Network Working Group                                         C. Ellison
Request for Comments: 2693                                         Intel
Category: Experimental                                         B. Frantz
                                                    Electric Communities
                                                              B. Lampson
                                                               Microsoft
                                                               R. Rivest
                                     MIT Laboratory for Computer Science
                                                               B. Thomas
                                                       Southwestern Bell
                                                               T. Ylonen
                                                                     SSH
                                                          September 1999

SPKI Certificate Theory

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

Abstract

   The SPKI Working Group has developed a standard form for digital
   certificates whose main purpose is authorization rather than
   authentication.  These structures bind either names or explicit
   authorizations to keys or other objects.  The binding to a key can be
   directly to an explicit key, or indirectly through the hash of the
   key or a name for it.  The name and authorization structures can be
   used separately or together.  We use S-expressions as the standard
   format for these certificates and define a canonical form for those
   S-expressions.  As part of this development, a mechanism for deriving
   authorization decisions from a mixture of certificate types was
   developed and is presented in this document.

   This document gives the theory behind SPKI certificates and ACLs
   without going into technical detail about those structures or their
   uses.

   1. Overview of Contents.......................................3
   1.1 Glossary..................................................4
   2. Name Certification.........................................5
   2.1 First Definition of CERTIFICATE...........................6
   2.2 The X.500 Plan and X.509..................................6
   2.3 X.509, PEM and PGP........................................7
   2.4 Rethinking Global Names...................................7
   2.5 Inescapable Identifiers...................................9
   2.6 Local Names..............................................10
   2.6.1 Basic SDSI Names.......................................10
   2.6.2 Compound SDSI Names....................................10
   2.7 Sources of Global Identifiers............................11
   2.8 Fully Qualified SDSI Names...............................11
   2.9 Fully Qualified X.509 Names..............................12
   2.10 Group Names.............................................12
   3. Authorization.............................................12
   3.1 Attribute Certificates...................................13
   3.2 X.509v3 Extensions.......................................13
   3.3 SPKI Certificates........................................14
   3.4 ACL Entries..............................................15
   4. Delegation................................................15
   4.1 Depth of Delegation......................................15
   4.1.1 No control.............................................15
   4.1.2 Boolean control........................................16
   4.1.3 Integer control........................................16
   4.1.4 The choice: boolean....................................16
   4.2 May a Delegator Also Exercise the Permission?............17
   4.3 Delegation of Authorization vs. ACLs.....................17
   5. Validity Conditions.......................................18
   5.1 Anti-matter CRLs.........................................18
   5.2 Timed CRLs...............................................19
   5.3 Timed Revalidations......................................20
   5.4 Setting the Validity Interval............................20
   5.5 One-time Revalidations...................................20
   5.6 Short-lived Certificates.................................21
   5.7 Other possibilities......................................21
   5.7.1 Micali's Inexpensive On-line Results...................21
   5.7.2 Rivest's Reversal of the CRL Logic.....................21
   6. Tuple Reduction...........................................22
   6.1 5-tuple Defined..........................................23
   6.2 4-tuple Defined..........................................24
   6.3 5-tuple Reduction Rules..................................24
   6.3.1 AIntersect.............................................25
   6.3.2 VIntersect.............................................27
   6.3.3 Threshold Subjects.....................................27
   6.3.4 Certificate Path Discovery.............................28

   6.4 4-tuple Reduction........................................28
   6.4.1 4-tuple Threshold Subject Reduction....................29
   6.4.2 4-tuple Validity Intersection..........................29
   6.5 Certificate Translation..................................29
   6.5.1 X.509v1................................................29
   6.5.2 PGP....................................................30
   6.5.3 X.509v3................................................30
   6.5.4 X9.57..................................................30
   6.5.5 SDSI 1.0...............................................30
   6.5.6 SPKI...................................................31
   6.5.7 SSL....................................................31
   6.6 Certificate Result Certificates..........................32
   7. Key Management............................................33
   7.1 Through Inescapable Names................................33
   7.2 Through a Naming Authority...............................33
   7.3 Through <name,key> Certificates..........................34
   7.4 Increasing Key Lifetimes.................................34
   7.5 One Root Per Individual..................................35
   7.6 Key Revocation Service...................................36
   7.7 Threshold ACL Subjects...................................36
   8. Security Considerations...................................37
   References...................................................38
   Acknowledgments..............................................40
   Authors' Addresses...........................................41
   Full Copyright Statement.....................................43

1. Overview of Contents

   This document contains the following sections:

   Section 2: history of name certification, from 1976 on.

   Section 3: discussion of authorization, rather than authentication,
   as the desired purpose of a certificate.

   Section 4: discussion of delegation.

   Section 5: discussion of validity conditions: date ranges, CRLs, re-
   validations and one-time on-line validity tests.

   Section 6: definition of 5-tuples and their reduction.

   Section 7: discussion of key management.

   Section 8: security considerations.

   The References section lists all documents referred to in the text as
   well as readings which might be of interest to anyone reading on this
   topic.

   The Acknowledgements section, including a list of contributors
   primarily from the start of the working group.  [The archive of
   working group mail is a more accurate source of contributor
   information.]

   The Authors' Addresses section gives the addresses, telephone numbers
   and e-mail addresses of the authors.

1.1 Glossary

   We use some terms in the body of this document in ways that could be
   specific to SPKI:

   ACL: an Access Control List: a list of entries that anchors a
   certificate chain.  Sometimes called a "list of root keys", the ACL
   is the source of empowerment for certificates.  That is, a
   certificate communicates power from its issuer to its subject, but
   the ACL is the source of that power (since it theoretically has the
   owner of the resource it controls as its implicit issuer).  An ACL
   entry has potentially the same content as a certificate body, but has
   no Issuer (and is not signed).  There is most likely one ACL for each
   resource owner, if not for each controlled resource.

   CERTIFICATE: a signed instrument that empowers the Subject.  It
   contains at least an Issuer and a Subject.  It can contain validity
   conditions, authorization and delegation information.  Certificates
   come in three categories: ID (mapping <name,key>), Attribute (mapping
   <authorization,name>), and Authorization (mapping
   <authorization,key>).  An SPKI authorization or attribute certificate
   can pass along all the empowerment it has received from the Issuer or
   it can pass along only a portion of that empowerment.

   ISSUER: the signer of a certificate and the source of empowerment
   that the certificate is communicating to the Subject.

   KEYHOLDER: the person or other entity that owns and controls a given
   private key.  This entity is said to be the keyholder of the keypair
   or just the public key, but control of the private key is assumed in
   all cases.

   PRINCIPAL: a cryptographic key, capable of generating a digital
   signature.  We deal with public-key signatures in this document but
   any digital signature method should apply.

   SPEAKING: A Principal is said to "speak" by means of a digital
   signature.  The statement made is the signed object (often a
   certificate).  The Principal is said to "speak for" the Keyholder.

   SUBJECT: the thing empowered by a certificate or ACL entry.  This can
   be in the form of a key, a name (with the understanding that the name
   is mapped by certificate to some key or other object), a hash of some
   object, or a set of keys arranged in a threshold function.

   S-EXPRESSION: the data format chosen for SPKI/SDSI.  This is a LISP-
   like parenthesized expression with the limitations that empty lists
   are not allowed and the first element in any S-expression must be a
   string, called the "type" of the expression.

   THRESHOLD SUBJECT: a Subject for an ACL entry or certificate that
   specifies K of N other Subjects.  Conceptually, the power being
   transmitted to the Subject by the ACL entry or certificate is
   transmitted in (1/K) amount to each listed subordinate Subject.  K of
   those subordinate Subjects must agree (by delegating their shares
   along to the same object or key) for that power to be passed along.
   This mechanism introduces fault tolerance and is especially useful in
   an ACL entry, providing fault tolerance for "root keys".

2. Name Certification

   Certificates were originally viewed as having one function: binding
   names to keys or keys to names.  This thought can be traced back to
   the paper by Diffie and Hellman introducing public key cryptography
   in 1976.  Prior to that time, key management was risky, involved and
   costly, sometimes employing special couriers with briefcases
   handcuffed to their wrists.

   Diffie and Hellman thought they had radically solved this problem.
   "Given a system of this kind, the problem of key distribution is
   vastly simplified.  Each user generates a pair of inverse
   transformations, E and D, at his terminal.  The deciphering
   transformation, D, must be kept secret but need never be communicated
   on any channel.  The enciphering key, E, can be made public by
   placing it in a public directory along with the user's name and
   address.  Anyone can then encrypt messages and send them to the user,
   but no one else can decipher messages intended for him." [DH]

   This modified telephone book, fully public, took the place of the
   trusted courier.  This directory could be put on-line and therefore
   be available on demand, worldwide.  In considering that prospect,
   Loren Kohnfelder, in his 1978 bachelor's thesis in electrical
   engineering from MIT [KOHNFELDER], noted: "Public-key communication
   works best when the encryption functions can reliably be shared among

   the communicants (by direct contact if possible).  Yet when such a
   reliable exchange of functions is impossible the next best thing is
   to trust a third party.  Diffie and Hellman introduce a central
   authority known as the Public File."

