






Understanding Trusted Distribution 






NCSC-TG-008
Understanding Trusted Distribution 



A Guide to Understanding Trusted Distribution in Trusted Systems


Library No. S-228,592
FOREWORD



"A Guide to Understanding Trusted Distribution in Trusted
Systems," is the latest in the series of technical guidelines
that are being published by the National Computer Security Center.
These publications are designed to provide insight to the Trusted
Computer Systems Evaluation Criteria requirements and guiance
for meeting each requirement.


The specific guidelines in this document provide a set of good
practices related to trusted distribution of the hardware, software,
and firmware portions, both originals and updates, of automated
data processing systems employed for processing classified and
other sensitive information. This technical guideline has been
written to help the vendor and evaluator community understand
what trusted distribution is, why it is important, and how an
effective trusted distribution system may be implemented to meet
the requirements of the Trusted Computer Systems Evaluation Criteria.


As the Director, National Computer Security Center, I invite your
recommendations for revision to this technical guideline. We plan
to review this document biannually. Please address any proposals
for revision through appropriate channels to:

 • National Computer Security Center
 • 9800 Savage Road
 • Fort George G. Meade, MD 20755-6000






 • Attention: Chief, Criteria and Guidelines
 • Patrick R. Gallagher ,Jr.                               15
December 1988
 • Director
 • National Computer Security Center





ACKNOWLEDGMENTS



Special recognition is extended to James N. Menendez, National
Computer Security Center (NCSC), as project manager and coauthor
of this document.  Recognition is also extended to Scott Wright,
Advanced Information Management (AIM), Inc., as coauthor and researcher
of this document.


Acknowledgment is also given to all those members of the computer
security community who contributed their time and expertise by
actively participating in the review of this document.
1. Introduction

1.1 Purpose



The Trusted Computer System Evaluation Criteria (TCSEC) is the
metric used for evaluating the effectiveness of security controls
built into Automated Data Processing (ADP) systems.  The TCSEC
is divided into four divisions: D, C, B, and A, ordered in a hierarchical
manner with the highest division, A, being reserved for systems
providing the best available level of assurance..  Within  division
C through A are a number of subdivisions known as classes, which
are also ordered in a hierarchical manner to represent different
levels of security.


At TCSEC class A1, trusted distribution of the hardware, software,
and firmware portions of the Trusted Computing Base (TCB) and
their updates shall be provided.  Trusted distribution includes
procedures to ensure that all of hte TCB configuration items,
such as the TCB software, firmware, hardware, and updates, distributed
to a customer site arrive exactly as intended by the vendor without
any alterations.  Additionally, trusted distribution may include
procedures that enable the customer site to determine that what
was received at the site was actually sent by the vendor.  The
purpose of this guideline is to provide guidance to vendors of
trusted systems on what trusted distribution is, why it is important,
and how to select and implement an effective trusted distribution
system to meet the TCSEC requirement.


Examples in this document are not to be construed as the only
implementations that will satisfy the TCSEC requirement.  The
examples are merely suggestions of appropriate implementations.
 The recommendations in this document are also not to be construed
as supplementary requirements to the TCSEC.  The TCSEC is the
only metric against which systems are to be evaluated.


This guideline is part of an ongoing program to provide helpful
guidance on TCSEC issues and the features they address.
1.2 Scope



An important assurance requirement of TCSEC class Al is that the
TCB software, firmware, hardware, and their updates be distributed
to a customer site in a trusted manner. This guideline is to be
used by vendors of trusted systems in the preparation of procedures,
techniques, and equipment to establish trusted distribution between
a vendor site and a customer site. This guideline will discuss
trusted distribution as it relates to computer systems and products
that are intended to meet the Al requirements of the TCSEC.
1.3 Control Objective



Trusted distribution focuses primarily on the assurance control
objective of the TCSEC. The assurance control objective states:


"Systems that are used to process or handle classified or
other sensitive information must be designed to guarantee correct
and accurate interpretation of the security policy and must not
distort the intent of that policy. Assurance must be provided
that correct implementation and operation of the policy exists
throughout the system's life cycle."[7]


Any alteration to the TCB at any time during the system life cycle
could result in a violation of the system security policy. Assurance
that the system security policy is correctly implemented and operational
throughout the system life cycle is provided by different TCSEC
requirements. At TCSEC class Al, trusted distribution, in conjunction
with configuration management, provides assurance that the TCB
software, firmware, and hardware, both original and updates, are
received by a customer site exactly as specified by the vendor's
master copy.  Trusted distribution also ensures that TCB copies
sent from other than legitimate parties are detected.
2. OVERVIEW OF TRUSTED DISTRIBUTION

2.1 Threats



The different divisions of the Trusted Computer System Evaluation
Criteria (TCSEC) were developed to protect against threats that
could be directed towards Automated Data Processing (ADP) systems.
Each higher class is required to provide additional features and
assurances over the next lower class, thus providing increasing
levels of trust. At the C level and above, passwords and audit
mechanisms provide ways to restrict access to a system and make
users accountable for their actions. At the TCSEC B level, the
addition of labeling mechanisms control access to data based on
clearances of subjects and classifications of objects.


