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Network Working Group S. Kent
Request for Comments: 1114 BBNCC
J. Linn
DEC
IAB Privacy Task Force
August 1989
Privacy Enhancement for Internet Electronic Mail:
Part II -- Certificate-Based Key Management
STATUS OF THIS MEMO
This RFC suggests a draft standard elective protocol for the Internet
community, and requests discussion and suggestions for improvements.
Distribution of this memo is unlimited.
ACKNOWLEDGMENT
This RFC is the outgrowth of a series of IAB Privacy Task Force
meetings and of internal working papers distributed for those
meetings. We would like to thank the members of the Privacy Task
Force for their comments and contributions at the meetings which led
to the preparation of this RFC: David Balenson, Curt Barker, Matt
Bishop, Morrie Gasser, Russ Housley, Dan Nessett, Mike Padlipsky, Rob
Shirey, and Steve Wilbur.
Table of Contents
1. Executive Summary 2
2. Overview of Approach 3
3. Architecture 4
3.1 Scope and Restrictions 4
3.2 Relation to X.509 Architecture 7
3.3 Entities' Roles and Responsibilities 7
3.3.1 Users and User Agents 8
3.3.2 Organizational Notaries 9
3.3.3 Certification Authorities 11
3.3.3.1 Interoperation Across Certification Hierarchy Boundaries 14
3.3.3.2 Certificate Revocation 15
3.4 Certificate Definition and Usage 17
3.4.1 Contents and Use 17
3.4.1.1 Version Number 18
3.4.1.2 Serial Number 18
3.4.1.3 Subject Name 18
3.4.1.4 Issuer Name 19
3.4.1.5 Validity Period 19
3.4.1.6 Subject Public Component 20
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RFC 1114 Mail Privacy: Key Management August 1989
3.4.1.7 Certificate Signature 20
3.4.2 Validation Conventions 20
3.4.3 Relation with X.509 Certificate Specification 22
NOTES 24
1. Executive Summary
This is one of a series of RFCs defining privacy enhancement
mechanisms for electronic mail transferred using Internet mail
protocols. RFC-1113 (the successor to RFC 1040) 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-1115 specifies
algorithms for use in processing privacy-enhanced messages, as called
for in RFC-1113. 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. A subsequent RFC, the fourth in this series, will
provide detailed specifications, paper and electronic application
forms, etc. for the key management infrastructure described herein.
The key management architecture described in this RFC is compatible
with the authentication framework described in X.509. The major
contributions of this RFC lie not in the specification of computer
communication protocols or algorithms but rather in procedures and
conventions for the key management infrastructure. This RFC
incorporates numerous conventions to facilitate near term
implementation. Some of these conventions may be superceded in time
as the motivations for them no longer apply, e.g., when X.500 or
similar directory servers become well established.
The RSA cryptographic algorithm, covered in the U.S. by patents
administered through RSA Data Security, Inc. (hereafter abbreviated
RSADSI) has been selected for use in this key management system.
This algorithm has been selected because it provides all the
necessary algorithmic facilities, is "time tested" and is relatively
efficient to implement in either software or hardware. It is also
the primary algorithm identified (at this time) for use in
international standards where an asymmetric encryption algorithm is
required. Protocol facilities (e.g., algorithm identifiers) exist to
permit use of other asymmetric algorithms if, in the future, it
becomes appropriate to employ a different algorithm for key
management. However, the infrastructure described herein is specific
to use of the RSA algorithm in many respects and thus might be
different if the underlying algorithm were to change.
Current plans call for RSADSI to act in concert with subscriber
organizations as a "certifying authority" in a fashion described
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RFC 1114 Mail Privacy: Key Management August 1989
later in this RFC. RSADSI will offer a service in which it will sign
a certificate which has been generated by a user and vouched for
either by an organization or by a Notary Public. This service will
carry a $25 biennial fee which includes an associated license to use
the RSA algorithm in conjunction with privacy protection of
electronic mail. Users who do not come under the purview of the RSA
patent, e.g., users affiliated with the U.S. government or users
outside of the U.S., may make use of different certifying authorities
and will not require a license from RSADSI. Procedures for
interacting with these other certification authorities, maintenance
and distribution of revoked certificate lists from such authorities,
etc. are outside the scope of this RFC. However, techniques for
validating certificates issued by other authorities are contained
within the RFC to ensure interoperability across the resulting
jurisdictional boundaries.
2. Overview of Approach
This RFC defines a key management architecture based on the use of
public-key certificates, in support of the message encipherment and
authentication procedures defined in RFC-1113. In the proposed
architecture, a "certification authority" representing an
organization applies a digital signature to a collection of data
consisting of a user's public component, various information that
serves to identify the user, and the identity of the organization
whose signature is affixed. (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 publically
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.) This establishes a binding between these user
credentials, the user's public component and the organization which
vouches for this binding. The resulting signed, data item is called
a certificate. The organization identified as the certifying
authority for the certificate is the "issuer" of that certificate.
In signing the certificate, the certification authority vouches for
the user's identification, especially as it relates to the user's
affiliation with the organization. The digital signature is affixed
on behalf of that organization and is in a form which can be
recognized by all members of the privacy-enhanced electronic mail
community. Once generated, certificates can be stored in directory
servers, transmitted via unsecure message exchanges, or distributed
via any other means that make certificates easily accessible to
message originators, without regard for the security of the
transmission medium.