2.1 First Definition of CERTIFICATE

   Kohnfelder then noted, "Each individual has a name in the system by
   which he is referenced in the Public File.  Once two communicants
   have gotten each other's keys from the Public File they can securely
   communicate.  The Public File digitally signs all of its
   transmissions so that enemy impersonation of the Public File is
   precluded."  In an effort to prevent performance problems, Kohnfelder
   invented a new construct: a digitally signed data record containing a
   name and a public key.  He called this new construct a CERTIFICATE.
   Because it was digitally signed, such a certificate could be held by
   non-trusted parties and passed around from person to person,
   resolving the performance problems involved in a central directory.

2.2 The X.500 Plan and X.509

   Ten years after Kohnfelder's thesis, the ISO X.509 recommendation was
   published as part of X.500.  X.500 was to be a global, distributed
   database of named entities: people, computers, printers, etc.  In
   other words, it was to be a global, on-line telephone book.  The
   organizations owning some portion of the name space would maintain
   that portion and possibly even provide the computers on which it was
   stored.  X.509 certificates were defined to bind public keys to X.500
   path names (Distinguished Names) with the intention of noting which
   keyholder had permission to modify which X.500 directory nodes.  In
   fact, the X.509 data record was originally designed to hold a
   password instead of a public key as the record-access authentication
   mechanism.

   The original X.500 plan is unlikely ever to come to fruition.
   Collections of directory entries (such as employee lists, customer
   lists, contact lists, etc.) are considered valuable or even
   confidential by those owning the lists and are not likely to be
   released to the world in the form of an X.500 directory sub-tree.
   For an extreme example, imagine the CIA adding its directory of
   agents to a world-wide X.500 pool.

   The X.500 idea of a distinguished name (a single, globally unique
   name that everyone could use when referring to an entity) is also not
   likely to occur.  That idea requires a single, global naming
   discipline and there are too many entities already in the business of
   defining names not under a single discipline.  Legacy therefore
   militates against such an idea.

2.3 X.509, PEM and PGP

   The Privacy Enhanced Mail [PEM] effort of the Internet Engineering
   Task Force [RFC1114] adopted X.509 certificates, but with a different
   interpretation.  Where X.509 was originally intended to mean "the
   keyholder may modify this portion of the X.500 database", PEM took
   the certificate to mean "the key speaks for the named person".  What
   had been an access control instrument was now an identity instrument,
   along the lines envisioned by Diffie, Hellman and Kohnfelder.

   The insistence on X.509 certificates with a single global root
   delayed PEM's adoption past its window of viability.  RIPEM, by Mark
   Riordan of MSU, was a version of PEM without X.509 certificates.  It
   was distributed and used by a small community, but fell into disuse.
   MOSS (a MIME-enhanced version of PEM, produced by TIS (www.tis.com))
   made certificate use optional, but received little distribution.

   At about the same time, in 1991, Phil Zimmermann's PGP was introduced
   with a different certificate model.  Instead of waiting for a single
   global root and the hierarchy of Certificate Authorities descending
   from that root, PGP allowed multiple, (hopefully) independent but not
   specially trusted individuals to sign a <name,key> association,
   attesting to its validity.  The theory was that with enough such
   signatures, that association could be trusted because not all of
   these signer would be corrupt.  This was known as the "web of trust"
   model.  It differed from X.509 in the method of assuring trust in the
   <name,key> binding, but it still intended to bind a globally unique
   name to a key.  With PEM and PGP, the intention was for a keyholder
   to be known to anyone in the world by this certified global name.

2.4 Rethinking Global Names

   The assumption that the job of a certificate was to bind a name to a
   key made sense when it was first published.  In the 1970's, people
   operated in relatively small communities.  Relationships formed face
   to face.  Once you knew who someone was, you often knew enough to
   decide how to behave with that person.  As a result, people have
   reduced this requirement to the simply stated: "know who you're
   dealing with".

   Names, in turn, are what we humans use as identifiers of persons.  We
   learn this practice as infants.  In the family environment names work
   as identifiers, even today.  What we learn as infants is especially
   difficult to re-learn later in life.  Therefore, it is natural for
   people to translate the need to know who the keyholder is into a need
   to know the keyholder's name.

   Computer applications need to make decisions about keyholders.  These
   decisions are almost never made strictly on the basis of a
   keyholder's name.  There is some other fact about the keyholder of
   interest to the application (or to the human being running the
   application).  If a name functions at all for security purposes, it
   is as an index into some database (or human memory) of that other
   information.  To serve in this role, the name must be unique, in
   order to serve as an index, and there must be some information to be
   indexed.

   The names we use to identify people are usually unique, within our
   local domain, but that is not true on a global scale.  It is
   extremely unlikely that the name by which we know someone, a given
   name, would function as a unique identifier on the Internet.  Given
   names continue to serve the social function of making the named
   person feel recognized when addressed by name but they are inadequate
   as the identifiers envisioned by Diffie, Hellman and Kohnfelder.

   In the 1970's and even through the early 1990's, relationships formed
   in person and one could assume having met the keyholder and therefore
   having acquired knowledge about that person.  If a name could be
   found that was an adequate identifier of that keyholder, then one
   might use that name to index into memories about the keyholder and
   then be able to make the relevant decision.

   In the late 1990's, this is no longer true.  With the explosion of
   the Internet, it is likely that one will encounter keyholders who are
   complete strangers in the physical world and will remain so.  Contact
   will be made digitally and will remain digital for the duration of
   the relationship.  Therefore, on first encounter there is no body of
   knowledge to be indexed by any identifier.

   One might consider building a global database of facts about all
   persons in the world and making that database available (perhaps for
   a fee).  The name that indexes that database might also serve as a
   globally unique ID for the person referenced.  The database entry
   under that name could contain all the information needed to allow
   someone to make a security decision.  Since there are multiple
   decision-makers, each interested in specific information, the
   database would need to contain the union of multiple sets of
   information.  However, that solution would constitute a massive
   privacy violation and would probably be rejected as politically
   impossible.

   A globally unique ID might even fail when dealing with people we do
   know.  Few of us know the full given names of people with whom we
   deal.  A globally unique name for a person would be larger than the
   full given name (and probably contain it, out of deference to a

   person's fondness for his or her own name).  It would therefore not
   be a name by which we know the person, barring a radical change in
   human behavior.

   A globally unique ID that contains a person's given name poses a
   special danger.  If a human being is part of the process (e.g.,
   scanning a database of global IDs in order to find the ID of a
   specific person for the purpose of issuing an attribute certificate),
   then it is likely that the human operator would pay attention to the
   familiar portion of the ID (the common name) and pay less attention
   to the rest.  Since the common name is not an adequate ID, this can
   lead to mistakes.  Where there can be mistakes, there is an avenue
   for attack.

   Where globally unique identifiers need to be used, perhaps the best
   ID is one that is uniform in appearance (such as a long number or
   random looking text string) so that it has no recognizable sub-field.
   It should also be large enough (from a sparse enough name space) that
   typographical errors would not yield another valid identifier.

2.5 Inescapable Identifiers

   Some people speak of global IDs as if they were inescapable
   identifiers, able to prevent someone from doing evil under one name,
   changing his name and starting over again.  To make that scenario
   come true, one would have to have assignment of such identifiers
   (probably by governments, at birth) and some mechanism so that it is
   always possible to get from any flesh and blood person back to his or
   her identifier.  Given that latter mechanism, any Certificate
   Authority desiring to issue a certificate to a given individual would
   presumably choose the same, inescapable name for that certificate.  A
   full set of biometrics might suffice, for example, to look up a
   person without danger of false positive in a database of globally
   assigned ID numbers and with that procedure one could implement
   inescapable IDs.

   The use of an inescapable identifier might be possible in some
   countries, but in others (such as the US) it would meet strong
   political opposition.  Some countries have government-assigned ID
   numbers for citizens but also have privacy regulations that prohibit
   the use of those numbers for routine business.  In either of these
   latter cases, the inescapable ID would not be available for use in
   routine certificates.

   There was a concern that commercial Certificate Authorities might
   have been used to bring inescapable names into existence, bypassing
   the political process and the opposition to such names in those
   countries where such opposition is strong.  As the (name,key)

   certificate business is evolving today, there are multiple competing
   CAs each creating disjoint Distinguished Name spaces.  There is also
   no real block to the creation of new CAs.  Therefore a person is able
   to drop one Distinguished Name and get another, by changing CA,
   making these names not inescapable.

2.6 Local Names

   Globally unique names may be politically undesirable and relatively
   useless, in the world of the Internet, but we use names all the time.

