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PKIX Working Group                                   R. Housley (SPYRUS)
Internet Draft                                          W. Ford (Nortel)
                                                           D. Solo (BBN)
expires in six months                                      February 1996


                   Internet Public Key Infrastructure

               Part I:  X.509 Certificate and CRL Profile

                  <draft-ietf-pkix-ipki-part1-00.txt>


Status of this Memo

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

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

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


Abstract

   This is the second draft of the Internet Public Key Infrastructure
   X.509 Certificate and CRL Profile.  This document was sections 1
   through 5 and section 11 of draft-ietf-pkix-ipki-00.txt.  That
   original document has been divided into four parts; it was simply too
   big.  This is the first part.  Many changes are the result of
   discussion at the Dallas IETF in December 1995 and discussion on the
   ietf-pkix@tandem.com mail list. The intent of this document is to
   generate further productive discussion and build concensus.


1  Executive Summary

   << Write this last. >>




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2  Requirements and Assumptions

   Goal is to develop a profile and associated management structure to
   facilitate the adoption/use of X.509 certificates within Internet
   applications for those communities wishing to make use of X.509
   technology. Such applications may include WWW, electronic mail, user
   authentication, electronic payment systems, IPSEC, as well as others.
   In order to relieve some of the obstacles to using X.509
   certificates, this document defines a profile to promote the
   development of certificate management systems; development of
   application tools; and interoperability determined by policy, as
   opposed to syntax.

   Some communities will need to supplement, or possibly replace, this
   profile in order to meet the requirements of specialized application
   domains or environments with additional authorization, assurance, or
   operational requirements.  However, for basic applications, common
   representations of frequently used attributes are defined so that
   application developers can obtain necessary information without
   regard to the issuer of a particular certificate or certificate
   revocation list (CRL).

   As supplemental authorization and attribute management tools emerge,
   such as attribute certificates, it may be appropriate to limit the
   authenticated attributes that are included in a certificate.  These
   other management tools may be more appropriate method of conveying
   many authenticated attributes.

2.1  Communication and Topology

   The users of certificates will operate in a wide range of
   environments with respect to their communication topology, especially
   users of secure electronic mail.  This profile supports users without
   high bandwidth, real-time IP connectivity, or high connection
   availablity.  In addition, the profile allows for the presence of
   firewall or other filtered communication.

2.2  Acceptability Criteria

   The goal of the Internet Public Key Infrstructure (PKI) is to meet
   the needs of deterministic, automated identification, authentication,
   access control, and authorization functions.  Support for these
   services determines the attributes contained in the certificate as
   well as the ancillary control information in the certificate such as
   policy data and certification path constraints.






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2.3  User Expectations

   Users of the Internet PKI are people and processes who use client
   software and are the subjects named in certificates.  These uses
   include readers and writers of electronic mail, the clients for WWW
   browsers, and the key manager for IPSEC within a router.  This
   profile recognizes the limitations of the platforms these users
   employ and the sophistication/attentiveness of the users themselves.
   This manifests itself in minimal user configuration responsibility
   (e.g., root keys, rules), explicit platform usage constraints within
   the certificate, certification path constraints which shield the user
   from many malicious actions, and applications which sensibly automate
   validation functions.

2.4  Administrator Expectations

   As with users, the Internet PKI profile is structured to support the
   individuals who generally operate Certification Authorities (CAs).
   Providing administrators with unbounded choices increases the chances
   that a subtle CA administrator mistake will result in broad
   compromise.  Also, unbounded choices greatly complicates the software
   that must process and validate the  certificates created by the CA.

3  Overview of Approach

   Following is a simplified view of the architectural model assumed by
   the PKIX specifications.

      +---+
      | C |                       +------------+
      | e | <-------------------->| End entity |
      | r |       Operational     +------------+
      | t |       transactions         ^
      |   |      and management        |  Management
      | / |       transactions         |  transactions
      |   |                            |
      | C |    PKI users               v
      | R |             -------+-------+--------+------
      | L |   PKI management   ^                ^
      |   |      entities      |                |
      |   |                    v                |
      | R |                 +------+            |
      | e | <-------------- | RA   | <-----+    |
      | p |   certificate   |      |       |    |
      | o |       publish   +------+       |    |
      | s |                                |    |
      | i |                                v    v
      | t |                            +------------+



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      | o | <--------------------------|     CA     |
      | r |   certificate publish      +------------+
      | y |           CRL publish             ^
      |   |                                   |
      +---+                                   |    Management
                                              |    transactions
                                              v
                                          +------+
                                          |  CA  |
                                          +------+

                          Figure 1 - PKI Entities

   The components in this model are:

   end entity:  user of PKI certificates and/or end user system that
                the PKI certifies;
   CA:          certification authority;
   RA:          registration authority, i.e., an optional system to
                which a CA delegates certain manaagement functions;
   repository:  a system or collection of distributed systems that
                store certificates and CRLs and serves as a means of
                distributing these certificates and CRLs to end entities.

3.1  X.509 Version 3 Certificate

   Application of public key technology requires the user of a public key to be confident that the public key
   belongs to the correct remote subject (person or system) with which an encryption or digital signature
   mechanism will be used.  This confidence is obtained through the use of public key certificates, which are
   data structures that bind public key values to subject identities.  The binding is achieved by having a
   trusted certification authority (CA) digitally sign each certificate.  A certificate has a limited valid lifetime
   which is indicated in its signed contents.  Because a certificate's signature and timeliness can be
   independently checked by a certificate-using client, certificates can be distributed via untrusted
   communications and server systems, and can be cached in unsecured storage in certificate-using systems.

   The standard known as ITU-T X.509 (formerly CCITT X.509) or ISO/IEC 9594-8, which was first
   published in 1988 as part of the X.500 Directory recommendations, defines a standard certificate format.
   The certificate format in the 1988 standard is called the version 1 (v1) format.  When X.500 was revised
   in 1993, two more fields were added, resulting in the version 2 (v2) format.  These two fields are used to
   support directory access control.

   The Internet Privacy Enhanced Mail (PEM) proposals, published in 1993, include specifications for a
   public key infrastructure based on X.509 v1 certificates [RFC 1422].  The experience gained in
   attempts to deploy RFC 1422 made it clear that the v1 and v2 certificate formats are deficient in several
   respects.  Most importantly, more fields were needed to carry information which PEM design and
   implementation experience has proven necessary.  In response to these new requirements, ISO/IEC and
   ANSI X9 developed the X.509 version 3 (v3) certificate format.  The v3 format extends the v2 format by
   adding provision for additional extension fields.  Particular extension field types may be specified in



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   standards or may be defined and registered by any organization or community.  In August 1995,
   standardization of the basic v3 format was completed [ISO TC].

   ISO/IEC and ANSI X9 have also developed a set of standard extensions for use in the v3 extensions field
   [ISO DAM, ANSI X9.55].  These extensions can convey such data as additional subject identification
   information, key attribute information, policy information, and certification path constraints.

   However, the ISO/IEC and ANSI standard extensions are very broad in their applicability.  In order to
   develop interoperable implementations of X.509 v3 systems for Internet use, it is necessary to specify
   a profile for use of the X.509 v3 extensions tailored for the Internet.  It is one goal of this document to
   specify a profile for Internet WWW, electronic mail, and IPSEC applications.  Environments with
   additional requirements may build on this profile or may replace it.

