CA1188739A - Analog voice privacy device for a secure communications system - Google Patents

Abstract

TITLE
ANALOG VOICE PRIVACY DEVICE FOR A
SECURE COMMUNICATIONS SYSTEM

INVENTOR
Brian Bryden ABSTRACT OF THE DISCLOSURE
The voice privacy device divides the voice waveform into equal segments and transmits the segments in an unnatural order. In order to synchronize the terminals and unscramble the transmitted segments, the transmitter also sends a binary random sequence which it has generated in a register and which is duplicated in the receivers once initiated by the received sequence. A day code determines the ports of the register that are to be connected to a key generator which generates key numbers for controlling the order in which segments are to be transmitted. Synchronization of the receivers can occur at any point in the transmission, allowing the voice privacy device to be used under adverse conditions on mobile radio systems, as well as the dial-up telephone network.

Description

'7~'13 Back~round_ _the Invention This invention is directed to an analog voice privacy device for a secure communicatlons system in which a message is scrambled, transmitted by radio or on a narrow band line, and descrambled at the receiver.
An analog voice privacy device consists of a voice scrambler which renders a spoken message unintelliglble, and a descrambler which reconstructs the message.
Many scrambling ~echnlques exist which attempt ~o render voice signals unintelligible. Among the most common, i9 time domain scrambling wherein the voice waveform is divided lnto segments which are generally vf equal length and then the segments are rearranged f or transmission in a quasi-random fashion. United States Patent 3,824,467 which issued on July 16, 19749 to R.C. French and United State~ Patent 4,160,123 which issued on July 39 1979 to G. Guanella et al, teach such ! systems wherein the voice segments of equal lengths are placed in stor-age and selected out to form an unintelligible signal. As each element is removed from storage, a new element takes its place.
In order to be abla to rearrange the elements lnto an intel-ligible voice signal at the receiver, both Guanella et al and French transmit an initial signal at the start of transmlssion to preset an arithmetic device in the receivers~ In these systems, if the initial signal is not received, the receivers cannot descramble the transmis-sion. Also, if fading occurs, and the signal is lost for a time, the receivers may not resynchronize until the beginning of ~he next transmission.
Summary of the Invention I~ is therefore an object of this invention to provlde a voice privacy device which is highly immune to unauthorlzed reception while maintaining a substan~ially continuous communication with author-ized terminals under adver~e transmlssion conditions.

Thls and other objects are achieved in a voice communication system having a number of terminals, wherein each terminal transmits and receives voice waveEorms which are scrambled using a sliding window time segment permutation technlque, wherein the voice wavefor~ is divided lnto segments and transmit~:ed in an unnatural order. I~e scrambler in each terminal includes a memory having a number of storage positions each capable of storing a voice segment such that the storage posltion, once selected by a key number, transmits its volce segment.
The key numbers associated w-lth segments left in ~he memory are sequenced, and the next segment in the voice waveform whLch i8 placed in memory is associated with the lowest key number. The scrambler also includes a sequ&nce generator which generates a binary sequence ln an n-position register for transmission to all termina~s. A day code circuit, when loaded with a day code, selects the outputs of a fraction o the n-positions in ~he register and applies them to a key generator which generates a sequence of key numbers for selecting the storage positions. The key generator may include a shaping circuit to increase ~he probability distribution of the larger key numbers. A monitor detects the ~ength of time each voice segment remains in the store so as to assure that all segments will be transmitted before a predeter-- mined maximum delay.
The trans~itter and receiver both generate identical binary sequences once the receiver has received a fixed number of error free binary sequence bits from the ~ransmitterO Once filled, the transmit-ter sequence register is run backward through a pre-generated sequence and then forward through the sequence to determine a history of key numbers before scrambling to preset the transmitter. Once the binary sequence is self-generating in the receiver, its register is also run backward to determine a history of key numbers. This occurs at any time that a receiver starts to receive transmissions, whether at ~he beginning or during a transmission by the transmitter~

