RFC 2704
Network Working Group M. Blaze
Request for Comments: 2704 J. Feigenbaum
Category: Informational J. Ioannidis
AT&T Labs - Research
A. Keromytis
U. of Pennsylvania
September 1999
The KeyNote Trust-Management System Version 2
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This memo describes version 2 of the KeyNote trust-management system.
It specifies the syntax and semantics of KeyNote `assertions',
describes `action attribute' processing, and outlines the application
architecture into which a KeyNote implementation can be fit. The
KeyNote architecture and language are useful as building blocks for
the trust management aspects of a variety of Internet protocols and
services.
1. Introduction
Trust management, introduced in the PolicyMaker system [BFL96], is a
unified approach to specifying and interpreting security policies,
credentials, and relationships; it allows direct authorization of
security-critical actions. A trust-management system provides
standard, general-purpose mechanisms for specifying application
security policies and credentials. Trust-management credentials
describe a specific delegation of trust and subsume the role of
public key certificates; unlike traditional certificates, which bind
keys to names, credentials can bind keys directly to the
authorization to perform specific tasks.
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A trust-management system has five basic components:
* A language for describing `actions', which are operations with
security consequences that are to be controlled by the system.
* A mechanism for identifying `principals', which are entities that
can be authorized to perform actions.
* A language for specifying application `policies', which govern the
actions that principals are authorized to perform.
* A language for specifying `credentials', which allow principals to
delegate authorization to other principals.
* A `compliance checker', which provides a service to applications
for determining how an action requested by principals should be
handled, given a policy and a set of credentials.
The trust-management approach has a number of advantages over other
mechanisms for specifying and controlling authorization, especially
when security policy is distributed over a network or is otherwise
decentralized.
Trust management unifies the notions of security policy, credentials,
access control, and authorization. An application that uses a
trust-management system can simply ask the compliance checker whether
a requested action should be allowed. Furthermore, policies and
credentials are written in standard languages that are shared by all
trust-managed applications; the security configuration mechanism for
one application carries exactly the same syntactic and semantic
structure as that of another, even when the semantics of the
applications themselves are quite different.
Trust-management policies are easy to distribute across networks,
helping to avoid the need for application-specific distributed policy
configuration mechanisms, access control lists, and certificate
parsers and interpreters.
For a general discussion of the use of trust management in
distributed system security, see [Bla99].
KeyNote is a simple and flexible trust-management system designed to
work well for a variety of large- and small-scale Internet-based
applications. It provides a single, unified language for both local
policies and credentials. KeyNote policies and credentials, called
`assertions', contain predicates that describe the trusted actions
permitted by the holders of specific public keys. KeyNote assertions
are essentially small, highly-structured programs. A signed
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assertion, which can be sent over an untrusted network, is also
called a `credential assertion'. Credential assertions, which also
serve the role of certificates, have the same syntax as policy
assertions but are also signed by the principal delegating the trust.
In KeyNote:
* Actions are specified as a collection of name-value pairs.
* Principal names can be any convenient string and can directly
represent cryptographic public keys.
* The same language is used for both policies and credentials.
* The policy and credential language is concise, highly expressive,
human readable and writable, and compatible with a variety of
storage and transmission media, including electronic mail.
* The compliance checker returns an application-configured `policy
compliance value' that describes how a request should be handled
by the application. Policy compliance values are always
positively derived from policy and credentials, facilitating
analysis of KeyNote-based systems.
* Compliance checking is efficient enough for high-performance and
real-time applications.
This document describes the KeyNote policy and credential assertion
language, the structure of KeyNote action descriptions, and the
KeyNote model of computation.
We assume that applications communicate with a locally trusted
KeyNote compliance checker via a `function call' style interface,
sending a collection of KeyNote policy and credential assertions plus
an action description as input and accepting the resulting policy
compliance value as output. However, the requirements of different
applications, hosts, and environments may give rise to a variety of
different interfaces to KeyNote compliance checkers; this document
does not aim to specify a complete compliance checker API.
2. KeyNote Concepts
In KeyNote, the authority to perform trusted actions is associated
with one or more `principals'. A principal may be a physical entity,
a process in an operating system, a public key, or any other
convenient abstraction. KeyNote principals are identified by a
string called a `Principal Identifier'. In some cases, a Principal
Identifier will contain a cryptographic key interpreted by the
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KeyNote system (e.g., for credential signature verification). In
other cases, Principal Identifiers may have a structure that is
opaque to KeyNote.
Principals perform two functions of concern to KeyNote: They request
`actions' and they issue `assertions'. Actions are any trusted
operations that an application places under KeyNote control.
Assertions delegate the authorization to perform actions to other
principals.
Actions are described to the KeyNote compliance checker in terms of a
collection of name-value pairs called an `action attribute set'. The
action attribute set is created by the invoking application. Its
structure and format are described in detail in Section 3 of this
document.
KeyNote provides advice to applications about the interpretation of
policy with regard to specific requested actions. Applications
invoke the KeyNote compliance checker by issuing a `query' containing
a proposed action attribute set and identifying the principal(s)
requesting it. The KeyNote system determines and returns an
appropriate `policy compliance value' from an ordered set of possible
responses.
The policy compliance value returned from a KeyNote query advises the
application how to process the requested action. In the simplest
case, the compliance value is Boolean (e.g., "reject" or "approve").
Assertions can also be written to select from a range of possible
compliance values, when appropriate for the application (e.g., "no
access", "restricted access", "full access"). Applications can
configure the relative ordering (from `weakest' to `strongest') of
compliance values at query time.
Assertions are the basic programming unit for specifying policy and
delegating authority. Assertions describe the conditions under which
a principal authorizes actions requested by other principals. An
assertion identifies the principal that made it, which other
principals are being authorized, and the conditions under which the
authorization applies. The syntax of assertions is given in Section
4.
A special principal, whose identifier is "POLICY", provides the root
of trust in KeyNote. "POLICY" is therefore considered to be
authorized to perform any action.
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Assertions issued by the "POLICY" principal are called `policy
assertions' and are used to delegate authority to otherwise untrusted
principals. The KeyNote security policy of an application consists
of a collection of policy assertions.
