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SPKI Requirements

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2692.
Author Carl Ellison
Last updated 2013-03-02 (Latest revision 1999-05-28)
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SPKI Requirements                                        Carl M. Ellison
INTERNET-DRAFT                                                     Intel
Expires: 3 December 1999

                                                             28 May 1999

                           SPKI Requirements
                           ---- ------------


Status of This Document

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   This document supersedes draft-ietf-spki-cert-req-02.txt.

   It is a compilation of the requirements for SPKI [Simple Public Key
   Infrastructure] certificates gathered from the discussion on the SPKI
   Working Group mailing list.  It is provided for information only.

   Distribution of this document is unlimited.  Comments should be sent
   to the SPKI (Simple Public Key Infrastructure) Working Group mailing
   list <> or to the author.

   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-

   Internet-Drafts are draft documents valid for a maximum of six
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   ``working draft'' or ``work in progress.''

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   Copyright (C) The Internet Society 1999. All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published and

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   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the  purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


   The IETF Simple Public Key Infrastructure [SPKI] Working Group is
   tasked with producing a certificate structure and operating procedure
   to meet the needs of the Internet community for trust management in
   as easy, simple and extensible a way as possible.

   The SPKI Working Group first established a list of things one might
   want to do with certificates (attached at the end of this document),
   and then summarized that list of desires into requirements.  This
   document presents that summary of requirements.

Charter of the SPKI working group

   Many Internet protocols and applications which use the Internet
   employ public key technology for security purposes and require a
   public key infrastructure to manage public keys.

   The task of the working group will be to develop Internet standards
   for an IETF sponsored public key certificate format, associated
   signature and other formats, and key acquisition protocols.  The key
   certificate format and associated protocols are to be simple to
   understand, implement, and use. For purposes of the working group,
   the resulting formats and protocols are to be known as the Simple
   Public Key Infrastructure, or SPKI.

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   The SPKI is intended to provide mechanisms to support security in a
   wide range of Internet applications, including IPSEC protocols,
   encrypted electronic mail and WWW documents, payment protocols, and
   any other application which will require the use of public key
   certificates and the ability to access them. It is intended that the
   Simple Public Key Infrastructure will support a range of trust

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Table Of Contents

      Status of This Document....................................1
      Charter of the SPKI working group..........................2

      Table Of Contents..........................................4


      General Requirements.......................................6
      Validity and CRLs..........................................7
      Implementation of Certificates.............................7

      List of Certificate Uses...................................9
      Open Questions............................................14

      Author's Address..........................................16
      Expiration and File Name..................................16

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   The term certificate traces back to the MIT bachelor's thesis of
   Loren M. Kohnfelder [KOHN].  Kohnfelder, in turn, was responding to a
   suggestion by Diffie and Hellman in their seminal paper [DH].  Diffie
   and Hellman noted that with public key cryptography, one no longer
   needs a secure channel over which to transmit secret keys between
   communicants.  Instead, they suggested, one can publish a modified
   telephone book -- one with public keys in place of telephone numbers.
   One could then look up his or her desired communication partner in
   the directory, find that person's public key and open a secure
   channel to that person.  Kohnfelder took that suggestion and noted
   that an on-line service has the disadvantage of being a performance
   bottleneck.  To replace it, he proposed creation of digitally signed
   directory entries which he called certificates.  In the time since
   1978, the term certificate has frequently been assumed to mean a
   binding between name and key.

   The SPKI team directly addressed the issue of <name,key> bindings and
   realized that such certificates are of extremely limited use for
   trust management.  A keyholder's name is one attribute of the
   keyholder, but as can be seen in the list of needs in this document,
   a person's name is rarely of security interest.  A user of a
   certificate needs to know whether a given keyholder has been granted
   some specific authorization.

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General Requirements

   We define the term KEYHOLDER of a public key to refer to the person
   or other entity that controls the corresponding private key.

   The main purpose of an SPKI certificate is to authorize some action,
   give permission, grant a capability, etc. to or for a keyholder.