The class of system needed by a specific site should be determined
by the types of threats that are faced by that site. Generally,
the class of system is dependent upon the sensitivity level of
the data that are being processed by the site and the clearance
of the system users. According to the Guideline for Applying the
Department of Defense Trusted Computer System Evaluation Criteria
in Specific Environments[5], a risk index corresponding to the
minimum clearance or authorization of system users and the maximum
sensitivity of data processed by the system can be used to determine
the minimum evaluation class required by a site.


The features and assurances in lower class systems protect against
the threat that someone will tamper with a system while it is
in operation, but it is at TCSEC class Al that the threat of subversion
is addressed. Computer subversion is the name generally applied
to deliberate, malicious modification of executable code or hardware
within a computer system. The subversion can be accomplished at
any time during the system's life from the earliest stages of
design to the last day of its use. A component can be subverted
either to permit later penetration or as an act of sabotage-to
nullify or to degrade the system's capability. Forms of subversion
include the trap door, the Trojan horse, and computer viruses.
The threat of subversion exists at all classes; however, the benefit
of providing assurance features to guard against it in lower class
systems may not justify the cost of this type of protection.


There are basically two threats that trusted distribution protects
against.


The first is the threat of someone tampering with a system during
its movement from a vendor site to a customer site. Throughout
this document the term "vendor site" will be used to
mean the point from which the TCB hardware, software, and firmware
to be distributed are sent. The term "customer site"
is the point at which the distributed material is accepted. Specifically,
any system component that participates in the enforcement of the
security policy of the system is what needs to be protected. For
example, someone may break into a delivery truck and insert malicious
code into a trusted system before it reaches the customer site.
The system or update being delivered, when installed at the site,
will contain the harmful code that could cause compromise of the
system's security policy. Trusted distribution may include protective
packaging methods that protect against changes being made to a
system (see Section 4.1 Protective Packaging). Trusted distribution
may also include methods of detecting if a system and/or its updates
have been accidentally or maliciously altered during or after
distribution through validation tests (see Section 4.7 Site Validation).


The second threat protected against by trusted distribution is
that the TCB is a counterfeit; that is the system or update did
not come from the vendor site.  Trusted distribution includes
procedures for the distribution of TCB components, such as requiring
the vendor to notify a customer site of an impending delivery.
By doing this, trusted distribution protects against the threat
of sites receiving systems that were not distributed by the vendor
and provides assurance that the vendor was the actual sender of
the product or update. Trusted distribution should be able to
answer the question, "Was what I received really sent from
the vendor or an imposter?" Without trusted distribution,
all a penetrator needs to do is package a system or update containing
a virus or Trojan horse so that it looks as though it came from
an actual vendor. The receiving site, upon seeing the packaging,
assumes that the system or update is genuine, unaware that it
is really a counterfeit. To prevent this from happening, the vendor
and customer sites may establish procedures requiring them to
agree on when deliveries will be made and what they will contain.
2.2 Purpose of Trusted Distribution



Hostile attacks may occur on computer systems when they are in
use, but it is also possible for computer systems to be attacked
even before they are installed at a customer site. The TCSEC requires
that assurance mechanisms be in place throughout the life cycle
of a system to prevent modifications being made to the TCB which
could adversely affect the security policy of a system.  One such
assurance requirement for class Al systems is trusted distribution.
Trusted distribution maintains the integrity of the current TCB
software, firmware, and hardware as well as any updates to these
by ensuring that any changes made to the TCB during the distribution
process do not go unnoticed.


"Trap doors can be inserted during the distribution phase.
If updates are 


sent via insecure communications - either U.S. Mail or insecure


telecommunications, the penetrator can intercept the update and
subtly


modify it. The penetrator could also generate his own updates
and distribute


them using forged stationery. "[3]


The main purpose of trusted distribution is to protect a trusted
system as it is being distributed, and consequently to provide
protection for the information that will be processed on the system.
Trusted distribution provides assurance that a trusted system
arrives at a customer site with all of its security properties
intact and that the system or update that is received at the customer
site is the, "same system or update which was produced from
the master copy of the system evaluated against the TCSEC."[6]
 Any tampering with the TCB software, firmware, hardware, whether
originals or updates, from the time they leave the vendor site
to the time they arrive at the customer site may permit the security
policy of the system to be circumvented.


