Abstract:
A signal transmitted from a first device to a second device is encrypted using an encryption key generated from a preceding part of the signal itself. The signal is decrypted in the second device using a decryption key generated from a preceding part of the received and decrypted signal. This encryption method provides effective privacy protection, because the encryption and decryption keys are constantly changing. Since the transmitted signal provides its own encryption and decryption keys, the method is inexpensive to implement, and can be used in systems such as packet transmission systems that transmit signals intermittently.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a system that encrypts and decrypts signals, and transmits and receives the encrypted signals, more particularly to a system, such as a packet transmission system, in which the encrypted signals are transmitted intermittently.  
         [0002]     One well-known type of encrypted transmission system has the general structure shown in  FIG. 8 , comprising a first device  220  and a second device  280  linked by a communication channel  300 . The communication channel  300  may be a wireline link comprising copper wire, optical fibers, or the like, or a wireless link comprising radio waves, infrared beams, or the like.  
         [0003]     The first device  220  has a transmitting section and a receiving section. The transmitting section includes a scrambling circuit  221  that scrambles a transmit signal A to obtain a scrambled signal CA, a pseudo-random pattern generating circuit  222  that supplies a pseudo-random pattern RAl to the scrambling circuit  221 , and a transmitting circuit  223  that transmits a modulated signal MCA, modulated by the scrambled signal CA. The receiving section includes a receiving circuit  230  that receives and demodulates a modulated signal MCB to obtain a scrambled signal CB, a descrambling circuit  231  that descrambles the scrambled signal CB to obtain a receive signal B, and a pseudo-random pattern generating circuit  232  that supplies a pseudo-random pattern RA 2  to the descrambling circuit  231 .  
         [0004]     The second device  280  also has a transmitting section and a receiving section. The transmitting section includes a scrambling circuit  281  that scrambles a transmit signal B to obtain a scrambled signal CB, a pseudo-random pattern generating circuit  282  that supplies pseudo-random pattern RA 2  to the scrambling circuit  281 , and a transmitting circuit  283  that transmits the modulated signal MCB, which is modulated by the scrambled signal CB. The receiving section includes a receiving circuit  290  that receives and demodulates the modulated signal MCA to obtain a scrambled signal CA, a descrambling circuit  291  that descrambles the scrambled signal CA to obtain a receive signal A, and a pseudo-random pattern generating circuit  292  that supplies pseudo-random pattern RAl to the descrambling circuit  291 .  
         [0005]     When signal A is transmitted from the first device  220  to the second device  280 , the scrambling circuit  221  uses the pseudo-random pattern RAl supplied by the pseudo-random pattern generating circuit  222  to alter the contents of signal A in a seemingly random fashion, typically by taking the exclusive logical OR of corresponding bits of A and RA 1 . As a result, if the modulated signal MCA is intercepted by a third party, the intercepted signal is unintelligible. The descrambling circuit  291  uses the same pseudo-random pattern RA 1 , supplied by the pseudo-random pattern generating circuit  292 , to perform the reverse alteration on the scrambled signal CA (typically by performing another exclusive logical OR operation), thereby obtaining the original signal A.  
         [0006]     When signal B is transmitted from the second device  280  to the first device  220 , it is similarly scrambled and descrambled, using pseudo-random pattern RA 2 , which may differ from pseudo-random pattern RA 1 .  
         [0007]     In the system in  FIG. 8 , the pseudo-random patterns RA 1 , RA 2  are hard-wired into the pseudo-random pattern generating circuits, which are typically manufactured in large quantities. Moreover, the pseudo-random patterns are of finite length, and repeat cyclically. Under these conditions, it is difficult to ensure that an intercepted transmission cannot be descrambled by the intercepting party, who may be in possession of equipment with a similar pseudo-random pattern generating circuit. The only defense is to use a very long pseudo-random pattern, but this requires a comparatively large and therefore expensive pseudo-random pattern generating circuit, and leads to difficulties in maintaining synchronization between the pseudo-random patterns generated in the first and second devices  220 ,  280 .  