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~ ny other ob~ects and aspects of the invention wlll be clear from the detailed description of the drawings.
Brief Description of the Drawings Figure 1 illustrates the segments into which the voice wave-form is divided;
Flgure 2 is a block diagram of the analog privacy device;
Figure 3 illustra~es tap selec~ion of ~he sequence generator;
Figure 4 illustrates segment probability of delay curves;
Figure 5 illus~rates the key number probability distributLon curve; and Figure 6 illustrates a time diagram Eor two-way communi-cation.
Detailed Description The scrambling technique used for scrambling the voice wave-form in the present invention is known as a sliding window time segment permutation.; me speech waveform is divided into segments, Sl,S~,S3, ..... , as illustrated schematically as A in figure 1, these segments are shown to be of equal duration, however, they may also be made to vary in dura~ion for more complex systems. Each consecutive segment is stored in a scrambler and after being held for varying lengths of time, the segments are transmitted in an unnatural order. The range between the mdnimum and maximum delay which can be imparted to each segment ~s known as the window which is set at some number of seg~ents in dura-tion. The window for segment S1 is labelled B, the window for seg~
ment S2 is labelled C, and so on. The windows B, C,... are all of the same duratlon, however, each subsequent window is shifted in time by the length of the segment.
In the receiver, the segments received ln the unnatural order are stored in a descrambler where an additional delay is added to each segment so that the delays of all segments are made equal, thereby reconstructing the sp~ech waveform~
In the preferred embodiment of the analog voice privacy '7~9 device in accordance with the present lnvention, the segments Sl,S2,S3, ..~ are all of the same duration, the window has a duration of 16 seg-men~s for a total delay oE from 1 ~o 17 segments since the initial seg-ment is used to reverse each segement so that it is transmitted back~
ward. It, therefore, cannot be transmitted immediately.
In the analog voice privacy device, in accordance with the present inven~ion, the segments in the scrambler are each delayed by an interval of time be~ore they are transmitted~ these intervals are pre-ferably random to prevent unauthorized descrambling. In the descram-bler, however, the segments must further be delayed so that ~he delays for all segments are made equal~ The receiver must therefore be syn-chronized with the transmitter and have the necessary information to descramble the scrambled speech. With the synchronlza~ion data, it can determine the random delay sequence for the segments. Figure 2 illus-trates the analog privacy device in accordance with the present inven-tion in which synchronization data is sent to receivers continuously throughout transmission to allow them to synchronize with the transmit-ter and the random delays of the segments are such that the recelvers with the synchronizaeion data can determine the rando~ delay sequence for the segmen~s.
In the privacy device transmitter 30, the clear speech is converted in an ~nalog ~o digital conver~er 31 and placed ~n a memory 32 under the control of con~roller 33. Controller 33 determines the order in which the speech segments are to be transmi~ted. The scram-bled speech segments are passed through a digital to analog converter 34 and the resul~ing analog signal is transmltted in a voice bandwidt`h to any receiver. A synchronization signal is se~t simultaneously with the scrambled voice signal using frequency multiplex. For example, the synchronization signal may be modulated by modulator 35 to be transmit~

