Patent Publication Number: US-8542828-B2

Title: Cryptographic secret key distribution

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to cryptographic secret key distribution between a transmission side and a receiving side. 
     BACKGROUND OF THE INVENTION 
     WO 2007/031089 discloses a method for secure communication in a wireless communication system. In a key generation mode, an access point equipped with an ESPAR antenna forms a beam pattern and sends a packet for measurement. The terminal receives that packet with an omnidirectional pattern and acquires a Received Signal Strength Indication (RSSI) value after averaging in order to equalize the influence of noise. Next, a packet for measurement is transmitted using the omnidirectional pattern by the regular user. The access point receives that packet by a pattern, which is identical to the original pattern, and acquires the RSSI value after averaging. There are K different RSSI values acquired by repeating the measurement of the RSSI K times and changing the beam pattern of the access point. An iteration K is simply set according to key length. Next, a threshold value is set up for the RSSI value of the K pieces, and it becomes 1, if it is higher than a threshold value and 0 if it is under the threshold value. After binarization, the same key is generated in the access point and the regular user, and key agreement can be achieved. 
     In wireless communication systems, secret-key cryptography is used because its processing speed can deal with bulk data. Secret-key cryptography is sometimes referred to as symmetric cryptography. It is a traditional form of cryptography, in which a single key can be used to encrypt and decrypt a message. Secret-key cryptography not only deals with encryption, but it also deals with authentication. One such technique is called message authentication codes (MACs). The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transformation to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link. An advantage of secret-key cryptography is that it is generally faster than public-key cryptography. 
     Other terms for symmetric-key encryption are secret-key, single-key, shared-key, one-key and private-key encryption. However, use of the latter term conflicts with the term private key in public-key cryptography. 
     A major problem with secret-key cryptosystems is getting the sender and receiver to agree on the secret key without anyone else finding out. This requires a method by which the two parties can communicate without fear of eavesdropping. An important question is thus how to achieve the initial key exchange. 
     A first approach resides in the use of bi-directional LQI/RSSI (link quality indicator/received signal strength indicator) measurements to assess variations of the attenuation of the signal path between two transceivers, in order to establish a shared secret between two nodes. Thanks to the reciprocity theorem of radio wave propagation between two communication parties, it is possible for them to calculate common information by using the fluctuation characteristics of the channels. This approach can provide a secret key agreement scheme without any key distribution processes. Because this scheme can provide a onetime key when it is needed, it is an excellent method to solve the problems of key distribution and key management. 
     A second approach resides in the sending of a set of random numbers, optionally at low transmit power, and combining (e.g. XOR) them all together to generate a “key”. An attacker is unlikely to hear all of them correctly. 
     The first approach has an optional variant in which the established shared secret is used to secure a 128-bit random key generated by one of the devices. This helps to protect against future attacks against the amount of “randomness” in the attenuation of the signal. Additionally, the first approach makes it difficult for an attacker to get the keys because the attacker&#39;s receiver will have a different path attenuation between itself and each of the targets to that that they have between one another. As such the link quality figures (LQI, typically assessed as received signal strength indication, RSSI) assessed in each direction between the two nodes will be strongly correlated, whereas LQI/RSSI to a 3 rd  node will typically be very weakly correlated. 
     The second approach makes it difficult for an attacker to get the keys because they would either need to have a radio receiver device very nearby and specifically configured to be snooping the right channel at the time of the installation. Nevertheless, one of the risks with the second approach (combining multiple keys) is that attackers might leave a snooping device running all the time, possibly with a high quality receiver. They could then trawl through log files later and may, if they are lucky, receive all the keying information. This may optionally be mitigated by transmissions being at low transmit power, hopefully reducing the risk of attacks to a level that manufacturers are content to deploy products using it. 
     However, both approaches have a weakness in that they require a lot of transactions to generate a key that is strong enough for use in all cases. For example, the first proposal might require an exchange of 300 messages to give 128-bit security. For many applications this is excessive. A particular example of an application where this would be unsuitable is an energy scavenging device. It is becoming possible for devices, such as light switches, to generate sufficient power from the action of pressing the switch that they are able to enable their transceiver and microprocessor for a short period. This period would probably not be sufficient for the exchange of tens or hundreds of messages. 
     Latency is also an important consideration—if a user presses a key and nothing happens for e.g. 3 seconds, he may press another button. This is likely to be more important for devices that need to join a network frequently, perhaps including point-of-sale applications. 