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Prior to sending an encrypted message, 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
(described later) or is itself distributed in a certificate to which
this validation procedure is applied recursively.
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) that is used to encrypt the message itself.
The resulting encrypted DEK is incorporated into the X-Key-Info field
of the message header. Upon receipt of an encrypted message, a
recipient employs his secret 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 secret component of his public-key pair,
and includes the resulting value in the message header in the X-MIC-
Info field. The certificate of the originator is also included in
the header in the X-Certificate field as described in RFC-1113, 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, 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 (see RFC-1113) in the Internet environment.
The architecture describes procedures for ordering certificates from
issuers, for generating and distributing certificates, and for "hot
listing" of revoked certificates. Concurrent with the issuance of
this RFC, RFC 1040 has been updated and reissued as RFC-1113 to
describe the syntax and semantics of new or revised header fields
used to transfer certificates, represent the DEK and MIC in this
public-key context, and to segregate algorithm definitions into a
separate RFC to facilitate the addition of other algorithms in the
future. This RFC focuses on the management aspects of certificate-
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based, public-key cryptography for privacy enhanced mail while RFC-
1113 addresses representation and processing aspects of such mail,
including changes required by this key management technology.
The proposed architecture imposes conventions for certification paths
which are not strictly required by the X.509 recommendation nor by
the technology itself. The decision to impose these conventions is
based in part on constraints imposed by the status of the RSA
cryptosystem within the U.S. as a patented algorithm, and in part on
the need for an organization to assume operational responsibility for
certificate management in the current (minimal) directory system
infrastructure for electronic mail. Over time, we anticipate that
some of these constraints, e.g., directory service availability, will
change and the procedures specified in the RFC will be reviewed and
modified as appropriate.
At this time, we propose a system in which user certificates
represent the leaves in a shallow (usually two tier) certification
hierarchy (tree). Organizations which act as issuers are represented
by certificates higher in the tree. This convention minimizes the
complexity of validating user certificates by limiting the length of
"certification paths" and by making very explicit the relationship
between a certificate issuer and a user. Note that only
organizations may act as issuers in the proposed architecture; a user
certificate may not appear in a certification path, except as the
terminal node in the path. 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.
The RFC proposes that RSADSI act as a "co-issuer" of certificates on
behalf of most organizations. This can be effected in a fashion
which is "transparent" so that the organizations appear to be the
issuers with regard to certificate formats and validation procedures.
This is effected by having RSADSI generate and hold the secret
components used to sign certificates on behalf of organizations. The
motivation for RSADSI's role in certificate signing is twofold.
First, it simplifies accounting controls in support of licensing,
ensuring that RSADSI is paid for each certificate. Second, it
contributes to the overall integrity of the system by establishing a
uniform, high level of protection for the private-components used to
sign certificates. If an organization were to sign certificates
directly on behalf of its affiliated users, the organization would
have to establish very stringent security and accounting mechanisms
and enter into (elaborate) legal agreements with RSADSI in order to
provide a comparable level of assurance. Requests by organizations
to perform direct certificate signing will be considered on a case-
by-case basis, but organizations are strongly urged to make use of
the facilities proposed by this RFC.
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Note that the risks associated with disclosure of an organization's
secret component are different from those associated with disclosure
of a user's secret component. The former component is used only to
sign certificates, never to encrypt message traffic. Thus the
exposure of an organization's secret component could result in the
generation of forged certificates for users affiliated with that
organization, but it would not affect privacy-enhanced messages which
are protected using legitimate certificates. Also note that any
certificates generated as a result of such a disclosure are readily
traceable to the issuing authority which holds this component, e.g.,
RSADSI, due to the non-repudiation feature of the digital signature.
The certificate registration and signing procedures established in
this RFC would provide non-repudiable evidence of disclosure of an
organization's secret component by RSADSI. Thus this RFC advocates
use of RSADSI as a co-issuer for certificates until such time as
technical security mechanisms are available to provide a similar,
system-wide level of assurance for (distributed) certificate signing
by organizations.
We identify two classes of exceptions to this certificate signing
paradigm. First, the RSA algorithm is patented only within the U.S.,
and thus it is very likely that certificate signing by issuers will
arise outside of the U.S., independent of RSADSI. Second, the
research that led to the RSA algorithm was sponsored by the National
Science Foundation, and thus the U.S. government retains royalty-free
license rights to the algorithm. Thus the U.S. government may
establish a certificate generation facilities for its affiliated
users. A number of the procedures described in this document apply
only to the use of RSADSI as a certificate co-issuer; all other
certificate generation practices lie outside the scope of this RFC.
This RFC specifies procedures by which users order certificates
either directly from RSADSI or via a representative in an
organization with which the user holds some affiliation (e.g., the
user's employer or educational institution). Syntactic provisions
are made which allow a recipient to determine, to some granularity,
which identifying information contained in the certificate is vouched
for by the certificate issuer. In particular, organizations will
usually be vouching for the affiliation of a user with that
organization and perhaps a user's role within the organization, in
addition to the user's name. In other circumstances, as discussed in
section 3.3.3, a certificate may indicate that an issuer vouches only
for the user's name, implying that any other identifying information
contained in the certificate may not have been validated by the
issuer. These semantics are beyond the scope of X.509, but are not
incompatible with that recommendation.
