Folks,
Here is the newest version of RFC 1114 (now up to E?),
representing a very significant revision since the ID version
published in June 1991. I hope to devote the PEM WG meeting at in San
Diego to a detailed review of this document. With luck, we can
proceed to make needed revisions after this review, tidy up
references, and then publsih the document as an ID for final review
and comment. I'd like to ask everyone who is plannign to attend the
IETf meeting in San Diego to read the document prior to the PEM WG
meetings (it's a long flight from the east coast), and be prepared to
discuss the document. For those who will not be attending, please
review the document and get back with comments by ythe end of March,
so that a revised verison can be published as an ID in April.
Steve
--------------------------------------------------------------------------
9 March 1992
Privacy Enhancement for Internet Electronic Mail:
Part II: Certificate-Based Key Management
STATUS OF THIS MEMO
This draft document will be submitted to the RFC editor as a
standards document, and is submitted as a proposed successor to RFC
1114. References within the text of this Internet-Draft to this
document as an RFC, or to related Internet-Drafts cited as "RFC
[1113E]", "RFC [1115C]", and "RFC [FORMS-C]" are not intended to
carry any connotation about the progression of these Internet-Drafts
through the IAB standards-track review cycle. Distribution of this
memo is unlimited. This specification was developed by the PEM
Working Group of the IETF, based on work initiated in the Internet
Research Task Force's Privacy and Security Research Group. Comments
should be sent to <pem-dev(_at_)tis(_dot_)com>.
This RFC specifies a key management infrastructure for use the
Internet community, and requests discussion and suggestions for
improvements.
ACKNOWLEDGMENT
This RFC is the outgrowth of a series of meetings of the Privacy and
Security Research Group of the IRTF and the PEM Work Group of the
IETF. I would like to thank the members of the PSRG and the PEM WG
for their comments and contributions at the meetings which led to the
preparation of this RFC. I also would like to thank contributors to
the PEM-DEV mailing list who have provided valuable input which is
reflected in this RFC.
1 Executive Summary
This is one of a series of RFCs defining privacy enhancement
mechanisms for electronic mail transferred using Internet mail
protocols. RFC [1113E] prescribes protocol extensions and processing
procedures for RFC-822 mail messages, given that suitable
cryptographic keys are held by originators and recipients as a
necessary precondition. RFC [1115C] specifies algorithms, modes and
associated identifiers for use in processing privacy-enhanced
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PEM-1114E Certificate-Based Key Management March 1992
messages, as called for in RFC [1113E] and this RFC. This RFC
defines a supporting key management architecture and infrastructure,
based on public-key certificate techniques, to provide keying
information to message originators and recipients. RFC [FORMS-C]
provides additional specifications for services in conjunction with
the key management infrastructure described herein.
The key management architecture described in this RFC is compatible
with the authentication framework described in CCITT 1988 X.509 [2].
This RFC goes beyond X.509 by establishing procedures and conventions
for a key management infrastructure for use with Privacy Enhanced
Mail (PEM) and with other protocols, from both the TCP/IP and OSI
suites, in the future. There are several motivations for
establishing these procedures and conventions (as opposed to relying
only on the very general framework outlined in X.509):
- It is important that a certificate management infrastructure
for use in the Internet community accommodate a range of
clearly-articulated certification policies for both users and
organizations in a well-architected fashion. Mechanisms
must be provided to enable each user to be aware of the
policies governing any certificate which the user may
encounter. This requires the introduction and
standardization of procedures and conventions that are
outside the scope of X.509.
-The procedures for authenticating originators and recipient in
the course of message submission and delivery should be
simple, automated and uniform despite the existence of
differing certificate management policies. For example,
users should not have to engage in careful examination of a
complex set of certification relationships in order to
evaluate the credibility of a claimed identity.
-The authentication framework defined by X.509 is designed to
operate in the X.500 directory server environment. However
X.500 directory servers are not expected to be ubiquitous in
the Internet in the near future, so some conventions are
adopted to facilitate operation of the key management
infrastructure in the near term.
-Public key cryptosystems are central to the authentication
technology of X.509 and those which enjoy the most widespread
use are patented in the U.S. Although this certification
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management scheme is compatible with the use of different
digital signature algorithms, it is anticipated that the RSA
cryptosystem will be used as the primary signature algorithm
in establishing the Internet certification hierarchy.
Special license arrangements have been made to facilitate the
use of this algorithm in the U.S. portion of Internet
environment.
The infrastructure specified in this RFC establishes a single root
for all certification within the Internet, the Internet Certification
Authority (ICA). The ICA establishes global policies, described in
this RFC, which apply to all certification effected under this
hierarchy. Beneath ICA root are Policy Certification Authorities
(PCAs), each of which establishes and publishes (in the form of an
informational RFC) its policies for registration of users or
organizations. Each PCA is certified by the ICA. (1) Below PCAs,
Certification Authorities (CAs) will be established to certify users
and subordinate organizational entities (e.g., departments, offices,
subsidiaries, etc.). Initially, we expect the majority of users will
be registered via organizational affiliation, consistent with current
practices for how most user mailboxes are provided. In this sense
the registration is analogous to the issuance of a university or
company ID card.
Some CAs are expected to provide certification for residential users
in support of users who wish to register independent of any
organizational affiliation. Over time, we anticipate that civil
government entities which already provide analogous identification
services in other contexts, e.g., driver's licenses, may provide
this service. For users who wish anonymity while taking advantage of
PEM privacy facilities, one or more PCAs will be established with
policies that allow for registration of users, under subordinate CAs,
who do not wish to disclose their identities.
_______________
(1) It is desirable that there be a relatively small number of
PCAs, each with a substantively different policy, to facilitate
user familiarity with the set of PCA policies. However there is
no explicit requirement that the set of PCAs be limited in this
fashion.
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2 Overview of Approach
This RFC defines a key management architecture based on the use of
public-key certificates, primarily in support of the message
encipherment and authentication procedures defined in RFC [1113E].
The concept of public-key certificates is defined in X.509 and this
architecture is a compliant subset of that envisioned in X.509.
Briefly, a (public-key) certificate is a data structure which
contains the name of a user (the "subject"), the public component (2)
of that user, and the name of an entity (the "issuer") which vouches
that the public component is bound to the named user. This data,
along with a time interval over which the binding is claimed to be
valid, is cryptographically signed by the issuer using the issuer's
private component. The subject and issuer names in certificates are
Distinguished Names (DNs) as defined in the directory system (X.500).
Once signed, certificates can be stored in directory servers,
transmitted via non-secure message exchanges, or distributed via any
other means that make certificates easily accessible to message
system users, without regard for the security of the transmission
medium. Certificates are used in PEM to provide the originator of a
message with the (authenticated) public key of each recipient and to
provide each recipient with the (authenticated) public key of the
originator. The following brief discussion illustrates the
procedures for both originator and recipients.