ted in the bandwidth of 150-300 hz and the voice signal limited to the bandwidth 350-3100 hz.
The synchronization signal is generated by an n-stage sequence generator 36 which includes a shift register 37 that is initi-ally fLlled through switch 38 by the leas~ significant bit of the speech signal digltized in A/D 31. After register 37 is filled in this or any other manner, the input to the shift register is produced by an adder 39 which combines the contents of a number of binary storage ele-ments in the register 37. This sequence is fed back into the register 37 through switch 38 as well as to modulator 35 for transmission to the receiver 30'. In the present embocliment, register 37 has twenty-four binary positLorls, which, by adding two of the binary storage elements, provides a sequence that would repeat itself every 65 minutes.
A time period or day code selector 40 selects a number of the binary positions in register 37 and connects them to the key generator 41. In the present embodiment, six of the 24 binary positions are selected for a total number of possible day codes of:
24 x 23 ~ 22 x 21 x 20 x 19 = 96909120 An eight digit decimal number that is between 00000000 and ~6909119 inclusive will therefore select six unique tap positions TSl-TS6 from the 24 taps T1-T24 that are available. The labelling of the 2~ taps is shown in figure 3.
The key generator 41 receives the six outputs TS1... TS6 from the se~uence generator 36 and uses them to generate key signals which are used to eventually control memory 32 such that the order of trans mitted segments is randomized as much as possible. An ideal key gener-ator 41 from a statistical point of view, will generate key numbers such that the probability of a segment receiving any one of the delays of 1 to 16 would be equal. This probability distribution is sl1own as curve 43 on figure 4. This type of probability distribution will result in the most effective scramble. In any practical key generator, it is necessary to modify the key numbers on occasion to take into account the age of the segments which are stored in the transmitter 30.
This is not desirable in the present key generator because the age of t~

the segments stored ln the transmitter memory 32 i8 not directly known to the receLver 30'.
The key generator 41, shown in figure 2, may be any known type of standard key generator which has the deslred characteristics.
The key generator 41 should operate such that each of the possLble selections of day code which when applied to the input of the key generator 41 wiLl produce scrambled speech which is distinctly differ-ent from that produced by all of the other possible day code seLec-tions. It is particularly importarlt for ad~acent day codes, i.e, those differirlg by only one tap selection, to produce scrambles that have no statistical similaritles. The key generator 41 should be such that if scrambled, speech is descrambled by a brute force method such as cutting and rearranging a tape recording in order to derive the age of the transmitted segments to thereby decypher the text, it must still be impossible for the cryptanalyst to calculate the plain text and, thereby, the day code without an exhaustive search through all possible day codes. Such a key generator is described in the Federal Informa-tion Processing Standards Publication 46 dated January 1977.
In the 6 bit cypher text output from the key generator 41, each bit has an equal probability of 0 or 1. l~is cypher text is fed to a shaping or ~eighting circuit 42 which converts the cypher text to a 3 bit final key number from 0 to 7, which determines the segment of the speech in ~emory 32 that is to be transmitted. The final key num-ber is given a weighted uneven probability distribution which is greater for the higher key numbers. This assures that the older seg-ments are more likely to be transmitted before reaching the maximum allowable age.
In the present embodiment, the final key number is generated in circuit 42 by first splitting the cypher text into two 3-bit "dice"
and then adding the dice arithmetically to give a raw key number.

C5C4C3 2 lCo y 73~

Since the two dice may be numbers from 0 to 7 with a flat probability distribution, the raw key number may be from 0 to 14 with the probabil-ity distribution of a dice roll. The raw key numbers 0 to 14 would therefore have a probability out of 64 of 1, 2, 3, 4, 5, 6, 7, 8, 7, 6, 5, 4, 3, 2, and 1, respectively. The raw key number is then converted to a 3-bit tentative key number 0 to 7 by converting the raw key numbers 0 and 10 to 0, 1 and l1 ~o 19 2 and 12 to 2, 3 and 13 to 3, 4 and 14 to 4, 5 and 9 to 6, 6 and 8 to 7, and 7 to 5, reæul~ing in a ten~atlve key number 0-7 probability distribution curve 45 of 6, 6, 6, 6, 6, 8, 12, 14, shown in figure 5.
In the trar1smitter 30, the memory 32 stores a number of speech segments at one timeO These storage areas can be considered to be bins add~essed by the key numbers. In the present embodiments, eight storage areas or bins, B0 - B7, are utili~ed. The memory 32 receives the clear speech segments, these are sequenced into the bins B0 - B7. the key number 0-7 received from shaping circuit 42 by con~
~ 33 ff troller ~ determines the bin B0 to B7 from which a speech segmen~ is fed to the receiver 30'. After a segment is transmitted, the remaining segments are sequenced ~o as to remain ordered according to age and a new segment is placed in the first bin B0. In addition, the controller 33 monitors each speech segment from the time it enters the memory 32 until lt is transmitted keeping track of the age of each segment.
Since stored speech segments of lncreasing age have increasingly higher bin numbers, i~e. they are age ordered, the jump in probability of selection of bin 5 is evident in the distribution curve 46 of segment delays in figures 4, wherein the probability of delay starts to decrease. Nevertheless, the oldest segments in bins B6 and B7 need not be transmitted until they reach an advanced age, up to 16.
If the history of key numbers is such tha~ a speech segment is not naturally selected for transmission before reaching age 17 or more, the memory controller 33 intervenes and changes tha "ten~ative 73~