When a principal is identified by a public key, it can digitally sign
assertions and distribute them over untrusted networks for use by
other KeyNote compliance checkers. These signed assertions are also
called `credentials', and serve a role similar to that of traditional
public key certificates. Policies and credentials share the same
syntax and are evaluated according to the same semantics. A
principal can therefore convert its policy assertions into
credentials simply by digitally signing them.
KeyNote is designed to encourage the creation of human-readable
policies and credentials that are amenable to transmission and
storage over a variety of media. Its assertion syntax is inspired by
the format of RFC822-style message headers [Cro82]. A KeyNote
assertion contains a sequence of sections, called `fields', each of
which specifies one aspect of the assertion's semantics. Fields
start with an identifier at the beginning of a line and continue
until the next field is encountered. For example:
KeyNote-Version: 2
Comment: A simple, if contrived, email certificate for user mab
Local-Constants: ATT_CA_key = "RSA:acdfa1df1011bbac"
mab_key = "DSA:deadbeefcafe001a"
Authorizer: ATT_CA_key
Licensees: mab_key
Conditions: ((app_domain == "email") # valid for email only
&& (address == "mab@research.att.com"));
Signature: "RSA-SHA1:f00f2244"
The meanings of the various sections are described in Sections 4 and
5 of this document.
KeyNote semantics resolve the relationship between an application's
policy and actions requested by other principals, as supported by
credentials. The KeyNote compliance checker processes the assertions
against the action attribute set to determine the policy compliance
value of a requested action. These semantics are defined in Section
5.
An important principle in KeyNote's design is `assertion
monotonicity'; the policy compliance value of an action is always
positively derived from assertions made by trusted principals.
Removing an assertion never results in increasing the compliance
value returned by KeyNote for a given query. The monotonicity
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property can simplify the design and analysis of complex network-
based security protocols; network failures that prevent the
transmission of credentials can never result in spurious
authorization of dangerous actions. A detailed discussion of
monotonicity and safety in trust management can be found in [BFL96]
and [BFS98].
3. Action Attributes
Trusted actions to be evaluated by KeyNote are described by a
collection of name-value pairs called the `action attribute set'.
Action attributes are the mechanism by which applications communicate
requests to KeyNote and are the primary objects on which KeyNote
assertions operate. An action attribute set is passed to the KeyNote
compliance checker with each query.
Each action attribute consists of a name and a value. The semantics
of the names and values are not interpreted by KeyNote itself; they
vary from application to application and must be agreed upon by the
writers of applications and the writers of the policies and
credentials that will be used by them.
Action attribute names and values are represented by arbitrary-length
strings. KeyNote guarantees support of attribute names and values up
to 2048 characters long. The handling of longer attribute names or
values is not specified and is KeyNote-implementation-dependent.
Applications and assertions should therefore avoid depending on the
the use of attributes with names or values longer than 2048
characters. The length of an attribute value is represented by an
implementation-specific mechanism (e.g., NUL-terminated strings, an
explicit length field, etc.).
Attribute values are inherently untyped and are represented as
character strings by default. Attribute values may contain any non-
NUL ASCII character. Numeric attribute values should first be
converted to an ASCII text representation by the invoking
application, e.g., the value 1234.5 would be represented by the
string "1234.5".
Attribute names are of the form:
:: {Any string starting with a-z, A-Z, or the
underscore character, followed by any number of
a-z, A-Z, 0-9, or underscore characters} ;
That is, an begins with an alphabetic or underscore
character and can be followed by any number of alphanumerics and
underscores. Attribute names are case-sensitive.
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The exact mechanism for passing the action attribute set to the
compliance checker is determined by the KeyNote implementation.
Depending on specific requirements, an implementation may provide a
mechanism for including the entire attribute set as an explicit
parameter of the query, or it may provide some form of callback
mechanism invoked as each attribute is dereferenced, e.g., for access
to kernel variables.
If an action attribute is not defined its value is considered to be
the empty string.
Attribute names beginning with the "_" character are reserved for use
by the KeyNote runtime environment and cannot be passed from
applications as part of queries. The following special attribute
names are used:
Name Purpose
------------------------ ------------------------------------
_MIN_TRUST Lowest-order (minimum) compliance
value in query; see Section 5.1.
_MAX_TRUST Highest-order (maximum) compliance
value in query; see Section 5.1.
_VALUES Linearly ordered set of compliance
values in query; see Section 5.1.
Comma separated.
_ACTION_AUTHORIZERS Names of principals directly
authorizing action in query.
Comma separated.
In addition, attributes with names of the form "_", where is
an ASCII-encoded integer, are used by the regular expression matching
mechanism described in Section 5.
The assignment and semantics of any other attribute names beginning
with "_" is unspecified and implementation-dependent.
The names of other attributes in the action attribute set are not
specified by KeyNote but must be agreed upon by the writers of any
policies and credentials that are to inter-operate in a specific
KeyNote query evaluation.
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By convention, the name of the application domain over which action
attributes should be interpreted is given in the attribute named
"app_domain". The IANA (or some other suitable authority) will
provide a registry of reserved app_domain names. The registry will
list the names and meanings of each application's attributes.
The app_domain convention helps to ensure that credentials are
interpreted as they were intended. An attribute with any given name
may be used in many different application domains but might have
different meanings in each of them. However, the use of a global
registry is not always required for small-scale, closed applications;
the only requirement is that the policies and credentials made
available to the KeyNote compliance checker interpret attributes
according to the same semantics assumed by the application that
created them.
For example, an email application might reserve the app_domain
"RFC822-EMAIL" and might use the attributes named "address" (the
email address of a message's sender), "name" (the human name of the
message sender), and any "organization" headers present (the
organization name). The values of these attributes would be derived
in the obvious way from the email message headers. The public key of
the message's signer would be given in the "_ACTION_AUTHORIZERS"
attribute.
Note that "RFC822-EMAIL" is a hypothetical example; such a name may
or may not appear in the actual registry with these or different
attributes. (Indeed, we recognize that the reality of email security
is considerably more complex than this example might suggest.)
4. KeyNote Assertion Syntax
In the following sections, the notation [X]* means zero or more
repetitions of character string X. The notation [X]+ means one or
more repetitions of X. The notation * means zero or more
repetitions of non-terminal . The notation + means one or more
repetitions of X, whereas ? means zero or one repetitions of X.