         [0008]     A known solution to these problems is given in Japanese Unexamined Patent Application No. 05-007202, which discloses an encrypted transmission system that is both simpler and more secure. In place of the pseudo-random patterns employed in  FIG. 8 , this system uses signal A to encrypt signal B, and signal B to encrypt signal A.  
         [0009]     Referring to  FIG. 9 , this system comprises a first device  200  and a second device  260  linked by a communication channel  300 . The transmitting section of the first device  200  includes a converter  201  that uses a received signal B′ as an encryption key to convert a transmit signal A to an encrypted signal CA, and a transmitting circuit  203  that converts the encrypted signal CA to a modulated signal MCA for transmission on the communication channel  300 . The receiving section includes a receiving circuit  210  that receives and demodulates a modulated signal MCB and outputs an encrypted signal CB, a deconverter  211  that decrypts the encrypted signal CB to obtain the receive signal B′, and a memory  202  that stores the transmit signal A sent to the converter  201  and supplies the stored signal A as a decryption key to the deconverter  211 .  
         [0010]     The second device  260  has a similar structure. Its transmitting section includes a converter  261  that uses a received signal A′ as an encryption key to convert a transmit signal B to an encrypted signal CB, and a transmitting circuit  263  that converts the encrypted signal CB to a modulated signal MCB for transmission on the communication channel  300 . The receiving section includes a receiving circuit  270  that receives and demodulates a modulated signal MCA and outputs an encrypted signal CA, a deconverter  271  that decrypts the encrypted signal CA to obtain the receive signal A′, and a memory  262  that stores the transmit signal B and supplies it as a decryption key to the deconverter  271 .  
         [0011]     Because it uses the receive signals A′ and B′ as encryption keys, and the transmit signals A, B as decryption keys, this system does not require separate circuits for generating pseudo-random patterns. A high level of security is provided, even if a simple encryption procedure is used, because the encryption and decryption keys are constantly changing. Encryption by the exclusive logical OR operation, for example, provides better security in  FIG. 9  than in  FIG. 8 .  
         [0012]     The system in  FIG. 9  has the disadvantage, however, of requiring synchronization between the transmit signals, so it cannot be used when A and B are intermittent signals.  
         [0013]     If the first device  200  encrypts the transmit signal A by performing exclusive logical OR operations, for example, then for each bit of A, the converter  201  uses a corresponding bit of the receive signal B′. If the second device  260  transmits signal B intermittently, the required bits of the receive signal B′ may not be available when they are needed. Similarly, if signal A is not transmitted continuously (Al, A 2 , A 3 , . . . ), the receive signal A′ may not be available when needed for encrypting transmit signal B.  
         [0014]     The system shown in  FIG. 9 , accordingly, cannot be used in packet communication systems, which include the numerous systems employing the internet protocol (IP).  
       SUMMARY OF THE INVENTION  
       [0015]     An object of the present invention is to protect the privacy of signals that are transmitted intermittently.  
         [0016]     When a signal is transmitted from a first device to a second device according to the invention, the first device generates an encryption key from the signal, delays the encryption key with respect to the signal, and uses the delayed encryption key to encrypt the signal. When the second device receives the encrypted signal, it uses a delayed decryption key to decrypt the encrypted signal, generates a decryption key from the decrypted signal, and delays the decryption key with respect to the decrypted signal, thereby obtaining the delayed decryption key.  
         [0017]     Because the encryption and decryption keys are generated from the signal itself, the signal may be transmitted intermittently. In particular, the signal may be transmitted in a series of packets.  
         [0018]     The second device preferably detects transmission errors in the encrypted signal, and transmits an initialization control signal to the first device when a transmission error is detected. Upon receiving the initialization control signal, the first device initializes the encryption key. The first device then preferably transmits an initialization reply signal to the second device. Upon receiving the initialization reply signal, the second device initializes the decryption key.  