3.2  Certification Paths and Trust

   A user of a security service requiring knowledge of a public key generally needs to obtain and validate a
   certificate containing the required public key.  If the public-key user does not already hold an assured copy
   of the public key of the CA that signed the certificate, then it might need an additional certificate to obtain
   that public key.  In general, a chain of multiple certificates may be needed, comprising a certificate of the
   public key owner (the end entity) signed by one CA, and zero or more additional certificates of CAs
   signed by other CAs.  Such chains, called certification paths, are required because a public key user is
   only initialized with a limited number (often one) of assured CA public keys.

   There are different ways in which CAs might be configured in order for public key users to be able to find
   certification paths.  For PEM, RFC 1422 defined a rigid hierarchical structure of CAs.  There are three
   types of PEM certification authority:

   (a)  Internet Policy Registration Authority (IPRA):  This authority, operated under the auspices of the
   Internet Society, acts as the root of the PEM certification hierarchy at level 1.  It issues certificates only for
   the next level of authorities, PCAs.  All certification paths start with the IPRA.

   (b)  Policy Certification Authorities (PCAs):  PCAs are at level 2 of the hierarchy, each PCA being
   certified by the IPRA.  A PCA must establish and publish a statement of its policy with respect to
   certifying users or subordinate certification authorities.  Distinct PCAs aim to satisfy different user needs.
   For example, one PCA (an organizational PCA) might support the general electronic mail needs of
   commercial organizations, and another PCA (a high-assurance PCA) might have a more stringent policy
   designed for satisfying legally binding signature requirements.

   (c)  Certification Authorities (CAs):  CAs are at level 3 of the hierarchy and can also be at lower levels.
   Those at level 3 are certified by PCAs.  CAs represent, for example, particular organizations, particular
   organizational units (e.g., departments, groups, sections), or particular geographical areas.

   RFC 1422 furthermore has a name subordination rule which requires that a CA can only issue certificates
   for entities whose names are subordinate (in the X.500 naming tree) to the name of the CA itself.  The
   trust associated with a PEM certification  path is implied by the PCA name.  The name subordination rule
   ensures that CAs below the PCA are sensibly constrained as to the set of subordinate entities they can
   certify (e.g., a CA for an organization can only certify entities in that organization's name tree).
   Certificate user systems are able to mechanically check that the name subordination rule has been



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   followed.

   The RFC 1422 CA hierarchical model has been found to have several deficiencies, including:

   (a)  The pure top-down hierarchy, with all ertification paths starting from the root, is too restrictive for
   many purposes.  For some applications, verification of certification paths should start with a public key of
   a CA in a user's own domain, rather than mandating that verification commence at the top of a hierarchy.
   In many environments, the local domain is often the most trusted.  Also, initialization and key-pair-update
   operations can be more effectively conducted between an end entity and a local management system.

   (b)  The name subordination rule introduces undesirable constraints upon the X.500 naming system an
   organization may use.

   (c)  Use of the PCA concept requires knowledge of individual PCAs to be built into certificate chain
   verification logic.  In the particular case of Internet mail, this is not a major problem -- the PCA name can
   always be displayed to the human user who can make a decision as to what trust to imply from a particular
   chain.  However, in many commercial applications, such as electronic commerce or EDI, operator
   intervention to make policy decisions is impractical.  The process needs to be automated to a much higher
   degree.  In fact, the full process of certificate chain processing needs to be implementable in trusted
   software.

   Because of the above shortcomings, it is proposed that more flexible CA structures than the RFC 1422
   hierarchy be supported by the PKIX specifications.  In fact, the main reason for the structural restrictions
   imposed by RFC 1422 was the restricted certificate format provided with X.509 v1.  With X.509 v3, most
   of the requirements addressed by RFC 1422 can be addressed using certificate extensions, without a need
   to restrict the CA structures used.  In particular, the certificate extensions relating to certificate policies
   obviate the need for PCAs and the constraint extensions obviate the need for the name subordination rule.

3.3  Revocation

   When a certificate is issued, it is expected to be in use for its entire validity period.  However, various
   circumstances may cause a certificate to become invalid prior to the expiration of the validity period.
   Such circumstances might include change of name, change of association between subject and CA (e.g.,
   an employee terminates employment with an organization), and compromise or suspected compromise of
   the corresponding private key.  Under such circumstances, the CA needs to revoke the certificate.

   X.509 defines one method of certificate revocation.  This method involves each CA periodically issuing a
   signed data structure called a certificate revocation list (CRL).  A CRL is a time stamped list identifying
   revoked certificates which is signed by a CA and made freely available in a public repository.  Each
   revoked certificate is identified in a CRL by its certificate serial number.  When a certificate-using system
   uses a certificate (e.g., for verifying a remote user's digital signature), that system not only checks the
   certificate signature and validity but also acquires a suitably-recent CRL and checks that the certificate
   serial number is not on that CRL.  The meaning of "suitably-recent" may vary with local policy, but it
   usually means the most recently-issued CRL.  A CA issues a new CRL on a regular periodic basis (e.g.,
   hourly, daily, or weekly).  Entries are added to CRLs as revocations occur, and an entry may be removed
   when the certificate expiration date is reached.

   An advantage of this revocation method is that CRLs may be distributed by exactly the same means as



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   certificates themselves, namely, via untrusted communications and server systems.

   One limitation of the CRL revocation method, using untrusted communications and servers, is that the
   time granularity of revocation is limited to the CRL issue period.  For example, if a revocation is reported
   now, that revocation will not be reliably notified to certificate-using systems until the next periodic CRL is
   issued -- this may be up to one hour, one day, or one week depending on the frequency that the CA issues
   CRLs.

   Another potential problem with CRLs is the risk of a CRL growing to an entirely unacceptable size.  In
   the 1988 and 1993 versions of X.509, the CRL for the end-user certificates needed to cover the entire
   population of end-users for one CA.  It is desirable to allow such populations to be in the range of
   thousands, tens of thousands, or possibly even hundreds of thousands of users.  The end-user CRL is
   therefore at risk of growing to such sizes, which present major communication and storage overhead
   problems.  With the version 2 CRL format, introduced along with the v3 certificate format, it becomes
   possible to arbitrarily divide the population of certificates for one CA into a number of partitions, each
   partition being associated with one CRL distribution point (e.g., directory entry or URL) from which
   CRLs are distributed.  Therefore, the maximum CRL size can be controlled by a CA.  Separate CRL
   distribution points can also exist for different revocation reasons.  For example, routine revocations (e.g.,
   name change) may be placed on a different CRL to revocations resulting from suspected key compromises,
   and policy may specify that the latter CRL be updated and issued more frequently than the former.

   As with the X.509 v3 certificate format, in order to facilitate interoperable implementations from multiple
   vendors, the X.509 v2 CRL format needs to be profiled for Internet use.  It is one goal of this document to
   specify such profiles.

   Furthermore, it is recognized that on-line methods of revocation notification may be applicable in some
   environments as an alternative to the X.509 CRL.  On-line revocation checking eliminates the latency
   between a revocation report and CRL the next issue.  Once the revocation is reported, any query to the on-
   line service will correctly reflect the certificate validation impacts of the revocation.  Therefore, this
   document will also consider standard approaches to on-line revocation notification.

3.4  Operational Protocols

   Operational protocols are required to deliver certificates and CRLs to certificate using client systems.
   Provision is needed for a variety of different means of certificate and CRL delivery, including
   request/delivery procedures based on E-mail, http, X.500, and WHOIS++.  These specifications include
   definitions of, and/or references to, message formats and procedures for supporting all of the above
   operational environments, including definitions of or references to appropriate MIME content types.

