NBSIR 73-324 (fi

VOICE PRIVACY EQUIPMENT FOR LAW ENFORCEMENT

COMMUNICATION SYSTEMS

George R. Sugar

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

September 1973

Prepared for

National Institute of Law Enforcement snd Criminal Justice

Law Enforcement Assistance Administration

U.S. Department of Justice

Washington, D.C.

NBSIR 73-324

VOICE PRIVACY EQUIPMENT FOR LAW ENFORCEMENT

COMMUNICATION SYSTEMS

George R. Sugar

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Points of view or opinions stated in this document

do not represent the official position or policy

of the Department of Justice.

September 1973

Prepared for

National Institute of Law Enforcement and Criminal Justice

Law Enforcement Assistance Administration

U.S. Department of Justice

Washington, D.C.

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

CONTENTS

Page

1. INTRODUCTION 1 2. DEFINITIONS 2

3. VOICE PRIVACY TECHNIQUES 3 3.1 General Approaches 3 3.2 Scramblers with Fixed Codes 3 3.2.1 Inversion 3 3.2.2 Band-Splitting - 4 3.2.3 Masking 5

3-. 2.4 Time Division - 6 3.3 Scramblers with Continually Changing Codes- 6 4. INHERENT DIFFICULTIES IN PROVIDING VOICE PRIVACY

OVER LAW ENFORCEMENT COMMUNICATION CHANNELS 6 5. POTENTIAL PERFORMANCE MEASURES --- 7 5.1 Intelligibility 7 5.2 Speech Quality 9 5.3 Privacy 10

5.3.1 The Threat Concept 10

5.3.2 Privacy in Scramblers Having Fixed

Codes -- 10

5.3.3 Privacy in Scramblers Using Continually

Changing Codes 11

5.3.4 Measuring Privacy 11

5.4 General System Characteristics 12 5.4.1 Electrical Characteristics 12 5.4.2 Environmental Characteristics 13

5.4.3 Installation Characteristics 13

6. COMMERCIALLY AVAILABLE SCRAMBLERS 13

6.1 Common Characteristics and Features 13

6.2 Description of Tables 14

7. HINTS FOR THE PROSPECTIVE PURCHASER 15

8. ACKNOWLEDGMENTS 15

9. BIBLIOGRAPHY 19

10. REFERENCES 20

ILLUSTRATIONS AND TABLES

Figure 1. Scrambling by inversion 4 Figure 2. Scrambling by band-splitting 5 Table 1. Fixed inverters and fixed band- splitters 16

Table 2. Other types 17

Table 3. Manufacturers 18

iii
FOREWORD

The National Bureau of Standards (NBS) has established a Law Enforcement Standards

Laboratory (LESL) for the National Institute of Law Enforcement and Criminal Justice

(NILECJ) of the Law Enforcement Assistance Administration, Department of Justice.

This report was prepared under the direction of Marshall J. Treado, Program Manager

for Communication Systems and Jacob J. Diamond, Chief of LESL. LESL is conducting

research to develop national voluntary standards to assist law enforcement agencies

in their selection and procurement of equipment. Voluntary standards produced by

LESL will be disseminated by NILECJ to law enforcement agencies and other interested

part ies

The Electromagnetics Division of NBS is participating in this effort through the

development of voluntary standards for law enforcement communications equipment. In addition, standard procedures and measurement techniques are being developed for com- munications equipment evaluation, and a program for the accreditation of testing

laboratories will be defined.

This document, Voice Privacy Equipment for Law Enforcement Communication Systems , is

a report of preliminary work on the preparation of a standard on voice privacy equipment selection and application. It is intended to serve as a background reference

or information guide for users and prospective users of voice privacy equipment.

iv
VOICE PRIVACY EQUIPMENT FOR

LAW ENFORCEMENT COMMUNICATION SYSTEMS

George R. Sugar

ABSTRACT

Law enforcement agencies are finding increasing need for voice -scrambling

equipment to provide privacy on their two-way radio communication systems. Work

is underway at the National Bureau of Standards, under the sponsorship of the

National Institute of Law Enforcement and Criminal Justice (NILECJ) , to develop

performance standards for voice scramblers. The four areas being considered are intelligibility, voice quality, privacy, and general system characteristics.

This report first defines a set of special terms, then describes a number of

types of scramblers that are now available for law enforcement use. Some inherent

problems and weaknesses are discussed. The concepts of intelligibility and

privacy are explored in detail. The potential contents of the standard are described, and some problems inherent in preparing and using the standard are discussed. The report concludes with some material intended to be of immediate

assistance to the prospective purchaser of scramblers: a survey of units now

on the market, some hints on how to proceed in the absence of a standard, and a bibliography of technical publications on voice scrambling.

Key words: Intelligibility; LESL ; NILECJ; performance standard; privacy;

scramblers; speech quality; speech scramblers; survey; testing; voice privacy.

1. INTRODUCTION

Law enforcement activities throughout the United States are being seriously hampered

by a lack of privacy on two-way radio systems. Police radio communications are now

continually intercepted, both by casual eavesdroppers and by lawbreakers. Simple

police-band receivers cost as little as $20 and for $100-$200 one can purchase a scanning

receiver that continually monitors up to 8 different channels. The ready availability

of these receivers has virtually eliminated the privacy of communication often essential

for effective police work.

One of the ways to increase the privacy of communications is to use electronic

devices that encode voice signals for transmission and decode them at the receiver.

These devices, loosely referred to as "scramblers", rearrange the voice signals so that,

in principle, they are unintelligible to a listener who is not equipped with the proper

decoding equipment. Scramblers were used in the 1920 's to provide a measure of privacy

on commercial radio- telephone circuits and were substantially improved and used during

World War II for military and diplomatic [1]* communications. Since then, the use of

voice privacy equipment has spread into many areas of the industrial and commercial

world as well as into the various law enforcement and security agencies. At present

there are approximately 15 companies in the U.S. offering voice scramblers at unit

prices from $260 to over $6,000.

Many law enforcement agencies have expressed the need to acquire scramblers. A

recent survey [2] reports that 9% of the 428 departments that responded have scramblers

available, and an additional 58% need them. However, agencies are confronted with the

following when investigating scrambler purchases:

a. It is often difficult to obtain factual information on the design characteristics and performance of given units. There are both elements of secrecy

and a scarcity of performance data on these scramblers.

b. There are almost no objective ways to compare the performance of different

units, thus making it difficult to determine the relative value of each model

of scrambler. It is even more difficult to determine the amount of privacy

obtained for the money spent.

c. Scramblers often must be tailored to work with specific radio units. In general, the performance of a specific scrambler - transce iver combination

cannot be accurately predicted from the listed characteristics of the

separate units

.

*References are listed in section 10.
d. Scramblers from different manufacturers are usually not compatible, thus inhibiting their use for communications between different agencies in the same locality.

Under present circumstances it is difficult for a law enforcement agency to make opti- mum decisions in acquiring scramblers, and it is difficult for scrambler suppliers to compete with each other on the basis of the merits of their products.

