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PKIX Working Group                               H. Kikuchi (Tokai Univ)
Internet Draft                                          M. Sakurai (NEC)
                                                 H. Hattori (Meiji Univ)
                                             Y. Sameshima (Hitachi Soft)
expires in six months                                         March 1997


                  Internet Public Key Infrastructure:

                Web-based Certificate and CRL Repository


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet- Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).



Abstract

   This document provides a specification how to publish and retrieve
   X.509 certificates and certificate revocation lists (CRLs).  In the
   proposed method, the World Wide Web (WWW) is used for securely
   distributing certificates across a firewall in both human and machine
   readable syntax. A various certificate concerning information that
   includes certificates, CRLs, and certification authority (CA) policy
   are retrieved from an integrated single authority access point
   specified in X.509 version 3 extensions. The information access point
   accepts certification and revocation requests in the uniform access
   method based on the standard WWW.







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1. Introduction

   The first attempt for security enhancement of the Internet
   applications was electrionic mail. The Privacy Enhanced Mail (PEM)
   defined in [RFC-1421, 1422, 1423 and 1424] proposed X.509 public key
   certificates and hierarchically structured certification authorities
   (CAs), which are then adopted in MOSS [MOSS] and S/MIME [S/MIME].
   These security protocols, however, require that a sender has to
   convey many certificates needed to certify its validity from a
   receiver because of the lack of certificates repository. Therefore, a
   big challenge to establish Public Key Infrastructure (PKI) is made by
   specifying profiles of the X.509 version 3 certificate and version 2
   CRL [PKIX-1]. The PKI working group also proposed certificate
   management protocols in [PKIX-3], in which wide range of CA
   information format called PKI messages are defined by ASN.1 [ASN.1]
   that makes processing easier for management software, and simple
   socket based transport protocols.

   However, in the practical point of view, the socket based transport
   protocol is problematic. First, most commercial organization have a
   firewall, which prevents intruder from gaining access to internal LAN
   and might reject PKI message transfer. For this reason, they shall
   provide a proxy service for each protocol. Second problem is
   confidentiality of PKI messages. Although most PKI message are public
   information, the initializing message such as certification or
   revocation requests require an extra capability of message encryption
   for achieving confidentiality. The third problem is the scaleability,
   which makes the service available in the wide scale networks.  The
   requirement involves reducing traffic cost by a certificates cashing
   or a distributed database. The last problem is ASN.1 based definition
   which forces Basic Encoding Rule (BER) to transfer a PKI messages.
   For easier implementation, human readable encoding rule is
   appropriate.

   To meet these requirements, this document defines World Wide Web
   (WWW) based certification and CRL repository. Since WWW is now one of
   the major application in the internet, almost all internet users can
   use it even if the site they belong has a firewall against intruders.
   The WWW provides some useful facilities for PKI; an information
   cashing by both a proxy server and client software, a secure
   transport layer service for confidentiality, a flexible request
   forwarding which can be used in CA and CA communication, and human
   readable and easier manipulating message format.








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2. Basic Definition, Requirements and Assumptions

2.1 Overview

   This document suppose simple restricted hierarchical certificate
   infrastructure rather than complicated CA web.  Figure 1 shows an
   example of hierarchy of Publishing Authority (PAs), where PA2 has two
   subordinary PAs, PA1 and PA2, and PA1 has two users having
   certificate Cert1 and Cert2, respectively.

   Every certificate has one information access point (IAP) from which
   any information with regards to users can be retrieved.  The PA is
   responsible for all information it publishes. Thus, it also provides
   on-line validation service with and without CRLs.  The PA
   certificate, which may be identical to the standard CA certificate,
   also has upper IAP entry. There is no different in IAP syntax among
   end entity, PA and CA. None of user and PAs publish certificate by
   theirselves, that is, subject information access extension is not
   necessary. The IAP specification is defined in Section 2.2.