     Conversely, higher demands for security require more messages to be sent in order to decrease the probability of an attacker being successful. The conflicting demands of higher security and low operating power/latency cannot be met adequately by the available systems. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an enhanced cryptographic secret key distribution scheme which provides flexibility in terms of the above-mentioned conflicting demands. The invention is defined by the independent claims. The dependent claims define advantageous embodiments. The invention provides a system for transmitting or receiving encrypted data using a cryptographic secret key, in which a setting function is provided for setting an iteration number; and a number of messages to be exchanged during generating the cryptographic secret key, is controlled based on the iteration number. 
     Accordingly, one or both of the nodes, devices, transmission sides or transmission ends may vary, decide or influence the number of messages transferred during the generation of a cryptographic secret key. This renders the present invention much more adaptable to needs than the prior art of WO 2007/031089, according to which the iteration is simply set according to key length. 
     In the present invention, the term “setting unit” is to be understood within the context of the application text. This means that the setting unit can be any unit or functionality which effectively sets the iteration number. It may be implemented in a variety of manners, e.g. by setting the iteration number autonomously (e.g. by generating a number (e.g. a (pseudo) random number) within a particular range), by reading a pre-determined number, and/or by setting the iteration number based on external input (e.g. by receiving an iteration number from another device). 
     According to a first implementation, the iteration number can be provided or generated in response to an initiation of a key establishment. Thus, a fast initiation of the key generation process can be ensured. 
     According to a second implementation, which could be combined with the first implementation, the iteration number can be generated based on an input operation provided at the key generation apparatus. This option provides flexibility to a user in that security and latency can be individually controlled, e.g. based on application requirements. 
     According to a third implementation, which may be combined with at least one of the first and second implementations, the iteration number can be generated based on a type of the encrypted data. Thus, security and/or latency can be automatically controlled based on the requirements of the type of encrypted data. 
     According to a fourth implementation, which may be combined with at least one of the first to third implementations, the iteration number can be received from another transmission side. According to a fifth implementation, which may be combined with at least one of the first to fourth implementations, the iteration number can be transmitted to another transmission side. Thereby, it can be ensured that both transmission sides use the same number of messages to generate the key. 
     According to a sixth implementation, which may be combined with at least one of the first to fifth implementations, a memory may be provided for storing the iteration value. This ensures that the iteration value remains available and does not get lost after receipt. 
     According to a seventh implementation, which may be combined with at least one of the first to sixth implementations, a counter may be provided for counting the number of messages to be exchanged during generating the cryptographic secret key. 
     According to an eighth implementation, which may be combined with at least one of the first to seventh implementations, the cryptographic secret key may be generated based on recorded received signal strength indicator values obtained from the number of messages to be exchanged. 
     According to a ninth implementation, which may be combined with at least one of the first to eighth implementations, the cryptographic secret key may be generated based on a combination of random numbers obtained from said number of messages to be exchanged. According to a specific implementation example, the combination may be a logic exclusive-or (XOR) combination. 
     According to a tenth implementation, which may be combined with at least one of the first to ninth implementations, the cryptographic secret key may be generated based on a transmission of subsequent cryptographic secret keys secured with a previous cryptographic secret key, and wherein the previous cryptographic secret key and the subsequent cryptographic secret keys are transmitted by using the number of messages to be exchanged. In an advantageous modification of the tenth implementation, the cryptographic secret keys may be transmitted on more than one transmission channel, to thereby further enhance security of key exchange. 
     According to an eleventh implementation, which may be combined with at least one of the first to tenth implementations, the apparatus may be arranged to concatenate a number of bits of said cryptographic secret key into groups, to compare each group with a set of sample symbols, to classify groups according to which is the most closely matching sample symbol, and to reject groups that could not be classified with confidence above a predetermined threshold. Thereby, small number of bit errors caused by noise can be avoided, and a shared secret can be better extracted from the shared data sets. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a signaling diagram of an RSSI-based random number generation according to a first embodiment; 
         FIG. 2  shows a schematic diagram indicating typical RSSI values received at different transmission sides; 
         FIG. 3  shows a signaling diagram of a transmission of multiple keys with subsequent combination according to a second embodiment; 
         FIG. 4  shows a signaling diagram of a transmission of keys secured with a previous key on more than one channel, according to a third embodiment; 
         FIG. 5  shows a schematic block diagram of a television (TV) device of a key distribution system in which the embodiments can be implemented; 
         FIG. 6  shows a schematic block diagram of a remote control device of the key distribution system in which the embodiments can be implemented; 
         FIG. 7  shows a schematic block diagram of a hardware implementation at both transmission ends of an RSSI-based random number generation system according to a fourth embodiment; 