The key management architecture described in this RFC has been
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RFC 1114 Mail Privacy: Key Management August 1989
designed to support privacy enhanced mail as defined in this RFC,
RFC-1113, and their successors. Note that this infrastructure also
supports X.400 mail security facilities (as per X.411) and thus paves
the way for transition to the OSI/CCITT Message Handling System
paradigm in the Internet in the future. The certificate issued to a
user for the $25 biennial fee will grant to the user identified by
that certificate a license from RSADSI to employ the RSA algorithm
for certificate validation and for encryption and decryption
operations in this electronic mail context. No use of the algorithm
outside the scope defined in this RFC is authorized by this license
as of this time. Expansion of the license to other Internet security
applications is possible but not yet authorized. The license granted
by this fee does not authorize the sale of software or hardware
incorporating the RSA algorithm; it is an end-user license, not a
developer's license.
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. The
public-key certificate approach defined in X.509 has also been
adopted in CCITT 1988 X.411 in support of the message handling
application.
This RFC interprets the X.509 certificate mechanism to serve the
needs of privacy-enhanced mail 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.
In large part constraints have been levied in order to simplify
certificate validation in the absence of a widely available, user-
level directory service. The certification hierarchy proposed here
also embodies semantics which are not explicitly addressed by X.509,
but which are consistent with X.509 precepts. The additional
semantic constraints have been adopted to explicitly address
questions of issuer "authority" which we feel are not well defined in
X.509.
3.3 Entities' Roles and Responsibilities
One way to explain the architecture proposed by this RFC is to
examine the various roles which are defined for various entities in
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RFC 1114 Mail Privacy: Key Management August 1989
the architecture and to describe what is required of each entity in
order for the proposed system to work properly. The following
sections identify three different types of entities within this
architecture: users and user agents, organizational notaries, and
certification authorities. For each class of entity we describe the
(electronic and paper) 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. Note that the
infrastructure described here applies to the situation wherein RSADSI
acts as a co-issuer of certificates, sharing the role of
certification authority as described later. Other certifying
authority arrangements may employ different procedures and are not
addressed by this RFC.
3.3.1 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).
A UA process supporting privacy-enhanced mail processing must protect
the private component of its associated entity (ordinarily, a human
user) from disclosure. We anticipate that a user will employ
ancillary software (not otherwise associated with the UA) to generate
his public/private component pair and to compute the (one-way)
message hash required by the registration procedure. The public
component, along with information that identifies the user, will be
transferred to an organizational notary (see below) for inclusion in
an order to an issuer. The process of generating public and private
components is a local matter, but we anticipate Internet-wide
distribution of software suitable for component-pair generation to
facilitate the process. The mechanisms used to transfer the public
component and the user identification information must preserve the
integrity of both quantities and bind the two during this transfer.
This proposal establishes two ways in which a user may order a
certificate, i.e., through the user's affiliation with an
organization or directly through RSADSI. In either case, a user will
be required to send a paper order to RSADSI on a form described in a
subsequent RFC and containing the following information:
1. Distinguished Name elements (e.g., full legal name,
organization name, etc.)
2. Postal address
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3. Internet electronic mail address
4. A message hash function, binding the above information to the
user's public component
Note that the user's public component is NOT transmitted via this
paper path. In part the rationale here is that the public component
consists of many (>100) digits and thus is prone to error if it is
copied to and from a piece of paper. Instead, a message hash is
computed on the identifying information and the public component and
this (smaller) message hash value is transmitted along with the
identifying information. Thus the public component is transferred
only via an electronic path, as described below.
If the user is not affiliated with an organization which has
established its own "electronic notary" capability (an organization
notary or "ON" as discussed in the next section), then this paper
registration form must be notarized by a Notary Public. If the user
is affiliated with an organization which has established one or more
ONs, the paper registration form need not carry the endorsement of a
Notary Public. Concurrent with the paper registration, the user must
send the information outlined above, plus his public component,
either to his ON, or directly to RSADSI if no appropriate ON is
available to the user. Direct transmission to RSADSI of this
information will be via electronic mail, using a representation
described in a subsequent RFC. The paper registration must be
accompanied by a check or money order for $25 or an organization may
establish some other billing arrangement with RSADSI. The maximum
(and default) lifetime of a certificate ordered through this process
is two years.
The transmission of ID information and public component from a user
to his ON is a local matter, but we expect electronic mail will also
be the preferred approach in many circumstances and we anticipate
general distribution of software to support this process. Note that
it is the responsibility of the user and his organization to ensure
the integrity of this transfer by some means deemed adequately secure
for the local computing and communication environment. There is no
requirement for secrecy in conjunction with this information
transfer, but the integrity of the information must be ensured.
3.3.2 Organizational Notaries
An organizational notary is an individual who acts as a clearinghouse
for certificate orders originating within an administrative domain
such as a corporation or a university. An ON represents an
organization or organizational unit (in X.500 naming terms), and is
assumed to have some independence from the users on whose behalf
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certificates are ordered. An ON will be restricted through
mechanisms implemented by the issuing authority, e.g., RSADSI, to
ordering certificates properly associated with the domain of that ON.
For example, an ON for BBN should not be able to order certificates
for users affiliated with MIT or MITRE, nor vice versa. Similarly,
if a corporation such as BBN were to establish ONs on a per-
subsidiary basis (corresponding to organization units in X.500 naming
parlance), then an ON for the BBN Communications subsidiary should
not be allowed to order a certificate for a user who claims
affiliation with the BBN Software Products subsidiary.
