NAME
keynote - a trust-management system
SYNOPSIS
#include <sys/types.h>
#include <regex.h>
#include <keynote.h>
Link options: -lkeynote -lm -lcrypto
DESCRIPTION
For more details on KeyNote, see RFC 2704.
Details on the API, assertion syntax, and command-line tool are given in
the man pages listed at the end of this manual.
Trust management, introduced in the PolicyMaker system, 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.
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 the papers listed at the end of this manual.
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 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.
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 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 elsewhere of this document.
KeyNote provides advice to applications on 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 keynote(5).
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.
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. A KeyNote assertion contains a sequence of
sections, called ‘fields,’ each of which specifying 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"
For the exact meanings of all the fields, see the RFC reference at the
end of this manual, and/or keynote(5).
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 later in this
document.
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 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.
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. Applications and assertions should therefore avoid
depending on the use of attributes with names or values longer than 2048
characters.
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".
An <AttributeID> begins with an alphabetic or underscore character and
can be followed by any number of alphanumerics and underscores.
Attribute names are case-sensitive.
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:
_MIN_TRUST
Lowest-order (minimum) compliance value in query.
_MAX_TRUST
Highest-order (maximum) compliance value in query.
_VALUES
Linearly ordered set of compliance value in query.
_ACTION_AUTHORIZERS
Names of principals directly authorizing action in query. Comma
separated.
In addition, attributes with names of the form "_<N>", where <N> is an
ASCII-encoded integer, are used by the regular expression matching
mechanism described in keynote(5).
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 mail
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.
QUERY SEMANTICS
The discussion in the following sections assume some familiarity with
assertion syntax. Please refer to keynote(5) for more details on the
syntax.
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).
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.
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.
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).
POLICY COMPLIANCE VALUE CALCULATION
The Policy Compliance Value of a query is the Principal Compliance Value
of the principal named "POLICY".
PRINCIPAL COMPLIANCE VALUE
The Compliance Value of a principal <X> is the highest order (maximum)
of:
- the Direct Authorization Value of principal <X>; and
- the Assertion Compliance Values of all assertions identifying
<X> in the Authorizer field.
DIRECT AUTHORIZATION VALUE
The Direct Authorization Value of a principal <X> is _MAX_TRUST if <X> is
listed in the query as an authorizer of the action. Otherwise, the Direct
Authorization Value of <X> is _MIN_TRUST.
ASSERTION COMPLIANC 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.
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 described previously.
If an assertion’s Conditions field is missing entirely, its Conditions
Compliance Value is considered to be the _MAX_TRUST value, as defined
previously.
The set of successful test clause values is calculated as follows:
Recall from the grammar of the Conditions field (see keynote(5) for more
details) 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
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";
Notice that 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 <RegExpr> is considered a runtime error and causes the test in
which it occurs to be considered false.
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 (see keynote(5)) is evaluated as follows:
* 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 "<K>-of(<List>)" expression has the K-th highest order
compliance value listed in <list>. 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.
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.
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.
EXAMPLES
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);
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"
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);
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"
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"
FILES
keynote.h
libkeynote.a
SEE ALSO
keynote(1), keynote(3), keynote(5)
‘‘The KeyNote Trust-Management System, Version 2’’
M. Blaze, J. Feigenbaum, A. D. Keromytis, Internet Drafts, RFC
2704.
‘‘Decentralized Trust Management’’
M. Blaze, J. Feigenbaum, J. Lacy, 1996 IEEE Conference on
Privacy and Security
‘‘Compliance-Checking in the PolicyMaker Trust Management System’’
M. Blaze, J. Feigenbaum, M. Strauss, 1998 Financial Crypto
Conference
AUTHOR
Angelos D. Keromytis (angelos@dsl.cis.upenn.edu)
WEB PAGE
http://www.cis.upenn.edu/~keynote
BUGS
None that we know of. If you find any, please report them at
keynote@research.att.com