3.5  Management Protocols

   Management protocols are required to support on-line interactions between Public Key Infrastructure
   (PKI) components.  For example, management protocol might be used between a CA and a client system
   with which a key pair is associated, or between two CAs which cross-certify each other.  The set of
   functions which potentially need to be supported by management protocols include:

   (a)  registration:  This is the process whereby a user first makes itself known to a CA, prior to that CA
   issuing  a certificate or certificates for that user.



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   (b)  initialization:  Before a client system can operate securely it is necessary to install in it necessary key
   materials which have the appropriate relationship with keys stored elsewhere in the infrastructure.  For
   example, the client needs to be securely initialized with the public key of a CA, to be used in validating
   certificate paths.  Furthermore, a client typically needs to be initialized with its own key pair(s).

   (c)  certification:  This  is the process in which a CA issues a certificate for a user's public key, and returns
   that certificate to the user's client system and/or posts that certificate in a public repository.

   (d)  key pair recovery:  As an option, user client key materials (e.g., a user's private key used for
   encryption purposes) may be backed up by a CA or a key backup system associated with a CA.  If a user
   needs to recover these backed up key materials (e.g., as a result of a forgotten password or a lost key chain
   file), an on-line protocol exchange may be needed to support such recovery.

   (e)  key pair update:  All key pairs need to be updated regularly, i.e., replaced with a new key pair, and
   new certificates issued.

   (f)  revocation request:  An authorized person advises a CA of an abnormal situation requiring certificate
   revocation.

   (g)  cross-certification:  Two CAs exchange the information necessary to establish cross-certificates
   between those CAs.

   Note that on-line protocols are not the only way of implementing the above functions.  For all functions
   there are off-line methods of achieving the same result, and this specification does not mandate use of on-
   line protocols.  For example, when hardware tokens are used, many of the functions may be achieved
   through as part of the physical token delivery.  Furthermore, some of the above functions may be
   combined into one protocol exchange.  In particular, two or more of the registration, initialization, and
   certification functions can be combined into one protocol exchange.

   Part 3 of the PKIX series of specifications defines a set of standard message formats supporting the
   above functions.  The protocols for conveying these messages in different environments (on-line,
   e-mail, and WWW) are also specified.

4  Certificate and Certificate Extensions Profile

   As described above, the goal of this section is to create a profile for X.509 v3 certificates that will foster
   interoperability and a reusable public key infrastructure.  To achieve this goal, some assumptions need to
   be made about the nature of information to be included along with guidelines for how extensibility will be
   employed.

   Certificates may be used in a wide range of applications and environments covering a broad spectrum of
   interoperability goals and a broader spectrum of operational and assurance requirements.  The goal of this
   section is to establish a common baseline for generic applications requiring broad interoperability and
   limited special purpose requirements.  In particular, the emphasis will be on supporting the use of X.509
   v3 certificates for informal internet electronic mail, IPSEC, and WWW applications.  This section defines
   a baseline set of information, common locations within a certificate for this information, and common
   representations for this information.  Environments with additional requirements may build on this
   profile or may replace it.



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4.1  Basic Certificate Fields

   The X.509 v3 certificate Basic syntax follows.  For signature calculation, the certificate is ASN.1
   DER encoded [reference X.509?].  ASN.1 DER encoding is a tag, length, value encoding system for each
   element.

   Certificate  ::=  SIGNED  {  SEQUENCE  {
        version         [0]  Version DEFAULT v1,
        serialNumber         CertificateSerialNumber,
        signature            AlgorithmIdentifier,
        issuer               Name,
        validity             Validity,
        subject              Name,
        subjectPublicKeyInfo SubjectPublicKeyInfo,
        issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,
                             -- If present, version must be v2 or v3
        subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL,
                             -- If present, version must be v2 or v3
        extensions      [3]  Extensions OPTIONAL
                             -- If present, version must be v3
        }  }

   Version  ::=  INTEGER  {  v1(0), v2(1), v3(2)  }

   CertificateSerialNumber  ::=  INTEGER

   Validity  ::=  SEQUENCE  {
        notBefore            UTCTime,
        notAfter             UTCTime  }

   UniqueIdentifier  ::=  BIT STRING

   SubjectPublicKeyInfo  ::=  SEQUENCE  {
        algorithm            AlgorithmIdentifier,
        subjectPublicKey     BIT STRING  }

   The following items describe a proposed use of the X.509 v3
   certificate for the Internet.

4.1.1  Version

   This field describes the version of the encoded certificate.  When
   extensions are used, as expected in this profile, use X.509 version 3
   (value is 2).  If no extensions are present, but a UniqueIdentifier
   is present, use version 2 (value is 1).  If only basic fields are
   present, use version 1 (the value is absent).

   Implementations should be prepared to accept any version certificate.



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   In particular, at a minimum, implementations should recognize version
   3 certificates; determine whether any critical extensions are
   present; and accept certificates without critical extensions even if
   they don't recognize any extensions.  A certificate with an
   unrecognized critical extension must always be rejected.

   Generation of version 2 certificates is not expected by CAs using
   this profile.

4.1.2  Serial number

   The serial number is an integer assigned by the CA to each
   certificate.  It must be unique for each certificate issued by a CA
   (i.e., the issuer name and serial number identify a unique
   certificate).

   << Do we want to define a maximum value for the serial number? >>

4.1.3  Signature

   This field contains the algorithm identifier for the algorithm used
   to sign the certificate.  Section 7.2 of this profile lists the
   supported signature algorithms.

4.1.4  Issuer Name

   The issuer name (combined with the IssuerUniqueID, if present)
   provides a globally unique identifier of the authority signing the
   certificate.  Reliance on the IssuerUniqueID is strongly discouraged.
   The syntax of the issuer name is an X.500 distinguished name.  A name
   in the certificate may provide semantic information, may provide a
   reference to an external information store or service, provides a
   unique identifier, may provide authorization information, or may
   provide a basis for managing the CA relationships and certificate
   paths (other purposes are also possible).  This strawman suggests
   that the issuer (and subject) name fields must provide a globally
   unique identifier.  In addition, they should contain semantic
   information identifying the issuer/subject (e.g. a full name,
   organization name, etc.). Access information will be provided in a
   separate extension (when other than via X.500 directory) and internet
   specific identities (electronic mail address, DNS name, and URLs)
   will be carried in alternative name extensions.

   << Further discussion of naming guidelines for internet use is
   needed. >>






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4.1.5  Validity

   This field indicates the dates on which the certificate becomes valid
   (notBefore) and on which the certificate ceases to be valid
   (notAfter).

   The UTCTime (Coordinated Universal Time) values included in this
   field shall be expressed in Greenwich Mean Time (Zulu) and include
   granularity to the minute, even though finer granularity can be
   expressed in the UTCTime format.  That is, UTCTime should be
   expressed as YYMMDDHHMMZ.

   Implementors are warned that no DER is defined for UTCTime, thus
   transformation between local time representations and the DER
   transfer syntax must be performed carefully when computing the hash
   value for a certificate signature.  For example, a UTCTime value
   which includes explict, zero values for seconds will not produce the
   same hash value as one in which the seconds are omitted.  UTCTime
   expresses the value of a year modulo 100, with no indication of
   century, hence comparisons involving dates in different centuries
   must be performed with care.

4.1.6  Subject Name

   The purpose of the subject name (combined with the SubjectUniqueID,
   if present) is to provide a unique identifier of the subject of the
   certificate.  Reliance on the IssuerUniqueID is discouraged.  The
   syntax of the subject name is an X.500 distinguished name.  The
   discussion in section 4.1.4 on issuer names applies to subject names
   as well.