It can be assumed that the set of ONs changes relatively slowly and
that the number of ONs is relatively small in comparison with the
number of users. Thus a more extensive, higher assurance process may
reasonably be associated with ON accreditation than with per-user
certificate ordering. Restrictions on the range of information which
an ON is authorized to certify are established as part of this more
elaborate registration process. The procedures by which
organizations and organizational units are established in the RSADSI
database, and by which ONs are registered, will be described in a
subsequent RFC.
An ON is responsible for establishing the correctness and integrity
of information incorporated in an order, and will generally vouch for
(certify) the accuracy of identity information at a granularity finer
than that provided by a Notary Public. We do not believe that it is
feasible to enforce uniform standards for the user certification
process across all ONs, but we anticipate that organizations will
endeavor to maintain high standards in this process in recognition of
the "visibility" associated with the identification data contained in
certificates. An ON also may constrain the validity period of an
ordered certificate, restricting it to less than the default two year
interval imposed by the RSADSI license agreement.
An ON participates in the certificate ordering process by accepting
and validating identification information from a user and forwarding
this information to RSADSI. The ON accepts the electronic ordering
information described above (Distinguished Name elements, mailing
address, public component, and message hash computed on all of this
data) from a user. (The representation for user-to-ON transmission
of this data is a local matter, but we anticipate that the encoding
specified for ON-to-RSADSI representation of this data will often be
employed.) The ON sends an integrity-protected (as described in
RFC-1113) electronic message to RSADSI, vouching for the correctness
of the binding between the public component and the identification
data. Thus, to support this function, each ON will hold a
certificate as an individual user within the organization which he
represents. RSADSI will maintain a database which identifies the
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users who also act as ONs and the database will specify constraints
on credentials which each ON is authorized to certify. The
electronic mail representation for a user's certificate data in an ON
message to RSADSI will be specified in a subsequent RFC.
3.3.3 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". This alternate expansion for the acronym "CA" is
roughly equivalent to that contemplated as a "central authority" in
RFC-1040 and RFC-1113. The only difference is that in X.509 there is
no requirement that a CA be a distinguished entity or that a CA serve
a large number of users, as envisioned in these RFCs. Rather, any
user who holds a certificate can, in the X.509 context, act as a CA
for any other user. As noted above, we have chosen to restrict the
role of CA in this electronic mail environment to organizational
entities, to simplify the certificate validation process, to impose
semantics which support organizational affiliation as a basis for
certification, and to facilitate license accountability.
In the proposed architecture, individuals who are affiliated with
(registered) organizations will go through the process described
above, in which they forward their certificate information to their
ON for certification. The ON will, based on local procedures, verify
the accuracy of the user's credentials and forward this information
to RSADSI using privacy-enhanced mail to ensure the integrity and
authenticity of the information. RSADSI will carry out the actual
certificate generation process on behalf of the organization
represented by the ON. Recall that it is the identity of the
organization which the ON represents, not the ON's identity, which
appears in the issuer field of the user certificate. Therefore it is
the private component of the organization, not the ON, which is used
to sign the user certificate.
In order to carry out this procedure RSADSI will serve as the
repository for the private components associated with certificates
representing organizations or organizational units (but not
individuals). In effect the role of CA will be shared between the
organizational notaries and RSADSI. This shared role will not be
visible in the syntax of the certificates issued under this
arrangement nor is it apparent from the validation procedure one
applies to these certificates. In this sense, the role of RSADSI as
the actual signer of certificates on behalf of organizations is
transparent to this aspect of system operation.
If an organization were to carry out the certificate signing process
locally, and thus hold the private component associated with its
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organization certificate, it would need to contact RSADSI to discuss
security safeguards, special legal agreements, etc. A number of
requirements would be imposed on an organization if such an approach
were persued. The organization would be required to execute
additional legal instruments with RSADSI, e.g., to ensure proper
accounting for certificates generated by the organization. Special
software will be required to support the certificate signing process,
distinct from the software required for an ON. Stringent procedural,
physical, personnel and computer security safeguards would be
required to support this process, to maintain a relatively high level
of security for the system as a whole. Thus, at this time, it is not
recommended that organizations pursue this approach although local
certificate generation is not expressly precluded by the proposed
architecture.
RSADSI has offered to operate a service in which it serves as a CA
for users who are not affiliated with any organization or who are
affiliated with an organization which has not opted to establish an
organizational notary. To distinguish certificates issued to such
"non-affiliated" users the distinguished string "Notary" will appear
as the organizational unit name of the issuer of the certificate.
This convention will be employed throughout the system. Thus not
only RSADSI but any other organization which elects to provide this
type of service to non-affiliated users may do so in a standard
fashion. Hence a corporation might issue a certificate with the
"Notary" designation to students hired for the summer, to
differentiate them from full-time employees. At least in the case of
RSADSI, the standards for verifying user credentials that carry this
designation will be well known and widely recognized (e.g., Notary
Public endorsement).
To illustrate this convention, consider the following examples.