bin number generated by circuit 42 to the bLn number 7. This "Joker"
option is used on about 8.5% of "naturally" generated bin numbers. The type of shaping or weighting described abo~e is required such that the use by the controller 33 of the "Joker" option is minimized so that a controller in the receiver 30' is able to predict the age o~ each seg-ment as it is received.
The age of a speech segment stored :ln any particular storage bin may vary by as much as 270 ms, depending on the previous "history"
of ~ey numbers~ It is imyortant for continuous synchronization that this dependence on the history of key nl1mbers be limited, so that the descrambler may calculate the age of the segments it is receiving. The age of a segment stored in a particular bin is influenced by the prev-ious 90 ~or so) key number selections. ~lowever, the influence of key selections is most prominent due to the latest 30 (or so) key numbers.
The receiver 30' includes the same components as the trans-mitter 30. The modulated data signal is demodulated in demodulator 35' and is fed into register 37' of the sequence generator 36' through a switch 38'. Once the sequence generator 36' receives 34 error free bits of the transmitted data sequence, then sequence generator 36' is in synchronism and contlnues to generate an identical data stream to that of generator 36~ This generated data stream is continuously ~om-pared to the received data stream to determine if synchronization is being maintained. In the remainder of the receiver 30'~ the day code selector 40' wi-th the key generator 41' and the shaping circuit 42' will generate key numbers identical to those from shaping circuit 42, thus allowing the memory con~roller 33' to predict the age of the seg-ments as they are being received in memory 32', so that the speech seg-ments may each be further delayed by a time which when combined with the initial delay for the segment in transmitter 30, equalizes the 7~

overall delay of each of the segments resulting in clear speech at the output of the D/A converter 34'.
Tables 1 and 2 provide an example of the operation of the memories 32, 32', and the memory controllers 33, 331, as segments are read into memory 32 and read out of memory 32'- In table 1, the input segment, the segments contained ln the speech storage areas AGI in the memory 32, the key number K which controls the operation, the corre-sponding bin numbers for the segments in storage, the age of the seg-ments in storage, and the segment which is transmitted, are tabulated.
The steps of the opertion are as follows:
1) A key number K is generated, this starts the process which operates on the segments present in the memory.

2) The age of the segment in bin M1mber 7 is checkedl if it is 16 ie is transmitted~ if not, the segment in bin number K is transmitted.

3) Each bin number below the bln number K of the segment transmitted is incremented by 1. The bin ~ust vacated is et to 8.

4) The age of each segment remaining is incremented by 1~
S) A new input segment is read into the memory location vacated by the previous transmission.
As an example, assume key number 4 is generated. Bin 7 is checked, at this point bin 7 represents position G in the memory 32 where element 141 is stored with an age of 14. Since the age is not 16, bin 4 with element 150 is transmitted (see last column, line 2).
Also, in line 2, lt is seen that all of the bin numbers below 4 are incremented by 1, i.e. 1 to 2, 2 to 3, 3 to 4, and 0 to 1. Bin prev-~0 ~
- iously labelled 8 is n~t- labelled 0 and a new segment, 155, is posi-tioned in this location, i.e. B, in the memory. The bin previously labelled 4 is now labelled 8. At the same time, the age of all of the segments remaining in memory are incremented by 1 as can be seen by comparing line 2 to line 1. This complates the cycle. When a new key number, is generated the entire process is repeated. The letters D and M in front of the first column sign~fy default condition and segmant _g~