Nonterminal grammar symbols are enclosed in angle brackets. Quoted
strings in grammar productions represent terminals.
4.1 Basic Structure
:: ? ?
? ?
? ? ;
All KeyNote assertions are encoded in ASCII.
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KeyNote assertions are divided into sections, called `fields', that
serve various semantic functions. Each field starts with an
identifying label at the beginning of a line, followed by the ":"
character and the field's contents. There can be at most one field
per line.
A field may be continued over more than one line by indenting
subsequent lines with at least one ASCII SPACE or TAB character.
Whitespace (a SPACE, TAB, or NEWLINE character) separates tokens but
is otherwise ignored outside of quoted strings. Comments with a
leading octothorp character (see Section 4.2) may begin in any
column.
One mandatory field is required in all assertions:
Authorizer
Six optional fields may also appear:
Comment
Conditions
KeyNote-Version
Licensees
Local-Constants
Signature
All field names are case-insensitive. The "KeyNote-Version" field,
if present, appears first. The "Signature" field, if present,
appears last. Otherwise, fields may appear in any order. Each field
may appear at most once in any assertion.
Blank lines are not permitted in assertions. Multiple assertions
stored in a file (e.g., in application policy configurations),
therefore, can be separated from one another unambiguously by the use
of blank lines between them.
4.2 Comments
:: "#" {ASCII characters} ;
The octothorp character ("#", ASCII 35 decimal) can be used to
introduce comments. Outside of quoted strings (see Section 4.3), all
characters from the "#" character through the end of the current line
are ignored. However, commented text is included in the computation
of assertion signatures (see Section 4.6.7).
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4.3 Strings
A `string' is a lexical object containing a sequence of characters.
Strings may contain any non-NUL characters, including newlines and
nonprintable characters. Strings may be given as literals, computed
from complex expressions, or dereferenced from attribute names.
4.3.1 String Literals
:: "\"" {see description below} "\"" ;
A string literal directly represents the value of a string. String
literals must be quoted by preceding and following them with the
double-quote character (ASCII 34 decimal).
A printable character may be `escaped' inside a quoted string literal
by preceding it with the backslash character (ASCII 92 decimal)
(e.g., "like \"this\"."). This permits the inclusion of the double-
quote and backslash characters inside string literals.
A similar escape mechanism is also used to represent non-printable
characters. "\n" represents the newline character (ASCII character
10 decimal), "\r" represents the carriage-return character (ASCII
character 13 decimal), "\t" represents the tab character (ASCII
character 9 decimal), and "\f" represents the form-feed character
(ASCII character 12 decimal). A backslash character followed by a
newline suppresses all subsequent whitespace (including the newline)
up to the next non-whitespace character (this allows the continuation
of long string constants across lines). Un-escaped newline and
return characters are illegal inside string literals.
The constructs "\0o", "\0oo", and "\ooo" (where o represents any
octal digit) may be used to represent any non-NUL ASCII characters
with their corresponding octal values (thus, "\012" is the same as
"\n", "\101" is "A", and "\377" is the ASCII character 255 decimal).
However, the NUL character cannot be encoded in this manner; "\0",
"\00", and "\000" are converted to the strings "0", "00", and "000"
respectively. Similarly, all other escaped characters have the
leading backslash removed (e.g., "\a" becomes "a", and "\\" becomes
"\"). The following four strings are equivalent:
"this string contains a newline\n followed by one space."
"this string contains a newline\n \
followed by one space."
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"this str\
ing contains a \
newline\n followed by one space."
"this string contains a newline\012\040followed by one space."
4.3.2 String Expressions
In general, anywhere a quoted string literal is allowed, a `string
expression' can be used. A string expression constructs a string
from string constants, dereferenced attributes (described in Section
4.4), and a string concatenation operator. String expressions may be
parenthesized.
:: "." /* String concatenation */
| /* Quoted string */
| "(" ")"
| /* See Section 4.4 */
| "$" ; /* See Section 4.4 */
The "$" operator has higher precedence than the "." operator.
4.4 Dereferenced Attributes
Action attributes provide the primary mechanism for applications to
pass information to assertions. Attribute names are strings from a
limited character set ( as defined in Section 3), and
attribute values are represented internally as strings. An attribute
is dereferenced simply by using its name. In general, KeyNote allows
the use of an attribute anywhere a string literal is permitted.
Attributes are dereferenced as strings by default. When required,
dereferenced attributes can be converted to integers or floating
point numbers with the type conversion operators "@" and "&". Thus,
an attribute named "foo" having the value "1.2" may be interpreted as
the string "1.2" (foo), the integer value 1 (@foo), or the floating
point value 1.2 (&foo).
Attributes converted to integer and floating point numbers are
represented according to the ANSI C `long' and `float' types,
respectively. In particular, integers range from -2147483648 to
2147483647, whilst floats range from 1.17549435E-38F to
3.40282347E+38F.
Any uninitialized attribute has the empty-string value when
dereferenced as a string and the value zero when dereferenced as an
integer or float.
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Attribute names may be given literally or calculated from string
expressions and may be recursively dereferenced. In the simplest
case, an attribute is dereferenced simply by using its name outside
of quotes; e.g., the string value of the attribute named "foo" is by
reference to `foo' (outside of quotes). The "$" construct
dereferences the attribute named in the string expression .
For example, if the attribute named "foo" contains the string "bar",
the attribute named "bar" contains the string "xyz", and the
attribute "xyz" contains the string "qua", the following string
comparisons are all true:
foo == "bar"
$("foo") == "bar"
$foo == "xyz"
$(foo) == "xyz"
$$foo == "qua"
If evaluates to an invalid or uninitialized attribute name,
its value is considered to be the empty string (or zero if used as a
numeric).
The token is defined as:
:: ;
4.5 Principal Identifiers
Principals are represented as ASCII strings called `Principal
Identifiers'. Principal Identifiers may be arbitrary labels whose
structure is not interpreted by the KeyNote system or they may encode
cryptographic keys that are used by KeyNote for credential signature
verification.
::
| ;
4.5.1 Opaque Principal Identifiers
Principal Identifiers that are used by KeyNote only as labels are
said to be `opaque'. Opaque identifiers are encoded in assertions as
strings (see Section 4.3):
:: ;
Opaque identifier strings should not contain the ":" character.
