SPKI Certificate Theory Carl M. Ellison
INTERNET-DRAFT Intel
Expires: 29 April 1999
Bill Frantz
Electric Communities
Butler Lampson
Microsoft
Ron Rivest
MIT Laboratory for Computer Science
Brian M. Thomas
Southwestern Bell
Tatu Ylonen
SSH
24 October 1998
SPKI Certificate Theory
---- ----------- ------
<draft-ietf-spki-cert-theory-03.txt>
Status of This Document
This draft supersedes the draft filed under the name draft-ietf-spki-
cert-theory-02.txt. It has been completely rewritten from that
previous version.
This document is one of four. SPKI structure definitions are to be
found in draft-ietf-spki-cert-structure-*.txt and examples of
certificate uses are to be found in draft-ietf-spki-cert-
examples-*.txt. This work derives from the requirements gathered at
the beginning of the working group and documented in draft-ietf-spki-
cert-req-*.txt.
Distribution of this document is unlimited. Comments should be sent
to the SPKI (Simple Public Key Infrastructure) Working Group mailing
list <spki@c2.net> or to the authors.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted by
Ellison, et al. [Page 1]
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other documents at any time. It is not appropriate to use Internet-
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net (East USA), ftp.isi.edu (West USA),
nic.nordu.net (North Europe), ftp.nis.garr.it (South Europe),
munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa).
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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.
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Table of Contents
Status of This Document....................................1
Abstract...................................................3
Table of Contents..........................................4
1. Overview of Contents....................................6
1.1 Glossary...............................................6
2. Name Certification......................................8
2.1 First Definition of CERTIFICATE........................8
2.2 The X.500 Plan and X.509...............................9
2.3 X.509, PEM and PGP.....................................9
2.4 Rethinking Global Names...............................10
2.5 Inescapable Identifiers...............................11
2.6 Local Names...........................................12
2.6.1 Basic SDSI Names....................................12
2.6.2 Compound SDSI Names.................................13
2.7 Sources of Global Identifiers.........................13
2.8 Fully Qualified SDSI Names............................14
2.9 Fully Qualified X.509 Names...........................14
2.10 Group Names..........................................15
3. Authorization..........................................16
3.1 Attribute Certificates................................16
3.2 X.509v3 Extensions....................................16
3.3 SPKI Certificates.....................................17
3.4 ACL Entries...........................................18
4. Delegation.............................................19
4.1 Depth of Delegation...................................19
4.1.1 No control..........................................19
4.1.2 Boolean control.....................................19
4.1.3 Integer control.....................................20
4.1.4 The choice: boolean.................................20
4.2 May a Delegator Also Exercise the Permission?.........20
4.3 Delegation of Authorization vs. ACLs..................20
5. Validity Conditions....................................22
5.1 Anti-matter CRLs......................................22
5.2 Timed CRLs............................................23
5.3 Timed Revalidations...................................23
5.4 Setting the Validity Interval.........................23
5.5 One-time Revalidations................................24
5.6 Short-lived Certificates..............................24
5.7 Other possibilities...................................24
5.7.1 Micali's Inexpensive On-line Results................25
5.7.2 Rivest's Reversal of the CRL Logic..................25
6. Tuple Reduction........................................26
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6.1 5-tuple Defined.......................................27
6.2 4-tuple Defined.......................................27
6.3 5-tuple Reduction Rules...............................28
6.3.1 AIntersect..........................................28
6.3.2 VIntersect..........................................30
6.3.3 Threshold Subjects..................................30
6.3.4 Reduction Direction.................................31
6.3.5 5-tuple Reduction Infinite Loops....................32
6.4 4-tuple Reduction.....................................32
6.5 Certificate Translation...............................33
6.5.1 X.509v1.............................................33
6.5.2 PGP.................................................34
6.5.3 X.509v3.............................................34
6.5.4 X9.57...............................................34
6.5.5 SDSI................................................34
6.5.6 SPKI................................................35
6.6 Implicit Authorizations...............................35
7. Key Management.........................................36
7.1 Through Inescapable Names.............................36
7.2 Through a Naming Authority............................36
7.3 Through <name,key> Certificates.......................37
7.4 Increasing Key Lifetimes..............................37
7.5 One Root Per Individual...............................38
7.6 Key Revocation Service................................38
8. Security Considerations................................40
References................................................41
Acknowledgments...........................................44
Authors' Addresses........................................44
Expiration and File Name..................................45
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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, listing contributors primarily at the
start of the working group. This section has not been augmented in
many months. The archive of working group mail is a more accurate
source of such 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: 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 that power (since it
theoretically has the owner of the resource being controlled 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
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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 passing 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".
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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.
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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.
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.
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. 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
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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.
Therefore, it was natural for people to translate the need to know
who the keyholder was 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, it is as an index into
some database (or human memory) of that other information.
