the correct CA.
Housley, Ford, Farrell, Solo [Page 29]
INTERNET DRAFT November 1995
9.2 User Registration
9.2.1 Overview of Exchange
This protocol exchange is used for a user to request registration
with a CA. It is a first step in the establishment of key materials
and certificates between client and CA for that user. Assuming the
CA accepts the request, it will be necessary to follow-up this
exchange with a User Initialization/Certification exchange as
described in 9.3 or 9.4. At the time this request is issued, it is
not necessary for the client to have any established key materials.
9.2.2 Detailed Description
A single message is used for a user to request registration with a
CA.
RegisterUserRequest:: client-to-CA
{
protocol version
message type
[transaction identifier]
[client system time]
user unique name (DN)
user attributes
[certificate policy]
} Signature (signed with user signature private key)
No specific message is defined to respond to this request. It is
asumed that the procedure defined in 9.3 or 9.4 will follow.
9.3 User Initialization/Certification with Client-Generated Encryption
Key Pair
9.3.1 Overview of Exchange
This protocol exchange is used to support client initialization,
including certificate issuance, for one user, with provision for
simultaneously establishing and certifying separate key pairs for
digital signature and encryption (or encryption key exchange)
purposes. Both key pairs are generated by the client and no private
key is exposed to the CA. Generation and certification of the
encryption key pair is optional.
Prior to conducting this exchange, the user must have registered with
the CA, either using the user registration exchange defined in 9.2 or
by other means.
Housley, Ford, Farrell, Solo [Page 30]
INTERNET DRAFT November 1995
Following registration, the CA creates a secret data item, called an
authorization code, and transfers this data item by out-of-band
means to the user. The authorization code is used to establish
authentication and integrity protection of the user
initialization/certification on-line exchange. This is done by
generating a symmetric key based on the authorization code and using
this symmetric key for generating Message Authentication Codes (MACs)
on all exchanges between client and CA.
In the first two messages exchanged, the client sends its user
signature public key (and, optionally, a client-generated encryption
public key) to the CA and the CA returns the currently valid CA
certificate(s). This exchange of public keys allows the client and CA
to authenticate each other.
9.3.2 Detailed Description
The user receives a reference number and a secret machine-generated
authorization code from the CA administrator. Both pieces of
information are transferred to the user in a secure manner which
preserves their integrity and confidentiality. The reference number
is used to uniquely identify the client at the CA and the
authorization code is used to secure the exchange integrity-wise. The
reference number is used instead of a DN to uniquely identify the
client because a DN may be lengthy and difficult for a user to
manually type without error.
After the reference number and authorization code have been entered
by the user, the client generates:
- a client random number,
- (if a new user signature key pair is required) a new user
signature key pair,
- (if a new client-generated encryption key pair is required) a
new encryption key pair.
The client securely stores locally any new signature private key
and/or client-generated encryption private key. The client then
sends the message InitClientRequest to the CA. The entire structure
is protected from modification with a MAC based on the authorization
code.
InitClientRequest:: client-to-CA
{
protocol version
message type
[transaction identifier]
[client system time]
Housley, Ford, Farrell, Solo [Page 31]
INTERNET DRAFT November 1995
client random number
reference number
user signature public key
[user signature key id]
[client-generated encryption public key]
[client-generated encryption key id]
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the InitClientRequest structure, if the CA recognizes
the reference number and if the protocol version is valid, it saves
the client random number, generates its own random number (CA random
number), and validates the MAC. Then for the user encryption public
key, it creates:
- a new certificate for the user?s digital signature public key,
- (if a new client-generated encryption key pair is required) a
new certificate.
The CA responds to the client with the message InitClientResponse.
The entire structure is protected from modification with a MAC based
on the authorization code.
InitClientResponse:: CA-to-client
{
message type
[transaction identifier]
client random number
CA random number
CA signature public key certificate
new user signature public-key certificate
[new user encryption public-key certificate]
CA system time
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the InitClientResponse structure, the client checks
that its own system time is sufficiently close to the CA system time,
checks the client random number, and validates the MAC. The client
then securely stores the new certificates and acknowledges the
transaction by sending back the message InitClientConfirm. The fields
in this message are protected from modification with a MAC based on
the authorization code.
InitClientConfirm:: client-to-CA
{
message type
[transaction identifier]
Housley, Ford, Farrell, Solo [Page 32]
INTERNET DRAFT November 1995
client random number
CA random number
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the InitClientConfirm structure, the CA checks the
random numbers and validates the MAC. If no errors occur, the CA
archives the new user public-key certificate(s).