         FIG. 8  shows a flow diagram of a key distribution procedure at a remote control side of an RSSI-based random number generation system according to a fifth embodiment; 
         FIG. 9  shows a flow diagram of a key distribution procedure at a TV side of the RSSI-based random number generation system according to the fifth embodiment; 
         FIG. 10  shows a schematic block diagram of a hardware implementation at a remote control side of a multiple-key combination system according to a sixth embodiment; 
         FIG. 11  shows a schematic block diagram of a hardware implementation at a TV side of the multiple-key combination system according to the sixth embodiment; 
         FIG. 12  shows a flow diagram of a key distribution procedure at a remote control side of a multiple-key combination system according to a seventh embodiment; 
         FIG. 13  shows a flow diagram of a key distribution procedure at a TV side of the multiple-key combination system according to the seventh embodiment; 
         FIG. 14  shows a schematic block diagram of a hardware implementation at a remote control side of a secured multiple-channel multiple-key transmission system according to an eighth embodiment; 
         FIG. 15  shows a schematic block diagram of a hardware implementation at a TV side of the secured multiple-channel multiple-key transmission system according to the eighth embodiment; 
         FIG. 16  shows a flow diagram of a key distribution procedure at a remote control side of a secured multiple-channel multiple-key transmission system according to a ninth embodiment; and 
         FIG. 17  shows a flow diagram of a key distribution procedure at a TV side of the secured multiple-channel multiple-key transmission system according to the ninth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following, various embodiments of the present invention are described on the basis of a key distribution between a remote control device and a TV device. 
     The embodiments are directed to modifications of either or both of the two initially described approaches (and/or their variants) to permit one or both of the participating nodes to vary the number of messages transferred. However, the invention is not limited to these approaches and may be applied to any key distribution mechanism. 
     As an implementation example, a TV remote control powered from a small solar panel might permit a maximum of e.g. 5 messages to be exchanged during key establishment. This would provide lower security than a typical device, but would allow the exchange to complete before power was exhausted. 
     As a further implementation example, a TV and a digital versatile disk (DVD) player establishing a secure relationship might exchange e.g. 1000 messages, as power efficiency is less important and resistance to a security compromise might be more important. 
     An implementation based on a wireless protocol could define the range of valid values for a number N of messages transferred. Typical values of N might range from 0 to 1000. 
     For the above RSSI based first approach, setting N=0 would result in the “shared secret” being NULL. Using this shared secret to transport a 128-bit key would be equivalent to sending the key in the clear. 
     For the XOR based second approach, setting N=1 would also be functionally equivalent to sending the key in the clear. 
     The first embodiment is directed to an implementation using RSSI-based generation of a shared secret. 
       FIG. 1  shows a signaling diagram of an RSSI-based key establishment mechanism according to a first embodiment. A first node or device A (e.g. a remote control) initiates communication with a second node or device B (e.g. a TV). Each device is permitted to carry out the key exchange protocol through some user interaction. For example a user presses a button on the second device B and then on the first device A, or vice versa. In a first step, the first device A sends an initiating message INI, including an indication of the number N of messages to exchange. Then, in a second step, the second device B sends an acknowledgement Ack, and in a third step the first device A sends a test RSSI message T_rssi containing no payload. In a fourth step, the second device B records the RSSI of the received test RSSI message T_rssi received from the first device A and sends an acknowledgement Ack. In a fifth step, the second device B sends a test RSSI message T_rssi, containing no payload. In a sixth step, the first device A records the RSSI of the received test RSSI message T_rssi and sends an acknowledgement Ack. Then, after a short delay D, the third to sixth steps are repeated until each node has sent and received N messages. 
     Readings above a predetermined threshold T are classified as “1” and those below that threshold T are classified as “0”. Then, “marginal” RSSI readings are identified and a bit field of marginal bits is created. In a seventh step, the first device A constructs and sends a marginal readings message MR containing the bit field of marginal bits. In an eighth step, the second device B sends an acknowledgement Ack. Then, in a ninth step, the second device B constructs and sends a marginal readings message MR containing a bit field of marginal bits. In a final tenth step, the first device A sends an acknowledgement Ack. 
     Now, both devices A and B combine the two bit fields of marginal bits e.g. by a logical OR combination, and all RSSI samples considered marginal are rejected by either device. The remaining, non-marginal, bits are concatenated into a shared secret. Optionally, this shared secret could be used to transport a random number from the first device A to the second device B. The key can be verified as per other key exchange algorithms. 
     If messages are not received as expected, the transmission ends may timeout and abort the process, and the user may have to retry later. 
     Possible frame formats for the above message exchange could be arranged as follows: 
     Format of Initiate Message INI: 
                                                    Bytes: variable   2   2           Header information   N   Checksum                        
Format of Acknowledgement Message Ack:
 