         [0019]     The first and second devices may also generate a pseudo-random pattern, by which the signal is scrambled before encryption in the first device, and by which the decrypted signal is descrambled in the second device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     In the attached drawings:  
         [0021]      FIG. 1  is a block diagram of a transmission system embodying the invention;  
         [0022]      FIG. 2  is a block diagram illustrating a uni-directional version of the transmission system in  FIG. 1 ;  
         [0023]      FIGS. 3A, 3B , and  3 C illustrate the operation of the transmission system in  FIG. 2 ;  
         [0024]      FIG. 4  is a block diagram of another transmission system embodying the invention;  
         [0025]      FIG. 5  is a block diagram illustrating a uni-directional version of the transmission system in  FIG. 4 ;  
         [0026]      FIG. 6  is a communication sequence diagram illustrating the operation of the transmission system in  FIG. 5 ;  
         [0027]      FIG. 7  is a block diagram of yet another transmission system embodying the invention;  
         [0028]      FIG. 8  is a block diagram of a conventional transmission system; and  
         [0029]      FIG. 9  is a block diagram of another conventional transmission system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Embodiments of the invention will be described with reference to the attached drawings.  
         [0031]     The first embodiment, shown in  FIG. 1 , comprises a first device  1  having a transmitting section  10  and a receiving section  20 , a second device  6  having a receiving section  60  and a transmitting section  70 , and a bi-directional communication channel  300  linking the first device  1  and second device  6 .  
         [0032]     The transmitting section  10  comprises an encryption circuit  11 , a transmitting circuit  12 , an encryption key generator  13 , and a memory  14 . The transmitting section  10  receives a transmit signal A comprising successive packets, which arrive intermittently and are supplied to the encryption circuit  11  and encryption key generator  13 . From the transmit signal A, the encryption key generator  13  generates an encryption key KA. The encryption key KA is a signal similar to the transmit signal A, divided into packets. The encryption key KA is stored in the memory  14  and thereby delayed. The encryption circuit  11  reads the delayed encryption key DKA from the memory  14  and uses it to encrypt the transmit signal A. The transmitting circuit  12  receives the encrypted signal CA and converts it to a modulated signal MCA for transmission on the communication channel  300 .  
         [0033]     The receiving section  20  comprises a receiving circuit  21 , a decryption circuit  22 , a decryption key generator  23 , and a memory  24 . The receiving circuit  21  receives a modulated signal MCB from the communication channel  300  and demodulates it. The resulting demodulated signal is an encrypted signal CB, which comprises successive packets that are received intermittently. The decryption circuit  22  decrypts the encrypted signal CB by use of a delayed decryption key DKB read from the memory  24 , and outputs the decrypted signal as a receive signal B′. From the receive signal B′, the decryption key generator  23  generates a decryption key KB. The memory  24  stores and thereby delays the decryption key KB, which becomes the delayed decryption key DKB.  
         [0034]     The receiving section  60  in the second device  6  comprises a receiving circuit  61 , a decryption circuit  62 , a decryption key generator  63 , and a memory  64 , which are similar to the corresponding elements in the receiving section  20  of the first device  1 . The receiving circuit  61  receives the modulated signal MCA and outputs an encrypted signal CA, which is decrypted by the decryption circuit  62  to obtain a receive signal A′. The decryption key generator  63  generates an encryption key KA from the receive signal A′, and the memory  64  stores the encryption key KA, supplying a delayed encryption key DKA to the decryption circuit  62 . The transmitting section  70  comprises an encryption circuit  71 , a transmitting circuit  72 , an encryption key generator  73 , and a memory  74 , which are similar to the corresponding elements in the transmitting section  10  of the first device  1 . The encryption circuit  71  encrypts a transmit signal B, which comprises intermittent packets. The transmitting circuit  72  modulates the signal MCB according to the encrypted signal CB. The encryption key generator  73  converts the transmit signal B to an encryption key KB, which is stored in the memory  74  and supplied to the encryption circuit  71  as a delayed encryption key DKB.  
         [0035]     Various encryption methods can be employed, such as the exclusive logical OR method mentioned above, or another method involving reversible arithmetic and logic operations. Signals A and B may be encrypted by different methods.  
