United States Patent [191
`Bremer et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,924,516
`May 8, 1990
`
`[54] METHOD AND SYSTEM FOR A
`SYNCHRONIZED PSEUDO-RANDOM
`PRIVACY MODEM
`[75] Inventors:
`Gordon Bremer, Clearwater; William
`L. Betts, St. Petersburg, both of Fla.
`[73] Assignee: AT&T Paradyne, Largo, Fla.
`[21] Appl. No.: 356,080
`[22] Filed:
`May 23, 1989
`
`........ .. H04L 9/22
`Int. c1.5
`[51]
`[52] US. Cl. ........................................ .. 380/46; 380/9;
`380/48
`[58] Field of Search .............................. .. 380/9, 48, 46
`[56]
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,752,953 7/1988 Paik et al. ............................. .. 380/9
`4,837,821 6/1989 Kage ................................... .. 380/48
`Primary Examiner—Salvatore Cangialosi
`Attorney, Agent, or Firm-Kane, Dalsimer, Sullivan,
`Kurucz, Levy, Eisele & Richard
`
`ABSTRACT
`[57]
`A method and system for a synchronized pseudoran
`dom privacy modem makes use of a pseudorandom
`signal generator to index into a table of N signal spaces
`thereby altering gain and phase modi?ers so or to mod
`ify both the gain and phase of portions of the data signal
`being transmitted from a QAM modem and thus trans
`mit an encrypted signal pattern to the other end of the
`communications channel. The modem transmitter cir
`cuit includes a data scrambler, and QAM signal point .
`mapper, connected in series, as well as a pair of mixers
`having inputs from the pseudorandom signal generator
`and functioning to modify the gain and phase of por
`tions of the QAM signal point mapper output signal
`before they are pulse amplitude modulated, ?ltered,
`converted to analog form, and transmitted over the
`communications channel. Complementary circuit ele
`ments in the receiver, which have been synchronized
`with those of the transmitter by baud rate counters at
`both locations, demodulate and decode the received
`encrypted signal.
`
`9 Claims, 2 Drawing Sheets
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`Page 1 of 6
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`US. Patent May 8,1990
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`Sheet 1 of 2
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`Page 3 of 6
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`

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`1
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`4,924,516
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`METHOD AND SYSTEM FOR A SYNCHRONIZED
`PSEUDO-RANDOM PRIVACY MODEM
`
`2
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic of the transmitter circuit used to
`facilitate the method of the present invention.
`FIG. 2a shows a conventional signal space for a
`nonencrypted data signal.
`FIG. 2b shows a data signal which has undergone
`rotational modi?cation by having pseudorandom
`changes applied to the phase of various signal points.
`FIG. 2c shows a signal space in which various signal
`points have been pseudorandomly modi?ed by adjust
`ments in their gain.
`FIG. 2d shows a signal which has been modi?ed by
`pseudorandom changes in'both the gain and phase char
`acteristics of at least some of its signal points.
`FIG. 3 is a schematic showing details of the pseudo
`random generator of FIG. 1.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIG. 1 shows a data input from a source such as a
`DTE (Data Terminal Equipment) to data scrambler 2.
`The output of data scrambler 2 is connected to the input
`of quadrature amplitude modulated signal space genera—
`tor 3. Signal space generator 3 includes a QAM signal
`point mapper 4, a gain modi?er 5 and a phase modi?er
`6. The gain modi?er 5 and the phase modi?er 6 are
`responsive to the outputs of gain selector/ phase selector
`7. Gain selector/phase selector 7 includes table in mem
`ory of N gains and N phases, each corresponding to one
`of N possible signal spaces. USA Pseudorandom Gener
`ator 8 generates pseudorandom number between 1 and
`N each symbol time. These pseudorandom numbers are
`transmitter to gain selector/phase selector 7 which was
`the pseudorandom numbers as an index into the table in
`memory of N signal spaces so as to generate the outputs
`of selector 7. After the signal has been encrypted by
`modi?ers 5 and 6, the signal is conveyed to module 9
`wherein it is pulse modulated, ?ltered and modulated by
`quadrature carriers. Finally, the signal is changed to
`analog form by converter 10 and is conveyed to com
`munications channel 11 for transmission to a receiver
`having complementary demodulation modi?ers to de
`code the encrypted signal.
`A typical modem, which would include all of the
`elements of FIG. 1 except elements 5, 6, 7 and 8, has
`several ?xed transmit functions which manipulate data
`prior to analog transmission. Complementary ?xed re
`ceiver functions are provided to convert this analog
`transmission into the original (error-free) digital data.
