NCSC TECHNICAL REPORT - 005
Volume 3/5
Library No. S-243,039
FOREWARD
This report is the third of five companion documents to the Trusted Database Management System
Interpretation of the Trusted Computer System Evaluation Criteria. The companion documents
address topics that are important to the design and development of secure database management
systems, and are written for database vendors, system designers, evaluators, and researchers. This
report addresses polyinstantiation issues in multilevel secure database management systems.
___________________________________
Keith F. Brewster May 1996
Acting Chief, Partnerships and Processes
ACKNOWLEDGMENTS
The National Computer Security Center extends special recognition to the authors of this
document. The initial version was written by Barbara Blaustein, Sushil Jajodia, and Alfred
Paradise of the MITRE Corporation. The final version was written by Gary Smith, Larry Halme,
and David Wichers of Arca Systems, Inc.
The documents in this series were produced under the guidance of Shawn P. O'Brien of the
National Security Agency, LouAnna Notargiacomo and Barbara Blaustein of the MITRE
Corporation, and David Wichers of Arca Systems, Inc.
We wish to thank the members of the information security community who enthusiastically gave
of their time and technical expertise in reviewing these documents and in providing valuable
comments and suggestions.
TABLE OF CONTENTS
SECTION PAGE
1.0 INTRODUCTION
1.1 BACKGROUND AND PURPOSE
1.2 SCOPE
1.3 INTRODUCTION TO POLYINSTANTIATION
1.4 AUDIENCES OF THIS DOCUMENT
1.5 ORGANIZATION OF THIS DOCUMENT
2.0 BACKGROUND
2.1 TERMINOLOGY AND NOTATION
2.2 TUPLE VERSUS ELEMENT POLYINSTANTIATION
2.3 POLYHIGH VERSUS POLYLOW POLYINSTANTIATION
3.0 POLYINSTANTIATION CONSIDERATIONS
3.1 PROBLEMS CAUSED BY POLYlNSTANTIATION
3.1.1 Loss of Real-World Entity Integrity
3.1.2 Increased Database Complexity to the User
3.1.3 Increased Database Administration
3.2 A SIMPLE, BUT UNACCEPTABLE, ALTERNATIVE TO POLYINSTANTIATION
3.3 AUTOMATIC VERSUS INTENTIONAL POLYINSTANTIATION
3.4 ARCHITECTURAL CONSIDERATIONS IN SUPPORTING POLYINSTANTIATION
4.0 POLYINSTANTIATION APPROACHES
4.1 AN EXAMPLE FOR COMPARING POLYINSTANTIATION APPROACHES
4.2 PROPAGATION APPROACH
4.3 DERIVED VALUES APPROACH
4.4 VISIBLE RESTRICTIONS APPROACH
4.4.1 Belief Approach
4.4.2 Insert-Low Approach
4.4.3 Prevention Approach
4.4.4 Explicit Alternatives Approach
4.4.5 Multilevel Relational Data Model Approach
5.0 CURRENT COMMERCIAL APPROACHES TO POLYINSTANTIATION
6.0 SUMMARY
REPERENCES
LIST OF FIGURES
FIGURE PAGE
1.1: INTRODUCTION TO POLYINSTANTIATION
2.1: A MULTILEVEL RELATION WITH TUPLE POLYINSTANTIATION
2.2: MULTILEVEL RELATION WITH ELEMENT POLYINSTANTIATION
2.3: POLYINSTANTIATED MULTILEVEL RELATION WITH TUPLE-LEVEL
LABELING
3.1: NO MAC PRIVILEGES ARCHITECTURE
3.2: TRUSTED SUBJECT ARCHITECTURE
4.1: MULTILEVEL SCHEME FOR THE RELATION SODC
4.2: U-INSTANCE OF SODC
4.3: EIGHT S-INSTANCES OF SODC
4.4: TS-INSTANCE OF SOD WITH FOUR MISSIONS
4.5: PROPAGATED TUPLES OF SODC
4.6: JOINS IN GDSS
4.7: EXAMPLE OF SOD IN THE BELIEF MODEL
SECTION 1
INTRODUCTION
This document is the third volume in the series of companion documents to the Trusted Database
Management System Interpretation of the Trusted Computer System Evaluation Criteria [TDI 91;
DoD 85]. This document examines polyinstantiation issues in multilevel secure (MLS) database
management systems and summarizes the research to date in this area.
1.1 BACKGROUND AND PURPOSE
In 1991 the National Computer Security Center published the Trusted Database Management
System Interpretation (TDI) of the Trusted Computer System Evaluation Criteria (TCSEC). The
TDI, however, does not address many topics that are important to the design and development of
secure database management systems (DBMSs). These topics (such as inference, aggregation, and
database integrity) are being addressed by ongoing research and development. Since specific
techniques in these topic areas had not yet gained broad acceptance, the topics were considered
inappropriate for inclusion in the TDI.
The TDI is being supplemented by a series of companion documents to address these issues
specific to secure DBMSs. Each companion document focuses on one topic by describing the
problem, discussing the issues, and summarizing the research that has been done to date. The intent
of the series is to make it clear to DBMS vendors, system designers, evaluators, and researchers
what the issues are, the current approaches, their pros and cons, how they relate to a TCSEC/TDI
evaluation, and what specific areas require additional research. Although some guidance may be
presented, nothing contained within these documents should be interpreted as criteria.
These documents assume the reader understands basic DBMS concepts and relational database
terminology. A background in security sufficient to use the TDI and TCSEC is also assumed;
however, fundamentals are discussed whenever a common understanding is important to the
discussion.
1.2 SCOPE
This document addresses polyinstantiation issues in multilevel secure DBMSs. It is the third of five
volumes in the series of TDI companion documents, which includes the following documents:
· Inference and Aggregation Issues in Secure Database Management Systems [Inference
96]
· Entity and Referential Integrity Issues in Multilevel Secure Database Management
Systems [Entity 96]
· Polyinstantiation Issues in Multilevel Secure Database Management Systems
· Auditing Issues in Secure Database Management Systems [Audit 96]
· Discretionary Access Control Issues in High Assurance Secure Database Management
Systems [DAC 96]
This series of documents uses terminology from the relational model to provide a common basis
for understanding the concepts presented. For most of the topics covered in this series, the concepts
presented should apply to most database modeling paradigms, depending on the specifics of each
model. This document specifically addresses relational DBMSs.
This document is related to the documents Inference and Aggregation Issues in Secure Database
Management Systems [Inference 96] and Entity and Referential Integrity Issues in Multilevel
Secure Database Management Systems [Entity 96]. Much of the discussion of the relationship
between enforcement of integrity constraints and multilevel security centers on the inference
channels which integrity constraints can introduce. In most cases, the enforcement of any integrity
constraint referring to multilevel data will create a signaling channel [Meadows 88]. One way to
avoid these channels caused by enforcing entity integrity is to use polyinstantiation.
1.3 INTRODUCTION TO POLYNISTANTIATION
The goal of mandatory security is to prevent the unauthorized disclosure of data by prohibiting
users (or automated programs working on behalf of users) from accessing data for which they are
not cleared. MLS DBMSs utilize mandatory access controls (MAC) based on the access class of
the subject that is acting on behalf of (or in the name of) some named, accountable user. However,
low-level users may be able to infer high-level information from only low-level data, real world
knowledge of what the database is modeling, and knowledge of how the database functions.
Besides preventing direct access to high data values, the TCB must also inhibit users from drawing
inferences from the perceived existence (or absence) of high data, or from any noticeable effects
from the manipulation of this data. This section introduces these concepts, which are examined in
greater detail in later sections of this document.
Suppose that a multilevel database relation with element level labeling is constructed to summarize
potentially sensitive flight mission information, as depicted in Figure 1.1. This relation's schema
specifies a primary key of Flight ID and three attributes that describe the Origin, Destination, and
Cargo of each flight. Associated with each element is a security label. Flight personnel on the
ground, who are cleared to Secret, may need to be privy to Flight ID and Origin/Destination
information. However, it may be desired that the nature of the flight manifest be at times hidden to
all but Top Secret users. For example, Flight 007's cargo of alien spacecraft debris may be
considered Top Secret. However, ground personnel may learn weight and size specifics, and may
even be witness to the real-world loading process. The absence of data in the Cargo field would
increase the likelihood of low users correctly inferring the presence of a Top Secret cargo with
resulting natural curiosity as to its nature. It is possible that such an inference would threaten the
success of the mission. Instead, a "cover story" could intentionally be created so that low users are
placated by the plausible, but erroneous statement that the cargo consists of weather balloon debris.
The question then becomes how to store and manage this conflicting information in the database.
Flight ID
Origin
Destination
Cargo
. . .
007 S
Roswell S
Area 51 S
Weather Balloon Debris S
007 S
Roswell S
Area 51 S
Alien Spacecraft Debris TS
. . .
Figure 1.1: Introduction to Polyinstantiation
A broader issue than cover stories that arises with a multilevel database is how the DBMS should
address attempts by subjects to insert or change multilevel database objects. A low-level user may
attempt to modify a tuple that, unbeknownst to him, already contains high data. If the database
were to permit this, the low-level user would be able to change data of which that user is not even
aware. This is likely to be undesirable. If instead, the database were to notify the user of the
collision, the user would be able to infer that high data exists, and a downward signaling channel
would result.
In a like manner, if a high user attempts to insert data in a field which already contains low data,
there are a similar set of inadequate approaches. The database can permit the update, but this would
violate confidentiality with a direct write down from a high subject. The database can prohibit the
subject from making the update, but it would be impractical to always disallow updates by high
users. Finally, the database can perform the update, but automatically raise the classification of the
data to the higher level of the update. However, since this would effectively make this field
disappear from the low-level user's perspective, a downward signaling channel would again be
created.
In order to avoid the drawbacks of these update approaches, an alternative is for the database to
support multiple instances of records with the same primary key at different levels.
Polyinstantiation is a solution that controls inference and signaling channels, and that also offers a
way to implement cover stories. Polyinstantiation permits the creation of multiple instances of data
records -- a low user sees the tuple associated with a particular primary key populated with one set
of element values; a high user may see the "same" tuple with perhaps different values for some of
the elements or multiple tuples of different levels with different data values. By supporting
multiple instances, the database could allow a low subject to insert data and not change the data in
the existing higher level record. Likewise, a high subject is permitted to insert data without
disclosing it to low subjects
Polyinstantiation expands the notion of primary key to include the security label so that more than
one tuple may possess the same "apparent primary key" if they are at different security levels. By
its nature, polyinstantiation thus necessitates the violation of integrity across levels: Users at
different security levels querying on a record with the same apparent primary key will not be
ensured of viewing the same data. The DBMS ensures database integrity only with respect to the
data at a given level. Integrity across levels cannot be enforced if multilevel confidentiality is
strictly enforced.
The impetus for supporting polyinstantiation is the attempt to reconcile conflicting requirements
within an MLS DBMS -- the fundamental conflict between data integrity (which is not required by
the TCSEC) and confidentiality (which is required by the TCSEC). Confidentiality, or secrecy,
refers to protecting the data from unauthorized disclosure, while integrity refers to protecting the
data from unauthorized alteration or destruction.
It is important to note that the TDI does not explicitly mandate that polyinstantiation be supported
(or implemented). It only requires that confidentiality be enforced correctly, and that an evaluated
DBMS address in some manner the threat to confidentiality that can arise from integrity constraints
defined over data at more than one security level. Enforcing integrity constraints can open channels
by which high-level information can be transmitted down to low-level users, either through some
form of signaling, or through inference. Signaling channels can covertly transfer information
through permitting either detectable changes to a low-level storage object to reflect high-level data
(storage channel), or timing variations, detectable at the low-level which reflect the encoding of
high-level data (timing channel). Guidelines on acceptable bandwidths for covert channels are
given in Section 8 of the TCSEC. The potential speed of signaling makes attention to these
channels important for even TCSEC Class B1 DBMS products to address as an unacceptable
design flaw. Inference channels permit low-level users to infer high-level information from only
low-level data and knowledge about the real world and how the DBMS works. Polyinstantiation is
an approach to enforcing confidentiality while retaining some data integrity. In the wider database
community, integrity refers more generally to the correctness, accuracy, and internal consistency
of data. The issue becomes one of stretching the traditional non-secure data model, with its built-
in data integrity safeguards, to accommodate the needs of the MLS environment.
