TRUSTED NETWORK INTERPRETATION ENVIRONMENTS GUIDELINE



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Table of Contents



     FOREWORD

     ACKNOWLEDGMENTS

     1 Introduction

          1.1 Background

          1.2 Trusted Network Technology Publications

          1.3 Purpose

          1.4 Scope

          1.5 Structure of the Document

     2 Terminology

          2.1 Automated Information System

          2.2 Network Trusted Computing Base

          2.3 System and Component

               2.3.1 TNI Introduction (definition not used in TNIEG)

               2.3.2 TNI - Appendix A (definition not used in TNIEG)

               2.3.3 Discussion

               2.3.4 Definitions

          2.4 Evaluation

          2.5 Certification

          2.6 Accreditation

          2.7 Two Types of Networks

               2.7.1 Unified Networks

               2.7.2 Interconnected Accredited AIS

          2.8 Network Security Architecture and Design

          2.9 Protocol Layer Approach

          2.10 Part II Security Services

     3 Network Security Architecture and Design (NSAD)

          3.1 Composing an NSAD

          3.2 Memorandum of Agreement

     4 TNI Part I Security Requirements

          4.1 Risk Management

          4.2 Determination of Network Risk

     5 TNI Part II Security Requirements

          5.1 Relationship of TNI Part II Services to Part I and Appendix A

          5.2 Specification and Evaluation of Security Services

          5.3 Evaluation Ratings

          5.4 Selecting Security Services

               5.4.1 Strength of Mechanism

               5.4.2 Assurance

               5.4.3 Functionality

     6 Interconnecting AIS

          6.1 Agreement Between Accreditors

               6.1.1 Accreditation Range

               6.1.2 Device Range

          6.2 Interconnection Rule

               6.2.1 A Complex Example

          6.3 Risk Factors

               6.3.1 Propagation of Local Risk

               6.3.2 The Cascading Problem

                    6.3.2.1 Tests for Cascading.

                    6.3.2.2 Solutions.

     APPENDIX A: Tests for the Cascading Problem

     APPENDIX B: Background References

     APPENDIX C: Encryption

     LIST OF REFERENCES

     ACRONYMS



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NCSC-TG-011

Library No. 5-235,465

Version 1



FOREWORD



The National Computer Security Center is issuing the Trusted Network

Interpretation Environments Guideline as part of our Technical Guidelines

Program, through which the "Rainbow Series" is produced. The Technical

Guidelines Program ensures that the features of the Trusted Computer Systems

Evaluation Criteria (DOD 5200.28-STD) are discussed in detail and that

guidance is provided for meeting each requirement. The National Computer

Security Center, through its Trusted Product Evaluation Program, analyses

the security features of commercially produced and supported computer

systems. Together, these programs ensure that organisations are capable of

protecting their important data with trusted computer systems.



The Trusted Network Interpretation Environments Guideline is a companion to

the Trusted Network Interpretation of the. Trusted Computer System

Evaluation Criteria (NCSC-TG~O5), published 31 July 1987. The Trusted

Network Interpretation Environments Guideline provides insight into the

issues relevant when integrating, operating, and maintaining trusted

computer networks. This document identifies the minimum security protection

required in different network environments such that network certifiers,

integrators, and accreditors can determine what protection mechanisms and

assurances are minimally required in specific network environments.



This document parallels Computer Security Requirements - Guidance for

Applying the Department of Defense Trusted Computer System Evaluation

Criteria in Specific Environments (CDC-STD~O3-85) and its technical

rationale (CSC STD~0485). It also provides a descriptive presentation of the

security issues that exist in networked computer systems as the networked

computer system environment is inherently more complex and requires

additional protection considerations over stand-alone computer systems.



As the Director, National Computer Security Center, I invite you suggestions

for revising this document. We plan to review this document as the need

arises.



PATRICK R. GALLAGHER JR. 1 August 1990



Director



National Computer Security Center





















ACKNOWLEDGMENTS



The National Computer Security Center extends special recognition and

acknowledgment for their contributions to this document to Dr. Marshall D.

Abrams, Renee Child, Annabelle Lee, Dr. Jonathan K. Millen, Samuel I.

Schaen, and Dr. Martin W. Schwartz, of The MITRE Corporation, as authors;

Richard Wilmer, also of The MITRE Corporation, as technical editor; and to

Alfred Arsenault, David Chizmadia, id Rick Siebenaler of the National

Computer Security Center, who managed the effort ad participated in the

development.



Special thanks are extended to the many members of the computer security

community who enthusiastically gave their time and expertise in reviewing

the material and providing valuable comments and suggested changes. Special

thanks are extended to James P. Anderson of James P. Anderson Co., Donald L.

Brinkley of Gemini Computers, Inc., Dr. Eric Roskos of The Institute for

Defense Analysis, Dr. Tien Tao of Gemini Computers, Inc., and Dr. John M.

Vasak of The MITRE Corporation for their extensive suggestions and

recommendations.



For sale by the Superintendent of Documents, U.S. Government



Printing Office Washington DC 20402































































1 Introduction



This Trusted Network Interpretation (TNI) Environments Guideline (TNIEG)

addresses many issues in determining the security protection required in

different network environments. It complements the TNI, just as the Trusted

Computer System Evaluation Criteria (TCSEC) Environments Guideline [1]

complements the TCSEC. The TNI interprets the TCSEC for networks; it

contains all of the criteria in the TCSEC, adding interpretation and

rationale to applying trust technology to network systems. In the same way

that the TCSEC Environments Guideline provides guidance on applying the

TCSEC, this TNIEG provides guidance on the use of the TNI. The TCSEC and its

Environments Guideline constitute the foundation on which the TNI and TNIEG

add network applicability.



1.1 Background



The National Computer Security Center (NCSC) is responsible for establishing

and maintaining technical standards and criteria for the evaluation of

trusted computer systems. As part of this responsibility, the NCSC is

developing guidance on how new security technology should be used. Two

objectives of this guidance are:



   * Establishing a metric for categorizing systems according to the

     security protection they provide, and

   * Identifying the minimum security protection required in different

     environments.



The TCSEC [2] helps to satisfy the first objective by categorizing computer

systems into hierarchical classes based on evaluation of their security

features and assurances. The TCSEC Environments Guideline [1] helps to

satisfy the second objective by identifying the minimum classes appropriate

for systems in different risk environments. These two documents, however,

apply to stand-alone computer systems.



The TNI [3] satisfies the first objective by interpreting the TCSEC for

networks. The TNI also provides guidance for selecting and specifying other

security services (e.g., communications integrity, denial of service,

transmission security). The TNIEG is the first step toward addressing the

second objective.



1.2 Trusted Network Technology Publications



The NCSC has decided to provide guidance concerning security in networks and

distributed Automated Information Systems (AISs)1 in a series of

publications. The subject area is collectively identified as Trusted Network

Technology (TNT). The TNI is the first TNT publication. This TNIEG is the

second TNT publication. It contains the best guidance that is available at

this time; as technology advances and more experience is gained in

implementing trusted networks, more specific guidance will be provided. This

TNIEG provides elaboration and clarification on the TNI. Guidance concerning





1 Definitions of terms particularly important to this document are given in

Section 2.

Interconnected AIS which initially appeared in the TNI, Appendix C, has been

revised and incorporated into this document (see Section 6 and Appendix A).

This document does not address all of the security issues that are excluded

from the TNI. Other topics, such as composing a trusted system from

evaluated components, will be discussed in future TNT publications.



1.3 Purpose



The overall purpose of this TNIEG is to assist program managers,

integrators, certifiers, and Accreditors with identifying the minimum

security protection needed for different trusted computer network

environments. For brevity, this audience is referred to as security

managers. Not all questions can be answered at this time. The NCSC invites

suggestions for topics to be addressed in future TNT publications.



This guideline is not a tutorial on security and networking; it is assumed

that the reader will have some background in both areas. Suggested

background references are identified in Appendix B. This guideline is

designed to be self contained; a fair amount of background information that

can be found in the TNI is also included here. The interested reader may

consult the TNI and other documents referenced in this guideline for further

detail.



1.4 Scope



This document describes an environmental assessment process that helps

determine the minimum level of trust recommended for a specific network

operational environment. The primary focus of this document (and also of the

TNI) is on the AIS hardware, firmware, and software aspects of security; 
therefore, neither this guideline nor the TNI address all the security 
requirements that may be imposed on a network. Depending on the particular 
environment, communications security (COMSEC), emanations security 
(TEMPEST), physical security, personnel security, administrative security, and 
other information security (INFOSEC) measures or safeguards are also required. 
This document applies to networks that are entrusted with the processing of 
information, regardless of whether that information is classified, 
sensitive, or otherwise relevant to national security.



1.5 Structure of the Document



Section 2 of this document defines terms and Section 3 discusses Network

Security Architecture and Design. Section 4 guides security managers in

applying Part I of the TNI; Section 5 does the same for Part II. Section 6

addresses security issues that arise when AIS are interconnected. Appendix A

discusses the Cascade Condition in greater detail; Appendix B provides

tutorial and background references on network security; and Appendix C

discusses encryption and encryption mechanisms.



2 Terminology



Many of the terms used in the TNI are drawn from diverse specialization

areas. Their special meaning in context may differ from common English

usage. In this section we explain how such terms are used in the TNI and how

these definitions have been refined in this document. Terms are printed in

boldface when they are defined.



2.1 Automated Information System



An AIS is defined in DODD 5200.28 as "an assembly of computer hardware,

software, and/or firmware configured to collect, create, communicate,

compute, disseminate, process, store, and/or control data or information"

[4]. This is both a broad definition and a new one, since DODD 5200.28 was

published after the TNI. The TNI states that "automatic data processing

(ADP) systems, referred to in this [TNI] document as Automated Information

System (AIS)...", and equates AIS and ADP. We will use the DODD 5200.28

definition since it is broader and more authoritative. We also note that

DODD 5200.28 tends to pluralize AIS as AISs while the TNI considers AIS to

be a collective noun. We have followed the latter convention.



2.2 Network Trusted Computing Base



The Network Trusted Computing Base (NTCB) is the totality of protection

mechanisms within a network system2-including hardware, firmware, and

software- the combination of which is responsible for enforcing a security

policy. The NTCB is the network generalization of the trusted computing base

(TCB). An NTCB Partition is the totality of mechanisms within a single

network subsystem3 for enforcing the network policy, as allocated to that

subsystem; it is the part of the NTCB within a single network subsystem.



The distinction between a system and a subsystem is a matter of the

viewpoint of the observer. One observer's system may be another observer's

subsystem. For example, the vendor of a local area network may regard his

product as a system, while the customer's network architect may consider it

to be a subsystem along with hosts, workstations, etc.



The THI uses component in the definition of NTCB Partition. We use subsystem

to be consistent in this document.



2.3 System and Component



The terms system add component need to be related to each other.

Unfortunately, the TNI is not completely consistent in its use of these

terms. We will first cite the relevant sections from the TNI; then we will

reconcile them as used in this guideline. As discussed below, we define the

relationship as follows: A component is a physical unit contained within a

system.



2.3.1 TNI Introduction (definition not used in TNIEG)



The TNI Introduction states (emphasis added):



     A network system is the entire collection of hardware, firmware, and

     software necessary to provide a desired functionality. A component is

     any part of a system that, taken by itself, provides all or a portion

     of the total functionality required of a system. A component is

     recursively defined to be an individual unit, not useful to further

     subdivide, or a collection of components up to and including the entire

     system.



