rfc2692.txt

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Network Working Group                                         C. Ellison
Request for Comments: 2692                                         Intel
Category: Experimental                                    September 1999


                           SPKI Requirements

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

   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.

Table of Contents

   Charter of the SPKI working group................................2
   Background.......................................................2
   General Requirements.............................................3
   Validity and CRLs................................................4
   Implementation of Certificates...................................4
   List of Certificate Uses.........................................5
   Open Questions..................................................11
   References......................................................12
   Security Considerations.........................................12
   Author's Address................................................13
   Full Copyright Statement........................................14








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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.

   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
   models.

Background

   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



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   name construct.  [Note: SPKI and SDSI have merged.]

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



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   embedded systems.  The code to process them and the memory to store
   them should both be as small as possible.

   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.

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
         checks.

      -  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.









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      -  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
         it.

      -  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 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



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            directory of keys.  It says nothing about how I certified
            the binding to my own satisfaction between the key and my
            friend.

      -  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 certificate.

      -  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 source.

      -  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

      -  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



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