The topic of polyinstantiation remains contentious because it introduces additional complexity into
a database and it increases confusion as to what in a database reflects correctly modeled real-world
values. For example, a polyinstantiated real-world entity may be modeled by multiple database
tuples, and cases can easily arise where no single one of these tuples will contain all the correct
element values to properly describe the real-world entity. Further, the relational theory of
normalization is at odds with polyinstantiation enforcement: for example, a relation with
polyinstantiated elements is not even in first normal form. This document reviews polyinstantiation
research, examines these concerns, and summarizes different approaches to address the underlying
database integrity-secrecy conflict.
1.4 AUDIENCES OF THIS DOCUMENT
This document is targeted at four primary audiences: the security research community, database
application developers/system integrators, trusted product vendors, and product evaluators. In
general, this document is intended to present a basis for understanding the necessity for supporting
polyinstantiation or a suitable alternative in MLS DBMSs. Implemented approaches and ongoing
research are examined. Members of the specific audiences should expect to get the following from
this document:
Researcher
This document describes the basic issues associated with polyinstantiation. Important research
contributions are discussed as various topics are examined. By presenting current theory and
debate, this discussion will help the research community understand the scope of the issue and
highlight polyinstantiation approaches and alternatives. For additional relevant work, the
researcher should consult two associated TDI companion documents: Inference and Aggregation
Issues in Secure Database Management Systems [Inference 96] and Entity and Referential
Integrity Issues in Multilevel Secure Database Management Systems [Entity 96].
Database Application Developer/System Integrator
This document highlights the potential hazards and added complexity of management caused by
polyinstantiation in MLS applications. It describes techniques to aid the application developer to
minimize the occurrence of polyinstantiation and facilitate the efficient locating and cleanup of
undesired polyinstantiation aftereffects. Intentional polyinstantiation in the form of cover stories is
introduced, and the issues associated with cover story support are examined.
Trusted Product Vendor
This document describes the conflict between integrity and secrecy. It examines approaches to
polyinstantiation enforcement in an MLS database and the benefits and drawbacks of these
approaches. This is discussed in the context of tuple level as well as element level labeling.
Approaches to polyinstantiation adopted by currently evaluated commercial MLS DBMS products
are examined.
Evaluator
This document presents an understanding of polyinstantiation issues to facilitate evaluation of a
candidate MLS DBMSs implementation of polyinstantiation or an alternative.
1.5 ORGANIZATION OF THIS DOCUMENT
The organization of the remainder of this document is as follows:
· Section 2 provides background by defining terminology and notation adopted by this
document, and by introducing concepts basic to the discussion of polyinstantiation.
· Section 3 describes the issues associated with polyinstantiation in more detail and
discusses the architectural considerations that affect polyinstantiation.
· Section 4 introduces an example and uses it to examine a number of different
polyinstantiation and polyinstantiation-avoidance approaches.
· Section 5 presents the polyinstantiation practices of commercially available DBMS
products.
· Section 6 summarizes the polyinstantiation issues which were presented in the document.
SECTION 2
BACKGROUND
This section provides background necessary for discussing polyinstantiation. Terminology and
notation used by this report are introduced in Section 2.1. Section 2.2 compares polyinstantiation
enforced at the granularity of a tuple versus at the granularity of an element. Section 2.3 compares
the triggering of polyinstantiation due to an action on the part of a user at the high level versus by
one at the low level.
2.1 TERMINOLOGY AND NOTATION
MLS DBMSs utilize MAC to prevent the unauthorized disclosure of high-level data to low-level
users. In an MLS DBMS, it is necessary to hide the actions (inserts, deletes, updates) of high
subjects from low subjects, and thereby prevent signaling channels that could disclose high-level
data. It is also important to prevent low-level user actions from overwriting high-level data in the
DBMS. Inference is another threat to MAC policy: low-level users may be able to infer high-level
information from only low-level data, real world knowledge, and knowledge about how the DBMS
works, such as what actions it normally allows or disallows. One method to reduce or eliminate
some potential inference channels is to upgrade the classification of some key data element in the
inferential chain and thus remove access to it by the low-level user [Inference 96]. However, other
inferential chains to the high-level information may be available using an alternate path or too
much information may have already been disseminated and thus no longer be explicitly
represented in (controlled by) the database [Garvey 91]. Polyinstantiation (and support for cover
stories) addresses these concerns.
The term polyinstantiation was coined by the Secure Data Views (SeaView) project and refers to
the simultaneous existence within an MLS DBMS of multiple data objects with the same name,
where the multiple instantiations are distinguished by their security level [Denning 87]. The
relational data model is the formalism used in describing polyinstantiation. We assume readers
possess basic familiarity with the relational data model [Date 81, 83]. To illustrate the notation we
use throughout this paper, we give some brief definitions pertinent to MLS DBMSs and discussion
of polyinstantiation approaches.
A standard relational database can be perceived by its users as a collection of relations. Relations
are composed of tuples. A tuple (v1, v2, ..., vk) has k components where the i-th component is vi.
A relation may be viewed as a table, with rows called tuples, and the columns called attributes. A
relation has well-defined mathematical properties and consists of a relation scheme (which defines
the attributes and name of the relation) and a relation instance. A relation schema R (A1, ..., An)
consists of a relation name R and a set of attribute names A1, ..., An. An instance R of the schema
R consists of a set of tuples t, each of which contains a single value t[Ai] for each attribute Ai.
In an MLS environment, relations may consist of values at different classification levels. These
classification levels are ordered according to a security lattice. The reader should be familiar with
classification levels and the standard models of secure information flow [Fernandez 81, Denning
82]. Our examples will refer to the levels Top Secret (TS), Secret (S), Confidential (C), and
Unclassified (U) as applied to both data and to user sessions. For example, an "S-user" is a user
performing actions on the database from a session operating at the Secret level.
A multilevel relation schema Rc is an augmented version of a relation schema R (A1,...,An in which
security labels are associated with certain attributes, as constrained in the multilevel schema. These
are maintained by the DBMS TCB for each tuple. In a system that requires all values in a single
tuple to be uniformly classified, Rc may simply have a security label, TC, which is associated with
each tuple. If values within a tuple may be classified at different levels, then Rc includes security
labels C1,...,Cn, associated with each one of the attributes A1,...,An, respectively, within a tuple. (If
it is known that certain attributes will always be classified at the same level within a single tuple,
fewer security labels may be stored.) The inclusion of security labels C1,...,Cn represents the most
general case and is therefore used in this document.
The basic model for multilevel relations needs to be defined with a MAC policy in mind. The MAC
policy for MLS databases is often based on the Bell-LaPadula model [Bell 76], which is stated in
terms of subjects and objects. A subject is an active entity, such as a process that can request access
to objects, whereas an object is a passive entity, such as a record, a file, or a field within a record.
Every subject is assigned a clearance level and every object a classification level. Classification
levels and clearances are collectively referred to as security levels, and form a lattice. Each security
level has two components: a hierarchical component and a set (possibly empty) of unordered
categories. A security level c1 is said to dominate security level c2, in the induced partial order, if
(1) the hierarchical component of c1 is greater than or equal to that of c2, and (2) all categories in
c2 are included in those of c1. A security level c1 strictly dominates security level c2, in the partial
order, if (1) c1 dominates c2, and (2) c1 does not equal c2.
The following are two necessary conditions in the Bell-LaPadula model [Bell 76, DoD 85]:
1. The Simple Security Property or "No Read Up": A subject can only read objects at a
security level dominated by the subject's level, and
2. The *-Property (Star Property) or "No Write Down": A subject can only write objects at
a security level that dominates the subject's level.
To apply these concepts to a DBMS, it is necessary to determine the granularity of the objects
protected by MAC, i.e., the storage objects. Security levels are then associated with these storage
objects. Work on MLS databases has focused on four choices for assigning security levels to data
stored in a relation. One can assign security levels to entire relations, to individual tuples (rows) of
a relation, to individual attributes (columns) of a relation, or to individual elements of a relation.
Much of the research on polyinstantiation has examined the case where security levels are assigned
to individual data elements stored in relations. However, since tuple level labeling is used in all
MLS DBMS products evaluated to date, the concept of polyinstantiation must also be considered
in the (simplified) context of tuple level labeling. Polyinstantiation is not an issue when all values
of a given attribute are uniformly classified as is the case with relation level or attribute level
labeling [Hinke 75].
A candidate key of a relation schema is a minimal set of attributes that uniquely determine the other
attributes. There may be a number of candidate keys for a relation schema R, i.e., there may be
distinct sets of attributes that could be chosen to be a key for the relation. One candidate key is
identified as the primary key. The notion of a primary key is a fundamental concept in the world
of single-level relational databases. The primary key is used to facilitate storage and retrieval, and
maintain the integrity of relations. Entity integrity requires that the primary key serve as a unique
identifier of each tuple in the relation and that it does not contain a null value.
While the notion of a primary key is simple and well understood for classical (single-level)
relations, it does not have a straightforward extension to multilevel relations. A primary key's
uniqueness requirement can create signaling channels [Meadows 88]. One approach to avoiding
these channels involves augmenting the user-defined primary key with security labels associated
with the primary key attributes. The concept of an apparent primary key was introduced by
Denning et al., to refer to the unaugmented user-defined primary key [Denning 87].
A multilevel relation is said to be polyinstantiated when it contains two or more tuples with the
same apparent primary key values. Therefore, the real primary key (i.e., the minimal set of
attributes unique in each tuple) of the multilevel relation is obtained by additionally considering
the security labels associated with the attributes of the apparent primary key. The exact manner in
which this is done is closely related to the precise polyinstantiation behavior of the relation
[Cuppens 92]. The discussion of different approaches to resolving the polyinstantiation problem
provided in Sections 4 and 5 includes descriptions of how real primary keys are defined.
2.2 TUPLE VERSUS ELEMENT POLYINSTANTIATION
Two types of polyinstantiation may be considered. Tuple polyinstantiation refers to
polyinstantiation at the granularity of the data tuple (which normally represents a realworld entity).
Element polyinstantiation refers to polyinstantiation at the granularity of a data element.
A relation with tuple polyinstantiation contains multiple tuples with the same apparent primary key
values, but with different access class values. As an example, consider the relation Starship-
Objective-Destination (SOD) illustrated in Figure 2.1. The named "starship" is tasked with
performing a particular mission objective at a specified destination in the galaxy. In this relation,
it may be considered important to the success of a mission that a particular objective or destination
not be accessible to users not cleared up to a certain security level. It may even be desired that the
existence of the craft itself remain secret to uncleared users.
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise S
Spying S
Rigel S
S
Figure 2.1: A Multilevel Relation With Tuple Polyinstantiation
Here we demonstrate the SOD relation using an element-level labeling database in which each
element has a classification as well as a value. In an element-level labeling database, the
classification of a tuple is computed to be the least upper bound of the classifications of the
individual data elements in the tuple. This computed value is denoted in our examples within a
bolded column to distinguish it from data which must be stored in the database. The attribute
Starship is the apparent primary key of SOD. This example and many of those following are drawn
from Jajodia et al. [Jajodia 94].