   The communication between entities was based on Hyper Text Transfer
   Protocol (HTTP) and its variation. The Hyper Text Markup Language is
   used as message format. Note that a subordinate entity is subject to
   be message sender and the higher entity just response to the
   requester.  Thus, the coordinate entities never get direct
   communication with them.  This assumption is convenient for
   conformance.  The end entity and PA protocol is defined in Section 3.

                                                    +-----+
                                                    |     |
                                                PA3 V     |
                                             IAP3+---+    |
                                           +---->| *------+
                                   PA1     |     +---+
                              IAP1 +---+   |
                          +------->| *-----+
               Cert1      |        +---+   |
                   +---+  |                |
                   | *----+                |
                   +---+  |        PA2     |
               Cert2      |        +---+   |
                   +---+  |        | *-----+
                   | *----+        +---+
                   +---+

                     Figure 1: Example of PAs hierarchy





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2.2 Requirement for PKI Application

   The PKI application needs certificates and CRL repository in the
   following three cases;

   1. certificate retrieval.
      For example, a sender of secure message wants to retrieve
      the recipient's public key by which the message is to be
      encrypted.

   2. certificate verification.
      The recipient of secure message check if the sender's certificate
      is not revoked by examining the corresponding CRL or asking
      the CA directly.

   3. certificate and CRL publication.
      Soon after a certificate is issued by a CA, the new certificate
      shall be got access for anybody who wants it.


2.3 Requirement for X.509 Version 3 Certificate and Extensions

   This proposal supposes that subset of X.509 Version 3 is used to form
   public key certificates. According to [PKIX1], the subject and issuer
   names in X.509 (v1) may be an empty sequence and subjectAltName and
   issuerAltName extensions (v3) shall be specified instead of the
   field. Even if the subject and issuer names are specified, the
   subjectAltName shall be given as an identification of certificate
   retrieval.

   Most PKI applications require many kinds of information about
   certificate including policy information, CRL, and CA information. In
   [PKIX1], several access methods are defined for each kind of
   information.  However, it is not so often case that a certain
   application has multiple access methods to PKI. Therefore, this
   document assumes that PKI application has an uniform access method of
   HTTP for for the simplification of PKI protocol.

   As the same reason, the subjectInfoAccess, the authrotyInfoAccess and
   the caInfoAccess can be unified to the authorityInfoAccess.  The
   subjectInfoAccess may be meaningless because a PKI user needs the
   information access point in two cases; (1) when it wants to verify
   the sender's certificate after it receives a secure message, or (2)
   when it wants to retrieve its recipient's certificate before it
   sends.  In the first case, he/she cannot believe any information
   provided by the subjectInfoAccess, which the sender itself specify,
   and thus may be altered. Hence, he/she shall use the
   authrotyInfoAccess instead of the subjectInfoAccess.  The other case,



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   he/she has not yet known the subject information access point which
   is to be specified the recipient's certificate, which shall be
   retrieved from any authrotyInfoAccess point she/he knows.  Once PKI
   user gets the recipient's certificate, the subjectInfoAccess is no
   longer necessary for him/her.  Consequently, the subjectInfoAccess is
   useless.

   The AuthorityInfoAccess contains at least one AccessDescription in
   which the accessMethod and accessLocation shall specify httpID and
   appropriately URL that accepts "POST" method.

   If this proposal is used, a standard certificate must specify

      - authorityInfoAccess,

   shall specify

      - subjectAltName,
      - issuerAltName,

   may specify

      - authorityKeyIdentifier,
      - subjectKeyIdentifier,
      - keyUsage,
      - privateKeyUsagePeriod,
      - certificagtePolicies,
      - basicConstraints,
      - nameConstraints,
      - policyConstraints,

   must not specify for avoiding confusion

      - cRLDistributionPoints,
      - policyMappings,
      - subjectDirectoryAttributes,
      - subjectInfoAccess,
      - authorityInfoAccess,
      - caInfoAccess.