One of the long-term goals of the present NILECJ program on voice privacy equipment is to provide a performance standard for evaluating scramblers, thereby hopefully

reducing the present confusion regarding scramblers. The general content of the

standard will be

a. a definition of which characteristics of scramblers are important for proper

performance

b. a description of the methods to be used in measuring these characteristics;

and

c. acceptable performance levels for the various characteristics.

Four technical areas are now under study. These are intelligibility, speech quality,

privacy, and general system characteristics. The present focus is on voice units that

will work with existing police radio equipment.

This report has been prepared as an advisory discussion of scramblers. It defines

a set of special terms, and then describes a number of types of scramblers that are available for law enforcement use. Some inherent problems and weaknesses are discussed.

The concepts of intelligibility and privacy are explored in detail. The potential

contents of the proposed standard are described, and some problems inherent in preparing

and using it are discussed.

The report concludes with some material intended to be of immediate assistance to the prospective purchaser of scramblers: a survey of units now on the market, some hints on how to proceed in the absence of a standard, and a bibliography of technical

publications on voice scrambling.

2. DEFINITIONS

attack . An attempt by an unauthorized person to unscramble a scrambled message, or a specific procedure for doing so. Also, a test to determine the relative privacy of

a scrambler by subjecting it to various unscrambling procedures.

clear . Not scrambled.

code . Any one of the set of fixed ways of rearranging voice signals by a specific

scrambler. See key . intell igibil ity . The ability of a voice communication system to convey the content of

a transmitted message to the intended listener.

key . A specification or setting that controls the sequence in which codes change in a scrambler with continually changing codes. ( Key and code are not well-defined in

the scrambler industry and are sometimes used interchangeably. This report distinguishes between them in accordance with these definitions.)

opponent . An unauthorized person who attempts to intercept and use scrambled messages

for his own purposes.

privacy . The ability of a communication system to conceal the content of a message from

unauthorized persons.

scrambler . A device for providing voice privacy by systematic modification of a voice

signal before transmission.

2
3. VOICE PRIVACY TECHNIQUES

3 • 1 General Approaches

Many different methods of providing voice privacy have been proposed, but relatively few of these are appropriate for current law enforcement use. Five categories

of techniques are:

a. frequency -domain

b . t ime -domain

c. masking

d. vocoder

e

.

digital

Frequency -domain systems rearrange the various frequency components of the voice signal

so as to produce unintelligible sounds.

Time -domain systems divide the voice signal into brief time elements and transmit the

various elements in a rearranged sequence.

Masking systems add extraneous signals and noise to the voice signal, thereby making it more difficult to understand. Masking alone has not proven to be a satisfactory

scrambling technique.

Vocoder systems analyze the basic speech elements present in voice signals and transmit

a set of signals representing these basic elements.

Digital systems convert the voice signal directly into an equivalent number stream and

transmit these numbers in place of the voice signal.

At present, all the systems marketed for law enforcement use are frequency-domain

systems or combined frequency -domain and masking systems. Time -domain systems are under study in England [3] and may become available in the near future. Vocoder systems are still too expensive for this application. Present digital systems require

more than the nominal 3000-Hz bandwidth now available in VHF police radio systems.

Some of the common approaches to scrambling are discussed below. This section also

includes some discussion of scrambler codes -- the different device settings that can be used to make two scramblers of the same type incompatible with each other. In addition, limited discussion of the ability of some types of scramblers to provide privacy

is included to provide some background for more general considerations of system privacy.

3.2 Scramblers with Fixed Codes

3.2.1 Inversion

The simplest scrambler now in use is one that interchanges low voice frequencies

and high ones. This device is commonly called an inverter. It operates by changing

each frequency component present in a voice signal to a new frequency, where the new

frequency is the difference between the original frequency and a reference or inversion

frequency. For example, for a reference frequency of 3000 Hz, a voice component at 750 Hz would be converted to a component at 3000 minus 750 or 2250 Hz. Figure 1 illustrates how a more complex signal would be changed by an inverter.

3
i

I

1

0 1000 2000 3000

Frequency, Hz

!

>s

aj tr

0 1000 2000 3000

Frequency, Hz

Figure 1. Scrambling by inversion.

Unscrambling the scrambled signal is done by using a second inverter that has the same reference frequency as the first inverter. Using the same example again, a scrambled voice component at 2250 Hz, when subtracted from a reference frequency of

3000 Hz, produces a 750-Hz component, thus restoring the original voice component.

As might be suspected from the simplicity of this scheme, it is easy for an opponent to unscramble inverted speech. All he has to do is use an inverter with an adjustable reference -frequency oscillator and tune this oscillator until the speech

is intelligible. Since single -sideband radio receivers have such a capability included

as a normal operating feature, simple inversion provides no privacy whatsoever for

single -sideband transmission systems.

When first heard by a novice, inverted speech sounds quite unintelligible. However,

with careful listening some words can be distinguished. It appears that with concen- trated attention one learns to understand inverted speech directly. In some recent

experiments [4] , people who were able to talk to each other only through inverters

learned to communicate with only four hours of practice. Forty years ago, workers at the Bell Telephone Laboratories demonstrated an ability to speak in *an inverted form

[5,6]. Their speech was intelligible only when "unscrambled" by an inverter. There is

thus little doubt that a trained observer can understand inverted speech directly.

Coding of inverters can be done, at least in principle, by using different

reference frequencies in the two inverters that might otherwise form a scrambler/unscrambler pair. If the two frequencies are sufficiently different, the speech is not intelligible. In practice, this coding is quite limited. Although frequency offsets as little as 10 Hz are noticeable, offsets as great as 200 Hz produce only moderate distortion [7] . Thus an offset of substantially more than 200 Hz would be needed for each

different code. This requirement conflicts with a need to work with speech input signals that are limited to a nominal 300-Hz to 3000-Hz range and to keep the inverted

output nominally within this same range. It therefore appears that only one or two

additional codes for inverters can be obtained by the use of different inversion frequencies. Some commercial inverters provide a number of codes by using tone-controlled

selective calling to unblock the squelch in the selected units [8]

3.2.2 Band-Splitting

A second form of voice scrambling now in use divides the nominal 300-Hz to 3000-Hz voice band into several subbands, and then interchanges the signals in these

subbands, or inverts them, or both. Figure 2 illustrates such a process. This

approach is commonly called band-splitting.

4
(a) Original voios signat

6 IOOO 2000

Fttqu«ncy, Hz 3000

1000 20 Frtqutncy, Hz 3000

(b) Scrambled signal. Natl

that bonds I, 2, and 5

havs bssn invwtsd as

well as displaced

Figure 2. Scrambling by band-splitting.