   The names we use are local names.  These are the names we write in
   our personal address books or use as nicknames or aliases with e-mail
   agents.  They can be IDs assigned by corporations (e.g., bank account
   numbers or employee numbers).  Those names or IDs do not need to be
   globally unique.  Rather, they need to be unique for the one entity
   that maintains that address book, e-mail alias file or list of
   accounts.  More importantly, they need to be meaningful to the person
   who uses them as indexes.

   Ron Rivest and Butler Lampson showed with SDSI 1.0 [SDSI] that one
   can not only use local names locally, one can use local names
   globally.  The clear security advantage and operational simplicity of
   SDSI names caused us in the SPKI group to adopt SDSI names as part of
   the SPKI standard.

2.6.1 Basic SDSI Names

   A basic SDSI 2.0 name is an S-expression with two elements: the word
   "name" and the chosen name.  For example,

        george:  (name fred)

   represents a basic SDSI name "fred" in the name space defined by
   george.

2.6.2 Compound SDSI Names

   If fred in turn defines a name, for example,

        fred:  (name sam)

   then george can refer to this same entity as

        george:  (name fred sam)

2.7 Sources of Global Identifiers

   Even though humans use local names, computer systems often need
   globally unique identifiers.  Even in the examples of section 2.6.2
   above, we needed to make the local names more global and did so by
   specifying the name-space owner.

   If we are using public key cryptography, we have a ready source of
   globally unique identifiers.

   When one creates a key pair, for use in public key cryptography, the
   private key is bound to its owner by good key safeguarding practice.
   If that private key gets loose from its owner, then a basic premise
   of public key cryptography has been violated and that key is no
   longer of interest.

   The private key is also globally unique.  If it were not, then the
   key generation process would be seriously flawed and we would have to
   abandon this public key system implementation.

   The private key must be kept secret, so it is not a possible
   identifier, but each public key corresponds to one private key and
   therefore to one keyholder.  The public key, viewed as a byte string,
   is therefore an identifier for the keyholder.

   If there exists a collision-free hash function, then a collision-free
   hash of the public key is also a globally unique identifier for the
   keyholder, and probably a shorter one than the public key.

2.8 Fully Qualified SDSI Names

   SDSI local names are of great value to their definer.  Each local
   name maps to one or more public keys and therefore to the
   corresponding keyholder(s).  Through SDSI's name chaining, these
   local names become useful potentially to the whole world.  [See
   section 2.6.2 for an example of SDSI name chaining.]

   To a computer system making use of these names, the name string is
   not enough.  One must identify the name space in which that byte
   string is defined.  That name space can be identified globally by a
   public key.

   It is SDSI 1.0 convention, preserved in SPKI, that if a (local) SDSI
   name occurs within a certificate, then the public key of the issuer
   is the identifier of the name space in which that name is defined.

   However, if a SDSI name is ever to occur outside of a certificate,
   the name space within which it is defined must be identified.  This
   gives rise to the Fully Qualified SDSI Name.  That name is a public
   key followed by one or more names relative to that key.  If there are
   two or more names, then the string of names is a SDSI name chain.
   For example,

        (name (hash sha1 |TLCgPLFlGTzgUbcaYLW8kGTEnUk=|) jim therese)

   is a fully qualified SDSI name, using the SHA-1 hash of a public key
   as the global identifier defining the name space and anchoring this
   name string.

2.9 Fully Qualified X.509 Names

   An X.509 Distinguished Name can and sometimes must be expressed as a
   Fully Qualified Name.  If the PEM or original X.500 vision of a
   single root for a global name space had come true, this wouldn't be
   necessary because all names would be relative to that same one root
   key.  However, there is not now and is not likely ever to be a single
   root key.  Therefore, every X.509 name should be expressed as the
   pair

        (name <root key> <leaf name>)

   if all leaf names descending from that root are unique.  If
   uniqueness is enforced only within each individual CA, then one would
   build a Fully Qualified Name chain from an X.509 certificate chain,
   yielding the form

        (name <root key> <CA(1)> <CA(2)> ... <CA(k)> <leaf name>).

2.10 Group Names

   SPKI/SDSI does not claim to enforce one key per name.  Therefore, a
   named group can be defined by issuing multiple (name,key)
   certificates with the same name -- one for each group member.

3. Authorization

   Fully qualified SDSI names represent globally unique names, but at
   every step of their construction the local name used is presumably
   meaningful to the issuer.  Therefore, with SDSI name certificates one
   can identify the keyholder by a name relevant to someone.

   However, what an application needs to do, when given a public key
   certificate or a set of them, is answer the question of whether the
   remote keyholder is permitted some access.  That application must
   make a decision.  The data needed for that decision is almost never
   the spelling of a keyholder's name.

   Instead, the application needs to know if the keyholder is authorized
   for some access.  This is the primary job of a certificate, according
   to the members of the SPKI WG, and the SPKI certificate was designed
   to meet this need as simply and directly as possible.

   We realize that the world is not going to switch to SPKI certificates
   overnight.  Therefore, we developed an authorization computation
   process that can use certificates in any format.  That process is
   described below in section 6.

   The various methods of establishing authorization are documented
   below, briefly.  (See also [UPKI])

3.1 Attribute Certificates

   An Attribute Certificate, as defined in X9.57, binds an attribute
   that could be an authorization to a Distinguished Name.  For an
   application to use this information, it must combine an attribute
   certificate with an ID certificate, in order to get the full mapping:

        authorization -> name -> key

   Presumably the two certificates involved came from different issuers,
   one an authority on the authorization and the other an authority on
   names.  However, if either of these issuers were subverted, then an
   attacker could obtain an authorization improperly.  Therefore, both
   the issuers need to be trusted with the authorization decision.

3.2 X.509v3 Extensions

   X.509v3 permits general extensions.  These extensions can be used to
   carry authorization information.  This makes the certificate an
   instrument mapping both:

        authorization -> key

   and

        name -> key

   In this case, there is only one issuer, who must be an authority on
   both the authorization and the name.

   Some propose issuing a master X.509v3 certificate to an individual
   and letting extensions hold all the attributes or authorizations the
   individual would need.  This would require the issuer to be an
   authority on all of those authorizations.  In addition, this
   aggregation of attributes would result in shortening the lifetime of
   the certificate, since each attribute would have its own lifetime.
   Finally, aggregation of attributes amounts to the building of a
   dossier and represents a potential privacy violation.

   For all of these reasons, it is desirable that authorizations be
   limited to one per certificate.

3.3 SPKI Certificates

   A basic SPKI certificate defines a straight authorization mapping:

        authorization -> key

   If someone wants access to a keyholder's name, for logging purposes
   or even for punishment after wrong-doing, then one can map from key
   to location information (name, address, phone, ...) to get:

        authorization -> key -> name

   This mapping has an apparent security advantage over the attribute
   certificate mapping.  In the mapping above, only the

        authorization -> key

   mapping needs to be secure at the level required for the access
   control mechanism.  The

        key -> name

   mapping (and the issuer of any certificates involved) needs to be
   secure enough to satisfy lawyers or private investigators, but a
   subversion of this mapping does not permit the attacker to defeat the
   access control.  Presumably, therefore, the care with which these
   certificates (or database entries) are created is less critical than
   the care with which the authorization certificate is issued.  It is
   also possible that the mapping to name need not be on-line or
   protected as certificates, since it would be used by human
   investigators only in unusual circumstances.

3.4 ACL Entries

   SDSI 1.0 defined an ACL, granting authorization to names.  It was
   then like an attribute certificate, except that it did not need to be
   signed or issued by any key.  It was held in local memory and was
   assumed issued by the owner of the computer and therefore of the
   resource being controlled.

   In SPKI, an ACL entry is free to be implemented in any way the
   developer chooses, since it is never communicated and therefore does
   not need to be standardized.  However, a sample implementation is
   documented, as a certificate body minus the issuer field.  The ACL
   entry can have a name as a subject, as in SDSI 1.0, or it can have a
   key as a subject.  Examples of the latter include the list of SSL
   root keys in an SSL capable browser or the file .ssh/authorized_keys
   in a user's home UNIX directory.  Those ACLs are single-purpose, so
   the individual entries do not carry explicit authorizations, but SPKI
   uses explicit authorizations so that one can use common authorization
   computation code to deal with multiple applications.

4. Delegation

   One of the powers of an authorization certificate is the ability to
   delegate authorizations from one person to another without bothering
   the owner of the resource(s) involved.  One might issue a simple
   permission (e.g., to read some file) or issue the permission to
   delegate that permission further.

   Two issues arose as we considered delegation: the desire to limit
   depth of delegation and the question of separating delegators from
   those who can exercise the delegated permission.

4.1 Depth of Delegation

   There were three camps in discussing depth of delegation: no control,
   boolean control and integer control.  There remain camps in favor of
   each of these, but a decision was reached in favor of boolean
   control.

4.1.1 No control

   The argument in favor of no control is that if a keyholder is given
   permission to do something but not the permission to delegate it,
   then it is possible for that keyholder to loan out the empowered
   private key or to set up a proxy service, signing challenges or
   requests for the intended delegate.  Therefore, the attempt to
   restrict the permission to delegate is ineffective and might back-
   fire, by leading to improper security practices.