Trusted distribution provides protection against tampering and
a means of detecting that a system has been altered; it accomplishes
this through physical devices (locks), electronic devices (encryption),
and procedures (bonding of couriers). It also provides procedures
for site validation so that if the protection mechanism[s] should
fail, any modification to the TCB software, firmware, hardware,
both originals and updates, will not go unnoticed. If sufficient
trust can be placed in either the protection methods or the customer
site validation, it is possible to use that method exclusively.
If not, it may be necessary to apply techniques for both the protection
of the shipment and validation.
2.3 Life-Cycle Assurance



Trusted distribution is one link in a chain of assurances provided
by trusted systems. It is helpful to take a look at all of the
other activities that take place to ensure that the system in
operation is the one that the vendor and customer agree upon.


The following is a summary of the assurances that are needed to
ensure that the product delivered to a customer site is operating
under a correct implementation of the system's security policy:

 •  Assurance that the product evaluated is the one the
manufacturer built
 •  Assurance that the product built is the one that was
sent
 •  Assurance that the product sent is the one the customer
site received.







Long before a system is boxed and prepared to be sent to a customer
site, assurance needs to be provided that the system is being
built as specified. The TCSEC class Al design specification and
verification requirements require a formal top-level specification
(FTLS) to be maintained that accurately describes the system.
The FTLS is shown to be consistent with the TCB interface. Additionally,
specification to code mapping provides assurance that the design
has been properly implemented.


Configuration management provides control over the design and
development of a system, ensuring that the system is built to
specification. The main purpose of configuration management is
to ensure that any changes to the system during design, development,
or during the system life cycle "take place in an identifiable
and controlled environment and that they do not adversely affect
the implementation of the security policy of the TCB."[4]
More information on configuration management can be found in A
Guide to Understanding Configuration Management in Trusted Systems
(NCSC-TG-006).


Once the system has been built, security testing shall be performed
to ensure that the system works exactly as claimed in the system
documentation. The security testing shall demonstrate that the
TCB implementation is consistent with the FTLS.


Trusted distribution provides assurance that the security policy
of a system is not violated during distribution of the system.
For trusted distribution to be successful, the TCB shall have
been under configuration management throughout the development
process, ensuring that the integrity of the developer's master
copy of the TCB has been maintained. Without configuration management
in place during the design and development of a system, the assurance
provided by trusted distribution will be suspect. True, it will
still protect against any tampering with a system during delivery,
but if the system being delivered has already been tampered with
during development then the damage has already been done.


Once the system is in operation at a customer site, assurance
shall be provided throughout the system life cycle that the security
policy of the system is correctly implemented. Configuration management
continues throughout the system life cycle ensuring that any changes
to the system take place in a controlled environment. It is said
that "a chain is only as strong at its weakest link,"
and if any of these assurances fails during the system life cycle,
the probability that the security policy of the system could be
violated increases.
2.3.1 Assurance for Different Types of Production



Not every distribution is as simple as having the entire computer
system designed, developed, and assembled in a vendor's facility
and then shipped to a customer site. Most computer systems, particularly
large-scale computer systems, comprise several parts which are
produced separately and need to be assembled.


The distribution process for a trusted system may be as simple
as shipping from vendor sites to customer sites, or may be as
complex as from vendor to central sites to other sites, or from
software development center sites to other sites, etc. If development
is done at different sites and then integrated at yet another
site, the pieces of the system transferred between these sites
during development should be subject to the same trusted distribution
requirements as the final TCB product.  The item, at the point
of transfer to a customer site, should be a true reflection of
the vendor's master copy.


The manner in which the TCB components are handled prior to distribution
will have an impact on the trusted distribution process. The following
is a recommendation for how a vendor may provide assurance before
the components are actually shipped. "There are basically
two types of production that companies employ: mass production,
and production per request basis. In either case, there will probably
be idle time where the system(s) will be waiting for delivery
probably in some type of warehouse. Strict accounting procedures
and physical controls must be placed on the system(s) both during
the delivery to and stay in the warehouse to ensure that no unauthorized
modifications are made. For example, controls must exist which
log any entry into the warehouse, authorizations for the alteration
of any system or range of serial numbers within the warehouse
must be properly documented, etc."  The "trusted warehouse"
spoken of in the preceding paragraph is not a TCSEC requirement,
but when considering that trusted distribution really extends
further than just providing assurance from vendor loading dock
to customer site loading dock, it should be considered. The assurance
that trusted distribution provides is dependent on a system not
being altered before being loaded onto a delivery truck. The control
of the system during "idle time" should be performed
by configuration management practices, emphasizing the importance
of configuration management in trusted distribution.
3. THE TCSEC REQUIREMENTS FOR TRUSTED DISTRIBUTION



This section lists the TCSEC requirements for trusted distribution.
These requirements have been extracted from the TCSEC and have
been listed separately and numbered. How these requirements can
be met will be discussed in the following sections of this document.
This section is designed to serve as a quick reference for the
TCSEC requirements for trusted distribution.


