APPARATUS FOR ENCIPHERING AND/OR DECIPHERING DATA SIGNALS

Abstract: Enciphering/deciphering apparatus having two pseudo-random bit generators (16, 28) which are cross-coupled in the enciphering mode by logic circuitry (10, 14, 18, 20, 22, 26). An input data signal is directed to the input of each bit generator by a pair of summing devices (18, 20). The data signal is also logically combined with the output of each bit generator by a second pair of summing devices (30, 32). In the deciphering mode the pseudo-random bit generators are connected in a feedback configuration by logic circuitry (12, 14, 18, 20, 24, 26). The to-be-decoded signal is directed to the input of each bit generator by the first pair of summing devices (18, 20). The to-be-decoded signal is also logically combined with the output of each bit generator by the second pair of summing devices (30, 32) to provide the decoded signal.

Description: APPARATUS FOR ENCIPHERING AND/OR DECIPHERING DATA SIGNALS Technical Field This invention relates to apparatus for en- ciphering and/or deciphering data signals. Background Art Data communication over unsecured communication links such as telephone lines is subject to detection and modification with a possible end result being the loss of a preferred position. In order to reduce the risk of a compromise of a transmitted data message, var¬ ious forms of enciphering the message have been devised. Once the data is enciphered, it is still not necessarily secure from unauthorized access. If the code used in the enciphering process is relatively simplistic, it is just a matter of trial and error before the code is broken and unauthorized access is gained. In order to be secure from deciphering attempts, which attempts now take advantage of the high power available in large- scale computers, it is necessary to make, for example, the trial and error process extremely long and uncom¬ plicated which in turn makes the attempt financially unsound. It is of course the ultimate goal to encipher data signals in such a manner that deciphering by un- authorized means is impossible even with an unlimited budget of time and computing power. In enciphering data signals, the use of one or more pseudo-random bit generators is known as a means for generating a cipher which appears random. U.S. Patent 3,911,216 discloses configurations of pseudo-random bit generators in enciphering apparatus wherein outputs from one bit generator are applied as inputs to another bit generator. The output from the bit generators is combined with a data signal to provide an enciphered data signal. Although such enciphered O PI data signal is difficult to decipher, for some applica¬ tions even greater security may be desirable. Disclosure of the Invention It is an object of the present invention to provide improved apparatus for enciphering and/or de¬ ciphering data signals wherein the risk of unauthorized deciphering of data signals is particularly low. The present invention provides apparatus for enciphering and deciphering data signals, including first and second pseudo-random bit generators, charac- terized by logic means operative in an enciphering mode for combining the output signal of each of said bit generators with a data input signal and for applying the combined signal to the input of the other one of said generators, and operative in a deciphering mode for combining the output signal of each of said bit gener¬ ators with an enciphered data input signal and for applying the combined signal to the input of the respec¬ tive bit generator, and combining means operative in each mode of operation for combining the relevant data input signal with the outputs of said bit generators to provide an enciphered data signal in the enciphering mode of operation or a deciphered data signal in the deciphering mode of operation. The present invention extends to apparatus for enciphering data signals, including first and second pseudo-random bit generators, characterized by means for combining the output signal of each of said bit gener¬ ators with a data input signal and for applying the combined signal to the input of the other one of said bit generators, and means for combining said data signal with the output signals of each of said bit generators to provide an enciphered data signal. The present invention also extends to apparatus for deciphering data signals, including first and second pseudo-random bit generators, characterized by means for ^ OMPI I? o combining the output signal of each of said bit gener¬ ators with an enciphered data input signal and for applying the combined signal to the input