4.1.2 Boolean control

   The argument in favor of boolean control is that one might need to
   specify an inability to delegate.  For example, one could imagine the
   US Commerce Department having a key that is authorized to declare a
   cryptographic software module exportable and also to delegate that
   authorization to others (e.g., manufacturers).  It is reasonable to
   assume the Commerce Department would not issue permission to delegate
   this further.  That is, it would want to have a direct legal
   agreement with each manufacturer and issue a certificate to that
   manufacturer only to reflect that the legal agreement is in place.

4.1.3 Integer control

   The argument in favor of integer control is that one might want to
   restrict the depth of delegation in order to control the
   proliferation of a delegated permission.

4.1.4 The choice: boolean

   Of these three, the group chose boolean control.  The subject of a
   certificate or ACL entry may exercise any permission granted and, if
   delegation is TRUE, may also delegate that permission or some subset
   of it to others.

   The no control argument has logical appeal, but there remains the
   assumption that a user will value his or her private key enough not
   to loan it out or that the key will be locked in hardware where it
   can't be copied to any other user.  This doesn't prevent the user
   from setting up a signing oracle, but lack of network connectivity
   might inhibit that mechanism.

   The integer control option was the original design and has appeal,
   but was defeated by the inability to predict the proper depth of
   delegation.  One can always need to go one more level down, by
   creating a temporary signing key (e.g., for use in a laptop).
   Therefore, the initially predicted depth could be significantly off.

   As for controlling the proliferation of permissions, there is no
   control on the width of the delegation tree, so control on its depth
   is not a tight control on proliferation.

4.2 May a Delegator Also Exercise the Permission?

   We decided that a delegator is free to create a new key pair, also
   controlled by it, and delegate the rights to that key to exercise the
   delegated permission.  Therefore, there was no benefit from
   attempting to restrict the exercise of a permission by someone
   permitted to delegate it.

4.3 Delegation of Authorization vs. ACLs

   One concern with defining an authorization certificate is that the
   function can be performed by traditional <authorization,name> ACLs
   and <name,key> ID certificates defining groups.  Such a mechanism was
   described in SDSI 1.0.  A new mechanism needs to add value or it just
   complicates life for the developer.

   The argument for delegated authorization as opposed to ACLs can be
   seen in the following example.

   Imagine a firewall proxy permitting telnet and ftp access from the
   Internet into a network of US DoD machines.  Because of the
   sensitivity of that destination network, strong access control would
   be desired.  One could use public key authentication and public key
   certificates to establish who the individual keyholder was.  Both the
   private key and the keyholder's certificates could be kept on a
   Fortezza card.  That card holds X.509v1 certificates, so all that can
   be established is the name of the keyholder.  It is then the job of
   the firewall to keep an ACL, listing named keyholders and the forms
   of access they are each permitted.

   Consider the ACL itself.  Not only would it be potentially huge,
   demanding far more storage than the firewall would otherwise require,
   but it would also need its own ACL.  One could not, for example, have
   someone in the Army have the power to decide whether someone in the
   Navy got access.  In fact, the ACL would probably need not one level
   of its own ACL, but a nested set of ACLs, eventually reflecting the
   organization structure of the entire Defense Department.

   Without the ACLs, the firewall could be implemented in a device with
   no mass storage, residing in a sealed unit one could easily hold in
   one hand.  With the ACLs, it would need a large mass storage device
   that would be accessed not only while making access control decisions
   but also for updating the ACLs.

   By contrast, let the access be controlled by authorization
   certificates.  The firewall would have an ACL with one entry,
   granting a key belonging to the Secretary of Defense the right to
   delegate all access through the firewall.  The Secretary would, in

   turn, issue certificates delegating this permission to delegate to
   each of his or her subordinates.  This process would iterate, until
   some enlisted man would receive permission to penetrate that firewall
   for some specific one protocol, but not have permission to delegate
   that permission.

   The certificate structure generated would reflect the organization
   structure of the entire Defense Department, just as the nested ACLs
   would have, but the control of these certificates (via their issuance
   and revocation) is distributed and need not show up in that one
   firewall or be replicated in all firewalls.  Each individual
   delegator of permission performs a simple task, well understood.  The
   application software to allow that delegation is correspondingly
   simple.

5. Validity Conditions

   A certificate, or an ACL entry, has optional validity conditions.
   The traditional ones are validity dates: not-before and not-after.
   The SPKI group resolved, in discussion, that on-line tests of various
   kinds are also validity conditions.  That is, they further refine the
   valid date range of a certificate.  Three kinds of on-line tests are
   envisioned: CRL, re-validation and one-time.

   When validity confirmation is provided by some online test, then the
   issuer of those refinements need not be the issuer of the original
   certificate.  In many cases, the business or security model for the
   two issuers is different.  However, in SPKI, the certificate issuer
   must specify the issuer of validity confirmations.

5.1 Anti-matter CRLs

   An early form of CRL [Certificate Revocation List] was modeled after
   the news print book that used to be kept at supermarket checkout
   stands.  Those books held lists of bad checking account numbers and,
   later, bad credit card numbers.  If one's payment instrument wasn't
   listed in the book, then that instrument was considered good.

   These books would be issued periodically, and delivered by some means
   not necessarily taking a constant time.  However, when a new book
   arrived, the clerk would replace the older edition with the new one
   and start using it.  This design was suited to the constraints of the
   implementation: use of physical books, delivered by physical means.
   The book held bad account numbers rather than good ones because the
   list of bad accounts was smaller.

   An early CRL design followed this model.  It had a list of revoked
   certificate identifiers.  It also had a sequence number, so that one
   could tell which of two CRLs was more recent.  A newer CRL would
   replace an older one.

   This mode of operation is like wandering anti-matter.  When the
   issuer wants to revoke a certificate, it is listed in the next CRL to
   go out.  If the revocation is urgent, then that CRL can be released
   immediately.  The CRL then follows some dissemination process
   unrelated to the needs of the consumers of the CRL.  If the CRL
   encounters a certificate it has listed, it effectively annihilates
   that certificate.  If it encounters an older CRL, it annihilates that
   CRL also, leaving a copies of itself at the verifiers it encounters.

   However, this process is non-deterministic.  The result of the
   authorization computation is at least timing dependent.  Given an
   active adversary, it can also be a security hole.  That is, an
   adversary can prevent revocation of a given certificate by preventing
   the delivery of new CRLs.  This does not require cryptographic level
   effort, merely network tampering.

   SPKI has ruled out the use of wandering anti-matter CRLs for its
   certificates.  Every authorization computation is deterministic,
   under SPKI rules.

5.2 Timed CRLs

   SPKI permits use of timed CRLs.  That is, if a certificate can be
   referenced in a CRL, then the CRL process is subject to three
   conditions.

   1.  The certificate must list the key (or its hash) that will sign
       the CRL and may give one or more locations where that CRL might
       be fetched.

   2.  The CRL must carry validity dates.

   3.  CRL validity date ranges must not intersect.  That is, one may
       not issue a new CRL to take effect before the expiration of the
       CRL currently deployed.

   Under these rules, no certificate that might use a CRL can be
   processed without a valid CRL and no CRL can be issued to show up as
   a surprise at the verifier.  This yields a deterministic validity
   computation, independent of clock skew, although clock inaccuracies
   in the verifier may produce a result not desired by the issuer.  The
   CRL in this case is a completion of the certificate, rather than a
   message to the world announcing a change of mind.

   Since CRLs might get very large and since they tend to grow
   monotonically, one might want to issue changes to CRLs rather than
   full ones.  That is, a CRL might be a full CRL followed by a sequence
   of delta-CRLs.  That sequence of instruments is then treated as a
   current CRL and the combined CRL must follow the conditions listed
   above.

5.3 Timed Revalidations

   CRLs are negative statements.  The positive version of a CRL is what
   we call a revalidation.  Typically a revalidation would list only one
   certificate (the one of interest), although it might list a set of
   certificates (to save digital signature effort).

   As with the CRL, SPKI demands that this process be deterministic and
   therefore that the revalidation follow the same rules listed above
   (in section 5.2).

5.4 Setting the Validity Interval

   Both timed CRLs and timed revalidations have non-0 validity
   intervals.  To set this validity interval, one must answer the
   question: "How long are you willing to let the world believe and act
   on a statement you know to be false?"

   That is, one must assume that the previous CRL or revalidation has
   just been signed and transmitted to at least one consumer, locking up
   a time slot.  The next available time slot starts after this validity
   interval ends.  That is the earliest one can revoke a certificate one
   learns to be false.

   The answer to that question comes from risk management.  It will
   probably be based on expected monetary losses, at least in commercial
   cases.

5.5 One-time Revalidations

   Validity intervals of length zero are not possible.  Since
   transmission takes time, by the time a CRL was received by the
   verifier, it would be out of date and unusable.  That assumes perfect
   clock synchronization.  If clock skew is taken into consideration,
   validity intervals need to be that much larger to be meaningful.