Trusted distribution is required at TCSEC class Al, and the requirement
can be broken down into two parts.


Requirement 1    - "A trusted ADP system control and distribution
facility shall be provided formaintaining the integrity of the
mapping between the master data describing the current version
of the TCB and the on-site master copy of the code for the current
version."[7]


Requirement 2    - "Procedures (e.g., site security acceptance
testing) shall exist for assuring that the TCB software, firmware,
and hardware updates distributed to a customer are exactly as
specified by the master copies."[7]
4. IMPLEMENTATION METHODS



This section describes various implementation methods that can
be used to establish a trusted distribution system and the advantages
and disadvantages of each method. When choosing a protective packaging
system or a customer site validation process, remember that some
devices or techniques provide a higher degree of protection than
others. The higher the threat to the distribution system, the
greater the need for more stringent measures or multiple levels
of trusted distribution methods.  In many situations, multiple
methods for trusted distribution should be used, such as a protective
packaging system during distribution and site validation upon
receipt. This layered approach will counter the insider threat
or the threat of collusion where employees themselves alter the
contents of a package before it is distributed. Each situation
that requires trusted distribution is unique and requires that
the system be addressed individually.


For a National Computer Security Center Al evaluation, a vendor
must submit a plan that describes the trusted distribution method(s)
for the system under evaluation. This plan should include a description
of the procedures to be followed and the mechanisms (type of packaging)
to be used for distribution of both initial versions and updates.
Any deviation from the trusted distribution plan submitted could
jeopardize the evaluation rating.


There are other requirements in the TCSEC which are related to
trusted distribution, namely those concerning configuration management.
A vendor should be sure that the system sitting on the loading
dock is the system that the vendor thinks it is. At TCSEC class
Al, the configuration management system shall include, "a
combination of technical, physical, and procedural safeguards"
to be "used to protect from unauthorized modification or
destruction the master copy or copies of all materials used to
generate the TCB."[7] All of the implementation methods that
follow are contingent upon prior performance of configuration
management, ensuring that the system has not been maliciously
altered before being distributed.


A related concern that plays a part in trusted distribution is
that communication should be established between the vendor and
customer sites for both the initial and updated distribution of
the TCB software, firmware, and hardware. The procedures used
should include agreement between the vendor and customer sites
as to the methods to be used for distribution and the time frame
in which the distribution will be made. The sender (the vendor)
and receiver (the customer) should be uniquely identified prior
to distribution for the trusted distribution system to function
successfully. It is essential that this identification be made
and that procedures be in place for manufacturers to notify users
of pending shipments.  Included in this identification should
be the unique identification of every component to be shipped.
This identification will allow for the detection of any system
configuration changes. Additionally, the customer should have
procedures in place that will keep the customer advised of the
latest changes from the vendor. At a minimum the customer should
enforce a policy that forbids the use of any new TCB software,
firmware, or hardware without prior notification from the vendor
of the most current change, to include the date each change to
the TCBo was sent and the means by which the TCB software, firmware,
and hardware was sent.


Another concern is the reliance on the individuals involved in
the design, development, manufacturing, and distribution of the
trusted system.  Each individual who is involved in the system
prior to and during distribution should be subject to review that
would verify his or her trustworthiness and reliability.  Particularly
for distribution, all individuals who play a significant role
in the establishment of the control at the shipping end, or the
validation at the receiving end should be worthy of the trust
placed in them to perform their roles reliably.  Without this
step, any process for protection is subject to an insider compromise.
4.1 Protective Packaging



Protective packaging is a way of wrapping an item so that it cannot
be opened and resealed without leaving some obvious indication
that the package has been tampered with. Protective packaging
can be provided to limit access to the TCB software, firmware,
and hardware during shipment and to guarantee their delivery in
an unaltered form. Protective packaging ranges from simple shrink
wrapping to complex fiber-optic techniques. The wrapping for shipments
should allow the sender and the package contents to remain anonymous.
Techniques such as double wrapping the materials to be distributed
or an absence of exterior markings when using shrink wrapping
could accomplish this. The technique used for packaging the TCB
components for distribution shall be documented in the trusted
distribution plan.


Protective packaging not only limits access to the materials being
distributed, but also aids in protection against environmental
factors, such as dust and water. It is not possible to present
every protective packaging technique; however, the examples that
follow are provided to present some of the methods that are currently
in use.
4.1.1 Shrink Wrapping



Shrink wrapping is one method of trusted distribution which consists
of enclosing the product in a plastic film. This method may be
used for TCB hardware, software, or firmware, but since it does
involve the use of heat, it should not be used for computer-related
equipment that is very sensitive to heat.