9.4 User Initialization/Certification with Centrally-Generated
Encryption Key Pair
9.4.1 Overview of Exchange
This protocol exchange is used to support client initialization,
including certificate issuance, for one user, with provision for
simultaneously establishing and certifying separate key pairs for
digital signature and encryption (or encryption key exchange)
purposes. The digital signature key pair is generated by the client.
Optionally, a new encryption key pair is generated by (and,
optionally, backed up by) a central facility associated with the CA.
Prior to conducting this exchange, the user must have registered with
the CA, either using the user registration exchange defined in 9.2 or
by other means.
Following registration, the CA creates a secret data item, called an
authorization code, and transfers this data item by out-of-band
means to the user. The authorization code is used to establish
authentication and integrity protection of the user
initialization/certification on-line exchange. This is done by
generating a symmetric key based on the authorization code and using
this symmetric key for generating Message Authentication Codes (MACs)
on all exchanges between client and CA.
In the first two messages exchanged, the client sends its user
signature public key to the CA and the CA returns the currently valid
CA certificate(s). This exchange of public keys allows the client and
CA to authenticate each other.
If a centrally-generated encryption key pair is to be established,
the private key of the newly generated key pair is sent from the CA
to the client. The client first generates a protocol encryption key
pair and sends the public protocol encryption key to the CA. The CA
creates a random symmetric key called the session key and encrypts
the user encryption private key with it and then encrypts the session
key with the public protocol encryption key it received from the
client. The CA sends the encrypted user encryption private key and
Housley, Ford, Farrell, Solo [Page 33]
INTERNET DRAFT November 1995
encrypted session key back to the client. The client uses its private
protocol decryption key to decrypt the session key and then uses the
session key to decrypt the encryption private key. The protocol
encryption key pair and session key are discarded after the exchange.
9.4.2 Detailed Description
The user receives a reference number and a secret machine-generated
authorization code from the CA administrator. Both pieces of
information are transferred to the user in a secure manner which
preserves their integrity and confidentiality. The reference number
is used to uniquely identify the client at the CA and the
authorization code is used to secure the exchange integrity-wise. The
reference number is used instead of a DN to uniquely identify the
client because a DN may be lengthy and difficult for a user to
manually type without error.
After the reference number and authorization code have been entered
by the user, the client generates:
- a client random number,
- (if a new user signature key pair is required) a new user
signature key pair,
- (if a new centrally-generated encryption key pair is required)
a protocol encryption key pair.
The client securely stores locally any new signature private key
and/or client-generated encryption private key. The client then
sends the message InitCentralRequest to the CA. The entire structure
is protected from modification with a MAC based on the authorization
code.
InitCentralRequest:: client-to-CA
{
protocol version
message type
[transaction identifier]
[client system time]
client random number
reference number
user signature public key
[user signature key id]
[protocol encryption key]
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the InitCentralRequest structure, if the CA
recognizes the reference number and if the protocol version is valid,
Housley, Ford, Farrell, Solo [Page 34]
INTERNET DRAFT November 1995
it saves the client random number, generates its own random number
(CA random number), and validates the MAC. It then creates:
- a new certificate for the user?s digital signature public key,
- (if a new centrally-generated encryption key pair is required)
a session key, a new user encryption key pair, and a new
certificate for the user encryption public key.
The CA responds to the client with the message InitCentralResponse.
If a new centrally-generated encryption key pair is being generated,
the user encryption private key is encrypted using the session key
and the session key is encrypted with the protocol encryption public
key. The entire structure is protected from modification with a MAC
based on the authorization code.
InitCentralResponse:: CA-to-client
{
message type
[transaction identifier]
client random number
CA random number
CA signature public key certificate
new user signature public-key certificate
[new user encryption public-key certificate]
[new user encryption private key encrypted with session key]
[session key encrypted with protocol encryption key]
CA system time
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the InitCentralResponse structure, the client checks
that its own system time is sufficiently close to the CA system time,
checks the client random number, and validates the MAC. If a new
centrally-generated encryption key pair is included, the client
decrypts the encryption private key. The client then securely stores
the new certificates and encryption private key (if present) and
acknowledges the transaction by sending back the message
InitCentralConfirm. The fields in this message are protected from
modification with a MAC based on the authorization code.