Employees of RSADSI will hold certificates which indicate "RSADSI" as
the organization in both the issuer field and the subject field,
perhaps with no organizational unit specified. Certificates obtained
directly from RSADSI, by user's who are not affiliated with any ON,
will also indicate "RSADSI" as the organization and will specify
"Notary" as an organizational unit in the issuer field. However,
these latter certificates will carry some other designation for
organization (and, optionally, organizational unit) in the subject
field. Moreover, an organization designated in the subject field for
such a certificate will not match any for which RSADSI has an ON
registered (to avoid possible confusion).
In all cases described above, when a certificate is generated RSADSI
will send a paper reply to the ordering user, including two message
hash functions:
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1. a message hash computed on the user's identifying information
and public component (and sent to RSADSI in the registration
process), to guarantee its integrity across the ordering
process, and
2. a message hash computed on the public component of RSADSI, to
provide independent authentication for this public component
which is transmitted to the user via email (see below).
RSADSI will send to the user via electronic mail (not privacy
enhanced) a copy of his certificate, a copy of the organization
certificate identified in the issuer field of the user's certificate,
and the public component used to validate certificates signed by
RSADSI. The "issuer" certificate is included to simplify the
validation process in the absence of a user-level directory system;
its distribution via this procedure will probably be phased out in
the future. Thus, as described in RFC-1113, the originator of a
message is encouraged, though not required, to include his
certificate, and that of its issuer, in the privacy enhanced message
header (X-Issuer-Certificate) to ensure that each recipient can
process the message using only the information contained in this
header. The organization (organizational unit) identified in the
subject field of the issuer certificate should correspond to that
which the user claims affiliation (as declared in the subject field
of his certificate). If there is no appropriate correspondence
between these fields, recipients ought to be suspicious of the
implied certification path. This relationship should hold except in
the case of "non-affiliated" users for whom the "Notary" convention
is employed.
In contrast, the issuer field of the issuer's certificate will
specify "RSADSI" as the organization, i.e., RSADSI will certify all
organizational certificates. This convention allows a recipient to
validate any originator's certificate (within the RSADSI
certification hierarchy) in just two steps. Even if an organization
establishes a certification hierarchy involving organizational units,
certificates corresponding to each unit can be certified both by
RSADSI and by the organizational entity immediately superior to the
unit in the hierarchy, so as to preserve this short certification
path feature. First, the public component of RSADSI is employed to
validate the issuer's certificate. Then the issuer's public
component is extracted from that certificate and is used to validate
the originator's certificate. The recipient then extracts the
originator's public component for use in processing the X-Mic-Info
field of the message (see and RFC-1113).
The electronic representation used for transmission of the data items
described above (between an ON and RSADSI) will be contained in a
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subsequent RFC. To verify that the registration process has been
successfully completed and to prepare for exchange of privacy-
enhanced electronic mail, the user should perform the following
steps:
1. extract the RSADSI public component, the issuer's certificate
and the user's certificate from the message
2. compute the message hash on the RSADSI public component and
compare the result to the corresponding message hash that was
included in the paper receipt
3. use the RSADSI public component to validate the signature on
the issuer's certificate (RSADSI will be the issuer of this
certificate)
4. extract the organization public component from the validated
issuer's certificate and use this public component to
validate the user certificate
5. extract the identification information and public component
from the user's certificate, compute the message hash on it
and compare the result to the corresponding message hash
value transmitted via the paper receipt
For a user whose order was processed via an ON, successful completion
of these steps demonstrates that the certificate issued to him
matches that which he requested and which was certified by his ON.
It also demonstrates that he possesses the (correct) public component
for RSADSI and for the issuer of his certificate. For a user whose
order was placed directly with RSADSI, this process demonstrates that
his certificate order was properly processed by RSADSI and that he
possesses the valid issuer certificate for the RSADSI Notary. The
user can use the RSADSI public component to validate organizational
certificates for organizations other than his own. He can employ the
public component associated with his own organization to validate
certificates issued to other users in his organization.
3.3.3.1 Interoperation Across Certification Hierarchy Boundaries
In order to accommodate interoperation with other certification
authorities, e.g., foreign or U.S. government CAs, two conventions
will be adopted. First, all certifying authorities must agree to
"cross-certify" one another, i.e., each must be willing to sign a
certificate in which the issuer is that certifying authority and the
subject is another certifying authority. Thus, RSADSI might generate
a certificate in which it is identified as the issuer and a
certifying authority for the U.S. government is indentified as the
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subject. Conversely, that U.S. government certifying authority would
generate a certificate in which it is the issuer and RSADSI is the
subject. This cross-certification of certificates for "top-level"
CAs establishes a basis for "lower level" (e.g., organization and
user) certificate validation across the hierarchy boundaries. This
avoids the need for users in one certification hierarchy to engage in
some "out-of-band" procedure to acquire a public-key for use in
validating certificates from a different certification hierarchy.
The second convention is that more than one X-Issuer-Certificate
field may appear in a privacy-enhanced mail header. Multiple issuer
certificates can be included so that a recipient can more easily
validate an originator's certificate when originator and recipient
are not part of a common CA hierarchy. Thus, for example, if an
originator served by the RSADSI certification hierarchy sends a
message to a recipient served by a U.S. government hierarchy, the
originator could (optionally) include an X-Issuer-Certificate field
containing a certificate issued by the U.S. government CA for RSADSI.