                                                Bytes: variable   2           Header information   Checksum                        
Format of Test RSSI Message T_rssi:
 
                                                Bytes: variable   2           Header information   Checksum                        
Format of Marginal Bits Message:
 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Bytes: variable 
                 Variable 
                 2 
               
               
                   
                 Header information 
                 Marginal Bits 
                 Checksum 
               
               
                   
                   
               
            
           
         
       
     
     After the short delay D both devices A and B have a set of RSSI values. The two sets are correlated, but generally not identical, as illustrated below. 
       FIG. 2  shows a schematic diagram indicating typical RSSI values received at different transmission sides, e.g. at the devices A and B. Each RSSI sample is categorized as “1” (if above the threshold T) and as “0” (if below the threshold T). Samples in the marginal region MR are flagged as “marginal”. These flagged RSSI samples are depicted as hatched circles, while black circles indicate RSSI samples received at the second device B and white circles indicate RSSI samples received at the first device A. The protocol rejects all samples flagged as marginal by either of the devices A and B. In the above example of  FIG. 2 , both device A and device B agree on all the non-rejected samples, and have a shared secret of value “0b110”. 
     It should be noted that various other algorithms could be used for extracting a shared secret from two correlated data sets. The above is given as a straightforward example. 
     One other example could be the use of signal variation, e.g.: 1=getting stronger, 0=getting weaker. Some algorithms are able to extract more than one bit of data from each message exchanged, for example by having three threshold lines between high, upper, lower, bottom levels. A further algorithm which may be applied involves concatenation of a number of bits into groups, and comparison of each group with a set of sample “symbols”. Groups can be classified according to which is the most closely matching sample symbol. Groups with marginal classifications (i.e. groups that could not be classified with confidence above a predetermined threshold) at either side are rejected by both sides in a similar way to bit classifications. This helps to avoid a small number of bit errors caused by noise, and allows more efficient extraction of a shared secret from the shared data sets. 
     These and other algorithms would also benefit in the same way from the ability to vary the number N of messages exchanged. 
       FIG. 3  shows a signaling diagram of a transmission of multiple keys with subsequent combination (e.g. XOR) according to a second embodiment. In the second embodiment, a first device A (e.g. a remote control device) initiates communication with a second device B (e.g. a TV). Each device is permitted to carry out a key exchange protocol through some user interaction, for example the user presses a button on the second device B then on the first device A, or vice versa. 
     In a first step, the first device A sends an initiating message INI, including an indication of the number N of messages to exchange. Then, in a second step, the second device B sends an acknowledgement Ack. In a third step, the first device A generates a 128-bit random number, constructs a key material message KM containing that number, and sends the key material message KM to the second device B, optionally using a reduced transmission power. In a fourth step, the second device B records the random number in the message received from the first device A and sends an acknowledgement Ack. The third and fourth steps are repeated until the first device A has sent N messages. 
     Then, both devices A and B combine a number “0” in a logical XOR manner with each random number sent and received in turn. Thus, the two devices A and B now share a shared secret. The obtained key can then be verified as per other key exchange algorithms. 
     If a device fails to receive an acknowledgement Ack to a given frame, then it may retry it e.g. in accordance with the Media Access Control (MAC) protocol. If all retries of any frame fail, the transmitter may abort the process and the receiver may eventually timeout and also abort the process. The user may then retry later. 
     Possible frame formats for the above message exchange could be arranged as follows: 
     Format of Initiating Message INI: 
                                                    Bytes: variable   2   2           Header information   N   Checksum                        
Format of Acknowledgement Message Ack:
 
                                                Bytes: variable   2           Header information   Checksum                        
Format of Key Material Message KM:
 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Bytes: variable 
                 16 (=128 bits) 
                 2 
               
               
                   
                 Header information 
                 Key material 
                 Checksum 
               
               
                   
                   
               
            
           
         
       
     
     The above procedure of the second embodiment may be modified in that an initial key is sent in the clear, and, after this, the next key is sent secured by the previous key. This is repeated a fixed or variable number of times. Again an attacker missing any single message will not have the final key. 
     As a further modification the channel may be changed between transmissions of the separate keys. This makes it harder for an attacker to capture all the required keys if they are equipped only with a simple radio snooper device, as these typically operate on a single channel. 
       FIG. 4  shows a signaling diagram of a transmission of keys secured with a previous key on more than one channel, according to a third embodiment. Again, a first device A (e.g. a remote control) initiates communication with a second device B (e.g. a TV). Each of the devices A and B may be permitted to carry out a key exchange protocol through some user interaction, for example the user presses a button on the second device B then on the first device A, or vice versa. 
     In a first step, the first device A sends an initiating message INI, including an indication of the number N of messages to exchange. Then, in a second step, the second device B sends an acknowledgement Ack. In a third step, the first device A generates a 128-bit random key kn, selects a new channel at random, constructs a key transport message KT containing the key kn and the number ch 1  of the new channel, and sends the message to the second device B, optionally using a reduced transmission power. It then waits for an acknowledgement Ack and switches to the new channel. 
     In a fourth step, the second device B records the key in the message received from the first device A, sends an acknowledgement Ack, and switches to the new channel. In a fifth step, the first device A generates another 128-bit random key, selects a further new channel at random, constructs a key transport message KT containing the key and the number ch 2  of the new channel, secures it using the previously transported key, and sends the message to the second device B, optionally using a reduced transmission power. It then waits for an acknowledgement Ack and switches to the new channel. 
     In a sixth step, the second device B sends an acknowledgement Ack and then checks if the received command was appropriately secured. If it was appropriately secured, then it removes the security from the message and then extracts the new key from the message and records it. It then switches to the new channel. 
     The fifth and sixth steps are repeated until the first device A has sent N key transport messages. The current key can be verified as per other key exchange algorithms and used for future communications. If a device fails to receive an acknowledgement Ack to a given frame, then it may retry it in accordance with the MAC protocol. If all retries of any frame fail, the transmitter may abort the process and the receiver may eventually timeout and also abort the process. The user may then retry later. 
     Possible frame formats for the above message exchange could be arranged as follows: 
     Format of Initiating Message INI: 
                                                    Bytes: variable   2   2           Header information   N   Checksum                        
Format of Acknowledgement Message Ack:
 
                                                Bytes: variable   2           Header information   Checksum                        
Format of Key Transport Message KT:
 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
             
            
               
                 Bytes: variable 
                 Variable 
                 16 (=128 bits) 
                 1 
                 Variable 
                 2 
               
               
                 Header 
                 Security header 
                 New key 
                 New 
                 Security footer 
                 Checksum 
               
               
                 information 
                 (Optional) 
                   
                 channel 
                 (Optional) 
               
               
                   
               
            
           
         
       
     