   The keyholder is most directly identified by the public key itself,
   although for convenience or other purposes some indirection (delayed
   binding) may be employed.  That indirection can be via a collision-
   free hash of the public key or via a name, later to be resolved into
   a key.

   The definition of attributes or authorizations in a certificate is up
   to the author of code which uses the certificate.  The creation of
   new authorizations should not require interaction with any other
   person or organization but rather be under the total control of the
   author of the code using the certificate.

   Because SPKI certificates might carry information that the keyholder
   might not want to publish, we assume that certificates will be
   distributed directly by the keyholder to the verifier.  If the
   keyholder wishes to use a global repository, such as LDAP, the global
   PGP key server or the DNS database, that is up to the keyholder and
   not for the SPKI WG to specify.

   Because SPKI certificates will carry information that, taken together
   over all certificates, might constitute a dossier and therefore a
   privacy violation, each SPKI certificate should carry the minimum
   information necessary to get a job done.  The SPKI certificate is
   then to be like a single key rather than a key ring or a single
   credit card rather than a whole wallet.  The keyholder should be able
   to release a minimum of information in order to prove his or her
   permission to act.

   It is necessary for at least some certificates to be anonymous.

   Because one use of SPKI certificates is in secret balloting and
   similar applications, an SPKI certificate must be able to assign an
   attribute to a blinded signature key.

   One attribute of a keyholder is a name.  There are names the
   keyholder prefers to be called and there are names by which the
   keyholder is known to various other keyholders.  An SPKI certificate
   must be able to bind a key to such names.  The SDSI work of Rivest
   and Lampson has done an especially good job of defining and using
   local name spaces, therefore if possible SPKI should support the SDSI
   name construct.  [Note: SPKI and SDSI have merged.]

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Validity and CRLs

   An SPKI certificate, like any other, should be able to carry a
   validity period: dates within which it is valid.  It may also be
   necessary to have on-line refinement of validity.  This is frequently
   achieved via a Certificate Revocation List (CRL) in previous
   certificate designs.

   A minimal CRL contains a list of revoked certificates, identified
   uniquely, a sequence number and a signature.  Its method of
   transmission is not specified.  If it encounters some certificate
   that it lists, then it annihilates that certificate.  If it
   encounters a previous CRL, as indicated by sequence number, then it
   annihilates that previous CRL.  Such a CRL leads to non-deterministic
   program behavior.  Therefore, we take as a requirement that if SPKI
   uses CRLs, then the certificate that uses it must explicitly tell the
   verifier where to find the CRL, the CRL must carry explicit validity
   dates and the dates of a sequence of CRLs must not overlap.  Under
   this set of requirements, behavior of certificate validation is
   deterministic (aside from the question of clock skew).

   A CRL is a negative statement.  It is the digital equivalent of the
   little paper books of bad checks or bad credit cards that were
   distributed to cashiers in the 1970's and before.  These have been
   replaced in the retail world by positive statements -- on-line
   validation of a single check, ATM card or credit card.

   SPKI should support both positive and negative on-line validations.

   Any CRL or revalidation instrument must have its own lifetime.  A
   lifetime of 0 is not possible because of communication delays and
   clock skews, although one can consider an instrument whose lifetime
   is "one use" and which is delivered only as part of a
   challenge/response protocol.

Implementation of Certificates

   The authorization certificates that are envisioned for SPKI (and
   needed to meet the demands of the list given at the end of this
   document) should be generated by any keyholder empowered to grant or
   delegate the authorization in question.  The code to generate
   certificates should be written by many different developers,
   frequently persons acting alone, operating out of garages or dorm
   rooms.  This leads to a number of constraints on the structure and
   encoding of certificates.  In addition, SPKI certificates should be
   usable in very constrained environments, such as smart cards or small
   embedded systems.  The code to process them and the memory to store
   them should both be as small as possible.