In this fashion the recipient could employ his public component for
the U.S. government CA to validate this certificate for RSADSI, from
which he would extract the RSADSI public component to validate the
certificate for the originator's organization, from which he would
extract the public component required to validate the originator's
certificate. Thus, more steps can be required to validate
certificates when certification hierarchy boundaries are crossed, but
the same basic procedure is employed. Remember that caching of
certificates by UAs can significantly reduce the effort required to
process messages and so these examples should be viewed as "worse
case" scenarios.
3.3.3.2 Certificate Revocation
X.509 states that it is a CA's responsibility to maintain:
1. a time-stamped list of the certificates it issued which have
been revoked
2. a time-stamped list of revoked certificates representing
other CAs
There are two primary reasons for a CA to revoke a certificate, i.e.,
suspected compromise of a secret component (invalidating the
corresponding public component) or change of user affiliation
(invalidating the Distinguished Name). As described in X.509, "hot
listing" is one means of propagating information relative to
certificate revocation, though it is not a perfect mechanism. In
particular, an X.509 Revoked Certificate List (RCL) indicates only
the age of the information contained in it; it does not provide any
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basis for determining if the list is the most current RCL available
from a given CA. To help address this concern, the proposed
architecture establishes a format for an RCL in which not only the
date of issue, but also the next scheduled date of issue is
specified. This is a deviation from the format specified in X.509.
Adopting this convention, when the next scheduled issue date arrives
a CA must issue a new RCL, 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 RCLs are issued by that CA. Note
that this does not preclude RCL issuance on a more frequent basis,
e.g., in case of some emergency, but no Internet-wide mechanisms are
architected for alerting users that such an unscheduled issuance has
taken place. This scheduled RCL issuance convention allows users
(UAs) to determine whether a given RCL is "out of date," a facility
not available from the standard RCL format.
A recent (draft) version of the X.509 recommendation calls for each
RCL to contain the serial numbers of certificates which have been
revoked by the CA administering that list, i.e., the CA that is
identified as the issuer for the corresponding revoked certificates.
Upon receipt of a RCL, a UA should compare the entries against any
cached certificate information, deleting cache entries which match
RCL entries. (Recall that the certificate serial numbers are unique
only for each issuer, so care must be exercised in effecting this
cache search.) The UA should also retain the RCL to screen incoming
messages to detect use of revoked certificates carried in these
message headers. More specific details for processing RCL are beyond
the scope of this RFC as they are a function of local certificate
management techniques.
In the architecture defined by this RFC, a RCL will be maintained for
each CA (organization or organizational unit), signed using the
private component of that organization (and thus verifiable using the
public component of that organization as extracted from its
certificate). The RSADSI Notary organizational unit is included in
this collection of RCLs. CAs operated under the auspices of the U.S.
government or foreign CAs are requested to provide RCLs conforming to
these conventions, at least until such time as X.509 RCLs provide
equivalent functionality, in support of interoperability with the
Internet community. An additional, "top level" RCL, will be
maintained by RSAD-SI, and should be maintained by other "top level"
CAs, for revoked organizational certificates.
The hot listing procedure (expect for this top level RCL) will be
effected by having an ON from each organization transmit to RSADSI a
list of the serial numbers of users within his organization, to be
hot listed. This list will be transmitted using privacy-enhanced
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mail to ensure authenticity and integrity and will employ
representation conventions to be provided in a subsequent RFC.
RSADSI will format the RCL, sign it using the private component of
the organization, and transmit it to the ON for dissemination, using
a representation defined in a subsequent RFC. Means for
dissemination of RCLs, both within the administrative domain of a CA
and across domain boundaries, are not specified by this proposal.
However, it is anticipated that each hot list will also be available
via network information center databases, directory servers, etc.
The following ASN.1 syntax, derived from X.509, defines the format of
RCLs for use in the Internet privacy enhanced email environment. See
the ASN.1 definition of certificates (later in this RFC or in X.509,
Annex G) for comparison.
revokedCertificateList ::= SIGNED SEQUENCE {
signature AlgorithmIdentifier,
issuer Name,
list SEQUENCE RCLEntry,
lastUpdate UTCTime,
nextUpdate UTCTime}
RCLEntry ::= SEQUENCE {
subject CertificateSerialNumber,
revocationDate UTCTime}
3.4 Certificate Definition and Usage
3.4.1 Contents and Use
A certificate contains the following contents:
1. version
2. serial number
3. certificate signature (and associated algorithm identifier)
4. issuer name
5. validity period
6. subject name
7. subject public component (and associated algorithm identifier)
This section discusses the interpretation and use of each of these
certificate elements.
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3.4.1.1 Version Number
The version number field is intended to facilitate orderly changes in
certificate formats over time. The initial version number for
certificates is zero (0).
3.4.1.2 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 RCLs to identify revoked certificates instead of including entire
certificates. Thus each certificate generated by an issuer must
contain a unique serial number. It is suggested that these numbers
be issued as a compact, monotonic increasing sequence.
3.4.1.3 Subject Name
A certificate provides a representation of its subject's identity and
organizational affiliation in the form of a Distinguished Name. The
fundamental binding ensured by the privacy enhancement mechanisms is
that between public-key and the user identity. CCITT Recommendation
X.500 defines the concept of Distinguished Name.