When heat is applied to the plastic film in shrink wrapping, the
film contracts, or shrinks, forming a tight seal around the product.
If the shrink-wrapped seal is broken, this is an indication that
the product being shipped has been tampered with. Not only is
shrink wrapping used for the trusted distribution of TCB components,
but many industries including the food and drug industry use shrink
wrapping to provide consumer protection that products have not
been tampered with. Additional special shrink-wrap techniques
are available that may increase the reliability of the shrink
wrap.


The size and construction of the materials to be packaged are
important considerations when selecting a shrink-wrapping technique.
For example, shrink wrapping products the size of mainframe central
processing units (CPUs) or mainframe disk drives is currently
not done because of their large size. Techniques will need to
be improved to make shrink wrapping a feasible packaging option
for these types of products.
4.1.2 Active Systems



The use of active systems for protective packaging is a viable
method for the distribution of TCB software, firmware, and hardware.
This method was originally intended to secure personal computers
and electronic equipment. Conceptually, an active system could
include looping a fiber-optic cable through a latch on the opening
of a container and attaching the cable to a control unit via an
alarm. An active system provides assurance that a package cannot
be entered without triggering the alarm.


An advantage to the active systems method is that there is a real-time
notification of any tampering with the TCB hardware, software,
or firmware as they are being delivered. It is very possible that
anyone tampering with the product could be caught before having
a chance to modify the system. This real-time notification can
be circumvented, though, by interfering with the alarm so that
the signal will not be picked up. In these cases, the break-in
will be detected but not until the delivery truck reaches its
destination. True, it will presumably be too late to catch those
responsible for the attack, but the detection provided by active
systems will prevent an altered component from being used that
could result in a security policy compromise.
4.1.3 Tamper-Resistant Seals



The use of tamper-resistant seals is another method of protective
packaging that may be used to protect the distribution of large
TCB software, firmware, and hardware items. One example of a tamper-resistant
seal for large items shipped by truck is a special-purpose truck
seal. This device generates a random 4-digit number when installed
on a truck door. When the door is opened a new random number is
generated. The device is encased in metal and epoxy resin to prevent
tampering. By sealing the truck at the vendor facility and sending
the number to the customer site by secure means, it is possible
for the user to determine whether or not the truck cargo has been
opened.


Other forms of locking devices and seals, such as a high impact
security lock or company registered seals, can also be applied
before shipment and verified prior to opening. The different sealing
devices used provide different degrees of assurance and should
be selected based on the needs of the item being distributed.
4.2 Couriers



The use of a courier service is a possible way to establish the
trusted distribution of TCB software, hardware, firmware, both
originals and updates.  Couriers provide the advantage of constant
surveillance of the materials they are transporting. The use of
couriers increases the reliability of any delivery system and
can easily be used in conjunction with other protective methods
such as locking devices and protective packaging.


There are several commercial firms that can supply bonded services,
or manufacturers may use their own internal courier service, if
available. Within the military, the Defense Courier Service (DCS),
formerly known as the Armed Forces Courier Service (ARFCOS), is
an alternative.


It is possible to transport the most sensitive materials by means
of couriers.


The TCB products to be distributed should be considered to be
as sensitive as the data they are being used to process and should
be treated accordingly. Depending upon the sensitivity of the
material to be distributed, the vendor and/or customer site may
want to establish regulations regarding who may act as a courier,
what type of materials they may transport, and where the material
may be delivered. A partial list of guidelines for the use of
courier service is provided below.


Persons acting as couriers should be trusted to the level of the


material they are transporting

 •  Material should remain in their personal custody at
all times
 •  Vendor as consignor should be responsible for the safety
of the material
 •  Vendor should notify the customer site of the nature
of the shipment, the means of the shipment, number of seals (if
used), and the anticipated time and date of arrival by separate
communication at least 24 hours in advance of the arrival of the
shipment in order that the customer site may take appropriate
steps to receive and protect the shipment.







For the use of a courier service to provide trusted distribution
successfully, assurance should exist that the TCB software, firmware,
or hardware is not modified while stored in a warehouse before
being picked up by the courier.  Otherwise, the assurance provided
by the courier will have been defeated.
4.3 Registered Mail



Registered mail can be part of the trusted distribution of TCB
hardware, software, and firmware to a customer site without any
undetected disclosure or loss.  Although registered mail alone
is not sufficient for meeting the TCSEC trusted distribution requirement,
it does not preclude it from being a part of the trusted distribution
process. The reason that registered mail alone does not satisfy
the TCSEC requirement is that although the customer site has to
verify its identity before being allowed to receive a package
via registered mail, the sender does not have to show any form
of identification to mail a package. This can result in the scenario
detailed earlier in which someone can duplicate a vendor's wrapping
and markings and mail a malicious update or system. Provided that
the registered mail is supplemented by other adequate mechanisms
to compensate for its shortcomings, such as an unforgeable internal
signature to ensure the identity of the sender, it can meet the
trusted distribution requirement and provide assurance that what
was delivered through the mail actually came from the vendor.