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4.5.2 Cryptographic Principal Identifiers
Principal Identifiers that are used by KeyNote as keys, e.g., to
verify credential signatures, are said to be `cryptographic'.
Cryptographic identifiers are also lexically encoded as strings:
:: ;
Unlike Opaque Identifiers, however, Cryptographic Identifier strings
have a special form. To be interpreted by KeyNote (for signature
verification), an identifier string should be of the form:
:: ":" ;
"ALGORITHM" is an ASCII substring that describes the algorithms to be
used in interpreting the key's bits. The ALGORITHM identifies the
major cryptographic algorithm (e.g., RSA [RSA78], DSA [DSA94], etc.),
structured format (e.g., PKCS1 [PKCS1]), and key bit encoding (e.g.,
HEX or BASE64). By convention, the ALGORITHM substring starts with
an alphabetic character and can contain letters, digits, underscores,
or dashes (i.e., it should match the regular expression "[a-zA-Z][a-
zA-Z0-9_-]*"). The IANA (or some other appropriate authority) will
provide a registry of reserved algorithm identifiers.
"ENCODEDBITS" is a substring of characters representing the key's
bits, the encoding and format of which depends on the ALGORITHM. By
convention, hexadecimal encoded keys use lower-case ASCII characters.
Cryptographic Principal Identifiers are converted to a normalized
canonical form for the purposes of any internal comparisons between
them; see Section 5.2.
Note that the keys used in examples throughout this document are
fictitious and generally much shorter than would be required for
security in practice.
4.6 KeyNote Fields
4.6.1 The KeyNote-Version Field
The KeyNote-Version field identifies the version of the KeyNote
assertion language under which the assertion was written. The
KeyNote-Version field is of the form
:: "KeyNote-Version:" ;
::
| ;
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where is an ASCII-encoded string. Assertions in
production versions of KeyNote use decimal digits in the version
representing the version number of the KeyNote language under which
they are to be interpreted. Assertions written to conform with this
document should be identified with the version string "2" (or the
integer 2). The KeyNote-Version field, if included, should appear
first.
4.6.2 The Local-Constants Field
This field adds or overrides action attributes in the current
assertion only. This mechanism allows the use of short names for
(frequently lengthy) cryptographic principal identifiers, especially
to make the Licensees field more readable. The Local-Constants field
is of the form:
:: "Local-Constants:" ;
:: /* can be empty */
| "=" ;
is an attribute name from the action attribute
namespace as defined in Section 3. The name is available for use as
an attribute in any subsequent field. If the Local-Constants field
defines more than one identifier, it can occupy more than one line
and be indented. is a string literal as described in
Section 4.3. Attributes defined in the Local-Constants field
override any attributes with the same name passed in with the action
attribute set.
An attribute may be initialized at most once in the Local-Constants
field. If an attribute is initialized more than once in an
assertion, the entire assertion is considered invalid and is not
considered by the KeyNote compliance checker in evaluating queries.
4.6.3 The Authorizer Field
The Authorizer identifies the Principal issuing the assertion. This
field is of the form
:: "Authorizer:" ;
::
| ;
The Principal Identifier may be given directly or by reference to the
attribute namespace (as defined in Section 4.4).
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4.6.4 The Licensees Field
The Licensees field identifies the principals authorized by the
assertion. More than one principal can be authorized, and
authorization can be distributed across several principals through
the use of `and' and threshold constructs. This field is of the form
:: "Licensees:" ;
:: /* can be empty */
| ;
:: "(" ")"
| "&&"
| "||"
| "-of(" ")" /* Threshold */
|
| ;
::
|
| "," ;
:: {Decimal number starting with a digit from 1 to 9} ;
The "&&" operator has higher precedence than the "||" operator.
is an ASCII-encoded positive decimal integer. If a
contains fewer than principals, the entire assertion is omitted
from processing.
4.6.5 The Conditions Field
This field gives the `conditions' under which the Authorizer trusts
the Licensees to perform an action. `Conditions' are predicates that
operate on the action attribute set. The Conditions field is of the
form:
:: "Conditions:" ;
:: /* Can be empty */
| ";" ;
:: "->" "{" "}"
| "->"
| ;
:: ;
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:: ;
:: "(" ")" /* Parentheses */
| "&&" /* Logical AND */
| "||" /* Logical OR */
| "!" /* Logical NOT */
|
|
|
| "true" /* case insensitive */
| "false" ; /* case insensitive */
:: "=="
| "!="
| "<"
| ">"
| "<="
| ">=" ;
:: "<"
| ">"
| "<="
| ">=" ;
:: "==" /* String equality */
| "!=" /* String inequality */
| "<" /* Alphanum. comparisons */
| ">"
| "<="
| ">="
| "~=" ; /* Reg. expr. matching */
:: "+" /* Integer */
| "-"
| "*"
| "/"
| "%"
| "^" /* Exponentiation */
| "-"
| "(" ")"
|
| "@" ;
:: "+" /* Floating point */
| "-"
| "*"
| "/"
| "^" /* Exponentiation */
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| "-"
| "(" ")"
|
| "&" ;
:: {Decimal number of at least one digit} ;
:: "." ;
is a quoted string as defined in Section 4.3
is defined in Section 3.
The operation precedence classes are (from highest to lowest):
{ (, ) }
{unary -, @, &, $}
{^}
{*, /, %}
{+, -, .}
Operators in the same precedence class are evaluated left-to-right.
Note the inability to test for floating point equality, as most
floating point implementations (hardware or otherwise) do not
guarantee accurate equality testing.
Also note that integer and floating point expressions can only be
used within clauses of condition fields, but in no other KeyNote
field.
The keywords "true" and "false" are not reserved; they can be used as
attribute or principal identifier names (although this practice makes
assertions difficult to understand and is discouraged).
is a standard regular expression, conforming to the POSIX
1003.2 regular expression syntax and semantics.
Any string expression (or attribute) containing the ASCII
representation of a numeric value can be converted to an integer or
float with the use of the "@" and "&" operators, respectively. Any
fractional component of an attribute value dereferenced as an integer
is rounded down. If an attribute dereferenced as a number cannot be
properly converted (e.g., it contains invalid characters or is empty)
its value is considered to be zero.