     Other implementation variations are possible. For example, the channel might be changed according to some prearranged schedule, such as sequential channels or a pseudo-random sequence. 
     Furthermore, key verification may carried out by sending messages secured with that key between the two devices A and B, and checking on reception that the message from the other was secured correctly. 
       FIG. 5  shows a schematic block diagram of a TV device of a key distribution system in which the embodiments of the present invention can be implemented. The TV device comprises a screen or display (e.g. liquid crystal display (LCD) or the like)  10 , a display driver  11 , a front panel keypad  12  with control buttons, other audio and video inputs  14 , a tuner  15 , a power supply  16 , a volatile memory (e.g. random access memory (RAM))  17 , a non-volatile memory (e.g. flash memory)  18 , a central processing unit (CPU)  19 , and a transceiver  20  to which an antenna  21  is connected. An interconnection between various of the above components is achieved by a communication line (e.g. system bus)  13 . 
     In operation, media data comes in via the tuner  15  and the other audio/video inputs  14 . The media data is output to the display  10  via the display driver  11 . The interface between the display  10  and the display driver  11  may also be used for control and configuration of the TV settings. The CPU  19  runs control software and may provide mechanisms of the embodiments of the present invention. The transceiver  20  might run the for example the IEEE 802.15.4 MAC protocol. It may also implement the IEEE 802.15.4 PHY layer protocol. The front panel keypad  12  may have a button as referred to in the embodiments for initiation of key establishment. The memories  17  and  18  can be used for storing the control software and also for implementing the IEEE 802.15.4 stack. 
       FIG. 6  shows a schematic block diagram of a remote control device of the key distribution system in which the embodiments of the present invention can be implemented. 
     The remote control device comprises a keypad  22  with control buttons, a power supply  24 , a volatile memory (e.g. random access memory (RAM))  25 , a non-volatile memory (e.g. flash memory)  26 , a central processing unit (CPU)  27 , and a transceiver  28  to which an antenna  29  is connected. An interconnection between various of the above components is achieved by a communication line (e.g. system bus)  23 . 
     The keypad  22  represents the buttons the user might press. It may include a button to initiate key establishment, as described in the embodiments of the present invention. The CPU  27  runs control software and may provide mechanisms of the embodiments of the present invention. The transceiver  28  might run the IEEE 802.15.4 MAC protocol. It may also implements the 802.15.4 PHY layer protocol. The memories  25  and  26  can be used for the control software and also for implementing the IEEE 802.15.4 stack. 
       FIG. 7  shows a schematic block diagram of a hardware implementation at both transmission ends of an RSSI-based random number generation system according to a fourth embodiment. The blocks depicted in  FIG. 7  may be implemented as discrete hardware circuits implemented in a module, arranged on a circuit board, or integrated on a single or multiple chip device. A control logic (CTRL)  33  which may be realized as a software-controlled CPU or as a discrete logic circuit has access to a counter (C)  30  and a memory (MEM)  31 , and can be controlled by an initiating button (IB)  32 . The control logic  33  stores the transmitted value of the number N of permitted iterations in the memory  31  and controls the counter  30  to count the number of received or transmitted messages during key establishment. Based on a comparison of the count value at the counter  30  and the value of the number N stored in the memory  31 , the control logic  33  can determine when the permitted number of iterations has been reached. Furthermore, the control logic  33  controls an RSSI categorization circuit or block (RSSI-C)  34  and a key generation circuit or block (KG)  35  which receives information from the RSSI categorization block  34 . Input/output signals are received/transmitted via a transceiver (TRX)  36  and an antenna  38 . An RSSI measurement circuit or block (RSSI-M)  37  received RSSI samples from the transceiver  36 , measures them, and forwards measurement results to the RSSI categorization block  34 . RSSI samples received by the transceiver are compared to the predetermined threshold T at the RSSI categorization block  34  which categorizes the measurement results similar to the procedure described above in connection with  FIGS. 1 and 2 . Based on the categorization of the N RSSI samples, the key generation block  35  generates the shared secret. 
       FIG. 8  shows a flow diagram of a key distribution procedure at a remote control side of an RSSI-based key establishment system according to a fifth embodiment. The procedure of  FIG. 8  may be implemented as a software routine controlling e.g. the CPU  27  of  FIG. 6 . 
     In step S 100  the initiation button for key establishment is pressed on the remote control device. Then, in step S 101 , a value for the iteration number or number N of messages used to generate the encryption key or secret key is selected. This may be achieved based on the type of data to be encrypted (e.g. the specific application of the secured transmission), or an individual user, manufacturer, or operator setting. In step S 102 , an initiating message including the selected value of the number N is sent to a TV to be controlled by the remote control device, and the remote control device then waits for an acknowledgement from the TV. In the subsequent step S 103 , the remote control device sends a test RSSI (e.g. “T_rssi” in  FIG. 1 ) message to the controlled TV and waits for an acknowledgement. Then, in step S 104  it waits for a test RSSI message from the controlled TV. Thereafter, in step S 105 , it records the received RSSI sample and sends an acknowledgement. After a short delay in step S 106 , the remote control device checks in step S 107  if the signaled number N of messages have been received. If the number N of messages have not been received yet, the procedure jumps back to step S 103  in order to send the next test RSSI message. Otherwise, if the number N of messages have been received, the recorded RSSI samples or values are classified and marginal bits are identified in step S 108 . Then, in step S 109 , a marginal reading message is sent to the controlled TV and it is waited for an acknowledgement. In the subsequent step S 110 , the remote control device waits for a marginal reading message from the controlled TV and sends an acknowledgement after receipt thereof. In step S 111  the marginal bit fields are OR-combined and all marginal bits are rejected. Additionally, the remaining non-marginal bits are concatenated to form the shared secret. In step S 112 , a random key is generated, secured by using the shared secret, and sent to the controlled TV. In step S 113 , the remote control device generates some message, secures it using the random key, and sends it to the controlled TV. Then, in step S 114 , the remote control device waits for a secure message from the controlled TV, and checks if the security of this received message is ok. If it determines in step S 115  that the security is not ok, the procedure jumps back to step S 102  in order to send a new initiating message. Otherwise, if it is determined in step S 115  that the security is ok, it is concluded in step S 116  that key establishment is finalized. 