2.3.2 TNI - Appendix A (definition not used in TNIEG)



Appendix A of the TNI presents the view:



     a trusted network represents a composition of trusted components....

     The approach to evaluation of a network suggested by this view is to

     partition the system into components, rate each component to determine

     its security-relevant characteristics, and then evaluate the

     composition of the components to arrive at an overall rating class for

     the network. This approach... allows for the evaluation of components

     which in and of themselves do not support all the policies required by

     the TCSEC, ... contribute[s] to the overall evaluation of any network

     which uses them and allows for the reuse of the evaluated component in

     different networks without the need for a re-evaluation of the

     component.



Appendix A goes on to state that:



     The set of policy-related features to be supported by the component

     need not be the complete set required for a stand-alone system:

     features not supplied by one component for the system are supplied by

     another.



2.3.3 Discussion



We see a difference between the Introduction and Appendix A of the TNI. We

will use the definition of component as an individual unit that does not

provide a complete set of end-user services. As a consequence, a subsystem

can operate on its own and a component will require an external connection

to perform a useful function.



Appendix C of the TNI uses component, as follows, where we would use

subsystem:



     Any AIS that is connected to other AIS must enforce an "Interconnection

     Rule" that limits the sensitivity levels of information that it may

     send or receive. Using the component connection view, each component

     responsible for maintaining the separation of multiple levels of

     information must decide locally whether or not information ca be sent

     or received.



A component may support all the policy and accountability requirements: M,

D, I, ad A4; however, as defined above, this is not applicable to

determining whether an individual unit is a component. A component which

supports some part of the security policy contains an NTCB partition. In the

extreme, a component may not have any security-relevant function; in this

case it doesn't support any TCSEC policy and doesn't contain an NTCB

partition.



2.3.4 Definitions



To summarize the previous discussions, following are definitions for

component ad system/subsystem.



   * Component: An individual physical unit that does not provide a complete

     set of end-user services.

   * System/subsystem: A collection of hardware, firmware, and software

     necessary configured to collect, create, communicate, compute,

     disseminate process, store, and/or control data ad information [4].



2.4 Evaluation



NCSC-evaluation refers specifically to the process in which the NCSC

determines whether a commercial-off-the-shelf (COTS) product satisfies the

TCSE~C. Application of the TCSEC to a particular product may be assisted by

an interpretation guideline such as the TNI [5]. In such a case, the

guideline clarifies the meaning of the TCSEC's language with regard to a

particular type of product, but in no case circumvents or grants exception

to the requirements of the TCSEC. The purpose of an NCSC- evaluation is as

follows:



     The primary goal of the NCSC is to encourage the widespread

     availability of trusted computer systems. This goal is realized, in

     large measure, through the NCSC's Commercial Product Evaluation

     Program. This program is focused on the technical evaluation of the

     protection capabilities of off-the- shelf, commercially produced and

     supported systems that meet the computer security needs of government

     departments and agencies. This product evaluation culminates in the

     publication of an Evaluated Products List (EPL) report... [6].



An NCSC evaluation places a product in one of four divisions: D, C, B, or A.

Division D is for systems that have been evaluated but fail to meet the

requirements for a higher NCSC evaluation rating. Division C has two

classes: C1 and C2, which require discretionary (need-to-know) protection.

Division B has three classes: B1, B2, and B3, which require support for

sensitivity labels and increasing robustness of system architecture.

Division A has only class Al, which requires additional assurance through

formal verification methods.



4 Mandatory access control, discretionary access control, identification and

authentication, and audit, respectively.



2.5 Certification



Certification is defined as "the technical evaluation of a system's security

features, made as part of and in support of the approval/accreditation

process, that establishes the extent to which a particular system's design

and implementation meet a set of specified security requirements" [7]. In

this definition, the word evaluation is used in the generic sense and should

not be confused with NCSC evaluation. The primary distinction is that

certification is an evaluation with respect to specified requirements, and

NCSC evaluation is an evaluation against the TCSEC (and the TNI).



Certification is conducted in support of the accreditation decision. For

most systems, the hardware, system software, applications software,

communications equipment, and the operational facility must be configured

and tested during certification. Certification should be performed by

technical personnel independent of the development organization, according

to an acceptable methodology. Certification should identify the level of

security protection with regard to a procedure, program, or system.

Certification results in the issuance of the Certification Statement, which

states whether system security requirements are met, describes all known

remaining vulnerabilities, and advises the Accreditor relative to the

accreditation decision. If requirements are no met, the Certification

Statement lists problem areas and identifies suggested solutions (if known).

A certification documentation package, called the Certification Report of

Findings, submitted to the Accreditor includes the Certification Statement,

certification analysis, results of Security Test and Evaluation, id results

of Operational Test and Evaluation.



2.6 Accreditation



Accreditation is "the managerial authorization and approval granted to an

ADP system or network to process sensitive data in an operational

environment, made on the basis of a certification by designated technical

personnel..." [3]. Accreditation is a management decision to operate a

system or network employing specific safeguards, against a defined threat,

at an acceptable level of risk, under a stated operational concept, with

stated interconnections, in a specific operational environment, with a

specific security mode of operation. Other terms have been used to identify

the formal managerial approval for operation; in this document we use the

term Accreditation. FIPS PUB 102 defines Accrediting Officials as "the

agency officials who have authority to accept an application's security

safeguards and issue an accreditation statement that records that decision.

The Accrediting Officials must also possess authority to allocate resources

to achieve acceptable security and to remedy security deficiencies" [7]. The

ultimate responsibility for system security rests with the Accreditor. DODD

5200.28 employs the term Designated Approving Authority (DAA) to refer to

the same officials or officers [4].



All AIS must be accredited before they may process or use sensitive or

classified information, unless a written waiver is granted by the

Accreditor. Accreditation is based on a technical investigation and a formal

review of the certification Report of Findings. Before authorizing an AIS to

operate, the Accreditor must ensure that satisfactory security measures have

been installed and that any residual risk is within acceptable limits.

Often, the Accreditor must weigh the technical shortcomings of an AIS

against operational necessity. Lacking other ways to accomplish an

operational mission, the Accreditor may determine that it is preferable to

accept a residual security risk than to preclude the mission. To ensure that

the accreditation goals and objectives are adequately met, the Accreditor

must be involved throughout the system life cycle.



2.7 Two Types of Networks



A network may be defined as either an interconnection of accredited AIS or

as a Unified Network. When it is not necessary to differentiate in this

guideline, the term network is used.



2.7.1 Unified Networks



The TNI defines a Network Single Trusted System while DODD 5200.28 Enclosure

(5) defines a Unified Network; this TNIEG conforms to the latter usage. The

section of Enclosure (5) that addresses Unified Networks is brief and

instructive 5:



     Some networks may be accredited as a whole without prior accreditation

     of their component AIS. It is necessary to treat a network as unified

     when some of its AIS subsystems are so specialized or dependent on

     other subsystems of the network for security support that individual

     accreditation of such subsystems is not possible or meaningful with

     respect to secure network operation. In order to be accredited, a

     Unified Network shall possess a coherent network architecture and

     design, and it should be developed with an attention to security

     requirements, mechanisms, and assurances commensurate with the range of

     sensitivity of information for which it is to be accredited.

     The recommended approach for accrediting a Unified Network is to apply

     Enclosure 4 to the entire network to derive an evaluation class.

     Requirements to meet that evaluation class then are obtained from an

     applicable interpretation of DOD5200.28-STD [the TCSEC], such as NCSC-

     TG-005 [the TNI].



2.7.2 Interconnected Accredited AIS



     Enclosure (5) of DODD 5200.28 also discusses Interconnected Accredited

     AIS:

     If a network consists of previously accredited AIS, a Memorandum of

     Agreement6 [MOA] is required between the DAA of each DOD Component AIS

     and the DAA responsible for the network ... The network DAA must ensure

     that interface restrictions and limitations are observed for

     connections between DOD Component AIS. ... In particular, connections

     between accredited AIS must be consistent with the mode of operation of

     each AIS, the specific sensitivity level or range of sensitivity levels

     for which each AIS is accredited, any additional interface constraints

     associated with the particular interface device used for the

     connection, and any other restrictions required by the MOA.



2.8 Network Security Architecture and Design



Network Security Architecture and Design (NSAD) applies to all networks. The

NSAD identifies how the NTCB is partitioned and how the trusted system

requirements are met. Security engineering, including the development of the

NSAD, is a specialty area within systems engineering. The security engineer

is responsible for ensuring that the system being built meets the security

requirements of the organization. The security engineer ensures that the AIS

security conforms to applicable regulations and policy, and implements the

system security requirements [8].



---------------------------



5 As mentioned in the introduction and the definitions, this TNIEG differs

from DODD 5200.28 and the TNI in the usage of AIS and the definition of

component. This quotation has been slightly edited to conform to the usage

in this guideline. she content of a Memorandum of Agreement is discussed in

Section 3.2

The security policy includes the set of laws, rules, and practices that

govern how an organization manages, protects, and distributes sensitive

information (including classified information). The overall security policy

is addressed in a family of related documents consisting of a system

security policy, a security policy model, and security requirements. The

system security policy is developed first, followed by the other two. A

system security policy interprets and applies regulations to systems. The

security policy model defines subjects and objects and the accesses between

the two. The security requirements document identifies valuable user

requirements for a secure system.



The security architecture is that part of the system architecture that

describes the required security services and features. The security

architecture shows how the required level of assurance for the system is to

be met. A mapping of security requirements to functional elements is

documented in the security architecture; therefore, the security

architecture is used to document security design decisions.



2.9 Protocol Layer Approach



This guideline discusses networks in terms of the Open System

Interconnection (0SI) reference model [9] because that model provides a

well-understood terminology and is used in the TNI. This guideline, however,

is independent of the actual protocol reference model used.



An NTCB implementation need not include all protocol layers. The precise

security services and their granularity will depend on the highest protocol

layer at which an NTCB partition is implemented.7 For example, in a Unified

Network where layer 3 (the network layer) is the highest layer that

implements the NTCB, the network will be able to enforce mandatory access

control (MAC) and discretionary access control (DAC) decisions on the

granularity of network addresses[8]. The network system being evaluated knows

only about the range of classifications that the host (or other network) is

permitted to handle and the hosts (or other networks) that are permitted to

communicate with each other. Finer distinctions must be made by the hosts

(or other networks) involved. When a trusted network provides all seven

layers, the network is aware of and enforces MAC and DAC at the granularity

of individual users.







______________________



7 Since the publication of the TNI, the policy environment has changed.

"User", as defined in DODD 5200.28, ad peer.entity, as defined in the 051

reference model, are comparable. Therefore, the TNIEG applies to all layers

of the 051 architecture.



8 A network address refers to either a host or another network.



9 Sections 4 and 5 of this document should guide the security manager in

selecting those security services and safeguards that are appropriate for

the given operational environment.



A network device might not provide a complete set of end-user services

through layer 7. Products that do not provide all system services through

layer 7 may be NCSC-evaluated as components and subsequently used with other

components to compose a network.





2.10 Part II Security Services



The terms functionality, strength of mechanism, and assurance are used to

rate TNI Part II services. Their meanings in that context are described

below.



     Functionality refers to the objective and approach of a security

     service; it includes features, mechanism, and performance. Alternative

     approaches to achieving the desired functionality may be more suitable

     in different applications environments.