The assumption that names are valid identifiers remains true in much
of daily life, but 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
better when addressed by name, but they are almost never globally
unique. Therefore 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. In that case, a globally unique name would not
succeed in accessing information about a person encountered for the
first time on the net because there is no physical-world relationship
giving information to be accessed and there is no digital
relationship yet established through which cyberspace information can
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be gathered.
One might remedy this situation by assigning everyone a globally
unique and unchangeable name and then using that name to index a
global database of facts about the person. This might bring us back
to the mode of operation we were accustomed to in small towns.
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.
Perhaps the best globally unique identifier 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.
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Such an inescapable identifier would be a great boon to a police
state and for that reason alone one can expect it to be defeated
politically in one way or another. In the US, this falls under the
category of a national ID card or national ID number and has been
strongly opposed politically. In some countries that do have
national ID numbers, there are laws against being required to
disclose those numbers except for official government purposes. In
those countries, such a number would uniquely identify a person but
not become universally used. In particular, it would not be
available for use in 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. 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 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.
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,
(name fred) // george
is a basic SDSI name, where the comment indicates that the name is in
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the name space defined by george.
2.6.2 Compound SDSI Names
If fred in turn defines a name, for example,
(name sam) // fred
then one can refer to this same entity as
(name fred sam) // george
relative to george's name space (assuming the definition in 2.6.1).
2.7 Sources of Global Identifiers
Even though humans use local names, computer systems often need
globally unique identifiers.
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.
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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 relative 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 a single root key 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>).
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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.
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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 some 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.
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:
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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.
As Steve Kent has observed, this aggregation of attributes would also
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
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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.
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 however 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.
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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.
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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 private key, 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.
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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.
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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.
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.
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 whether the CRL was more recent than the one currently on
the virtual checkout stand. 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 copy of itself at the verifier it has
encountered.
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.
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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 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.
5.3 Timed Revalidations
CRLs are a negative statement. The positive version of this
statement is one 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.
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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.
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
If validity intervals are greater than zero, and if the on-line test
result is signed by the same key as the issuer of the certificate,
then it makes sense to issue short-lived certificates instead of a
long-lived certificate and a short-lived on-line test result. The
issuer ends up generating fewer signatures this way.
An on-line test result message might be smaller than a full
certificate, so communication costs might still encourage the
separation of these functions, but this is a decision that must be
made on a per-application and per-issuer basis. The decision will
almost certainly hinge on performance, since all of these options
have equivalent security semantics.
5.7 Other possibilities
There are other possibilities to be considered when choosing a
validity condition model to use.
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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.
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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).
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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:
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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,
I1 = S2
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
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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 and below 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.
(* 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.
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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 write) )
evaluates to (tag write)
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.
6.3.3 Threshold Subjects
Given the 5-tuple:
<I1,S1,D1,A1,V1>
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if S1 is a threshold subject, then each individual subject in that
threshold list is a candidate for reduction. That is, for example,
<I1,(k-of-n #2# #3# S11 S12 S13),D1,A1,V1> + <I2,S2,D2,A2,V2>
would map to
<I1,(k-of-n #2# #3# S2 S12
S13),D3,AIntersect(A1,A2),VIntersect(V1,V2)>
if I2 = S11.
The delegation field is temporarily ternary, with the following logic
table:
D1 D2
TRUE FALSE
+------+------+
TRUE | TRUE | MID |
+------+------+
MID | MID | MID |
+------+------+
FALSE | STOP | STOP |
+------+------+
That is, if D1 = FALSE, the reduction stops and there is no D3 value.
D3 takes on the value MID if it is intended to become FALSE, once the
threshold subject has been reduced.
Whenever the k-of-n list has K subjects the same, the k-of-n
structure is replaced by that single subject. This applies to nested
threshold subjects as well as to single-level. Once the subject of
the tuple has no thresholds, then the delegation field is mapped back
to a boolean value by mapping MID to FALSE.
6.3.4 Reduction Direction
It is anticipated that most if not all reduction operations will be
in the order provided by the prover. However, the rules above are
defined as if one were dealing with a pool of tuples and wanted to
reduce that pool. That mode of operation is available to the
developer if it is seen as desirable for some reason.
The tuple reduction rules are independent of direction of operation.
One can start at either end of a chain of 5-tuples. However, it is
likely that the right side of that chain will be free from *-forms.
Since the intersection of a *-form with something free from *-forms
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is likely to be free from *-forms, the reduction from right to left
may be easier. However, this is a question left to the individual
developer to resolve according to taste.
6.3.5 5-tuple Reduction Infinite Loops
It is possible for someone to define loops of certificates.
If one is processing certificates in order, as provided by a caller,
then the caller will have avoided any loop and presented a finite
amount of work for the verifier to do. This is likely to be the most
common case.