of. the res¬ pective bit generator, and means for combining said en- ciphered data input signal with the output signals of each of said bit generators to provide a deciphered data signal. By employing, in accordance with the invention, pseudo-random bit generators for enciphering data wherein the input to one generator is determined by the data input signal combined with the output of the other gen¬ erator, the resultant cipher is complex and extremely difficult to decipher by unauthorized persons. Since, in enciphering data signals, the output of each pseudo-random bit generator determines in part the input to the other generator, the generators can be said to be cross-coupled. In deciphering data signals, the output signal of each generator determines in part the input to the respective generator, and the gener- ators can be said to be connected in a feedback con¬ figuration. Preferably each of said pseudo-random bit generators of an apparatus in accordance with the in¬ vention is implemented to satisfy a polynomial, prefer- ably a primitive polynomial. The following two polynomials are examples of primitive polynomials which may be used: X 16 + X 12 + X 11 + X 9 + X 8 + X 4 + X 3 + X 2 + 1; X 16 + X 12 + X 3 + X + 1. An initializing key comprised of a series of arbitrarily-chosen bits having an arbitrarily-chosen length is directed to the cross-coupled pseudo-random bit generators in the enciphering mode prior to receipt of the clear text data signal. The bit generators respond to the bit sequence of the key by setting to an initializing state. The initialized state of the bit generators is also established prior to the receipt of an enciphered signal in order to ensure accurate deci¬ phering. The pseudo-random bit generators may comprise ' shift registers connected in a known feedback manner for generating pseudo-random bit signals, each register having an input, an output and a preset input for re¬ ceiving a preset signal to set the register to a present condition. Brief Description of the Drawing One embodiment of the invention will now be described with reference to the accompanying drawing which is a schematic logic diagram of apparatus for enciphering and deciphering data signals. Best Mode for Carrying Out the Invention Referring to the drawing, the apparatus shown therein includes an ENCIPHER terminal, so labeled, which is operatively connected to the gating input of AND gate 10 and of AND gate 22. A DECIPHER terminal, so labeled, is operatively connected to the gating input of AND gate 12 and of AND gate 24. A data input terminal, so labeled, is operatively connected by means of a summing junction 18 to the gated input of AND gate 12 and of AND gate 22 and by means of a summing junction 20 to the gated input of AND gate 24 and of AND gate 10. The outputs of AND gates 10 and 12 are connected to the inputs of an OR gate 14. The output of OR gate 14 is denoted as D'l, signalwise, and is coupled to the input of a pseudo-random bit sequence generator 16, herein re¬ ferred to as a PN generator. The outputs of AND gates 22 and 24 are operatively connected to the input of OR gate 26. The output of OR gate 26 is denoted as D'2, signalwise, and is coupled to the input of a PN gener¬ ator 28. The output of PN generator 16, denoted Rl. , is directed to an input of the summing junction 18 and to an input of a summing junction 32. The output of the PN generator 28 denoted R2, is directed to a summing input of the summing junction 20 and to a summing input of a summing junction 30. The summed output from sum¬ ming junction 30 is directed to a summing input of the summing junction 32. The output of the summing junction 32 is the apparatus output, denoted DO. The data input terminal labeled DI is operatively connected to a summing input of summing junctions 18, 20 and 30. In the pre¬ ferred embodiment of the invention, the PN generators 16 and 28 are each 16-bit shift registers which are connec¬ ted in the well-known feedback method to implement the following primitive polynomials: (Gen. 16) X 16 + X 12 + X 11 + X 9 + X 8 + X 4 + X 3 + X 2 + 1 (Gen. 28) X 16 + X 12 + X 3 + X + 1 The above polynomials receive more detailed attention in the publication entitled: "Error-Correcting Codes" by Peterson, MIT Press, copyright 1961, page 261. The summing junctions 18, 20, 30 and 32 may be EXCLUSIVE-OR „ gates which perform binary addition. In operation, the PN generators 16 and 28 are initialized to a predetermined state by the application of a preset signal to the preset input. An ENCIPHER mode select signal, for example a binary 1 level sig¬ nal, is applied to the ENCIPHER