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   An SPKI certificate should be as simple as possible.  There should be
   a bare minimum of fields necessary to get the job done and there
   should be an absolute minimum of optional fields.  In particular, the
   structure should be specific enough that the creator of a certificate
   is constrained by the structure definition, not by complaints (or
   error messages) from the reader of a certificate.

   An SPKI certificate should be described in as simple a method as
   possible, relating directly to the kind of structures a C or PASCAL
   programmer would normally write.

   No library code should be required for the packing or parsing of SPKI
   certificates.  In particular, ASN.1 is not to be used.

   A certificate should be signed exactly as it is transmitted.  There
   should be no reformatting called for in the process of checking a
   certificate's signature (although one might canonicalize white space
   during certificate input, for example, if the format is text).

   For efficiency, if possible, an SPKI certificate should be encoded in
   an LR(0) grammar.  That is, neither packing nor parsing of the
   structure should require a scan of the data.  Data should be read
   into the kind of structure a programmer would want to use without
   touching the incoming bytes more than once.

   For efficiency, if possible, an SPKI certificate should be packed and
   parsed without any recursion.

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List of Certificate Uses

   The list below is a brainstorming list, accumulated on the SPKI
   mailing list, of uses of such certificates.

      - I need a certificate to give me permission to write electronic

      - My bank would need a certificate, proving to others that it is a
        bank capable of cashing electronic checks and permitted to give
        permission to people to write electronic checks.

      - My bank would issue a certificate signing the key of a master
        bank certifier -- perhaps NACHA -- so that I could follow a
        certificate chain from a key I know (my bank's) to the key of
        any other bank in the US and, similarly, to any other bank in
        the world.

      - I might generate a certificate (a ``reputation voucher'') for a
        friend to introduce him to another friend -- in which
        certificate I could testify to my friend's political opinion
        (e.g., libertarian cypherpunk) or physical characteristics or
        anything else of interest.

      - I might have a certificate giving my security clearance, signed
        by a governmental issuing authority.

      - I want a certificate for some software I have downloaded and am
        considering running on my computer -- to make sure it hasn't
        changed and that some reputable company or person stands behind

      - I need certificates to bind names to public keys:

           - [traditional certificate] binding a key to a name, implying
             "all the attributes of the real person having this name are
             transferred to this key by this certificate".  This
             requires unique identification of a person (which is
             difficult in non-digital space, as it is) and someone
             trustworthy binding that unique name to the key in
             question.  In this model, a key starts out naked and
             acquires attributes, permissions and authority from the
             person bound to it.

           - [direct certificate] binding a name to a key, implying "I
             (the person who is able to use the associated private key
             to make this signature) declare that I go by the name of
             XXXXXXX."  The unique identification of the key is

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             automatic -- from the key itself or a cryptographic hash of
             the key.  The binding is done by the key itself -- in a
             self-signed certificate.  In this model, a key is loaded
             with attributes, permissions and authority directly by
             other certificates, not indirectly through some person's
             name, and this certificate declares only a name or nickname
             by which the key's owner likes to be addressed.

           - [personal binding] binding a key to a nickname.  This kind
             of certificate is signed by me, singing someone else's key
             and binding it to a nickname by which I know that person.
             It is for my use only -- never given out -- and is a signed
             certificate to prevent tampering with my own private
             directory of keys.  It says nothing about how I certified
             the binding to my own satisfaction between the key and my

      - I might be doing genealogy and be collecting what amounts to 3x5
        cards with facts to be linked together.  Some of these links
        would be from one content to another reference [e.g., indexing
        and cross-referencing].  Others might be links to the researcher
        who collected the fact.  By rights, the fact should be signed by
        that researcher.  Viewing only the signature on the fact and the
        link to the researcher, this electronic 3x5 card becomes a

      - I want to sign a contract to buy a house.  What kind of
        certificate do I need?

      - I have found someone on the net and she sounds really nice.
        Things are leading up to cybersex.  How do I make sure she's not
        really some 80-year-old man in a nursing home?

      - I have met someone on the net and would like a picture of her
        and her height, weight and other measurements from a trustworthy

      - Can I have a digital marriage license?