Prior to sending an encrypted message (using PEM), an originator must
acquire a certificate for each recipient and must validate these
certificates. Briefly, validation is performed by checking the
digital signature in the certificate, using the public component of
the issuer whose private component was used to sign the certificate.
The issuer's public component is made available via some out of band
means (for the ICA) or is itself distributed in a certificate to
which this validation procedure is applied recursively. In the
_______________
(2) Throughout this RFC we have adopted the terms "private
component" and "public component" to refer to the quantities
which are, respectively, kept secret and made publicly available
in asymmetric cryptosystems. This convention is adopted to avoid
possible confusion arising from use of the term "secret key" to
refer to either the former quantity or to a key in a symmetric
cryptosystem.
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latter case, the issuer of a user's certificate becomes the subject
in a certificate issued by another certifying authority (or a PCA),
thus giving rise to a certification hierarchy. The validity interval
for each certificate is checked and Certificate Revocation Lists
(CRLs) are checked to ensure that none of the certificates employed
in the validation process has been revoked by an issuer.
Once a certificate for a recipient is validated, the public component
contained in the certificate is extracted and used to encrypt the
data encryption key (DEK), which, in turn, is used to encrypt the
message itself. The resulting encrypted DEK is incorporated into the
Key-Info field of the message header. Upon receipt of an encrypted
message, a recipient employs his private component to decrypt this
field, extracting the DEK, and then uses this DEK to decrypt the
message.
In order to provide message integrity and data origin authentication,
the originator generates a message integrity code (MIC), signs
(encrypts) the MIC using the private component of his public-key
pair, and includes the resulting value in the message header in the
MIC-Info field. The certificate of the originator is (optionally)
included in the header in the Certificate field as described in RFC
[1113E]. This is done in order to facilitate validation in the
absence of ubiquitous directory services. Upon receipt of a privacy
enhanced message, a recipient validates the originator's certificate
(using the ICA public component as the root of a certification path),
checks to ensure that it has not been revoked, extracts the public
component from the certificate, and uses that value to recover
(decrypt) the MIC. The recovered MIC is compared against the locally
calculated MIC to verify the integrity and data origin authenticity
of the message.
3 Architecture
3.1 Scope and Restrictions
The architecture described below is intended to provide a basis for
managing public-key cryptosystem values in support of privacy
enhanced electronic mail in the Internet environment. The
architecture describes procedures for registering certification
authorities and users, for generating and distributing certificates,
and for generating and distributing CRLs. RFC [1113E] describes the
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syntax and semantics of header fields used to transfer certificates
and to represent the DEK and MIC in this public-key context.
Definitions of the algorithms, modes of use and associated
identifiers are separated in RFC [1115C] to facilitate the adoption
of additional algorithms in the future. This RFC focuses on the
management aspects of certificate-based, public-key cryptography for
privacy enhanced mail.
The proposed architecture imposes conventions for the certification
hierarchy which are not strictly required by the X.509 recommendation
nor by the technology itself. These conventions are motivated by
several factors, primarily the need for authentication semantics
compatible with automated validation and the automated determination
of the policies under which certificates are issued.
Specifically, the architecture proposes a system in which user (or
mailing list) certificates represent the leaves in a certification
hierarchy. This certification hierarchy is largely isomorphic to the
X.500 directory naming hierarchy, with two exceptions: the ICA forms
the root of the tree (the root of the X.500 DIT is not instantiated
as a node), and a number of Policy Certification Authorities (PCAs)
form the "roots" of subtrees, each of which represents a different
certification policy.
Not every level in the directory hierarchy need correspond to a
certification authority. For example, the appearance of geographic
entities in a distinguished name (e.g., countries, states, provinces,
localities) does not require that various governments become
certifying authorities in order to instantiate this architecture.
However, it is anticipated that, over time, a number of such points
in the hierarchy will be instantiated as CAs in order to simplify
later transition of management to appropriate governmental
authorities.
These conventions minimize the complexity of validating user
certificates, e.g., by making explicit the relationship between a
certificate issuer and the user (via the naming hierarchy). Note that
in this architecture, only PCAs may be certified by the ICA and every
CA must be certified by a PCA. If a CA is certified by more than one
PCA, each certificate issued by a PCA for the CA must contain a
distinct public component. These conventions result in a
certification hierarchy which is a compatible subset of that
permitted under X.509, with respect to both syntax and semantics.
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Although the key management architecture described in this RFC has
been designed primarily to support privacy enhanced mail, this
infrastructure also may, in principle, be used to support X.400 mail
security facilities (as per 1988 X.411) and X.500 directory
authentication facilities. Thus establishment of this infrastructure
paves the way for use of these and other OSI protocols in the
Internet in the future. In the future, these certificates also may
be employed in the provision of security services in other protocols
in the TCP/IP and OSI suites as well.
3.2 Relation to X.509 Architecture
CCITT 1988 Recommendation X.509, "The Directory - Authentication
Framework", defines a framework for authentication of entities
involved in a distributed directory service. Strong authentication,
as defined in X.509, is accomplished with the use of public-key
cryptosystems. Unforgeable certificates are generated by
certification authorities; these authorities may be organized
hierarchically, though such organization is not required by X.509.
There is no implied mapping between a certification hierarchy and the
naming hierarchy imposed by directory system naming attributes.
This RFC interprets the X.509 certificate mechanism to serve the
needs of PEM in the Internet environment. The certification
hierarchy proposed in this RFC in support of privacy enhanced mail is
intentionally a subset of that allowed under X.509. This
certification hierarchy also embodies semantics which are not
explicitly addressed by X.509, but which are consistent with X.509
precepts. An overview of the rationale for these semantics is
provided in Section 1.
3.3 Certificate Definition
Certificates are central to the key management architecture for X.509
and PEM. This section provides an overview of the syntax and a
description of the semantics of certificates. Appendix A includes
the ASN.1 syntax for certificates. A certificate includes the
following contents:
1. version
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2. serial number
3. signature (algorithm ID and parameters)
4. issuer name
5. validity period
6. subject name
7. subject public key (and associated algorithm ID)
3.3.1 Version Number
The version number field is intended to facilitate orderly changes in
certificate formats over time. The initial version number for
certificates used in PEM is the X.509 default which has a value of
zero (0), indicating the 1988 version. PEM implementations are
encouraged to accept later versions as they are endorsed by
CCITT/ISO.
3.3.2 Serial Number
The serial number field provides a short form, unique identifier for
each certificate generated by an issuer. The serial number is used
in CRLs to identify revoked certificates, as described in Section
3.4.3.4. Although this attribute is an integer, PEM processing of
this attribute need not involve any arithmetic operations. All PEM
implementations must be capable of processing serial numbers up to 48
bits in length and support for larger serial numbers is encouraged.