2.4 Requirement for Publishing Authority

   Since the number of PKI user increases step by step, the set of CAs
   always have to keep communicating with each other. Moreover, the
   number of CA also increases slightly, so, the hierarchical CAs
   structure is proposed in [RFC1422]. Where, the root CA is required to
   update all CAs and to manage the access path.



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   However, in practice, at the entrance to the Internet every
   organization has a firewall facility which restricts internet access
   to a particular application, service, and host in order for security
   consideration. Thus, generally, an CA runs within the firewall and
   only communicates with internal PKI users.  Therefore, we need a
   publishing authority (PA) that is set up for each CA and works as a
   certificate repository outside of the firewall.  Figure 2 shows this
   structure.

   The transaction between particular entities can be easily restricted
   by a firewall, thus, it does not spoil the security of CA.  An
   internal static information access point provides a simple and
   uniform access method for PKI users. Any information stored in
   external PA is not secret information and may be different to that of
   internal PA.

                                           [root PA]
                            The Internet       |
                           ---+------+---------+-
                              |      |
                         |    |      |   |
                   [CA]--+---[PA]  [PA]--+--[CA]
                   /|    |               |    |
             PKI users   |               |   PKI users
                        firewall    firewall

              Figure 2. Relationship between PA and CA



3. Transport Protocol

3.1 Information Access
   Information access point (IAP) is specified by the subjectInfoAccess
   field in certificate extension. The IAP is a point from which
   certificates are distributed and on-line verification service is
   provided.

   The PA server can be implemented as a standard HTTP server which
   enables CGI facility. The IAP server works as PA, CRL distribution
   point, policy repository, verification server, and certificate
   repository.  The PA server management certificates in a specific
   closed organization, and communicates with upper PA server which
   knows the all subordinate PA server's location.

   A PKI application points at least one IAP so as to retrieve locations
   of other IAPs.




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   An information transfer is based on HTTP with method POST.  Thus, the
   typical query is formated as follows;

           POST IAP/queryType HTTP/1.0
           name1=value1&name2=value2&...&namen=valuen

   where "queryType" is a type of query and a pair of "name" and "value"
   are used to send PKI message. All fields are subject to be formed in
   standard encoding rule defined in [HTTP].

           HTTP/1.0 200 OK
           Date: Wednesday
           MIME-version: 1.0
           Content-type: text/html

           <HEAD><TITLE>queryType</TITLE></HEAD>
           <BODY>
           <H3> statusCode </H3>
           <H1> statusMessage </H1>
                information
           </BODY>

   Where, "queryType" is the same as the type given in sending request.
   The "statusCode" contains a status with three digit codes.  With at
   least one space, the "statusMessage" and "information" are
   interpreted in corresponding semantics.  The syntax of "information"
   depends on the query type, and will be defined section 5.


3.2 Type of Query

   This document defines the followings query types.

           1. certreq         2. revokereq         3. lookupreq
           4. verifyreq


3.2.1 Initial Registration type "certreq"

   Type "certreq" is used for sending certification request to CA.  This
   query may be used only if the PA works as CA or has some means to
   relay the request to the corresponding CA.  The certreq request shall
   be sent with some of the following pairs of name and value.

           name            value
           ----            -----
           o               Organization
           ou1             Organizational unit



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           ou2             Organizational unit 2
           ou3             Organizational unit 3
           ou4             Organizational unit 4
           cn              Common name
           n               public component of RSA cryptosystem
           email           RFC822 style email address
           info            URL of user's home page
           key             public key encoded in corresponding BER
           acct            User identification
           pass            User password
           loginname       Manager identification
           password        Manager password

   The pair "o", "cn", "n", "email" are critical pairs.  The name "n"
   implicitly identifies a modulo value when public component of 0x10001
   is used in RSA cryptosystem. Otherwise, name "key" shall be used to
   convey public key information which is encoded in the corresponding
   BER rule. Either of "n" and "key" must be given.  The "acct" and
   "pass" may be used for user authentication.  If these pairs are
   missing, the response shall contain the other pairs as hidden
   attribute     and be used as confirmation of user information.
   Alternatively, the "loginname" and "password" may be used by the CA
   manage.