Unscrambling this signal is achieved by interchanging the signals in the subbands

and reinverting them as required. A unit similar to that used for scrambling is re- quired, with only minor internal differences. Many different code settings are possible

with band splitters, corresponding to how the different Subbands are rearranged in the

scrambling process. For example, a system that uses 5 subbands can have 3840 different codes, since there are 3840 distinct ways to shuffle and invert 5 subbands. Not

all of these are useful codes, however. One of these 3840 possible code settings produces no scrambling at all. Others produce relatively little scrambling and are thus

of little value. In the 5-band band-splitter (called the A-3) installed by the

American Telephone and Telegraph Company (A.T.§T.) for radio telephone service in 1937

only 11 of the 3840 possible codes were considered suitable for privacy [9] . Thus

availability of a large number of codes in a scrambler should not be interpreted to mean that all of these codes are equally useful.

The band- splitter can provide more privacy than an inverter. Unlike the case of

the inverter, information has not been found suggesting that one can learn to directly

understand the scrambled output of a band- splitter that uses appropriate codes. However, it is reported that an average of 47 percent of the words scrambled by the A-3

were recovered simply by listening to a message several times [10] . In one case intelligibility rose to 76%. It is also possible for an opponent to eavesdrop by

using equipment that returns just one of the subbands to its proper place [11] . Thus

it appears that while band-splitters offer somewhat more privacy than inverters, they

are by no means secure against relatively simple attacks.

The privacy offered by inverters and band- splitters is sometimes increased by

adding extraneous tones or noise or both in the scrambler to mask the speech. These

need to be added in such a way that they can be filtered out at the unscrambler. In practice, removing this masking at the unscrambler is always less than perfect; and the

intended listener is always at least aware that extraneous signals are present. The

difficulties of filtering these signals out at the scrambler are aggravated by the

presence of harmonic distortion in the radio system, since such distortion will generate

noise and tones at new frequencies where they cannot be removed without removing some of the desired voice signals also. Furthermore, the addition of masking signals to the

scrambled voice signal will usually require that the voice signal be reduced in amplitude so that the peak value of the composite signal does not exceed the transmission

channel specifications. This reduction, of itself, is likely to degrade overall intelligibility of the system. Thus the addition of masking increases the privacy of a "system but reduces message intelligibility for the intended listener.

3.2.3 Masking

5
Tones used for masking sometimes also serve to synchronize or control circuits in

the unscrambler. They can serve to lock the frequency of the reference oscillator in

the unscrambler to that used in the scrambler [12], to operate a selective -calling

feature, or to automatically switch the unscrambler in and out of the circuit for an intermixed set of clear and scrambled messages.

3.2.4 Time Division

The scramblers discussed above operate in the frequency domain. That is, they

change the frequencies of the voice input to new frequencies. Another approach is to leave voice frequencies unchanged but to change the time sequence of what is said.

For example, the time -domain equivalent of an inverter would generate speech backward.

In practice this is not done because such a scrambler would need to save the whole

message, and to wait until the end occurred before replaying the message in reverse. This would introduce unreasonably long time delays in the communication channel. A more practical approach is to divide the voice signal into small time segments (of

about 60 milliseconds or less) and delay these for varying brief intervals before re- producing them. This varying delay mixes the order of the voice segments and can make

the scrambled output unintelligible.

During World War II, much work was done with time -division scrambling techniques,

often in combination with frequency-domain scrambling [13]. However, the resulting

systems all used magnetic recording devices and were relatively large and expensive.

At present there are no time -division scramblers on the U.S. market. However, with the

advent of new compact digital and analog [14] semiconductor storage devices that can provide the required time delays, there appears to be renewed interest in this area. A principal problem of time -division scramblers is similar to that of bandsplitters. While there are many ways to rearrange the normal voice sequence, many of

these ways do not result in adequate scrambling. The "scrambled" output is often

intelligible [15].

3 . 3 Scramblers with Continually Changing Codes

Thus far, all of the scramblers discussed have used fixed codes. Even though some had a selection of codes, once selected the code was fixed until changed by the user. A number of types of scramblers use continually changing codes to make it more difficult

for an opponent to eavesdrop. The code changes typically occur from a few times per

second to 100 times per second. Such systems permit the use of different code sequences

at different times, thus preventing eavesdropping even by persons who have an unscrambler but who do not know what code sequence (key) is being used at the time. A set of

switches is usually employed to adjust the key generator that controls the actual code

sequence. A scrambler and unscrambler must be set to the same key to function together.

In a properly designed system there will be enough different keys so that it is not

practical for an opponent to find the correct one simply by trying all of the different

possible switch settings. This is particularly true if the users change the key

periodically such as once per day and exercise adequate precautions to prevent dissemination of the key to unauthorized persons.

Special names are commonly given to scramblers with continually changing codes.

Two of these are frequency-hopping inverters and rolling-code band-splitters.

Some scramblers with continually changing codes also incorporate masking to in- crease privacy. The mask is sometimes scrambled independently to make the system less

susceptible to attacks.

4. INHERENT DIFFICULTIES IN PROVIDING VOICE PRIVACY OVER

LAW ENFORCEMENT COMMUNICATION CHANNELS

The designers of scramblers for police (narrow-band) radio channels have a substantial set of technical problems to overcome. Therefore it is not surprising that there

are so few Successful designs. A major part of the problem, the inherent robustness

of speech communication, has been succinctly described as follows [16]:

6
"Beginners in the study of privacy systems never fail to be amazed at the

difficulty of scrambling speech sufficiently to destroy the intelligence. The ear can tolerate or even ignore surprising amounts of noise, nonlinearity , frequency distortion, misplaced components, gaps, superpositions, and other forms

of interference. Very often partial or even complete intelligence can be

obtained from a privacy system by partial or imperfect decoding, and this in

turn can often be accomplished by operating on the scramble in some way which

the designer did not contemplate.

The fact that the ear is such a good decoding tool in combination with noncryptographic methods makes the production of privacy systems very difficult. Scrambling

systems which look very effective on paper sometimes turn out on trial to degrade

the intelligibility very little, although the scrambled speech usually sounds

unpleasant. Most methods pushed to the point where they succeed in hiding the

intelligibility so distort the speech that it is impossible to restore the speech

with good quality. In fact, there are very few speech privacy systems which

achieve a high degree of privacy with acceptable quality."

The limited audio bandwidth available in VHF communications channels also provides

a substantial obstacle to scrambler designers. The highly secure all -digital scramblers

will not fit into the presently available 3000-Hz audio bandwidths of police transceivers. The actual available bandwidth is usually even less than this nominal value.

The EIA standard [17] permits the overall frequency response at 300 Hz and 3000 Hz to vary from +3 dB to -11 dB relative to the value at 1000 Hz. This standard is not a mandatory one, and the response of actual units has been reported [18] to be as poor as -20 dB at 3000 Hz. The scrambler designer thus needs to achieve good performance in

widely varying bandwidths, with the widest bandwidth being less than optimum.

A further problem faced by designers is the great ingenuity exhibited by eaves- droppers in devising methods to unscramble transmissions. The history of voice

scramblers displays a continuing sequence of confident inventors and the Subsequent

"breaking" of their scrambling systems. In the first U.S. patent on scramblers [19]

the inventor states:

"Thus it will be seen that by my invention absolute secrecy in the transmission

of signals is insured --a desideratum long sought, but, so far as I am aware, now for the first time accomplished."