`However, in the present invention, by using USA gen
`erator 8 to provide pseudorandom time varying
`changes to some of the above transmit functions, it is
`possible to make data reception dif?cult or impossible
`unless the identical, synchronized complementary
`changes are provided in the receiver.
`It is the intent of this invention to identify functions
`which can be time varied, to provide a method for time
`varying these functions, and to provide methods for
`synchronizing the time varying changes. The method of
`the present invention also functions to identify the se
`lected ?xed function modi?ers.
`The privacy modem of the present invention will
`accept a privacy key consisting of P-bits where P is long
`enough to discourage eavesdropping events by trial and
`error method. A value of P=56 is used because this is
`the length of the DES (Data Encryption Standard) key.
`The bits of the privacy key will reside in a signal proces
`
`5
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`10
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`BACKGROUND OF THE INVENTION
`1. Field Of The Invention
`The present invention relates to a method for pseudo
`randomly manipulating data prior to its analog trans
`mission over a communications channel such as a tele
`phone line so as to provide secure communications
`between the ends of the communications channel.
`2. Description Of The Prior Art
`In general, modems have ?xed modulation and data
`handling methods so that every modern of a given type
`can communicate with all others of that type. When
`encryption has been needed to protect the privacy of
`communications, the prior art has generally accom
`plished the encryption by providing circuit functions
`implemented by hardware and software which is sepa
`rate from the modem function.
`Some “privacy” modems exist in the prior art, but it
`is doubtful if any contain powerful, pseudorandom,
`self-synchronizing modulation modi?ers such as are
`presented herein.
`OBJECTIVES AND SUMMARY OF THE
`INVENTION
`‘
`It is an object of the invention to provide pseudoran
`dom time varying changes to some of the several ?xed
`transmit functions of a typical modem so as to manipu
`late the data prior to analog transmission and to make
`data reception dif?cult or impossible unless the identi
`cal, synchronized, complementary changes are pro
`vided in the receiver.
`_
`it is further object of this invention to identify func
`tions which can be time varied, provide a method for
`time varying, and provide methods for synchronizing
`these changes.
`These and other objects and advantages of the inven
`tion will be made clear by the following description.
`The method of the present invention comprises passing
`a digital data input signal through conventional modem
`elements comprising a data scrambler, and quadrature
`45
`amplitude modulation (QAM) signal point mapper. At
`this point, in an ordinary modem, the outputs from the
`QAM signal point mapper would be conveyed to pulse
`amplitude modulation (PAM) ?lters, the outputs of
`which would modulate quadrature carriers which
`would then be combined to produce a QAM signal and
`then transmitted in analog form on the communications
`channel. However, in the method of the present inven
`tion, the signal from the QAM signal point mapper is
`encrypted by having various portions of the signal
`changed pseudorandomly in gain and/or phase by at
`least one USA (Unobtrusive Signature Analysis) pseu
`dorandom generator operating through a gain selector
`and phase selector module. The receiver at the other
`end of the communications channel must, of course, be
`equipped to perform changes on the received signal
`which are complementary to those produced by the
`USA pseudorandom generator(s) in order to correctly
`decode the encrypted data signal. The effect of the
`invention is to pseudorandomly select a QAM signal
`space for each data symbol from a limited set of signal
`spaces.
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`sor memory and groups of these bits will be utilized to
`FIG. 2d shows a signal pattern which at least some
`determine various ?xed and time varying signal pro
`points of a conventional signal pattern have been varied
`cessing modi?er functions.
`in both gain and phase due to signals generated by sig
`Further enhancement of the security of the method of
`nals emitted from generator 8 and conveyed through
`the present invention can be obtained by using two
`gain selector and phase selector module 7 to the respec
`privacy keys, one for each direction of communication
`tive mixers 5 and 6.
`of the channel.
`The pseudorandom pattern generator 8 disclosed in
`Certain bits of the privacy key can be used to select
`FIG. 1 is implemented for the purpose of time varying
`?xed parameter modi?ers. These include, but are not
`certain modulation and demodulation parameters. The
`limited to the following transmit functions. The com
`characteristics of the pseudorandom pattern generator 8
`plementary receive functions are also selected by bits
`are determined by a T-bits word (“SEED”) which is
`from the privacy key. The following are ?xed parame
`programmed into a random access memory (RAM) in
`generator 8. Generator 8 runs at the modem baud rate
`" or faster to vary the modulation parameters, the pattern
`being the function of the aforementioned “SEED”
`15
`word. A Controller loads the “SEED” word into the
`RAM in the USA generator 8. The Controller can self
`generate a new “SEED” or increment the previous
`“SEED” to vary the pattern (to be non-user interrup
`tive, these changes need to be synchronized at the end
`of the communication link).