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4.6.6 The Comment Field
The Comment field allows assertions to be annotated with information
describing their purpose. It is of the form
:: "Comment:" ;
No interpretation of the contents of this field is performed by
KeyNote. Note that this is one of two mechanisms for including
comments in KeyNote assertions; comments can also be inserted
anywhere in an assertion's body by preceding them with the "#"
character (except inside string literals).
4.6.7 The Signature Field
The Signature field identifies a signed assertion and gives the
encoded digital signature of the principal identified in the
Authorizer field. The Signature field is of the form:
:: "Signature:" ;
:: ;
The string should be of the form:
:: ":" ;
The formats of the "ALGORITHM" and "ENCODEDBITS" substrings are as
described for Cryptographic Principal Identifiers in Section 4.4.2
The algorithm name should be the same as that of the principal
appearing in the Authorizer field. The IANA (or some other suitable
authority) will provide a registry of reserved names. It is not
necessary that the encodings of the signature and the authorizer key
be the same.
If the signature field is included, the principal named in the
Authorizer field must be a Cryptographic Principal Identifier, the
algorithm must be known to the KeyNote implementation, and the
signature must be correct for the assertion body and authorizer key.
The signature is computed over the assertion text, beginning with the
first field (including the field identifier string), up to (but not
including) the Signature field identifier. The newline preceding the
signature field identifier is the last character included in
signature calculation. The signature is always the last field in a
KeyNote assertion. Text following this field is not considered part
of the assertion.
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RFC 2704 The KeyNote Trust-Management System September 1999
The algorithms for computing and verifying signatures must be
configured into each KeyNote implementation and are defined and
documented separately.
Note that all signatures used in examples in this document are
fictitious and generally much shorter than would be required for
security in practice.
5. Query Evaluation Semantics
The KeyNote compliance checker finds and returns the Policy
Compliance Value of queries, as defined in Section 5.3, below.
5.1 Query Parameters
A KeyNote query has four parameters:
* The identifier of the principal(s) requesting the action.
* The action attribute set describing the action.
* The set of compliance values of interest to the application,
ordered from _MIN_TRUST to _MAX_TRUST
* The policy and credential assertions that should be included in
the evaluation.
The mechanism for passing these parameters to the KeyNote evaluator
is application dependent. In particular, an evaluator might provide
for some parameters to be passed explicitly, while others are looked
up externally (e.g., credentials might be looked up in a network-
based distribution system), while still others might be requested
from the application as needed by the evaluator, through a `callback'
mechanism (e.g., for attribute values that represent values from
among a very large namespace).
5.1.1 Action Requester
At least one Principal must be identified in each query as the
`requester' of the action. Actions may be requested by several
principals, each considered to have individually requested it. This
allows policies that require multiple authorizations, e.g., `two
person control'. The set of authorizing principals is made available
in the special attribute "_ACTION_AUTHORIZERS"; if several principals
are authorizers, their identifiers are separated with commas.
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5.1.2 Ordered Compliance Value Set
The set of compliance values of interest to an application (and their
relative ranking to one another) is determined by the invoking
application and passed to the KeyNote evaluator as a parameter of the
query. In many applications, this will be Boolean, e.g., the ordered
sets {FALSE, TRUE} or {REJECT, APPROVE}. Other applications may
require a range of possible values, e.g., {No_Access, Limited_Access,
Full_Access}. Note that applications should include in this set only
compliance value names that are actually returned by the assertions.
The lowest-order and highest-order compliance value strings given in
the query are available in the special attributes named "_MIN_TRUST"
and "_MAX_TRUST", respectively. The complete set of query compliance
values is made available in ascending order (from _MIN_TRUST to
_MAX_TRUST) in the special attribute named "_VALUES". Values are
separated with commas; applications that use assertions that make use
of the _VALUES attribute should therefore avoid the use of compliance
value strings that themselves contain commas.
5.2 Principal Identifier Normalization
Principal identifier comparisons among Cryptographic Principal
Identifiers (that represent keys) in the Authorizer and Licensees
fields or in an action's direct authorizers are performed after
normalizing them by conversion to a canonical form.
Every cryptographic algorithm used in KeyNote defines a method for
converting keys to their canonical form and that specifies how the
comparison for equality of two keys is performed. If the algorithm
named in the identifier is unknown to KeyNote, the identifier is
treated as opaque.
Opaque identifiers are compared as case-sensitive strings.
Notice that use of opaque identifiers in the Authorizer field
requires that the assertion's integrity be locally trusted (since it
cannot be cryptographically verified by the compliance checker).
5.3 Policy Compliance Value Calculation
The Policy Compliance Value of a query is the Principal Compliance
Value of the principal named "POLICY". This value is defined as
follows:
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5.3.1 Principal Compliance Value
The Compliance Value of a principal is the highest order
(maximum) of:
- the Direct Authorization Value of principal ; and
- the Assertion Compliance Values of all assertions identifying
in the Authorizer field.
5.3.2 Direct Authorization Value
The Direct Authorization Value of a principal is _MAX_TRUST if
is listed in the query as an authorizer of the action.
Otherwise, the Direct Authorization Value of is _MIN_TRUST.
5.3.3 Assertion Compliance Value
The Assertion Compliance Value of an assertion is the lowest order
(minimum) of the assertion's Conditions Compliance Value and its
Licensee Compliance Value.
5.3.4 Conditions Compliance Value
The Conditions Compliance Value of an assertion is the highest-order
(maximum) value among all successful clauses listed in the conditions
section.
If no clause's test succeeds or the Conditions field is empty, an
assertion's Conditions Compliance Value is considered to be the
_MIN_TRUST value, as defined Section 5.1.
If an assertion's Conditions field is missing entirely, its
Conditions Compliance Value is considered to be the _MAX_TRUST value,
as defined in Section 5.1.