It is not evident whether this invention was ever used. However, it is clear from

examining the principle used that the scrambled signal would be at least 40% intelligible to an eavesdropper. On the other hand, it is reported [9] that in the 1920'

s

radio amateurs were able to eavesdrop on the inverted signals used then by A.T.^T. for

transatlantic radio-telephone service soon after this service was inaugurated.

5. POTENTIAL PERFORMANCE MEASURES

As indicated in the Introduction, four technical areas are being considered for

inclusion in the scrambler standard: intelligibility, speech quality, privacy, and

general system characteristics. In this section each of these areas will be discussed

in more detail.

5.1 Intelligibility

One of the first things that a new user observes is that speech transmitted through

a scrambler/unscrambler system is usually not as clear nor as easy to understand as it

is when the equipment is not used on the radio circuit. The relative ability of a listener to understand what is being said by a talker is called intelligibility. It is

quantified as the relative number of correctly heard items, measured in percent. For

example, if a talker reads a list of 200 words and a listener identifies 150 of them

correctly, the intelligibility for that test was 751. Thus intelligibility testing

provides a way of measuring the fidelity of a voice communication system.

Almost all communication systems tend to reduce the intelligibility of speech.

The extent of this reduction is dependent on many different factors such as the speaker's

voice and diction, the nature of what is being said, the hearing acuity of the listener

and his previous experience in listening to a given speaker and a given class of mes- sages and the testing environment. Thus intelligibility is a function of at least the

7
speaker, the listener, the message content, the transmission channel, and the environment. The usual procedure for measuring intelligibility of a communication channel

involves using enough speakers and listeners to average out their individual differences,

and controlling the environment, thus arriving at a measure representative of the

channel performance and the test material used. The importance of the test material

itself can be appreciated by considering some examples of the material used. These

include lists of:

single syllable words that sound nearly alike as [20]

hip, hick, hit, hiss

sail, pail, tail, fail

tan, ten, tin, teen

phonetically balanced (PB) word lists, words that contain a representative sample

of the sounds that make up speech [21]

PB -50 List 1 PB -50 List 2

1. cane L 0 . pes t 1. tang 26 . blush

2. there 9LI.7 5 J. lp 2. fate 27 . nab

3. dish 28. rub 3. suck 28 . bait

4. hid 29. feast 4. else 29 . bud

5. heap 30. deed 5. pit 30 . rap

6. pants 31. cleanse 6. gill 31 . moose

7. hunt 32. folk 7. charge 32 . trash

8. no 33. nook 8. bought 33 . gloss

9. bar 34. mange 9. cloud 34 . perk

10. pan 35. such 10. mute 35 . vamp

11. fuss 36. use (yews) 11. bean 36 . start

12. creed 37. crash 12 . scythe 37 . earl

13. box 38. ride 13. vast 38 . corpse

14. strife 39. pile 14. rib 39 . sludge

15. dike 40. rat 15 . pick 40 . tan

16. not 41. rag 16. hock 41 . ways

17. ford 42. is 17. our 42 . bounce

18. end 43. wheat 18. hit 43 . niece

19. then 44. rise 19. job 44 . awe

20. bask 45. hive 20. wish 45 . them

21

fraud 46. grove 21

nut 46 . need

22. smile 47. toe 22 . dab 47 . quart

23. death 48. plush 23. frog 48 . five

24. are 49. clove 24. log 49 . hire

25. bad 50. fe rn 25. snuff 50 . shoe

Phonetically balanced sentences [22]

1. The birch canoe slid on the smooth planks.

2. Glue the sheet to the dark blue background.

3. It's easy to tell the depth of a well.

4. These days a chicken leg is a rare dish.

5. Rice is often served in round bowls.

6. The juice of lemons makes fine punch.

7. The box was thrown beside the parked truck.

8. The hogs were fed chopped corn and garbage.

9. Four hours of steady work faced us.

10. A large size in stockings is hard to sell.

sentences that request a simple response [23]

1. Name a prominent millionaire of the country.

2. How large is the sun compared with the earth?

3. Why are flagpoles surmounted by lightning rods?

4. Give the abbreviations for January and February.

5. Name the tree on which bananas grow.

6. How often does the century plant bloom?

7. What description can you give of the bottom of the ocean?

8
8. Explain the difference between a hill and a mountain.

9. What is the chief purpose of industrial strikes?

10. Describe the shoes of the native Hollander.

random five-unit code groups read phonetically [24]

PAPA NINE ROMEO ONE UNIFORM

SIX BRAVO DELTA FOUR YANKEE

HOTEL MIKE PAPA QUEBEC ZERO

The various types of test material above are arranged approximately in order of

decreasing difficulty. Tests based on s ingle - syll able words will tend to be the most

difficult ones, i.e., give the lowest intelligibility scores, while those toward the

bottom of the list will tend to be the easiest ones and give the highest scores. While

it is possible to derive intercomparisons among the tests, this is a rather lengthy

process. In general, comparisons between different communication systems, specifically

different scramblers, are valid only when similar test material is used for all systems.

For example, a scrambler that provides high intelligibility on 5-unit code groups may

be nearly unintelligible for phonetically balanced sentences.

It is easy to conclude from the above discussion that one would like more objective

ways of measuring system intelligibility and this in effect has been done for conventional communication systems. For such systems the relations between intelligibility

and sound levels, noise levels, frequency response, distortion, and other factors, have

been extensively studied. The results of these studies are then applied to permit

specifying the performance of conventional communication systems in terms of performance characteristics such as frequency response, harmonic distortion, and noise level,

all of which are much more easily measured than intelligibility itself. In the case of

scramblers, the relations between intelligibility and these other characteristics have

not been established. Indeed, it is not yet clear that it is possible to establish a single set of relationships that will be adequately accurate for all types of scramblers.

Different relationships may be required for different types of scramblers. Thus, with

the present lack of data on scrambler performance, it is essential that intelligibility

be measured directly. For scramblers, there is no proven way to infer intelligibility

from more conventional measurements. Various special measuring devices have been

developed for measuring articulation index, a quantity closely related to intelligibility [25] . However, the validity of using these devices for testing scramblers has

not yet been established.

5 . 2 Speech Quality

While intelligibility refers to the accuracy of message transmission through a system, it excludes many other significant aspects of speech communication. Systems

that provide adequate intelligibility may differ in characteristics related to recognition of who is speaking, transmission of the emotional content of speech, conversational

effort, etc. Good performance of scramblers in some of these areas is important to some users. In particular, police agencies often depend on good speaker recognition

in their voice communication.

This general performance area is referred to as speech qual ity or listener

preference [26]. It tends to include intelligibility as one of its elements. The test

methods used [22] are similar to those used for intelligibility testing, but usually

are modified to have the observer rate the system quality numerically; or in terms of

categories such as excellent, good, fair, poor, and unsatisfactory; or by indicating

which of two units he prefers throughout a series of paired comparisons which compare

different units in rapid succession.

