Note: Descriptions are shown in the official language in which they were submitted.
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VOICE ENCRYPTION AND DECRYPTION SYSTEM
3ACK~ROUND OF THE INVENTION
The present invention relates to an encryption and
decryption system which facilitates synchronization
and, more particularly, to an encryption and decryption
system for an analog signal.
For communications privacy, various encryption
and decryption systems have heretofore been developed.
Particularly, numerous techniques are now available for
encrypting and decrypting analog signals. While
communications privacy may be insured if encryption
or sarambling is applied to a signal to be transmitted
in one mode or another, it is a prerequisite for a
higher level of privacy to vary the scrambling parameter
with respect to time. In order that a receiver may
obtain the transmitted signal by decryption or
descrambling, it has to perform decryption timed to
the encryption which i~ varied at the transmitter
with respect to time. Therefore, the receiver is
required to set up synchronization with the transmitter.
Implementations heretofore proposed for the
synchronization in analog signal encryption and decryp-
tion include~ one described in the paper "A Voice
Scrambler for Mobile Communication", IEEE Transactions
on Vehicular Technology, Vol. VT-29, No. 1, pp. 81-86,
1980. In accordance with the system described in this
paper, a synchronization signal is constantly transmit-
ted from a transmitter to a receiver together with
desired analog information. Because a synchronization
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signal corresponds to the period of encryption, a higher level
of privacy is unattainable unless the encryption occurs with a
long period. This would consume a substantial period of time
for setting up synchronization while requiring an intricate
synchronizing circuitry.
SU~RY OF THE INVENTION
It is therefore an object of the present invention to
provide a voice encryption and decryption system which eliminates
the need for a synchronization signal and, thereby, a synchroniz-
ing circuit.
It is another object of the present invention to providea voice encryption and decryption system which consumes a minimum
of period of time for synchronization for enciphering.
It is ano~her object of the present invention to pro-
vide a generally improved voice encryption and decryption system.
The present invention provides an analog signal encryp-
tion and decryption system including a transmitter and a receiver,
said transmitter comprising: random number sequence signal
generator means for generating a random number sequence signal;
discrete value signal generator means for generating a discrete
value signal by processing the random number sequence signal;
scrambler means for generating a scrambled analog information
signal by scrambling an initial analog information signal to be
transmitted in response to the discrete value signal; encipher-
ing means for generating a discrete output signal by enciphering
the random number sequence signal; and multiplexer means for
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generating a composite signal to be transmitted by multiplexing
the scrambled analog information signal and the discrete output
signal, said receiver being adapted to descramble said analog
signal in response to the discrete signal and inversely to the
scrambling by the scrambler means, whereby said system i5
operable without a separate synchronizing signal.
In accordance with the present invention, a random
number sequence is generated and, in response
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to a discrete value produced from the sequence, an
analog signal to be transmitted is scrambled to produce
a first analog signal. The first analog signal and a
second discrete signal produced by enciphering the
random number sequence are combined, the composite
signal being sent out to a receiver. At the receiver,
a procedure opposite to that performed by the trans-
mitter for combining the first and second signals
occurs to separate the first analog signal and the
second discrete signal. Decryption opposite to the
encryption at the transmitter proceeds in response to
a discrete value which is produced from a signal
provided by deciphering the second discrete signal.
The above and other objects, features and
advantages of the present invention will become
apparent from the following detailed description
taken with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of an encrypting
device forming part of a voice encryption and decryp-
tion system embodying the present invention;
Figure 2 is a block diagram of a discrete number
generator included in the encrypting device shown in
Figure l;
Figure 3 is a block diagram of a decrypting device
forming the other part of the system of the present
inventiOn;
Figure 4 is a block diagram of a practical example
of an enciphering circuit shown in Figure 1 and a
deciphering circuit shown in Figure 3;
Figure 5 is a spectrum diagram demonstrating the
operation of a multiplexer shown in Figure 1.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
While the voice encryption and decryption system
of the present invention is susceptible of numerous
physical embodiments, depending upon the environment
and requirements of use, a substantial number of the
herein shown and described embodiment have made, tested
and used, and all have performed in an eminently
satisfactory manner.
Referring to Figure 1 of the drawings, an encrypt-
ing device forming part of the system of the presentinvention is shown. An analog signal supplied to an
input terminal 10 is applied to a scrambler 12 to be
scrambled thereby for secrecy. While various modes are
available for the scrambling operation, there may be
selected a system which divides an input voice signal
into a plurality of sub-bands and then varies the order
of the sub-bands. Scrambling with such a principle is
practicable using, for example, a scrambler disclosed
in U.S. Patent 4,278,840 (Morgan et al). Various
combinations may be contemplated for the variation of
the order of the sub-bands. Specific one of such
combinations is selected by a signal which is produced
by a discre*e number generator 16, which is connected
to an output terminal of a random number sequence
generator 18.
The discrete number generator 16 may be constructed
as shown in Figure 2 by way of example. As shown, a
random number sequence is supplied from the random
number sequence generator 18 to an input terminal 20
of the discrete number generator 16. The random
number sequence is sequentially entered into three
stages 24, 26 and 28 of a shift register. Assuming a
two-level signal as a random number sequence, eight
different combinations are available with the discrete
number generator 16 due to the use of three-stage
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shift register. In Figure 2, a conversion circuit 30
is adapted to generate a control signal at an output
terminal 32 thereof in response to data stored in the shift
register stages 24, 26 and 28, the control signal being
applied to the scrambler 12.
Referring again to Figure l, part of the output
signal of the random number sequence generator 18 is
applied to an enciphering circuit 34 which may operate
with the self-synchronized enciphering principle, as
shown in Figure 4. In Figure 4, a random number
sequence arrived at an input terminal 36 is applied to
an adder 38 which performs modulo N addition. Also
supplied to the adder 38 is an output signal of a
function generator 40 which will be described later.