3.3.3 Signature
This field specifies the algorithm used by the issuer to sign the
certificate, and any parameters associated with the algorithm. (3)
_______________
(3) The certificate signature is appended to the data structure,
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The signature is validated by the UA processing a certificate, in
order to determine that the integrity of its contents have not been
modified subsequent to signing by a CA (ICA, or PCA). In this
context, a signature is effected through the use of a Certificate
Integrity Check (CIC) algorithm and a public-key encryption
algorithm. RFC [1115C] contains the definitions and algorithm IDs
for signature algorithms employed in this architecture.
3.3.4 Subject Name
A certificate provides a representation of its subject's identity in
the form of a Distinguished Name (DN). The fundamental binding
ensured by the key management architecture is that between the public
component and the user's identity in this form. A distinguished name
is an X.500 directory system concept and if a user is already
registered in an X.500 directory, his distinguished name is defined
via that registration. Users who are not registered in a directory
should keep in mind likely directory naming structure (schema) when
selecting a distinguished name for inclusion in a certificate.
3.3.5 Issuer Name
A certificate provides a representation of its issuer's identity, in
the form of a Distinguished Name. The issuer identification is used
to select the appropriate issuer public component to employ in
performing certificate validation. (4) The issuer is the certifying
_______________
as defined by the signature macro in X.509. This algorithm
identification information is replicated with the signature.
(4) If an issuer (CA) is certified by multiple PCAs, then the
issuer DN does not uniquely identify the public component used to
sign the certificate. In such circumstances it may be necessary
to attempt certificate validation using multiple public
components, from certificates held by the issuer under different
PCAs. If the 1992 version of a certificate is employed, the
issuer may employ distinct issuer UIDs in the certificates it
issues, to further facilitate selection of the right issuer
public component.
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authority (ICA, PCA or CA) who vouches for the binding between the
subject identity and the public key contained in the certificate.
3.3.6 Validity Period
A certificate carries a pair of date and time indications, indicating
the start and end of the time period over which a certificate is
intended to be used. The duration of the interval may be constant
for all user certificates issued by a given CA or it might differ
based on the nature of the user's affiliation. For example, an
organization might issue certificates with shorter intervals to
temporary employees versus permanent employees. It is recommended
that the UTCT values recorded here specify granularity to no more
than the minute, even though finer granularity can be expressed in
the format. It also recommended that all times be expressed as
Greenwhich Mean Time (Zulu), to simplify comparisons and avoid
confusion relating to daylight savings time.
The longer the interval, the greater the likelihood that compromise
of a private component or name change will render it invalid and thus
require that the certificate be revoked. Once revoked, the
certificate must remain on the issuer's CRL (see Section 3.4.3.4)
until the validity interval expires. PCAs may impose restrictions on
the maximum validity interval that may be elected by CAs operating in
their certification domain (see Appendix B).
3.3.7 Subject Public Key
A certificate carries the public component of its associated subject,
as well as an indication of the algorithm, and any algorithm
parameters, with which the public component is to be used. This
algorithm identifier is independent of that which is specified in the
signature field described above. RFC [1115C] specifies the algorithm
identifiers which may be used in this context.
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3.4 Roles and Responsibilities
One way to explain the architecture proposed by this document is to
examine the roles which are defined for various entities in the
architecture and to describe what is required of each entity in order
for the proposed system to work properly. The following sections
identify four types of entities within this architecture: users and
user agents, the Internet Certification Authority, Policy
Certification Authorities, and other Certification Authorities. For
each type of entity this document the procedures which the entity
must execute as part of the architecture and what responsibilities
the entity assumes as a function of its role in the architecture.
3.4.1 Users and User Agents
The term User Agent (UA) is taken from CCITT X.400 Message Handling
Systems (MHS) Recommendations, which define it as follows: "In the
context of message handling, the functional object, a component of
MHS, by means of which a single direct user engages in message
handling." UAs exchange messages by calling on a supporting Message
Transfer Service (MTS), e.g., the SMTP mail relays used in the
Interent.
3.4.1.1 Generating and Protecting Component Pairs
A UA process supporting PEM must protect the private component of its
associated entity (e.g., a human user or a mailing list) from
disclosure, though the means by which this is effected is a local
matter. It is essential that the user take all available precautions
to protect his private component as the secrecy of this value is
central to the security offered by PEM to that user. For example,
the private component might be stored in encrypted form, protected
with a locally managed symmetric encryption key (e.g., using DES).
The user would supply a password or passphrase which would be
employed as a symmetric key to decrypt the private component when
required for PEM processing (either on a per message or per session
basis). Alternatively, the private component might be stored on a
diskette which would be inserted by the user whenever he originated
or received PEM messages. Explicit zeroing of memory locations where
this component transiently resides could provide further protection.
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Other precautions, based on local operating system security
facilities, also should be employed.
It is recommended that each user employ ancillary software (not
otherwise associated with normal UA operation) or hardware to
generate his personal public-key component pair. Software for
generating user component pairs will be available as part of the
reference implementation of PEM distributed freely in the U.S.
portion of the Internet. It is critically important that the
component pair generation procedure be effected in as secure a
fashion as possible, to ensure that the resulting private component
is unpredictable. Introduction of adequate randomness into the
component pair generation procedure is potentially the most difficult
aspect of this process and the user is advised to pay particular
attention to this aspect.
There is no requirement imposed by this architecture that anyone
other than the user, including any certification authority, have
access to the user's private component. Thus a user may retain his
component pair even if his certificate changes, e.g., due to rollover
in the validity interval or because of a change of certifying
authority. Even if a user is issued a certificate in the context of
his employment, there is generally no requirement that the employer
have access to the user's private component. The rationale is that
any messages signed by the user are verifiable using his public
component. In the event that the corresponding private component
becomes unavailable, any ENCRYPTED messages directed to the user
would be indecipherable and would require retransmission.
Note that if the user stores messages in ENCRYPTED form, these
messages also would become indecipherable in the event that the
private component is lost or changed. To minimize the potential for
loss of data in such circumstances messages can be transformed into
MIC-ONLY or MIC-CLEAR form if cryptographically-enforced
confidentiality is not required for the messages stored within the
user's computer. Alternatively, these messages might be forwarded in
ENCRYPTED form to a (trivial) distribution list which serves in a
backup capacity and for which the user's employer holds the private
component.
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3.4.1.2 User Registration
Most details of user registration are a local matter, subject to
policies established by the user's CA and the PCA under which that CA
has been certified. In general a user must provide, at a minimum,
his public component and distinguished name to a CA, or a
representative thereof, for inclusion in the user's certificate.
(The user also might provide a complete certificate, minus the
signature, as described in RFC [FORMS-C].) The CA will employ some
means, specified by the CA in accordance with the policy of its PCA,
to validate the user's claimed identity and to ensure that the
public component provided is associated with the user whose
distinguished name is to be bound into the certificate. (In the case
of PERSONA certificates, described below, the procedure is a bit
different.) The certifying authority generates a certificate
containing the user's distinguished name and public component, the
authority's distinguished name and other information (see Section
3.3) and signs the result using the private component of the
authority.