         [0036]     Various methods can be used to generate encryption and decryption keys from the transmit and receive signals. For example, the order of bits in the transmit and receive signals can be permuted according to a fixed rule to generate the encryption and decryption keys.  
         [0037]     The transmission and reception of signal A are independent of the transmission and reception of signal B. If the receiving section  20  and transmitting section  70  are eliminated, as shown in  FIG. 2 , the resulting system is still capable of transmitting signal A in encrypted form from the first device  1  to the second device  6 . The following description will be confined to the simplified uni-directional system shown in  FIG. 2 , but the description applies equally to the transmission of signal B in  FIG. 1 .  
         [0038]     Initially, the memories  14 ,  64  store predetermined initial key values, such as all-zero values. Upon receiving the first packet of a transmission, the encryption circuit  11  reads the initial key value from the memory  14  and uses it to generate the first encrypted packet. At the same time, the encryption key generator  13  uses the first packet to generate a new encryption key, which is then stored in the memory  14 . The first encrypted packet is transmitted to the second device  6  and decrypted by use of the initial key stored in memory  64 . The decryption key generator  63  generates a new decryption key from the decrypted packet, and stores it in memory  64 .  
         [0039]     This process continues.  FIG. 3A  illustrates four successive packets A(n−2), A(n−1), A(n), A(n+1) of transmit signal A, where n is an arbitrary integer equal to or greater than three. As shown, the packets may be separated by unequal gaps of arbitrary length.  FIG. 3B  illustrates the corresponding encrypted packets CA(n−2) to CA(n+1);  FIG. 3C  illustrates the corresponding received packets A′ (n−2) to A′ (n+1).  
         [0040]     When the encryption circuit  11  receives packet A(n−2), it uses the key currently stored in memory  14  as an encryption key to generate an encrypted signal CA(n−2). At the same time, the encryption key generator  13  uses packet A(n−2) to generate a new encryption key KA(n−2). Next, this encryption key KA(n−2) is stored in the memory  14 , and the transmitting circuit  12  converts the encrypted signal CA(n−2) to a modulated signal MCA(n−2).  
         [0041]     In the second device  6 , the receiving circuit  61  demodulates the modulated signal MCA(n−2) to obtain the encrypted signal CA(n−2). The key currently stored in memory  64  matches the key that was read from memory  14  during the encryption of signal A(n−2). The decryption circuit  62  uses this key to decrypt the encrypted signal CA(n−2), obtaining a receive packet A′ (n−2) identical to the transmit packet A(n−2). The decryption key generator  63  generates a decryption key KA(n−2) from the receive signal A′ (n−2), and stores it in memory  64 . The decryption key KA(n−2) is identical to the encryption key KA(n−2) generated by the encryption key generator  13  in the first device  1 .  
         [0042]     When the encryption circuit  11  receives the next packet A(n−1), it uses the key now stored in memory  14  as an encryption key to generate an encrypted signal CA(n−1). This key is denoted DKA(n−2) in  FIGS. 3A and 3B , although it is identical to the key KA(n−2) written previously by the encryption key generator  13 . The corresponding modulated signal MCA(n−1) is transmitted on the communication channel  300  to the second device  6 , and demodulated by the receiving circuit  61 , which outputs the encrypted signal CA(n−1). The decryption circuit  62  uses the key now stored in memory  64 , denoted DKA(n−2) but identical to the key KA(n−2) written previously by the decryption key generator  63 , to decrypt the encrypted signal CA(n−1) and obtain the next receive packet A′ (n−1).  
         [0043]     In the meantime, the encryption key generator  13  in the first device  1  generates the next encryption key KA(n−1) from packet A(n−1) and stores it in memory  14 . The decryption key generator  63  in the second device  6  generates an identical decryption key KA(n−1) from the receive packet A′ (n−1) and stores it in memory  64 .  