At present, it appears that the measurement of speech quality of scramblers is

substantially more difficult than the measurement of intelligibility and that the re- sults of such measurements will be difficult to interpret in terms of users' needs.

The initial scrambler standard will probably not include speech quality performance as

a parameter to be measured.

9
5 . 3 Privacy

In a scrambler system, one desires that the intended listeners receive a perfectly

intelligible message. Correspondingly, one desires that other listeners, eavesdroppers,

etc., receive an unintelligible message. In this report, privacy is the term used to

label the ability of a scrambler system to prevent unauthorized interception of messages.

It is assumed that it is possible to assign numerical or relative ratings to the privacy

provided by different scramblers, and that standard measurement procedures can be devised to determine these ratings. These assumptions seem reasonable, but are yet to be

proven. The term security is sometimes used in addition to or in place of privacy . However, at least in federal government usage, a voice security system would be one safe enough to handle classified security information -- one that would require perhaps

years of expert effort to unscramble. Because of this special usage of the term

security, and because few, if any, of the scramblers now available for police use would

meet the criteria for a secure system, the term privacy has been used in this report.

5.3.1 The Threat Concept

In examining police needs for scramblers, particularly with a view to determining

how much privacy is required, the question arises of what is the "threat"? What level

of "attack" must the scrambler system withstand? If the only protection needed is

against casual eavesdroppers who have no criminal intentions, relatively simple scram- blers may be adequate. If, on the other hand, protection is needed against highly

organized criminal activities, scramblers offering a high degree of privacy may be

essential

For example, consider the consequences of a criminal acquiring an unscrambler and

installing it in his automobile. If the scrambler is one that uses fixed codes, it

probably will be relatively easy for him to determine which code is in use at any particular time, thereby possibly rendering the whole scrambler installation relatively

useless. Clearly, fixed-code scramblers do not offer adequate privacy under such circumstances. On the other hand, if the scrambler were one that used continually

changing codes, it probably would be much more difficult for the criminal to find the

correct key. It may be practically impossible to do so just by trying various combinations, especially if the key were changed daily. A possible next step for the criminal

would be to gain knowledge of the key from someone in the police agency who had access

to it. The protection against such a move would be to strongly limit the number of

agency people who had access to the keys. In part this could be done by having the key

setting switches in a locked compartment of the scrambler, accessible to only a few

highly trusted staff members.

It should be clear from the preceding discussion that the proper choice and use of

scramblers is very dependent on the level of the threat. Specifically:

a. From whom must the information be kept?

b. How long must the information be kept private?

c. What skills does the opponent have?

d. What resources does the opponent have?

Once the threat has been evaluated it should be possible to determine the level of

privacy required in a scrambler to withstand that threat. Since the costs of scramblers

tends to increase rapidly as privacy requirements increase, it is desirable to evaluate

the threat as realistically as possible and not purchase more privacy than is necessary.

5.3.2 Privacy in Scramblers Having Fixed Codes

Scramblers that employ fixed codes achieve privacy by requiring the intended receiver

of the message to have an unscrambler that is similar to the scrambler itself. Anyone

possessing the appropriate unscrambler can hear the message. It is helpful but not

essential to an eavesdropper to know which specific code is being used in systems that

have a number of codes available. For such scramblers it is usually possible for the

eavesdropper to tune in to the proper code just as one tunes a radio receiver to the

desired station. For example, in units that have multiple code-setting switches, such

as fixed-code band- splitte rs , it will probably be possible to adjust each of the switches

in succession until clear reception is obtained [27]. Few of the total number of possible codes need be tried.

10
For some fixed-code scramblers, it is possible to unscramble messages, probably

with some loss of intelligibility, with devices less complex than the normal unscrambler.

Thus an opponent may be able to eavesdrop by using equipment that is less expensive

than the equipment used by the police agency itself. For example, a radio technician

can construct an inverter from readily available parts costing only $20 [28]. In summary, scramblers that use fixed codes offer relatively low privacy because

they offer little or no protection against an opponent who possesses a similar unscram- bler or equivalent device.

5.3.3 Privacy in Scramblers Using Continually Changing Codes

Scramblers with continually changing codes apply cryptographic techniques to voice

scrambling in an attempt to achieve a substantially higher degree of privacy than is

possible with fixed codes. The scrambler designer attempts to force opponents to employ

cryptanalys is to unscramble the message and to make mere possession of an unscrambler

by an opponent be of relatively little value. As was previously discussed, this is done

by designing units to have so many different key settings available for use that the

opponent cannot find the correct key in any reasonable amount of time just by trying

different settings one after the other. If keys are changed daily then a few thousand

different settings may be adequate [11]. This is true because in many systems of this

type the key setting at the unscrambler must be correct at the start of a transmission

to synchronize the two units properly. Only one new key setting can be readily tested

on each transmission.

In practice, two factors can reduce the privacy of these scramblers: cryptographically weak systems and non-cryptographic attacks. It is well beyond the scope of

this report to include any discussion of cryptographic techniques. These are excellently covered in the literature [29,30,31]. However, one important fact is that the

number of different key settings available is not necessarily a direct measure of

cryptographic security. A system with 1,000,000 key settings may be easier to break

than one with 1,000 settings. The most significant consideration is likely to be how

the key stream is generated, not the number of key settings. A comment from the

literature on cryptanalys is is pertinent in this connection [32].

"Many inventors also invoke the vast number of combinations of keys afforded by

their system as proof of its invulnerability. To exhaust the possible solutions

would take eons, they contend... [erroneously]. For, as Shannon [33] has shown,

the cryptanalyst does not go after these possibilities one by one. He eliminates

them millions at a time. Moreover, the trials progress from the more probable

to the less probable hypotheses, increasing the cryptanalyst ' s chance of striking

the right one early. 'Whereas complete trial and error requires trials to the

order of the number of keys,' Shannon wrote, 'this subdividing trial and error

requires only trials to the order of the key size in bits,' a very much smaller

number .

It is thus clear that the number of key settings is an inadequate measure of privacy.

A scrambling system that is resistant to cryptographic attacks can sometimes be

unscrambled by other means. These non -cryptographic methods, which ignore the coding

and instead work directly on the scrambled voice signal, may allow an opponent to unscramble a message with relative ease [16].

In summary, scramblers that use continually changing codes have the potential for

offering a high level of privacy. However it is essential that their levels of privacy

be evaluated by actual tests and that resistance against both cryptographic and non- cryptographic attacks be determined in these tests.

5.3.4 Measuring Privacy

Consider now the question of how to define and measure the privacy of a scrambler,

either in a relative or an absolute sense. In the previous discussion it has been

assumed that privacy can be measured. It will be seen from the discussion below that

there are substantial problems in defining standard measurement procedures for privacy.

11
Part of the problem is that the present approach to determining scrambler privacy

is not one of measurement, but one of attack . Given a new scrambler, the expert will

try a variety of ways to unscramble the messages, starting with simple attacks and proceeding to more complex ones, or perhaps starting with an attack that previously worked

with that general type of scrambler. This approach is both difficult to specify and

difficult to measure. Some characteristics that are or may be measurable include:

a. the level of expertise required to break the system;

b. the amount, kind, and cost of the equipment required; and

c. the time required, both for the first time a new system is attacked and

subsequent times after the user changes the code or key setting.