   << How do we bind a public key to an Internet e-mail address?  One
   alternative is to make Subject Name as a unique identifier.  Or, it
   could be legal to have a null Subject Name.  Either way the
   SubjectAltName contains the e-mail address. >>

4.1.7  Subject Public Key Info

   This field is used to carry the public key and identify the algorithm
   with which the key is used.

4.1.8  Unique Identifiers

   The subject and issuer unique identifier are present in the
   certificate to handle the possibility of reuse of subject and/or
   issuer names over time.  This profile strongly recommends that names
   not be reused, thus certificates conforming to this profile do not
   make use of unique identifiers.



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4.2  Certificate Extensions

   The extensions already defined by ANSI X9 and ISO for X.509 v3
   certificates provide methods for associating additional attributes
   with users or public keys and for managing the certification
   hierarchy. The X.509 v3 certificate format also allows communities to
   define private extensions to carry information unique to those
   communities.  Each extension in a certificate may be designated as
   critical or non-critical. A certificate using system (an application
   validating a certificate) must reject the certificate if it
   encounters a critical extension it does not recognize.  A non-
   critical extension may be ignored if it is not recognized.  The
   following presents recommended extensions used within Internet
   certificates and standard locations for information.  Communities may
   elect to use additional extensions; however, caution should be
   exercised in adopting any critical extensions in certificates which
   might be used in a general context.

   << Need to add table of OIDs for all extensions from X.509 and X9.55.
   Say which are allowed in this profile, and which are prohibited in
   this profile. >>

4.2.1  Subject Alternative Name

   The altNames extension allows additional identities to be bound to
   the subject of the certificate.  Defined options include an rfc822
   name (electronic mail address), a DNS name, and a URL.  Each of these
   are IA5 strings.  Multiple instances may be included.  Whenever such
   identities are to be bound in a certificate, the subject alternative
   name (or issuer alternative name) field shall be used.  A form of
   such an identifier may also be present in the subject distinguished
   name; however, the altName field is the preferred location for
   finding such information.

   The following definition is an enhanced version of  the X9.55
   definition of GeneralName. This definition is anticipated to be used
   in the X.509 Amendment.

   rfc822Name, dNSName, url, and ipAddress are name forms expected to be
   used with this profile.  Such names are subject to the basic
   constraint extension for issuers which may restrict the names a given
   CA can certify (see section on Basic Constraint extension).

   The use of otherName should not be used in conjunction with this
   profile.

   AltNames  ::=  SEQUENCE OF GeneralName




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   GeneralName  ::=  CHOICE  {
        otherName       [0] INSTANCE OF OTHER-NAME,
        rfc822Name      [1] IA5String,
        dNSName         [2] IA5String,
        x400Address     [3] ORAddress,
        directoryName   [4] Name,
        ediPartyName    [5] IA5String,
        url             [6] IA5String,
        ipAddress       [7] OCTET STRING  }

4.2.2  Issuer Alternative Name

   As with 4.2.1, this extension is used to bind Internet style
   identities to the issuer name.

4.2.3  Certificate Policies

   The certificatePolicies extension contains one or more object
   identifiers (OIDs).  Each OID indicates the policy under which the
   certificate has been issued.  This profile expects that a simple OID
   will be present in each PolicyElementInfo.  The qualifier within the
   PolicyElementInfo should be absent.

   Implementations processing certificate policy fields are expected to
   have lists of those policies which they will accept.  The
   implementations compare the policy identifier(s) in the certificate
   to that list.  This field provides information to be used at the
   discretion of a relying party.  In contrast, the policy identifier(s)
   in the keyUsageRestriction is a mandate by the issuer that a
   certificate be used only in particular environments.

   CertificatePolicies  ::=  SEQUENCE OF PolicyInformation

   PolicyInformation  ::=  SEQUENCE OF PolicyElementInfo

   PolicyElementInfo  ::=  SEQUENCE  {
        policyElementId     OBJECT IDENTIFIER,
        qualifier           ANY DEFINED BY policyElementId OPTIONAL  }

4.2.4  Key Attributes

   The keyAttributes extension contains information about the key itself
   including a unique key identifier, a key usage period (lifetime of
   the private key as opposed to the lifetime of the certificate), and
   an intended key usage.  The Internet certificate should use the
   keyAttributes extension and contain a key identifier and private key
   validity to aid in system management.  The key usage field in this
   extension is intended to be advisory (as contrasted with the key



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   usage restriction extension which imposes mandatory restrictions).
   The key usage field in this extension should be used to differentiate
   certificates containing public keys for validating CA certificate
   signatures, for validating CA CRL signatures, and validating
   signatures on on-line transactions.  However, the nonrepudiation and
   dataEncipherment values should not be used. Where a reference to a
   public key identifier is needed (as with an Authority Key ID) and is
   not included in an attribute in the associated certificate, an SHA-1
   hash of the public key shall be used.

   The GeneralizedTime values included in this field shall be expressed
   in Greenwich Mean Time (Zulu) and include granularity to the minute,
   even though finer granularity can be expressed in the GeneralizedTime
   format.  That is, GeneralizedTime should be expressed as
   YYYYMMDDHHMMZ.

   Implementors are warned that no DER is defined for GeneralizedTime,
   thus transformation between local time representations and the DER
   transfer syntax must be performed carefully when computing the hash
   value for a certificate signature.  For example, a GeneralizedTime
   value which includes explict, zero values for seconds will not
   produce the same hash value as one in which the seconds are omitted.
   GeneralizedTime expresses the using four digits.  Remember that
   UTCTime represents the value of a year modulo 100, with no indication
   of century.

   KeyAttributes  ::=  SEQUENCE  {
        keyIdentifier           KeyIdentifier OPTIONAL,
        intendedKeyUsage        KeyUsage  OPTIONAL,
        privateKeyUsagePeriod   PrivateKeyValidity OPTIONAL  }

   KeyIdentifier  ::=  OCTET STRING

   PrivateKeyValidity  ::=  SEQUENCE  {
        notBefore           [0] GeneralizedTime OPTIONAL,
        notAfter            [1] GeneralizedTime OPTIONAL  }

   KeyUsage  ::=  BIT STRING  {
        digitalSignature    (0),
        nonRepudiation      (1),
        keyEncipherment     (2),
        dataEncipherment    (3),
        keyAgreement        (4),
        keyCertSign         (5),
        offLineCRLSign      (6)  }






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4.2.5  Key Usage Restriction

   The keyUsageRestriction extension defines mandatory restrictions on
   the use of the key contained in the certificate based on policy
   and/or usage (e.g., signature, encryption).  This field should be
   used whenever the use of the key is to be restricted based on either
   usage or policy (see discussion in policies).  The usage restriction
   would be employed when a multipurpose key is to be restricted (e.g.,
   when an RSA key should be used only for signing or only for key
   encipherment).

   The policy restriction in this field provides a mandate by the issuer
   that a certificate be used only in selected environments (for
   example, that a certificate be used only for a given type of
   financial transaction).  In contrast, the policy identifier in the
   certificatePolicies extension is information which may be used at the
   discretion of a relying party.

   keyUsageRestriction  ::=  SEQUENCE  {
        certPolicySet            SEQUENCE OF CertPolicyId OPTIONAL,
        restrictedKeyUsage       KeyUsage OPTIONAL  }

4.2.6  Basic Constraints

   The basicConstraints extension identifies whether the subject of the
   certificate is a CA or an end user.  In addition, this field can
   limit the authority of a subject CA in terms of the certificates it
   can issue. Discussion of certification path restriction is covered
   elsewhere in this draft.  The subject type field should be present in
   all Internet certificates.

   basicConstraints  ::=  SEQUENCE  {
        subjectType         SubjectType,
        pathLenConstraint       INTEGER OPTIONAL,
        permittedSubtrees       [0] SEQUENCE OF GeneralName OPTIONAL,
        excludedSubtrees        [1] SEQUENCE OF GeneralName OPTIONAL  }

   SubjectType  ::=  BIT STRING  {
        cA                  (0),
        endEntity           (1)  }

4.2.7  CRL Distribution Points

   The cRLDistributionPoints extension identifies the CRL distribution
   point or points to which a certificate user should refer to acertain
   if the certificate has been revoked.  This extenstion provides a
   mechanism to divide the CRL inot manageable pieces if the CA has a
   large constituency.  Further discussion of CRL management is



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   contained in section 5.