3.4.1.3 CRL Management
Mechanisms for managing a UA certificate cache are, in typical
standards parlance, a local matter. However, proper maintenance of
such a cache is critical to the correct, secure operation of a PEM
UA, as well as providing a basis for improved performance. The
following discussion provides a paradigm for one aspect of cache
management, namely the processing of CRLs, the functional equivalent
of which must be embodied in any PEM UA implementation compliant with
this RFC. The specifications for CRLs used with PEM are provided in
Section 3.5.
X.500 makes provision for the storage of CRLs as directory attributes
associated with CA entries. Thus, when X.500 directories become
widely available, UAs can retrieve CRLs from directories as required.
In the interim, a (replicated) database will be maintained by the ICA
which contains CRLs for all PCAs and CAs. Every PEM UA must provide
a facility for fetching CRLs from this database using the mechanisms
defined in RFC [FORMS-C]. Access to the CRL database may be
automated, e.g., as part of the certificate validation process (see
Section 3.6) or may be user directed. In addition, a "push" (vs.
"pull") model of CRL distribution is provided through the definition
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of a PEM header format specifically for CRL propagation (see RFC
[1113E]). As noted in RFC [1113E], every PEM UA must be capable of
processing CRLs distributed via such messages.
Upon receipt and validation of a CRL, (5) A PEM UA must compare the
CRL entries against any cached certificate information, and must mark
as revoked any cache entries which match CRL entries. (Recall that
the certificate serial numbers are unique only for each issuer, so
care must be exercised in effecting this cache search.) This
procedure applies to cache entries associated with PCAs and CAs, as
well as user entries. The UA also must retain each CRL to screen
incoming messages to detect use of revoked certificates carried in
PEM message headers. Thus a UA must be capable of processing and
retaining CRLs issued by the ICA (which will list revoked PCA
certificates), by any PCA (which will list revoked CA certificate
issued by that PCA), and by any CA (which will list revoked user or
CA certificates issued by that CA).
3.4.1.4 Facilitating Interoperation
In the absence of ubiquitous directory services or knowledge that a
recipient already possesses the necessary issuer certificates, it is
recommended that an originating (PEM) UA include appropriate
certificates (using the "Issuer-Certificate" field) when
communicating with a recipient who is certified by other that the
originator's CA. When an originator submits an ENCRYPTED message (as
per RFC [1113E], his UA must validate the certificates of the
recipients (see Section 3.6). In the course of performing this
validation the UA can determine if any of the recipients are
registered under a CA other than the originator's CA. The UA also
can determine if any recipients are certified under a PCA other than
the one under which the originator is certified. In these
circumstances, the originator's UA could include his CA and PCA
certificates to facilitate validation of the user's certificate by
the recipient's UA. It is recommended that PEM software include a
provision for the user to specify the automatic inclusion of the
_______________
(5) A CRL is validated in much the same manner as a certificate,
i.e., the CIC is calculated and compared against the decrypted
signature value obtained from the CRL. See Section 3.6 for
additional details related to validation of certificates.
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minimum set of appropriate certificates (using "Issuer-Certificate"
fields in the PEM header) when submitting an ENCRYPTED message,
either on a per-message or default basis.
Submission of a MIC-ONLY or MIC-CLEAR message (as per RFC [1113D)
does not entail validation of recipient certificates and thus it is
not possible for the originator's UA to determine automatically that
a recipient might require CA or PCA certificates to validate the
message signature. Thus it is recommended that PEM software provide
an interface to allow the user to explicitly include a certification
path back to the ICA root (using "Issuer-Certificate" fields in the
PEM header) when submitting MIC-CLEAR or MIC-ONLY messages. Here too
it is recommended that this facility be available on either a per-
message or default basis.
3.4.2 The Internet Certification Authority (ICA)
The ICA acts as the root of the certification hierarchy for the
Internet community. The public component of the ICA forms the
foundation for all certificate validation within this hierarchy. The
ICA will be operated under the auspices of the Internet Society, an
international, non-profit organization [ISOC92]. The ICA certifies
all PCAs, ensuring that they agree to abide by the Internet-wide
policy established by the ICA. This policy, and the services
provided by the ICA, are detailed below.
3.4.2.1 PCA Registration
The ICA certifies only PCAs, not CAs or users. Each PCA must file
with the ICA a description of its proposed policy. This document
will be published as an informational RFC. A copy of the document,
signed by the ICA (in the form of a PEM MIC-ONLY message) will be
made available via electronic mail access by the ICA. This
convention is adopted so that every Internet user has a reference
point for determining the policies associated with the issuance of
any certificate which he may encounter. The existence of a digitally
signed copy of the document ensures the immutability of the document.
Authorization of a PCA to operate in the Internet hierarchy is
signified by the publication of the policy document, and the issuance
of a certificate to the PCA, signed by the ICA. An outline for PCA
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policy statements is contained in Section 3.4.3 of this document.
As part of registration, a PCA specifies its distinguished name. The
ICA will take reasonable precautions to ensure that the distinguished
name claimed by a PCA is legitimate, e.g., requiring the PCA to
provide documentation supporting its claim to a DN. However, the
certification of a PCA by the ICA does not constitute a endorsement
of the PCA's claim to this DN outside of the context of this
certification system.
3.4.2.2 Ensuring the Uniqueness of Distinguished Names
A fundamental requirement of this certification scheme is that
certificates are not issued to distinct entities under the same
distinguished name. This requirement is crucial to the success of
distributed management for the certification hierarchy. The ICA will
not certify two PCAs with the same distinguished name. Since PCAs
are expected to certify CAs in widely disjoint portions of the
directory namespace, and since X.500 directories are not ubiquitous,
a facility is required for coordination among PCAs to ensure the
uniqueness of CA DNs.
In support of the uniqueness requirement, the ICA will establish and
maintain a database to detect potential, unintended duplicate
certification of CA distinguished names. This database will be made
accessible to all PCAs. Each entry in this database will consist of
a 4-tuple. The first element in each entry is a hash value, computed
on a canonical, ASN.1 encoded representation of a CA distinguished
name. The second element contains the public component of the CA.
The third element is the distinguished name of the PCA which
registered the entry. The fourth element consists of the date and
time at which the entry was made, as established by the ICA. This
database structure provides a degree of privacy for CAs registered by
PCAs, while providing a facility for ensuring global uniqueness of CA
DNs certified in this scheme.
In order to avoid conflicts, a PCA must query the database using a CA
DN hash value as a search key, prior to certifying a CA. The
database will return any entries which match the query, i.e., which
have the same CA DN. A PCA can submit a candidate entry, consisting
of the first three tuple values, and the database will register this
entry, supplying the time and date stamp, if the first two elements
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(the CA DN hash and the CA public component) together are unique. If
there is a conflict, the database will return the conflicting entry.