         [0044]     Packet A(n) is now encrypted by use of delayed key DKA(n−1), identical to KA(n−1), and the encrypted signal CA(n) is decrypted by use of the same delayed key DKA(n−1) to obtain receive packet A′ (n). New keys KA(n) are generated from A(n) and A′ (n) and stored in the memories  14 ,  64 . Then packet A(n+1) is encrypted by use of delayed key DKA(n), identical to KA(n), and the encrypted signal CA(n+1) is decrypted by use of the same delayed key DKA(n) to obtain receive packet A′ (n+1). In other words, after the first packet, each packet is encrypted and decrypted by use of a key generated from the preceding packet.  
         [0045]     This system is simple because it generates key information from the transmit and receive signals, and therefore does not require circuitry to generate random patterns. The system is secure in that the key is constantly changing. Moreover, the system enables a signal transmitted from the first device  1  to the second device  6  to be encrypted and decrypted without reliance on a signal transmitted from the second device  6  to the first device  1 , so it is particularly useful in packet transmission systems and other intermittent transmission systems.  
         [0046]     As a second embodiment of the invention,  FIG. 4  shows a system comprising a first device  2  having a transmitting section  30  and a receiving section  40 , and a second device  7  having a receiving section  80  and a transmitting section  90 . These sections and their constituent elements are equivalent to the corresponding elements in the first embodiment, but with additional functions and elements for detecting transmission errors and sending and receiving initialization command, control, and reply signals.  
         [0047]     The transmitting section  30  in the first device  2  has an encryption circuit  31 , a transmitting circuit  32 , an encryption key generator  33 , a memory  34 , and an initialization control signal generator  35 . The initialization control signal generator  35  receives an error detection signal EDB from the receiving section  40 , and supplies a corresponding initialization control signal IPB to the transmitting circuit  32  for transmission on the communication channel  300 . IPB is supplied and transmitted as a packet. The initialization control signal generator  35  also receives a report-of-initialization signal (RIA) from the receiving section  40 , and supplies a corresponding initialization reply signal (IRA) to the transmitting circuit  32  for transmission as a packet on the communication channel  300 .  
         [0048]     The receiving section  40  has a receiving circuit  41 , a decryption circuit  42 , a decryption key generator  43 , and a memory  44 . The receiving circuit  41  tests the validity of a frame check sequence (FCS) included in each packet to detect transmission errors, and generates the error detection signal EDB when an invalid FCS is detected. If the receiving circuit  41  receives an initialization control packet IPA from the second device  7 , it sends an initialization command ICA to the encryption circuit  31 , encryption key generator  33 , and memory  34  in the transmitting section  30 . If the receiving circuit  41  receives an initialization reply signal (IRB) from the second device  7 , it sends an initialization command RCB to the decryption circuit  42 , decryption key generator  43 , and memory  44 .  
         [0049]     The receiving section  80  in the second device  7  has a receiving circuit  81 , decryption circuit  82 , decryption key generator  83 , and memory  84 , which are similar to the corresponding elements in the receiving section  40  in the first device  2 . The transmitting section  90  in the second device  7  has an encryption circuit  91 , a transmitting circuit  92 , an encryption key generator  93 , a memory  94 , and an initialization control signal generator  95  which are similar to the corresponding elements in the transmitting section  30  in the first device  2 . The receiving circuit  81  generates an error detection signal EDA, a report-of-initialization signal (RIB), and initialization command signals RCA and ICB. The initialization control signal generator  95  generates an initialization control signal IPA and an initialization reply signal (IRB).  
         [0050]     The system in  FIG. 4  is bi-directional, transmitting a signal A from the first device  2  to the second device  7 , and a signal B from the second device  7  to the first device  2 .  FIG. 5  shows a variation of the second embodiment adapted for transmission of signal A without transmission of signal B. The first device  2  includes the encryption circuit  31 , transmitting circuit  32 , encryption key generator  33 , memory  34 , initialization control signal generator  35 , and receiving circuit  41  of  FIG. 4 , the initialization control signal generator  35  now being external to the transmitting section  30 . The second device  7  includes the receiving section  80 , transmitting circuit  92 , and initialization control signal generator  95  of  FIG. 4 . The operation of the second embodiment will be described in relation to the variation in  FIG. 5 , but similar operations take place when signal B is transmitted in  FIG. 4 .  