The set of successful test clause values is calculated as follows:
Recall from the grammar of section 4.6.5 that each clause in the
conditions section has two logical parts: a `test' and an optional
`value', which, if present, is separated from the test with the "->"
token. The test subclause is a predicate that either succeeds
(evaluates to logical `true') or fails (evaluates to logical
`false'). The value subclause is a string expression that evaluates
to one value from the ordered set of compliance values given with the
query. If the value subclause is missing, it is considered to be
_MAX_TRUST. That is, the clause
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foo=="bar";
is equivalent to
foo=="bar" -> _MAX_TRUST;
If the value component of a clause is present, in the simplest case
it contains a string expression representing a possible compliance
value. For example, consider an assertion with the following
Conditions field:
Conditions:
@user_id == 0 -> "full_access"; # clause (1)
@user_id < 1000 -> "user_access"; # clause (2)
@user_id < 10000 -> "guest_access"; # clause (3)
user_name == "root" -> "full_access"; # clause (4)
Here, if the value of the "user_id" attribute is "1073" and the
"user_name" attribute is "root", the possible compliance value set
would contain the values "guest_access" (by clause (3)) and
"full_access" (by clause (4)). If the ordered set of compliance
values given in the query (in ascending order) is {"no_access",
"guest_access", "user_access", "full_access"}, the Conditions
Compliance Value of the assertion would be "full_access" (because
"full_access" has a higher-order value than "guest_access"). If the
"user_id" attribute had the value "19283" and the "user_name"
attribute had the value "nobody", no clause would succeed and the
Conditions Compliance Value would be "no_access", which is the
lowest-order possible value (_MIN_TRUST).
If a clause lists an explicit value, its value string must be named
in the query ordered compliance value set. Values not named in the
query compliance value set are considered equivalent to _MIN_TRUST.
The value component of a clause can also contain recursively-nested
clauses. Recursively-nested clauses are evaluated only if their
parent test is true. That is,
a=="b" -> { b=="c" -> "value1";
d=="e" -> "value2";
true -> "value3"; } ;
is equivalent to
(a=="b") && (b=="c") -> "value1";
(a=="b") && (d=="e") -> "value2";
(a=="b") -> "value3";
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String comparisons are case-sensitive.
A regular expression comparison ("~=") is considered true if the
left-hand-side string expression matches the right-hand-side regular
expression. If the POSIX regular expression group matching scheme is
used, the number of groups matched is placed in the temporary meta-
attribute "_0" (dereferenced as _0), and each match is placed in
sequence in the temporary attributes (_1, _2, ..., _N). These
match-attributes' values are valid only within subsequent references
made within the same clause. Regular expression evaluation is case-
sensitive.
A runtime error occurring in the evaluation of a test, such as
division by zero or an invalid regular expression, causes the test to
be considered false. For example:
foo == "bar" -> {
@a == 1/0 -> "oneval"; # subclause 1
@a == 2 -> "anotherval"; # subclause 2
};
Here, subclause 1 triggers a runtime error. Subclause 1 is therefore
false (and has the value _MIN_TRUST). Subclause 2, however, would be
evaluated normally.
An invalid is considered a runtime error and causes the
test in which it occurs to be considered false.
5.3.5 Licensee Compliance Value
The Licensee Compliance Value of an assertion is calculated by
evaluating the expression in the Licensees field, based on the
Principal Compliance Value of the principals named there.
If an assertion's Licensees field is empty, its Licensee Compliance
Value is considered to be _MIN_TRUST. If an assertion's Licensees
field is missing altogether, its Licensee Compliance Value is
considered to be _MAX_TRUST.
For each principal named in the Licensees field, its Principal
Compliance Value is substituted for its name. If no Principal
Compliance Value can be found for some named principal, its name is
substituted with the _MIN_TRUST value.
The licensees expression (as defined in Section 4.6.4) is evaluated
as follows:
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* A "(...)" expression has the value of the enclosed subexpression.
* A "&&" expression has the lower-order (minimum) of its two
subexpression values.
* A "||" expression has the higher-order (maximum) of its two
subexpression values.
* A "-of()" expression has the K-th highest order
compliance value listed in . Values that appear multiple
times are counted with multiplicity. For example, if K = 3 and
the orders of the listed compliance values are (0, 1, 2, 2, 3),
the value of the expression is the compliance value of order 2.
For example, consider the following Licensees field:
Licensees: ("alice" && "bob") || "eve"
If the Principal Compliance Value is "yes" for principal "alice",
"no" for principal "bob", and "no" for principal "eve", and "yes" is
higher order than "no" in the query's Compliance Value Set, then the
resulting Licensee Compliance Value is "no".
Observe that if there are exactly two possible compliance values
(e.g., "false" and "true"), the rules of Licensee Compliance Value
resolution reduce exactly to standard Boolean logic.
5.4 Assertion Management
Assertions may be either signed or unsigned. Only signed assertions
should be used as credentials or transmitted or stored on untrusted
media. Unsigned assertions should be used only to specify policy and
for assertions whose integrity has already been verified as
conforming to local policy by some mechanism external to the KeyNote
system itself (e.g., X.509 certificates converted to KeyNote
assertions by a trusted conversion program).
Implementations that permit signed credentials to be verified by the
KeyNote compliance checker generally provide two `channels' through
which applications can make assertions available. Unsigned,
locally-trusted assertions are provided over a `trusted' interface,
while signed credentials are provided over an `untrusted' interface.
The KeyNote compliance checker verifies correct signatures for all
assertions submitted over the untrusted interface. The integrity of
KeyNote evaluation requires that only assertions trusted as
reflecting local policy are submitted to KeyNote via the trusted
interface.
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Note that applications that use KeyNote exclusively as a local policy
specification mechanism need use only trusted assertions. Other
applications might need only a small number of infrequently changed
trusted assertions to `bootstrap' a policy whose details are
specified in signed credentials issued by others and submitted over
the untrusted interface.
5.5 Implementation Issues
Informally, the semantics of KeyNote evaluation can be thought of as
involving the construction a directed graph of KeyNote assertions
rooted at a POLICY assertion that connects with at least one of the
principals that requested the action.
Delegation of some authorization from principal to a set of
principals is expressed as an assertion with principal given
in the Authorizer field, principal set given in the Licensees
field, and the authorization to be delegated encoded in the
Conditions field. How the expression digraph is constructed is
implementation-dependent and implementations may use different
algorithms for optimizing the graph's construction. Some
implementations might use a `bottom up' traversal starting at the
principals that requested the action, others might follow a `top
down' approach starting at the POLICY assertions, and still others
might employ other heuristics entirely.