     Strength of mechanism refers to how well a specific approach may be

     expected to achieve its objectives. In some cases the selection of

     parameters, such as number of bits used in a checksum or the number of

     permutations used in an encryption algorithm, can significantly affect

     strength of mechanism. With regard to inadvertent threats, strength

     refers to the ability to operate correctly during natural disasters,

     emergencies, operator errors, and accidents. Inadvertent threats are

     particularly critical to prevention of denial of service. As an

     example, for communications line outages, strength of mechanism may

     refer to the number of alternate routes that maybe used to bypass the

     outage. The misdelivery of messages is an example of an inadvertent

     threat that may disclose information to unauthorized individuals.

     Encryption can be used to prevent the unintended recipient from seeing

     the information.

     Assurance refers to a basis for believing that the functionality will

     be achieved; it includes tamper resistance, verifiability, and

     resistance against circumvention or bypass. Assurance is generally

     based on analysis involving theory, testing, software engineering,

     validation and verification, and related approaches. The analysis may

     be formal or informal.



3 Network Security Architecture and Design (NSAD)



Every network should have a Network Security Architecture and Design (NSAD).

This section helps the security manager in establishing the NSAD for the

network.



The NSAD, produced as part of the risk management process, documents the

security services. As mentioned in Section 1, the primary focus of this

TNIEG is on the AIS aspects of security. Depending on the particular

environment, communications security (COMSEC), emanations security

(TEMPEST), physical security, personnel security, administrative security,

and other information security (INFOSEC) measures or safeguards are also

incorporated in the NSAD. An NSAD results from a series of trade-offs among

cost, effectiveness, technical risk, mission requirements, and risk

management.



While the architecture part of the NSAD may be somewhat abstract, the design

part should be quite concrete. The design maps the selected security

services to system functional elements9. Next, these functional elements are

assigned to components and subsystems.



3.1 Composing an NSAD



The security manager is responsible for ensuring that an NSAD is defined

that applies to all the components or subsystems that constitute the

network. The NSAD for a network must address the applicable

security-relevant policies and may incorporate the NSADs of its constituent

components or subsystems. In some cases, such as when a component provides

part of the functionality of the network (e.g., a local area network (LAN)

providing 051 layer three communication services), the NSAD of the component

may be incorporated with little or no change into the NSAD for the network.

The component NSAD will probably require some modification to address the

applicable policy and environment constraints.



A typical network configuration will include multiple vendor's products.

When the network is created, the security manager must reconcile the diverse

NSADs of the constituents into a coherent NSAD for the configured network

and identify any restrictions or new security services and assurance that

must be added. The NSAD must implement national, service, and command

policies for the environment in which the network will operate. The same

process applies when previously accredited AIS are to be interconnected to

support the exchange of information.



In contrast to the networks described above, when a network is created from

scratch, the NSAD may be established before any devices are selected and may

be included as part of the criteria for selecting the network devices.



Note that the network may include components that are not security-relevant

and, therefore, do not have a component NSAD. The design decisions that

result in the inclusion of non-security-relevant components are documented

in the NSAD.



AIS may be combined into a network under conditions of a hierarchical

relationship of their security managers. In this case the NSAD of the

subordinate system must conform to the governing NSAD. For example, when a

host computer connects to a common user network, the host computer must

conform to the NSAD established by the security manager of the common user

network, who has a responsibility to the security managers of all other

connected AIS to maintain the network's trustworthiness. As discussed below,

this conformance is recorded in a Memorandum of Agreement (MOA).



AIS whose security managers are not hierarchically related may also be

combined to form a network. In this case, the security managers come to

agreement on the NSAD for the network; this agreement is also recorded in an

MOA.



3.2 Memorandum of Agreement



If a network consists of previously accredited AIS, a MOA is required

between the Accreditors for each subsystem. This MOA is part of the

documentation of the NSAD. The MOA discusses the accreditation requirements

for each subsystem that is to be interconnected to form the network [4],

i.e., defines all the terms and conditions of the security arrangements that

will govern the operation of the network [10]. The objective of the MOA is

to document the interconnection requirements and identify any requirements

that may be necessary to provide overall security safeguards for the entire

network. This network includes all the interconnected subsystems, the

communications devices, the users, and the data stored in the subsystem

[10]. A Memorandum of Record (MOR) is used when the subsystems have the same

Accreditor. A comprehensive M0A10 could constitute the entire NSAD for a

network; alternatively, the MOA could contain high level agreements, with

the details spelled out in supporting documents. Following is a list of

suggestions for the contents of the MOA and supporting documents. The items

towards the top of the list are more likely to occur in the MOA; those

towards the end of the list are more likely to occur in supporting

documents.



        o A general description of the information that will be transmitted

          to the network by each subsystem

        o A summary discussion of the trusted behavior that is expected from

          each subsystem

        o The details of the overall security plan for the network and the

          assignment of responsibility for producing and accepting the plan

        o A description of the overall network security policy

        o A description of additional security training and assignment of

          training responsibility

        o Specification of the security parameters that are to be

          transmitted between communicating subsystems

        o A discussion of security details that are relevant to the exchange

          of information among the subsystems.

        o A description of the user community, including the lowest

          clearance of any user who will have access to the network

        o Any special considerations for dial-up connections to any

          subsystem in the network, including potential security threats and

          the safeguards that will be used.

        o A description of the security protections provided by the data

          communications, both local to a subsystem and between

          communicating subsystem

        o A description of the information that each subsystem will log in

          the audit trail, and how the audit trail tasks will be divided

          among the subsystems

        o A description of the information security services to be offered

          to the network by each subsystem, including:

               - The types of processing provided by each subsystem, e.g.,

               file query, individual user, general processing

               - The modes of operation of all the subsystems, e.g.,

               dedicated, system high, multilevel

               - The sensitivity levels processed on all subsystems



________________



10 In this guideline, NOA is used to identify the agreement between

Accreditors and includes the NOR.





4 TNI Part I Security Requirements



This section assists the security manager in determining the recommended

minimum security requirements based on TNI Part I and Appendix A, which

interprets the TCSEC for networks.



The procedure for determining the minimum security requirements for a

network parallels the procedure for a stand-alone system, whereby the

highest classification of data and the lowest clearance among system users

are used in computing a risk index. The risk index is used to determine

which NCSC evaluation rating is required of the system to provide adequate

security. To emphasize, these are the minimum requirements. The TCSEC

Environments Guideline does not address environmental factors such as the

number of users and the percentage of users at different classification

levels. These factors may become more significant in a network environment.

Communications security risk in a network is addressed by the National

Security Agency (NSA) and other cognizant organizations and results in a set

of recommendations for the appropriate equipment or security procedures.

Other factors, such as the number of connections, the physical distance

between devices, the number of subsystems, the presence of encryption, and

the possible presence of intervening systems between the resources being

used and the ultimate user may result in more or less stringent

requirements.



4.1 Risk Management



Risk management is a methodology used to identify, measure, and control

events which adversely affect security; it involves cost-benefit analyses to

ensure appropriate cost-effectiveness of security safeguards. A risk

management program is mandated by Enclosure (3) of DODD 5200.28.



The literature on risk management is quite extensive. It is not the;Purpose

of this document to survey the field. The interested reader is referred to

FIPS PUB 65 [11]. The literature is constantly growing; a recent high-level

introduction to general concepts and terminology can be found in Bell [12]

and in the Proceedings of the First Invitational Workshop on Computer

Security Risk Management [13].



DODD 5200.28 Enclosure (4) mandates the use of a methodology, extracted from

the TCSEC Environments Guideline, to determine the recommended evaluation

class(or requirements of an evaluation class) based on a specific

environment. Enclosure (5) of the Directive also recommends this method to

determine minimum computer-based requirements in a network. This guideline

also uses that method. Use of a different method requires prior approval of

the Assistant Secretary of Defense (ASD) Command, Control, Communications,

and Intelligence (C31).



DODD 5200.28 Enclosure (4) contains six major steps in the risk assessment

procedure. These steps are listed below. DODD 5200.28 Enclosure (4) applies

in all steps.



Step 1. Determine system security mode of operation.



Step 2. Determine minimum user clearance or authorization rating.



Step 3. Determine maximum data sensitivity rating.



Step 4. Determine risk index.



Step 5. Determine minimum security evaluation class for computer-based

controls.



Step 6. Determine adjustments to computer security evaluation class

required.



An elaboration of step six given in Migues [14], involving a detailed

analysis of both environmental and architectural risk factors, is based on

Landwehr and Lubbes[15]. It presents a method which incorporates analysis of

the applications environment. This analysis includes such factors as whether

the system allows programming, or whether it is restricted to a limited set

of applications. This more detailed information supports a finer

determination of the level of trust required.



4.2 Determination of Network Risk



To apply the TCSEC Environments Guideline guidance, the security manager



must determine the following:



        o minimum clearance or authorization of the network users (see Table

          1 [11]),

_













































__________________



11 Tables 1 and 2 are adapted from DODD 5200.28 (enclosure 4).



Table 1:   Rating Scale for Minimum User Clearance (Rmin)

------------------------------------

Minimum User Clearance                   Rmin

Uncleared OR Not                           0

Authorized (U)

Not Cleared but

Authorized Access to                       1

Sensitive Classified

Information

Confidential (C)                           2

Secret(S)                                  3

Top Secret (TS) and/or

current Background                         4

Investigation (BI)

TS and/or current

Special Background                         5

Investigation (SBI)



One Category (IC)                          6

Multiple Categories (MC)                   7

------------------------------------



The number derived from Table 1 is used for Rmin; the one derived from Table

2 is used for Rmax. A risk index for the network is calculated using the

following formula:



Risk Index = Rmax - Rmin



Table 2:  Rating Scale for Maximum Data Sensitivity(Rmax)

----------------------------------------------------------------

 Maximum               Rating         Maximum Data               Rating

Sensitivity            (Rmax)    with categories 1,2 (Rmax)

Ratings

without

categories



Unclassified (U)         0               N/A 3



Not

Classified               1          N with one or more             2

Sensitive (N)4                          Categories



Confidential (C)         2        C with one or more Cate-         3

                                         gories



Secret (S)               3        S with one or more

                                  Categories only one              4

                                 Category containing S

                                  S with two or more               5

                                Categories containing S



Top Secret (TS)          55     TS with one or more Categories     6

                             only one Category containing S or

                                                                          

                                        TS                     

                               TS with two or more             7

                             Categories containing S

                                      or TS

----------------------------------------------------------------



1 Where the number of categories is large or where a highly sensitive

category is involved, a higher rating might be warranted.



2 The only categories of concern are those for which some users are not

authorized access. When counting the number of categories, count all

categories regardless of the sensitivity level associated with the data. If

a category is associated with more than one sensitivity level, it is only

counted at the highest level. Systems in which all data are in the same

category are treated as without categories.



3 Unclassified data by definition may not contain categories.



4 Examples of N data include financial, proprietary, privacy, and mission

sensitive data. In some situations (e.g., those involving extremely large

financial sums or critical mission-sensitive data), a higher rating may be

warranted. This table prescribes minimum ratings.



5 The rating increment between the Secret and Top Secret data sensitivity

levels is greater than the increment between other adjacent levels. This

difference derives from the fact that the loss of Top Secret data causes

EXCEPTIONALLY GRAVE damage to U.S. national security, whereas the loss of

Secret data causes SERIOUS damage.



Table 3:  Security Risk Index

------------------------------------------------------------------------

  Risk                 Security Mode       Minimum Security Class4

Index

0                      Dedicated 5           No Minimum Class 1,2

0                      System High                  C22

1                Multilevel Partitioned             B13

2                Multilevel Partitioned             B2

3                       Multilevel                  B3

4                       Multilevel                  A1

5                       Multilevel                   *

6                       Multilevel                   *

7                       Multilevel                   *



------------------------------------------------------------------------



1 Although there is no prescribed minimum class, the integrity and denial of

service requirements of many systems warrant at least class C2 protection.