         [0051]      FIG. 6  depicts the operation from a point at which a packet A(m) is encrypted in the first device  2 , using a key generated from the preceding packet A(m−1) as described in the first embodiment (step S 1 ). The encrypted packet CA(m) is transmitted as a modulated signal MCA(m) to. the second device  7  and decrypted using the same key, which is generated from the preceding received packet A′ (m−1), as also described in the first embodiment (step S 2 ). This process continues as long as no transmission errors are detected.  
         [0052]     If at some point a packet A(n) encrypted by the first device  2  (step  11 ) is corrupted in transmission, the error is detected by the FCS check performed by the receiving circuit  81  in the second device  7  (step S 12 ). The receiving circuit  81  then sends an error detection signal EDA to the initialization control signal generator  95 . The initialization control signal generator  95  generates an initialization control signal IPA and supplies it as a maintenance packet to the transmitting circuit  92  (step  13 ). The transmitting circuit  92  transmits this IPA packet to the first device  2 .  
         [0053]     If the IPA packet is received without error, the receiving circuit  41  in the first device  2  sends an initialization command signal ICA to the encryption circuit  31 , encryption key generator  33 , and memory  34 , thereby initializing the transmitting section  30  (step S 14 ). At the same time, the receiving circuit  41  reports the reception of the IPA packet by sending a report-of-initialization signal RIA to the initialization control signal generator  35 , which generates an initialization reply signal IRA and supplies it as a maintenance packet to the transmitting circuit  32 . The transmitting circuit  32  transmits this IRA packet to the second device  7 .  
         [0054]     If the IRA packet is received without error, the receiving circuit  81  in the second device  7  sends an initialization command signal RCA to the decryption circuit  82 , decryption key generator  83 , and memory  84 , thereby initializing the receiving section  80  (step S 15 ). Thus IPA is the trigger for initialization of the transmitting section  30 , while IRA is the trigger for initialization of the receiving section  80 . After this initialization, both memories  34 ,  84  hold the same initial key value, such as an all-zero value.  
         [0055]     The encryption circuit  31  in the first device  2  now encodes the next packet A(n+1), using the initial key (step S 16 ). The encrypted packet CA(n+1) is transmitted as a modulated signal MCA(n+1) to the second device  7  and decrypted, using the same initial key (step S 17 ).  
         [0056]     The next packet A(n+2) is encrypted in the normal way in the first device  2 , using a key generated from the preceding packet A(n+1) (step S 18 ). In the second device  7 , the encrypted packet CA(n+2) is decrypted by use of a key generated from the preceding received packet A′ (n+1) (step S 19 ).  
         [0057]     The second embodiment provides a way to recover from transmission errors without retransmitting the erroneous packet. The second embodiment is particularly useful in systems that must operate in real time and cannot afford to retransmit erroneous packets.  
         [0058]     If the system provides for retransmission of erroneous packets, either the first or the second embodiment can be employed.  
         [0059]     In a variation of the second embodiment, the receiving circuit  81  also detects missing packets, by use of packet serial numbers, for example, and generates an error detection signal EDA when a packet is either corrupted or missing.  
         [0060]     As a third embodiment of the invention,  FIG. 7  shows a uni-directional transmission system comprising a first device  3  having a transmitting section  50 , and a second device  8  having a receiving section  100 .  
         [0061]     The transmitting section  50  comprises an encryption circuit  51 , a transmitting circuit  52 , an encryption key generator  53 , and a memory  54 , which are similar to the corresponding elements in the first embodiment, and a pseudo-random pattern generating circuit  55  and scrambling circuit  56 . The receiving section  100  comprises a receiving circuit  101 , a decryption circuit  102 , a decryption key generator  103 , and a memory  104 , which are similar to the corresponding elements in the first embodiment, and a pseudo-random pattern generating circuit  105  and descrambling circuit  106 . The two pseudo-random pattern generating circuits  55 ,  105  generate identical pseudo-random patterns RA.  