It appears that the first of these characteristics, degree of expertise, will be the

most difficult to measure. In addition, the required equipment and time can depend

strongly on expertise, thus making them less reliable measures. Suppose that one could

eliminate the need to measure expertise by writing a set of procedures to be followed

in attacking a scrambler. These procedures would describe the various attacks in sufficient detail so that any adequately equipped laboratory could try first one and then

another attack to see which were successful and which were not. These results would

then be interpreted according to a predetermined rating system to indicate the privacy

level of the scrambler. This approach, if it can be made to work, solves the problem

of how to specify and measure privacy.

Now to address the second part of the problem. Who is allowed to have copies of

the standard for measuring privacy? Normally one would try to get wide distribution of

all of the standards so that buyers, sellers, and users all can have a common basis for

describing the performance of the equipment. But the privacy standard just described

would be very helpful to opponents, too, be they casual eavesdroppers or criminals,

since it would tell them exactly how to proceed to decode scrambled messages, what

equipment to use, and how long it would take. The standard which was designed to help

users get the scrambler performance they need would give their opponents just the information needed to defeat the system. It thus appears that if such a standard for privacy

is developed, the standard itself will have to be private and to receive only limited

distribution, thereby reducing its usefulness to users. At present it is not clear how

to resolve this dilemma.

5 .4 General System Characteristics

The fourth area under consideration includes the pertinent performance characteristics that do not fall directly into one of the first three categories. The items

tend to be similar to those that are usually contained in standards for more common communications equipment. In many cases they are closely related to intelligibility,

quality, and privacy in that they specify conditions that affect these three characteristics. For discussion purposes, the general system characteristics are divided

into three parts: electrical, environmental, and installation aspects. While the

focus may appear to be on mobile units, essentially the same considerations do apply

to base station and portable units.

5.4.1 Electrical Characteristics

a . Interface to transceiver

(1) Where does a scrambler connect to the transceiver?

(2) What signal levels are acceptable?

(3) Can a defective transceiver be replaced without readjusting the scrambler or transceiver?

(4) Can a defective scrambler be replaced without readjusting the transceiver or scrambler ?

12
b. Transceiver Performance

(1) Does the scrambler operating in the clear mode reduce the performance of the

transceiver?

(2) If the scrambler fails or is removed temporarily, can the transceiver con- tinue to be used?

(3) Does the scrambler permit proper operation of squelch circuits and selectivesignalling (tone) arrangements?

c. Scrambler performance

(1) What transceiver signal-to-noise ratio is required for proper operation?

(2) What audio bandwidth is required for proper operation? (Note that audio

bandwidth will depend on the characteristics of transceivers, base stations,

repeaters, satellite receivers, and telephone lines.

(3) How is synchronization between scrambler and unscrambler affected by system

noise, ignition noise, signal fades, very strong signals, and lengths of

transmissions ?

(4) How long is required to establish synchronization?

5.4.2 Environmental Characteristics

(1) What temperature and humidity ranges must be tolerated?

(2) What shock and vibration levels must be tolerated?

(3) What power supply variations must be tolerated?

(4) Does unit operate properly in the radio - frequency fields and conducted rf

signals from the transceiver?

5.4.3 Installation Characteristics

(1) Is the scrambler secured from theft or tampering?

(2) Are the key-setting switches protected from unauthorized access?

6. COMMERCIALLY AVAILABLE SCRAMBLERS

An extensive search has been conducted to identify all of the narrow-band voice

scramblers being offered on the open market for police use. Units that were not offered

as standard commercial items were excluded, as were devices that clearly required more

than 3000-Hz bandwidths. The results of this survey are summarized in tables 1 and 2. Manufacturers' complete names and addresses are given in table 3 as an aid to proper

identification of the various units. These data are presented to indicate the range of

equipment choice that exists and to illustrate what types of technical data are commonly

available. The listed units have not been examined or tested, nor has an attempt been

made to evaluate or validate any of the data supplied by the manufacturers. Features

and characteristics that are common to all or nearly all units are not given in the

tables but are presented below.

6 . 1 Common Characteristics and Features

The following list describes the characteristics and features shared by most or all

units in the tables. Exceptions are noted in the tables under the "Other" and "Special

Features and Options" headings.

Hal f -duplex -- The unit is capable of scrambling and unscrambling (transmission and

reception) but only one of these at any time. A minimum system contains two units,

one at each end of the radio or telephone link.

13
Compatibility -- In general, units from different manufacturers are not compatible with

each other. Most mobile units are available to operate with any mobile transceivers,

but the specific model must be identified.

Power supply -- Units for mobile operation operate from the normal 12 volt automobile

battery. Units for fixed operation operate from 115 V AC.

Mounting arrangements -- Mobile scramblers usually come as single units for under-dash

mounting or as two units, an under-the -dash control head and a trunk-mounted

electronic unit. Base-station scramblers usually come as single units for tabletop or relay rack mounting.

6 . 2 Description of Tables

In preparing the tables, scramblers were divided into three categories: fixedcode inverters, fixed-code band-splitters, and all other types. Data on the inverters

and band-splitters are presented in table 1, arranged alphabetically by manufacturer

for each of the two types. Data on all other types are given in table 2, alphabetically

by manufacturer. The listing of a unit in table 2 does not imply that it is superior

to units in table 1. Blanks in the tables indicate that the manufacturer has not

supplied the corresponding data.

Columns 1 and 2 give manufacturer and model number. Different models are grouped

together as a single entry when they represent merely different mechanical or electrical

versions of the same unit.

Column 3 lists the intended application. M designates units designed for use with

mobile transceivers and their associated base stations. T designates units for use with telephone systems. P designates units for use with personal/portable transceivers.

PS designates a personal portable transceiver with a self-contained scrambler. RT

(radio -telephone) designates units that appear to be designed for base station use only,

where no version is offered for mobile installation.

Column 4 lists the scrambling principle used. The terms used here are chosen to be self -consistent and may differ slightly from the manufacturers' terminology.

Columns 5 and 6 indicate how many different codes or keys are available and how

they are selected. The term plug-in module is used in a general sense to indicate all

plug-in elements, including elements that do not contain any electronic components but

simply act as a mechanical key or switch.

Column 7 lists the speech bandwidth that is accepted at the input to the scrambler.

It is assumed that voice frequencies outside of this band are rejected by the scrambler

and not utilized.

Column 8 lists the bandwidth of the scrambled speech signal. Note that while this

is a crucial characteristic in determining the compatibility between given scramblers

and transceivers, many manufacturers do not supply this information.

Column 9 lists the audio power output of those units designed to drive a loudspeaker directly. This is a less significant factor for units that interface in other

ways, and data for the other units have been omitted.

Column 10 lists carrier suppression; that is, the amount that internal reference

signals are reduced relative to the normal scrambled output. It is not clear that all

manufacturers measure this in the same way, and the values therefore may not be comparable.