The database does not, in itself, guarantee uniqueness of CA DNs as
it allows for two DNs associated with different public components to
be registered. Rather, it is the responsibility of PCAs to
coordinate with one another whenever the database indicates a
potential DN conflict and to resolve such conflicts prior to
certification of CAs. Details of the protocol used to access the
database are contained in Appendix B.
As noted earlier, a CA may be certified under more than one PCA,
e.g., because the CA wants to issue certificates under two different
policies. If a CA is certified by multiple different PCAs, the CA
must employ a different public key pair for each PCA. In such
circumstances the certificate issued to the CA by each PCA will
contain a different public component and thus will represent a
different entry in this database.
To complete the strategy for ensuring uniqueness of DNs, there is a
DN subordination requirement levied on CAs. In general, CAs are
expected to sign certificates only if the subject DN in the
certificate is subordinate to the issuer (CA) DN. This ensures that
certificates issued by a CA are syntactically constrained to refer to
subordinate entities in the X.500 directory information tree (DIT),
and this further limits the possibility of duplicate DN registration.
CAs may sign certificates which do not comply with this requirement
if the certificates are "cross-certificates" or "reverse
certificates" (see X.509) used with applications other than PEM.
The ICA also will establish and maintain a separate database to
detect potential duplicate certification of (residential) user
distinguished names. Each entry in this database will consist of 4-
tuple as above, substituting a user's distinguished name and public
component in lieu of a CA name and public component. This structure
provides a degree of privacy for users registered by CAs which
service residential users while providing a facility for ensuring
global uniqueness of user DNs certified under this scheme. The same
database access facilities are provided as described above for the CA
database. Here it is the responsibility of the CAs to coordinate
whenever the database indicates a potential conflict and to resolve
the conflict prior to (residential) user certification.
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3.4.2.3 Accuracy of Distinguished Names
As noted above, the ICA will make a reasonable effort to ensure that
PCA DNs are accurate. The procedures employed to ensure the accuracy
of a CA distinguished name, i.e., the confidence attached to the
DN/public component binding implied by a certificate, will vary
according to PCA policy. However, it is expected that every PCA will
make a good faith effort to ensure the legitimacy of each CA DN
certified by the PCA. Part of this effort should include a check
that the purported CA DN is consistent with any applicable national
standards for DN assignment, e.g., NADF recommendations within North
America [RFC-NADF].
3.4.2.4 Distinguished Name Conventions
A few basic DN conventions are included in the ICA policy. The ICA
will certify PCAs, but not CAs nor users. PCAs will certify CAs, but
not users. These conventions are required to allow simple
certificate validation within PEM, as described later. Certificates
issued by CAs (for use with PEM) will be for users or for other CAs,
either of which must have DNs subordinate to that of the issuing CA.
The attributes employed in constructing DNs will be specified in a
list maintained by the IANA, to provide a coordinated basis for
attribute identification for all applications employing DNs. This
list will initially be populated with attributes taken from X.520.
This document does not impose detailed restrictions on the attributes
used to identify different entities to which certificates are issued,
but PCAs may impose such restrictions as part of their policies.
PCAs, CAs and users are urged to employ only those DN attributes
which have printable representations, to facilitate display and
entry.
3.4.2.5 CRL Management
Among the procedures articulated by each PCA in its policy statement
are procedures for the maintenance and distribution of CRLs by the
PCA itself and by its subordinate CAs. The frequency of issue of
CRLs may vary according to PCA-specific policy, but every PCA and CA
must issue a CRL upon inception to provide a basis for uniform
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certificate validation procedures throughout the Internet hierarchy.
The format for these CRLs is that specified in Section 3.5.2 of the
document.
The ICA will maintain a CRL for all the PCAs it certifies and this
CRL will be updated monthly. Each PCA will maintain a CRL for all of
the CAs which it certifies and these CRLs will be updated biweekly.
The ICA will establish and maintain a database to hold CRLs for the
Internet hierarchy, i.e., the ICA CRL, PCA CRLs, and CRLs from all
CAs. This database will be accessible via email as specified in RFC
[FORMS-C], both for retrieval of (current) CRLs and for entering new
CRLs. Individual PCAs may elect to maintain CRL archives for their
CAs, but this is not required by this policy.
3.4.2.6 Public Key Algorithm Licensing Issues
This certification hierarchy is architecturally independent of any
specific digital signature (public key) algorithm. Some algorithms,
employed for signing certificates and validating certificate
signatures, are patented in some countries. The ICA will not grant a
license to any PCA for the use of any signature algorithm in
conjunction with the management of this certification hierarchy. The
ICA will acquire, for itself, any licenses needed for it to sign
certificates and CRLs for PCAs, for all algorithms which the ICA
supports. Every PCA will be required to represent to the ICA that
the PCA has obtained any licenses required to issue (sign)
certificates and CRLs in the environment(s) which the PCA will serve.
For example, the RSA cryptosystem is patented in the United States
and thus any PCA operating in the U.S. and using RSA to sign
certificates and CRLs must represent that it has a valid license to
employ the RSA algorithm in this fashion. In contrast, a PCA
employing RSA and operating outside of the U.S. would represent that
it is exempt from these licensing constraints.
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3.4.3 Policy Certification Authorities
The policy statement submitted by a prospceptive PCA must address the
topics in the following outline. Additional policy information may
be contained in the statement, but PCAs are requested not to use
these statements as advertising vehicles.
1. PCA Identity- The DN of the PCA must be specified. A postal
address, an Internet mail address, and telephone (and optional fax)
numbers must be provided. The date on which this statement is
effective, and its scheduled duration must be specified.
2. PCA Scope- Each PCA must describe the community which the PCA
plans to serve. A PCA should indicate if it will certify
organizational, residential, and/or PERSONA CAs. There is not a
requirement that a single PCA serve only one type of CA, but if a PCA
serves multiple types of CAs, the policy statement must specify
clearly how a user can distinguish among these classes. If the PCA
will operate CAs to directly serve residential or PERSONA users, it
must so state.
3. PCA Security & Privacy- Each PCA must specify the technical and
procedural security measures it will employ in the generation and
protection of its component pair. If any security requirements are
imposed on CAs certified by the PCA these must be specified as well.
A PCA also must specify what measures it will take to protect the
privacy of any information collected in the course of certifying CAs.
If the PCA operates one or more CAs directly, to serve residential or
PERSONA users, then this statement on privacy measures applies to
these CAs as well.
4. Certification Policy- Each PCA must specify the policy and
procedures which govern its certification of CAs and how this policy
applies transitively to entities (users or subordinate CAs) certified
by these CAs. For example, a PCA must state what procedure is
employed to verify the claimed identity of a CA, and the CA's right
to use a DN. Similarly, if any requirments are imposed on CAs to
validate the identity of users, these requirements must be specified.
Since all PCAs are required to cooperate in the resolution of
potential DN conflicts, each PCA is required to specify the procedure
it will employ to resolve such conflicts. If the PCA imposes a
maximum validity interval for the CA certificates it issues, and/or
for user (or subordinate CA) certifificates issued by the CAs it
certifies, then these restrictions must be specified.