InitCentralConfirm:: client-to-CA
{
message type
[transaction identifier]
client random number
CA random number
MAC algorithm id
} MAC (key based on authorization code)
Housley, Ford, Farrell, Solo [Page 35]
INTERNET DRAFT November 1995
Upon receipt of the InitCentralConfirm structure, the CA checks the
random numbers and validates the MAC. If no errors occur, the CA
archives the new user public-key certificate(s) and (if there is a
new centrally-generated encryption key pair and key recovery is to be
supported) the encryption private key.
9.5 Encryption Key-Pair Recovery
9.5.1 Overview of Exchange
This protocol exchange is used to support recovery in the event that
a client no longer has a valid signature key pair (due to expiration
or revocation), or client system key materials have been lost, e.g.,
as a result of a forgotten user password. This exchange assumes a
system in which an encryption key pair has been centrally generated
and backed up (by a central system associated with a CA).
This exchange is very similar to the exchange for User
Initialization/Certification with Centrally-Generated Encryption Key
Pair. The client and CA start without a way to trust one another,
i.e., they have no reliable shared key pairs.
9.5.2 Detailed Description
The user must first receive, by out-of-band means, a reference number
and a secret machine-generated authorization code from the CA
administrator. The on-line exchange then consists of a sequence of
KeyRecoverRequest, KeyRecoverResponse and KeyRecoverConfirm, which
are the same as the exchange in 9.4 except for two differences.
First, the CA does not generate (or archive) a new encryption key
pair and encryption public-key certificate for the user. Second, the
user?s entire encryption key history (list of encryption public-key
certificates and matching encryption private keys) are sent back to
the client with KeyRecoverResponse.
KeyRecoverRequest:: client-to-CA
{
protocol version
message type
[transaction identifier]
[client system time]
client random number
reference number
user signature public key
[user signature key id]
protocol encryption key
MAC algorithm id
} MAC (key based on authorization code)
Housley, Ford, Farrell, Solo [Page 36]
INTERNET DRAFT November 1995
KeyRecoverResponse:: CA-to-client
{
message type
[transaction identifier]
client random number
CA random number
CA certificate(s)
user encryption private key history encrypted with session key
session key encrypted with protocol encryption key
user encryption public-key certificate history
new user signature public-key certificate
CA system time
MAC algorithm id
} MAC(key based on authorization code)
KeyRecoverConfirm:: client-to-CA
{
message type
[transaction identifier]
client random number
CA random number
MAC algorithm id
} MAC (key based on authorization code)
9.6 Key Pair Update for Client-Generated Key Pair(s)
9.6.1 Overview of Exchange
This exchange is used to update the signature key pair and/or
client-generated encrypyion key pair of a user, (e.g., as a result of
routine cryptoperiod expiry).
A user must have a valid signature key pair in order to do this
exchange. It is up to the client to determine when a new signature
key pair should be generated; this has to be done prior to the
expiration of its signature public-key certificate.
A key pair update request from a client is digitally signed using the
original user signature private key, this signature being verifiable
using an existing signature certificate. If the key pair update is
for a new user digital signature key, then the client signs the
request message once more (including the first signature), this time
using the new signature private key. The reason for this second
signature is to prove to the CA that the client possesses both the
new and old private keys.
The request is verified at the CA by using the matching user
signature public key. A protocol signature key pair is used to
Housley, Ford, Farrell, Solo [Page 37]
INTERNET DRAFT November 1995
authenticate messages from the CA to the client. CA responses are
signed with the protocol signature private key.
A CA response is validated at the client by using a protocol
signature public-key certificate which is included in the CA
response. The protocol signature public-key certificate can be
validated by using the CA certificate stored at the client. A new
user initialization (as in 9.3) or key pair recovery (as in 9.4) must
be done if the user signature key pair becomes invalid.
In some client system implementations, local key materials are stored
in an encrypted key data disk file. A user may have several copies of
this key data file on different computers. It is possible that a key
update could occur and the user could forget to copy the updated key
data file to all the computers they use. To help keep the client
using the latest keys, the client sends the CA the serial number of
the latest user signature public-key certificate it has in the key
update request. Serial numbers are sent so that the CA can check if
the client has the latest key pair. If the client does not have the
latest signature private key and the signature public-key certificate
serial number is equal to that of a previous certificate, the CA
sends back an error code which indicates that the client has an old
version of the key data file. After this, the client can either find
the latest key data file or, if that fails, key recovery can be done.