2 Automated markings on output must not be relied on to be accurate unless

at least class B1 is used.



3 Where an AIS handles classified or compartmented data and some users do

not have at least a Confidential clearance, or when there are more than two

types of compartmented information being handled, at least a class B2 is

required.



4 The asterisk (*) indicates that computer protection for environments with

that risk index is considered to be beyond the state of current computer

security technology.



5 Most embedded systems and desk top computers operate in the dedicated

mode.



Table 3 [12] is used, along with the Risk Index calculated above, to determine

a minimum NCSC-evaluation rating for the system. Note that some terms that

appear in the TCSEC Environments Guideline are no longer defined in DODD

5200.28. (Limited Access Mode, and Compartmented Mode fall under the heading

of Partitioned Mode. Controlled Mode comes under the heading Multilevel. The

prevt.ou:°sly used terms referred to the equivalent of the BI and B2

evaluation classes. In DODD 5200.28, Partitioned Mode is used in place of

Compartmented Mode.)







5 TNI Part II Security Requirements



This section contains a discussion of TNI Part II which describes

qualitative evaluations of security services in terms of functionality,

strength of mechanism, and assurance. Part II of the TNI describes

additional security concerns and services that arise in conjunction with

networks, but that do not normally arise in stand-alone computers.



Part II of the TNI focuses on those threats that occur between end systems

(hosts) on the network. These security services include protection against

compromise, denial of service, and unauthorized modification. In discussing

these services, the TNI borrows heavily from the International Standards

Organization (150) 051 Basic Reference Model [9] and Security Architecture

[16]. The services discussed are closely related to those found in the

latter reference. The TNI goes beyond the 051 Security Architecture in

several respects. First, the 051 document does not address the relative

strengths of different mechanisms nor the assurances that they operate as

intended. Second, the protection against denial of service threats is not

specifically addressed by 051 but is an important consideration in the TNI.



5.1 Relationship of TNI Part II Services to Part I and Appendix A



The Part II services are not as well understood as the features in TN1 Part

I. The fact that Part 11 services have not been supported by equally well

developed theories and detailed evaluation criteria should not be

interpreted to imply that their security problems do not have to be

evaluated as rigorously as TNI Part I and Appendix A services. Some Part II

services may not be part of the NTCB. For example, to make the NTCB as small

as possible, some of the protocol software may be outside the NTCB.



____________________

12 Table 3 is adapted from the TCSEC Environments Guideline

Therefore, the protocol-based protection against denial of service is likely

to be outside the NTCB. Nonetheless, it must rely on some of the fundamental

NTCB assurances since the protocols invoke portions of the subsystem's

operating system.



It is important to recognize that many Part II security services depend on

(embedded) AIS. These AIS should be evaluated using Part I and Appendix A of

the TNI. Encryption systems, for example, are highly dependent upon AIS;

they are addressed in Appendix B of the TNI and Appendix C of this

guideline. Appendix C presents some considerations concerned with applying

Tables 1, 2, and 3 to encryption systems.



For security services that do not depend strongly on A!S, a qualitative

evaluation approach is used. For functionality, a question and answer format

is presented in Section 5.4.3. For strength of mechanism and assurance, the

concept of a risk index is used in Sections 5.4.1 and 5.4.2.





5.2 Specification and Evaluation of Security Services



Specifying and evaluating Part II security services is quite different from

a TN! Part! evaluation even though both parts are concerned with the same

three aspects of security services or capabilities: functionality, strength

of mechanism, and assurance. For clarity these terms are defined as follows:



          Functionality refers to the objective and approach of a security

          service.

          Strength of mechanism refers to how well a specific approach may

          be expected to achieve its objectives.

          Assurance refers to a basis for believing that the functionality

          will be achieved.





5.3 Evaluation Ratings



Part II evaluations are qualitative, as compared with the hierarchically

ordered ratings (e.g., C1, C2, ...) from the TCSEC. The results of a Part II

evaluation for offered services are generally summarized using the terms

none, minimum, fair, and good. For some services, functionality is

summarized using none or present because gradations are not meaningful. The

term none is used to mean the security service fails to distinguish the

strength of mechanism. The term not offered is used when a security service

is not offered. For example, if a certain network did not include non-

repudiation as one of its security services, that network would be rated not

offered with respect to non-repudiation. Table 4 represents the evaluation

structure of Part II as a matrix. It identifies a set of security services.

It also shows the possible evaluation ranges for each service in terms of

its functionality, strength of mechanism, and assurance.





5.4 Selecting Security Services



Part II enumerates representative security services that an organization may

choose to employ in a specific situation. Not all security services will be

equally important for a specific environment, nor will their relative

importance be the same



Table 4:  Evaluation Structure for Network Security Services

-------------------------------------------------------------

 Network Security          Criterion        Evaluation

Service

Communications                      Functionality           None present

Integrity                           Strength                None - good

Authentication                      Assurance               None - good

Communications Field                Functionality           None - good

Integrity                           Strength                None - good

                                    Assurance               None - good

Non-repudiation                     Functionality           None present

                                    Strength                None - good

                                    Assurance               None - good

Denial of Service                   Functionality           None - good

Continuity of Operations            Strength                None - good

                                    Assurance               None - good

Protocol Based                      Functionality           None - good

Protection                          Strength                None - good

                                    Assurance               None - good

Network Management                  Functionality           None present

                                    Strength                None - good

                                    Assurance               None - good

Compromise Protection               Functionality           None present

Data Confidentiality                Strength                Sensitivity level

                                    Assurance               None - good

Traffic Flow Confidenti             Functionality           None present

ality                               Strength                Sensitivity level

                                    Assurance               None - good

Selective Routing                   Functionality           None present

                                    Strength                None - good

                                    Assurance               None - good



-------------------------------------------------------------



among different environments. Selecting security services is a management

decision, with assistance provided by this guideline.



Ordinarily, the security manager would first determine whether a particular

service is required and what functionality is needed (if there are

distinctions) through a series of questions provided in Section 5.4.3. A

separate set of questions is provided for each service shown in Table 4.



Once the functionality has been determined, the strength of mechanism and

appropriate level of assurance must be determined. The process is similar to

the determination of Part I risk in Section 4 of this guideline. Since the

strength of mechanism and assurance determination do not differ for the

various services, these topics are addressed first.



5.4.1 Strength of Mechanism



Determination of strength of mechanism for each service has two components.

The inadvertent threat and the malicious threat should be analyzed

separately. In many cases, the malicious threat will dominate the

inadvertent threat; malicious users can often duplicate the circumstances of

an inadvertent threat. The required strength of mechanism is determined

using a risk index similar to that used in Part I.



For inadvertent threats, traditional risk management techniques are used.

While some countermeasures may be the same for inadvertent and malicious

threats, others may be effective only against the former. The security

manager must determine the likelihood of a particular threat, the dollar

cost of a countermeasure, and the residual risk if the countermeasure is put

into effect. The manager concerned with these inadvertent threats is

referred to the references on risk assessment previously cited.



For malicious threats, we consider the most sensitive information contained

on the system and the lowest clearance of user who can gain physical access

to some device in the system, including access to wireless transmissions.

Some devices in the system may be physically protected in buildings that

require a clearance for admittance. Other devices in the system, such as

long-haul transmission lines, may have no physical protection.



Protection of the information in the network system is a combination of

physical, administrative, procedural, and technical protections. The TNIl is

concerned only with the AIS hardware, firmware, software, and configuration

management protections. Various service or agency regulations describe

methods for implementing the other protections.



The various devices in the system must be considered separately; for each

device, the risk index will be based on the most sensitive information on

the network system and the minimum clearance to gain physical access to the

device. Note that this is different from the Part I risk index calculation

(where the lowest cleared user is of concern). For some devices in the

system (e.g., the communications media), the clearance of individuals with

physical access may be lower than that for authorized users. For

convenience, all the necessary tables are included here. Table 5, Minimum

Clearance for Physical Access, is identical to Table 1. For each device in

the system, the lowest clearance of individuals with physical access to that

device is used. Table 6 for Maximum Data Sensitivity is identical to Table

2.



Table 5:  Minimum Clearance for Physical Access

--------------------------------

 Minimum Use Clearance                Rmin

Uncleared OR Not                       0

Authorized (U)

Not Cleared but

Authorized Access to                   1

SensitiveUnclassified

Information (N)

Confidential (C)                       2

Secret (S)                             3

Top Secret (TS) and/or                 4

current Background

Investigation (BI)

TS and/or current                      5

Special Background

Investigation (SBI)

One Category (IC)                      6

Multiple `Categories                   7

(MC)

--------------------------------



Table 6:  Maximum Data Sensitivity

---------------------------------------------------------------

Maximum                               Maximum Data

Sensitivity Ratings     Rating      Sensitivity with        Rating

without categories     (Rmax)         Categories1, 2         (Rmax)



Unclassified (U)         0                N/A 3



Not Classified but       1          N with one or more         2

Sensitive (N)4                         Categories



Confidential (C)         2          C with one or more         3

                                       Categories



Secret(S)                3          S with two or more         4

                                   Categories only one

                                   Category containing S                    

                                    S with two or more         5

                                  Categories containing S



Top Secret (TS)          55         TS with one or more        6

                                Categories only one Cate-

                                 gory containing S or TS                  

                                   TS with two or more         7

                                        Categories

---------------------------------------------------------------



1 Where the number of categories is large or where a highly sensitive

category is involved, a higher rating might be warranted.



2 The only categories of concern are those for which some users are not

authorized access. When counting the number of categories, count all

categories regardless of the sensitivity level associated with the data. If

a category is associated with more than one sensitivity level, it is only

counted at the highest level. Systems in which all data are in the same

category are treated as without categories.



3 Unclassified data by definition may not contain categories.



4 Examples of N data include financial, proprietary, privacy, and

mission-sensitive data. In some situations (e.g., those involving extremely

large financial sums or critical mission-sensitive data), a higher rating

may be warranted. This table prescribes minimum ratings.



5 The rating increment between the Secret and Top Secret data sensitivity

levels is greater than the increment between other adjacent levels. This

difference derives from the fact that the loss of Top Secret data causes

EXCEPTIONALLY GRAVE damage to U.S. national security, whereas the loss of

Secret data causes SERIOUS damage.



Table 7 now gives the strength of mechanism requirement based on the risk

index calculated as



Risk Index = Rmax - Rmin



Table 7:  Minimum Strength of Mechanism Requirement

------------------------

Risk Index        Strength of

                   Mechanism



O                    None

1                   Minimum

2                    Fair

>2                   Good



------------------------



5.4.2 Assurance



Assurance is a very important concept in the TCSEC and TN!. This section

discusses the need for assurance and the ways in which it may be achieved.



One salient property of trusted systems is the reliance on a TCB. Similarly,

trusted network systems rely on an NTCB. In addition to its other

responsibilities, the NTCB prevents unauthorized modification to objects

within the network system. In particular, the NTCB maintains the integrity

of the programs which provide security services, thus ensuring that their

assurance is continued. The NTCB provides an execution environment that is

extremely valuable in enhancing the assurance of security services.

Discretionary and mandatory access controls can be employed to segregate

unrelated services. Thus, service implementation that is complex and

error-prone or obtained from an unevaluated supplier can be prevented from

degrading the assurance of other services implemented in the same device.

