TRUSTED NETWORK INTERPRETATION ENVIRONMENTS GUIDELINE

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

Library No. 5-235,465

Version 1

FOREWORD

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

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

1.1 Background

• Establishing a metric for categorizing systems according to the security protection they provide, and
  
• Identifying the minimum security protection required in different environments.
  

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

1.3 Purpose

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

1 Definitions of terms particularly important to this document are given in Section 2.

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

2 Terminology

2.1 Automated Information System

2.2 Network Trusted Computing Base

2~e 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.

~e THI uses component in the definition of NTCB Partition. We use subsystem to be consistent in this document.

2.3 System and Component

2.3.1 TNI Introduction (definition not used in TNIEG)

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)

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.

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

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.

2.3.4 Definitions

• 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

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].

4 Mandatory access control, discretionary access control, identification and authentication, and audit, respectively.

2.5 Certification

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

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

2.7.1 Unified Networks

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~05 [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.

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

2.8 Network Security Architecture and Design

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

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 addresses8. 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.

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

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.

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.

3 Network Security Architecture and Design (NSAD)

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

___________________

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 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

• A general description of the information that will be transmitted to the network by each subsystem
  
• A summary discussion of the trusted behavior that is expected from each subsystem
  
• The details of the overall security plan for the network and the assignment of responsibility for producing and accepting the plan
  
• A description of the overall network security policy
  
• A description of additional security training and assignment of training responsibility
  
• Specification of the security parameters that are to be transmitted between communicating subsystems
  
• A discussion of security details that are relevant to the exchange of information among the subsystems.
  
• A description of the user community, including the lowest clearance of any user who will have access to the network
  
• Any special considerations for dial-up connections to any subsystem in the network, including potential security threats and the safeguards that will be used.
  
• A description of the security protections provided by the data communications, both local to a subsystem and between communicating subsystem
  
• 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
  
• 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

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

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

must determine the following:

• 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                        7  
(MC)                                   
------------------------------------
      

Risk Index = Rmax - Rmin

Table 2:  Rating Scale for Maximum Data Sensitivity(Rmax)
----------------------------------------------------------------
 Maximum          Rating         Maximum Data               Rating   
Sensitivity       (Rmax)    with categories1,2 (Rmax)   
Ratings                                                            
without                                                            
categories                                                         
Unclassi-                      0                       N/A 3                                    
fied (U)                                                           
Not                                                                
Classified                     1                       N with one or more             2 
but                                                        Categories                               
Sensitive                                                          
N4                                                                 
Confiden-                      2                     C with one or more Cate-         3          
tial C                                                        ories                                    
                                                     S with one or more                       
Secret(S)                      3                     Categories only one              4           
                                                     Category containing S                    
                                                     S with two or more               5           
                                                     Categories containing S                  
                                                                   
                                                     TS with one or more                      
Top Secret                     5 5      Categories only one              6           
(TS)                                                 Category containing S or                 
                                                              TS                      7           
                                                     TS with two or more                      
                                                     Categories containing S                  
                                                             or TS                                    
----------------------------------------------------------------

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                                *                         
                                                                            
------------------------------------------------------------------------

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.)

____________________

12 Table 3 is adapted from the TCSEC Environments Guideline

5 TNI Part II Security Requirements

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

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

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

5.4 Selecting Security Services

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.

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

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                        
SensitivityRatings     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 Cate-                 3        
                                                       ories                               
Secret(S)                          3                   S with two or more                       4         
                                                       Categories only one                 
                                                       Category containing S                    5        
                                                       S with two or more                  
                                                       Categories containing S             
Top Secret (TS)                    55     TS with one or more                      6        
                                                       Categories only one Cate-           
                                                       gory containing S or TS                  7        
                                                       TS with two or more                 
                                                       Categories                          
---------------------------------------------------------------

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

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

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

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

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.
   

1. Do you have a requirement to protect communication against unauthorized modification?
   

If no, skip to Non-Repudiation.


1. 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.


1. 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.


1. 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.


1. 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.

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.


1. 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.


1. 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.

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.


1. 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.


1. 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.


1. 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.


1. 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.


1. 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.

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.


1. 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.


1. 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.

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.

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.


1. 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.


1. 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

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.

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

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 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

___________

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.)

6.1.1 Accreditation Range

The range must be a single level in the case of a system high or dedicated device14. 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.

Figure 1

Information Levels and Accreditation Ranges

N/A TS

S-C S

N/A 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

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.

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

Accreditation Ranges of Two Interconnected Sub systems

Subsystem X Subsystem Y

N/A 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 subsystem15. 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.

______________

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.

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.

6.1.2 Device Range

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.

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

Subsystem X Subsystem Y

Figure5

Compatibility Labeling

Subsystem X Subsystem Y

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

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

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

6.3 Risk Factors

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

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

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.
   

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.

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.

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

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

TS Subsystem B

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

[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

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.
   

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