   For those who want to set the validity interval to zero, SPKI defines
   a one-time revalidation.

   This form of revalidation has no lifetime beyond the current
   authorization computation.  One applies for this on-line, one-time
   revalidation by submitting a request containing a nonce.  That nonce
   gets returned in the signed revalidation instrument, in order to
   prevent replay attacks.  This protocol takes the place of a validity
   date range and represents a validity interval of zero, starting and
   ending at the time the authorization computation completes.

5.6 Short-lived Certificates

   A performance analysis of the various methods of achieving fine-grain
   control over the validity interval of a certificate should consider
   the possibility of just making the original certificate short-lived,
   especially if the online test result is issued by the same key that
   issued the certificate.  There are cases in which the short-lived
   certificate requires fewer signatures and less network traffic than
   the various online test options.  The use of a short-lived
   certificate always requires fewer signature verifications than the
   use of certificate plus online test result.

   If one wants to issue short-lived certificates, SPKI defines a kind
   of online test statement to tell the user of the certificate where a
   replacement short-lived certificate might be fetched.

5.7 Other possibilities

There are other possibilities to be considered when choosing a
validity condition model to use.

5.7.1 Micali's Inexpensive On-line Results

   Silvio Micali has patented a mechanism for using hash chains to
   revalidate or revoke a certificate inexpensively.  This mechanism
   changes the performance requirements of those models and might
   therefore change the conclusion from a performance analysis [ECR].

5.7.2 Rivest's Reversal of the CRL Logic

   Ron Rivest has written a paper [R98] suggesting that the whole
   validity condition model is flawed because it assumes that the issuer
   (or some entity to which it delegates this responsibility) decides
   the conditions under which a certificate is valid.  That traditional
   model is consistent with a military key management model, in which
   there is some central authority responsible for key release and for
   determining key validity.

   However, in the commercial space, it is the verifier and not the
   issuer who is taking a risk by accepting a certificate.  It should
   therefore be the verifier who decides what level of assurance he
   needs before accepting a credential.  That verifier needs information
   from the issuer, and the more recent that information the better, but
   the decision is the verifier's in the end.

   This line of thought deserves further consideration, but is not
   reflected in the SPKI structure definition.  It might even be that
   both the issuer and the verifier have stakes in this decision, so
   that any replacement validity logic would have to include inputs from
   both.

6. Tuple Reduction

   The processing of certificates and related objects to yield an
   authorization result is the province of the developer of the
   application or system.  The processing plan presented here is an
   example that may be followed, but its primary purpose is to clarify
   the semantics of an SPKI certificate and the way it and various other
   kinds of certificate might be used to yield an authorization result.

   There are three kinds of entity that might be input to the
   computation that yields an authorization result:

    1.  <name,key> (as a certificate)

    2.  <authorization,name> (as an attribute certificate or ACL entry)

    3.  <authorization,key> (as an authorization certificate or ACL
        entry)

   These entities are processed in three stages.

    1.  Individual certificates are verified by checking their
        signatures and possibly performing other work.  They are then
        mapped to intermediate forms, called "tuples" here.

        The other work for SPKI or SDSI certificates might include
        processing of on-line test results (CRL, re-validation or one-
        time validation).

        The other work for PGP certificates may include a web-of-trust
        computation.

        The other work for X.509 certificates depends on the written
        documentation for that particular use of X.509 (typically tied
        to the root key from which the certificate descended) and could

        involve checking information in the parent certificate as well
        as additional information in extensions of the certificate in
        question.  That is, some use X.509 certificates just to define
        names.  Others use X.509 to communicate an authorization
        implicitly (e.g., SSL server certificates).  Some might define
        extensions of X.509 to carry explicit authorizations.  All of
        these interpretations are specified in written documentation
        associated with the certificate chain and therefore with the
        root from which the chain descends.

        If on-line tests are involved in the certificate processing,
        then the validity dates of those on-line test results are
        intersected by VIntersect() [defined in 6.3.2, below] with the
        validity dates of the certificate to yield the dates in the
        certificate's tuple(s).

    2.  Uses of names are replaced with simple definitions (keys or
        hashes), based on the name definitions available from reducing
        name 4-tuples.

    3.  Authorization 5-tuples are then reduced to a final authorization
        result.

6.1 5-tuple Defined

   The 5-tuple is an intermediate form, assumed to be held in trusted
   memory so that it doesn't need a digital signature for integrity.  It
   is produced from certificates or other credentials via trusted
   software.  Its contents are the same as the contents of an SPKI
   certificate body, but it might be derived from another form of
   certificate or from an ACL entry.

   The elements of a 5-tuple are:

    1.  Issuer: a public key (or its hash), or the reserved word "Self".
        This identifies the entity speaking this intermediate result.

    2.  Subject: a public key (or its hash), a name used to identify a
        public key, the hash of an object or a threshold function of
        subordinate subjects.  This identifies the entity being spoken
        about in this intermediate result.

    3.  Delegation: a boolean.  If TRUE, then the Subject is permitted
        by the Issuer to further propagate the authorization in this
        intermediate result.

    4.  Authorization: an S-expression.  [Rules for combination of
        Authorizations are given below.]

    5.  Validity dates: a not-before date and a not-after date, where
        "date" means date and time.  If the not-before date is missing
        from the source credential then minus infinity is assumed.  If
        the not-after date is missing then plus infinity is assumed.

6.2 4-tuple Defined

   A <name,key> certificate (such as X.509v1 or SDSI 1.0) carries no
   authorization field but does carry a name.  Since it is qualitatively
   different from an authorization certificate, a separate intermediate
   form is defined for it.

   The elements of a Name 4-tuple are:

    1.  Issuer: a public key (or its hash).  This identifies the entity
        defining this name in its private name space.

    2.  Name: a byte string

    3.  Subject: a public key (or its hash), a name, or a threshold
        function of subordinate subjects.  This defines the name.

    4.  Validity dates: a not-before date and a not-after date, where
        "date" means date and time.  If the not-before date is missing
        from the source credential then minus infinity is assumed.  If
        the not-after date is missing then plus infinity is assumed.

6.3 5-tuple Reduction Rules

   The two 5-tuples:

      <I1,S1,D1,A1,V1> + <I2,S2,D2,A2,V2>

   yield

         <I1,S2,D2,AIntersect(A1,A2),VIntersect(V1,V2)>

   provided

       the two intersections succeed,

       S1 = I2

   and

       D1 = TRUE

If S1 is a threshold subject, there is a slight modification to this
rule, as described below in section 6.3.3.

6.3.1 AIntersect

   An authorization is a list of strings or sub-lists, of meaning to and
   probably defined by the application that will use this authorization
   for access control.  Two authorizations intersect by matching,
   element for element.  If one list is longer than the other but match
   at all elements where both lists have elements, then the longer list
   is the result of the intersection.  This means that additional
   elements of a list must restrict the permission granted.

   Although actual authorization string definitions are application
   dependent, AIntersect provides rules for automatic intersection of
   these strings so that application developers can know the semantics
   of the strings they use.  Special semantics would require special
   reduction software.

   For example, there might be an ftpd that allows public key access
   control, using authorization certificates.  Under that service,

       (ftp (host ftp.clark.net))

   might imply that the keyholder would be allowed ftp access to all
   directories on ftp.clark.net, with all kinds of access (read, write,
   delete, ...).  This is more general (allows more access) than

       (ftp (host ftp.clark.net) (dir /pub/cme))

   which would allow all kinds of access but only in the directory
   specified.  The intersection of the two would be the second.

   Since the AIntersect rules imply position dependency, one could also
   define the previous authorization string as:

       (ftp ftp.clark.net /pub/cme)

   to keep the form compact.

   To allow for wild cards, there are a small number of special S-
   expressions defined, using "*" as the expression name.

   (*)
             stands for the set of all S-expressions and byte-strings.
             In other words, it will match anything.  When intersected
             with anything, the result is that other thing.  [The
             AIntersect rule about lists of different length treats a

             list as if it had enough (*) entries implicitly appended to
             it to match the length of another list with which it was
             being intersected.]

   (* set <tag-expr>*)
             stands for the set of elements listed in the *-form.

   (* prefix <byte-string>)
             stands for the set of all byte strings that start with the
             one given in the *-form.

   (* range <ordering> <lower-limit>? <upper-limit>?)
             stands for the set of all byte strings lexically (or
             numerically) between the two limits.  The ordering
             parameter (alpha, numeric, time, binary, date) specifies
             the kind of strings allowed.

   AIntersect() is normal set intersection, when *-forms are defined as
   they are above and a normal list is taken to mean all lists that
   start with those elements.  The following examples should give a more
   concrete explanation for those who prefer an explanation without
   reference to set operations.