terminal and a signal of opposite level, i.e., a binary 0 is applied to the DECIPHER terminal. The binary 1 level signal on the inputs of AND gates 10 and 22 enable these gates. The binary 0 level signal on the inputs of AND gates 12 and 24 disables these gates. A sequence of bit signals, of arbitrary length, referred to as a key bit stream, is applied to the data input terminal. The data bits of the key bit stream are summed on a bit-by-bit basis with the output bits from a respective PN generator, by the respective summing junction 18 or 20. Identifying the signal on the data input terminal as DI, and the signal on the data output terminal as DO the following equations set forth the logic state of the signals into the PN generator 16 and 28 and the signals present at the output: ENCIPHER MODE DECIPHER MODE 1. D0=DIΦR1 1 *R2 1 1. D0=Dl€t>Rl 1 «*R2. 3. D , 2 =DIΦR1 1 3. D 1 =DI*R2 1 In the ENCIPHER mode the output bits Rl, from the PN generator 16 are summed with the clear text data signal Dl in the summing junction 18 and are directed as the input D* of the PN generator 28 via AND gate 22 and OR gate 26. In a like manner, the output bits R2, from the PN generator 28 are summed with the clear text data signal Dl in the summing junction 20 and directed as the. input D'l of the PN generator 16 via AND gate -10 and OR gate 14. The summing junction 30, in the ENCIPHER mode, combines the clear text bit stream with the pseudo-random bit sequence from PN generator 28, on a- bit-by-bit basis to provide a partially encoded bit sequence. Summing junction 32 combines the partially encoded bit sequence with the pseudo-random bit se¬ quence from PN generator 16, on a bit-by-bit basis to provide the completed enciphered signal, DO, to the data output terminal. To convert from the ENCIPHER mode to the DECIPHER mode it is first necessary to initialize the PN generators 16 and 28. This is accomplished by applying a preset signal to their PRESET inputs. During initialization an enabling signal is applied to the ENCIPHER input and a disabling signal is applied to the DECIPHER input. The key bit sequence is applied to the data input terminal to drive the PN generators, 16 and 28, to their predetermined states. To decipher an enciphered signal an enabling signal, logic level 1, is applied to the DECIPHER r i OMPI Vv'IPO - input terminal, for effecting the enablement of gates 12 and 24. A disabling signal, logic level 0, is applied to the ENCIPHER labeled input terminal for effecting the disablement of gates 10 and 22. With gates 10 and 22 disabled the cross- coupling connections of PN generators 16 and 18 are terminated. The enabling of gates 12 and 24 effec¬ tively connects the output of each PN generator to its respective input in a feedback configuration. The signal to be deciphered Dl is applied to the data input terminal. The inputs- to summing junc¬ tions 18, 20 and 30 each receive the to-be-deciphered signal. The to-be-deciphered signal is combined with the output bits present at the output of PN generators 16 and 28 and the combined signal is directed to the inputs of the respective PN generators; The signal at the input of each PN generator determines the next state of the PN generator output signal. The to-be-decoded signal is combined on a bit-by-bit basis first with the output of PN generator 28, in summing junction 30, the combined -signal then being combined with the output of PN generator 16. This combining results in a decoding of the enciphered signal to provide at the data output terminal the clear text data signal. It should be understood that since the signal to be deciphered Dl is equivalent to the signal D Q in the enciphering mode the feedback connections of PN generators 16, 28 act to ensure that, in the deci¬ phering mode, the same signals exist at the inputs of generators 16, 28 as exist in the enciphering mode, thereby causing correct decoding. Further encoding may be accomplished by con¬ necting in cascade the data output terminal of a first encoding and decoding apparatus to the data input of a second encoding and decoding apparatus. The same or a different key bit sequence could be utilized to initialize the PN generators of the first and the second encoding and decoding apparatus. It will be appreciated that the apparatus described above with reference to the accompanying draw¬ ing will encipher a string of data independent of the length of the string. Other features of the apparatus are that the next state of the apparatus is partially determined by the state of input data bits, that the key is destroyed after the data is enciphered and that the key may be any arbitrarily chosen bit stream. • ^ Oλ4P WIP