Furthermore, an NTCB ensures that the basic protection of the security and

integrity information entrusted to the network is not diluted by various

supporting security services.



The relationship of the risk index to the required assurance is expressed in

Table 8.



Table 8:  Minimum Assurance Requirements

----------------------

Risk Index          Part II

                   Assurance

                   Rating



0                    None

1                  Minimum

2                    Fair

>2                Good



----------------------



Assurance of the design and implementation of Part II mechanisms is also

related to the assurance requirements in Part I because service integrity

depends on protection by the NTCB or TCBs. Table 9 expresses this

dependency. The second column identifies the minimum Part I evaluation which

supports the Part II assurance requirement. Note that Table 9 is applicable

only to those Part II services not strongly dependent on A!S; the AIS

implementing those services can be directly evaluated under Part I and

Appendix A of the TNI.



Note that the Evaluation Class calculation in Part I will not necessarily be

the same as the Minimum Part I Evaluation in Table 9. This is because the

Rmin for Part II may be different from that of Part I! since the Part II!

protections are oriented towards outsiders (those with physical access)

rather than towards users. Depending on the particular environment, either

the Part I! requirement or the Part II requirement may dominate. The latter

would be the case if a system were operated in the system high mode-where

all users were cleared to see the most sensitive information-but the network

was exposed to lower clearance individuals.



5.4.3 Functionality



This section asks questions about each of the security services contained in

Part II of the TNI. These questions are designed to help the security

manager identify the functionality required for each security service. The

questions should be answered in sequence, unless the answer to one question

contains an instruction to skip ahead.



Authentication



       1. Is there a requirement to determine what individual, process or

          device is at the other end of a network communication? If yes,

          document this requirement.



Table 9:  Part II Assurance Rating

-----------------------------------

Part II            Minimum

Assurance          Part I

Rating             Evaluation



Minimum                  C1

Fair                     C2

Good                     B2

-----------------------------------



If no, skip to Communications Field Integrity.



       1. Do you have a requirement to identify and authenticate the

          specific hardware device at the distant end-point involved in the

          network communication? If yes, then you have a functionality

          requirement for authentication. This functionality may be

          implemented at one or more protocol layer. For example, a specific

          control character, ENQ (enquiry or who-are-you) may be used to

          return immediately a stored terminal identifier.

       2. Do you have a requirement to identify and authenticate the

          location of the hardware at the distant end-point or in any

          intermediate system involved in the network communication? If yes,

          then you have a functionality requirement for authentication at

          protocol layer 2, the Link Layer or layer 3, the Network Layer.

       3. Do you have a requirement to identify and authenticate the

          specific operating system or control program at the distant

          end-point or in any intermediate system involved in the network

          communication? If yes, then you have a functionality requirement

          for authentication at protocol layer 4, the Transport Layer.

       4. Do you have a requirement to identify and authenticate the subject

          (process/domain pair) at the distant end-point involved in the

          network communication? If yes, then you have a functionality

          requirement for authentication at protocol layer 4 or above.

       5. Do you have a requirement to identify ad authenticate the

          application or user at the distant end-point involved in the

          network communication? If yes, then you have a functionality

          requirement for authentication above protocol layer 7, the

          Applications Layer. The Applications Layer provides an interface

          to the application. Authentication information may pass over this

          interface. Authentication of a user is addressed in Part I of the

          TN!.Application process authentication is outside the scope of the

          05! Security Architecture, but does fall within the scope of TN!

          Part II Security Services Have you chosen to use some mechanism

          other than encryption to provide authentication? If so, your

          strength of mechanism is shown in Table 7. If your authentication

          mechanism is encryption based, see the appropriate encryption

          authority (e.g., NSA). Even if encryption is used, some supporting

          processes may need to satisfy the strength of mechanism shown in

          Table 7 (depending on the architecture). For example, a database

          that relates encryption keys to specific users may need to be

          trusted.



Communications Field Integrity



            1. Do you have a requirement to protect communication against

               unauthorized modification?

               If no, skip to Non-Repudiation.

            2. Are your protection requirements the same for all parts of

               the information communicated?

               If no, then you should identify the separate parts and answer

               the rest of the questions in this section separately for each

               part. Each part is known as a field.

               There are two major fields: protocol-information, wherein the

               network is informed of the destination of the information and

               any special services required; and user-data. Not every

               protocol data unit (PDU) contains user- data, but

               protocol-information is necessary. Each of these fields may

               be divided into additional fields; depending on your

               application, protection requirements for fields may differ.

            3. Do you have a requirement for detecting unauthorized

               modification to part or all of a PDU?

               If yes, you have a requirement for at least minimum

               functionality.

            4. Do you have a requirement for detecting any of the following

               forms of message stream modification: insertion, deletion, or

               replay?

               If yes, you have a requirement for at least fair

               functionality. In addition, your functionality must be

               incorporated in a connection oriented protocol.

            5. Do you require that, if message stream modification is

               detected, recovery correction) should be attempted?

               If yes, you have a requirement for good functionality. In

               addition, you must implement integrity in a reliable

               transport (layer 4) mechanism.



Non-repudiation



            1. Do you have a requirement to be able to prove (to a third

               party) that a specific message transfer actually occurred?

               If no, skip to Denial of Service.

            2. Do you have a requirement for proving that a specific message

               was sent?

               Specific message means that the identity of the subject

               sending the message, the host computer and/or mail

               agent/server, time and date, and contents are all uniquely

               and unalterably identified.

               If yes, then you have a functionality requirement for

               non-repudiation with proof of origin.

            3. Do you have a requirement for proving that a specific message

               was received?

               Specific message means that the identity of the subject to

               which the message was delivered, the host computer and/or

               mail agent/server, time and date, and contents are all

               uniquely and unalterably identified.

               If yes, then you have a functionality requirement for

               non-repudiation with proof of delivery.



Denial of Service



            1. Do you have a requirement to assure the availability of

               communications service or to determine when a Denial of

               Service (DOS) condition exists? A DOS condition is defined to

               exist whenever throughput falls below a pre-established

               threshold, or when access to a remote entity is unavailable,

               or when resources are not available to users on an equitable

               basis. For a DOS condition to occur, the user must have

               priority to access the system or resources.

               If no, skip to Data Confidentiality.

            2. Do you have a requirement to detect conditions that would

               degrade service below a pre-selected minimum and to report

               such degradation to the network operators?

               If yes, you have a requirement for at least minimum denial of

               service functionality.

            3. Could failure of the system to operate for several minutes

               lead to personal injury or large financial loss?

               If yes, you have a requirement for at least fair denial of

               service functionality.

            4. Do you have a requirement for service resiliency that would

               continue-perhaps in a degraded or prioritized mode-in the

               event of equipment failure and/or unauthorized actions?

               If yes, you have a requirement for at least fair denial of

               service functionality.

            5. Could failure of your system to operate for several minutes

               lead to loss of life?

               If yes, you have a requirement for good denial of service

               functionality.

            6. Do you have a requirement for automatic adaptation upon

               detection of a denial- of-service condition?

               If yes, you have a requirement for good denial of service

               functionality.



Protocol Based DOS Protection



            1. Do you want advanced knowledge of unavailability of service?

               If no, skip to Network Management.

               If yes, do you want to implement alternatives?

               If yes, you should employ this alternative basis and skip to

               Network Management.

            2. In general, ordinary protocol mechanisms don't provide

               protection against malicious attacks or bizarre errors. Do

               you have a requirement to detect a DOS condition which cannot

               be met by the protocols used as part of normal

               communications?

               If no, you do not have a functional requirement for

               protocol-based DOS protection and should skip to Network

               Management.

            3. The TNI suggests the following protocol-based mechanisms:

                    a) Measure the transmission rate between peer entities

                    under conditions of input queuing, and compare the

                    measured transmission rate with a rate previously

                    identified as the minimum acceptable;

                    b) Employ a request-response polling mechanism, such as

                    "are-you-there" and "here-I-am" messages, to verify that

                    an open path exists between peer entities.

               If you have identified any additional mechanisms, include

               them in your list of required mechanisms.



Network Management



            1. Do you have a requirement for (at least) detecting a denial

               of service condition that affects more than a single instance

               of communication or attempted communication?

               If no, skip to Data Confidentiality.

               If yes, you have a functional requirement for network

               management denial of service protection.



Data Confidentiality



            1. Do you have a requirement to protect any part of transmitted

               data from disclosure to unauthorized persons?

               If no, skip to Traffic Flow Confidentiality.

            2. Is your requirement for confidentiality limited to selected

               field of user-data within a PDU?

               If no, then you require confidentiality for the entire data

               portion of each PDU.

               Continue with Traffic Flow Confidentiality.

            3. Is there a reason to encrypt only selected fields (e.g., cost

               savings, legal requirements)?

               If yes, you require selected field confidentiality.

               If no, you require full confidentiality on the data portion

               of each PDU



Traffic Flow Confidentiality



            1. Do you have a requirement to prevent analysis of message

               length, frequency, ad protocol components (such as addresses)

               to prevent information disclosure through inference (traffic

               analysis)?

               If no, skip to Selective Routing.

               If yes, you have a functional requirement for traffic flow

               confidentiality.



Selective Routing



            1. Do you have a requirement to choose or avoid specific

               networks, links, relays, or other devices for any reason at

               any time?

               If yes, you have a functional requirement for selective

               routing.



6 Interconnecting AIS



The definition of Interconnected Accredited AIS recognizes that parts of a

network may be independently created, managed, and accredited. AIS in

different security domains [13] generally operate under different security

policies, consequently, it is difficult to combine them into a Unified

Network. For example, AIS operated by the U.S. DOD and NATO cannot be

combined into a Unified Network, since they enforce different policies and

do not have a common authority.



Interconnecting systems that support different security domains (e.g.,

classified, sensitive unclassified) adds additional complexity. Exchange of

information among these different security domains requires identification

of the markings and protection given to information when transmitted to

another domain. For example, several evolving approaches to the protection

of sensitive unclassified information [17] consider "that sensitive

___________



13 A security domain is a collection of AIS under the control of a single

administrator that enforces or operates under a specified policy. There can

be sub-domains, (e.g., Army and Air Force are sub-domains under the

Department of Defense.)

information is not part of the same hierarchy as classified information".



There are technical criteria for judging the trustworthiness of

Interconnected Accredited AIS: an Interconnection Rule, which ensures that

information conveyed between subsystems is labeled properly, and risk

factors such as propagation of local risk and the cascading problem. These

criteria are examined in some detail below.





6.1 Agreement Between Accreditors



Interconnection of AIS between security domains requires a documented

agreement identifying the interconnection requirements and all safeguards.

This agreement will have many similarities to the MOA discussed in Section

3.2. It will probably have to reconcile different security policies and

philosophies of protection, identifying the conditions under which specified

classes of information can be exchanged among domains. In addition to the

information included in the MOA, this agreement between managers of

different security domains should address the mappings of policy and

countermeasures between the domains. In many ways this agreement takes on

the character of an NSAD for the agreed upon information exchange between

domains.



6.1.1 Accreditation Range



An accreditation designates a system's mode of operation and range of data

sensitivity' levels. The accreditation range reflects the Accreditor's

judgment on the subsystem's ability to exchange information within an

acceptable level of risk, with respect to its network connections, and in

accord with the designated sensitivity levels.



The range must be a single level in the case of a system high or dedicated

device[14]. If the accreditation range comprises more than a single level, the

system is trusted to reliably segregate data by level within its

accreditation range, and label it accurately for transmission over

multilevel interfaces. The accreditation range will be specified in the MOA.