When sending TCB software, firmware, hardware, originals or updates,
via registered mail, the products should be considered to be as
sensitive as the data they are being used to process and should
be treated accordingly. Some procedures to be observed when using
registered mail for trusted distribution include:

 •  Material to be transmitted should be enclosed in opaque
inner and outer containers
 •  If the material is in a hard-copy form and is of such
size as to permit the use of envelopes for wrapping, the contents
should be protected from direct contact with the inner container
by a cover sheet or by folding inward.





4.4 Message Authentication Codes



Message Authentication Codes provide an effective means for transmitting
segments of TCB software. The banking system is one of the largest
users of electronic distribution, and has successfully used Message
Authentication Codes for several years. A Message Authentication
Code employs an encryption process for a data stream and from
this process develops a unique code that is appended to the data
stream. It is important to note that only the appended code is
actually encrypted, and the message remains in plaintext. The
process, repeated by the recipient, must then produce the identical
code.


If the codes are not the same, this is an indication that the
code being


transmitted has been tampered with. The length of the appended
code can vary,


but the strength of the process is directly related to the length
of the code. As with


all encryption-based processes, the management and protection
of the key ancinor


algorithm is a critical factor that shall be addressed in the
design documentation.
4.5 Encryption



Encryption of the entire text is an effective way of protecting
data from compromise or modification attacks. Encryption has the
advantage not only of preventing undetected changes to the TCB
software, but also of preventing viewing of the code for any person
without the key. Encryption of TCB software with public or private
key techniques is a viable method of trusted distribution, provided
that the keys are properly managed, protected, and changed at
frequent intervals. In the event that the keys are compromised,
it would be possible to alter the TCB software without detection
of the alteration.


As stated before, the success of encryption is dependent upon
the protection of the key. For this reason, the key should be
subject to some form of trusted distribution, such as courier,
to ensure that it is not compromised. Encryption provides a significant
increase in the quality of assurance; however, management of the
system, for example, key generation and distribution, can become
a very complex and time-consuming activity that needs to be well
defined in the design documentation.
4.6 Site Validation



Site validation is validation by the customer site that the TCB
hardware, software, and firmware received are exactly as specified
in the master copy. Site validation is most commonly performed
on the TCB hardware items that are shipped, but TCB software and
firmware items should also be subject to some type of validation
testing upon receipt. Site validation includes methods for validating
that a system has not been tampered with during its movement from
vendor site to the customer site. Site validation provides a second
layer of assurance for trusted distribution. In the event that
any of the TCB components were altered during distribution, site
validation procedures should detect the alteration before the
system is installed and any compromise of security policy can
take place.
4.6.1 Checksum Programs



Checksum programs provide an acceptable means to detect TCB software
and firmware changes during electronic or physical distribution.
In this validation method, a group of digits are summed and then
checked against a previously computed sum to verify that no digits
have been changed since the last summation.  Any difference in
the two sums would indicate that the piece of software being checked
had been modified. The following is taken from the Final Evaluation
Report of SCOMP [2] and describes how SCOMP used the checksum
method for software distribution.


"When a site purchases a SCOMP system, a description of the
desired configuration must be sent to Honeywell. A set of configuration
files are then set up and the desired release, with the site specific
configuration files, is generated. A checksum algorithm is then
applied to the executable code.  The executable code is sent to
the site along with a checksum generation program. The checksum
that was originally generated is then sent to the site. The site
can run the checksum generation program and compare the result
with the checksum delivered through the mail. The two checksums
provide a means whereby the site can ascertain that the system
that they received was the same system that Honeywell sent to
them."[2] It should be noted that there are ways to improve
this approach.  The checksum program should not be distributed
with the TCB software. Trusted distribution implementing checksums
should consist of sending one package containing the checksum
generation program and checksum result and another package containing
the TCB software. Both packages should be protected by appropriate
means of trusted distribution such as courier or registered mail.
 Anyone having access to both the checksum generator and the TCB
software could retrieve the output from the checksum program through
a print command and alter the system so that the change would
go unnoticed by saving the original checksum value, modifying
the system, and running the checksum program until it matches
the original checksum, adding modules to inconsequential areas
of the system when necessary. It may take some time to get the
checksum of the modified software to equal the original checksum,
but with a 16-bit or smaller checksum it may be a practical method
for modifying TCB software.