   The response consists of statusCode, statusMessage and information.

       statusCode  statusMessage           information
       ----------  -------------           -----------
           200     "accept your request"   the issued certificate
                                           encoded in Base64 encoding rule.
           301     "incomplete request"    the missing pair
                                           and the other information
           302     "duplicate request"     the certificate has already been
                                           issued.
           303     "reject your request"   any other reason why the request
                                           failed.
           304     "service not available" the PA does not accept this request.


3.2.2 Certificate Revocation "revokereq"

   The "revokereq" is a request to revoke a certificate.  To prevent
   malicious PKI user from revoking other's certificate, this request
   should be sent with a proof of possession of the secret key. The
   simplest way is to use conventical application that supports digital
   signature.

           name            value



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           ----            -----
           sig             digitally signed revocation message

       statusCode  statusMessage           information
       ----------  -------------           -----------
           200     "accept your request"   nothing (the certificate is revoked)
           301     "invalid signature"     nothing
           302     "duplicate request"     nothing (the certificate has already
                                           been revoked)
           303     "reject your request"   any other reason why the request
                                           failed.
           304     "service not available" the PA does not accept this request.

3.2.3 Certificate Distribution "lookupreq"

   The "lookupreq" type is used for retrieving and searching
   certificate, CRLs, and any other information.  The PA server may
   forward a request to other PA server when it does not has sufficient
   information to response to the request.

   Certificate is identified by either of the following names;

           a. email address         b. Distinguished Name

   Both identifiers must be fully specified because a substring matching
   rule might violate a privacy issue when the PA is the outside of
   firewall.  The lookup query is sent with the following pairs of names
   and value.

           name            value
           ----            -----
           o               Organization
           ou1             Organizational unit
           ou2             Organizational unit 2
           ou3             Organizational unit 3
           ou4             Organizational unit 4
           cn              Common name
           dn              subject/CA Distinguished Name.
           email           subject/CA email address
           object          "policy" (policy description and mapping)
                           "cert"  (CA or subject certificate)
                           "crl"   (CA CRL)

   The pairs of  "o", "ou1", "ou2", "ou3", "ou4", and "cn" specify
   intend distinguished name. Alternatively, the "dn" pair may be used
   to specify distinguished names. When the specification is incomplete,
   the PA may reject it for privacy issue or accept it as substring
   matching.  Whether the request is for subject or for CA can be



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   identified by the distinguished name. For example, a distinguished
   name containing "cn=Authority" is a request for CA, otherwise for
   subject.  The "object" specifies what information is required and
   takes "cert" in default.  For instance, a request with "object=crl"
   is equivalent to the request by crlDistributionPoint extension.

   The response consists of statusCode, statusMessage and information.

       statusCode  statusMessage           information
       ----------  -------------           -----------
           200     "accept your request"   the certificate, policy or CRL
                                           encoded in Base64 encoding rule.

           301     "revoked"               the reason of revoked
           302     "reject your request"   the reason why the request failed.
           303     "service not available" the PA does not accept this request.


3.2.4 Certificate Verification "verifyreq"

   Type "verifyreq" is used for validation check of certificate.  This
   document does not support path validation.  The verifyreq request
   shall be sent with the following pairs of name and value.

           name            value
           ----            -----
           cert            certificate embedded in CGI coding rule
           certsig         certificate embedded in CGI coding rule
           crl             DN of intended certificate
           sn              serial number of a certificate
           time            GENERALIZED TIME when certificate
                           should be checked to be valid.
           dn              request originator's distinguished name

   The "certsig" request has the PA digitally signed its response.  The
   PKI user specify a certificate by either sending the whole
   certificate with "cert" or just sending "sn" when the implicit PA
   server can be determined.  The "time" is optional pair, which means
   when the certificate to be examined. This option is used when one
   wants to verify if an old message was valid at that time.