Claims: CLAIMS : 1. Apparatus for enciphering data signals, including first and second pseudo-random bit generators (16, 28), characterized by means (10, 14, 18, 20, 22, 26) for combining the output signal of each of said bit generators with a data input signal and for applying the combined signal to the input of the other one of said bit generators, and means (30, 32) for combining said data signal with the output signals of each of said bit generators to provide an enciphered data signal. 2. Apparatus for deciphering data signals, including first and second pseudo-random bit generators (16, 28), characterized by means (12, 14, 18, 20, 24, 26) for combining the output signal of each of said bit generators with an enciphered data input signal and for applying the combined signal to the input of the respec¬ tive bit generator, and means (30, 32) for combining said enciphered data input signal with the output signals of each of said bit generators to provide. a deciphered data signal. 3. Apparatus for enciphering, and deciphering data signals, including first and second pseudo-random bit generators (16, 28), characterized by logic means (10, 12, 14, 18, 20, 22, 24, 26) operative in an enci- phering mode for combining the output signal of each of said bit generators with a data input signal and for applying the combined signal to the input of the other one of said generators, and operative in a deciphering mode for combining the output signal of each of said bit generators with an enciphered data input signal and for applying the combined signal to the input of the respective bit generator, and combining means (30, 32) operative in each mode of operation for combining the relevant data input signal with the outputs of said bit 3. (concluded) generators to provide an enciphered data signal in the enciphering mode of operation, or a deciphered data signal in the deciphering mode of operation. 4. Apparatus according to claim 3, charac¬ terized in that said logic means includes first and second summing means (18, 20) having first inputs respectively connected to the outputs of said first and second bit generators (16, 28), second inputs for re¬ ceiving a data input signal, and outputs coupled to gating means (10, 12, 14, 22, 24, 26), said gating means being operative in said deciphering mode to connect the outputs of said first and second summing means (18, 20) to the inputs of said first and second bit generators respectively, and being operative in said enciphering mode to connect the outputs of said first and second summing means to the inputs of said second and first bit generators respectively. 5. Apparatus according to.claim 4, charac¬ terized in that said combining means (30, 32) includes third and fourth summing means, each of said third and fourth summing means having an input connected to the output of a respective one of said bit generators, said third summing means (30) having a second input connected to receive a data input signal, and a second input of said fourth summing means (32) being connected to the output of said third summing means to provide at the output of said fourth summing means the enciphered or deciphered data signal. 6. Apparatus according to claim 4, charac¬ terized in that said gating means includes first and second AND gates (10, 22) responsive to an enciphering mode signal for connecting the outputs of said first and second summing means (18, 20) to the inputs of said : 6. ( concluded ) second and first bit generators (28, 16) respectively, and third and fourth AND gates (12, 24) responsive to a deciphering mode signal for connecting the outputs of said first and second summing means (18, 20) to the in- puts of said first and second bit generators (16, 28) respectively, the outputs of said first and third AND gates (10, 12) being connected to the input of said first bit generator (16) via a first OR gate (14), and the outputs of said second and fourth AND gates (22, 24) being connected to the input of said second bit genera¬ tor (28) via a second OR gate (26). 7. Apparatus according to claim 5, charac¬ terized in that each of said summing means is an EX- CLUSIVE-OR gate. 8. Apparatus according to any one of the preceding claims, characterized in that each of said first and second bit generators (16, 28) is a shift register having an input, an output and a preset input for receiving a preset signal to set the register to a preset condition. OMPI

Inventors: OOSTERBAAN D, WILLIAMS G
Applicants: NCR CO, NCR CORP
Application year: 1980
Main classification: H04L9
IPC codes: H04L9