9.6.2 Detailed Description
The client initiates the exchange by creating a new signature and/or
encryption key pair and generating a random number (client random
number). The client then sends the CA the message
UpdateClientKeyRequest. The fields in this message are protected from
modification and authenticated by a digital signature using the pre-
existing user signature private key. If the update includes a new
signature key pair, the result is additionally signed using the new
user signature private key.
UpdateClientKeyRequest:: client-to-CA
{
protocol version
message type
[transaction identifier]
[client system time]
client random number
user unique name (DN)
[new user signature public key]
[new user signature key id]
[new user encryption public key]
[new user encryption key id]
Housley, Ford, Farrell, Solo [Page 38]
INTERNET DRAFT November 1995
serial number of latest signature public-key certificate
} Signature (signed with pre-existing user signature private key)
[Signature (signed with new user signature private key)]
Upon receipt of the UpdateClientKeyRequest structure, the CA checks
the protocol version, checks the serial number, saves the client
random number, generates its own random number (CA random number) and
verifies the signature using the previous user verification key which
is archived at the CA. If a user digital signature key pair is being
updated, the CA also checks the second signature. It then generates
new user signature and/or encryption public-key certificate(s). The
CA responds with the message UpdateClientKeyResponse. The fields in
this message are protected from modification and authenticated by a
digital signature using the CA protocol signature private key.
UpdateClientKeyResponse:: CA-to-client
{
message type
[transaction identifier]
client random number
CA random number
protocol signature public-key certificate
[new user signature public-key certificate]
[new user encryption public-key certificate]
CA system time
} Signature (signed with protocol signature private key)
Upon receipt of the UpdateClientKeyResponse structure, the client
verifies the digital signature using the protocol verification key
contained in the protocol signature public-key certificate, checks
that its own system time is close to the CA system time, and checks
the received client random number. The client then securely stores
locally the new user public-key certificate(s). It responds with the
message UpdateClientKeyConfirm. The fields in this message are
protected from modification and authenticated by a digital signature
using the pre-existing user signature private key.
UpdateClientKeyConfirm:: client-to-CA
{
message type
[transaction identifier]
client random number
CA random number
} Signature (signed with pre-existing user signature private key)
Upon receipt of the UpdateClientKeyConfirm structure, the CA checks
that the client and CA random numbers are the same as the ones
initially generated, and verifies the received signature using the
Housley, Ford, Farrell, Solo [Page 39]
INTERNET DRAFT November 1995
previous user signature public key which is archived at the CA. The
CA then archives the new user public-key certificate(s) and updates
its data stores to reflect the new status of the user.
9.7 Key Pair Update for Centrally-Generated Encryption Key Pair
9.7.1 Overview of Exchange
This exchange is used to update the encryption key pair of an user,
under the assumption that encryption key pairs are generated (and,
optionally, backed up) centrally. A user must have a valid signature
key pair in order to do this exchange. It is up to the client to
determine when a new encryption key pair should be generated; this
has to be done some time before the expiration date in its encryption
public-key certificate.
9.7.2 Detailed Description
The client initiates the exchange by generating a random number
(client random number) and a protocol encryption key pair. The client
then sends the CA the message UpdateEncKeyRequest1. The fields in
this message are protected from modification and authenticated by a
digital signature using the latest user signature private key.
UpdateCentralKeyRequest:: client-to-CA
{
protocol version
message type
[transaction identifier]
[client system time]
client random number
user unique name (DN)
latest user encryption public-key certificate serial number
latest user signature public-key certificate serial number
protocol encryption key
} Signature (signed with latest user signature private key)
Upon receipt of the UpdateCentralKeyRequest structure, the CA checks
the protocol version, checks the serial numbers, saves the client
random number, generates its own random number (CA random number),
generates a session key, and verifies the received signature using
the latest user signature public key which is archived at the CA. It
then generates a new end-user encryption key pair and encryption
public-key certificate for the user. In the case where the encryption
public-key certificate serial number is the second latest, the CA
does not generate any keys and uses the latest encryption public-key
certificate and encryption private key that it has. The CA responds
with the message UpdateEncKeyResponse1. In this message, the new or
Housley, Ford, Farrell, Solo [Page 40]
INTERNET DRAFT November 1995
latest encryption private key is encrypted with the session key and
the session key is encrypted with the protocol encryption key. The
fields in this message are protected from modification and
authenticated by a digital signature using the protocol signature
private key.