   AIntersect( (tag (ftp ftp.clark.net cme (* set read write))),
               (tag (*)) )

   evaluates to (tag (ftp ftp.clark.net cme (* set read write)))

   AIntersect( (tag (* set read write (foo bla) delete)),
               (tag (* set write read) ) )

   evaluates to (tag (* set read write))

   AIntersect( (tag (* set read write (foo bla) delete)),
               (tag read ) )

   evaluates to (tag read)

   AIntersect( (tag (* prefix http://www.clark.net/pub/)),
               (tag (* prefix http://www.clark.net/pub/cme/html/)) )

   evaluates to (tag (* prefix http://www.clark.net/pub/cme/html/))

   AIntersect( (tag (* range numeric ge #30# le #39# )), (tag #26#) )

   fails to intersect.

6.3.2 VIntersect

   Date range intersection is straight-forward.

       V = VIntersect( X, Y )

   is defined as

       Vmin = max( Xmin, Ymin )

       Vmax = min( Xmax, Ymax )

   and if Vmin > Vmax, then the intersection failed.

   These rules assume that daytimes are expressed in a monotonic form,
   as they are in SPKI.

   The full SPKI VIntersect() also deals with online tests.  In the most
   straight-forward implementation, each online test to which a
   certificate is subject is evaluated.  Each such test carries with it
   a validity interval, in terms of dates.  That validity interval is
   intersected with any present in the certificate, to yield a new,
   current validity interval.

   It is possible for an implementation of VIntersect() to gather up
   online tests that are present in each certificate and include the
   union of all those tests in the accumulating tuples.  In this case,
   the evaluation of those online tests is deferred until the end of the
   process.  This might be appropriate if the tuple reduction is being
   performed not for answering an immediate authorization question but
   rather for generation of a summary certificate (Certificate Result
   Certificate) that one might hope would be useful for a long time.

6.3.3 Threshold Subjects

   A threshold subject is specified by two numbers, K and N [0<K<=N],
   and N subordinate subjects.  A threshold subject is reduced to a
   single subject by selecting K of the N subjects and reducing each of
   those K to the same subject, through a sequence of certificates.  The
   (N-K) unselected subordinate subjects are set to (null).

   The intermediate form for a threshold subject is a copy of the tuple
   in which the threshold subject appears, but with only one of the
   subordinate subjects.  Those subordinate tuples are reduced
   individually until the list of subordinate tuples has (N-K) (null)
   entries and K entries with the same subject.  At that point, those K
   tuples are validity-, authorization- and delegation- intersected to
   yield the single tuple that will replace the list of tuples.

6.3.4 Certificate Path Discovery

   All reduction operations are in the order provided by the prover.
   That simplifies the job of the verifier, but leaves the job of
   finding the correct list of reductions to the prover.

   The general algorithm for finding the right certificate paths from a
   large set of unordered certificates has been solved[ELIEN], but might
   be used only rarely.  Each keyholder who is granted some authority
   should receive a sequence of certificates delegating that authority.
   That keyholder may then want to delegate part of this authority on to
   some other keyholder.  To do that, a single additional certificate is
   generated and appended to the sequence already available, yielding a
   sequence that can be used by the delegatee to prove access
   permission.

6.4 4-tuple Reduction

   There will be name 4-tuples in two different classes, those that
   define the name as a key and those that define the name as another
   name.

    1.  [(name K1 N) -> K2]

    2.  [(name K1 N) -> (name K2 N1 N2 ... Nk)]

   As with the 5-tuples discussed in the previous section, name
   definition 4-tuples should be delivered in the order needed by the
   prover.  In that case, the rule for name reduction is to replace the
   name just defined by its definition.  For example,

        (name K1 N N1 N2 N3) + [(name K1 N) -> K2]

             -> (name K2 N1 N2 N3)

   or, in case 2 above,

        (name K1 N Na Nb Nc) + [(name K1 N) -> (name K2 N1 N2 ... Nk)]

             -> (name K2 N1 N2 ... Nk Na Nb Nc)

   With the second form of name definition, one might have names that
   temporarily grow.  If the prover is providing certificates in order,
   then the verifier need only do as it is told.

   If the verifier is operating from an unordered pool of tuples, then a
   safe rule for name reduction is to apply only those 4-tuples that
   define a name as a key.  Such applications should bring 4-tuples that
   started out in class (2) into class (1), and eventually reduce all
   names to keys.  Any naming loops are avoided by this process.

6.4.1 4-tuple Threshold Subject Reduction

   Some of a threshold subject's subordinate subjects might be names.
   Those names must be reduced by application of 4-tuples.  The name
   reduction process proceeds independently on each name in the
   subordinate subject as indicated in 6.3.3 above.

   One can reduce individual named subjects in a threshold subject and
   leave the subject in threshold form, if one desires.  There is no
   delegation- or authorization-intersection involved, only a validity-
   intersection during name reduction.  This might be used by a service
   that produces Certificate Result Certificates [see 6.7].

6.4.2 4-tuple Validity Intersection

   Whenever a 4-tuple is used to reduce the subject (or part of the
   subject) of another tuple, its validity interval is intersected with
   that of the tuple holding the subject being reduced and the
   intersection is used in the resulting tuple.  Since a 4-tuple
   contains no delegation or authorization fields, the delegation
   permission and authorization of the tuple being acted upon does not
   change.

6.5 Certificate Translation

   Any certificate currently defined, as well as ACL entries and
   possibly other instruments, can be translated to 5-tuples (or name
   tuples) and therefore take part in an authorization computation.  The
   specific rules for those are given below.

6.5.1 X.509v1

   The original X.509 certificate is a <name,key> certificate.  It
   translates directly to a name tuple.  The form

        [Kroot, (name <leaf-name>), K1, validity]

   is used if the rules for that particular X.509 hierarchy is that all
   leaf names are unique, under that root.  If uniqueness of names
   applies only to individual CAs in the X.509 hierarchy, then one must
   generate

        [Kroot, (name CA1 CA2 ... CAk <leaf-name>), K1, validity]

   after verifying the certificate chain by the rules appropriate to
   that particular chain.

6.5.2 PGP

   A PGP certificate is a <name,key> certificate.  It is verified by
   web-of-trust rules (as specified in the PGP documentation).  Once
   verified, it yields name tuples of the form

        [Ki, name, K1, validity]

   where Ki is a key that signed that PGP (UserID,key) pair.  There
   would be one tuple produced for each signature on the key, K1.

6.5.3 X.509v3

   An X.509v3 certificate may be used to declare a name.  It might also
   declare explicit authorizations, by way of extensions.  It might also
   declare an implicit authorization of the form (tag (*)).  The actual
   set of tuples it yields depends on the documentation associated with
   that line of certificates.  That documentation could conceptually be
   considered associated with the root key of the certificate chain.  In
   addition, some X.509v3 certificates (such as those used for SET),
   have defined extra validity tests for certificate chains depending on
   custom extensions.  As a result, it is likely that X.509v3 chains
   will have to be validated independently, by chain validation code
   specific to each root key.  After that validation, that root-specific
   code can then generate the appropriate multiple tuples from the one
   certificate.

6.5.4 X9.57

   An X9.57 attribute certificate should yield one or more 5-tuples,
   with names as Subject.  The code translating the attribute
   certificate will have to build a fully-qualified name to represent
   the Distinguished Name in the Subject.  For any attribute
   certificates that refer to an ID certificate explicitly, the Subject
   of the 5-tuple can be the key in that ID certificate, bypassing the
   construction of name 4-tuples.

6.5.5 SDSI 1.0

A SDSI 1.0 certificate maps directly to one 4-tuple.

6.5.6 SPKI

   An SPKI certificate maps directly to one 4- or 5- tuple, depending
   respectively on whether it is a name certificate or carries an
   authorization.

6.5.7 SSL

   An SSL certificate carries a number of authorizations, some
   implicitly.  The authorization:

        (tag (ssl))

   is implicit.  In addition, the server certificate carries a DNS name
   parameter to be matched against the DNS name of the web page to which
   the connection is being made.  That might be encoded as:

        (tag (dns <domain-name>))

   Meanwhile, there is the "global cert" permission -- the permission
   for a US-supplied browser to connect using full strength symmetric
   cryptography even though the server is outside the USA.  This might
   be encoded as:

        (tag (us-crypto))

   There are other key usage attributes that would also be encoded as
   tag fields, but a full discussion of those fields is left to the
   examples document.

   An ACL entry for an SSL root key would have the tag:

        (tag (* set (ssl) (dns (*))))

   which by the rules of intersection is equivalent to:

        (tag (* set (ssl) (dns)))

   unless that root key also had the permission from the US Commerce
   Department to grant us-crypto permission, in which case the root key
   would have:

        (tag (* set (ssl) (dns) (us-crypto)))

   A CA certificate, used for SSL, would then need only to communicate
   down its certificate chain those permissions allocated in the ACL.
   Its tag might then translate to:

        (tag (*))

   A leaf server certificate for the Datafellows server might, for
   example, have a tag field of the form:

        (tag (* set (ssl) (dns www.datafellows.com)))

   showing that it was empowered to do SSL and to operate from the given
   domain name, but not to use US crypto with a US browser.