4.2.8  Authority Key Identifier

   The authority key identifier extension provides a means of
   identifying the particular public key used to sign a certificate.
   The identification can be based on either the key identifier (from
   the key Attributes extension) or on the issuer name and serial
   number.  The key identifier method is recommended in this profile.
   This extension would be used where an issuer has multiple signing
   keys (either due to multiple concurrent key pairs or due to
   changeover).  In general, this extension should be included in
   certificates. If the issuer name/serial number approach is used, both
   the certIssuer and certSerialNumber fields must be present.

   authorityKeyId  ::=  SEQUENCE  {
        keyIdentifier       [0] KeyIdentifier OPTIONAL,
        certIssuer          [1] Name OPTIONAL,
        certSerialNumber    [2] CertificateSerialNumber OPTIONAL  }

4.2.9  Subject Directory Attributes

   The DAM provides an extension for subject directory attributes.  This
   extension may hold any information about the subject where that
   information has a defined X.500 Directory attribute.  This extension
   is not recommended as an essential part of this profile but may be
   used in local environments.  This extension is always non-critical.

   subjectDirectoryAttributes  ::=  SEQUENCE OF Attribute

4.2.10  Information Access

   The informationAccess field is proposed as a private extension to
   tell how information about a subject or CA (or ancillary CA services)
   may be accessed.  For example, this field might provide a pointer to
   information about a user (e.g., a URL) or might tell how to access CA
   information such as certificate status or on-line validation
   services.

   In many cases, the accuracy of this information is not certified by
   the CA.

   << Can IssuerAltNames and SubjectAltNames be used instead of some of
   this information?  If not, then add a paragraph describing each of
   the optional components? >>

   informationAccess  ::=  SEQUENCE  {
        certRetrieval        GeneralName OPTIONAL,



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        certValidation       GeneralName OPTIONAL,
        caInfo               GeneralName OPTIONAL,
        userInfo             GeneralName OPTIONAL  }

   Url  ::=  IA5String

4.2.11  Other extensions

   The X.509 DAM defines additional extensions; however, this
   specification does not include them in the profile.

   << policyMappings?  We could say this optional.  It is non-critical,
   so not problematical. >>

   << nameConstraints.  We should add a paragraph that strictly forbids
   use of this extensions.  >>

   << policyConstraints? We should encourage support of this extension.
   Since it is critical, we should include it in our profile so that all
   implementations are prepared to process it.  It will be needed for
   interoperability in the future. >>

4.3  Examples

   << Certificate samples including descriptive text and ASN.1 encoded
   blobs will be inserted. >>

5  CRL and CRL Extensions Profile

   As described above, one goal of this X.509 v2 CRL profile is to
   foster the creation of an interoperable and reusable Internet PKI.
   To achieve this goal, guidelines for the use of extensions are
   specified, and some assumptions are made about the nature of
   information included in the CRL.

   CRLs may be used in a wide range of applications and environments
   covering a broad spectrum of interoperability goals and an even
   broader spectrum of operational and assurance requirements.  This
   profile establishes a common baseline for generic applications
   requiring broad interoperability.  Emphasis is placed on support for
   X.509 v2 CRLs.  The profile defines a baseline set of information
   that can be expected in every CRL.  Also, the profile defines common
   locations within the CRL for frequently used attributes, and common
   representations for these attributes.

   Environments with additional or special purpose requirements may
   build on this profile or may replace it.




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5.1  CRL Fields

   The X.509 v2 CRL syntax is as follows.  For signature calculation,
   the data that is to be signed is ASN.1 DER encoded.  ASN.1 DER
   encoding is a tag, length, value encoding system for each element.

   CertificateList  ::=  SIGNED  {  SEQUENCE  {
        version                 Version OPTIONAL,
                                     -- if present, must be v2
        signature               AlgorithmIdentifier,
        issuer                  Name,
        thisUpdate              UTCTime,
        nextUpdate              UTCTime,
        revokedCertificates     SEQUENCE OF SEQUENCE  {
             userCertificate         CertificateSerialNumber,
             revocationDate          UTCTime,
             crlEntryExtensions      Extensions OPTIONAL  }  OPTIONAL,
        crlExtensions           [0]  Extensions OPTIONAL  }  }

   Version  ::= INTEGER  {  v1(0), v2(1), v3(2)  }

   AlgorithmIdentifier  ::=  SEQUENCE  {
        algorithm               OBJECT IDENTIFIER,
        parameters              ANY DEFINED BY algorithm OPTIONAL  }
                                     -- contains a value of the type
                                     -- registered for use with the
                                     -- algorithm object identifier value

   CertificateSerialNumber  ::=  INTEGER

   Extensions  ::=  SEQUENCE OF Extension

   Extension  ::=  SEQUENCE  {
        extnId                  OBJECT IDENTIFIER,
        critical                BOOLEAN DEFAULT FALSE,
        extnValue               OCTET STRING  }
                                     -- contains a DER encoding of a value
                                     -- of the type registered for use with
                                     -- the extnId object identifier value

   The following items describe the proposed use of the X.509 v2 CRL for
   in Internet PKI.

5.1.1  Version

   This field describes the version of the encoded CRL.  When extensions
   are used, as expected in this profile, use version 2 (the integer
   value is 1).  If neither CRL extensions nor CRL entry extensions are



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   present, use version 1 (the integer value must be omitted).

5.1.2  Signature

   This field contains the algorithm identifier for the algorithm used
   to sign the CRL.  Section 7.2 lists the signature algorithms used in
   the Internet PKI.

5.1.3  Issuer Name

   The issuer name provides a globally unique identifier of the
   certification authority signing the CRL.  The syntax of the issuer
   name is an X.500 distinguished name.

5.1.4  Last Update

   This field indicates the issue date of this CRL.

   The UTCTime (Coordinated Universal Time) value included in this field
   shall be expressed in Greenwich Mean Time (Zulu) and include
   granularity to the minute, even though finer granularity can be
   expressed in the UTCTime format.  That is, UTCTime should be
   expressed as YYMMDDHHMMZ.

   Implementors are warned that no DER is defined for UTCTime, thus
   transformation between local time representations and the DER
   transfer syntax must be performed carefully when computing the hash
   value for a CRL signature.  For example, a UTCTime value which
   includes explict, zero values for seconds will not produce the same
   hash value as one in which the seconds are omitted.  UTCTime
   expresses the value of a year modulo 100, with no indication of
   century, hence comparisons involving dates in different centuries
   must be performed with care.

5.1.5  Next Update

   This field indicates the date by which the next CRL will be issued.
   The next CRL could be issued before the indicated date, but it will
   not be issued any later than the indicated date.

   The same restrictions associated with UTCTime for Last Update apply
   to Next Update.

5.1.6  Revoked Certificates

   Revoked certificates are listed.  The revoked certificates are named
   by their serial numbers.  Certificates are uniquely identified by the
   combination of the issuer name and the user certificate serial



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   number.  The date on which the revocation occured is specified.  The
   same restrictions associated with UTCTime for Last Update apply to
   the revocation date.  CRL entry extensions are discussed in section
   5.3.