UpdateCentralKeyResponse:: CA-to-client
{
message type
[transaction identifier]
client random number
CA random number
new or latest user encryption private key encrypted with session key
new or latest user encryption public-key certificate
session key encrypted with protocol encryption key
protocol signature public-key certificate
CA system time
} Signature (signed with protocol signature private key)
Upon receipt of the UpdateCentralKeyResponse structure, the client
verifies the digital signature using the protocol signature public-
key certificate, makes sure its own system time is close to the CA
system time, and checks the received client random number. The client
then decrypts the new or latest encryption private key and securely
stores locally the new or latest user encryption public-key
certificate and encryption private key. It responds with the message
UpdateCentralKeyConfirm. The fields in this message are protected
from modification and authenticated by a digital signature using the
latest user signature private key.
UpdateCentralKeyConfirm:: client-to-CA
{
message type
[transaction identifier]
client random number
CA random number
} Signature (signed with latest user signature private key)
Upon receipt of the UpdateClientKeyConfirm structure, the CA checks
that the client and CA random numbers are correct and verifies the
signature using the latest user signature public key which is
archived at the CA. If no errors occur, the CA archives the new user
encryption public-key certificate and encryption private key, and
updates its data stores to reflect the new status of the user.
Housley, Ford, Farrell, Solo [Page 41]
INTERNET DRAFT November 1995
9.8 Key Pair Update (Centrally-Initiated)
9.8.1 Overview of Exchange
This exchange is used to update the encryption key pair of an user,
under the assumption that encryption key pairs are generated (and,
optionally, backed up) centrally. This exchange differs from the
preceding exchange (Key Pair Update for Centrally-Generated
Encryption Key Pair) in that the exchange is initiated by the CA
rather than the client.
9.8.2 Detailed Description
<< To be supplied >>
9.9 Revocation Request
9.9.1 Overview of Exchange
This protocol exchange is used to support a revocation request from a
user or other authorized party.
9.9.2 Detailed Description
<< To be supplied >>
9.10 Cross-Certification
9.10.1 Overview of Exchange
The cross certification exchange allows two CAs to simultaneously
certify each other. This means that each CA will create a certificate
that contains the CA verification key of the other CA.
Cross certification is initiated at one CA known as the responder.
The CA administrator for the responder identifies the CA it wants to
cross certify and the responder CA equipment generates an
authorization code. The responder CA administrator passes this
authorization code by out-of-band means to the requester CA
administrator. The requester CA administrator enters the
authorization code at the requester CA in order to initiate the on-
line exchange.
The authorization code is used for authentication and integrity
purposes. This is done by generating a symmetric key based on the
authorization code and using the symmetric key for generating Message
Authentication Codes (MACs) on all messages exchanged.
Housley, Ford, Farrell, Solo [Page 42]
INTERNET DRAFT November 1995
Serial numbers and protocol version are used in the same manner as in
the above CA-client exchanges.
9.10.2 Detailed Description
The requester CA initiates the exchange by generating a random number
(requester random number). The requester CA then sends the responder
CA the message CrossCertifyRequest. The fields in this message are
protected from modification with a MAC based on the authorization
code.
CrossCertifyRequest:: requester CA to responder CA
{
protocol version
message type
[transaction identifier]
requester random number
requester CA unique name (DN)
requester CA public key
[requester CA key id]
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the CrossCertifyRequest structure, the responder CA
checks the protocol version, saves the requester random number,
generates its own random number (reponder random number) and
validates the MAC. It then generates and archives a new requester
certificate which contains the requester CA public key and is signed
with the responder CA signature private key. The responder CA
responds with the message CrossCertifyResponse. The fields in this
message are protected from modification with a MAC based on the
authorization code.
CrossCertifyResponse:: responder CA to requester CA
{
message type
[transaction identifier]
requester random number
reponder random number
requester certificate (requester CA is the subject, signed by responder CA)
responder CA public key
[responder CA key id]
responder CA system time
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the CrossCertifyResponse structure, the requester CA
checks that its own system time is close to the responder CA system
Housley, Ford, Farrell, Solo [Page 43]
INTERNET DRAFT November 1995
time, checks the received random numbers and validates the MAC. It
then generates and archives a new responder certificate which
contains the responder CA public key and is signed by the requester
CA signature private key. The requester CA responds with the message
CrossCertifyConfirm. The fields in this message are protected from
modification with a MAC based on the authorization code.
CrossCertifyConfirm:: requester CA to responder CA
{
message type
[transaction identifier]
requester random number
reponder random number
reponder certificate (responder CA is the subject, signed by requester CA)
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the CrossCertifyConfirm structure, the responder CA
checks the random numbers, archives the reponder certificate, and
validates the MAC. It writes both the request and reponder
certificates to the Directory. It then responds with the message
CrossCertifyFinish. The fields in this message are protected from
modification with a MAC based on the authorization code.