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5. CRL Management- Each PCA must specifiy any constraints it imposes
in the frequency of issue of CRLs by the CAs it certifies, or by
entities certified by these CAs. Both maximum and minimum
constraints should be specified. Since the ICA policy calls for a
copy of each CRL issued by a CA to be forwarded to the cognizant PCA,
each PCA must specify a mailbox to which CRLs are to be transmitted.
If the PCA offers any additional CRL managmement services, e.g.,
archiving of old CRLs, then procedures for invoking these services
must be specified. If the PCA requires CAs to provide any additional
CRL management services, such services must be specified here.
6. Naming Conventions- If the PCA imposes any conventions on DNs used
by the CAs it certifies, or by entities certified by these CAs, these
conventions must be specified. If any sematics are associated with
such conventions, these semantics must be specified.
7. Business Issues- If a legal agreement must be executed between a
PCA and the CAs it certifies, reference to that agreement must be
noted, but the agreeement itself ought not be a part of the policy
statement. Similarly, if any fees are charged by the PCA this should
be noted, but the fee structure per se ought not be part of this
policy statement.
8. Other- Any other topics the PCA deems relevant to a statement of
its policy can be included. However, the PCA should be aware that a
policy statement is considered to be an immutable, long lived
doucument and thus considerable care should be exercised in deciding
what material is to be included in the statement.
3.4.4 Certification Authorities
In X.509 the term "certification authority" is defined as "an
authority trusted by one or more users to create and assign
certificates". X.509 imposes few constraints on CAs, but practical
implementation of a worldwide certification system requires
establishment of technical and procedural conventions by which all
CAs are expected to abide. Such conventions are established
throughout this RFC.
It is critical that the private component of a CA be afforded a high
level of security, otherwise the authenticity guarantee implied by
certificates signed by the CA is voided. Some PCAs may impose
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stringent requirements on CAs within their purview to ensure that a
high level of security is afforded the certificate signing process,
but not all PCAs are expected to impose such constraints.
3.4.4.1 Organizational CAs
Many of the CAs certified by PCAs are expected to represent
organizations. A wide range of organizations are encompassed by this
model: commercial, governmental, educational, non-profit,
professional societies, etc. The common thread is that the entities
certified by these CAs have some form of affiliation with the
organization. The object classes for organizations, organizational
units, organizational persons, organizational roles, etc., as defined
in X.521, form the models for entities certified by such CAs. The
affiliation implied by organizational certification motivates the DN
subordination requirement cited in Section 3.4.2.4.
As an example, an organizational user certificate might contain a
subject DN of the form: C = "US" SP = "Massachusetts" L = "Cambridge"
O = "Bolt Beranek and Newman" OU = "Communications Division" CN =
"Steve Kent". The issuer of this certificate might have a DN of the
form: C = "US" SP = "Massachusetts" L = "Cambridge" O= "Bolt Beranek
and Newman". Note that the organizational unit attribute is omitted
from the issuer DN, implying that there is no CA dedicated to the
"Communications Division".
3.4.4.2 Residential CAs
Users may wish to obtain certificates which do not imply any
organizational affiliation but which do purport to accurately and
uniquely identify them. Such users can be registered as residential
persons and the DN of such a user should be consistent with the
attributes of the corresponding X.521 object class. Over time we
anticipate that such users will be accommodated by civil government
entities who will assume electronic certification responsibility at
geographically designated points in the naming hierarchy. Until
civil authorities are prepared to issue certificates of this form,
residential user CAs will accommodate such users.
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As an example, a residential user certificate might include a subject
name of the form: C = "US" SP = "Massachusetts" L = "Boston" PA = "19
North Square" CN = "Paul Revere." The issuer of that certificate
might have a DN of the form: C = "US" S = "Massachusetts" L =
"Boston". Note that the issuer DN is superior to the subject DN, as
required by the ICA policy described earlier.
3.4.4.3 PERSONA CAs
One or more CAs will be established to accommodate users who wish to
conceal their identities while making use of PEM security features,
e.g., to preserve the anonymity offered by "arbitrary" mailbox names
in the current mail environment. In this case the certifying
authority is explicitly NOT vouching for the identity of the user.
All such certificates are issued under a PERSONA CA, subordinate to a
PCA with a PESONA policy, to warn users explicitly that the subject
DN is NOT a validated user identity. To minimize the possibility of
syntactic confusion with certificates which do purport to specify an
authenticated user identity, a PERSONA certificate is issued as a
form of organizational user certificate, not a residential user
certificate. There are no explicit, reserved words used to identify
PERSONA user certificates.
A CA issuing PERSONA certificates must institute procedures to ensure
that it does not issue the same subject DN to multiple users (a
constraint required for all certificates of any type issued by any
CA). There are no requirements on an issuer of PERSONA certificates
to maintain any other records that might bind the true identity of
the subject to his certificate. However, a CA issuing such
certificates must establish procedures (not specified in this RFC) in
order to allow the holder of a PESRONA certificate to request that
the certificate be revoked (i.e., listed on a CRL).
As an example, a PERSONA user certificate might include a subject DN
of the form: C = "US" SP = "Massachusetts" L = "Boston" O =
"Pseudonyms R US" CN = "Paul Revere." The issuer of this certificate
might have a DN of the form: C = "US" S = "Massachusetts" L =
"Boston" O = "Pseudonyms R US". Note the differences between this
PERSONA user certificate for "Paul Revere" and the corresponding
residential user certificate for the same common name.
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3.4.4.4 CA Responsibilities for CRL Management
As X.500 directory servers become available, CRLs should be
maintained and accessed via these servers. However, prior to
widespread deployment of X.500 directories, this RFC adopts some
additional requirements for CRL management by CAs and PCAs. As per
X.509, each CA is required to maintain a CRL (in the format specified
by this RFC in Appendix A) which contains entries for all
certificates issued and later revoked by the CA. Once a certificate
is entered on a CRL it remains there until the validity interval
expires. Each PCA is required to maintain a CRL for revoked CA
certificates within its domain. The interval at which a CA issues a
CRL is not fixed by this RFC, but the PCAs may establish minimum and
maximum intervals for such issuance.
As noted earlier, the ICA will operate a database containing CRLs for
all PCAs and CAs. In support of this requirement, each CA must
supply its current CRL to its PCA in a fashion consistent with CRL
issuance rules imposed by the PCA and with the next scheduled issue
date specified by the CA (see Section 3.5.1) Each CA will
simultaneously forward its CRL to the ICA-maintained database. CAs
may transfer CRLs to subordinate UAs using the CRL processing type
available in PEM messages (see RFC [1113E]). CAs also may provide
access to CRLs via the database mechanism described in RFC [FORMS-C]
and alluded to immediately above.
3.5 Certificate Revocation
3.5.1 X.509 CRLs
X.509 states that it is a CA's responsibility to maintain: "a time-
stamped list of the certificates it issued which have been revoked."