The accreditation range is used in determining whether communication between

systems is permitted.



























14 Often in the discussion it is not appropriate to distinguish between a

component and a subsystem; in that case we use the term device.



Figure 1



Information Levels and Accreditation Ranges





      S-C                      TS

                                S

                                C



      C2   Evaluation Class    B3



        

       S Accreditation Range   TS-C



      SH   Operating Mode      MLS



As shown in Figure 1, an AIS may contain information at levels that are

below its accreditation range. For example, a C2 host which contsuns Secret

(S) and Confidential (C) information, is not trusted to segregate this

confidential and Secret information. Therefore, it is accredited to operate

in system high (SH) mode at Secret (the highest sensitivity level of

information on the system), and its accreditation range is the single level

Secret. All exported information must be labeled with the system high

sensitivity label until there is a manual review to assign the information a

lower classification level. In contrast, a B3 multilevel secure (MLS) host, 
which contains Top Secret (TS), Secret, and Confidential information could be

assigned an accreditation range equal to the entire set of levels processed.

In this case, the label of the exported data is equal to the actual level of

the exported data, unless unclassified data is to be exported.



Figure 2 illustrates the accreditation ranges of two interconnected

subsystems. Although Subsystem Y is able to separate its three levels of

information, it may exchange information with Subsystem X only at the S and

C levels.



Figure 2



Accreditation Ranges of Two Interconnected Subsystems



Subsystem X                    Subsystem Y



                                    TS



     S                               S



     C                               C



Class B1                          Class B3













In a network, an accreditation range bounds the sensitivity levels of

information that may be sent (exported) to or received (imported) from each

interconnected subsystem[15]. For example, if a network consists of only

dedicated and system high subsystems, each subsystem will be assigned

single-valued accreditation ranges (i.e., an accreditation range consisting

of one sensitivity level).



When the same communications channel processes information at different

levels, the data must be labeled through some protocol agreed upon by the

communicating systems.



     DODD 5200.28 Enclosure (5) also addresses A!5 that have not been

     accredited:

     Untrusted, unaccredited AIS ... may be components of a network.... Only

     unclassified information, which does not include sensitive unclassified

     information, may be sent to ad from untrusted, unaccredited AIS.



This trust requirement is satisfied by restricting the accreditation rage of

the untrusted, unaccredited AIS to Unclassified (U).



6.1.2 Device Range



A network subsystem is typically connected to another subsystem through some

kind of 1/0 network interface or device (see Figures 3~) ad the same device

may provide connection to multiple subsystems.



Although a 1/0 device is part of a subsystem, it may be designated to

process a more restricted set of sensitivity levels than the accreditation

rage of the subsystem as a whole. This leads to the concept of a device

range. Each 1/0 device in a subsystem that is used to communicate with other

subsystems in the network must have a device rage. The device rage may be

single level, or it may be a set of levels (in which case the device is

referred to as multilevel), and it must be included within the subsystem

accreditation range. The TCSEC states that "systems that have been evaluated

at classes B2 ad above must support minimum ad maximum security labels for

all multilevel 1/0 devices". The purpose of device labels is to document the

constraints placed on the security levels of information authorized for the

devices.



Each physically attached multilevel system (if any) has a minimum ad maximum

sensitivity level associated with it. A B1 or higher system interconnected

to a second system must ensure that both imported and exported information

is contained within appropriate sensitivity levels. Information Transfer

Restrictions



The following points summarize the discussion on the restrictions imposed on

information transfer between interconnected devices.



______________

15 Note that information exported or imported to a subsystem having a single

level accreditation range is implicitly labeled at that level. It is also

possible for a subsystem with a multilevel accreditation range to employ

network interface devices with single-level ports, in which case the data

transferred over such ports is also implicitly labeled.



Information exported or imported using a single-level device is labeled

implicitly by the security level of the device. As shown in Figure 3, any

information transferred between the single-level (S) devices on Subsystems X

and Y is implicitly labeled S.



Figure 3



Implicit Labeling



Subsystem X ---------- Subsystem Y



Information exported from one multilevel device and imported at another

multilevel device must be labeled through an agreed-upon protocol, unless it

is labeled implicitly by using a communications link that always carries a

single level. For instance, in Figure 4, Secret and Confidential information

may be transferred between the multilevel devices.



Figure 4



Protocol Labeling

 

                  (C)

Subsystem X ------------- Subsystem Y

                  (S)





Figure5



Compatibility Labeling



              (S,C)

Subsystem X -------------  Subsystem Y

               (S)



Information exported at a given security level can be sent only to a

importing device whose device rage contains that level or a higher level. In

Figure 5,Subsystem X is allowed to export only Secret information to

Subsystem Y's multilevel device. Subsystem Y is allowed to export Secret ad

Confidential information to Subsystem X, because the device rage Subsystem X

is TS-S. If the importing device rage dominates the exported level, the

information is implicitly or explicitly relabeled upon receipt at a higher

level within the importing device range.



Figure 6



Relabeling



             (C) -- (S)

Subsystem X -------------   Subsystem Y



In Figure 6, Subsystem Y relabels information imported from Subsystem X. The

information transfer restrictions also permit one-way communication (i.e.,

no acknowledgments) from one device to another whose rages have no level in

common, as long as each level in the sending device rage is dominated by

some level in the receiving device rage. It is never permitted to send

information at a given level to a device whose rage does not contain a

dominating level.



In most interconnected subsystems, the bidirectional flow of information is

permitted. In this environment, the sensitivity level of any transmitted

message must be within the accreditation range of both the sending and

receiving systems. In some networks, an additional restriction on

information flow may be unidirectional communications. This restriction may

enhance security. The following discussions refer to Figure 7.



Figure 7



Bidirectional and Unidirectional Information Flow



Subsystem X Subsystem Y



TS --------------------(C or U)--------------- C







SH (TS)              Subsystem Z    (C) 

                                 MLS (C-U)

 

                        TS

                        S

                        C



                     MLS (TS-C)



The system high mode is usually assigned to A!S that are unevaluated or are

NCSC evaluated in class C. These AIS do not employ explicit labels because

they cannot be trusted to differentiate between sensitivity levels. All

information within these AIS is implicitly labeled. When exported on a

single level channel, by default the information is labeled implicitly by

the level of the channel. Human-readable output must be labeled at the

system high level; it may be manually downgraded by an authorized reviewer.



Explicit labels are required on a multilevel channel. In order to export

explicit labels, Subsystem X would normally be expected to be NCSC-evaluated

at BI or above or employ an 1/0 device, such as those shown in Figure 6,

NCSC evaluated at BI or above. Also, Subsystem X or the 1/0 device should be

used as specified in Section 4 of this guideline. Lacking such NCSC

evaluation, the MOA between the Accreditors would have to specifically

address these labels.



Subsystem X can import a message from Subsystem Y, but cannot acknowledge

receipt of that message, because an exported acknowledgment (labeled TS)

cannot be imported by Subsystem Y, which can only receive C or U

information. Transmitting an acknowledgment from Subsystem X to Subsystem Y

would constitute a write-down (i.e., writing information at a lower

sensitivity level-generally a security violation.)



Subsystems Y and Z can exchange information at C since this level is in the

accreditation range of each subsystem. When only unidirectional

communication (no acknowledgment) is utilized between two subsystems, write

up is permitted if each sensitivity level in the source subsystem is

dominated by a sensitivity level in the destination subsystem. The receiving

subsystem must change the sensitivity level of the message when the message

is received. For instance, U information sent from Subsystem Y will be

labeled C by Subsystem Z.



6.2 Interconnection Rule



The Interconnection Rule states that each device in the network must be

separately accredited to operate in an approved security mode of operation

and with a specific accreditation range. The device is accredited to

participate in the network at those levels and only those levels. This means

that information exported at a given sensitivity level can be sent only to

an importing device whose accreditation range contains that level or a

higher level. Information is relabeled, implicitly or explicitly, upon

reception at a higher level within the importing device accreditation range

only if the original level is not in that range.



According to the Interconnection Rule, a multilevel network may contain

devices with different operating modes: dedicated, system high, partitioned,

and multilevel. Also the devices may differ in the sensitivity levels and

categories which they process, and the formal access approvals of their

users (some users may not have access to all information).



Figure 7 illustrates the flexibility of the Interconnection Rule. For

example, the Interconnection Rule will allow, with certain restrictions, a

multilevel subsystem to communicate with a single-level subsystem and with

another multilevel subsystem (and the two MLS subsystems may have different

accreditation ranges). It also allows for one-way communication to a

higher-level system. It is intended to be a non-restricting rule and yet

ensure that each system receives only information that it can properly mark

and handle. Interconnection in the context of the Interconnection Rule means

only direct connections, that is, without any intermediate accredited

subsystem.



The Interconnection Rule alone does not guarantee that classified

information will not be exposed to greater risks in a network than in a

stand-alone environment. One problem in networks that is dealt with at some

length below is the cascading problem.



Figure 8



A Complex Interconnection



TS: P,A -------------   TS: Q,C



Subsystem X           Subsystem Y



6.2.1 A Complex Example



The Interconnection Rule and device range allow for some rather challenging

situations. Consider, for example, the connection depicted in Figure 8. The

system on the left processes TS information of two types: categories A and P

(where P is the union of categories C and D, P = C U D). The system on the

right processes the categories C and Q (where Q is the union of categories A

and B, Q = A U B). The two devices have no sensitivity levels in common. Yet

this is a legitimate connection as long as only TS ,A and TS ,C information

is transferred. Any information sent must be relabeled upon receipt.

Information in category A is relabeled Q when received on the right, ad

information in category C is relabeled P when received on the left



6.3 Risk Factors



There are two global considerations that affect the interconnection of

systems. The first is called propagation of local risk and the second is the

cascading problem. Before discussing these considerations, the concepts of

subsystem connection view and global network view need to be introduced.



As discussed in the previous section, any subsystem that is connected to

other subsystems must enforce the Interconnection Rule. Using the subsystem

connection view, each subsystem responsible for maintaining the separation

of multiple levels of information must decide locally whether or not

information can be sent or received. This view, then, does not require a

subsystem to know the accreditation ranges of all the subsystems on the

network; only of those with which it can communicate without an

intermediary.



The Interconnection Rule applies to communication between any two (or more)

accredited systems. However, even when the Interconnection Rule is followed,

there may be other potential security problems that will require the

implementation of additional constraints on the network. In order to address

these problems, it is necessary to adopt a global view of the network. This

view requires a knowledge of the accreditation ranges of all the subsystems

in the network. That is, it is no longer determinable locally whether or not

a constraint is being satisfied. These accreditation ranges are taken into

account when determining whether or not a subsystem should be allowed to

connect to the network. In this way, the potential damage that can occur

when information is compromised or modified can be limited to an acceptable

level.



Two global concerns are discussed below. One concern is the propagation of

local risk; the other is the cascading problem.



6.3.1 Propagation of Local Risk



The term Propagation of Local Risk comes from the notion of jeopardizing the

security of a system as a result of weaknesses in other systems to which it

may be connected. Table 3 in Section 4 recommends minimum classes of trusted

systems for specific environments. Unfortunately, in many cases, operational

needs have led to the accreditation of systems for multilevel operation that

would not meet the requirements of the recommended class. While this

increased risk may be accepted by the Accreditor of a particular system,

connection of such a system to a network exposes users of all other

subsystems in the network to the additional risk. Consequently, when an

unevaluated system, or one that does not meet the class recommended for its

accreditation, is proposed for connection to a network, constraints should

be considered, such as one-way connections, manual review of transmissions,

cryptographic isolation, or other measures to limit the risk it introduces.