       FIG. 9  shows a flow diagram of a key distribution procedure at a TV side of the RSSI-based key establishment system according to the fifth embodiment. The procedure of  FIG. 9  may be implemented as a software routine controlling e.g. the CPU  19  of  FIG. 5 . 
     In step S 200  the initiation button for key establishment is pressed at the TV device. Then, in step S 201 , the TV device waits for an initiating message from the remote control device, and sends an acknowledgement after receipt thereof. In step S 202  a received value for the iteration number or number N of messages is recorded in order to set the iteration number for key generation. In the subsequent step S 203 , the TV device waits for a test RSSI message, records an RSSI value after receipt thereof, and waits for an acknowledgement. In step S 204 , the TV device sends a test RSSI message to the remote control device and waits for an acknowledgement. Then, in step S 205  the TV device checks if the signaled number N of messages have been received. If the number N of messages have not been received yet, the procedure jumps back to step S 203  in order to wait for the next test RSSI message. Otherwise, if the number N of messages have been received, the recorded RSSI samples or values are classified and marginal bits are identified in step S 206 . Then, in step S 207 , the TV device waits for a marginal reading message sent by the remote control device and sends an acknowledgement after receipt thereof. In the subsequent step S 208 , the TV device sends a marginal reading message to the remote control device and waits for an acknowledgement. In step S 209  the marginal bit fields are OR-combined and all marginal bits are rejected. Additionally, the remaining non-marginal bits are concatenated to form the shared secret. In step S 210 , the TV device waits for a message from the remote control secured with the shared secret, removes the security, and records the conveyed key. In step S 211 , the TV device waits for a message from the remote control device secured with the recorded key, and checks if the security of this received message is ok. If it determines in step S 212  that the security is not ok, the procedure jumps back to step S 201  in order to wait for a new initiating message. Otherwise, if it is determined in step S 212  that the security is ok, the TV device generates in step S 213  some message, secures it using the recorded key, and sends it to the remote control device. Then, it is concluded in step S 214  that key establishment is finalized. 
       FIG. 10  shows a schematic block diagram of a hardware implementation at a remote control side of a multiple-key combination system according to a sixth embodiment. The blocks depicted in  FIG. 10  may be implemented as discrete hardware circuits implemented in a module, arranged on a circuit board, or integrated on a single or multiple chip device. A control logic (CTRL)  43  which may be realized as a software-controlled CPU or as discrete logical circuit has access to a counter (C)  40  and a memory (MEM)  41 , and can be controlled by an initiating button (IB)  42 . The control logic  43  stores the transmitted value of the number N of permitted iterations in the memory  41  and controls the counter  40  to count the number of received or transmitted messages during key establishment. Based on a comparison of the count value at the counter  40  and the value of the number N stored in the memory  41 , the control logic  43  can determine when the permitted number of iterations has been reached. Furthermore, the control logic  43  controls a random number generation circuit or block (RNG)  44 , a key storage or key memory (KMEM)  45  for keying material, and an XOR combination circuit or block  47  for key generation. Input/output signals are received/transmitted via a transceiver (TRX)  46  and an antenna  48 . The random number generation block  44  generates random numbers and supplies them to the transceiver  46  for transmission to the controlled TV device. The generated and signaled random numbers are memorized or recorded in the key memory  45  and XOR-combined at the XOR combination block  47  to generate the shared secret. 
       FIG. 11  shows a schematic block diagram of a hardware implementation at a TV side of the multiple-key combination system according to the sixth embodiment. The blocks depicted in  FIG. 11  may be implemented as discrete hardware circuits implemented in a module, arranged on a circuit board, or integrated on a single or multiple chip device. A control logic (CTRL)  53  which may be realized as a software-controlled CPU or as discrete logical circuit has access to a counter (C)  50  and a memory (MEM)  51 , and can be controlled by an initiating button (IB)  52 . The control logic  53  stores the received value of the number N of permitted iterations in the memory  51  and controls the counter  50  to count the number of received or transmitted messages during key establishment. Based on a comparison of the count value at the counter  50  and the value of the number N stored in the memory  51 , the control logic  53  can determine when the permitted number of iterations has been reached. Furthermore, the control logic  53  controls a key storage or key memory (KMEM)  55  for keying material, and an XOR combination circuit or block  57  for key generation. Input/output signals are received/transmitted via a transceiver (TRX)  56  and an antenna  58 . Random numbers received by the transceiver  56  from the remote control device are memorized or recorded in the key memory  55  and XOR-combined at the XOR combination block  57  to generate the shared secret. 