CrossCertifyFinish:: responder CA to requester CA
{
message type
[transaction identifier]
requester random number
responder random number
MAC algorithm id
} MAC (key based on authorization code)
Upon receipt of the CrossCertifyFinish message, the requester CA
checks the random numbers and validates the MAC. The requester CA
writes both the requester and reponder certificates to the Directory.
10 Management Protocol Transport
10.1 On-line Management Protocol
<< To be supplied. This subsection will specify a means for
conveying ASN.1-encoded messages for the protocol exchanges described
in Section 9 over a TCP connection. >>
Housley, Ford, Farrell, Solo [Page 44]
INTERNET DRAFT November 1995
10.2 Management Protocol via E-mail
<< To be supplied. This subsection will specify a means for
conveying ASN.1-encoded messages for the protocol exchanges described
in Section 9 via Internet mail. >>
10.3 Management Protocol via HTTP
<< To be supplied. This subsection will specify a means for
conveying ASN.1-encoded messages for the protocol exchanges described
in Section 9 over WWW browser-server links, employing HTTP or related
WWW protocols. >>
11 Algorithm Support
11.1 One-way Hash Functions
One-way hash functions are also called message digest algorithms.
MD5 and SHA-1 will be the most popular one-way hash functions used in
the Internet PKI. However, PEM uses MD2 for certificates [RFC1422,
RFC1423]. For this reason, MD2 will continue to be used in
certificates for many years.
11.1.1 MD5 One-way Hash Function
MD5 was developed by Ron Rivest, and RSA Data Security has placed the
MD5 algorithm in the public domain. MD5 is fully described in RFC
1321.
MD5 is the one-way hash function of choice for use with the RSA
signature algorithm.
11.1.2 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 MD5 is preferred. MD2 is fully described
in RFC 1319.
11.1.3 SHA-1 One-way Hash Function
SHA-1 was developed by the U.S. Government. SHA-1 is fully described
in FIPS 180-1.
SHA-1 is the one-way hash function of choice for use with the DSA
signature algorithm.
Housley, Ford, Farrell, Solo [Page 45]
INTERNET DRAFT November 1995
11.2 Signature Algorithms
RSA and DSA will be the most popular signature algorithms used in the
Internet PKI.
There is some ambiguity in 1988 X.509 document regarding the
definition of the SIGNED macro regarding, 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
function output value) is first ASN.1 encoded as an OCTET STRING and
the result is encrypted (e.g., using RSAEncryption) to form the
signed quantity, which is then ASN.1 encoded as a BIT STRING.
11.2.1 RSA Signature Algorithm
The RSA algorithm is named for it's inventors: Rivest, Shamir, and
Adleman. The RSA signature algorithm is defined in PKCS #1. It
combines the either the MD2 or the MD5 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 }
md4WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 4 }
<< Should we permit RSA with SHA-1? >>
When this object identifier is used with the ASN.1 type
AlgorithmIdentifier, the parameters component of that type is the
ASN.1 type NULL.
11.2.2 DSA Signature Algorithm
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. The ASN.1 object
identifiers used to identify this signature algorithm is:
dsaWithSHA-1 OBJECT IDENTIFIER ::= {
joint-iso-ccitt(2) country(16) US(840) organization(1)
us-government(101) dod(2) infosec(1) algorithms(1) 2 }
Housley, Ford, Farrell, Solo [Page 46]
INTERNET DRAFT November 1995
When this object identifier is used with the ASN.1 type
AlgorithmIdentifier, the parameters component of that type is
optional. If it is absent, the DSA parameters p, q, and g are
assumed to be known, otherwise the parameters are included using the
following ASN.1 structure:
Dss-Parms ::= SEQUENCE {
p OCTET STRING,
q OCTET STRING,
g OCTET STRING }
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
Bell-Northern Research
PO Box 3511, Station C
Ottawa, Ontario
Canada KY 4H7
wford@bnr.ca
Stephen Farrell
Software and Systems Engineering Ltd
7984 Fitzwilliam Court
Dublin 2
IRELAND
stephen.farrell@sse.ie
David Solo
BBN
150 CambridgePark Drive
Cambridge, MA 02140
USA
solo@bbn.com
Housley, Ford, Farrell, Solo [Page 47]