`A fundamental requirement for the method and sys
`tem of the present invention is the establishment of an
`absolute timing synchronization between the transmit
`ter and the remote receiver. This is established using the
`unobtrusive signature method mentioned with regard to
`the aforementioned US. application Ser. No. 083,696,
`which is extended to long elaborate pseudorandom
`patterns.
`At the end of the transmit training sequence, the
`generator 8 and associated baud counter are started.
`The remote receiver of the system, upon detecting the
`end of receiver training, starts its complementary pseu
`dorandom generator and baud counter. Thus, both the
`transmitterand the receiver then have identical pseudo
`random generating signals and baud counts available.
`Loss of synchronization and/or baud count will cause a
`loss of receiver demodulation and institute a round
`robin retraining sequence which will reestablish syn
`chronization.
`The pseudorandom generator 8 may be a feedback
`shift register, multiple register, or a counter. In any of
`these embodiments, the con?guration and starting pat
`terns are function of the aforementioned “SEED”
`word. “SEED” needs to have a minimum of 24 bits. As
`stated previously, the output of generator 8 is used to
`rotate the transmitter signal space. The choice of rota
`tion may be binary or multiphased. This rotation can be
`viewed as pseudorandom selection of two possible sig
`nal cases (the binary case) or many possible signal pha
`ses (the multiphase case) or many possible signal phases
`(the multiphase case). The degree of rotation needs to
`be large enough to cause bit “errors” in the receiver as
`well as marginal decision region “errors” to assure
`scrambled receiver data for each baud.
`In the method of the present invention the baud
`counter will output a periodic pulse to request a new
`“SEED”. The Controller will use this as a noti?cation
`to either (1) send a binary information bit back to the
`generator 8 or (2) send a whole new “SEED” to the
`generator 8. At the next “SEED” request the USA will
`increment its internal “SEED” word. Since the baud
`counters are synchronized at the transmitter and remote
`receiver, no loss of user data will occur.
`FIG. 3 shows details of an implementation of the
`above mentioned function including details of pseudo
`random generator 8 wherein it is shown that the genera
`tor comprises a transmit module 20 which outputs gain
`
`ter modi?ers:
`‘
`1. Scrambler
`a. Self-sync or non-self-sync
`b. Register Length
`c. Con?guration
`d. Bits scrambled
`e. Speed
`f. Starting vector
`g. Multiple scrambling at different rates
`2. Bit to signal space assignment mapper
`In addition to the above mentioned ?xed parameter
`modi?ers, the method of the present invention contem
`plates a much more sophisticated security con?guration
`which is provided by including time varying modi?ers.
`The time varying modi?cation can affect the ?xed pa
`rameters above. However, it creates a much more se
`cure system by providing pseudorandom signal space
`rotation and amplitude changes. This method is feasible
`due to the “Unobtrusive Signature For Modulated Sig
`nals” method invented by Betts and Martinez and de
`scribed in US. patent application Ser. No. 083,696
`which was filed on Aug. 7, 1987.
`The use of combinations of the above mentioned
`modi?er techniques makes it not only possible to pro
`vide tandem nonlinear digital encryption, but also pre
`vents improperly keyed modem receiver demodulation
`by causing the receiver to totally interrupt data during
`attempts to coordinate the transmitter and receiver
`(receiver retraining).
`The pseudorandom generator 8 shown in FIG. 1
`makes use of unobtrusive signature analysis (USA) to
`establish an arbitrarily long transmitter/receiver syn
`chronization pseudorandom sequence. Loss of this syn
`chronization totally destroys the ability of the receiver
`to demodulate data. Thus, if the receiver is not properly
`keyed, the receiver will present totally encrypted and
`interrupted data as well as interrupted control signal
`ling.
`The USA generator 8 of FIG. 1 is used to make ran
`dom phase changes of the signal space up to i180°.
`These changes are quantized by an amount such that
`invalid signal points are received for a phase change
`other than 0°. The USA generator is also used to make
`random gain changes.