The relation given in Figure 2.1 contains two tuples for a starship that has the same name, resulting
in tuple polyinstantiation. The apparent primary key values are identical in both tuples; however,
these values have different classification levels. These tuples can be regarded as pertaining to two
different real-world entities (e.g., if the existence of the secret Enterprise starship is unknown to a
U-user who then independently chooses to christen an unclassified starship "Enterprise"). It can
also be regarded as pertaining to two representations of a single real-world entity (e.g., an
unclassified cover story for a secret mission). It is important to note that the relation itself does not
indicate which of these two interpretations is true.
Figure 2.2 shows a relation with element polyinstantiation. A relation with element
polyinstantiation contains two or more tuples with identical values both for the apparent primary
key and the associated classification level element, but these tuples have different values for one
or more other elements, as illustrated in Figure 2.2. Both tuples in this relation refer to the same
starship Enterprise; however, the objectives and destinations of these tuples appear different to
users at the Secret level.
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise S
Spying S
Rigel S
S
Figure 2.2: Multilevel Relation With Element Polyinstantiation
As mentioned before, even when multilevel relations are labeled by tuple instead of element,
explicit polyinstantiation is still possible. Consider the same relation as in Figure 2.1 with the tuple-
level labeling that is offered by all currently evaluated commercial MLS DBMS products. The S-
user will see the multilevel relation shown in Figure 2.3. In this case the TC column is not bolded.
Because there are no security labels associated with individual elements, the classification of the
tuple must be stored explicitly (or implicitly) in the DBMS.
Starship
Objective
Destination
TC
Enterprise
Exploration
Talos
U
Enterprise
Spying
Rigel
S
Figure 2.3: Polyinstantiated Multilevel Relation With Tuple-Level Labeling
Notice that when tuple-level labeling is used, there is no differentiation between tuple
polyinstantiation and element polyinstantiation. In the example above, both tuples may possibly
pertain to a single starship. In this case, the U-tuple may be a cover story which was purposely
inserted to appease low user curiosity, or it may represent data about the starship which was entered
by a low user (whether erroneously or from another Unclassified source such as a news feed) or it
may have been left unchanged when an update was performed by a high user. On the other hand,
it is also possible that the tuples refer to two different starships that happen to have the same name,
although this situation may have arisen by error.
2.3 POLYHIGH VERSUS POLYLOW POLYINSTANTIATION
Tuple and element polyinstantiation can be triggered in two different ways: these are called
polyhigh and polylow for mnemonic convenience.
1. Polyhigh occurs when a subject at a high level attempts to insert a tuple with the same
apparent primary key as a low tuple or attempts to modify a low tuple. Although the high
subject could be notified of this collision, the low-level data cannot be modified.
Modifying the data and permitting access to it at the lower level would result in a direct
write down in violation of the MAC policy. Modifying the data and raising its level to
match the higher level of the subject would create a downward signaling channel, as the
data would disappear at the low level. Polyhigh polyinstantiation leaves the old tuple
unmodified, and instead inserts a new tuple which contains the new higher level data.
2. Polylow occurs when a subject at a low level attempts to insert a tuple with the same
apparent primary key as a high tuple or modify an attribute of a tuple which already
contains high data. In this case the low subject cannot be notified of the collision: if the
update were rejected, there would be a downward signaling channel. Therefore, a new
tuple is added with the lower level data.
The following examples show polyhigh and polylow using element polyinstantiation; tuple
polyinstantiation is similar.
To illustrate polyhigh, suppose that a subject attempts to update the following relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
null U
U
Suppose that the following sequence of updates occurs.
1. A U-user updates the destination of the Enterprise to be Talos. The result of this update is:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
2. Next, an S-user wishes to update the destination of the Enterprise to be Rigel. The system
cannot reject this update without denying legitimate privileges to the user, but there would
be a downward signaling channel if the U-level data were replaced by S-level data because
U-users would see data disappearing, as a result of the data being upgraded. Therefore, a
new tuple is added, making the relation polyinstantiated. U-users would see the following
unchanged relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
S-users, however, could see the following modified relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Spying U
Rigel S
S
S-users may interpret this polyinstantiated relation in two ways: (1) the tuples represent a real-
world truth, namely that the U-Level destination is a cover story for the S-level destination, or (2)
that there is some error or inconsistency in the database that must be repaired.
To illustrate polylow, suppose that the two updates above occur in the opposite order. Starting from
the same initial relation as above, suppose the updates occur in the following sequence.
1. An S-user updates the destination of the Enterprise to be Rigel. U-users see the following
unchanged relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
null U
U
S-users, however, see the following modified relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Rigel S
S
2. A U-user wishes to update the destination of the Enterprise to be Talos. The system cannot
reject this update or even notify the U-user without causing a downward signaling channel,
so the U-tuple is modified. U-users see the following modified relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
S-users see the following polyinstantiated relation, identical to the one at the end of the
polyhigh example:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Spying U
Rigel S
S
In some applications, permitting the denial of service (in the case of polyhigh) or the overwriting
of high data with low data (in the case of polylow), may be tolerable. Generally, however,
polyinstantiation is required to enforce a multilevel security policy which prevents signaling
channels as well as denial of service. Polyinstantiation may introduce complexity and even
ambiguity into a database. These and other issues associated with polyinstantiation are examined
in the following section.
SECTION 3
POLYINSTANTIATION CONSIDERATIONS
This section summarizes issues associated with polyinstantiation. Section 3.1 examines particular
problems that polyinstantiation introduces. Section 3.2 presents a straightforward but flawed
alternative to polyinstantiation. Section 3.3 examines the issue of enforcing automatic
polyinstantiation versus user requested polyinstantiation. Limiting polyinstantiation to occur only
when an authorized user explicitly requests it (i.e., to enforce cover stories) simplifies enforcement,
eliminating many of the problems associated with polyinstantiation. Section 3.4 examines the
architectural implications of supporting polyinstantiation.
3.1 PROBLEMS CAUSED BY POLYINSTANTIATION
This section discusses problems that enforcement of polyinstantiation presents to the database
designer and maintainer, as well as the database application designer and database user. A core
problem associated with polyinstantiation is the fundamental conflict between maintaining the
secrecy of the information within a database and the integrity of the database. A database designer
has added responsibility to see to the continued consistency of the database despite the need to
present multiple views to its users. Enforcing secrecy through polyinstantiation results in the loss
of real-world entity integrity and increases the complexity of the data relationships within the
database. Due to this added complexity, as well as the unintentional ways in which
polyinstantiation may be mistakenly invoked, increased administrative burden is also a problem.
3.1.1 Loss of Real-World Entity Integrity
Intuitively, a standard relational database represents a single non-contradictory view of the real
world. Adding multilevel security and polyinstantiation support means that the relational database
must now represent multiple views, potentially one or more for each classification level
represented in the database. Whereas in a non-MLS database each tuple in a relation would
normally represent a unique instance of a real-world entity, in an MLS DBMS this one-for-one
relationship does not necessarily hold true and ambiguity becomes a problem.
To address secrecy-threatening signaling issues, low level users must be prevented from learning
any information about higher level views. Polyinstantiation provides a means of hiding high views
from low users. However, with polyinstantiation, realworld entity integrity may now only be
ensured within a level and not across multiple levels. Further, a user of the database whose
authorization spans more than one classification level will now be in the position of having to
choose from multiple views. If the database presents views which contain contradictory data
without associated explanation, the user may be faced with an inconsistent situation [Cholvy 94].
As described in Section 2.0, multiple tuples with the same apparent primary key could represent
either (1) separate real-world entities modeled with the same key in the database, or (2) a single
real-world entity modeled differently at different security levels. In non-MLS DBMSs, the first
case violates the property that primary keys be unique. A non-MLS DBMS would simply prohibit
the addition of a second tuple with the same key as an existing tuple. However, in an MLS
environment, this prohibition may conflict with confidentiality requirements. If the existing tuple
is at a higher security level than the proposed second tuple, prohibiting the addition of the second
tuple will signal the existence of the higher level tuple. This consequence is at the core of the tuple
polyinstantiation problem. Since the user inserting the new tuple cannot be informed of the
existence of the higher level tuple, one cannot determine whether the user is entering data about
the same real-world entity.
The relational theory of normalization has been studied as a way to address polyinstantiation and
the relationship of integrity to secrecy [Qian 92, 94]. The notion of integrity is problematic for
MLS DBMSs. Qian argues that if integrity constraints serve as a filter on how much low-level data
can flow high then the case of a low datum contradicting a high datum would neither be apparent
at a particular level (violating integrity) nor would either datum need to be disallowed (causing loss
of information and introducing a signaling channel). Likewise, the notion of secrecy is problematic
in the presence of integrity constraints. Again, it is argued that if update semantics are adopted that
permit high-level users to gain access to low-level data only as long as integrity is preserved, then
signaling channels are eliminated: Low updates are not denied due to high data, and high updates
will not cause low data to change. Qian concludes that the detection of deductive inference
channels is a special case of integrity protection: integrity implies the absence of deductive
inference channels. Thus, according to Qian, the relationship between integrity and secrecy are not
necessarily in fundamental conflict.
3.1.2 Increased Database Complexity to the User
Polyinstantiation can result from a number of scenarios:
1. A user intentionally introducing a cover story against data stored at a higher level.
2. Automatic polylow or polyhigh polyinstantiation correctly acting to counter the signaling
threat which would result from announcing the existence of higher data when a user (or
malicious code acting on a user's behalf) modifies or inserts data already stored at another
level.
3. A user who, by failing to check whether the data being added is referenced already at a
lower level, mistakenly modifies a relation at a level higher than necessary. (This is
especially a concern associated with executing at the wrong level an automated program
possessing only limited checks.)
4. A data regrade in progress in which a high user adds a tuple at the new level (low or high),
but has not yet deleted the old tuple.
Given the wide range of reasons which can be the cause of a database demonstrating the effects of
polyinstantiation, the increased complexity to interpret what is actually signified by what is stored
in the database is obvious.
For example, consider the following scenario using a tuple level labeling DBMS. Given the
following relation:
Starship
Objective
Destination
TC
Defiant
Repair
Talos
S
Enterprise
Exploration
Talos
U
An operator is provided the Secret information that the Enterprise should prepare for a spying
mission. By mistake, the operator updates this information as a TS-user, creating the following
relation:
Starship
Objective
Destination
TC
Enterprise
Spy
Talos
TS
Defiant
Repair
Talos
S
Enterprise
Exploration
Talos
U
Another operator is relayed the information that the destination of the Secret Enterprise mission is
Rigel. Logging in as an S-user and making this straightforward addition creates the following table:
Starship
Objective
Destination
TC
Enterprise
Spy
Talos
TS
Enterprise
Exploration
Rigel
S
Defiant
Repair
Talos
S
Enterprise
Exploration
Talos
U
On a far outpost, a new supply ship is christened with the name Defiant and a U-user adds this
information as the ship sets out for Talos, unaware this name is already taken by a secret military
ship.
Starship
Objective
Destination
TC
Enterprise
Spy
Talos
TS
Enterprise
Exploration
Rigel
S
Defiant
Repair
Talos
S
Defiant
null
Talos
U
Enterprise
Exploration
Talos
U
Now suppose the Romulans attack Talos and in this crisis situation you are given the task of
querying the database to determine which resources are near the front. How difficult will it be to
determine what is reality vs. cover story vs. database entry error?
An operational polyinstantiating database may be subject to a number of inputs over its lifetime
which can create similar ambiguous and/or spurious tuples. Mechanisms and enforcement of
administrative procedures will be needed to periodically cleanup undesired polyinstantiation
effects. The additional burden to database administration is discussed in the following section.
3.1.3 Increased Database Administration
Real-world use of databases enforcing polyinstantiation will demand additional duties of the
database administrator, as well as greater effort on the part of the users. Users must be careful not
to cause unwanted polyinstantiation. Inadvertent and unwanted polyinstantiation may result due to
clumsy user error, incorrect use of functionality, or *-property anomalies which result from
operations being performed from the wrong level by a user or automated function. Undesired
polyinstantiation will create additional clean-up issues for the database administrator. All of these
increased database administration issues are experienced with tuple-level labeling databases, and
become ever more problematic for element-level labeling databases.
