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
Copyright (C) The Internet Society (1999). All Rights Reserved.
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.
Table of Contents
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", RFC1114, 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", RFC2045, December 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC2046,
December 1996.
[RFC2047] K. Moore, "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII
Text", RFC2047, December 1996.
[RFC2065] Eastlake, D. and C. Kaufman, "Proposed Standard for DNS
Security", RFC2065, January 1997.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC2104,
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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