In the special case of a common user network such as DDN, it may be

necessary to provide communications capabilities among systems that do not

conform to the security requirements established by the network Accreditor

(i.e., a system meeting no security requirements may be connected to a

network.) One common way to provide network service to these non-conforming

systems while still protecting the other, conforming, systems would be to

segregate the non-conforming systems into closed communities that could not

directly communicate with conforming systems. This approach is discussed in

detail in the Defense Data Network Security Architecture [18].



6.3.2 The Cascading Problem



One of the problems that the Interconnection Rule does not address is the

cascading problem, discussed in Appendix C of the INI. The cascading problem

exists when an attacker can take advantage of network connections to reduce

the nominal system resistance against leaking information across a range of

sensitivity levels. Most multilevel systems, evaluated or not, are

vulnerable to some risk that information can be leaked from a higher to a

lower level supported on the system. The accreditation range of a subsystem

represents a judgment that the risk is acceptable for that range of

classifications. The size of the range is one indication of the

attractiveness of the system as a target, so larger ranges call for more

care in system design and management. In particular, Section 4 of this

guideline discusses computation of a risk index calculation based on the

accreditation range, and recommends a minimum evaluation class for a given

risk index.



The cascading problem results from the observation that subsystems may be

connected in such a way that the network covers a larger sensitivity level

range than the individual systems are accredited to handle. Depending on the

actual topology of the interconnection and the characteristics of the

installations, the amount of effort required for an attacker to take

advantage of residual vulnerabilities may be less than what is required for

the network sensitivity range.



To see how this is possible, consider two systems, each of which is

accredited to handle two adjacent classifications of information, as shown

in Figure 9. Subsystem A processes Secret and Top Secret information, and

all users are cleared to at least the Secret level. Subsystem B processes

Confidential and Secret information, and all users are cleared to at least

the Confidential level.



The network as a whole has three levels of information. However, the leakage

resistance of the network is only that offered by two systems qualified for

only two levels. To make Top Secret information available to Confidential

users, an attacker might attempt to:



       1. Install a Trojan horse in Subsystem A to leak some Top Secret

          information to Secret

       2. Send that information across the network link to Subsystem B

       3. Install a Trojan horse in Subsystem B to leak the original Top

          Secret information, now labeled Secret, to Confidential.



The path from Top Secret in Subsystem A to Confidential in Subsystem B is

referred to as a cascading path, with three steps. Step 1 is from TS to S in

Subsystem A, Step 2 is the network link, and Step 3 is from S to C in

Subsystem B. Steps (1) and (3), the downgrading steps, offer resistance

commensurate with strictly smaller ranges. Step (2), the network link,

offers no additional resistance, given that the two Trojan horses have been

written and installed.



Figure 9



Cascading Problem



Subsystem A



     TS                         Subsystem B



      S                              S



                                     C



The question is, whether subverting two systems qualified for two levels of

information is as hard as defeating one system qualified for three levels of

information. In some cases it might be. Lee [19] gives an argument that if

two systems have probabilistically independent flaw sources, "...the

resistance to threat of a cascade of two B2 systems is approximately the

same as, or even better than, that of a B3 system."



But Lee also remarks that demonstrating effective independence of flaw

sources m a practical case is not easy, and that two systems may have the

same or equivalent flaws, particularly if their TCBs are the same, or are

separate implementations of a single flawed design. Exploitation of the

flaws on two or more systems does present additional resistance to the

attacker, but it should be kept in mind that physical access to all

interconnected systems is not necessary to send untrusted software to them,

as our experience with viruses shows unmistakably.



6.3.2.1 Tests for Cascading.



For a relatively large and complex interconnection of systems, it might not

be obvious whether a cascading problem exists. Appendix C of the TNI

includes three approaches, with different degrees of complexity and

precision, for recognizing a potential cascading problem. These range from a

simple test that is rather pessimistic, called the nesting condition, to a

complex procedure. Appendix A of this TNIEG reviews the nesting condition,

and presents additional information concerning tests for the cascading

problem.



Whichever test for cascading is employed, its result is to focus attention

on certain subsystems and their network connections that might potentially

be subject to a cascading threat. The next step is to determine whether the

systems involved are actually vulnerable to the multiple coordinated attack

that is necessary for cascading, ad, if so, to consider countermeasures.



6.3.2.2 Solutions.



There are several ways to tackle a cascading problem. Since cascading

depends on cooperative action by malicious software on the participating

subsystems, one approach is to institute configuration controls to prevent

installation of unscrutinized software, or perhaps isolating it from network

usage.



Another solution is to use a more trusted subsystem at appropriate nodes in

the network, so that an attacker will be forced to overcome a protection

mechanism commensurate with the seriousness of the potential compromise. In

Figure 9, for example, if either subsystem A or subsystem B is

NCSC-evaluated at class B3, which is sufficient according to Table 3 in

Section 4 of this guideline for a range of Top Secret to Confidential, then

the attacker is presented with an acceptable level of difficulty.



A cascading threat can also be interdicted by eliminating certain network

connections, to break paths by which an attacker could compromise

information with insufficient resistance. This solution is practical only

when the links to be eliminated are not needed for operational reasons.

Sometimes end-to-end encryption can be used to address a cascading threat

while preserving necessary connectivity, by reducing the level of

information available to intermediate systems on a communication path.



























































APPENDIX A: Tests for the Cascading Problem



The cascading problem was discussed in Section 6. This appendix reviews the

approaches presented in Appendix C of the TN! for testing whether a

cascading problem exists in an interconnection of accredited subsystems.

Three criteria are given there: the nesting condition, the cascade

condition, and a heuristic procedure. The nesting condition is a simple but

pessimistic test that can, in some cases, dismiss the possibility of a

cascading problem. When it fails, there is not necessarily a cascading

problem; other, more accurate, tests should then be applied to confirm and

locate it. This appendix first summarizes the nesting condition, and then

discusses other approaches briefly. A forthcoming report will provide

further guidance on computational approaches for the cascading problem.



A.l Nesting Condition



The nesting condition is evaluated solely on the basis of the accreditation

ranges of the subsystems. In the form given both here and in the TNI, it is

applicable only when all sensitivity levels are totally ordered - that is,

if they can be placed in order such that each one is higher than the one

before it. This is true, in particular, if they are pure classifications,

with no categories or compartments.



The nesting condition is satisfied, by definition, if the accreditation

ranges of each pair of subsystems are either disjoint or nested. A pair of

accreditation ranges is disjoint if they have no levels in common. They are

nested if one range is included (as a subset) in the other. All possible

pairs (not just those of adjacent subsystems) must be considered, but some

pairs may be nested while others are disjoint.



If the nesting condition is satisfied, there is no cascading problem.

Because the nesting condition does not take into account which network

subsystems are actually connected to one another, it can sometimes give a

pessimistic result, i.e., there are cases when the nesting condition fails,

but there is actually no cascading problem.



Figure A-I



Accreditation Ranges of Two Interconnected Sub systems



                                        Subsystem Y



Subsystem X                                  TS



    S                                         S



    C                                         C



Class B1                                  Class B3



Example 1: Consider the situation illustrated in Figure A-I. The

accreditation range of Subsystem X is nested within that of Subsystem Y

(i.e., C-S is completely contained within C-TS). Therefore, the nesting

condition is satisfied, and there is no cascading problem.



Figure A-2



Cascading Problem



Subsystem A



                        Subsystem B

     TS



      S                        S



                               C                        



Example 2: Consider the situation illustrated in Figure A-2. The

accreditation ranges of Subsystem A and Subsystem B are not disjoint;

neither is one completely contained within the other. Therefore, the nesting

condition fails, ad a cascading problem is possible. Note, however, that the

nesting condition would still fail even if the two subsystems were not

connected to one another, yet in that case there would be, no cascading

problem.



The situation is more complex when sensitivity levels are drawn from a

partially ordered set, so that the accreditation ranges of some subsystems

have sensitivity levels that are incomparable. Two sensitivity levels are

incomparable when neither is greater than or equal to the other. Sensitivity

levels with category sets are, in general, incomparable. An extended form of

the nesting condition has been devised that applies to partially ordered

sensitivity level sets. [20]



A.2 Other Approaches



Appendix C of the TNI contains two other criteria for the cascading problem:

the cascade condition, which is a mathematical characterization of the

problem, and a heuristic procedure. These criteria have been superseded by

improved methods since the publication of the TNI. The new approach is

described in a separate report, in order to give adequate scope to the

presentation of background and context necessary to apply it appropriately.



The need for a new approach arose from a recognition of the limitations of

the existing criteria. The cascade condition is accurate but it is not, in

itself, a computational procedure. It is limited by its assumption that all

of the interconnected subsystems have been given evaluation classes. The

heuristic procedure is believed to provide a conservative approximate test

for cascading, but only when applied to interconnections in which all

communication paths are two-way, i.e., capable of both sending and

receiving. A simpler procedure is now available.















APPENDIX B: Background References



Neither the TNI nor this TNIEG contain tutorial information on security and

networking; it is assumed that the reader will have some background in both

areas. There is considerable literature available. Following are some

references that provide background and related information concerning

security in networks:



[1]

     M. D. Abrams and H. J. Podell, Computer and Network Security, a Tutori

     al, IEEE Computer Society Press 1987.

[2]

     D. W. Davies and W. L. Price, Security for Computer Networks, John

     Wiley & Sons 1984.

[3]

     D. E. Denning, Cryptography and Data Security, Addison-Wesley 1983.

[4]

     M. Gasser, Building a Secure Computer System, Van Nostrand Reinhold

     Company 1988.

[5]

     International Standards Organization, Information Processing Systems -

     Open System Interconnection - Basic Reference Model, 15 October 1984.

     International Standard 7498. Part 2: Security Architecture, February

     1989.1507498-2-1988(E).

[6]

     Charles P. Pfleeger, Security in Computing, Prentice-Hall 1989.

[7]

     Andrew S. Tanenbaurn, Computer Networks, Second Edition, Prentice-Hall

     1988.



















































APPENDIX C: Encryption



May networks today use or plan to use encryption as a fundamental mechanism

for providing security services. The encryption devices provide a security

perimeter at the protocol layer at which they provide service (typically the

Network or Transport Layer). This section presents some information on how

an encryption system can be part of the NSAD. It discusses MAC and DAC, use

of encryption to reduce the number of AIS, and the risk index of the

encryption system.



C.1 Use of Encryption



As indicated in the TN!, an encryption mechanism is evaluated differently

than other mechanisms. Evaluating encryption mechanisms has a long history

predating the TNI. Evaluation of an encryption mechanism is part of COMSEC.

Generally, encryption mechanisms receive a rating of the highest level of

classified information which may be protected using that mechanism.

Therefore, the only rating applicable to an encryption mechanism is the

classification level of the information that is to be protected. This

classification level also establishes the requirement.



In general, organizations using the TNI and this document select their

encryption mechanisms from a list provided by an organization which is

responsible for evaluating such mechanisms. In many cases, that organization

is the NSA.



A more complicated situation exists when encryption is employed above the

Link Protocol Layer, layer 2. At layers 3 and 4 the protocols are concerned

with the end systems or intermediate devices (e.g., hosts, network switches)

that the links connect. Higher layers are concerned with other peer

entities. Traditionally, encryption applied above layer 2 has been termed

end-to-end encryption, or E3.