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with maximum allowable age transmitted, respectively.
In the receiver, for descrambling, table 2, in addition to the received segment, segments in memory, etc., the total age of the segments in memory i5 determined by noting the age of the segment at the time it is received, i,e. line 2, segment 150, position D, had an age of 5 when transmission started and an age of 6 when Eu11y received.
This age oE 6 is then incremented by 1 during each cycle until the age of 17 at line 13, when it ls fed out of memory. As seen in the decoded segment column, all of the segments are in clear speech order.
On the initiation of a transmission, the memory controller ~3 33, in the ~ransmitter 30' takes into account the key numbers that ~J would have been generated during the previous two seconds as if it had already been operating, i.e. had the shift register 37 been operating in internal feedback mode. The procedure is performed to set up the initial sequence in which digitized speech is to be transmitted. The ! receiver 30' performs an identlcal inltialization procedure to that performed by the transmitter 30 when it commences to generate its own binary sequence.
A key number needs to be calculated once per segment, i.e. at 30 ms intervals. However~ it is preferred that the shift regis~ers be clocked three times per segment (once every 10 ms) so as to speed up the synchronization procedure. In this case9 every third bit in the transmitted sequence may be ~nverted to inform the receiver 30' when to calculate key numbers. If left continuously operating, the random sequence will repeat itself only after 65 minutes.
When a receiver either looses synchronism with the transmit-ter during the transmission due to fading or other interrupting factor, or when a receiver is switched on during a transmission, ~ the receiver 30' will go through the synchronization procedure, which results in a short period of transmission being lost which does not occur when the transmitter and receiver synchronize on initial trans~
mission.

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me synchronization procedure includes the following sequence of steps:
1) 34 consecutive error-free random sequence bits are received from the transmitter 30 shift regisler 37 after which switch 38' is switched to feed its output to its input. Ihis step talces a minlmum of 340 ms.
2) Shift registers 37, 37' are run backward for 180 shifts of its sequence to predict the pre-generated sequence. This step takes

5 ms~
3) An arbitrary age value is assumed for all of the segments stored in the memory 32 by both the transmitter 30 and the receiver 30', the arbitrary age value need not be the same in both.
; 4) The shift registers 37, 37' are run forward to their current value.
The ages of the segments are updated as the registers 37, 37' are shifted would normally be done in controllers 33~ 33'. As the ~, ~ shift registers 379 37' get to their current value, the ages of the ; segments as they are transmitted will converge with the ages of segments as predic~ed in the receiver. This takes 60 ms.
5) After ~his ~nitial 405 ms, the received scrambled speech signal starts to be descrambled.
When only the receiver 30' follows ~his procedure3 the first hal of one second of received speech is lost.
In scrambling the speech signal by rearrangement of time segments of the speech in ~umbled order, individual segments must be shorter than a syllable. The range of time over which individual segments may be displaced must be sufflciently large to yield an unintelligible text, but on the other hand, not so large as to give an unnecessarily long transmission delay. In the present embodiment, the segment size was chosen as 30 ms, and the maximum time displacement as 480 ms.
~13-7~3~3 In the transmitter 30, speech segments are read out of digi~
tal memory 32 with a delay of n x 30 ms (n = 1 to 16). At the receiver 30', this delay is increased to 480 ms for each segment and an extra 30 ms is added because segments are transmitted backwards. The total system dependent delay time is, therefore, 510 ms.
The time diagram for two~way communication is shown in flgure , 6. With communication ini~iated at location-~, a synchronization pre-amble 480 ms is shown aEter which the message commences to be ~ransmit-ted. A system delay of 510 ms occurs be.Eore the clear speech message is delivered at the output at location ~. For a return message, from location 2 to 1, the synchronization preamble again takes up 480 ms and the system delay a further 510 ms before the clear speech message is delivered at ~he output of location ~.
Many modifications in the above described embodiments oE the lnvention can be carried out without departing from the scope thereof l: and~ therefore, the scope of the present invention is intended to be limited only by the appended claims.