      - Can I have a digital divorce decree?

      - ..a digital Voter Registration Card?

      - There are a number of cards one carries in a typical wallet
        which could become certificates attached to a public key:

      - health insurance card

      - prescription drug card

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      - driver's license (for permission to drive)

      - driver's license (for permission to buy alcohol)

      - supermarket discount card

      - supermarket check-cashing card [I know -- anachronism]

      - Blockbuster Video rental card

      - ATM card

      - Credit card

      - membership card in the ACLU, NRA, Republican party, Operation
        Rescue, NARAL, ACM, IEEE, ICAR....

      - Red Cross blood donor card

      - Starbuck's Coffee buy-10-get-1-free card

      - DC Metro fare card

      - Phone calling card

      - Alumni Association card

      - REI Membership card

      - Car insurance card

      - claim check for a suitcase

      - claim check for a pawned radio

      - authorization for followup visits to a doctor, after surgery

      - Better Business Bureau [BBB] style reputation certificates
        [testimonies from satisfied customers]

      - BBB-style certificate that no complaints exist against a
        business or doctor or dentist, etc.

      - LDS Temple Recommend

      - Stock certificate

      - Stock option

      - Car title

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      - deed to land

      - proof of ownership of electronic equipment with an ID number

      - time card certificate [activating a digital time clock]

      - proof of degree earned [PhD, LLD, MD, ...]

      - permission to write digitally signed prescriptions for drugs

      - permission to spend up to $X of a company's money

      - permission to issue nuclear launch codes

      - I'm a sysadmin, I want to carry a certificate, signed by SAGE,
        that says I'm good at the things sysadmins are good at.

      - I'm that same sysadmin, I want an ephemeral certificate that
        grants me root access to certain systems for the day, or the
        week, or...

        Certain applications *will* want some form of auditing, but the
        audit identity should be in the domain of the particular
        application...  For instance an "is a system administrator of
        this host" certificate would probably want to include an audit
        identity, so you can figure out which of your multiple admins
        screwed something up.

      - I'm an amateur radio operator.  I want a signed certificate that
        says I'm allowed to engage in amateur radio, issued by the DOC.
        [I currently have a paper version of one].  This would be useful
        in enforcing access policies to the amateur spectrum; and in
        tracking abuse of that same spectrum.  Heck!  extend this
        concept to all licensed spectrum users.

      - I'm the a purchasing agent for a large corporation.  I want to
        posses a certificate that tells our suppliers that I'm
        authorized to make purchases up to $15,000.  I don't want the
        suppliers to know my name, lest their sales people bug me too
        much.  I don't want to have to share a single "Megacorp
        Purchasing Department Certificate" with others doing the same
        job [the private key would need to be shared--yuck!].

      - "This signed-key should be considered equivalent to the
        certifying-key until this certificate expires for the following
        purposes ..."
           [This is desirable when you wish to reduce the exposure of
           long-term keys.  One way to do this is to use smart cards,
           but those typically have slow processors and are connected
           through low-bandwidth links; however, if you only use the

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           smart card at "login" time to certify a short-term key pair,
           you get high performance and low exposure of the long term

           I'll note here that this flies in the face of attempts to
           prevent delegation of certain rights..  Maybe we need a
           "delegation-allowed" bit -- but there's nothing to stop
           someone who wishes to delegate against the rules from also
           loaning out their private key..].

      - "I am the current legitimate owner of a particular chunk of
        Internet address space."
           [I'd like to see IPSEC eventually become usable, at least for
           privacy, without need for prior arrangement between sites,
           but I think there's a need for a "I own this address"/"I own
           this address range" certificate in order for IPSEC to coexist
           with existing ip-address-based firewalls]

      - "I am the current legitimate owner of a this DNS name or

      - "I am the legitimate receiver of mail sent to this rfc822 email
        address.  [this might need to be signed by a key which itself
        had been certified by the appropriate "DNS name owner"
           [This is in case I know someone owns a particular e-mail
           address but I don't know their key.]