   The use of (* set) for the two attributes in this example could have
   been abbreviated as:

        (tag (ssl www.datafellows.com))

   while CA certificates might carry:

        (tag (ssl (*))) or just (tag (*))

   but separating them, via (* set), allows for a future enhancement of
   SSL in which the (ssl) permission is derived from one set of root
   keys (those of current CAs) while the (dns) permission is derived
   from another set of root keys (those empowered to speak in DNSSEC)
   while the (us-crypto) permission might be granted only to a root key
   belonging to the US Department of Commerce.  The three separate tests
   in the verifying code (e.g., the browser) would then involve separate
   5-tuple reductions from separate root key ACL entries.

   The fact that these three kinds of permission are treated as if ANDed
   is derived from the logic of the code that interprets the permissions
   and is not expressed in the certificate.  That decision is embodied
   in the authorization code executed by the verifying application.

6.6 Certificate Result Certificates

   Typically, one will reduce a chain of certificates to answer an
   authorization question in one of two forms:

    1.  Is this Subject, S, allowed to do A, under this ACL and with
        this set of certificates?

    2.  What is Subject S allowed to do, under this ACL and with this
        set of certificates?

   The answer to the second computation can be put into a new
   certificate issued by the entity doing the computation.  That one
   certificate corresponds to the semantics of the underlying
   certificates and online test results.  We call it a Certificate
   Result Certificate.

7. Key Management

   Cryptographic keys have limited lifetimes.  Keys can be stolen.  Keys
   might also be discovered through cryptanalysis.  If the theft is
   noticed, then the key can be replaced as one would replace a credit
   card.  More likely, the theft will not be noticed.  To cover this
   case, keys are replaced routinely.

   The replacement of a key needs to be announced to those who would use
   the new key.  It also needs to be accomplished smoothly, with a
   minimum of hassle.

   Rather than define a mechanism for declaring a key to be bad or
   replaced, SPKI defines a mechanism for giving certificates limited
   lifetimes so that they can be replaced.  That is, under SPKI one does
   not declare a key to be bad but rather stops empowering it and
   instead empowers some other key.  This limitation of a certificate's
   lifetime might be by limited lifetime at time of issuance or might be
   via the lifetime acquired through an on-line test (CRL, revalidation
   or one-time).  Therefore, all key lifetime control becomes
   certificate lifetime control.

7.1 Through Inescapable Names

   If keyholders had inescapable names [see section 2.5, above], then
   one could refer to them by those names and define a certificate to
   map from an inescapable name to the person's current key.  That
   certificate could be issued by any CA, since all CAs would use the
   inescapable name for the keyholder.  The attribute certificates and
   ACLs that refer to the keyholder would all refer to this one
   inescapable name.

   However, there are no inescapable names for keyholders.  [See section
   2.5, above.]

7.2 Through a Naming Authority

   One could conceivably have a governmental body or other entity that
   would issue names voluntarily to a keyholder, strictly for the
   purpose of key management.  One would then receive all authorizations
   through that name.  There would have to be only one such authority,

   however.  Otherwise, names would have to be composed of parts: an
   authority name and the individual's name.  The authority name would,
   in turn, have to be granted by some single global authority.

   That authority then becomes able to create keys of its own and
   certificates to empower them as any individual, and through those
   false certificates acquire access rights of any individual in the
   world.  Such power is not likely to be tolerated.  Therefore, such a
   central authority is not likely to come to pass.

7.3 Through <name,key> Certificates

   Instead of inescapable names or single-root naming authorities, we
   have names assigned by some entity that issues a <name,key>
   certificate.  As noted in sections 2.8 and 2.9, above, such names
   have no meaning by themselves.  They must be fully qualified to have
   meaning.

   Therefore, in the construct:

        (name (hash sha1 |TLCgPLFlGTzgUbcaYLW8kGTEnUk=|) jim)

   the name is not

        "jim"

   but rather

        "(name (hash sha1 |TLCgPLFlGTzgUbcaYLW8kGTEnUk=|) jim)"

   This name includes a public key (through its hash, in the example
   above).  That key has a lifetime like any other key, so this name has
   not achieved the kind of permanence (free from key lifetimes) that an
   inescapable name has.  However, it appears to be our only
   alternative.

   This name could easily be issued by the named keyholder, for the
   purpose of key management only.  In that case, there is no concern
   about access control being subverted by some third-party naming
   authority.

7.4 Increasing Key Lifetimes

   By the logic above, any name will hang off some public key.  The job
   is then to increase the lifetime of that public key.  Once a key
   lifetime exceeds the expected lifetime of any authorization granted
   through it, then a succession of new, long-lifetime keys can cover a
   keyholder forever.

   For a key to have a long lifetime, it needs to be strong against
   cryptanalytic attack and against theft.  It should be used only on a
   trusted machine, running trusted software.  It should not be used on
   an on-line machine.  It should be used very rarely, so that the
   attacker has few opportunities to find the key in the clear where it
   can be stolen.

   Different entities will approach this set of requirements in
   different ways.  A private individual, making his own naming root key
   for this purpose, has the advantage of being too small to invite a
   well funded attack as compared to the attacks a commercial CA might
   face.

7.5 One Root Per Individual

   In the limit, one can have one highly protected naming root key for
   each individual.  One might have more than one such key per
   individual, in order to frustrate attempts to build dossiers, but let
   us assume only one key for the immediate discussion.

   If there is only one name descending from such a key, then one can
   dispense with the name.  Authorizations can be assigned to the key
   itself, in raw SPKI style, rather than to some name defined under
   that key.  There is no loss of lifetime -- only a change in the
   subject of the certificate the authorizing key uses to delegate
   authority.

   However, there is one significant difference, under the SPKI
   structure.  If one delegates some authorization to

        (name (hash sha1 |TLCgPLFlGTzgUbcaYLW8kGTEnUk=|) carl)

   and a different authorization to

        (hash sha1 |TLCgPLFlGTzgUbcaYLW8kGTEnUk=|)

   directly, both without granting the permission to delegate, that key
   can delegate at will through <name,key> certificates in the former
   case and not delegate at all in the latter case.

   In the case of key management, we desire the ability to delegate from
   a long lived, rarely used key to a shorter lived, often used key --
   so in this case, the former mechanism (through a SDSI name) gives
   more freedom.

7.6 Key Revocation Service

   In either of the models above, key |TLCgPLFlGTzgUbcaYLW8kGTEnUk=|
   will issue a certificate.  In the first model, it will be a
   <name,key> certificate.  In the second, it will be an authorization
   certificate delegating all rights through to the more temporary key.

   Either of those certificates might want an on-line validity test.
   Whether this test is in the form of a CRL, a re-validation or a one-
   time test, it will be supplied by some entity that is on-line.

   As the world moves to having all machines on-line all the time, this
   might be the user's machine.  However, until then -- and maybe even
   after then -- the user might want to hire some service to perform
   this function.  That service could run a 24x7 manned desk, to receive
   phone calls reporting loss of a key.  That authority would not have
   the power to generate a new key for the user, only to revoke a
   current one.

   If, in the worst case, a user loses his master key, then the same
   process that occurs today with lost wallets would apply.  All issuers
   of authorizations through that master key would need to issue new
   authorizations through the new master key and, if the old master key
   had been stolen, cancel all old authorizations through that key.

7.7 Threshold ACL Subjects

   One can take extraordinary measures to protect root keys and thus
   increase the lifetimes of those keys.  The study of computer fault-
   tolerance teaches us that truly long lifetimes can be achieved only
   by redundancy and replacement.  Both can be achieved by the use of
   threshold subjects [section 6.3.3], especially in ACL entries.

   If we use a threshold subject in place of a single key subject, in an
   ACL (or a certificate), then we achieve redundancy immediately.  This
   can be redundancy not only of keys but also of algorithms.  That is,
   the keys in a threshold subject do not need to have the same
   algorithm.

   Truly long lifetimes come from replacement, not just redundancy.  As
   soon as a component fails (or a key is assumed compromised), it must
   be replaced.

   An ACL needs to be access-controlled itself.  Assume that the ACL
   includes an entry with authorization

       (tag (acl-edit))

   Assume also that what might have been a single root authorization
   key, K1, is actually a threshold subject

       (k-of-n #03# #07# K1 K2 K3 K4 K5 K6 K7)

   used in any ACL entry granting a normal authorization.

   That same ACL could have the subject of an (acl-edit) entry be

       (k-of-n #05# #07# K1 K2 K3 K4 K5 K6 K7)

   This use of threshold subject would allow the set of root keys to
   elect new members to that set and retire old members.  In this
   manner, replacement is achieved alongside redundancy and the proper
   choice of K and N should allow threshold subject key lifetimes
   approaching infinity.