   When "cert" is specified, the certificate to be verified is embedded
   into the value in CGI coding rule.  The "certsig" is same as the
   "cert" except but it responses with message integrity check code.

   When "crl" is specified, PA server only make sure if the intended
   certificate is already revoked.




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   The "dn" is used when the PA provides verification service only to
   restricted users.

   The response of "cert" request is as follows.

       statusCode  statusMessage           information
       ----------  -------------           -----------
           200     "valid"                 nothing (the certificate is valid)
           201     "not revoked"           nothing (the certificate is not revoked)

           301     "revoked"               the reason of revoked
           302     "hold"                  the reason of hold

           303     "reject your request"   any other reason why the request
                                           failed.
           304     "service not avialable" the PA does not accept this request.

   Some example of reason of revoked are such that the key was
   compromised, or the affiliation was changed.


3.3 Correspondence to preceding PKI draft

   This document corresponds to PKI management protocol defined in
   [PKIX3].  Table 1 shows the correspondence and side effect occurred
   by a request.

                   Table 1. Correspondence of methods

           PKI method              PA method       Side effect
           ------------            ----------      -----------
           certStatus              verify          no
           certRetrieval           lookup          no
           caPolicy                lookup          no
           caCert                  lookup          no

           CRLDistributionPoint    lookup          no

           certReq                 certreq         yes
           revokeReq               revokereq       yes


3.4 Inter-PA Protocol

   If a request to a PA concerns information not stored in the PA, the
   PA shall manage to obtain it by relying the request to an appropriate
   PA. This section define an inter-PA transaction.




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3.4.1 PA Model

   Suppose that there are PA1, PA2 and RootPA, and PA1 has a request for
   retrieving information from PA2. The PA1 and PA2 does not have their
   locations but the access point to RootCA.  There are two
   possibilities for PA1 to get access to PA2 (Figure 3).

    - Model 1. [referral] PA1 sends the request to Root PA (1), which
   then         replies to PA1 with the access point to PA2 (2).
           PA1 sends it to PA2 again (3), and finally PA1 gets
           the information from PA2.

    - Model 2. [chaining] PA1 sends the request to Root PA (1), which
           redirects it to PA2 on behalf of PA1 (2). PA2 answers
           to Root PA (3), which forwards it to PA1 (4).


              +---->[RootPA]                          +---->[RootPA](2)---+
              | +---(2)                               | +---(4)     <---+ |
              | |PA2                                  | |               | |
              | |                                     | |               | |
           (1)| V                                  (1)| V            (3)| V
             [PA1] (3)-----------> [PA2]             [PA1]             [PA2]
                   <-----------(4)

                   Referral                                Chaining

                           Figure 3. Inter-PA models


3.4.2 Referral Model

   To redirect a request to another PA server, the root PA responds to
   the requester with the following HTTP redirect message consisting
   HTTP header and HTTP body.

       HTTP/1.0 302 Found
       Date: Monday, 20-Jan-97 13:33:29 GMT
       Server: NCSA/1.1
       MIME-version: 1.0
       Location: http://xxx.yy:zz/method
       Content-type: text/html

       <HEAD><TITLE>Document moved</TITLE></HEAD>
       <BODY><H1>Document moved</H1>
       This document has moved <A HREF="http://xxx.yy:zz">here</A>.<P>
       </BODY>




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   Where the URL "http://xxx.yy:zz/method" specified by the Location
   HTTP header     provides information where the query to be sent next.
   This is a control message used in the standard HTTP transaction
   [HTTP].

   The root PA must respond with either correct PA location or error
   message to mean that there is no certificate.  To do this, all
   correspondences between identifiers and IAP locations should be
   notified by the root PA.

   In this model, PKI application or PA must support HTTP redirect
   message. Each round trip time is short, but PA has to send the same
   query to several servers.


3.4.3 CGI Chaining Model

   To implement CGI chaining model, the CGI script in root PA produces
   an extra CGI message before it responds to the request originator.