There are two primary reasons for a CA to revoke a certificate, i.e.,
suspected compromise of a private component (invalidating the
corresponding public component) or change of user affiliation
(invalidating the DN). The use of Certificate Revocation Lists
(CRLs) as defined in X.509 is one means of propagating information
relative to certificate revocation, though it is not a perfect
mechanism. In particular, an X.509 CRL indicates only the age of the
information contained in it; it does not provide any basis for
determining if the list is the most current CRL available from a
given CA.
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The proposed architecture establishes a format for a CRL in which not
only the date of issue, but also the next scheduled date of issue is
specified. Adopting this convention, when the next scheduled issue
date arrives a CA (6) will issue a new CRL, even if there are no
changes in the list of entries. In this fashion each CA can
independently establish and advertise the frequency with which CRLs
are issued by that CA. Note that this does not preclude CRL issuance
on a more frequent basis, e.g., in case of some emergency, but no
system-wide mechanisms are architected for alerting users that such
an unscheduled issuance has taken place. This scheduled CRL issuance
convention allows users (UAs) to determine whether a given CRL is
"out of date," a facility not available from the (1988) X.509 CRL
format.
The description of CRL management in the text and the format for CRLs
specified in X.509 (1988) are inconsistent. For example, the latter
associates an issuer distinguished name with each revoked certificate
even though the text states that a CRL contains entries for only a
single issuer (which is separately specified in the CRL format). The
CRL format adopted for PEM is a (simplified) format consistent with
the text of X.509, but not identical to the accompanying format. The
ASN.1 format for CRLs used with PEM is provided in Appendix A.
X.509 also defines a syntax for the "time-stamped list of revoked
certificates representing other CAs." This syntax, the
"AuthorityRevocationList" (ARL) allows the list to include references
to certificates issued by CAs other than the list maintainer. There
is no syntactic difference between these two lists except as they are
stored in directories. Since PEM is expected to be used prior to
widespread directory deployment, this distinction between ARLs and
CRLs is not syntactically significant. As a simplification, this RFC
specifies the use the CRL format defined below for revocation both of
user and of CA certificates.
_______________
(6) Throughout this section, when the term "CA" is employed, it
should be interpreted broadly, to include the ICA and PCAs as
well as organizational, residential, and PERSONA CAs.
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3.5.2 PEM CRL Format
Appendix A contains the ASN.1 description of CRLs specified by this
RFC. This section provides an informal description of CRL components
analogous to that provided for certificates in Section 3.3.
1. signature (signature algorithm ID and parameters)
2. issuer
3. last update
4. next update
5. revoked certificates
The "signature" is a data item completely analogous to the signature
data item in a certificate. Similarly, the "issuer" is the DN of the
CA which signed the CRL. The "last update" and "next update" fields
contain time and date values (UTCT format) which specify,
respectively, when this CRL was issued and when the next CRL is
scheduled to be issued. Finally, "revoked certificates" is a
sequence of ordered pairs, in which the first element is the serial
number of the revoked certificate and the second element is the time
and date of the revocation for that certificate.
The semantics for this second element are not made clear in X.509.
For example, the time and date specified might indicate when a
private component was thought to have been compromised or it may
reflect when the report of such compromise was reported to the CA.
For uniformity, this RFC adopts the latter convention, i.e., the
revocation date specifies the time and date at which a CA formally
acknowledges a report of a compromise or a change or DN attributes.
As with certificates, it is recommended that the UTCT values be of no
finer granularity than minutes and that all values be stated in terms
of Zulu.
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3.6 Certificate Validation
3.6.1 Validation Basics
Every UA must contain the public component of the ICA as the root for
its certificate validation database. Public components associated
with PCAs must be identified as such, so that the certificate
validation process described below can operate correctly. Whenever a
certificate for a PCA is entered into a UA cache, e.g., if
encountered in a PEM message encapsulated header, the certificate
must NOT be entered into the cache automatically. Rather, the user
must be notified and must explicitly direct the UA to enter any PCA
certificate data into the cache. This precaution is essential
because introduction of a PCA certificate into the cache implies user
recognition of the policy associated with the PCA.
Validating a certificate begins with verifying that the signature
affixed to the certificate is valid, i.e., that the hash value
computed on the certificate contents matches the value that results
from decrypting the signature field using the public component of the
issuer. In order to perform this operation the user must possess the
public component of the issuer, either via some integrity-assured
channel, or by extracting it from another (validated) certificate.
In order to rapidly terminate this recursive validation process, we
recommend each PCA sign certificates for all CAs within its domain,
even CAs which are certified by other, superior CAs in the
certification hierarchy.
The public component needed to validate certificates signed by the
ICA is made available to each user as part of the registration or via
the PEM installation process. Thus a user will be able to validate
any PCA certificate immediately. CAs are certified by PCAs, so
validation of a CA certificate requires processing a validation path
of length two. User certificates are issued by CAs (either
immediately subordinate to PCAs or subordinate to other CAs), thus
validation of a user certificate may require three or more steps.
Local caching of validated certificates by a UA can be used to speed
up this process significantly.
Consider the situation in which a user receives a privacy enhanced
message from an originator with whom the recipient has never
previously corresponded, and assume that the message originator
includes all the requisite certificates in the PEM message header.
First the recipient can use the ICA's public component to validate a
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PCA certificate contained in an Issuer-Certificate field. Using the
PCA's public component extracted from this certificate, the CA
certificate in an Issuer-Certificate field also can be validated.
This process cam be repeated until the certificate for the
originator, from the User-Certificate field, is validated.
Having performed this certificate validation process, the recipient
can extract the originator's public component and use it to decrypt
the content of the MIC-Info field. By comparing the decrypted
contents of this field against the MIC computed locally on the
message the user verifies the data origin authenticity and integrity
of the message. It is recommended that implementations of privacy
enhanced mail cache validated public components (acquired from
incoming mail) to speed up this process. If a message arrives from
an originator whose public component is held in the recipient's
cache, the recipient can immediately employ that public component
without the need for the certificate validation process described
here. Also note that the arithmetic required for certificate
validation is considerably faster than that involved in digitally
signing a certificate, so as to minimize the computational burden on
UAs.
3.6.2 Display of Certificate Validation Data
PEM provides authenticated identities for message recipients and
originators expressed in the form of distinguished names. Mail
systems in which PEM is employed may not employ DNs as the primary
means of identifying recipients or originators. Thus, in order to
benefit from these authentication facilities, each PEM implementation
must employ some means of binding native mail system identifiers to
distinguished names in a fashion which does not undermine this basic
PEM functionality.
For example, if a human user interacts directly with PEM, then the
full DN of the originator of any message received using PEM should be
displayed for the user. Merely displaying the PEM-protected message
content, containing an originator name from the native mail system,
does not provide equivalent security functionality and could allow
spoofing. If the recipient of a message is a forwarding agent such
as a list exploder or mail relay, display of the originator's DN is
not a relevant requirement. In all cases the essential requirement
is that the ultimate recipient of a PEM message be able to ascertain
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the identity of the originator based on the PEM certification system,
not on unauthenticated identification information, e.g., extracted
from the native message system.