         [0062]     The transmit signal A received by the transmitting section  50  is first scrambled by the scrambling circuit  56 , using the pseudo-random pattern RA supplied by the pseudo-random pattern generating circuit  55 . Various well-known scrambling methods can be used, such as the exclusive logical OR method described above. The resulting scrambled signal SA is then encrypted by the encryption circuit  51 , using a delayed encryption key DKSA read from the memory  54 . The transmitting circuit  52  converts the encrypted signal CSA to a modulated signal MCSA for transmission to the second device  8 . The encryption key generator  53  generates a new key KSA from the scrambled signal SA, and stores the new key in the memory  54 , from which it will be read as the delayed encryption key DKSA for the next scrambled packet.  
         [0063]     In the second device  8 , the receiving circuit  101  demodulates the modulated signal MCSA to obtain the encrypted signal CSA, which is decrypted by the decryption circuit  102 , using a delayed decryption key DKSA read from the memory  104 . The decrypted signal is the scrambled signal SA, from which the decryption key generator  103  generates a new decryption key KSA. The new decryption key KSA is stored in the memory  104 , from which it will be read as the next delayed decryption key. The scrambled signal SA is descrambled by the descrambling circuit  106 , using the pseudo-random pattern RA supplied by the pseudo-random pattern generating circuit  105 , to obtain the receive signal A′.  
         [0064]     In the first and second embodiments, a party intercepting the communication between the first device and the second device may attempt to decrypt each packet on the assumption that it was encrypted with the initial key value. In this way, the intercepting party may succeed in decrypting the first packet transmitted in the first embodiment, or a packet transmitted after a transmission error in the second embodiment. If the decrypted packet includes a text message, for example, the intercepting party will know that he has decrypted the packet successfully because the decrypted message will be in plain text. The intercepting party may then be able to determine how the key is generated and decrypt the succeeding packets as well.  
         [0065]     In the third embodiment, even if an intercepted packet is correctly decrypted, the intercepting party obtains only a scrambled message, and cannot easily recognize that the packet has been decrypted correctly. This makes it extremely difficult for the intercepting party to determine how the key is generated, and how the packets have been scrambled.  
         [0066]     The third embodiment accordingly provides a higher level of security than the first and second embodiments. This higher level of security can moreover be obtained with a comparatively short pseudo-random pattern, because the key changes from packet to packet, so that even if two packets are scrambled in the same way, they will not be encrypted in the same way. Differing from the prior art, the third embodiment does not require long pseudo-random patterns or complex and expensive pseudo-random pattern generating circuits in order to protect the privacy of communications.  
         [0067]     The third embodiment can be modified for bi-directional communication, by adding a receiving section to the first device  3  and a transmitting section to the second device  8 .  
         [0068]     The third embodiment can also be varied by providing for initialization in case of transmission errors, as in the second embodiment.  
         [0069]     In the description of the first embodiment, the delay of the encryption and decryption keys in the memories  14 ,  64  was assumed to be equal to the length of one packet, but this is not a requirement. The delay can have any fixed value, expressed as a fixed number of bits with respect to the transmit and receive signals.  
         [0070]     In the description of the first embodiment, the packets were implicitly assumed to be of equal length, but this is not a requirement either. The packets may have variable length.  
         [0071]     The encryption key used to encrypt the transmit signal need not be identical to the decryption key used to decrypt the encrypted signal. The decryption key may be complementary to the encryption key, for example.  
         [0072]     Any of the preceding embodiments can be modified for communication among more than two devices. The invention can be used in a packet-switching network, for example.  
         [0073]     The invention can also be used in systems that transmit signals continuously, instead of intermittently. The invention is particularly useful in uni-directional systems, as illustrated in  FIGS. 2 and 5 .  
         [0074]     Those skilled in the art will recognize that further variations are possible within the scope claimed below.