An E3 system may be provided as (part of) an NTCB. When the E3 system is

integral to the NTCB, the use of the E3 system requires evaluation under the

TCSEC with interpretations in the TNI. The evaluation must consider (1) the

accreditation rage of the user interface, (2) the risk index for the bypass

in the E3 device, and (3) the risk index between the highest sensitivity

data ad the lowest clearance of user on the network.



Depending on the design, devices of an E3system may satisfy all requirements

for a system evaluated under Part! of the TN!. MAC may be provided either

explicitly or implicitly. Explicit MAC is provided if the packets sent by

the encryption device include a security label. Implicit MAC is provided if

the security level must be inferred from the encryption key used to protect

the data. All data protected by that key must be classified at a single

security level.



DAC is often provided in an E3 system as well. Typically, keys for

exchanging data are provided to the E3 devices only after DAC has been

applied. The encryption devices can provide identification ad

authentication. While identification is generally done explicitly (by

transmitting an identifier), authentication can be done implicitly (i.e., by

the use of a unique key). The encryption devices may perform certain types

of auditing as well. Typically, a device collects information and forwards

it to another device for storage. Information collected may include:

connections established, connections refused, packets with inappropriate

labels, and misrouted packets. The granularity provided by these E3

mechanisms is determined by the protocol layer at which the service is

offered.



Figure C-1



Typical Interconnected AIS



Host LAN Gateway WAN Gateway LAN Host



AIS 1 AIS 2 AIS 3 AIS 4 AIS 5 AIS 6 AIS 7



In a typical network there will be a number of A!S. For example, two hosts

are often attached to separate local networks connected by a wide area

network (WAN). As shown in Figure C-1, the path between the hosts (without

E3) may involve 7 separate interconnected AIS.



E3 can help reduce the number of A!S. By placing E3 devices between each

host and the LAN to which it is connected ad incorporating suitable key

distribution components, the LANs and WAN collapse into a single network

system and the path now traverses only three AIS, as shown in Figure C-2.

A!S 2 provides security services to the hosts, therefore, it may be part of

the NTCB.



Figure C-2



Using End-to-End Encryption to Reduce Number of AIS



Host E3 LAN Gateway WAN Gateway LAN E3 Host



AIS 1 AIS 2 AIS 3



There may be a hierarchy of trusted system views. For example, E3 may be

provided at protocol layers 3,4, and 7. Depending on the architecture, the

layers of E3 could constitute a single NTCB or each could be a separate

NTCB. In the latter case, the devices supporting different layers would be

part of different AIS and the interconnection rules would be applied between

higher and lower protocol layers.



In general, an A!S at a higher protocol layer encompasses more devices than

one at a lower protocol layer. The granularity of services offered is also

finer at the higher protocol layer.



In a situation where the higher protocol layer encryption system also has a

higher evaluation class, the lower protocol layers might be considered less

trusted just as current E3 designs treat the subnetwork as untrusted.

Continuing the analogy, just as certain physical security requirements are

imposed on the untrusted subnetwork, the higher protocol layer encryption

might rely on characteristics of the lower protocol layers.



However, one may be faced with a dilemma that the higher protocol layer E3

system has a lower security evaluation than the lower-protocol-layer trusted

system. For example, a WAN with E3 at layer 3 might be evaluated Al. The

system might also provide E3 at layer 4, but an NTCB that includes layer 4

might. only be rated B2. In this case, treating the subsystems constituting

the separate layers as separate AIS and using the Interconnection Rule to

accredit the network as a whole could prove advantageous, as illustrated in

Figure C-3.



Figure C-3



Separate Layers Treated as Separate AIS



B2 B2



(N-C) (N-C)



A1



0SI Layer 3 Network



(N-TS)



B2 B2



(S-TS) (S-TS)



C.2 Encryption Mechanisms



In a trusted AIS, the recommended evaluation class is determined using a

risk index based on the highest data classification and the lowest user

clearance. In considering an E3 subsystem in a network, three separate

indexes must be considered [21]:



  1. The subscriber's range of sensitivity levels. The clear text side of

     the encryption subsystem must be sufficiently trusted to maintain

     separation among the clear text data streams sent and received by the

     subscriber attached to the encryption subsystem. A risk index is based

     on the highest and lowest sensitivity levels sent or received by the

     subscriber through this encryption subsystem.

  2. The bypass. In an E3 system, protocol control information must be sent

     around the encryption unit through a bypass. The software and hardware

     to implement the bypass must be trusted not to send user data through

     the bypass. A risk index can be computed based on the difference

     between the sensitivity level of the clear text information and the

     sensitivity level of the untrusted subsystems of the network.

  3. The range of sensitivity levels across the network. This risk index is

     concerned with the difference between the highest level of information

     any host attached to the network and the lowest clearance of a user

     that could potentially get access to that information. Depending on the

     characteristics of the network, this risk index could be larger than

     either 1. or 2. above. The worst case scenario occurs when some users

     have lower clearances than the level at which the network backbone is

     physically protected. For example, there are currently plans to allow

     some uncleared users on the DISNET segment of the DDN [22] which will

     be physically protected at the Secret level. In that case, the risk

     index for the bypass works the opposite of the normal case: the

     ciphertext side will be the higher of the two ratings.



The subsystem which performs access control and key distribution must also

be concerned with this risk range since improper key distribution could lead

to compromise across the entire network. An erroneous distribution could

potentially permit the lowest cleared user to access the highest

classification of information.



























































































LIST OF REFERENCES



[1]

     Department of Defense Computer Security Center, Computer Security

     Requirements - Guidance for Applying the Department of Defense

     Trusted,Computer System Evaluation Criteria in Specific Environments,

     25 June 1985. CSC-STD~03-85

[2]

     Department of Defense, Department of Defense Trusted Computer System

     Evaluation Criteria, 15 August 1983. Department of Defense 5200.28-STD

[3]

     National Computer Security Center, Trusted Network Interpretation of

     the Trusted Computer System Evaluation Criteria, Version 1, July 1987.

     NCSC-TG-005

[4]

     Department of Defense, Security Requirements for Automative Data

     Processing (ADP) Systems, March 1988. Department of Defense Directive

     5200.28

[5]

     National Computer Security Center, Trusted Product Evaluation, A Guide

     for Vendors, draft 1 March 1988 (or most recent edition). NCSCTG~02,

     Version 1

[6]

     National Security Agency, Information Security Products and Services

     Catalogue , (quarterly updates).

[7]

     National Institute of Standards and Technology, United States

     Department of Commerce, Guideline for Computer Security Certification

     and Accreditation, 27 September 1983. FIPS PUB 102

[8]

     V. A. Ashby, Thomas Gregg, and Annabelle Lee, "Security Approach for

     Rapid Prototyping in Multilevel Secure Systems," Fifth Annual Computer

     Security Applications Conference, December 1989.

[9]

     International Standards Organization, Information processing systems -

     Open Systems Interconnection - Basic Reference Model, 15 October 1984.

     International Standard 7498

[10]

     Defense Intelligence Agency, Security Manual for the Uniform Protection

     of Intelligence Processed in Automated Information Systems and Networks

     (U), Supplement to Director of Central Intelligence Directive (DCID)

     1/16 (U), SECRET, 19 July 1988.

[11]

     National Institute of Standards and Technology, United States

     Department of Commerce, Guideline for Automatic Data Processing Risk

     Analysis, August 1979. FIPS PUB 65

[12]

     T. E. Bell, "Managing Murphy's Law: Engineering a Minimum-Risk System,"

     IEEE Spectrum, pp. 2i27, June 1989.

[13]

     National Computer Security Center, Proceedings of the 1988 Computer

     Security Risk Management Model Builders Workshop, Fort Meade, MD, May

     1988.

[14]

     Sammy Migues, "A Guide to Effective Risk Management," Third Aerospace

     Computer Security Conference Proceedings, December 1987.

[15]

     Carl E. Laudwehr and H.O. Lubbes, An Approach to Determining Computer

     Security Requirements for Navy Systems, 13 May 1987.

[16]

     International Standards Organization, Information Processing Systems -

     Open Systems Interconnection - Basic Reference Model - Part 2: Security

     Architecture, October 1988. International Standard 7498-2-1988(E)

[17]

     Ingrid M. Olson, Eugene F. Troy, Milan S. Kuchta, and Brian W.

     McKenney, Disclosure Protection of Sensitive Information," 13th

     National Computer Security Conference Proceedings, 1A October 1990.

[18]

     Robert W. Shirey, "Defense Data Network Security Architecture,"

     Computer Communication Review, April 1990.

[19]

     T. M. P. Lee, "Statistical Models of Trust: TCB's vs. People," IEEE

     Symposium on Security and Privacy, 1989.

[20]

     Jonathan K. Millen and Martin W. Schwartz, "The Cascading Problem for

     Interconnected Networks," Fourth Aerospace Computer Security Conference

     Proceedings, December 1988.

[21]

     R. W. Shirey and S.!. Schaen, "ARCHWAY Program Preliminary Planning,"

     MTR~7W00093~2, The M!TRE Corporation, December 1987. Not currently in

     the public domain.

[22]

     G. R. Mundy and R. W. Shirey, "Defense Data Network Security

     Architecture," MILCOM `87 Proceedings, 21 October 1987.

















































ACRONYMS



AIS Automated Information System

ASD Assistant Secretary of Defense

AUTODIN Automated Digital Network

BI Background Investigation

C Confidential

C&A Certification and Accreditation

COMPUSEC Computer Security

COMSEC Communications Security

COTS Commercial-off The-Shelf

CPU Central Processing Unit

CBS Central Security Service

CI Command, Control, Communications, and Intelligence

CSSII Computer Security Subsystem Interpretation

DAA Designated Approving Authority

DAC Discretionary Access Control

DCA Defense Communications Agency

DDN Defense Data Network

DIA Defense Intelligence Agency

DISNET Defense Integrated Secure Network

DOD Department of Defense

DODD Department of Defense Directive

DOS Denial of Service

E3 End-to-end Encryption

ENQ Enquiry

EPL Evaluated Products List

FIPS PUB Federal Information Processing Standards Publication

GOSIP Government 0SI Profile

I&A Identification and Authentication

INFOSEC Information Security

IPC Inter-Process Communication

1S0 International Standards Organization

1SS0 Information System Security Officer

JCS Joint Chiefs of Staff

LAN Local Area Network

MAC Mandatory Access Control

MLS Multilevel Secure

MOA Memorandum of Agreement

MOR Memorandum of Record

N Not Classified but Sensitive

NACSI National Communications Security Instruction

NCSC National Computer Security Center

NDI Non-Development Item

NIU Network Interface Unit

NSA National Security Agency

NSAD Network Security Architecture and Design

NSAP Network Service Access Point

NTCB Network Trusted Computing Base

0SI Open System Interconnection

OT&E Operational Test and Evaluation

PDS Protected Distribution System

PDU Protocol Data Unit (a.k.a. packet, datagram)

POSIX Portable Operating System Interface for Computer

Environments

RFP Request for Proposal

RI Risk Index

S SECRET

SBI Special Background Investigation

SCI Special Compartmented Information

SDNS Secure Data Network System

SH System High

SlOPESI Single Integrated Operational Plan-Extremely Sensitive

Information

ST&E Security Test and Evaluation

STS Single Trusted System

TCB Trusted Computing Base

TCP/IP Transmission Control Protocol/Internet Protocol

TCSEC Trusted Computer System Evaluation Criteria

TEMPEST (Not an acronym)

TNI Trusted Network Interpretation

TNIEG TNI Environments Guideline

TS TOP SECRET

TSAP Transport Service Access Point

WAN Wide Area Network