   When a CA wishes to revoke a certificate that it issued to another
   CA, the revocation shall appear on the CRL.  The revocation should
   also appear on the ARL.  The CA is revoking a certificate that it
   issued.

5.2  CRL Extensions

   The extensions already defined by ANSI X9 and ISO for X.509 v2 CRLs
   provide methods for associating additional attributes with CRLs.  The
   X.509 v2 CRL format also allows communities to define private
   extensions to carry information unique to those communities.  Each
   extension in a CRL may be designated as critical or non-critical.  A
   CRL validation must fail if it encounters an critical extension which
   it does not know how to process.  However, an unrecognized non-
   critical extension may be ignored. The following presents those
   extensions used within Internet CRLs.  Communities may elect to use
   additional extensions; however, caution should be exercised in
   adopting any critical extensions in CRLs which might be used in a
   general context.

   << Need to add table of OIDs for all extensions from X.509 and X9.55.
   Say which are allowed in this profile, and which are prohibited in
   this profile. >>


5.2.1  Authority Key Identifier

   The authorityKeyIdentifier is a non-critical CRL extension that
   identifies the CA's key used to sign the CRL.  This extension is
   useful when a CA uses more than one key; it allows distinct keys
   differentiated (e.g., as key updating occurs).  The key may be
   identified by an explicit key identifier, by identification of a
   certificate for the key (giving certificate issuer and certificate
   serial number), or both.  If both are used then the CA issuer shall
   ensure that all three fields are consistent.

   AuthorityKeyId  ::=  SEQUENCE  {
        keyIdentifier    [0] KeyIdentifier OPTIONAL,
        certIssuer       [1] Name OPTIONAL,
        certSerialNumber [2] CertificateSerialNumber OPTIONAL  }
           -- certIssuer and certSerialNumber constitute a logical pair,
           -- and if either is present both must be present.  Either this
           -- pair or the keyIdentifier field or all shall be present



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   KeyIdentifier  ::=  OCTET STRING

5.2.2  Issuer Alternative Name

   The issuerAltName is a non-critical CRL extension that provides
   additional CA names.  Multiple instances may be included.  The syntax
   for the issuerAltName is the same as described in section 4.2.1.
   Whenever such alternative names are included in a CRL, the issuer
   alternative name field shall be used.  Implementations which
   recognize this extension are not required to be able to process all
   the alternative name formats.  Unrecognized alternative name formats
   may be ignored by an implementation.

   The following definition is an enhanced version of  the X9.55
   definition of GeneralName. This definition is anticipated to be used
   in the X.509 Amendment.

   rfc822Name, dNSName, url, and ipAddress are name forms expected to be
   used with this profile.  Such names are subject to the basic
   constraint extension for issuers which may restrict the names a given
   CA can certify (see section on Basic Constraint extension).

   The use of otherName should not be used in conjunction with this
   profile.

   AltNames  ::=  SEQUENCE OF GeneralName

   GeneralName  ::=  CHOICE  {
        otherName       [0] INSTANCE OF OTHER-NAME,
        rfc822Name      [1] IA5String,
        dNSName         [2] IA5String,
        x400Address     [3] ORAddress,
        directoryName   [4] Name,
        ediPartyName    [5] IA5String,
        url             [6] IA5String,
        ipAddress       [7] OCTET STRING  }

5.2.3  CRL Number

   The cRLNumber is a non-critical CRL extension which conveys a
   monotonically increacing sequence number for each CRL issued by a
   given CA through a specific CA X.500 Directory entry or CRL
   distribution point.  This extension allows users to easily determine
   when a particular CRL superceeds another CRL.  CAs conforming to this
   profile shall include this CRL.

   CRLNumber  ::=  INTEGER




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5.2.4  Issuing Distribution Point

   The issuingDistributionPoint is a critical CRL extension that
   identifiers the CRL distribution point for this particular CRL, and
   it indicates whether the CRL covers revocation for end entity
   certificates only, CA certificates only, or a limitied set of reason
   codes.  Support for CRL distribution points is strongly encouraged.
   The use of certificateHold is strictly prohibited in this profile.

   Only the following reason codes may be used in conjunction with this
   profile.  The use of keyCompromise (1) shall be used to indicate
   compromise or suspected compromise.  The use of affiliationChanged
   (3), superseded (4), or cessationOfOperation (5)shall be used to
   indicate routine compromise.

   << Does anyone see a use for (2)? >>

   The CRL is signed by the CA's key.  CRL Distribution Points do not
   have their own key pairs.  If the CRL is stored in the X.500
   Directory, it is stored entry corresponding to the CRL distribution
   point, which may be different that the directory entry of the CA.

   CRL distribution points, if used, should be partitioned the CRL on
   the basis of compromise and routine revocation.  That is, the
   revocations with reason code (1) shall appear in one distribution
   point, and the revocations with reason codes (3), (4), and (5) shall
   appear in another distribution point.

   DistributionPoint  ::=  SEQUENCE  {
        distributionPoint    DistributionPointName,
        reasons              ReasonFlags OPTIONAL  }

   DistributionPointName  ::=  CHOICE  {
        fullName               [0] Name,
        nameRelativeToCA       [1] RelativeDistinguishedName,
        generalName            [2] GeneralName  }

   GeneralName  ::=  CHOICE  {
        otherName              [0] INSTANCE OF OTHER-NAME,
        rfc822Name             [1] IA5String,
        dNSName                [2] IA5String,
        x400Address            [3] ORAddress,
        directoryName          [4] Name,
        ediPartyName           [5] IA5String,
        uniformResourceLocator [6] IA5String  }

   OTHER-NAME  ::=  TYPE-IDENTIFIER




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   ReasonFlags  ::=  BIT STRING  {
        unused                 (0),
        keyCompromise          (1),
        caCompromise           (2),
        affiliationChanged     (3),
        superseded             (4),
        cessationOfOperation   (5),
        certificateHold        (6)  }

5.2.5  Delta CRL Indicator

   The deltaCRLIndicator is a critical CRL extension that identifies a
   delta-CRL.  The use of delta-CRLs can significantly improve
   processing time for applications which store revocation information
   in a format other than the CRLstructure.  This allows changes to be
   added to the local database while ignoring unchanged information that
   is already in the local databse.

   CAs are shall always issue a complete CRL when a delta-CRL is issued.

   The value of BaseCRLNumber identifies the CRL number of the base CRL
   that was used as the starting point in the generation of this delta-
   CRL.  The delta-CRL contains the changes between the base CRL and the
   current CRL issued along with the delta-CRL.  It is the decision of a
   CA as to whether to provide delta-CRLs.  Again, a delta-CRL shall not
   be issued without a corresponding CRL.  The value of CRLNumber for
   both the delta-CRL and the corresponding CRL shall be identical.

   A CRL user constructing a locally held CRL from delta-CRLs shall
   consider the constructed CRL incomplete and unusable if the CRLNumber
   of the received delta-CRL is more that one greater that the CRLnumber
   of the delta-CRL last processed.

5.3  CRL Entry Extensions

   The CRL entry extensions already defined by ANSI X9 and ISO for X.509
   v2 CRLs provide methods for associating additional attributes with
   CRL entries.  The X.509 v2 CRL format also allows communities to
   define private CRL entry extensions to carry information unique to
   those communities.  Each extension in a CRL entry may be designated
   as critical or non-critical.  A CRL validation must fail if it
   encounters an critical CRL entry extension which it does not know how
   to process.  However, an unrecognized non-critical CRL entry
   extension may be ignored.  The following presents recommended
   extensions used within Internet CRL entries and standard locations
   for information.  Communities may elect to use additional CRL entry
   extensions; however, caution should be exercised in adopting any
   critical extensions in CRL entries which might be used in a general



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   context.