   The request originator, PA1 or PKI application, does not have to send
   request many times, but have to wait longer time than that of
   referral model. According to [Mine], the estimated total round trip
   time is less than that of the referral model.  Since PA communicates
   with a particular PA, the access control at firewall can be easily
   set up.


4. Security Consideration



4.1 Confidentiality of transaction

   To prevent message from being eavesdropped, secure communication
   channel such as SSL shall be used. Especially, initial registration
   process is critical to eavesdropping. Since user authentication is
   checked by "uid" and "passwd", a client software is not required to
   have its own certificate.  Under the assumption, PKI message
   protection proposed in [PKIX3] need not here.


4.2 Non-Repudiation

   The verify request supports the time to be checked and digitally
   signed response. This can avoid a message sender from denying the
   message. To enable this service, any PA must manage all certificates
   which it has already issued, including revoked certificates.



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4.3 Privacy

   In the lookup request, the support of substring matching facility may
   distribute private information to outsiders, and thereby may be used
   for sending an advertisement via email.


5. ASN.1 encoding rule in HTML

5.1 Definition

   A certificate management protocol is defined in ASN.1 syntax in
   [PKIX3].  The BER is not human readable but is better for security
   enhancements such as an integrity checking, whereas the ASCII text is
   human readable but not suitable for machine processing. Therefore,
   this document defines ASN.1 encoding rule in HTML, which can be both
   human and machine readable encoding.

   In the BER, any data type is formed with three elements, tag, length,
   and value. Instead of the length field, the HTML encoding identifies
   the value field by specifying the data start tag and the data end
   tag.  The printable string data type and UTC time type are specified
   by the <H3> and </H3> tag.  The other data type is defined by the
   <H5> and </H5> tag.  The structured types, SET and SEQUENCE, are
   defined by the <UL> tag and <OL> tag, respectively.  For the inverse
   function of the encoding, the corresponding data tag number follows
   the tag name in two octet hexadecimal numbers.

   The printable string data value is represented in ASCII.  The other
   data value is represented in hexadecimal octet string.  The document
   does not define a means to encode bit string data.  For easier
   implementation, this document define a set of data value encoded in
   Base64, which is identified by <Blockquote> and </Blockquote> tag.
   This notation is useful when PKI application transfers the whole
   certificate without interpreting the contents.

   The encoding rule allows no optional notation, no tagged type and no
   default value.  Every data type is specified explicitly in order to
   for uniquely distinguishing data types.

   Table 2 shows main data type and the encoding format.  Where, "n",
   "s" are examples of numbers and string, and "a" and "b" are of any
   ASN.1 data type.  For example, an integer 12 is coded by "<H5
   03>0C</H5>".

                   Table 2. HTML Encoding Rule

           ASN.1 Data Type         HTML Encoding



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           ---------------         --------------
           BOOLEAN                 <H5 01> n </H5>
           INTEGER                 <H5 02> n </H5>
           BIT STRING              <H5 03> n </H5>
           OCTET STRING            <H5 04> n </H5>
           NULL                    <H5 05> </H5>
           OBJECT IDENTIFIER       <H5 06> n </H5>

           PrintableString         <H3 13> s </H3>
           IA5String               <H3 16> s </H3>
           UTCTime                 <H3 17> s </H3>
           GeneralizedTime         <H3 18> s </H3>

           SEQUENCE {a, b}         <OL 30> <LI> a <LI> b </OL>
           SET {a, b}              <UL 31> <LI> a <LI> b </UL>

           xxx (Base64 format)     <BlockQuote> xxx </BlockQuote>

5.2 Example


   A PKI message format and the corresponding encoding are as follows.
   Note that the tagged data protection and extraCerts are not omitted.
       PKIMessage ::= SEQUENCE {
         header           PKIHeader,
         body             PKIBody,
         protection   [0] PKIProtection OPTIONAL,
         extraCerts   [1] SEQUENCE OF Certificate OPTIONAL
       }

