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Privacy enhancement for Internet electronic mail: Part II - certificate-based key management :: RFC1114

Network Working Group                                            S. Kent
Request for Comments:  1114                                        BBNCC
                                                                 J. Linn
                                                  IAB Privacy Task Force
                                                             August 1989

           Privacy Enhancement for Internet Electronic Mail:
              Part II -- Certificate-Based Key Management


   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.


   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  Interoperation Across Certification Hierarchy Boundaries  14  Certificate Revocation                                    15
   3.4  Certificate Definition and Usage                              17
   3.4.1  Contents and Use                                            17  Version Number                                            18  Serial Number                                             18  Subject Name                                              18  Issuer Name                                               19  Validity Period                                           19  Subject Public Component                                  20

Kent & Linn                                                     [Page 1]
RFC 1114              Mail Privacy: Key Management           August 1989  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

Kent & Linn                                                     [Page 2]
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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RFC 1114              Mail Privacy: Key Management           August 1989

   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

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-

Kent & Linn                                                     [Page 4]
RFC 1114              Mail Privacy: Key Management           August 1989

   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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RFC 1114              Mail Privacy: Key Management           August 1989

   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

Kent & Linn                                                     [Page 6]
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

   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

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

Kent & Linn                                                     [Page 7]
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

Kent & Linn                                                     [Page 8]
RFC 1114              Mail Privacy: Key Management           August 1989

      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

Kent & Linn                                                     [Page 9]
RFC 1114              Mail Privacy: Key Management           August 1989

   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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RFC 1114              Mail Privacy: Key Management           August 1989

   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

   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

      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

      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.  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.  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.

Kent & Linn                                                    [Page 17]
RFC 1114              Mail Privacy: Key Management           August 1989  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).  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.  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.  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.  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.  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.  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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   required for certificates issued by other CAs as described in section

   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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RFC 1114              Mail Privacy: Key Management           August 1989

   encrypted hash.  The encrypted hash is formed and processed as

      1.  X, the hash, is computed as a function of the certificate

      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

   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.


  [1]  CCITT Recommendation X.411 (1988), "Message Handling Systems:
       Message Transfer System: Abstract Service Definition and

  [2]  CCITT Recommendation X.509 (1988), "The Directory Authentication

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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

Kent & Linn                                                    [Page 25]