Claims (11)

CLAIMS:

1. A privacy communications system having a number of terminals, wherein each terminal transmits and receives intelligence bearing signals wherein the intelligence of the said signals is disrupted before transmission by a scrambler which divides the said signals into segments of equal length and which then alters the original time sequence of said segments using a sliding window time segment permutation technique; the scrambler in each terminal comprising:
- memory means having a number of storage areas, each storage area being capable of storing a segment of the intelligence bearing signals;
- sequence generator means for generating a random binary sequence for transmission to all terminals along with the scrambled intelligence bearing signals - key generator means for generating a random sequence of key numbers from said random binary sequence wherein said key numbers are integers limited in range to the number of storage areas of said memory means allocated for segment storage, one said key number being generated during each of a plurality of segment intervals and wherein said key generator means includes:
(1) time period code means for ensuring the generation of a distinct key number sequence from said random binary sequence for each time period code selection, the number of possible time period code selections being quite large;
(2) weighting means for modifying the generation of the key numbers such that the probability distribution of said key numbers increases with larger key numbers;

controller means including:
(1) means for storing a predetermined age number assigned to each, segment of the said intelligence bearing signals as it is stored in said memory means, said predetermined age number being the integer 1 indicating that the segment to which it is assigned has just been stored in said memory means;
(2) means for incrementing the value of each age number during each successive segment interval such that the instantaneous value of each age number indicates the number of segment intervals that its associated segment has been in storage in said memory means;
(3) means for assigning and storing a bin number for each stored segment of the said intelligence bearing signals during each successive segment interval, said bin numbers being integers selected and assigned such as to represent the order of the said stored segments according to their current respective age numbers;
(4) means for removing said stored segment from said memory means for transmission during each segment interval, the segment selected during any given segment interval being the segment whose corresponding bin number compares to the key number obtained from said key generator means for that given segment interval;
(5) means for overriding said key number obtained front said key generator means when any age number associated to the said stored segments has reached a predetermined maximum age value, the stored segment whose age number has reached said maximum age value being selected by default for transmission.

2. A privacy communications system as defined in claim 1 wherein said sequence generator means includes an n-position register.

3. A privacy communications system as defined in claim 2 wherein said time period code means includes means for selecting the outputs of a fraction of the n positions of said n position register of said sequence generator means in accordance with a preselected time period code.

4. A privacy communications system as defined in claim 3 including means for presetting said scrambler prior to each message transmission, said preset means comprising:
- means for erasing the contents of every segment storage area of said memory means;
- means for loading said n-position register of said sequence generator means with a non-zero initial state;
- means for clocking said sequence generator means, once loaded with a proper initial state, backward at higher than normal operating speed for a fixed predetermined number of steps;
- means for assigning an arbitrary initial age number to each segment storage area of said memory means, each age number being a distinct integer less than or equal to said maximum age value;
- means for clocking said sequence generator, key generator means and controller means forward at higher than normal operating speed until said sequence generator means reaches said initial state, all segment storage or retrieval being inhibited during said accelerated clocking.

5. A privacy communications system as defined in claim 4 wherein the said fixed predetermined number of steps is sufficient to ensure convergence to the same steady state sequence of age number vectors regardless of the arbitrary initial age number vector used by said preset means, said fixed predetermined number of steps being a function of the number of default selections made by said controller means which in turn depends on the probability distribution of the said key numbers.