Implementations are encouraged to employ mechanisms for recording
exceptions (such as division by zero or syntax error), and reporting
them to the invoking application if requested. Such mechanisms are
outside the scope of this document.
6. Examples
In this section, we give examples of KeyNote assertions that might be
used in hypothetical applications. These examples are intended
primarily to illustrate features of KeyNote assertion syntax and
semantics, and do not necessarily represent the best way to integrate
KeyNote into applications.
In the interest of readability, we use much shorter keys than would
ordinarily be used in practice. Note that the Signature fields in
these examples do not represent the result of any real signature
calculation.
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1. TRADITIONAL CA / EMAIL
A. A policy unconditionally authorizing RSA key abc123 for all
actions. This essentially defers the ability to specify
policy to the holder of the secret key corresponding to
abc123:
Authorizer: "POLICY"
Licensees: "RSA:abc123"
B. A credential assertion in which RSA Key abc123 trusts either
RSA key 4401ff92 (called `Alice') or DSA key d1234f (called
`Bob') to perform actions in which the "app_domain" is
"RFC822-EMAIL", where the "address" matches the regular
expression "^.*@keynote\.research\.att\.com$". In other
words, abc123 trusts Alice and Bob as certification
authorities for the keynote.research.att.com domain.
KeyNote-Version: 2
Local-Constants: Alice="DSA:4401ff92" # Alice's key
Bob="RSA:d1234f" # Bob's key
Authorizer: "RSA:abc123"
Licensees: Alice || Bob
Conditions: (app_domain == "RFC822-EMAIL") &&
(address ~= # only applies to one domain
"^.*@keynote\\.research\\.att\\.com$");
Signature: "RSA-SHA1:213354f9"
C. A certificate credential for a specific user whose email
address is mab@keynote.research.att.com and whose name, if
present, must be "M. Blaze". The credential was issued by the
`Alice' authority (whose key is certified in Example B
above):
KeyNote-Version: 2
Authorizer: "DSA:4401ff92" # the Alice CA
Licensees: "DSA:12340987" # mab's key
Conditions: ((app_domain == "RFC822-EMAIL") &&
(name == "M. Blaze" || name == "") &&
(address == "mab@keynote.research.att.com"));
Signature: "DSA-SHA1:ab23487"
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D. Another certificate credential for a specific user, also
issued by the `Alice' authority. This example allows three
different keys to sign as jf@keynote.research.att.com (each
for a different cryptographic algorithm). This is, in
effect, three credentials in one:
KeyNote-Version: "2"
Authorizer: "DSA:4401ff92" # the Alice CA
Licensees: "DSA:abc991" || # jf's DSA key
"RSA:cde773" || # jf's RSA key
"BFIK:fd091a" # jf's BFIK key
Conditions: ((app_domain == "RFC822-EMAIL") &&
(name == "J. Feigenbaum" || name == "") &&
(address == "jf@keynote.research.att.com"));
Signature: "DSA-SHA1:8912aa"
Observe that under policy A and credentials B, C and D, the
following action attribute sets are accepted (they return
_MAX_TRUST):
_ACTION_AUTHORIZERS = "dsa:12340987"
app_domain = "RFC822-EMAIL"
address = "mab@keynote.research.att.com"
and
_ACTION_AUTHORIZERS = "dsa:12340987"
app_domain = "RFC822-EMAIL"
address = "mab@keynote.research.att.com"
name = "M. Blaze"
while the following are not accepted (they return
_MIN_TRUST):
_ACTION_AUTHORIZERS = "dsa:12340987"
app_domain = "RFC822-EMAIL"
address = "angelos@dsl.cis.upenn.edu"
and
_ACTION_AUTHORIZERS = "dsa:abc991"
app_domain = "RFC822-EMAIL"
address = "mab@keynote.research.att.com"
name = "M. Blaze"
and
_ACTION_AUTHORIZERS = "dsa:12340987"
app_domain = "RFC822-EMAIL"
address = "mab@keynote.research.att.com"
name = "J. Feigenbaum"
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2. WORKFLOW/ELECTRONIC COMMERCE
E. A policy that delegates authority for the "SPEND" application
domain to RSA key dab212 when the amount given in the
"dollars" attribute is less than 10000.
Authorizer: "POLICY"
Licensees: "RSA:dab212" # the CFO's key
Conditions: (app_domain=="SPEND") && (@dollars < 10000);
F. RSA key dab212 delegates authorization to any two signers,
from a list, one of which must be DSA key feed1234 in the
"SPEND" application when @dollars < 7500. If the amount in
@dollars is 2500 or greater, the request is approved but
logged.
KeyNote-Version: 2
Comment: This credential specifies a spending policy
Authorizer: "RSA:dab212" # the CFO
Licensees: "DSA:feed1234" && # The vice president
("RSA:abc123" || # middle manager #1
"DSA:bcd987" || # middle manager #2
"DSA:cde333" || # middle manager #3
"DSA:def975" || # middle manager #4
"DSA:978add") # middle manager #5
Conditions: (app_domain=="SPEND") # note nested clauses
-> { (@(dollars) < 2500)
-> _MAX_TRUST;
(@(dollars) < 7500)
-> "ApproveAndLog";
};
Signature: "RSA-SHA1:9867a1"
G. According to this policy, any two signers from the list of
managers will do if @(dollars) < 1000:
KeyNote-Version: 2
Authorizer: "POLICY"
Licensees: 2-of("DSA:feed1234", # The VP
"RSA:abc123", # Middle management clones
"DSA:bcd987",
"DSA:cde333",
"DSA:def975",
"DSA:978add")
Conditions: (app_domain=="SPEND") &&
(@(dollars) < 1000);
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H. A credential from dab212 with a similar policy, but only one
signer is required if @(dollars) < 500. A log entry is made if
the amount is at least 100.
KeyNote-Version: 2
Comment: This one credential is equivalent to six separate
credentials, one for each VP and middle manager.
Individually, they can spend up to $500, but if
it's $100 or more, we log it.