   << Need to add table of OIDs for all extensions from X.509 and X9.55.
   Say which are allowed in this profile, and which are prohibited in
   this profile. >>


5.3.1  Reason Code

   The reasonCode is a non-critical CRL entry extension that identifies
   the reason for the certificate revocation.  CAs are strongly
   encouraged to include reason codes in CRL entries; however, some
   reasonCode values are strictly prohibited.  The reason code extension
   permits certificates to placed on hold or suspended.  The processing
   associated with suspended certificates greatly complicates
   certificate validation, therefore the use of reasonCode values
   certificateHold (6), certHoldRelease (7), and removeFromCRL (8) shall
   not be used.  Also, the reasonCode CRL entry extension should be
   absent instead of using the unspecified (0) reasonCode value.

   << Again, is there any reason to permit caCompromise (2)? >>

   CRLReason  ::=  ENUMERATED  {
        unspecified             (0),
        keyCompromise           (1),
        caCompromise            (2),
        affiliationChanged      (3),
        superseded              (4),
        cessationOfOperation    (5),
        certificateHold         (6),
        certHoldRelease         (7),
        removeFromCRL           (8)  }

5.3.2  Expiration Date

   The expirationDate is a non-critical CRL entry extension that
   indicates the expiration of a hold entry in a CRL.  The use of this
   extension is strictly prohibited by this profile.

5.3.3  Instruction Code

   The instructionCode is a non-critical CRL entry extension that
   provides a registered instruction identifier which indicates the
   action to be taken after encountering a certificate that has been
   placed on hold.  The use of this extension is strictly prohibited by
   this profile.





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5.3.4  Invalidity Date

   The invalidityDate is a non-critical CRL entry extension that
   provides the date on which it is known or suspected that the private
   key was compromised or that the certificate otherwise became invalid.
   This date may be earlier than the revocation date in the CRL entry,
   but it must be later than the issue date of the previously issued
   CRL.  Remember that the revocation date in the CRL entry specifies
   the date that the CA revoked the certificate.  Whenever this
   information is available, CAs are strongly encouraged to share it
   with CRL users.

   The GeneralizedTime values included in this field shall be expressed
   in Greenwich Mean Time (Zulu) and include granularity to the minute,
   even though finer granularity can be expressed in the GeneralizedTime
   format.  That is, GeneralizedTime should be expressed as
   YYYYMMDDHHMMZ.

   Implementors are warned that no DER is defined for GeneralizedTime,
   thus transformation between local time representations and the DER
   transfer syntax must be performed carefully when computing the hash
   value for a certificate signature.  For example, a GeneralizedTime
   value which includes explict, zero values for seconds will not
   produce the same hash value as one in which the seconds are omitted.
   GeneralizedTime expresses the using four digits.  Remember that
   UTCTime represents the value of a year modulo 100, with no indication
   of century.

   InvalidityDate  ::=  GeneralizedTime

5.4  Examples

   << CRL samples including descriptive text and ASN.1 encoded blobs
   will be inserted. >>

6  Certificate Path Validation

   Certification path processing verifies the binding between the
   subject distinguished name and subject public key.  The basic
   constraints and policy constraints extensions facilitate automated,
   self-contained implementation of certification path processing logic.

   The following is an outline of a procedure for validating
   certification paths.  An implementation shall be functionally
   equivalent to the external behaviour resulting from this procedure.
   Any algorithm may be used by a particular implementation so long as
   it derives the correct result.




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   The inputs to the certification path processing procedure are:

      (a)  a set of certificates comprising a certification path;

      (b)  a CA name and trusted public key value (or an identifier of
      such a key if the key is stored internally to the certification
      path processing module) for use in verifying the first certificate
      in the certification path;

      (c)  a set of initial-policy identifiers (each comprising a
      sequence of policy element identifiers), which identifies one or
      more certificate policies, any one of which would be acceptable
      for the purposes of certification path processing; and

      (d)  the current date/time (if not available internally to the
      certification path processing module).

   The outputs of the procedure are:

      (a)  an indication of success or failure of certification path
      validation;

      (b)  if validation failed, a reason for failure; and

      (c)  if validation was successful, a (possibly empty) set of
      policy qualifiers obtained from CAs on the path.

   The procedure makes use of the following set of state variables:

   (a)  acceptable policy set:  A set of certificate policy identifiers
   comprising the policy or policies recognized by the public key user
   together with policies deemed equivalent through policy mapping;

   (b)  constrained subtrees:  A set of root names defining a set of
   subtrees within which all subject names in subsequent certificates in
   the certification path shall fall; if no restriction is in force this
   state variable takes the special value unbounded; and

   (c)  excluded subtrees:  A set of root names defining a set of
   subtrees within which no subject name in subsequent certificates in
   the certification path may fall; if no restriction is in force this
   state variable takes the special value empty.

   The procedure involves an initialization step, followed by a series
   of certificate-processing steps.  The initialization step comprises:

      (a)  Initialize the constrained subtress to unbounded;




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      (b)  Initialize the excluded subtrees indicator to empty; and

      (c)  Initialize the acceptable policy set to the set of initial-
      policy identifiers.

   Each certificate is then processed in turn, starting with the
   certificate signed using the trusted CA public key which was input to
   this procedure.  The last certificate is processed as an end-entity
   certificate; all other certificates (if any) are processed as CA-
   certificates.

   The following checks are applied to all certificates:

      (a)  Check that the signature verifies, that dates are valid, that
      the subject and issuer names chain correctly, and that the
      certificate has not been revoked;

      (b)  If a key usage restriction extension is present in the
      certificate and contains a certPolicySet component, check that at
      least one member of the acceptable policy set appears in the
      field;

      (c)  Check that the subject name is consistent with the
      constrained subtrees state variables; and

      (d)  Check that the subject name is consistent with the excluded
      subtrees state variables.

   If any one of the above checks fails, the procedure terminates,
   returning a failure indication and an appropriate reason.  If none of
   the above checks fail on the end-entity certificate, the procedure
   terminates, returning a success indication together with the set of
   all policy qualifier values encountered in the set of certificates.

   For a CA-certificate, the following constraint recording actions are
   then performed, in order to correctly set up the state variables for
   the processing of the next certificate:

      (a)  If permittedSubtrees is present in the certificate, set the
      constrained subtrees state variable to the intersection of its
      previous value and the value indicated in the extension field.

      (b)  If excludedSubtrees is present in the certificate, set the
      excluded subtrees state variable to the union of its previous
      value and the value indicated in the extension field.

   Note:  It is possible to specify an extended version of the above
   certification path processing procedure which results in default



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   behaviour identical to the rules of Privacy Enhanced Mail [RFC 1422].
   In this extended version, additional inputs to the procedure are a
   list of one or more Policy Certification Authority (PCA) names and an
   indicator of the position in the certification path where the PCA is
   expected.  At the nominated PCA position, the CA name is compared
   against this list.  If a recognized PCA name is found, then a
   constraint of SubordinateToCA is implicitly assumed for the remainder
   of the certification path and processing continues.  If no valid PCA
   name is found, and if the certification path cannot be validated on
   the basis of identified policies, then the certification path is
   considered invalid.