Depending upon the specific decomposition and materialization algorithms with which
polyinstantiation is enforced, a database may also become cluttered with what may be considered
spurious tuples. In the original element-level label SeaView model [Lunt 90], a single update
operation could create a number of tuples equal to the product of the number of tuples in each of
the affected domains.
For example, in the original SeaView, an S-user issuing an update to the following relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
to state that the Enterprise's mission was actually spying at Rigel, would generate the following
tuples:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Exploration U
Rigel S
S
Enterprise U
Spying S
Talos U
S
Enterprise U
Spying S
Rigel S
S
The polyinstantiation integrity property enforced by SeaView requires that the key and its
classification determine each attribute and its classification via multivalued dependency (see
Section 4.2). If the user updating the relation believes the second and third tuples are spurious,
clean-up tasks are required to remove them. Further discussion of spurious tuples and techniques
to minimize them is presented in Section 4.2's detailed discussion of decomposition and
materialization algorithms.
The enforcement of polyinstantiation may also cause the database administrator to be subject to
complaints about database performance and increased requests for database tuning to improve it.
Depending upon the implementation and the number of instances across levels, the overhead
associated with polyinstantiation may significantly slow query performance.
A final administrative issue is interoperation with resident and subsequently acquired database
applications. Untrusted applications written for standard versions of DBMS products may be
incompatible with polyinstantiation enforcement, and may behave unpredictably when
polyinstantiation is encountered. A database administrator will have additional duties to ensure that
introduced tools or utilities will not "break" the database's integrity, and that the tools will not
break or produce erroneous results.
3.2 A SIMPLE, BUT UNACCEPTABLE, ALTERNATIVE TO
POLYINSTANTIATION
A simple solution may be theorized that enforces "secure" alternatives to both visible and invisible
polyinstantiations. These alternatives are secure in the sense of secrecy and information flow, and
preserve primary key requirements in multilevel relations; but unfortunately, they suffer from
denial of service and integrity problems. The following two rules describe this solution:
1. Whenever a high user makes an update which violates the uniqueness requirement, we
simply refuse that update.
2. Whenever a low user makes a change that conflicts with the uniqueness requirement, the
conflicting high data is overwritten in place by the low data.
It is not difficult to see that this simple alternative to polyinstantiation preserves the uniqueness
requirement in multilevel relations. This solution is also secure in the sense of secrecy and
information flow. Although this solution may be acceptable in some specific situations, it is clearly
unacceptable as a general solution because it introduces serious denial-of-service and integrity
problems.
3.3 AUTOMATIC VERSUS INTENTIONAL POLYINSTANTIATION
Two approaches to implementing polyinstantiation may be considered: instruct a system to
automatically polyinstantiate whenever an action by a high or low user opens a potential inference
channel, or polyinstantiate only when manually directed to by a high user and handle any
remaining inference channel concerns through some other means. The more general approach of
automatic polyinstantiation is the prevalent method which has been investigated, but restricting
polyinstantiation to occur only upon a user's request eliminates many of the automatic
polyinstantiation approach's shortcomings.
Intentional polyinstantiation refers to user-directed polyinstantiation, generally in support of
creating cover stories that lead to alternative explanations and the forestalling of an inferential
chain resulting in unauthorized disclosure of information [Garvey 91]. Cover stories must be
designed to purposely mislead the lower user with a plausible explanation and prevent the
inference of the classified value. To be effective, a cover story usually requires consistency.
Some researchers assert that cover stories are the only valid reason for the use of polyinstantiation
[Burns 90, 91]. The complexity and confusion caused by automatic polyinstantiation, especially
when likely blunders by unsophisticated users are acknowledged, supports this argument for
implementing only intentional polyinstantiation. The ability for complexity arising from
polyinstantiation to impact a modeled mission was illustrated in Section 3.1.2. These researchers
would advocate alternatives to polyinstantiation where cover stories are not explicitly needed. It
may be argued that polyinstantiation is not a fundamental property of multilevel databases. Rather,
polyinstantiation simply provides a powerful technique for supporting cover stories. Other
research has sought to demonstrate that polyinstantiation is not even essential for supporting cover
stories, and may be considered a poor technique since it is difficult to prevent spurious cover stories
from occurring [Wiseman 90].
The problem of maintaining global consistency remains in databases that restrict polyinstantiation
to intentional polyinstantiation. High users who query the database and are returned high level data
and low level cover stories without explanation are still faced with an inconsistent situation.
One approach to this complexity is to assume that the higher the level view of a database the more
reliable it is, and that the other views should be considered cover stories [Cholvy 94]. In the case
of databases with partial ordering, Cholvy and Cuppens suggest that topics be associated with the
data. Topics allow representation of semantic links between data, and may be used to parameterize
the order of the security levels and to merge data with this finer grain of preference. The database
administrator would define an order of preference for merging information related to a specific
topic.
For example, consider a database which includes Unclassified and Secret hierarchical
classifications and two compartments: Destination and Cargo. The topics dest and freight are
defined against the database. The order of preference specified for merging information related to
these two topics may vary. For instance, the total order of preference defined for merging
information related to the topic dest might be:
(S, {Destination, Cargo}) > dest(S, Destination) > dest(S, Cargo) > destU
while the total order of preference for the topic freight might be:
(S, (Destination, Cargo)) > freight(Si Cargo) > freight(Si Destination) > freightU
The order of preference between these two topics differ because, according to the specific need to
know of users at level (S, Cargo), information related to the topic freight is more reliable at level
(S, Cargo) than at level (S, Destination) and the opposite is true for information related to the topic
dest.
Another approach is to create two different attributes named Cargo and Freight which are classified
at different global levels. These could be used as a means of controlling the two different views
without ambiguity [Hinke 75].
3.4 ARCHITECTURAL CONSIDERATIONS IN SUPPORTING
POLYINSTANTIATION
An MLS DBMS may be designed and built using a number of different architectures. The adopted
architecture is closely tied to the range of polyinstantiation strategies available for a given system.
Two primary architectures may be defined based on whether the database is trusted with respect to
MAC [NAP 83]. In this section, we briefly discuss these two architectures in order to identify their
influence on polyinstantiation strategies.
Figures 3.1 and 3.2 illustrate the two approaches. Figure 3.1 illustrates the No MAC Privileges
(NMP) architecture. Separate databases store data at each classification level. Each DBMS
process can access all databases with data at or below its level.
A variation to the NMP approach is for each DBMS to contain data at a given level and replicated
data from all lower databases. The SINTRA database system prototype has adopted this replicated
distributed approach with MAC enforcement by a trusted frontend, physical separation as a
protection measure, and no modification to untrusted backend DBMSs [Kang 94]. The SINTRA
architecture boasts greater data retrieval performance because a user's view is materialized from a
single database, rather than across multiple single level database fragments.
Figure 3.1: No MAC Privileges Architecture
Figure 3.2 illustrates the Trusted Subject architecture. A single database is used to store data at
multiple levels, and the DBMS is trusted (i.e., has MAC privilege) to guard against illegal
information flows.
Figure 3.2: Trusted Subject Architecture
Polyinstantiation is a natural consequence of the NMP architecture. Since a lower-level DBMS has
no knowledge of higher-level data, there is no way to prevent lower-level subjects from making
updates that conflict with higher-level data. Requiring all keys to be classified at the lowest level
protects against tuple polyinstantiation, but element polyinstantiation could still occur.
Furthermore, this requirement may not be suitable for all applications. A specific approach to avoid
this problem is discussed in Section 4.4.2. Element polyinstantiation may be allowed by defining
logical relations that span multiple levels. The underlying databases would store single-level
fragments of the relations. Restrictions on fragmentation are the first method to control the types
of polyinstantiation semantics allowed within a system. If relations are fragmented, the fragments
must be integrated to provide a coherent response to a query. Sections 4 and 5 discuss a number of
approaches to integrating data.
The controls to prevent illegal information flow, which are built into the OS TCB in the NMP
architecture, must be implemented through trusted software in the DBMS part of the TCB in the
Trusted Subject architecture. For some applications it may be desirable to permit certain channels
and avoid polyinstantiation. In general, this is easier to do in a Trusted Subject architecture where
the DBMS portion of the TCB has access to all data levels, than in the NMP architecture where
each DBMS instance can only observe data at levels it dominates.
SECTION 4
POLYINSTANTIATION APPROACHES
A number of different approaches can be used to implement polyinstantiation in a database
management system reflecting divergent perspectives on the meaning and uses of polyinstantiation
within an MLS environment. Each perspective has its strengths and its weaknesses, and the correct
choice of approach depends on the requirements of specific applications. Different applications
will be built to model different understandings of and requirements for multilevel data. Some
approaches are only appropriate for databases enforcing element-level labeling, and as such may
not be applicable to current commercial DBMS efforts which enforce tuple-level labeling.
This section introduces an example which will be used to examine various polyinstantiation
approaches. Our discussion starts with approaches to MLS DBMS design that rely upon
polyinstantiation and propagation of tuples to reflect different meaningful combinations of
attribute values. Next, the section presents strategies that answer users' queries by using the
security levels of retrieved tuples to derive new tuples. The last group of approaches places explicit
restrictions on users' views of data, and includes approaches which permit complete avoidance of
polyinstantiation. While this section presents a wide range of options, no claim is made that its
exploration of polyinstantiation approaches is exhaustive. Likewise, alternative approaches toward
the express goal of avoidance of polyinstantiation may be theorized, including careful auditing of
channel usage, and constraints on the levels allowed in the DBMS.
4.1 AN EXAMPLE FOR COMPARING POLYINSTANTIATION APPROACHES
This section provides a detailed example that is used by the rest of Section 4 and Section 5 to
compare and contrast approaches to polyinstantiation. Some of these approaches use
polyinstantiation, while others add restrictions to eliminate the need for polyinstantiation. We use
the same relation as previously, SOD, with the attributes Starship, Objective, and Destination. We
assume for simplicity that the subjects and objects in our database may be described by simple
hierarchical levels -- for example U and S. Furthermore, we assume here that the Starship attribute
is always Unclassified. Therefore, the classification range of the Starship attribute has lower and
upper bounds of U. Suppose, however, that both the Objective and Destination attributes have a
classification range with a lower bound of U and an upper bound of S. Figure 4.1 shows the schema
of this new relation, SODc.
The Tuple Class security label, abbreviated as TC, gives us the classification of the entire tuple.
TC is a redundant (computed) security label whose value is the least upper bound of the
classifications associated with the individual attributes in a tuple. The range of TC is derived in an
obvious way from the classification ranges of the individual attributes.
Starship is the apparent primary key of SOD. Intuitively, this means that if SODc contained only
single-level data, then Starship would be the actual primary key of the relation. If SODc contains
Unclassified and Secret data, however, the actual primary key of SODc would be Starship along
with the classifications associated with the attributes. This primary key concept is central to the
polyinstantiation problem and is formally stated in the next section.
Attribute
Classification Range
Starship
[U, U]
Objective
[U, S]
Destination
[U, S]
Tuple Classification (TC)
[U, S]
Figure 4.1: Multilevel Scheme for the Relation SODc
An instance of SODc is likely to contain tuples at different levels. Therefore, it is important to
distinguish between the U-instance of SODc, visible to Unclassified users, and the S-instance,
visible to Secret users. Increasing a user's clearance level should keep all previously visible
information intact and perhaps add some new facts visible only at the higher level. For example,
consider the U-instance of SODc given in Figure 4.2. It contains exactly one tuple, meaning that,
as far as Unclassified users are concerned, the starship Enterprise has set out to explore Talos.