       <OL 30>
         <LI> PKIHeader
         <LI> PKIBody
         <LI> PKIProtection
         <LI> <OL 30>
                   <LI> Certificate 1
                   <LI> Certificate 2
              </OL>
       </OL>

   A distinguished name consisting of countryName="JP",
   organizationName="ICAT", and organizationalUnitName="Certification
   Authority" is encoded as follows.
       <OL 30><LI>
            <UL 31><LI>
               <OL 30><LI>
                  <H5 06>550406</H5><LI>
                  <H3 13>JP</H3>



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               </OL>
            </UL><LI>
            <UL 31><LI>
               <OL 30><LI>
                  <H5 06>55040a</H5><LI>
                  <H3 13>ICAT</H3>
               </OL>
            </UL><LI>
            <UL 31><LI>
               <OL 30><LI>
                  <H5 06>55040b</H5><LI>
                  <H3 13>Certification Authority</H3>
               </OL>
            </UL>
         </OL>


Acknowledgement

   The authors thank Mr. Ohbayashi, Mr. Kobayashi, Mr. Kuroda, Mr.
   Fujimoto, and Mr. Wada for their comments to this proposal.  The
   authors also thank the other researchers joining the Initiative
   Computer Authentication Technology (ICAT), and the Interauth working
   group of WIDE project.


Reference

           [PKIX-1] R. Housley, et. al., "Internet Public Key Infrastructure
                   Part I: X.509 Certificate and CRL Profile,"
                   <draft-ietf-pkix-ipki-part1-03.txt>, December 1996

           [PKIX-3] S. Farrell and C. Adams, "Internet Public Key Infrastructure
                   Part III: Certificate Management Protocols,"
                   <draft-ietf-pkix-ipki3cmp-02.txt>, December 1996

           [RFC1422] Kent, S.,  "Privacy Enhancement for Internet Electronic
                   Mail: Part II: Certificate-Based Key Management,"
                   RFC 1422, February 1993.

           [S/MIME] S. Dusse, "S/MIME Message Specification: PKCS Security
                   Services for MIME",  <draft-dusse-mime-msg-spec-00.txt>
                   September 1996

           [MOSS] S. Crocker, et. al., MIME Object Security Services,
                   RFC 1848, October 1995

           [HTTP] T. Berners-Lee, R. Fielding, H. Nielsen, "Hypertext Transfer



Kikuchi                                                        [Page 16]


Internet DRAFT                                                March 1997


                   Protocol -- HTTP/1.0", RFC 1945, May 1996

           [ASN.1] B. Kaliski, "A Layman's Guide to a Subset of ASN.1, BER,
                     and DER", ftp://ftp.rsa.com (layman.ps), June 1991

           [Mine]  M. Sakurai, et.al., A Design of Certificate or CRL
                   Distribution Architecture between Certification Authorities,
                   The 1997 Symp. on Cryptography and Information Security
                   (SCIS'97), 8D, January 1997


Security Considerations

   This entire memo is about security mechanisms.


Author Addresses:

   Hiroaki Kikuchi
   Dept. of Electrical Engineering
   Tokai University
   1117 Kitakaname, Hiratsuka, Kanagawa 259-12, Japan
   kikn@ep.u-tokai.ac.jp

   Mine Sakurai
   NEC Corporation
   garashi bldg., 2-11-5 Shibaura, Minato-ku, Tokyo 108, Japan
   m-sakura@ccs.mt.nec.co.jp

   Hiroyuki Hattori
   Meiji University
   1-1-1, Higashi-mita, Tama-ku, Kawasaki, Kanagawa 214, Japan
   hhat@isc.meiji.ac.jp

   Yoshiki Sameshima
   Hitachi Software Engineering Co., Ltd.
   6-81 Onoe-cho Naka-ku, Yokohama, Kanagawa 231, Japan
   same@ori.hitachi-sk.co.jp













Kikuchi                                                        [Page 17]


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