Version 2 of the U.S. Government Open Systems Interconnection Profile
(GOSIP) specifies maximum sizes for O/R Name attributes. Since most
of these attributes also appear in Distinguished Names, we have
adopted the O/R Name attribute size constraints specified in GOSIP
and noted below. Using these size constraints yields a maximum
Distinguished Name length (exclusive of ASN encoding) of two-hundred
fifty-nine (259) characters, based on the required and optional
attributes described below for subject names. The following
attributes are required in subject Distinguished Names for purposes
of this RFC:
1. Country Name in standard encoding (e.g., the two-character
Printable String "US" assigned by ISO 3166 as the identifier
for the United States of America, the string "GB" assigned as
the identifier for the United Kingdom, or the string "NQ"
assigned as the identifier for Dronning Maud Land). Maximum
ASCII character length of three (3).
2. Organizational Name (e.g., the Printable String "Bolt Beranek
and Newman, Inc."). Maximum ASCII character length of
sixty-four (64).
3. Personal Name (e.g., the X.402/X.411 structured Printable
String encoding for the name John Linn). Maximum ASCII
character length of sixty-four (64).
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The following attributes are optional in subject Distinguished Names
for purposes of this RFC:
1. Organizational Unit Name(s) (e.g., the Printable String "BBN
Communications Corporation") A hierarchy of up to four
organizational unit names may be provided; the least
significant member of the hierarchy is represented first.
Each of these attributes has a maximum ASCII character length of
thirty-two (32), for a total of one-hundred and twenty-eight
(128) characters if all four are present.
3.4.1.4 Issuer Name
A certificate provides a representation of its issuer's identity, in
the form of a Distinguished Name. The issuer identification is
needed in order to determine the appropriate issuer public component
to use in performing certificate validation. The following
attributes are required in issuer Distinguished Names for purposes of
this RFC:
1. Country Name (e.g., encoding for "US")
2. Organizational Name
The following attributes are optional in issuer Distinguished Names
for purposes of this RFC:
1. Organizational Unit Name(s). (A hierarchy of up to four
organizational unit names may be provided; the least significant
member of the hierarchy is represented first.) If the
issuer is vouching for the user identity in the Notary capacity
described above, then exactly one instance of this field
must be present and it must consist of the string "Notary".
As noted earlier, only organizations are allowed as issuers in the
proposed authentication hierarchy. Hence the Distinguished Name for
an issuer should always be that of an organization, not a user, and
thus no Personal Name field may be included in the Distinguished Name
of an issuer.
3.4.1.5 Validity Period
A certificate carries a pair of time specifiers, indicating the start
and end of the time period over which a certificate is intended to be
used. No message should ever be prepared for transmission with a
non-current certificate, but recipients should be prepared to receive
messages processed using recently-expired certificates. This fact
results from the unpredictable (and sometimes substantial)
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transmission delay of the staged-delivery electronic mail
environment. The default and maximum validity period for
certificates issued in this system will be two years.
3.4.1.6 Subject Public Component
A certificate carries the public component of its associated entity,
as well as an indication of the algorithm with which the public
component is to be used. For purposes of this RFC, the algorithm
identifier will indicate use of the RSA algorithm, as specified in
RFC-1115. Note that in this context, a user's public component is
actually the modulus employed in RSA algorithm calculations. A
"universal" (public) exponent is employed in conjunction with the
modulus to complete the system. Two choices of exponents are
recommended for use in this context and are described in section
3.4.3. Modulus size will be permitted to vary between 320 and 632
bits.
3.4.1.7 Certificate Signature
A certificate carries a signature algorithm identifier and a
signature, applied to the certificate by its issuer. The signature
is validated by the user of a certificate, in order to determine that
the integrity of its contents have not been compromised subsequent to
generation by a CA. An encrypted, one-way hash will be employed as
the signature algorithm. Hash functions suitable for use in this
context are notoriously difficult to design and tend to be
computationally intensive. Initially we have adopted a hash function
developed by RSADSI and which exhibits performance roughly equivalent
to the DES (in software). This same function has been selected for
use in other contexts in this system where a hash function (message
hash algorithm) is required, e.g., MIC for multicast messages. In
the future we expect other one-way hash functions will be added to
the list of algorithms designated for this purpose.
3.4.2 Validation Conventions
Validating a certificate involves 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 the proposed architecture this recursive operation is terminated
quickly by adopting the convention that RSADSI will certify the
certificates of all organizations or organizational units which act
as issuers for end users. (Additional validation steps may be
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RFC 1114 Mail Privacy: Key Management August 1989
required for certificates issued by other CAs as described in section
3.3.3.1.)
Certification means that RSADSI will sign certificates in which the
subject is the organization or organizational unit and for which
RSADSI is the issuer, thus implying that RSADSI vouches for the
credentials of the subject. This is an appropriate construct since
each ON representing an organization or organizational unit must have
registered with RSADSI via a procedure more rigorous than individual
user registration. This does not preclude an organizational unit
from also holding a certificate in which the "parent" organization
(or organizational unit) is the issuer. Both certificates are
appropriate and permitted in the X.509 framework. However, in order
to facilitate the validation process in an environment where user-
level directory services are generally not available, we will (at
this time) adopt this certification convention.