8. Security Considerations

   There are three classes of information that can be bound together by
   public key certificates: key, name and authorization.  There are
   therefore three general kinds of certificate, depending on what pair
   of items the certificate ties together.  If one considers the
   direction of mapping between items, there are six classes: name->key,
   key->name, authorization->name, name->authorization, authorization-
   >key, key->authorization.

   The SPKI working group concluded that the most important use for
   certificates was access control.  Given the various kinds of mapping
   possible, there are at least two ways to implement access control.
   One can use a straight authorization certificate:

       (authorization->key)

   or one can use an attribute certificate and an ID certificate:

       (authorization->name) + (name->key)

   There are at least two ways in which the former is more secure than
   the latter.

    1.  Each certificate has an issuer.  If that issuer is subverted,
        then the attacker can gain access.  In the former case, there is
        only one issuer to trust.  In the latter case, there are two.

    2.  In the second case, linkage between the certificates is by name.
        If the name space of the issuer of the ID certificate is
        different from the name space of the issuer of the attribute

        certificate, then one of the two issuers must use a foreign name
        space.  The process of choosing the appropriate name from a
        foreign name space is more complex than string matching and
        might even involve a human guess.  It is subject to mistakes.
        Such a mistake can be made by accident or be guided by an
        attacker.

   This is not to say that one must never use the second construct.  If
   the two certificates come from the same issuer, and therefore with
   the same name space, then both of the security differentiators above
   are canceled.

References

   [Ab97]       Abadi, Martin, "On SDSI's Linked Local Name Spaces",
                Proceedings of the 10th IEEE Computer Security
                Foundations Workshop (June 1997).

   [BFL]        Matt Blaze, Joan Feigenbaum and Jack Lacy, "Distributed
                Trust Management", Proceedings 1996 IEEE Symposium on
                Security and Privacy.

   [CHAUM]      D. Chaum, "Blind Signatures for Untraceable Payments",
                Advances in Cryptology -- CRYPTO '82, 1983.

   [DH]         Whitfield Diffie and Martin Hellman, "New Directions in
                Cryptography", IEEE Transactions on Information Theory,
                November 1976, pp. 644-654.

   [DvH]        J. B. Dennis and E. C. Van Horn, "Programming Semantics
                for Multiprogrammed Computations", Communications of the
                ACM 9(3), March 1966.

   [ECR]        Silvio Micali, "Efficient Certificate Revocation",
                manuscript, MIT LCS.

   [ELIEN]      Jean-Emile Elien, "Certificate Discovery Using SPKI/SDSI
                2.0 Certificates", Masters Thesis, MIT LCS, May 1998,
                <http://theory.lcs.mit.edu/~cis/theses/elien-masters.ps>
                [also .pdf and

   [HARDY]      Hardy, Norman, "THE KeyKOS Architecture", Operating
                Systems Review, v.19 n.4, October 1985. pp 8-25.

   [IDENT]      Carl Ellison, "Establishing Identity Without
                Certification Authorities", USENIX Security Symposium,
                July 1996.

   [IWG]        McConnell and Appel, "Enabling Privacy, Commerce,
                Security and Public Safety in the Global Information
                Infrastructure", report of the Interagency Working Group
                on Cryptography Policy, May 12, 1996; (quote from
                paragraph 5 of the Introduction).

   [KEYKOS]     Bomberger, Alan, et al., "The KeyKOS(r) Nanokernel
                Architecture", Proceedings of the USENIX Workshop on
                Micro-Kernels and Other Kernel Architectures, USENIX
                Association, April 1992. pp 95-112 (In addition, there
                are KeyKOS papers on the net available through
                <http://www.cis.upenn.edu/~KeyKOS/#bibliography>).

   [KOHNFELDER] Kohnfelder, Loren M., "Towards a Practical Public-key
                Cryptosystem", MIT S.B. Thesis, May. 1978.

   [LAMPSON]    B. Lampson, M. Abadi, M. Burrows, and E. Wobber,
                "Authentication in distributed systems: Theory and
                practice", ACM Trans. Computer Systems 10, 4 (Nov.
                1992), pp 265-310.

   [LANDAU]     Landau, Charles, "Security in a Secure Capability-Based
                System", Operating Systems Review, Oct 1989 pp 2-4.

   [LEVY]       Henry M. Levy, "Capability-Based Computer Systems",
                Digital Press, 12 Crosby Dr., Bedford MA 01730, 1984.

   [LINDEN]     T. A. Linden, "Operating System Structures to Support
                Security and Reliable Software", Computing Surveys 8(4),
                December 1976.

   [PKCS1]      PKCS #1: RSA Encryption Standard, RSA Data Security,
                Inc., 3 June 1991, Version 1.4.

   [PKLOGIN]    David Kemp, "The Public Key Login Protocol", Work in
                Progress.

   [R98]        R. Rivest, "Can We Eliminate Revocation Lists?", to
                appear in the Proceedings of Financial Cryptography
                1998, <http://theory.lcs.mit.edu/~rivest/revocation.ps>.

   [RFC1114]    Kent, S. and  J. Linn, "Privacy Enhancement for Internet
                Electronic Mail: Part II -- Certificate-Based Key
                Management", RFC 1114, August 1989.

   [RFC1321]    Rivest, R., "The MD5 Message-Digest Algorithm", RFC
                1321, April 1992.

   [RFC2045]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", RFC 2045, December 1996.

   [RFC2046]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part Two: Media Types", RFC 2046,
                December 1996.

   [RFC2047]    K. Moore, "MIME (Multipurpose Internet Mail Extensions)
                Part Three: Message Header Extensions for Non-ASCII
                Text", RFC 2047, December 1996.

   [RFC2065]    Eastlake, D. and C. Kaufman, "Proposed Standard for DNS
                Security", RFC 2065, January 1997.

   [RFC2104]    Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                Keyed-Hashing for Message Authentication", RFC 2104,
                February 1997.

   [SDSI]       Ron Rivest and Butler Lampson, "SDSI - A Simple
                Distributed Security Infrastructure [SDSI]",
                <http://theory.lcs.mit.edu/~cis/sdsi.html>.

   [SET]        Secure Electronic Transactions -- a protocol designed by
                VISA, MasterCard and others, including a certificate
                structure covering all participants.  See
                <http://www.visa.com/>.

   [SEXP]       Ron Rivest, code and description of S-expressions,
                <http://theory.lcs.mit.edu/~rivest/sexp.html>.

   [SRC-070]    Abadi, Burrows, Lampson and Plotkin, "A Calculus for
                Access Control in Distributed Systems", DEC SRC-070,
                revised August 28, 1991.

   [UPKI]       C. Ellison, "The nature of a useable PKI", Computer
                Networks 31 (1999) pp. 823-830.

   [WEBSTER]    "Webster's Ninth New Collegiate Dictionary", Merriam-
                Webster, Inc., 1991.

Acknowledgments

   Several independent contributions, published elsewhere on the net or
   in print, worked in synergy with our effort.  Especially important to
   our work were: [SDSI], [BFL] and [RFC2065].  The inspiration we
   received from the notion of CAPABILITY in its various forms (SDS-940,
   Kerberos, DEC DSSA, [SRC-070], KeyKOS [HARDY]) can not be over-rated.

   Significant contributions to this effort by the members of the SPKI
   mailing list and especially the following persons (listed in
   alphabetic order) are gratefully acknowledged: Steve Bellovin, Mark
   Feldman, John Gilmore, Phill Hallam-Baker, Bob Jueneman, David Kemp,
   Angelos D. Keromytis, Paul Lambert, Jon Lasser, Jeff Parrett, Bill
   Sommerfeld, Simon Spero.

Authors' Addresses

   Carl M. Ellison
   Intel Corporation
   2111 NE 25th Ave  M/S JF3-212
   Hillsboro OR 97124-5961 USA

   Phone: +1-503-264-2900
   Fax:   +1-503-264-6225
   EMail: carl.m.ellison@intel.com
          cme@alum.mit.edu
   Web:   http://www.pobox.com/~cme

   Bill Frantz
   Electric Communities
   10101 De Anza Blvd.
   Cupertino CA 95014

   Phone: +1 408-342-9576
   EMail: frantz@netcom.com

   Butler Lampson
   Microsoft
   180 Lake View Ave
   Cambridge MA 02138

   Phone: +1 617-547-9580 (voice + FAX)
   EMail: blampson@microsoft.com

   Ron Rivest
   Room 324, MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge MA 02139

   Phone: +1-617-253-5880
   Fax:   +1-617-258-9738
   EMail: rivest@theory.lcs.mit.edu
   Web:   http://theory.lcs.mit.edu/~rivest

   Brian Thomas
   Southwestern Bell
   One Bell Center, Room 34G3
   St. Louis MO 63101 USA

   Phone: +1 314-235-3141
   Fax:   +1 314-235-0162
   EMail: bt0008@sbc.com

   Tatu Ylonen
   SSH Communications Security Ltd.
   Tekniikantie 12
   FIN-02150 ESPOO
   Finland

   EMail: ylo@ssh.fi

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