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Figure 4.2: U-Instance of SODc
Figure 4.3 enumerates eight different S-instances of SODc, all of which are consistent with the U-
instance of Figure 4.2. Their common property is that the single tuple of the U-instance appears in
all eight S-instances. Each tuple in an instance of SODc defines a mission for the starship in
question. A U-instance of SODc allows only one mission per starship. S-instances, on the other
hand, allow up to four missions (pairs of Objective and Destination) per starship, three of which
are Secret and one Unclassified.
To gain further intuition into the polyinstantiation problem, consider instance 8 of Figure 4.3. This
instance contains four tuples for the starship Enterprise. The classification associated with the
Objective and Destination attributes makes each tuple distinct.
No.
Starship
Objective
Destination
TC
1
Enterprise U
Exploration U
Talos U
U
2 a
Enterprise U
Exploration U
Talos U
U
b
Enterprise U
Spying S
Talos U
S
3 a
Enterprise U
Exploration U
Talos U
U
b
Enterprise U
Exploration U
Rigel S
S
4 a
Enterprise U
Exploration U
Talos U
U
b
Enterprise U
Spying S
Rigel S
S
5 a
Enterprise U
Exploration U
Talos U
U
b
Enterprise U
Exploration U
Rigel S
S
c
Enterprise U
Spying S
Rigel S
S
6 a
Enterprise U
Exploration U
Talos U
U
b
Enterprise U
Spying S
Talos U
S
c
Enterprise U
Spying S
Rigel S
S
7 a
Enterprise U
Exploration U
Talos U
U
b
Enterprise U
Spying S
Talos U
S
c
Enterprise U
Exploration U
Rigel S
S
8 a
Enterprise U
Exploration U
Talos U
U
b
Enterprise U
Spying S
Talos U
S
c
Enterprise U
Exploration U
Rigel S
S
d
Enterprise U
Spying S
Rigel S
S
Figure 4.3: Eight S-Instances of SODc
The eight S-instances of SODc can be partitioned into three classes as follows:
1. Instance 1 has no polyinstantiation.
2. Instances 2, 3, and 4 have a single U-tuple (a) and a single S-tuple (b) for the Enterprise.
The U-tuple could be interpreted as a cover story for the correct information in the S-tuple.
Instances 2, 3, and 4 show a cover story applied against different aspects of higher
classified information. For example, instance 2 has a cover story for the objective but not
the destination, while instance 3 has a cover story for the destination but not the objective.
3. Instances 5, 6, 7, and 8 are, however, confusing to interpret if it is assumed that higher level
data correctly represent the real world. Each of these cases has more than one S-tuple for
the Enterprise, but only one U-tuple. Nonetheless, a meaningful and consistent
interpretation and update semantics for such relations may be developed [Jajodia 90a,
90c].
4.2 PROPAGATION APPROACH
One way to implement polyinstantiation is termed the propagation approach The perspective that
polyinstantiation is inherent in an MLS DBMS reflects the idea that, in the real world, people with
different security clearances may see different information about the same entity. Similarly, MLS
DBMS users at different levels may see different attribute values for the same real-world entity
(e.g., an Unclassified cover story for a starship's destination), and the users' updates will reflect
these different views. New tuples are added to reflect the different attribute values. The number of
polyinstantiated tuples may be quite large under this approach to polyinstantiation.
The propagation approach must meet two requirements:
1. Ensuring that keys still function to uniquely identify tuples in the database, and
2. Controlling the propagation of tuples to include only meaningful combinations of attribute
values.
The first requirement can be met by augmenting the apparent key with a security level and
enforcing the standard key uniqueness property over this augmented key. The second requirement
is more difficult to meet and researchers are still debating which types of combinations are
meaningful. In general, multivalued dependencies (see [Date 83] for a more detailed explanation)
are used to define the particular combinations allowed by a specific solution. While many variants
are possible, the SeaView project [Denning 87, 88a, 88b; Lunt 89, 90, 91] and the proposed
modifications of Jajodia and Sandhu [Jajodia 90b] provide the basis of this approach. First, we
present the original SeaView approach, then Jajodia's and Sandhu's proposed modification, and
finally some new techniques subsequently proposed by the SeaView project.
The goal of the SeaView project was to design an MLS relational database management system
that satisfies the TCSEC for Class A1 [DoD 85]. It is claimed that the SeaView design and
architecture can satisfy the A1 requirements simply by hosting it on an A1 operating system to
which Trusted Oracle has been ported. This has not been done because no A1 OS is available
[Hsieh 93, Lunt 94].
SeaView solves the problem of polyinstantiation of key attributes themselves by defining an entity
integrity property. This property requires all attributes in a key to be uniformly classified. That is,
for any instance Rc of a multilevel relation schema Rc, for any tuple t ¿ Rc, and for any attributes
Ai and Aj in the apparent primary key KR of R, t[Ci] = t[Cj]. Notice that this means it is possible
simply to define a single security label CK to represent the classification level of all attributes in
the apparent primary key. Further, no tuples may have null values for key attributes. This
restriction ensures that keys can be meaningfully specified and checked for uniqueness. In
addition, all non-key attribute classifications must dominate CK. This restriction guarantees that if
a user can see any part of a tuple, then he can see the key.
To meet the first challenge, that of using keys to determine when tuples model distinct real-world
entities, SeaView defines a polyinstantiation integrity property, described below. The formulation
of polyinstantiation integrity in SeaView consists of two distinct parts. The first part consists of a
functional dependency component whose effect is to prohibit polyinstantiation within the same
access class. The second part consists of a multivalued dependency requirement.
SeaView Polyinstantiation Integrity Property: A multilevel relation Rc satisfies
polyinstantiation integrity (PI) if and only if for every Rc there are for all Ai ¿ KR:
1. KR, CK, Ci > Ai
2. KR, CK >> Ai, Ci
(The above single arrow is simple functional dependency; the double arrow represents multivalued
functional dependency.) The PI property can be regarded as implicitly defining what is meant by
the primary key in a multilevel relation. The primary key of a multilevel relation is KR » CK u CR
(where CR is the set of classification security labels for data attributes not in K) since from PI it
follows that the functional dependency KR » CK » CR > AR holds (where AR consists of all
attributes that are not in KR). For example, considering a U instance for the schema displayed in
Figure 4.1, <the apparent primary key> (Starship) » <classification level of all attributes in the
apparent primary key> `U' » <set of classification security labels for data attributes not in the
primary key> `S' > (Objective, Destination).
The inclusion of the multivalued dependency in the definition of polyinstantiation integrity means
that one update may result in a number of tuples being added to the relation. To illustrate, consider
the situation in which an S-user attempts to go from S-instance 1 to S-instance 4 in Figure 4.3 by
inserting the Secret tuple specifying the secret mission of spying on Rigel. SeaView will interpret
this as a request to go from S-instance 1 to S-instance 8, thereby manufacturing two additional
missions for the Enterprise. Unfortunately, this increases the potential for such additional
information, that may not reflect true data, to be retrieved from the database by users with higher
clearances.
In fact, of the eight instances defined in Figure 4.3, SeaView's definition of polyinstantiation
integrity allows only two combinations of these eight instances within a single relation scheme.
Specifically, a SeaView relation can accommodate either instances 1, 2, 3, and 8 or instances 1 and
4 within a single scheme in the absence of the uniform classification constraint. SeaView admits
only instances 1 and 4 if the Objective and Destination attributes are uniformly classified (i.e.,
either both are classified U or both S).
It is easy to see that, in the worst case, the number of manufactured tuples grows at the rate of |
security-latticel | k, where k is the number of non-key attributes in the relation. For instance, Figure
4.4 shows a TS-instance of a relation similar to SOD, except that it has a range of four security
levels for the Objective and Destination attributes. The particular TS-instance shown there
describes four missions for the Enterprise, one each at the Unclassified, Confidential, Secret, and
Top Secret levels.
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Mining U
Sirius C
C
Enterprise U
Spying S
Rigel S
S
Enterprise U
Coup TS
Orion TS
TS
Figure 4.4: TS-Instance of SOD With Four Missions
The definition of polyinstantiation integrity in SeaView requires that this information be
represented by the sixteen missions shown in Figure 4.5. Users with clearances of U, C, S, and TS
will respectively see 1, 4, 9, and 16 missions with the SeaView approach.
Jajodia et al., propose dropping the multivalued dependency from the polyinstantiation integrity
property defined in the SeaView model [Jajodia 90a]. The authors argue that the multivalued
dependency prohibits the existence of relation instances that are desirable in practice. Specifically,
they argue that it is possible to accommodate all eight instances of Figure 4.3. Jajodia has
developed formal operational semantics for these update operations on multilevel relations
[Jajodia 91a, 91b, 91c].
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Exploration U
Sirius C
C
Enterprise U
Mining C
Talos U
C
Enterprise U
Mining C
Sirius C
C
Enterprise U
Exploration U
Rigel S
S
Enterprise U
Mining C
Rigel S
S
Enterprise U
Spying S
Talos U
S
Enterprise U
Spying S
Sirius C
S
Enterprise U
Spying S
Rigel S
S
Enterprise U
Exploration U
Orion TS
TS
Enterprise U
Mining C
Orion TS
TS
Enterprise U
Spying S
Orion TS
TS
Enterprise U
Coup TS
Talos U
TS
Enterprise U
Coup TS
Sirius C
TS
Enterprise U
Coup TS
Rigel S
TS
Enterprise U
Coup TS
Orion TS
TS
Figure 4.5: Propagated Tuples of SODc
Lunt and Hsieh of the SeaView team developed a semantics for the basic database manipulation
operations (insert, update, and delete) [Lunt 91]. Based on these semantics, they propose a different
definition for polyinstantiation integrity consisting of two separate pieces: (1) a state property
containing the same functional dependency component, and (2) a transition property concerning a
new dynamic multivalued dependency component. The latter property can be illustrated informally
by an example from [Lunt 91].
Consider the multilevel relation scheme SOD (Starship, CStarship, Objective, CObjective,
Destination, CDestination, TC), composed of three attributes with associated element-level security
labeling, and the computed tuple security label TC. The Starship attribute is the apparent primary
key of SOD. An instance SODc at a classification level c is assumed to satisfy the two constraints
of the PI property.
Now, consider the following relation instance SODU:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Suppose a Confidential user changes the value of Objective to "Mining," as shown here:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Mining C
Talos U
C
Under the update semantics of [Lunt 91], whenever an update involves some, but not all, of the
non-key attributes, certain dynamic multivalued dependencies are enforced in the multilevel
relations. In the example, the dynamic multivalued dependencies are:
Starship CStarship '' (Objective, CObjective) | (Destination, CDestination)
where the notation X '' Y | Z denotes the multivalued dependencies X '' Y and X '' Z.
Next, suppose a TS user updates the value of Destination to equal "Rigel." As before, since this
update involves some (but not all) of the nonkey attributes, the dynamic multivalued dependency
property causes two more tuples to be added to the relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Mining C
Talos U
C
Enterprise U
Exploration U
Rigel TS
TS
Enterprise U
Mining C
Rigel TS
TS
At this point, suppose a Secret user changes the value of the Objective to "Spying." The following
relation instance will result:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Mining C
Talos U
C
Enterprise U
Exploration U
Rigel TS
TS
Enterprise U
Mining C
Rigel TS
TS
Enterprise U
Spying S
Talos U
S
Enterprise U
Spying S
Rigel TS
TS
As stated in [Lunt 91], the way in which an update occurs determines whether or not the
multivalued dependency should be enforced. Essentially, if two or more attributes were updated in
a single update statement, the multivalued dependency would not be enforced. However, if the two
attributes were updated in two independent operations, the multivalued dependency would be
enforced. This dynamic approach has not been formalized, nor is it being incorporated into the
SeaView prototype.