The public component needed to validate certificates signed by RSADSI
(in its role as a CA for issuers) is transmitted to each user as part
of the registration process (using electronic mail with independent,
postal confirmation via a message hash). Thus a user will be able to
validate any user certificate (from the RSADSI hierarchy) in at most
two steps. Consider the situation in which a user receives a privacy
enhanced message from an originator with whom the recipient has never
previously corresponded. Based on the certification convention
described above, the recipient can use the RSADSI public component to
validate the issuer's certificate contained in the X-Issuer-
Certificate field. (We recommend that, initially, the originator
include his organization's certificate in this optional field so that
the recipient need not access a server or cache for this public
component.) Using the issuer's public component (extracted from this
certificate), the recipient can validate the originator's certificate
contained in the X-Certificate field of the header.
Having performed this certificate validation process, the recipient
can extract the originator's public component and use it to decrypt
the content of the X-MIC-Info field and thus verify the data origin
authenticity and integrity of the message. Of course,
implementations of privacy enhanced mail should cache validated
public components (acquired from incoming mail or via the message
from a user registration process) 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 users.
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A separate issue associated with validation of certificates is a
semantic one, i.e., is the entity identified in the issuer field
appropriate to vouch for the identifying information in the subject
field. This is a topic outside the scope of X.509, but one which
must be addressed in any viable system. The hierarchy proposed in
this RFC is designed to address this issue. In most cases a user
will claim, as part of his identifying information, affiliation with
some organization and that organization will have the means and
responsibility for verifying this identifying information. In such
circumstances one should expect an obvious relationship between the
Distinguished Name components in the issuer and subject fields.
For example, if the subject field of a certificate identified an
individual as affiliated with the "Widget Systems Division"
(Organizational Unit Name) of "Compudigicorp" (Organizational Name),
one would expect the issuer field to specify "Compudigicorp" as the
Organizational Name and, if an Organizational Unit Name were present,
it should be "Widget Systems Division." If the issuer's certificate
indicated "Compudigicorp" as the subject (with no Organizational Unit
specified), then the issuer should be "RSADSI." If the issuer's
certificate indicated "Widget Systems Division" as Organizational
Unit and "Compudigicorp" as Organization in the subject field, then
the issuer could be either "RSADSI" (due to the direct certification
convention described earlier) or "Compudigicorp" (if the organization
elected to distribute this intermediate level certificate). In the
later case, the certificate path would involve an additional step
using the certificate in which "Compudigicorp" is the subject and
"RSADSI" is the issuer. One should be suspicious if the validation
path does not indicate a subset relationship for the subject and
issuer Distinguished Names in the certification path, expect where
cross-certification is employed to cross CA boundaries.
It is a local matter whether the message system presents a human user
with the certification path used to validate a certificate associated
with incoming, privacy-enhanced mail. We note that a visual display
of the Distinguished Names involved in that path is one means of
providing the user with the necessary information. We recommend,
however, that certificate validation software incorporate checks and
alert the user whenever the expected certification path relationships
are not present. The rationale here is that regular display of
certification path data will likely be ignored by users, whereas
automated checking with a warning provision is a more effective means
of alerting users to possible certification path anomalies. We urge
developers to provide facilities of this sort.
3.4.3 Relation with X.509 Certificate Specification
An X.509 certificate can be viewed as two components: contents and an
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encrypted hash. The encrypted hash is formed and processed as
follows:
1. X, the hash, is computed as a function of the certificate
contents
2. the hash is signed by raising X to the power e (modulo n)
3. the hash's signature is validated by raising the result of
step 2 to the power d (modulo n), yielding X, which is
compared with the result computed as a function of certificate
contents.
Annex C to X.509 suggests the use of Fermat number F4 (65537 decimal,
1 + 2 **16 ) as a fixed value for e which allows relatively efficient
authentication processing, i.e., at most seventeen (17)
multiplications are required to effect exponentiation). As an
alternative one can employ three (3) as the value for e, yielding
even faster exponentiation, but some precautions must be observed
(see RFC-1115). Users of the algorithm select values for d (a secret
quantity) and n (a non-secret quantity) given this fixed value for e.
As noted earlier, this RFC proposes that either three (3) or F4 be
employed as universal encryption exponents, with the choice specified
in the algorithm identifier. In particular, use of an exponent value
of three (3) for certificate validation is encouraged, to permit
rapid certificate validation. Given these conventions, a user's
public component, and thus the quantity represented in his
certificate, is actually the modulus (n) employed in this computation
(and in the computations used to protect the DEK and MSGHASH, as
described in RFC-1113). A user's private component is the exponent
(d) cited above.
The X.509 certificate format is defined (in X.509, Annex G) 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
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Validity ::= SEQUENCE{
notBefore UTCTime,
notAfter UTCTime}
SubjectPublicKeyInfo ::= SEQUENCE{
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING}
AlgorithmIdentifier ::= SEQUENCE{
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL}
All components of this structure are well defined by ASN.1 syntax
defined in the 1988 X.400 and X.500 Series Recommendations, except
for the AlgorithmIdentifier. An algorithm identifier for RSA is
contained in Annex H of X.509 but is unofficial. RFC-1115 will
provide detailed syntax and values for this field.
NOTES:
[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".
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Authors' Addresses
Steve Kent
BBN Communications
50 Moulton Street
Cambridge, MA 02138
Phone: (617) 873-3988
EMail: kent@BBN.COM
John Linn
Secure Systems
Digital Equipment Corporation
85 Swanson Road, BXB1-2/D04
Boxborough, MA 01719-1326
Phone: 508-264-5491
EMail: Linn@ultra.enet.dec.com
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