6. A privacy communications system having a number of terminals wherein each terminal transmits and receives intelligence bearing signals wherein the intelligence of the said signals is disrupted before transmission by a scrambler which divides the said signals into segments of equal length and which then alters the original time sequence of said segments using a sliding window time segment:
permutation technique and wherein the intelligence of the said signals is reestablished after reception by a descrambler which equalizes the overall delay sustained by each segment; the descrambler in each terminal comprising:
- memory means having a number of storage areas, each storage area being capable of storing a segment of the scrambled intelligence bearing signals once received;
- sequence generator means for generating a random binary sequence identical to a random binary sequence received along with said scrambled intelligence bearing signals;
- key generator means for generating a random sequence of key numbers from said random binary sequence generated by said sequence generator means wherein said key numbers are integers identical in sequence and value to key numbers generated in said scrambler, one said key number being generated during each of a plurality of segment intervals and wherein said key generator includes:
(1) time period code means for ensuring the generation of a distinct key number sequence from said random binary sequence for each time period code selection, said time period code selection being identical to time period code selections in said scrambler;
(2) weighting means for modifying the generation of the, key numbers such that the probability distribution of said key numbers increases with larger key numbers as in said scrambler;
- controller means including:
(1) means for storing an age number corresponding to each segment of intelligence bearing signals stored in said scrambler from which the said scrambled intelligence bearing signals originate;
(2) means for assigning and storing a bin number corresponding to each stored segment in said scrambler during each successive segment interval, said bin numbers being integers selected and assigned such as to represent the order of the said stored segments according to their current respective age numbers;
(3) means for assigning and storing an age number for each segment of said scrambled intelligence bearing signals as it is stored in said memory means, said age number being an integer set to the same value as the age number corresponding to the segment stored in said scrambler whose bin number matches the key number obtained from said key generator means for that given segment interval;
(4) means for overriding said key number obtained from said key generator means when any age number corresponding to the segments stored in said scrambler has reached a predetermined maximum age value, the age number assigned to the segment of the said scrambled intelligence bearing signal being stored in said memory means being set by default to said maximum age value;
(5) means for incrementing the value of each age number during each successive segment interval such that the instantaneous value of each age number indicates the overall number of segment intervals that its associated segment has been stored since entering said scrambler;
(6) means for removing during each segment interval a segment of the said scrambled intelligence bearing signals stored in said memory means, the segment selected during any segment interval being the segment whose age number has reached the said maximum age value.

7. A privacy communications system as defined in claim 6 wherein said sequence generator means includes an n-position register.

8. A privacy communications system as defined in claim 7 wherein said time period code means includes means for selecting the outputs of a fraction of the n positions of said n-position register of said sequence generator means in accordance with a preselected time period code as in said scrambler.

9. A privacy communications system as defined in claim 8 including synchronization acquisition means comprising:
- means responsive to said random binary sequence received along with said scrambled intelligence bearing signals and by which said received random binary sequence is gated into said n-position register of said sequence generator means;
- means for detecting when the said n-position register of said sequence generator means is fully loaded and means for switching said sequence generator means to a self-generating mode;
- means for comparing said received random binary sequence with said random binary sequence generated by said sequence generator means operating in the self-generating mode to ensure that proper acquisition of the said received random binary sequence has been achieved;

- means for detecting the number of discrepancies between said received random binary sequence and said generated random binary sequence and means for reinitiating acquisition of said received random binary sequence when said discrepancies exceeds a predetermined percentage.

10. A privacy communications system as defined in claim 9 including means for presetting said descrambler once synchronization with the said received random binary sequence is achieved, said preset means comprising:
- means for erasing the contents of every segment storage areas of said memory means;
- means for clocking said sequence generator means backward at higher than normal operating speed for a fixed predetermined member of steps;
- means for storing an arbitrary initial age number corresponding to each segment storage area of said scrambler, each age number being a distinct integer less than or equal to said maximum age value;
- means for clocking said sequence generator means, key generator means and controller means forward at higher than normal operating speed until said sequence generator means is again in step with said received random binary sequence, all segment storage and retrieval being inhibited during said accelerated clocking.

11. A privacy communications system as defined in claim 10 wherein the said fixed predetermined number of steps is sufficient to ensure convergence to the same steady state sequence of age number vectors regardless of the arbitrary initial age number vector used by said preset means and wherein said convergence property enables said descrambler to decode said scrambled intelligence bearing signals starting from any point during the course of a given message transmission.