7  Algorithm Support

7.1  One-way Hash Functions

   One-way hash functions are also called message digest algorithms.
   SHA-1 is be the most popular one-way hash function used in the
   Internet PKI.  However, PEM uses MD2 for certificates [RFC1422,
   RFC1423]. For this reason, MD2 may continue to be used in
   certificates for many years.

7.1.1  MD2 One-way Hash Function

   MD2 was also developed by Ron Rivest, but RSA Data Security has not
   placed the MD2 algorithm in the public domain.  Rather, RSA Data
   Security has granted license to use MD2 for non-commerical Internet
   Privacy-Enhanced Mail.  For this reason, MD2 may continue to be used
   with PEM certificates, but MD5 is preferred.  MD2 is fully described
   in RFC 1319.

   << Add a paragraph about the MD2 flaw that was recently discovered.
   Urge MD2 replacement with SHA-1. >>

7.1.2  SHA-1 One-way Hash Function

   SHA-1 was developed by the U.S. Government.  SHA-1 is fully described
   in FIPS 180-1.

   SHA-1 is the one-way hash function of choice for use with both RSA
   the DSA signature algorithms.

7.2  Signature Algorithms

   RSA and DSA are the most popular signature algorithms used in the
   Internet.

   There is some ambiguity in 1988 X.509 document regarding the



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   definition of the SIGNED macro regarding, the representation of a
   signature in a certificate or a CRL.  The interpretation selected for
   the Internet requires that the data to be signed (e.g., the one-way
   function output value) is first ASN.1 encoded as an OCTET STRING and
   the result is encrypted (e.g., using RSA Encryption) to form the
   signed quantity, which is then ASN.1 encoded as a BIT STRING.

7.2.1  RSA Signature Algorithm

   A patent statement regarding the RSA algorithm can be found at the
   end of this profile.

   The RSA algorithm is named for it's inventors: Rivest, Shamir, and
   Adleman.  The RSA signature algorithm is defined in PKCS #1.  It
   combines the either the MD2 or the SHA-1 one-way hash function with
   the RSA asymmetric encryption algorithm.  As defined in PKCS #1, the
   ASN.1 object identifiers used to identify these signature algorithms
   are:

        md2WithRSAEncryption OBJECT IDENTIFIER  ::=  {
            iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
            pkcs-1(1) 2  }

        sha-1WithRSAEncryption OBJECT IDENTIFIER  ::=  {
            iso(1) identified-organization(3) oiw(14) secsig(3)
            algorithm(2) 29  }

   When either of these object identifiers is used within the ASN.1 type
   AlgorithmIdentifier, the parameters component of that type shall be
   absent or the ASN.1 type NULL.

7.2.2  DSA Signature Algorithm

   A patent statement regarding the DSA can be found at the end of this
   profile.

   The Digital Signature Algorithm (DSA) is also called the Digital
   Signature Standard (DSS).  DSA was developed by the U.S. Government,
   and DSA is used in conjunction with the the SHA-1 one-way hash
   function.  DSA is fully described in FIPS 186.  The ASN.1 object
   identifiers used to identify this signature algorithm is:

        dsaWithSHA-1 OBJECT IDENTIFIER  ::=  {
            joint-iso-ccitt(2) country(16) US(840) organization(1)
            us-government(101) dod(2) infosec(1) algorithms(1) 2  }

   When this object identifier is used with the ASN.1 type
   AlgorithmIdentifier, the parameters component of that type is



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   optional.  If it is absent, the DSA parameters p, q, and g are
   assumed to be known, otherwise the parameters are included using the
   following ASN.1 structure:

        Dss-Parms  ::=  SEQUENCE  {
            p             OCTET STRING,
            q             OCTET STRING,
            g             OCTET STRING  }

7.3  Subject Public Key Algorithms


   << Add a section that lists the public key algorithms that are
   supported by this profile.  Obviously, RSA, DSA, Diffie-Hellman, and
   KEA will be included.  Are there others? >>

   << Should a different algorithm identifier be assigned to RSA
   signature keys and RSA key management keys?  If so, there will be one
   subsection for each within this section.>>
Patent Statements

   The Internet PKI relies on the use of patented public key technology.
   The Internet Standards Process as defined in RFC 1310 requires a
   written statement from the Patent holder that a license will be made
   available to applicants under reasonable terms and conditions prior
   to approving a specification as a Proposed, Draft or Internet
   Standard.

   Patent statements for DSA, RSA, and Diffie-Hellman follow.  These
   statements have been supplied by the patent holders, not the authors
   of this profile.

   Digital Signature Algorithm (DSA)

      The U.S. Government holds patent 5,231,668 on the Digital
      Signature Algorithm (DSA), which has been incorporated into
      Federal Information Processing Standard (FIPS) 186.  The patent
      was issued on July 27, 1993.

      The National Institute of Standards and Technology (NIST) has a
      long tradition of supplying U.S. Government-developed techniques
      to committees and working groups for inclusion into standards on a
      royalty-free basis.  NIST has made the DSA patent available
      royalty-free to users worldwide.

      Regarding patent infringement, FIPS 186 summarizes our position;
      the Department of Commerce is not aware of any patents that would
      be infringed by the DSA.  Questions regarding this matter may be



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      directed to the Deputy Chief Counsel for NIST.

   RSA Signature and Encryption

      << Now that PKP has dissolved, a revised patent statement for RSA
      from RSADSI is needed. >>

   Diffie-Hellman Key Agreement

      << Now that PKP has dissolved, a revised patent statement for
      Diffie-Hellman from Cylink is needed. >>

   Obsolete PKP Patent Statement

      << This statement is included here until a replacement from RSADSI
      and Cylink can be obtained. >>

      The Massachusetts Institute of Technology and the Board of
      Trustees of the Leland Stanford Junior University have granted
      Public Key Partners (PKP) exclusive sub-licensing rights to the
      following patents issued in the United States, and all of their
      corresponding foreign patents:

         Cryptographic Apparatus and Method
         ("Diffie-Hellman")......................... No. 4,200,770

         Public Key Cryptographic Apparatus
         and Method ("Hellman-Merkle").............. No. 4,218,582

         Cryptographic Communications System and
         Method ("RSA")............................. No. 4,405,829

         Exponential Cryptographic Apparatus
         and Method ("Hellman-Pohlig").............. No. 4,424,414

      These patents are stated by PKP to cover all known methods of
      practicing the art of Public Key encryption, including the
      variations collectively known as El Gamal.

      Public Key Partners has provided written assurance to the Internet
      Society that parties will be able to obtain, under reasonable,
      nondiscriminatory terms, the right to use the technology covered
      by these patents.  This assurance is documented in RFC 1170 titled
      "Public Key Standards and Licenses".  A copy of the written
      assurance dated April 20, 1990, may be obtained from the Internet
      Assigned Number Authority (IANA).

      The Internet Society, Internet Architecture Board, Internet



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      Engineering Steering Group and the Corporation for National
      Research Initiatives take no position on the validity or scope of
      the patents and patent applications, nor on the appropriateness of
      the terms of the assurance.  The Internet Society and other groups
      mentioned above have not made any determination as to any other
      intellectual property rights which may apply to the practice of
      this standard.  Any further consideration of these matters is the
      user's own responsibility.

Security Considerations

   This entire memo is about security mechanisms.
Author Addresses:

   Russell Housley
   SPYRUS
   PO Box 1198
   Herndon, VA 22070
   USA
   housley@spyrus.com

   Warwick Ford
   Nortel Secure Networks
   PO Box 3511, Station C
   Ottawa, Ontario
   Canada KY 4H7
   wford@bnr.ca

   David Solo
   BBN
   150 CambridgePark Drive
   Cambridge, MA 02140
   USA
   solo@bbn.com

















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