       FIG. 12  shows a flow diagram of a key distribution procedure at a remote control side of a multiple-key combination system according to a seventh embodiment. The procedure of  FIG. 12  may be implemented as a software routine controlling e.g. the CPU  27  of  FIG. 6 . 
     In step S 300  the initiation button for key establishment is pressed on the remote control device. Then, in step S 301 , a value for the iteration number or number N of messages used to generate the encryption key or secret key is selected. This may be achieved based on the type of data to be encrypted (e.g. the specific application of the secured transmission), or an individual user, manufacturer, or operator setting. In step S 302 , an initiating message including the selected value of the number N is sent to a TV to be controlled by the remote control device, and the remote control device then waits for an acknowledgement from the TV. In the subsequent step S 303 , the remote control device generates a 128-bit random number, and sends it in step S 304  in a key material message to the controlled TV and waits for an acknowledgement. Then, in step S 305 , the remote control device checks if the signaled number N of messages have been transmitted. If the number N of messages have not been transmitted yet, the procedure jumps back to step S 303  in order to generate a new random number. Otherwise, if the number N of messages have been transmitted, the random numbers transmitted are XOR-combined in step S 306  to form a shared secret key. In step S 307 , the remote control device generates some message, secures it using the generated key, and sends it to the controlled TV. Then, in step S 308 , the remote control device waits for a secure message from the controlled TV, and checks if the security of the received message is ok. If it determines in step S 309  that the security is not ok, the procedure jumps back to step S 302  in order to send a new initiating message. Otherwise, if it is determined in step S 309  that the security is ok, it is concluded in step S 310  that key establishment is finalized. 
       FIG. 13  shows a flow diagram of a key distribution procedure at a TV side of the multiple-key combination system according to the seventh embodiment. The procedure of  FIG. 13  may be implemented as a software routine controlling e.g. the CPU  19  of  FIG. 5 . 
     In step S 400  the initiation button for key establishment is pressed at the TV device. Then, in step S 401 , the TV device waits for an initiating message from the remote control device. In step S 402 , a received value for the iteration number or number N of messages is recorded in order to set the iteration number for key generation. In the subsequent step S 403 , the TV device waits for a key material message from the remote control device and sends an acknowledgement after receipt thereof. Then, in step S 404 , the TV device records a 128-bit random number received from the remote control device. In step S 405 , the TV device checks if the signaled number N of messages have been received. If the number N of messages have not been transmitted yet, the procedure jumps back to step S 403  in order to wait for a new key material message. Otherwise, if the number N of messages have been transmitted, the random numbers received are XOR-combined in step S 406  to form a shared secret key. In step S 407 , the TV device waits for a secure message from the remote control device secured with the generated key, and checks if the security of the received message is ok. If it determines in step S 408  that the security is not ok, the procedure jumps back to step S 401  in order to wait for a new initiating message. Otherwise, if it is determined in step S 408  that the security is ok, the TV device generates in step S 409  some message, secures it using the generated key, and sends it to the remote control device. Then, it is concluded in step S 410  that key establishment is finalized. 
       FIG. 14  shows a schematic block diagram of a hardware implementation at a remote control side of a secured multiple-channel multiple-key transmission system according to an eighth embodiment. The blocks depicted in  FIG. 14  may be implemented as discrete hardware circuits implemented in a module, arranged on a circuit board, or integrated on a single or multiple chip device. A control logic (CTRL)  73  which may be realized as a software-controlled CPU or as discrete logical circuit has access to a counter (C)  70  and a memory (MEM)  71 , and can be controlled by an initiating button (IB)  72 . The control logic  73  stores the transmitted value of the number N of permitted iterations in the memory  71  and controls the counter  70  to count the number of received or transmitted messages during key establishment. Based on a comparison of the count value at the counter  70  and the value of the number N stored in the memory  71 , the control logic  73  can determine when the permitted number of iterations has been reached. Furthermore, the control logic  73  controls a random number generation circuit or block (RNG)  74  and a working key storage or working key memory (WKMEM)  75  for a working key. Input/output signals are received/transmitted via a transceiver (TRX)  76  and an antenna  78 . The random number generation block  74  generates random numbers and supplies them to the transceiver  76  for transmission to the controlled TV device. The generated and signaled random numbers are memorized or recorded in the working key memory  75  to generate the shared secret. Additionally, the control logic  73  controls a channel change circuit or block  77  which controls the channel used by the transceiver  76  to transmit/receive output/input signals. 
       FIG. 15  shows a schematic block diagram of a hardware implementation at a TV side of the secured multiple-channel multiple-key transmission system according to the eighth embodiment. The blocks depicted in  FIG. 15  may be implemented as discrete hardware circuits implemented in a module, arranged on a circuit board, or integrated on a single or multiple chip device. A control logic (CTRL)  83  which may be realized as a software-controlled CPU or as discrete logical circuit has access to a counter (C)  80  and a memory (MEM)  81 , and can be controlled by an initiating button (IB)  82 . The control logic  83  stores the received value of the number N of permitted iterations in the memory  81  and controls the counter  80  to count the number of received or transmitted messages during key establishment. Based on a comparison of the count value at the counter  80  and the value of the number N stored in the memory  81 , the control logic  83  can determine when the permitted number of iterations has been reached. Furthermore, the control logic  83  controls a key storage or key memory (WKMEM)  85  for a working key. Input/output signals are received/transmitted via a transceiver (TRX)  86  and an antenna  88 . Random numbers or keys received by the transceiver  56  from the remote control device are memorized or recorded in the working key memory  85  to generate the shared secret. Additionally, the control logic  83  controls a channel change circuit or block  87  which controls the channel used by the transceiver  86  to transmit/receive output/input signals. 