The summation output of the adder 38 appears at an
output terminal 42. Simultaneously, part of the
summation output is sequentially routed to shift register
stages 44, 46 and 48. The function generator 40,
therefore, produces a signal which is unconditionally
determined by data stored in the shift register stages 44,
46 and 48. Detailed operation of the function generator
40 will not be described herein because it is well
known in conjunction with enciphering techniques and
the like.
Referring to Figure 3, there is shown a decrypting
device which forms the other part of the system of the
present invention. The decrypting device includes a
deciphering circuit 50 the operation of which will be
described with reference to Figure 4. In Figure 4,
a code output from the enciphering circuit 34 is
received at an input terminal 52 of the deciphering
circuit 50. The input to the circuit 50 is partly
applied sequentially to shift register stages 44', 46' and
48' while, at the same time, it is subtracted by an output
of a function generator 40' by a subtracter 38'. The
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function generator 40' is common in construction and
operation to the previously described function
generator 40. The output of the subtracter 38'
is the same signal as one which is applied to the
enciphering circuit 34. To facilitate understanding
of such operation, assume a two-level (ZERO and ONE)
signal system. Neglecting a signal delay for
convenience, the shift register stages 44, 46 an~ 48
of the enciphering circuit 34 and the shift register
stages 44', 46' and 48' of the deciphering circuit
50 store same values and, therefore, the outputs
of the function generators 40 and 40' are identical.
This implies that a ZERO or a ONE is added to and
subtracted from the signal which is applied to the
input terminal 36 of the enciphering circuit 34.
As a result, a signal identical with the input
signal to the enciphering circuit 34 appears at
an output terminal 54 of the deciphering circuit
50. Therefore, if an eavesdropper succeeds to know
an output signal of the enciphering circuit 34
, while the transmitter and receiver are in communica-
tion, the original signal does not become intelligible
to the eavesdropper unless the operation of the
deciphering circuit 50, particularly that of the
function generator 40' is known.
The enciphering circuit 34 and deciphering
circuit 50 discussed above are individually operable
with the prior art encrypting and decrypting
principle.
Returning to Figure 1, the output of the
scrambler 12 and that of the enciphering circuit
34 are combined together by a multiplexer 56. An
output of the multiplexer 56 is transmitted to the
receiver via an output terminal 58. The multiplexer
56 may be constructed to combine the two inputs
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while separating them with respect to frequency
axis, as shown in Figure 5. In Figure 5, a portion
a represents a power spectrum of a scrambled analog
signal and a portion _, a spectrum of a signal
modulated by a enciphered discrete signal. It should
be noted that the modulation is merely adapted to
separate the frequency band and may be replaced
by simple frequency translation. Another approach
to combining the two inputs is the separation with
respect to time axis~ The gist is that the combining
method allows the receiver to separate the two
signals from each other.
In the receiver shown in Figure 3, a demultiplexer
62 separates the signal coming in through an input
terminal 60 into the scrambled analog signal and the
discrete enciphered signal, which have been mixed
at the transmitter. Where separat1on with respect
to frequency axis shown in Figure 5 is employed
at the transmitter, the demultiplexer 62 may be
constructed using a low pass filter and a high
pass filter, for example. If modulation by the
discrete signal is employed at the transmitter as
described, the demultiplexer 62 is ascumed to perform
demodulation as well. The discrete enciphered signal
is deciphered by the previously mentioned deciphering
circuit 50 into the transmitted random number sequence.
The random number sequence is applied to a discrete
number generator 64 an output of which is routed to
a descrambler 66 as a control signal.
The discrete number generator 64 may operate in
the same manner as the discrete number generator 16
by way of example. Data stored in the shift register
stages 24, 26 and 28 are identical with those stored in
the register stages in the evPnt of scrambling at the
transmitter. What is required is, therefore, simply assigning
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numerical values to the discrete number generators
at the transmitter and receiver which cause the scrambled
data to be returned to the original data by descrambling
for common register data. The transmitted analog signal
S is applied to an output terminal 68 as an output of
the descrambler 66.
In summary, it will be seen that the present
invention provides a voice encryption and decryption
system ~hich eliminates the need for transmitting a
synchronization signal because a transmitter in the
system sends out discrete information used for scrambl-
ing it, while a receiver descrambles it with information
provided by deciphering the code. The omission of a
synchronization signal renders time heretofore required
for establishing synchronization and a synchronization
circuit needless.
Because the synchronization is needlessl any
desired random number sequence, even those derived
from physical noise, are usable to promote the use of
infinitely close keys.
Besides the shown and described method of encipher-
ing a discrete signal, a synchronization type encipher-
ing process is known in the art and such a process is
also applicable to the present invention. In the
synchronization type system, it is necessary to set up
synchronization for deciphering. Nevertheless, no
limitation is imposed on the random number sequence
for encryption and the number of keys is not changed.
Further, the time period required by the system of
the present invention for synchronization is shorter
than one required by the conventional system because
the input signal to an enciphering circuit is a random
number sequence, which shortens the period of
synchronization.
The random number generator in accordance with the
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present invention is not limited in any respect and
may even use a data signal to be transmitted or a
signal provided by enciphering the data signal. In
such a case, the deciphering circuit at the receiver
produces the incoming data signal so that data can
be trans~itted in parallel with a conversation. While
the prior art privaey system has transmitted a
synehronization signal in the portion b in Figure 5,
for example, wasting khat portion of the band, even
such a band is effeetively usable for data transmission
with the above-described design.
Various modifications will beeome possible for
those skilled in the art after receiving the teaehings
of the present disclosure without departing from the
scope thereof.