4.3 DERIVED VALUES APPROACH
A second perspective on polyinstantiation is that although a multilevel relation may have several
tuples for the same real-world entity, there should be only one such tuple per classification level.
Instead of a classification level Ci associated with each attribute Ai, the schema Rc includes a single
classification level for each tuple, TC. When a user wants to update only certain attributes at a
particular level, the values of the other attributes will be derived from values at lower security
levels.
Consider the following relation SOD where Starship is the key:
Starship
Objective
Destination
TC
Enterprise
Exploration
Talos
U
Now suppose an S-user wishes to modify the destination of the Enterprise to be Rigel. He can
simply do so by inserting a new Secret tuple to SOD as follows:
(Enterprise, û, Rigel, S)
The symbol û is to be interpreted as follows: For this S-tuple the value of the Objective field is
identical to the corresponding U-tuple in SOD. As a consequence, when an S-user asks for the SOD
relation to be materialized, he sees the following:
Starship
Objective
Destination
TC
Enterprise
Exploration
Talos
U
Enterprise
Exploration
Rigel
S
The relation will appear unchanged to the U-user.
The Lock Data Views (LDV) project [Haigh 91] follows this derived data approach.
The derived data approach has been implemented for the United States Transportation Command
Air Mobility Command MLS Global Decision Support System (GDSS) [Nelson 91]. The MLS
GDSS implementation limits polyinstantiation in a multilevel relation to at most one tuple per
security class. Information is labeled at one of two levels, U or S. The design is based on the
organization's assumption that when S and U data are integrated into a single S response, the S data
takes precedence over the U data. This design can be extended to environments with more than two
strictly ordered security levels. Organizations for which this strict hierarchical rule does not apply,
such as many compartmented environments, would need to incorporate substantial changes into
this design in order to use it.
In the MLS GDSS application, trusted application software functionally extends the commercial
off-the-shelf (COTS) MLS DBMS to manage tuple-level polyinstantiation. Before inserting an S-
tuple, the trusted software ensures that a U-tuple exists with the same key. If it does not exist, the
insertion of an S tuple is not permitted. If a U-tuple with the same apparent primary key does exist,
the trusted application software examines each S-tuple attribute value, except the apparent key
value, and determines if it replicates the attribute's value in the U-tuple. If so, the value is not
replicated in the S-tuple but instead is set to null, minimizing data replication. The U-tuple thus
serves as the foundation upon which the S-tuple is built. The MLS GDSS solution is best explained
with several examples. Consider the following relation:
Starship
Objective
Destination
TC
Enterprise
Exploration
Talos
U
Now suppose an S-user wishes to modify the destination of the Enterprise to Rigel. The S-user
directs the system, through the trusted software, to insert an S-tuple into the SOD as follows:
S-USER:
Insert into SOD
(Starship, Objective, Destination)
Values (`Enterprise', `Exploration', `Rigel');
The U and S tuples are now stored in the relation as follows:
Starship
Objective
Destination
TC
Enterprise
Exploration
Talos
U
Enterprise
null
Rigel
S
By reducing the replication of data across polyinstantiated tuples, the probability of maintaining
the integrity of the database improves. Additionally, except for the key value, the sensitivity level
of all attribute values contained within the stored tuple are equivalent to the TC value. Given this
equivalence to the TC value, trusted application software derives attribute value labels from the TC
value. Users operating at the U-level are presented with a display showing the derived attribute
value labels as follows:
Starship
Objective
Destination
Enterprise U
Exploration U
Talos U
Users operating at the S-level are presented with a single composite display of a materialized tuple.
This materialized tuple comprises S and U data as follows:
Starship
Objective
Destination
Enterprise U
Exploration U
Rigel U
One of the major impacts of this derived values polyinstantiation approach, as implemented in the
MLS GDSS, involves the DBMS join operator at the S-level. Figure 4.6 illustrates the simplest
form of this problem which needed to be addressed by GDSS. A typical join operation between
two tables matches and retrieves rows based on the primary key Starship. In order to retrieve data
residing at the same security level, and thus permit proper collapsing of the rows into a materialized
tuple, the join is further qualified by the row's security label attribute TC:
S-USER:
Select * from Table 1, Table 2
where Table 1.Starship = Table 2.Starship and Table 1.TC = Table 2.TC
An important functional requirement in MLS GDSS is that S-users expect to see S-data as the end
product of a retrieval, if S-data exists; otherwise, U-data is returned. Case 1 in Figure 4.6 shows a
join between two tables that produces the correct materialized tuple for an S user. Case 2 illustrates
the anomaly associated with the join. In this case, Table 2 contains only U-data. Since the query
requires that the tuple labels must match, the query does not return the S-row of Table 1 to be joined
with the U-row of Table 2. Thus, if data does not exist at the same security levels in each table,
then information may be lost during the join operation.
In this simplified example, one might argue that removing the qualification that the tables must be
joined by tuple labels would permit joins. Doing this would return two rows in Case 2, one
containing only U information, and the other containing S and U information. If this approach were
taken, the tuple materialization process would become more complex and would need to extract
multiple tuple labels and assign them to the appropriate columns in the row that was returned. Also,
the join example shown in Case 1 would result in four rows of data returned from the server,
instead of just two. The complexity of the problem and the work required of the DBMS server
would increase significantly as more tables were joined. Database server performance would
decrease accordingly, perhaps to unacceptable levels.
Case 1:
Starship
Objective
Destination
TC
Enterprise
Exploration
Talos
U
Enterprise
null
Rigel
S
Table 1
Starship
Type
Propulsion
TC
Enterprise
Starship
Photon
U
Enterprise
Battlestar
Queller Drive
S
Table 2
Starship
Objective
Destination
Type
Propulsion
Enterprise U
Exploration U
Rigel S
Battlestar S
Queller Drive S
Result of Join at S Level
Case 2:
Starship
Objective
Destination
TC
Enterprise
Exploration
Talos
U
Enterprise
null
Rigel
S
Table 1
Starship
Type
Propulsion
TC
Enterprise
Starship
Photon
U
Table 2
Starship
Objective
Destination
Type
Propulsion
Enterprise U
Exploration U
Talos U
Starship U
Photon U
Result of Join at S Level
Figure 4.6: Joins in GDSS
In order to ensure the correct materialization of a logical joined tuple, MLS GDSS does not
currently use the join capabilities of the MLS COTS DBMS. Instead, tuples are selected from
individual tables and then joined outside the DBMS by GDSS application software. While this
operation results in some processing overhead, it ensures that data are not accidentally excluded
from the S-user without unsupported modification to the COTS DBMS itself.
4.4 VISIBLE RESTRICTIONS APPROACH
The third perspective on polyinstantiation is that users are made aware that data are restricted to
certain levels. In practice, this perspective means that users are cognizant of the levels of data that
they can see and update. The goal is to provide a more "honest" database without compromising
security. This perspective can lead to many different approaches; this section presents five
different possibilities, including techniques to eliminate the need for polyinstantiation.
4.4.1 Belief Approach
One visible restrictions approach to polyinstantiation is motivated by the idea that data at each level
should reflect the "beliefs" of users at that level about the real world [Smith 92]. For simplicity, we
call this work the belief approach. In this approach, users see all the data that they could read per
the Bell-LaPadula model, but believe the highest level data dominated by their operating level.
In this approach, updates reflect beliefs about the real world and they are regulated by the following
property:
Update Access Property: Data at a particular level can only be inserted, modified, or
deleted by users at that level.
Thus, data at each level reflects the beliefs of the users who maintain it. Users may see the data that
they believe as well as data believed by users at lower levels.
At the heart of this property is a model that takes a stand between tuple- and element-level
polyinstantiation. Keys may be classified at a different level than other attributes within the same
tuple, but all non-key attributes within a single tuple share a classification level.
Given a relation schema R, the multilevel relation Rc used in the belief model includes two
additional classification attributes: a key classification level (Kc) and a tuple classification level
(Tc). The model imposes two restrictions:
1. In any tuple, Tc must dominate Kc.
2. For the set of key attributes K and for all non-key attributes .Ai,...,An in Rc,
K, Kc, Tc ' Ai,...,An
Intuitively then, tuples with the same values for key attributes but different key classification levels
refer to different real-world entities. Tuples that are identical in key attributes and key
classification levels but differ in tuple classification levels represent different beliefs about the
same real-world entities. To maintain this distinction, users at a particular level are not allowed to
reuse key attribute values for new entities.
Given the relation SOD in Figure 4.7, in the belief model, U-users believe the first and second
tuples. C-users believe the third tuple, and S-users believe the fourth and fifth tuples. The second
and third tuples refer to the same real-world starship, but U- and C-users have different beliefs
about its objective and destination. The first and fifth tuples refer to different starships.
Starship
Kc
Objective
Destination
Tc
Voyager
U
Shipping
Mars
U
Enterprise
U
Exploration
Vulcan
U
Enterprise
U
Diplomat
Romulus
C
Zardor
S
Warfare
Romulus
S
Voyager
S
Spying
Rigel
S
Figure 4.7: Example of SOD in the Belief Model
U-users can see only the first two tuples in Figure 4.7, C-users can see the first three tuples, and S-
users can see all five tuples.
Although users are allowed to see all tuples at levels dominated by their belief levels, the query
language includes the optional keyword BELIEVED BY to allow users to restrict queries further.
Thus, S-users can ask to see all allowable tuples, or only those believed by C- and S-users, or
others.
So, the query "Display the destination of all starships named Enterprise" is expressed as:
SELECT Destination
FROM SOD
WHERE Starship = "Enterprise"
BELIEVED BY ANYONE
The result of this query, when issued against the relation in Figure 4.7, is:
Destination
Tc
Vulcan
U
for a U-user, and
Destination
Tc
Vulcan
U
Romulus
C
for all users at levels C or higher.
The query "Display the beliefs of U-users as to the destination of all starships named Enterprise"
is expressed as:
SELECT Destination
FROM SOD
WHERE Starship = "Enterprise"
BELIEVED BY U
The result of this query, when issued against the relation in Figure 4.7, is:
Destination
Tc
Vulcan
U
for all users.
The query "Display the classification level and destination of all starships named Voyager" is
expressed as:
SELECT Kc, Destination
FROM SOD
WHERE Starship = "Voyager"
BELIEVED BY ANYONE
The result of this query, when issued against the relation in Figure 4.7, is:
Kc
Destination
Tc
U
Mars
U
for U- and C-users, and
Kc
Destination
Tc
U
Mars
U
S
Rigel
S
for all users at levels S or higher.
4.4.2 Insert-Low Approach
Another variation of explicit restriction, the insert-low approach, has been adopted by the SWORD
project at the Royal Signals and Radar Establishment in England [Wood 92]. In this approach, each
relation is assigned a table usage classification, abbreviated as table class, at the time of its
creation. Each attribute is assigned a column classification that must dominate the table class.
The purpose of the table class is two fold: First, any insertion or deletion of tuples in a relation can
be made by users operating at the level of the table class of the relation. Second, the table class
controls exactly how the updates involving an access class that dominates the table class can be
made to the relation. This concept will be explained in greater detail below.
Consider once again the relation schema SOD. Say the table classification of SOD is U.
A typical instance of SOD is given as follows:
Starship
Objective
Destination
Enterprise U
Exploration U
Talos U
Voyager U
Spying S
Rigel TS
In this case, SWORD will show the entire relation to TS-users, while for those users at lower levels,
SWORD will substitute <not cleared> whenever a user has insufficient clearance to view a value.