Additionally, current Al systems should enhance their checksum
implementation by using cryptographically protected checksums.
Encryption of the checksum and result increases the assurance
provided, by preventing anyone from viewing the checksum result.
It also provides protection against the scenario described above
because no one would be able to view the checksum result without
possessing the proper cryptographic key.


A disadvantage to this implementation is that a checksum program
will not limit viewing of the TCB software; it will, however,
provide a level of assurance that the transmission has not been
altered.
4.6.2 Inventory



An inventory is a minimal way of performing customer site validation
of TCB hardware. Although this will not meet the TCSEC requirement
for trusted distribution, it may provide an acceptable level of
assurance for some sites. An inventory is a simple means of inspecting
for the presence or absence of a piece of hardware. It consists
of an inspection to see if each piece of equipment listed on the
inventory arrived at its destination. The assurance provided by
an inventory may be increased by inspecting the lower level elements
of the TCB hardware. These elements would include such things
as circuit boards and chips.


The disadvantage of conducting a physical inventory is that many
different hardware families use some of the same hardware components,
and a physical inventory of hardware at each end of the distribution
chain will not detect the substitution or change of these similar
components in the hardware. It will, however, serve to detect
any gross discrepancies between the items sent and received.


The advantage of performing an inventory is that it provides a
quick method of checking that the TCB components sent were the
ones requested. The assurance it provides will be minimal, but
a simple oversight or error in shipping or the loss of an item
may be detected in this manner. A copy of the invoice should always
be in a sealed envelope and a second copy should be sent by alternate
means, such as a courier, so that any invoice tampering can be
easily detected.
4.6.3 Engineering Inspection



An engineering inspection provides a more thorough check than
an inventory and may satisfy the TCSEC requirement for trusted
distribution of TCB hardware components. It differs from an inventory
in that it is a detailed inspection by a qualified technician
in a specific area. The technician should be capable of detecting
any changes to the inspected equipment that would affect the TCB.
Engineering inspections should be provided for critical parts
of the TCB hardware to ensure that the components of the hardware
are present and unchanged, such as ensuring, for example, physical
location and serial numbered parts, are as specified.


A disadvantage to an engineering inspection is that it can be
very time-consuming and it may not be possible for a technician
to inspect all of the TCB hardware components because of the locations
and construction of the equipment being inspected. This time element
may be offset by a simplified version of this safeguard consisting
of a confidential agreement between the vendor and the customer
as to which parts of the TCB hardware are to be inspected in detail.
This will reduce the amount of effort to a reasonable level and,
if the specific components are properly identified, will provide
an acceptable level of assurance that no changes have been made.
5. SAMPLE IMPLEMENTATION

5.1 Trusted Distribution of Hardware, Firmware and Software




The preceding sections of this document have addressed different
methods that may be used for trusted distribution, but none has
explicitly stated what will satisfy the TCSEC class Al requirement
for trusted distribution. The following paragraphs describe sample
methods for meeting the trusted distribution requirements. It
should be noted that these are not the only methods that will
satisfy the requirement. Through the guidance offered in this
document, it is hoped that vendors will be able to investigate
other creative methodologies to satisfy the trusted distribution
requirement.


When a system is directed to be transported, an acceptable methodology
would be the use of a bonded courier service. The courier would
accompany and be responsible for the safety of the system.  Alternate
methodologies would be protective packaging (protecting against
unauthorized modifications), registered mail, and, specifically
on software, the use of encrypted checksums.


Upon arrival of the system at the purchaser's site, the success
of the protective packaging methods provided by the vendor should
be validated. It is, however, the vendor's responsibility to provide
either the documentation or the manpower to assist the purchaser
in determining if the methods used were successful.  Additionally,
it is the purchaser's responsibility to provide configuration
management for the system throughout the remaining life cycle
of the system.
5.2 Trusted Distribution of Documentation



Trusted distribution shall be provided for the distribution of
the TCB hardware, software, and firmware. It can also be said
that trusted distribution is required for all of the TCB configuration
items as identified in the configuration management plan for a
system. When speaking of configuration items, one should include
the documentation for the system. Although not required by the
TCSEC, the documentation and configuration records for a TCSEC
class Al system should be delivered to the customer site through
trusted distribution. In the event that these documents are altered
during distribution, it is possible that the system could be configured
in a manner that would violate the security policy of the system.
For instance, documentation on a TCB mechanism could be altered
to allow the mechanism to be used in a harmful way. Trusted distribution
of the documentation of the system will ensure that the documentation
has not been altered during distribution and accurately describes
the system.
6. SUMMARY OF TRUSTED DISTRIBUTION



Trusted distribution is necessary to ensure that the TCB software,
firmware, and hardware developed by a vendor arrive at a customer
site exactly as specified by the master copy that has been evaluated
against the TCSEC. Modification of any TCB software, firmware,
or hardware, originals and updates, could result in a compromise
of the system's security policy. Trusted distribution is a part
of the life-cycle assurance required for trusted systems that
ensures that the security policy of a trusted system remains intact
throughout the life cycle of the system. Trusted distribution
provides assurance that the TCB components will not be altered
during their distribution from a vendor to a customer site. Generically,
this process of end-to-end control can be broken down into three
stages: post-production, transit, and delivery. Along with configuration
management and the other assurance requirements of the TCSEC,
assurance is provided that no violation of a system's security
policy can occur.