Column 11 lists the ambient temperature range specified for proper operation.

Column 12 indicates some other performance characteristics given by the manufacturer that may be important to scrambler performance but did not fit elsewhere in the

t ab 1 e s . Column 13 describes the intended methods of interfacing the scrambler with the

transceivers and base stations. Note that most suppliers will probably furnish units

to meet any interface requirements. Those listed here are the most readily available

one s

14
Column 14 indicates special features that are supplied as standard items with

specific units and optional items available at extra cost.

Column 15 indicates the advertised price per unit, in unit quantity, or the

corresponding range of prices for various versions of the same unit.

7. HINTS FOR THE PROSPECTIVE PURCHASER

As has already been indicated, this report has been prepared as one of the preliminary steps toward developing a standard for scramblers. It is hoped that the present

report will be useful both in understanding and applying the standard when it is

available, and in assisting agencies in purchasing scramblers in the interim. In this

latter regard the following suggestions are offered:

a. Evaluate the threat situation as realistically as possible and do not buy a more complex scrambler system than you need.

b. Get a factual explanation of the scrambler operating principle used, in terms

that will allow you to compare various scramblers. Assume that your opponents will have at least as much information about the units as you do.

c. Ask suppliers the questions listed above in Section 5.4 and compare their

answers with your needs wherever possible.

d. Determine whicn installation and performance aspects the supplier will take

responsibility for and which you must be responsible for.

e. Get a demonstration, using your entire communications system, under a variety

of normal and unusual operating conditions. (A one-week trial by users under

normal operating conditions can be very informative.) Such a demonstration

is essential before purchasing scramblers. During this demonstration, care- fully observe the installation procedures to see how much modification and

adjustment of scramblers and transceivers are required to achieve satisfactory

operat ion

.

f. Do not test scramblers with only your best transceivers if they must work with

average and poor transceivers. Adding scramblers will usually reduce the

intelligibility achieved in a system and this reduction may not be tolerable

in situations that were marginally acceptable without scramblers.

g. Be prepared to encounter significant installation and maintenance difficulties.

Until otherwise proven, plan to do more routine maintenance on transceivers

that use scramblers than on those that do not.

8. ACKNOWLEDGMENTS

NBS Electromagnetics Division staff members responsible for the preparation of

the report were George R. Sugar, project leader, and John P. Wakefield. Irma J. Starr

assisted in editing and copy preparation, and Janet R. Becker and Sharon C. Foote

typed the final manuscript.

15



TABLE



TABLE 2. OTHER TYPES

MANUFACTURER

PD101X

PD101XL

Ground Data 203

Ground Data 204

CODES OR KFYS

MODEL APPLICATION

ifqutncv-hoprLns i inverter with

one masking

- and band- 'o families

1.000,000/fanilv

16.000,000 full]

Frequencv-hoppin

inverter with independent fre- quency-hopping

I'l l 'fl,VH I

i ll.M'.Ai ll-.IMMII S ech Scrambled

Signal Audio bond , output

M/S, HI , 600:2/4,

C.TH

NOTES:

Colum j; Applications

«- Mobile transceiver

p;

e rsonai portable transci

Personal portable transci

integral scrambler

KT. Radiotelephone or base si 1 Telephone

•60 dB direct signal feedthrouoh Code changes 4/second

;0 ninu?os

S1",Chr0niSn f° f

"

l

' CaSI Code changes SO/sccond

Code changes 2/second

Sync time is 1 second

' »" a »laJ»le for JOO-Hi to 2B00-Hi

? °io A-n

°

n

T

J 400 "? 1 Chanr,cl response." Ma in t a i

p0lri 1 ^

rel at 1 v « to I Mil)

Column 13: Interface

C. Acoustic/magnetic coupler ti telephone handset

HI . High impedance

M/S, Microphone/speaker

T. Telephone instrument

TL. Telephone line (replaces telephone insi rumen t

TH. Replaces telephone handset

Can also operate as fixed 5-band band-splitter

Selective call ing

Full dupl

telephi

TABLE 3. MANUFACTURERS

Boeing Electronics Products

P.O. Box 24666

Seattle, Washington 98124

California Security Products, Inc. 21748 Devonshire Street

Chatsworth, California 91311

Com-U-Trol Division of DASA Corp.

4825 Scott Street

Schiller Park, Illinois 60176

Controlonics Corp.

One Adams Street

Littleton Common, Massachusetts 01460

Data Transmission Sciences, Inc. P.O. Box 1308

Danbury, Connecticut 06810

Datotek, Inc. P.O. Box 12388

Dallas, Texas 75225

E.F. Johnson Co. Waseca, Minnesota 56093

Ground/Data Corp.

4014 N.E. 5th Terrace

Fort Lauderdale, Florida 33308

Lynch Systems Inc. 204 Edison Way

Reno, Nevada 89502

MIECO, Inc. 1928 Green Springs Drive

Timonium, Maryland 21093

Motorola Communications and Electronics,

1301 E. Algonquin Road

Schaumburg, Illinois 60172

Pye TMC

15 Sheffield Street

Toronto, 385 Ontario

CANADA

RCA Mobile Communications Systems

Meadow Lands, Pennsylvania 15347

Scientific Radio Systems, Inc. 367 Orchard Street

Rochester, New York 14606

Singer Tele-Signal

250 Crossways Park Drive

Woodbury, Long Island, New York 11797

Technical Communications Corp.

442 Marrett Road

Lexington, Massachusetts 02173

18
9. BIBLIOGRAPHY

Arizona Public-Safety Communications Officers Association, 1969 Report on Public

Safety -- Voice Privacy Equipment , October, 1969 (Engineering and Research Committee , Arizona APCO)

Baker, H.C., Voice Privacy Transmission Techniques , Communication News, June 1972,

pp. 38-41.

Guanella, G., Methods for the Automatic Scrambling of Speech , Brown Boveri Review,

December 1941, pp. 397-408.

Carlson, R.L., Tellez, J.M. , Schreiber, W.L., Privacy of Voice Communication , Security

World, May 1972, pp. 49-53.

Detwiler, W., Scrambler Design Parameters , Communications, June 1970.

French, R.C., Computer Simulation of a Speech Scrambler , Conference on Digital Processing of Signals in Communications, Loughborough, Leies., England, 11-13 April 1972,

pp. 339-345 (London, England, IERE , 1972).

French R.C., Speech Scrambling , Electronics § Power, July 1972, pp. 263-264.

Gill, A.J., Privacy Systems for Radio Telephony , The Post Office Electrical Engineers'

Journal, October 1933, pp. 224-230.

McCalmont , A.M., and Eramo, W.S., Jr., Communications Privacy , Telecommunications,

October 1970.

Rompel , J.D., A Discussion of the Use of Speech Scramblers with Police Radio

Communications Systems , Electronics Research Laboratory, Montana State University

(Bozeman, Montana, March 1973).