Thus, for example, a C-user will see the following instance:
Starship
Objective
Destination
Enterprise U
Exploration U
Talos U
Voyager U
<not cleared>
<not cleared>
To see how SWORD avoids tuple polyinstantiation, consider once again the relation SOD with U
as its table class. Suppose the initial database state is as follows:
Starship
Objective
Destination
Enterprise U
Exploration U
Talos U
Suppose some U-user inserts the tuple (Voyager, S, Spying, U, Talos, U) into SOD. SWORD
allows lower level users to insert values at higher levels as long as the attribute value classifications
are dominated by the appropriate column classification. In this example, the column classification
for Starship would have to be S or higher. Furthermore, since the table classification of SOD is U,
this constitutes a legal insertion, and as a result, U-users and S-users will see the following states,
respectively:
Starship
Objective
Destination
Enterprise U
Exploration U
Talos U
<not cleared>
Spying U
Talos U
U-user
Starship
Objective
Destination
Enterprise U
Exploration U
Talos U
Voyager S
Spying U
Talos U
S-user
At this point, suppose a U-user wants to make an insertion (Freedom, U, Mining, U, Mars, U) to
SOD. Since the Starship attribute of tuples in SOD are not all visible to the U-user, there is always
a possibility that the Starship value of the tuple to be inserted equals that of the existing high tuple,
leading to element polyinstantiation (or tuple polyinstantiation in the case of attributes constituting
the primary key). SWORD avoids this by prohibiting U-users from inserting or modifying values
in this attribute. In the case of key attributes, like Starship, this means that all further insertions by
U-users will be forbidden. However, since the table classification is U, only U-users can insert
tuples into SOD. As a consequence, no further insertions can be made to SOD at all. In SWORD
applications, then, the column classifications for all attributes constituting the primary key must
equal the table class or users may be able to prohibit future insertions.
We next illustrate in more detail how element polyinstantiation is avoided in SWORD. Consider
the SOD instance:
Starship
Objective
Destination
Enterprise U
Exploration U
Talos U
Next, suppose a TS-user wishes to modify the destination of the Enterprise to be Rigel. This is
accomplished in two steps. First, the TS-user must log in as a U-user and change the classification
of Talos from U to TS. Having done so, the TS-user can log in at his level and then make the desired
update. As a result, the U instance and TS instance will become as follows:
Starship
Objective
Destination
Enterprise U
Exploration U
<not cleared>
U-user
Starship
Objective
Destination
Enterprise U
Exploration U
Rigel TS
TS-user
Given the database state shown immediately above, suppose an S-user wants to insert a Secret
destination for the Enterprise. He may do so by first logging in as a U-user, changing the
classification of the attribute Destination from TS to S. As a result of this change, all users,
including the TS-user, will see the following relation:
Starship
Objective
Destination
Enterprise U
Exploration U
<not cleared> S
Now, the S-user can log in at classification level S and make the appropriate change.
4.4.3 Prevention Approach
A third approach to explicit restriction relies on preventative techniques to eliminate tuple
polyinstantiation completely. Three basic techniques may be envisioned [Sandhu 91]:
1. Make all the keys visible. In this technique, the apparent primary key is required to be
labeled at the lowest level at which a relation is visible. For example, suppose that the
designer requires that all keys must be unclassified. Consequently, the following relation
would be forbidden:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise S
Spying S
Rigel S
S
Instead, note that the following two relations, called USOD and SSOD, represent the same
information:
UStarship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
USOD
SStarship
Objective
Destination
TC
Enterprise S
Spying S
Rigel S
S
SSOD
In other words, USOD and SSOD horizontally partition the original SOD relation, with all the U-
Starships in USOD and all the S-Starships in SSOD.
2. Partition the domain of the primary key. Another way to eliminate tuple
polyinstantiation is to partition the domain of the primary key among the various access
classes possible for the primary key. For our example, suppose that the application requires
that starships whose names begin with A-E be Unclassified, starships whose names begin
with F-T be Secret, and so on. Whenever a new tuple is inserted, the system enforces this
requirement as an integrity constraint. In this case, the Secret Enterprise must be renamed,
perhaps as follows:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Voyager S
Spying S
Rigel S
S
The DBMS can now reject any attempt by a U-user to insert a starship whose name begins
with F-Z, without causing any information leakage or integrity violation.
3. Limit insertions to be done by trusted subjects. A third way to eliminate tuple
polyinstantiation is to require that all insertions must be done by a system-high user, with
a write-down occurring as part of the insert operation. Strictly speaking, it is only
necessary to have a relation-high user (i.e., a user to whom all tuples are visible). In the
context of the example, this means that a U-user who wishes to insert the tuple (Enterprise,
Exploration, Talos) must request an S-user to do the insertion. The S-user will do so by
invoking a trusted subject that can check for key conflict, and if there is none, insert a U-
tuple by writing down. If there is a conflict the S-user informs the U-user about it, so the
U-user can, for example, change the name of the starship.
The first technique is available in any DBMS that allows a range of access classes for individual
attributes (or attribute groups) by simply limiting the classification range of the apparent key to be
a singleton set. The second technique is available to any DBMS that can enforce domain
constraints with adequate generality and trustworthiness. The third technique is always available
but requires the use of trusted code, and tolerates some leakage of information (although with a
human in the loop). The best technique will depend upon the characteristics of the DBMS and the
application, particularly concerning the frequency and source of insertions.
In addition to the above, Sandhu's prevention approach also proposes techniques to prevent
element polyinstantiation without compromising confidentiality, integrity, or denial-of-service
requirements. The basic idea is to introduce a special symbol denoted by "restricted" as the
possible value of a data element. The value "restricted" is distinct from any other value for that
element and is also different from "null." In other words, the domain of a data element is its natural
domain extended with "restricted" and "null." Sandhu then defines the semantics of "restricted" to
be able to eliminate both polyhigh and polylow polyinstantiation [Sandhu 91].
Consider again the polyhigh scenario of Section 2.3. Begin with the following relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Next, suppose an S-user attempts to modify the destination of the Enterprise to be Rigel. This
update does not cause any security violation, but if the new destination is classified Secret,
additional steps are required to prevent even temporary polyinstantiation. The prevention approach
requires an S-cleared user first to log in as a U-user and to mark the destination of the Enterprise
as "restricted," giving the following relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
restricted U
U
The meaning of <restricted, U> is that this field can no longer be updated by an ordinary U-user.
U-users can therefore infer that the true value of the Enterprise's destination is classified at some
level not dominated by U. The S-user then logs in as an S-subject and enters the destination of the
Enterprise as Rigel, giving us the following relations at the U- and S-levels, respectively:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
restricted U
U
Starship
Objective
Destination
TC
Enterprise U
Exploration U
restricted U
U
Enterprise U
Exploration U
Rigel S
S
Note that this protocol does not introduce a signaling channel from an S-subject to a U-subject.
There is an information flow, but from an S-user (logged in as a U-subject) to a U-subject. This is
an important distinction. This type of information flow, which includes humans in the process,
cannot be completely eliminated without cover stories. However, this protocol does prevent
malicious software from signaling information without the knowledge of users.
Next, consider how the polylow scenario of Section 2.3 works with the restricted requirement. In
this case, the Enterprise can have a Secret destination only if the destination has been marked as
being "restricted" at the Unclassified level. Thus, if the S- and U-users, respectively, see the
following instances of SOD:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
restricted U
U
Enterprise U
Exploration U
Rigel S
S
Starship
Objective
Destination
TC
Enterprise U
Exploration U
restricted U
U
then an attempt by a U-user to update the destination of the Enterprise to Talos will be
Starship
Objective
Destination
TC
Enterprise U
Exploration U
null U
U
rejected. Alternatively, if both S- and U-users see the following instance:
then the U-user update will be allowed (without causing polyinstantiation).
The concept of the "restricted" mark is straightforward, as long as the classification lattice is totally
ordered. In the general case of a partially ordered lattice, some subtleties arise. How to completely
eliminate polyinstantiation using "restricted" is discussed at length in [Sandhu 91]. In general,
updating the value of an element to "restricted" cannot cause polyinstantiation. On the other hand,
updating the value of an element to a data value, say, at the C-level, can be the cause of
polyinstantiation. If polyinstantiation is to be completely prohibited, this update must require that
the data element be restricted at all levels that do not dominate C. The fact that the data element is
restricted at all levels below C can be verified by the usual integrity-checking mechanisms in a
DBMS [Sandhu 90b]. However, it is difficult to guarantee this at levels incomparable with C. In
preparing to enter a data value at the C level, the system would need to start a system-low (actually,
a data-element-low) process that can then write up. A protocol for this purpose is described in
[Sandhu 91].
4.4.4 Explicit Alternatives Approach
A fourth approach to explicit restriction allows the application developer to choose among explicit
alternatives for polyinstantiation. Sandhu and Jajodia brought together a number of their
previously published ideas, along with some new ones, to define a particular semantics for
polyinstantiation called polyinstantiation for cover stories (PCS) [Sandhu 92]. PCS allows the
developer to specify whether an attribute (or attribute group) of a multilevel tuple will support: (1)
no polyinstantiation or (2) deliberate polyhigh polyinstantiation at the explicit request of a user to
whom the polyinstantiation is visible. PCS strictly limits the extent of polyinstantiation by
requiring that each real-world entity be modeled in a multilevel relation by at most one tuple per
security class. The goal of PCS is to provide a natural, intuitive, and useful technique for
implementing cover stories, with run-time flexibility regarding the use of cover stories. A
particular attribute may be used for cover stories for some tuples and not for others. Even for the
same real-world entity, a particular attribute may be polyinstantiated at some time and not at other
times.
PCS combines the "one tuple per tuple class" concept with the "restricted" concept of Section
4.4.3. The basic motivation for PCS can be appreciated by considering the following instance of
SOD:
Starship
Objective
Destination
TC
Enterprise U
restricted U
Talos U
U
Enterprise U
Spying S
Rigel S
S
In this case, the Destination attribute of the Enterprise is polyinstantiated, so that <Talos, U> is a
cover story for the real S destination of Rigel. The Objective is not polyinstantiated.
Consider the occurrence of polyinstantiation due to polylow, as discussed by the example in
Section 2.3. This example begins with S- and U-users, respectively, having the following views of
SOD:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Rigel S
S
Starship
Objective
Destination
TC
Enterprise U
Exploration U
null U
U
So far there is no polyinstantiation. Polyinstantiation occurs in the example when a U-user updates
the destination of the Enterprise to be Talos.
PCS takes a slightly different approach to this example. According to the PCS approach,
polyinstantiation does exist in the S-instance of SOD given above. PCS shows this instance as:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
null U
U
Enterprise U
Exploration U
Rigel S
S
In this approach, polyinstantiation already exists prior to the U-user updating the destination of the
Enterprise to be Talos. This update merely modifies an already polyinstantiated relation instance
to be:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Exploration U
Rigel S
S
With this approach, element polyinstantiation can occur only due to polyhigh. Polylow simply
cannot be the cause of element polyinstantiation. Consequently, polyinstantiation will occur only
by the deliberate action of a user to whom the polyinstantiation is immediately available. In other
words, element polyinstantiation does not occur as a surprise.
The PCS approach treats null values like any other data value (except in the apparent key fields
where null should not occur). Previous work on the semantics of null in polyinstantiated databases
has taken the view that nulls are subsumed by non-null values independent of the access class
[Jajodia 90b, Sandhu 90a]. In this case, the first tuple in the following relation available to S-users:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
null U
U
Enterprise U
Exploration U
Rigel S
S
is subsumed by the second tuple, resulting in the following relation for S-users used in the polylow
example of Section 2.3:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Rigel S
S
Under the explicit alternatives approach, the former relation is completely acceptable. The latter
can be acceptable, but only if the lower limit on the classification of the destination attribute is S.
To further illustrate the semantics of null in PCS, consider the following relation:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
null U
U
Enterprise U
Exploration U
null U
S
PCS considers this to be a polyinstantiated relation. The fact that there are nulls rather than data
values in the polyinstantiated field has no bearing on the treatment of this relation. In contrast, the
semantics of null in [Jajodia 90b] and [Sandhu 90a] require all null values to be classified at the
level of the apparent key (U in this case), thereby deeming the second tuple illegal.