       FIG. 16  shows a flow diagram of a key distribution procedure at a remote control side of a secured multiple-channel multiple-key transmission system according to a ninth embodiment. The procedure of  FIG. 16  may be implemented as a software routine controlling e.g. the CPU  27  of  FIG. 6 . 
     In step S 500  the initiation button for key establishment is pressed on the remote control device. Then, in step S 501 , a value for the iteration number or number N of messages used to generate the encryption key or secret key is selected. This may be achieved based on the type of data to be encrypted (e.g. the specific application of the secured transmission), or an individual user, manufacturer, or operator setting. In step S 502 , an initiating message including the selected value of the number N is sent to a TV to be controlled by the remote control device, and the remote control device then waits for an acknowledgement from the TV. In the subsequent step S 503 , the remote control device generates a 128-bit random number, and in step S 504  it generates a random channel number. Then, in step S 505  the remote control device sends a key transport message secured with a working key to the controlled TV and waits for an acknowledgement. In the next step S 506 , the remote control device changes to the previously selected new channel. In step S 507  it sets the working key to the value or pattern of the received random number, and then checks in step S 508  if the signaled number N of messages have been transmitted. If the number N of messages have not been transmitted yet, the procedure jumps back to step S 503  in order to generate a new random number. Otherwise, if the number N of messages have been transmitted, the working key is stored in step S 509  as the key shared with this TV. In step S 510 , the remote control device generates some message, secures it using the stored key, and sends it to the controlled TV. Then, in step S 511 , the remote control device waits for a secure message from the controlled TV, and checks if the security of the received message is ok. If it determines in step S 512  that the security is not ok, the procedure jumps back to step S 502  in order to send a new initiating message. Otherwise, if it is determined in step S 512  that the security is ok, it is concluded in step S 513  that key establishment is finalized. 
       FIG. 17  shows a flow diagram of a key distribution procedure at a TV side of the secured multiple-channel multiple-key transmission system according to the ninth embodiment. The procedure of  FIG. 17  may be implemented as a software routine controlling e.g. the CPU  19  of  FIG. 5 . 
     In step S 600  the initiation button for key establishment is pressed at the TV device. Then, in step S 601 , the TV device waits for an initiating message from the remote control device. In step S 602 , a received value for the iteration number or number N of messages is recorded in order to set the iteration number for key generation. In the subsequent step S 603 , the TV device waits for a key transport message from the remote control device, sends an acknowledgement after receipt thereof, and removes security from the received message. Then, in step S 604 , the TV device records a 128-bit random number retrieved from the received message, and in step S 605 , the TV device records a channel number retrieved from the received message. Then, in step S 606  the TV device changes to the new channel signaled from the remote control device, and sets in step S 607  the working key to the value of the transmitted random number. In step S 608 , the TV device checks if the signaled number N of messages have been received. If the number N of messages have not been transmitted yet, the procedure jumps back to step S 603  in order to wait for a new key transport message. Otherwise, if the number N of messages have been transmitted, the working key is stored in step S 609  as the key shared with this remote control device. In step S 610 , the TV device waits for a secure message from the remote control device secured with the stored key, and checks if the security of a received message is ok. If it determines in step S 611  that the security is not ok, the procedure jumps back to step S 601  in order to wait for a new initiating message. Otherwise, if it is determined in step S 611  that the security is ok, the TV device generates in step S 612  some message, secures it using the stored key, and sends it to the remote control device. Then, it is concluded in step S 613  that key establishment is finalized. 
     In summary, an apparatus and a method for performing cryptographic secret key distribution have been described, wherein a value for a number of iterations can be individually set, so that the number of messages to be exchanged during generating a cryptographic secret key can be varied based on the set value of the iteration number. 
     It is noted that the present invention is not restricted to the above embodiments and can be used for any key distribution scheme in any type of application, not only between a remote control device and a TV device, to provide a secure transport mechanism between devices in the network. Key management can be kept simple and transparent—thus minimizing the impact to the consumer experience. 
     In an LQI-based modification of the above RSSI-based embodiments, the same key may be generated on both devices, e.g. from dummy packets exchanged on the wireless or radio frequency (RF) link following the proposed signaling of the iteration number N. Acknowledged packets (of minimal data) are exchanged between devices, and the link quality is measured for every packet exchange. From the LQI variation over time, the key can be generated. The variation of the link quality over time could be “enhanced” by moving of at least one of the nodes or by changing the physical environment between the two devices. 
     Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality of elements or steps. A single processor or other unit may fulfill the functions of  FIGS. 7 to 17  and several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program used for controlling a processor to perform the claimed method features may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope thereof.