Teacher, C.F., Problems Encountered in the Design of Speech Privacy Systems , Proceedings

of the 1970 Carnahan Conference on Electronic Crime Countermeasures , pp. 44-58

(University of Kentucky, 1970) . Timothy, L.K., and Boll, S.F., A Secure Voice Communication System with Low Bit Rate

and High Voice Quality , Proceedings 1973 Carnahan Conference on Electronic Crime

Countermeasures, pp. 5-8 (University of Kentucky, 1973).

19
10. REFERENCES

Kahn, D. , The Codebreakers , pp. 551-560 (Macmillan Company, New York, 1967).

LEAA Police Equipment Survey: The Survey of Police Departments in the United States , NBSIR 73211 (National Bureau of Standards, Washington , D . C . , 1973).

Phillips, V.J., Lee, M.H. , and Thomas, J.E., Speech Scrambling by the Re -Ordering

of Amplitude Samples , The Radio and Electronic Engineer, V. 41_, No. 3, pp. 99-112,

March 1971.

Blesser, B.A., Inadequacy of a Spectral Description in Relationship to Speech

Perception , Acoustical Soc. America, 78th Meeting Abstracts, J. Speech and Hearing

Research, V. 1_5, p. 19 (Acoustical Society of America, New York, 1972).

Richey, J.L., as cited in Blesser, B.A., Speech Perception Under Conditions of

Spectral Transformation: I. Phonetic Characteristics , J~. Speech and Hearing

Research, Vol. 15 , p . 25 (Acous tical Society of America, New York, 1972).

Beers, Y.O., personal communication (1973).

Fletcher, H. , Speech and Hearing in Communication , p. 352 (D. Van Nostrand Company,

Inc., New York, 1953). ] McCalmont , A.M., private communication (1972).

Kahn, D., The Codebreakers , p. 554 (Macmillan Company, New York, 1967).

Ibid. , p. 558

.

McCalmont, A.M., Communications Security for Voice -- Techniques, Systems, and

Operat ions , Telecommunications, pp. 35 -42 , April 1973 . Alexander, D.M., Speech Privacy Circuit , Signetics Linear Phase Locked Loops Application Book, pp. 66-68 (Signetics Corp., Sunnyvale, Calif., 1972).

U.S. Office of Scientific Research and Development, Speech and Facsimile Scrambling

and Decoding , Summary Technical Report of Division 13, NDRC , V. 3, Washington, D.CT,

1946 . Motorola Solid State Technology 3_, No. 1, p. 53 , 1973.

Phillips, V.J., Lee, M.H., and Thomas, J.E., Speech Scrambling by the Re-Ordering

of Amplitude Samples , Radio and Electronic Engineer, V. 41_, No. 3, p. 112,

March 1971.

U.S. Office of Scientific Research and Development, Speech and Facsimile Scrambling

and Decoding , Summary Technical Report of Division 13, NDRC, V. 3, p. 40,

Washington, D.C., 1946.

EIA Standard, RS-237, Minimum Standard for Land-Mobile Communication Systems Using

FM or PM in the 25-470 MC Frequency Spectrum (Electronic Industries Association,

Washington, D.C., August 1960).

Miller, C.K., Voice Scramblers in Two-Way Systems , Communications News, pp. 32-33,

August 1972.

U.S. Patent No. 251,292, December 20, 1881.

Schubert, E.D., and Owens, E., CVC Words as Test Items , private communication.

American National Standards Institute, USA Standard Method for Measurement of

Monosyllabic Word Intelligibility , USAS S3. 2-1960 (American National Standards

Institute, New York).

1965 Revised List of Phonetically Balanced Sentences (Harvard Sentences) as given

in IEEE Standard No. 297, IEEE Recommended Practice for Speech Quality Measurements,

p. 15 (Institute of Electrical and Electronics Engineers, Inc., New York, 1969).

Fletcher, H., Speech and Hearing in Communication , p. 300 (D. Van Nostrand Company,

Inc., New York, 1953).

20
Smith, Ray, personal communication (1973).

Hecker, M.H.L., von Bismarck, G. , and Williams, C.E., Automatic Evaluation of TimeVarying Communication Systems , IEEE Transactions on Audio § Electroacoustics , AU-16 , No. 1, pp. 100-106, March 1968

Rothauser, E.H., A Comparison of Preference Measurement Methods , The Journal of the

Acoustical Society of America, V. 49, No. 4 (Part 2) , 1971

Maitland, P., Communications Security , 1972 Carnahan Conference on Electronic Crime

Countermeasures , private communication (University of Kentucky, 1972).

Rakes, CD., Builds e/e's Scramble Phone , Electronic Hobbyist, Spring-Summer 1973 , pp. 27-30, 119-120.

Twigg, T., Need to Keep Digital Data Secure? , Electronic Design 23, pp. 68-69,

Nov. 9, 1972.

Meyer, C.H., and Tuchman , W.L., Pseudorandom Codes Can Be Cracked , Electronic

Design 23, pp. 74-76, Nov. 9, 1972.

Geffe, Philip R. , How to Protect Data with Ciphers That are Really Hard To Break , Electronics, pp. 99-101, Jan. 4, 1973.

Kahn, D. , The Codebreakers , p. 776 (Macmillan Company, New York, 1967).

Shannon, C.E., Communication Theory of Secrecy Systems , Bell System Technical

Journal, V. 2_8, pp. 656-71 5; also Bell Monograph #172 7.

21
FORM NBS-114A (1-71) U.S. DEPT. OF comm. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA 1VTT, r, TT^ _ 0 ~ . No. sheet NBSIR 73- 324

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Voice Privacy Equipment for Law Enforcement

Communications Systems

5. Publication Date September 1973

6. Performing Organization Code

7. AUTHOR(S)

George R. Sugar

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/'Work Unit No. 2728414

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

National Institute of Law Enforcement and Criminal Justice

Law Enforcement Assistance Administration

U. So Department of Justice

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Law-enforcement agencies are finding increasing need for voice -scrambling equipment

to provide privacy on their two-way radio communication systems. Work is underway at

the National Bureau of Standards, under the sponsorship of the National Institute of

Law Enforcement and Criminal Justice (NILECJ), to develop performance standards for

, voice scramblers. The four areas being considered are intelligibility, voice quality,

privacy, and general system characteristics. This report first defines a set of special

terms, then describes a number of types of scramblers that are now available for law

enforcement use. Some inherent problems and weaknesses are discussed. The concepts

of intelligibility and privacy are explored in detail. The potential contents of the

standard are described, and some problems inherent in preparing and using the standard

are discussed „ The report concludes with some material intended to be of immediate

assistance to the prospective purchaser of scramblers: a survey of units now on the

market, some hints on how to proceed in the absence of a standard, and a bibliography

of technical publications on voice-scrambling.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Intelligibility; LESL; NILECJ; performance standard; privacy; scramblers; speech

quality; speech scramblers; survey; testing; voice privacy.

18. AVAILABILITY STATEMENT

I 1 UNLIMITED.

[XI FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

21. NO. OF PAGES

22. Price

UNCLASSIFIED

USCOMM-DC 66244-P71