The prover (caller) needs to present a string of certificates in the
proper order, if the verifier expects that. The prover might find
that string by examining an unordered pool of certificates, but it is
more likely that the prover will have been handed a certificate
string when granted the permission to do something and will merely
hand that string along to the verifier. If the prover delegates some
permission, then it appends the new delegation certificate (and any
name resolution certificates) to the string it had been handed, and
passes that new string along to the entity to whom permission was
being delegated.
If one is processing certificates from an unordered pool, then one
can work right to left, and start with a single 5-tuple (the desired
end result) as the right hand element of a pairwise reduction. The
active set of right hand elements would then acquire reduction
results and one could reduce the unordered set by choosing one tuple
in the active set as the right element and one tuple from the
inactive set as the left element. Each new element in the active set
would be tested against the inactive set only once. Therefore, the
process would terminate even if there were certification loops
present in the set.
There are other possible algorithms for breaking such loops. As was
mentioned elsewhere, the actual algorithm for tuple reduction is a
matter for individual developers to decide.
6.4 4-tuple Reduction
There will be name 4-tuples in two different classes:
1. [(name K1 N) -> K2]
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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)
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 tuples in class
(1). Such applications should bring 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.
6.5 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]
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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 each key that signed that PGP (UserID,key) pair.
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 tuples.
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
A SDSI certificate maps directly to one 4-tuple.
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6.5.6 SPKI
An SPKI certificate maps directly to one 5-tuple.
6.6 Implicit Authorizations
Some certificates in use today carry no authorization fields.
Perhaps the most commonly known example is an SSL server certificate.
It has the form of <name,key> certificate but is used to authorize
the establishment of an SSL connection while the name it carries is
not used in the authorization computation.
For this to work, the root key from which such a certificate descends
is in an ACL carrying that authorization (again, implicitly) and all
of the certificates in the chain are interpreted as having an
authorization field of
(tag (*))
This interpretation of that certificate is a function of the root key
and the software that interprets the certificate chain, in current
implementations. When such certificates are translated to 5-tuples,
they must generate authorization tuples rather than name tuples.
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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.
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. There are
not likely ever to be, considering the political dangers of such
names.
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.
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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.
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
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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 will probably have more than one such key per
individual, but let us assume only one 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 key 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.
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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.
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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 there is an increased chance of mistake in
choosing the proper name for the attribute certificate. Such a
mistake can be made by accident or 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.
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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.
[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
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INTERNET-DRAFT SPKI Certificate Theory 24 October 1998
[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", working draft,
06/12/1996.
[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] S. Kent, J. Linn, "Privacy Enhancement for Internet
Electronic Mail: Part II -- Certificate-Based Key Management", August
1989.
[RFC1321] R. Rivest, "The MD5 Message-Digest Algorithm", April 16
1992.
[RFC2045] N. Freed and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies", Dec 2
1996.
[RFC2046] N. Freed and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", Dec 2 1996.
[RFC2047] K. Moore, "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text", Dec 2
1996.
[RFC2065] D. Eastlake and C. Kaufman, "Proposed Standard for DNS
Security", Jan 1997.
[RFC2104] H. Krawczyk, M. Bellare and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", Feb 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,
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INTERNET-DRAFT SPKI Certificate Theory 24 October 1998
<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.
[WEBSTER] "Webster's Ninth New Collegiate Dictionary", Merriam-
Webster, Inc., 1991.
Ellison, et al. [Page 43]
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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 JF3-373
Hillsboro OR 97124-5961 USA
Telephone: +1-503-264-2900
FAX: +1-503-264-6225
EMail: carl.m.ellison@intel.com (work, Outlook)
cme@jf.intel.com (work, Eudora)
cme@alum.mit.edu, cme@acm.org (home, Eudora)
Web: http://www.pobox.com/~cme
Bill Frantz
Electric Communities
10101 De Anza Blvd.
Cupertino CA 95014
Telephone: +1 408-342-9576
Email: frantz@netcom.com
Butler Lampson
Microsoft
180 Lake View Ave
Cambridge MA 02138
Telephone: +1 617-547-9580 (voice + FAX)
EMail: blampson@microsoft.com
Ellison, et al. [Page 44]
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Ron Rivest
Room 324, MIT Laboratory for Computer Science
545 Technology Square
Cambridge MA 02139
Telephone: +1-617-253-5880
+1-617-258-9738(FAX)
Email: rivest@theory.lcs.mit.edu
Web: http://theory.lcs.mit.edu/~rivest
Brian Thomas
Southwestern Bell
One Bell Center, Room 23Q1
St. Louis MO 63101 USA
Telephone: +1 314-235-3141
+1 314-331-2755(FAX)
EMail: bt0008@entropy.sbc.com
Tatu Ylonen
SSH Communications Security Ltd.
Tekniikantie 12
FIN-02150 ESPOO
Finland
E-mail: ylo@ssh.fi
Expiration and File Name
This draft expires 29 April 1999.
Its file name is draft-ietf-spki-cert-theory-03.txt
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