Authorizer: "RSA:dab212" # From the CFO
Licensees: "DSA:feed1234" || # The VP
"RSA:abc123" || # The middle management clones
"DSA:bcd987" ||
"DSA:cde333" ||
"DSA:def975" ||
"DSA:978add"
Conditions: (app_domain="SPEND") # nested clauses
-> { (@(dollars) < 100) -> _MAX_TRUST;
(@(dollars) < 500) -> "ApproveAndLog";
};
Signature: "RSA-SHA1:186123"
Assume a query in which the ordered set of Compliance Values is
{"Reject", "ApproveAndLog", "Approve"}. Under policies E and G,
and credentials F and H, the Policy Compliance Value is
"Approve" (_MAX_TRUST) when:
_ACTION_AUTHORIZERS = "DSA:978add"
app_domain = "SPEND"
dollars = "45"
unmentioned_attribute = "whatever"
and
_ACTION_AUTHORIZERS = "RSA:abc123,DSA:cde333"
app_domain = "SPEND"
dollars = "550"
The following return "ApproveAndLog":
_ACTION_AUTHORIZERS = "DSA:feed1234,DSA:cde333"
app_domain = "SPEND"
dollars = "5500"
and
_ACTION_AUTHORIZERS = "DSA:cde333"
app_domain = "SPEND"
dollars = "150"
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However, the following return "Reject" (_MIN_TRUST):
_ACTION_AUTHORIZERS = "DSA:def975"
app_domain = "SPEND"
dollars = "550"
and
_ACTION_AUTHORIZERS = "DSA:cde333,DSA:978add"
app_domain = "SPEND"
dollars = "5500"
7. Trust-Management Architecture
KeyNote provides a simple mechanism for describing security policy
and representing credentials. It differs from traditional
certification systems in that the security model is based on binding
keys to predicates that describe what the key is authorized by policy
to do, rather than on resolving names. The infrastructure and
architecture to support a KeyNote system is therefore rather
different from that required for a name-based certification scheme.
The KeyNote trust-management architecture is based on that of
PolicyMaker [BFL96,BFS98].
It is important to understand the separation between the
responsibilities of the KeyNote system and those of the application
and other support infrastructure. A KeyNote compliance checker will
determine, based on policy and credential assertions, whether a
proposed action is permitted according to policy. The usefulness of
KeyNote output as a policy enforcement mechanism depends on a number
of factors:
* The action attributes and the assignment of their values must
reflect accurately the security requirements of the application.
Identifying the attributes to include in the action attribute set
is perhaps the most important task in integrating KeyNote into new
applications.
* The policy of the application must be correct and well-formed. In
particular, trust must be deferred only to principals that should,
in fact, be trusted by the application.
* The application itself must be trustworthy. KeyNote does not
directly enforce policy; it only provides advice to the
applications that call it. In other words, KeyNote assumes that
the application itself is trusted and that the policy assertions
it specifies are correct. Nothing prevents an application from
submitting misleading or incorrect assertions to KeyNote or from
ignoring KeyNote altogether.
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It is also up to the application (or some service outside KeyNote) to
select the appropriate credentials and policy assertions with which
to run a particular query. Note, however, that even if inappropriate
credentials are provided to KeyNote, this cannot result in the
approval of an illegal action (as long as the policy assertions are
correct and the the action attribute set itself is correctly passed
to KeyNote).
KeyNote is monotonic; adding an assertion to a query can never result
in a query's having a lower compliance value that it would have had
without the assertion. Omitting credentials may, of course, result
in legal actions being disallowed. Selecting appropriate credentials
(e.g., from a distributed database or `key server') is outside the
scope of the KeyNote language and may properly be handled by a remote
client making a request, by the local application receiving the
request, or by a network-based service, depending on the application.
In addition, KeyNote does not itself provide credential revocation
services, although credentials can be written to expire after some
date by including a date test in the predicate. Applications that
require credential revocation can use KeyNote to help specify and
implement revocation policies. A future document will address
expiration and revocation services in KeyNote.
Because KeyNote is designed to support a variety of applications,
several different application interfaces to a KeyNote implementation
are possible. In its simplest form, a KeyNote compliance checker
would exist as a stand-alone application, with other applications
calling it as needed. KeyNote might also be implemented as a library
to which applications are linked. Finally, a KeyNote implementation
might run as a local trusted service, with local applications
communicating their queries via some interprocess communication
mechanism.
8. Security Considerations
Trust management is itself a security service. Bugs in or incorrect
use of a KeyNote compliance checker implementation could have
security implications for any applications in which it is used.
9. IANA Considerations
This document contains three identifiers to be maintained by the
IANA. This section explains the criteria to be used by the IANA to
assign additional identifiers in each of these lists.
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9.1 app_domain Identifiers
The only thing required of IANA on allocation of these identifiers is
that they be unique strings. These strings are case-sensitive for
KeyNote purposes, however it is strongly recommended that IANA assign
different capitalizations of the same string only to the same
organization.
9.2 Public Key Format Identifiers
These strings uniquely identify a public key algorithm as used in the
KeyNote system for representing keys. Requests for assignment of new
identifiers must be accompanied by an RFC-style document that
describes the details of this encoding. Example strings are "rsa-
hex:" and "dsa-base64:". These strings are case-insensitive.
9.3 Signature Algorithm Identifiers
These strings uniquely identify a public key algorithm as used in the
KeyNote system for representing public key signatures. Requests for
assignment of new identifiers must be accompanied by an RFC-style
document that describes the details of this encoding. Example strings
are "sig-rsa-md5-hex:" and "sig-dsa-sha1-base64:". Note that all
such strings must begin with the prefix "sig-". These strings are
case-insensitive.
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RFC 2704 The KeyNote Trust-Management System September 1999
A. Acknowledgments
We thank Lorrie Faith Cranor (AT&T Labs - Research) and Jonathan M.
Smith (University of Pennsylvania) for their suggestions and comments
on earlier versions of this document.
B. Full BNF (alphabetical order)
:: {see section 4.4.2} ;
:: ? ?
? ?
? ? ;
:: "" | "="
;
:: {Any string starting with a-z, A-Z, or the
underscore character, followed by any number of
a-z, A-Z, 0-9, or underscore characters} ;
:: "Authorizer:" ;
:: | ;
:: "->" "{" "}"
| "->" | ;
:: "#" {ASCII characters} ;
:: "Comment:" {Free-form text} ;
:: "Conditions:" ;
:: "" | ";" ;
:: ;
:: {see section 4.4.2} ;
:: "+" | "-"
| "*" | "/"