The PCS approach leaves many of the choices of whether or not to polyinstantiate to the discretion
of application designer. It differentiates between updates that can and cannot cause
polyinstantiation by using two different keywords (UPDATE and PUPDATE) to make the
distinction explicit. The PCS approach also relies on the distinguished data value "restricted." The
meaning of this data value is that users at a classification level which returns the value "restricted"
for an attribute, may not modify the value of that attribute. As in the prevention approach (Section
4.4.3), PCS includes special privileges for imposing and lifting such restrictions.
4.4.5 Multilevel Relational Data Model Approach
A fifth approach described here is an extension of the work just described in Section 4.4.4. The
main benefits of this model, the Multilevel Relational (MLR) data model [Chen 95], over previous
models is that it retains the capability for upward information flow while eliminating ambiguity of
references by foreign keys.
This model, which supports attribute level labeling, requires that there be at most one tuple in each
access class for a given entity. It requires that if a tuple includes attributes which are classified
lower than the tuple's access class, then a tuple at this lower level must exist with the same attribute
value. Because of this requirement, and the fact that the minimum classification of an element may
be higher than that of the primary key, it allows classification attributes to be NULL for attributes
which have NULL values. The model also requires that a tuple exist at a given level in order to
recognize that tuple at that level (i.e., there is no automatic recognition of the validity of less
sensitive data elements). The main ideas behind the model are as follows:
1. The data accepted by a subject is divided into two parts: all data defined at the subject's
level and any data explicitly "borrowed" from lower levels. Allowing data-borrow ensures
that the MLR data model retains upward information flow because changes to lower level
data can be automatically propagated to higher levels.
2. A subject can see data which is accepted by subjects at its level or at the levels below it.
3. A tuple contains all the data accepted (either owned or borrowed) by subjects at the level
of the tuple. If a tuple at a given level does not exist, then subjects at that level do not accept
the existence of that tuple.
The MLR data model is more precisely defined by the following five integrity properties as
follows:
Entity Integrity:
Entity integrity (which comes from SeaView) protects the integrity of the apparent primary key
(AK). It basically has three requirements which must be met by each tuple:
1. No attributes in AK have a null value.
2. The attributes in AK are uniformly classified.
3. All non-key attributes must dominate the classification of AK.
Polyinstantiation Integrity:
This includes two requirements:
1. There can only be one tuple at a given classification level for each unique AK.
2. For each unique AK there can only be one attribute value at each attribute level (i.e., you
cannot have multiple values for the same attribute at the same attribute level for a given
AK).
Data Borrowed Integrity:
This property, which is new to this model, ensures that borrowed data actually exists and changes
to lower level data can be automatically propagated to higher levels. It basically states that:
1. In all tuples, for each non-null attribute that is less sensitive than the tuple classification,
there must exist a tuple at the level of the attribute with the same attribute value and
attribute classification (i.e., if you borrow less sensitive data, that data must exist).
Foreign Key Integrity:
This property, which also came from SeaView, basically states that:
1. For each foreign key, all attributes of that key are either null or non-null.
2. For each foreign key, all attributes of the key are uniformly classified.
Referential Integrity:
Referential integrity in the standard database model ensures that each foreign key references an
existing primary key [Entity 96]. In this model, referential integrity states that:
1. For each foreign key, there must be a matching AK.
2. The classification of the foreign key must dominate the classification of the AK.
3. The classification of the tuple containing the foreign key must equal the classification of
the tuple containing the AK.
This not only ensures that matching primary keys exist and are visible to the referencing tuple, but
also requires that the two tuples be classified at the same level. This last requirement means that
for any level c, c-tuples can only reference c-tuples. This follows from the restriction that only
tuples at a given level are accepted at that level.
The main benefit of this new restriction is that there is no ambiguity in any references by a foreign
key. Without this restriction, a tuple's foreign key could match the primary key of multiple
polyinstantiated tuples that are dominated by the tuple with the foreign key. This requirement
eliminates this potential ambiguity.
The model is illustrated in the following examples:
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Exploration U
Rigel S
S
Using our familiar example, the tuples above would be perfectly legal in the MLR data because
each tuple has a different access class and there is a U tuple that has the same Starship and
Objective attribute values as the S tuple (i.e., Enterprise and Exploration). The following would
not be legal for two reasons: one because it contains two tuples with the same apparent primary
key that have the same access class (i.e., S) and two because there is no matching Mining Objective
attribute in the U tuple (i.e., two attributes at the same level for the same primary key have
conflicting values).
Starship
Objective
Destination
TC
Enterprise U
Exploration U
Talos U
U
Enterprise U
Mining U
Rigel S
S
Enterprise U
Spying S
Rigel S
S
The following tuple illustrates the use of the "null" classification value capability. In this case, the
objective is null. This feature is provided because attributes may have a minimum classification
higher than the minimum classification of the primary key. In this example, the minimum
classification for Objective could be Confidential. Without allowing null attribute and label values,
there would be no way of creating a tuple at U which contains the Enterprise and Talos attribute
values at U.
Starship
Objective
Destination
TC
Enterprise U
null null
Talos U
U
Given this initial tuple, the following tuples could build upon this base as follows:
Starship
Objective
Destination
TC
Enterprise U
null null
Talos U
U
Enterprise U
Mining S
Rigel S
S
Enterprise U
Spying TS
Rigel S
TS
These tuples indicate that the actual purpose of the Enterprise mission is to perform Spying on
Rigel while cover stories of going to Talos or doing Mining are provided to less cleared
individuals. If the Rigel destination was changed by an S user, then it would automatically
propagate up to the TS tuple since it is being borrowed by that tuple. However, in the following
tuple, a change to the Rigel destination at the S level would not be propagated, since (although) the
destinations appear to be the same, that value was not borrowed from the lower level tuple.
Starship
Objective
Destination
TC
Enterprise U
null null
Talos U
U
Enterprise U
Mining S
Rigel S
S
Enterprise U
Spying TS
Rigel TS
TS
This model includes the addition of a new SQL statement which is used to indicate borrowing from
lower level tuples, the UPLEVEL statement. This statement is used to indicate which attributes to
borrow from a lower level tuple. This statement can be used to modify an existing tuple or create
a new one.
In summary, this model unifies a number of other models and then extends them in such a way as
to retain upward information flow without permitting downward flows, while eliminating semantic
ambiguity of foreign keys due to polyinstantiated elements within a reasonably flexible attribute
level labeling scheme.
SECTION 5
CURRENT COMMERCIAL APPROACHES TO POLYINSTANTIATION
Previous sections of this document discussed various issues surrounding polyinstantiation in MLS
DBMSs. This section summarizes the polyinstantiation approaches provided by three
commercially available MLS DBMSs: INFORMIX, Sybase, and Oracle; and the Trusted Rubix
DBMS commercial prototype.
Each of these DBMS products provides tuple level labeling, so element level labeling, and the
polyinstantiation issues associated with it, are not applicable.
INFORMIX:
The key for a tuple in INFORMIX OnLine/Secure automatically includes the tuple security label.
Thus, polyinstantiation is always possible and cannot be suppressed by the DBMS. INFORMIX
OnLine/Secure places no special requirements on suppressing polyhigh or polylow
polyinstantiation. It does not provide any tools for cleaning up polyinstantiation. Any
housecleaning of polyinstantiation effects will require a manual procedure or custom software.
[INFORMIX 92]
Sybase:
The tuple sensitivity in the Sybase Secure SQL Server is automatically part of all keys. Thus
polyinstantiation is always possible and cannot be suppressed by the DBMS. Sybase Secure SQL
Server does not provide any specific tools for cleaning up polyinstantiation. Any cleanup requires
a manual procedure or custom software. [Sybase 93]
Oracle:
Trusted Oracle can be configured to run in one of two modes. When run in DBMS MAC mode, a
single Trusted Oracle database can store information at multiple labels. In this mode, Trusted
Oracle can turn polyinstantiation on and off at the table level by requiring key integrity which does
not include the tuple label. When on, the primary key includes the tuple label, which allows
poLyinstantiation to occur. When off, the key does not include the tuple label, thus preventing
polyinstantiation. The ramifications of having polyinstantiation turned off is that high users cannot
insert tuples above lower level tuples that already exist with the same primary key and vice versa,
which causes denial of service (to some degree) for high users as well as a covert channel for low
users.
Including or not including the tuple label in the primary key is specified at table definition time.
The tuple label can be added to the primary key at some future date, which would then permit
polyinstantiation. However, once the tuple label is included in the primary key, it cannot be
removed. [Oracle 93]
When run in OS MAC mode, Trusted Oracle is capable of storing data at only a single label, and
the DBMS is constrained by the underlying OS MAC policy. Without any MAC privilege, the
DBMS cannot suppress polyinstantiation because a low DBMS will not be aware of any tuples
with the same primary key at a higher level, and a high DBMS cannot be trusted to modify low
data. As such, polyinstantiation cannot be prevented when Trusted Oracle is running in OS MAC
mode.
Trusted Rubix:
Trusted Rubix provides the most flexible polyinstantiation mechanism of the products surveyed by
its support of three polyinstantiation modes: POLYHIGH, POLYLOW, and what is referred to as
POLYNONE. The selected mode is defined on a per table basis, when each table is created. Once
a table had been created and the polyinstantiation discipline declared, it is not possible to change
that discipline for the remainder of the table's lifetime. [Rubix 93]
POLYNONE
When this mode is selected for a table, polyinstantiation is prevented in that table, thus introducing
a covert channel for low users and potentially preventing some inserts by high users.
POLYLOW
This mode causes HIGH keys to be replicated at LOW when a LOW subject attempts to insert an
item whose key matches that of an extant HIGH item. This is done to avoid the covert channel
problem. However, if a HIGH user attempts to insert a tuple which contains a key that already
exists at a lower level, that insert is prevented.
POLYHIGH
This mode causes LOW keys to be replicated at HIGH when a HIGH subject attempts to insert an
item whose key matches that of an extant LOW item. This is done to allow a high user to insert
HIGH tuples when a LOW version already exists, rather than preventing duplication of keys at
HIGH. This mode includes the characteristics offered by the POLYLOW mode since, in the
Trusted Rubix implementation, it is not possible to support POLYHIGH behavior without
supporting POLYLOW. This mode basically makes polyinstantiation of the Trusted Rubix table
behave like a table which has had polyinstantiation turned on under Trusted Oracle DBMS MAC
mode.
SECTION 6
SUMMARY
The design of an MLS DBMS must take into account the problem of polyinstantiation. When data
items exist at multiple classification levels, the potential exists for inconsistent values for the same
data item at different levels. Polyinstantiation may occur over tuples or elements, and it may arise
through updates at low or high classification levels. Researchers have developed a number of
different approaches to polyinstantiation; no one solution is best for all applications. This
document has outlined approaches to resolving the polyinstantiation problem:
1. Propagate polyinstantiated tuples to reflect valid combinations of values (Section 4.2).
Users at different levels may see different attribute values for the same realworld entity.
2. Identify derived tuples based on underlying polyinstantiated tuples (Section 4.3).
Although a multilevel relation may have several tuples for the same real-world entity, there
will only be one such tuple per classification level.
3. Indicate restrictions or inconsistencies present in the data so that polyinstantiation can be
controlled (Section 4.4). Users are made aware that data are restricted to certain levels, and
are cognizant of the levels of data that they can see and update.
There are no explicitly stated TCSEC requirements for automatic or intentional polyinstantiation
mechanisms. Evaluators will require that a vendor of an MLS DBMS address in some manner the
signaling channels which polyinstantiation prevents. Current commercial MLS DBMSs enforce
tuple level polyinstantiation. With appropriate (possibly complex/inconvenient) database design,
tuple level labeling can offer functionality comparable to research prototypes supporting element
level labeling and polyinstantiation.
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