Conversely, for the originator of an ENCRYPTED message, it is
important that recipient identities be linked to the DNs as expressed
in PEM certificates. This can be effected in a variety of ways by
the PEM implementation, e.g., by display of recipient DNs upon
message submission or by a tightly controlled binding between local
aliases and the DNs. Here too, if the originator is a forwarding
process this linkage might be effected via various mechanisms not
applicable to direct human interaction. Again, the essential
requirement is to avoid procedures which might undermine the
authentication services provided by PEM.
As described above, it is a local matter how and what certification
information is displayed for a human user in the course of submission
or delivery of a PEM message. Nonetheless all PEM implementations
must provide a user with the ability to display a full certification
path for any certificate employed in PEM upon demand. Implementors
are urged to not overwhelm the user with certification path
information which might confuse him or distract him from the critical
information cited above.
3.6.3 Validation Procedure Details
Every PEM implementation is required to perform the following
validation steps for every public component employed in the
submission of an ENCRYPTED PEM message or the delivery of an
ENCRYPTED, MIC-ONLY, or MIC-CLEAR PEM message. Each public component
may be acquired from an internal source, e.g., from a (secure) cache
at the originator/recipient or it may be obtained from an external
source, e.g., the PEM header of an incoming message or a directory.
The following procedures applies to the validation of certificates
from either type of source.
Validation of a public component involves constructing a
certification path between the component and the public component of
the ICA. The validity interval for every certificate in this path
must be checked. PEM software must, at a minimum, warn the user if
any certificate in the path fails the validity interval check, though
the form of this warning is a local matter. For example, the warning
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might indicate which certificate in the path was expired. Local
security policy may prohibit use of expired certificates.
Each certificate also must be checked against the current CRL from
the certificate's issuer to ensure that revoked certificates are not
employed. If the UA does not have access to the current CRL for any
certificate in the path, the user must be warned. Again, the form of
the warning is a local matter. For example, the warning might
indicate whether the CRL is unavailable or, if available but not
current, the CRL issue date should be displayed. Local policy may
prohibit use of a public component which cannot be checked against a
current CRL, and in such cases the user should receive the same
information provided by the warning indications described above.
If any revoked certificates are encountered in the construction of a
certification path, the user must be warned. The form of the warning
is a local matter, but it is recommended that this warning be more
stringent than those previously alluded to above. For example, this
warning might display the issuer and subject DNs from the revoked
certificate and the date of revocation and require the user to
provide a positive response before the submission or delivery process
may proceed. In the case of message submission, the warning might
display the identity of the recipient affected by this validation
failure and the user might be provided with the option to specify
that this recipient be dropped from recipient list processing without
affecting PEM processing for the remaining recipients. Local policy
may prohibit PEM processing if a revoked certificate is encountered
in the course of constructing a certification path.
Note that in order to comply with these validation procedures, a
certificate cache must maintain all of the information contained in a
certificate, not just the DNs and the public component. For example
the serial number and validity interval must be associated with the
cache entry to comply with the checks described above. Also note
that these procedures apply to human interaction in message
submission and delivery and are not directly applicable to forwarding
processes. When non human interaction is involved, a compliant PEM
implementation must provide parameters to enable a process to specify
whether certificate validation will succeed or fail if any of the
conditions arise which would result in warnings to a human user.
Finally, in the course of validating certificates as described above,
one additional check must be performed; the subject DN of every
certificate must be subordinate to the certificate issuer DN, except
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if the issuer is a PCA (hence another reasons to distinguish PCA
entries in a certificate cache). This requirement is levied upon all
PEM implementations as part maintaining the certification hierarchy
constraints defined in this document. Any certificate which does not
comply with these requirements is considered invalid and must be
rejected in PEM submission or delivery processing. The user must be
notified of the nature of this fatal error.
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A Appendix A: ASN.1 Syntax for Certificates and CRLs
A.1 Certificate Syntax
The X.509 certificate format is defined by the following ASN.1
syntax:
Certificate ::= SIGNED SEQUENCE{
version [0] Version DEFAULT v1988,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo}
Version ::= INTEGER {v1988(0)}
CertificateSerialNumber ::= INTEGER
Validity ::= SEQUENCE{
notBefore UTCTime,
notAfter UTCTime}
SubjectPublicKeyInfo ::= SEQUENCE{
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING}
AlgorithmIdentifier ::= SEQUENCE{
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL}
The components of this structure are defined by ASN.1 syntax defined
in the X.500 Series Recommendations. RFC [1115C] provides references
for and the values of AlgortihmIdentifiers used by PEM in the
subjectPublicKeyInfo the signature data items. There is also some
ambiguity in X.509 with regard to the representation of a signed
value, e.g., a certificate signature. The interpretation selected in
PEM requires that the data to be signed is first ASN.1 encoded as an
OCTET STRING and the result is encrypted to form the signed quantity,
which is then ASN.1 encoded as an OCTET STRING. Because the
certificate is a signed data object, the distinguished encoding rules
(see X.509, section 8.7) must be applied prior to signing.
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A.2 Certificate Revocation List Syntax
The following ASN.1 syntax, derived from X.509 and aligned with the
suggested format in recently submitted defect reports, defines the
format of CRLs for use in the PEM environment.
CertificateRevocationList ::= SIGNED SEQUENCE{
signature AlgorithmIdentifier,
issuer Name,
lastUpdate UTCTime,
nextUpdate UTCTime,
revokedCertificates
SEQUENCE OF CRLEntry OPTIONAL}
CRLEntry ::= SEQUENCE{
userCertificate SerialNumber,
revocationDate UTCTime}
B References
[1] CCITT Recommendation X.411 (1988), "Message Handling Systems:
Message Transfer System: Abstract Service Definition and Procedures".
[2] CCITT Recommendation X.509 (1988), "The Directory -
Authentication Framework".
[3] CCITT Recommendation X.520 (1988), "The Directory - Selected
Attribute Types". CCITT
[4] NIST Special Publication 500-183, "Stable Agreements for Open
Systems Interconnection Protocols," Version 4, Edition 1, December
1990.
[5] North American Directory Forum, ...
[6] RFC 1113E, Privacy Enhancement for Internet Electronic Mail: Part
I: Message Encryption and Authentication Procedures, J. Linn, ?,
1992.
[7] RFC 1115C, Privacy Enhancement for Internet Electronic Mail: Part
III: Algorithms, Modes, and Identifiers, D. Balenson, ?, 1992.
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[8] RFC FROMS-C, Privacy Enhancement for Internet Electronic Mail:
Part IV: Notary, Co-Issuer, CRL-Storing and CRL-Retrieving Services,
B. Kalaski, ?, 1992.
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