`'
`FIG. 2a shows a “normal” signal pattern which
`might be generated by a conventional modem having all
`of the elements of FIG. 1 except elements 5, 6, 7 and 8.
`FIG. 2b shows a signal pattern which has been oper~
`ated on by USA to the random generator 8 through the
`phase selector of gain selector and phase module 7 to
`rotate a conventional signal pattern in mixer 6.
`FIG. 20 shows a signal pattern wherein pseudoran
`dom generator 8 has applied modi?cation to the gain of
`65
`a various points of a conventional signal pattern
`through the gain selector portion of module 7 and mixer
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`and rotate signals to the rest of the modem circuitry, a
`transmitter baud counter 21 which is synchronized with
`the transmitter module 20 at the end of the training
`period, and a D-Q ?ip-?op 22 connected to an output of
`the baud counter and having a periodic output to ran
`dom access memory 12 which stores a “SEED” Word
`for transmission to the transmitter module 20 at the next
`pulse from baud counter 21.
`'
`Also shown in FIG. 3 is the receiver baud counter 13
`which is synchronized with both the initial transmit
`ter/receiver “SEED” word and with “SEED” incre
`ments.
`FIG. 3 also shows a Controller 15 having a memory
`module 16 which stores a “SEED” word therein for
`transmission to generator 8. Memory module 16 may
`have an input from a key expansion and transmitter
`formation module 17 which itself has an optional input
`of a “SEED” from a Controller or from a front panel
`module 18. Also on the front panel is a master key input
`which can be conducted to another terminal of module
`17. The DCP 15 transmits an initial transmitter/ receiver
`seed signal to the RAM 12 in generator 8 and also trans
`mits subsequent transmitter/receiver SEED increment
`as signals to flip-?op 22.
`Although several preferred embodiments have been
`disclosed and described in detail herein, it should be
`understood that this invention is in no sense limited
`thereby and its scope is to be determined by that of the
`appended claims.
`What is claimed is:
`1. A method for a synchronized pseudorandom pri
`vacy modem comprising the steps of:
`35
`storing a table with parameters corresponding to N
`signal spaces;
`generating a string of pseudorandom numbers;
`indexing into said table with said string of pseudoran
`dom numbers thereby generating a string of param
`eters corresponding to said N signal spaces;
`quadrature amplitude modulating a digital signal;
`altering the quadrature amplitude modulated digital
`signal responsive to said string of parameters corre
`sponding to said N signal spaces;
`converting the altered quadrature amplitude modu
`lated digital signal to an analog signal; and
`transmitting the analog signal over a communication
`channel.
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`4,924,516
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`2. The method of claim 1 wherein said altering step
`includes altering a gain of the quadrature amplitude
`modulated digital signal.
`3. The method of claim 1 wherein said altering step
`includes altering a phase of the quadrature amplitude
`modulated digital signal.
`4. The method of claim 1 wherein said altering step
`includes altering a gain and a phase of the quadrature
`amplitude modulated digital signal.
`5. A system for providing secure data transmission
`over a communication channel comprising:
`means for storing a table of parameters corresponding
`to N signal spaces;
`means for generating a string of pseudorandom num
`bers;
`means for indexing into said table with said string of
`pseudorandom numbers thereby generating a
`string of parameters corresponding to said N sig
`nal;
`means for quadrature amplitude modulating a digital
`signal;
`means for altering the quadrature amplitude modu
`lated digital signal responsive to said string of pa
`rameters corresponding to said N signal spaces;
`means for converting the altered quadrature ampli
`tude modulated digital signal to an analog signal;
`and
`means for transmitting the analog signal over a com
`munications channel.
`6. The system of claim 5 further comprising a data
`scrambler having at least one input to said quadrature
`amplitude modulating means.
`7. The system of claim 5 further comprising a phase
`selector module connected to an output of said pseudo
`random generating, means and a ?rst mixer connected
`to an output of said phase selector module and to an
`output of said quadrature amplitude modulating means.
`8. The system of claim 6 further comprising a gain
`selector in said phase selector module which has an
`input from said pseudorandom generating, means and
`further comprising a second mixer connected to an
`output of said gain selector and to said output of said
`quadrature amplitude modulating means.
`9. The system of claim 7 further comprising a pulse
`amplitude modulating and ?ltering module connected
`to outputs of said ?rst mixer, and a digital to analog
`converter connected to an output of said pulse ampli
`tude modulating and ?ltering module and to said com
`munications channel.
`* * *
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