Trusted distribution includes methods of protecting the TCB components
during distribution, and in the event of alteration, methods of
detecting that the system has been altered before it is installed
and compromise of the security policy occurs. In the latter case,
TCB software containing a virus could be distributed to a customer
site by an imposter with the intentions of compromising the data
processing facilities.


Advances in the ways of attacking a system and an increase in
insiders committing crimes necessitate greater degrees of protection
to be provided ADP systems. Therefore, a successful trusted distribution
system should consist of dual methods of protection and detection
and should not rely on any one technique.
GLOSSARY

Check Sum



A check in which groups of digits are summed, usually without
regard for overflow, and that sum checked against a previously
computed sum to verify that no digits have been changed since
the last summation. [9]
Configuration Item



The smallest component of hardware, software, firmware, documentation,
or any of its discrete portions, which is tracked by the configuration
management system. [4]
Configuration Management



The management of security features and assurances through control
of changes to a system's hardware, software, firmware, and documentation
throughout the development and operational life of the system.
[8]
Encryption



The process of transforming data to an unintelligible form in
such a way that the original data either cannot be obtained (one-way
encryption) or cannot be obtained without using the inverse decryption
process (two-way encryption).


[8]
Fiber-Optic Latches



An active system method available for trusted distribution. In
this method, a fiber optic cable is looped through a latch on
the opening of a container and attached to a control unit. The
control unit sends a light signal through the cable. If the light
is interrupted by the cutting or damaging of the cable in any
way, an alarm is set off. The alarm can be audible or telemetric.
Formal Top-Level Specification



A top-level specification that is written in a formal mathematical
language to allow theorems showing the correspondence of the system
specification to its formal requirements to be hypothesized and
formally proven.[7]
Message Authentication Code



A cryptographically computed number which is the result of passing
a message through the authentication algorithm using a specific
key. Lengths of from 8 to 16 hexadecimal characters can be used.[1]
System Life-Cycle



The period of time that a system is in existence, including its
design, development, implementation, transportation, installation,
maintenance, and disposal.
Trap Door



A hidden software or hardware mechanism that can be triggered
to permitsystem protection mechanisms to be circumvented. It is
activated in some innocent-appearing manner; e.g., special "random"
key sequence at a terminal. Software developers often introduce
trap doors in their code to


enable them to reenter the system and perform certain functions.[8]
Trojan Horse



A computer program with an apparently or actually useful function
that contains additional (hidden) functions that surreptitiously
exploit the legitimate authorizations of the invoking process
to the detriment of security or integrity. [8]


Trusted Computing Base (TCB)


The totality of protection mechanisms within a computer system-including
hardware, firmware, and software-the combination of which is responsible
for enforcing the security policy. A TCB consists of one or more
components that together enforce a unified security policy over
a product or system. The


ability of a TCB to correctly enforce a security policy depends
solely on the mechanisms within the TCB and on the correct input
by system administrative personnel of parameters (e.g., a user's
clearance) related to the security policy. [7]
REFERENCES


 • 1. American National Standards Institute, Financial Institution
Key Management (wholesale), X9.9, 1982.
 • 2. Department of Defense Computer Security Center, "Final
Evaluation of SCOMP, Secure Communications Processor, STOP Release
2.1," CSC-EPL-85-001, September 23,1985.
 • 3. Karger, 2LT Paul A. and Schell, Maj. Roger R., Multics
Security Evaluation, Vulnerability Analysis, Electronic System
Division, ESD-TR-74-193, June 1974.
 • 4. National Computer Security Center, A Guide to Understanding
Configuration Management in Trusted Systems, NCSC-TG-006, March
28,1988.
 • 5. National Computer Security Center, Computer Security Requirements
Guidance for Applying the Department of Defense Trusted Computer
System Evaluation Criteria in Specific Environments, CSC-STD-003-85,
1985.
 • 6. National Computer Security Center, Criterion Interpretation
Discussion #943, September 1986.
 • 7. National Computer Security Center, DoD Trusted Computer
System Evaluation Criteria, DoD 5200.28-STD, 1985.
 • 8. National Computer Security Center, Glossary of Computer
Security Terms, NCSC-TG-004, 1988.




9. Sippl, Charles J., Computer Dictionary, Howard W. Sams &
Co., Inc. Fourth Edition, 1985.



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