      - Encryption keys for E-mail and file encryption

      - Authentication of people or other entities

      - Digital signatures (unforgeability)

      - Timestamping / notary services

      - Host authentication

      - Service authentication

        Other requirements:

        - Trust model must be a web (people want to choose whom they
          trust).  People must be able to choose whom they trust or
          consider reliable roots (maybe with varying reliabilities).

        - Some applications (e.g., notary services) require highly
          trusted keys; generation complexity is not an issue here

        - Some applications (e.g., host authentication) require

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          extremely light (or no) bureaucracy.  Even communication with
          the central administrator may be a problem.

        - Especially in lower-end applications (e.g. host
          authentication) the people generating the keys (e.g.,
          administrators) will change, and you will no longer want them
          to be able to certify.  On the other hand, you will usually
          also not want all keys they have generated to expire.  This
          may imply a "certification right expiration" certificate
          requirement, probably to be implemented together with notary

        - Keys will need to be cached locally to avoid long delays
          fetching frequently used keys.  Cf. current name servers.  The
          key infrastructure may in future get used almost as often as
          the name server.  The caching and performance requirements are

        - Reliable distribution of key revocations and other
          certificates (e.g., the ceasing of the right to make new
          certificates).  May involve goals like "will have spread
          everywhere in 24 hours" or something like that.  This
          interacts with caching.

Open Questions

   Given such certificates, there remain some questions, most to do with
   proofs of the opposite of what a certificate is designed to do.
   These do not have answers provided by certificate definition or
   issuing alone.

      - Someone digitally signs a threatening e-mail message with my
        private key and sends it to  How do I
        prove that I didn't compose and send the message?  What kind of
        certificate characteristic might help me in this?

             This is an issue of (non-)repudiation and therefore a
             matter of private key protection.  Although this is of
             interest to the user of certificates, certificate format,
             contents or issuing machinery can not ensure the protection
             of a user's private key or prove whether or not a private
             key has been stolen or misused.

      - Can certificates help do a title scan for purchase of a house?

             Certificates might be employed to carry information in a

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             tamper-proof way, but building the database necessary to
             record all house titles and all liens is a project not
             related to certificate structure.

      - Can a certificate be issued to guarantee that I am not already
        married, so that I can then get a digital marriage license?

             The absence of attributes can be determined only if all
             relevant records are digitized and all parties have
             inescapable IDs.  The former is not likely to happen in our
             lifetimes and the latter receives political resistance.

             A certificate can communicate the 'positive' attribute "not
             already married" or "not registered as a voter in any other
             district".  That assumes that some organization is capable
             of determining that fact for a given keyholder.  The method
             of determining such a negative fact is not part of the
             certificate definition.

      - The assumption in most certificates is that the proper user will
        protect his private key very well, to prevent anyone else from
        accessing his funds.  However, in some cases the certificate
        itself might have monetary value [permission to prescribe drugs,
        permission to buy alcohol, ...].  What is to prevent the holder
        of such a certificate from loaning out his private key?

             This is a potential flaw in any system providing
             authorization and an interesting topic for study.  What
             prevents a doctor or dentist from selling prescriptions for
             controlled substances to drug abusers?

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   [DH] Diffie and Hellman, "New Directions in Cryptography", IEEE
   Transactions on Information Theory IT-22, 6 (Nov. 1976), 644-654

   [KOHN] Loren Kohnfelder, "Towards a Practical Public-key
   Cryptosystem", Bachelor's thesis, MIT, May, 1978

Author's Address

   Carl M. Ellison
   Intel Corporation
   2111 NE 25th Ave   M/S JF3-373
   Hillsboro OR 97124-5961 USA

   Telephone:      +1-503-264-2900
   Fax:            +1-503-264-6225
   EMail: (work, Outlook)
          (work, Eudora)
         , (home, Eudora)

Expiration and File Name

   This draft expires 3 December 1999.

   Its file name is draft-ietf-spki-cert-req-03.txt

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