Receiver that prevents stall conditions in a transmitter while maintaining compatibility and method therefor - Patents.com

Abstract

A receiver configured to receive protected content from a variety of transmitters, where some transmitters impose a maximum response time between transmitting a challenge to the receiver and receiving a response from the receiver, while other transmitters lack such a requirement, requiring the receiver to be compatible with both systems. Such a receiver is configured to receive protected content from a transmitter, where the transmitter is configured to transmit a challenge to the receiver and receive a response from the receiver, the receiver having a processor configured to execute some or all of a challenge response generator, a communications receiver configured to receive a challenge from the transmitter, and a communications transmitter configured to return a response to the transmitter, where the challenge response generator is configured to receive a challenge from the communications receiver and transmit a response to the transmitter after a response delay, where the processor is configured to select one of a first response and a second response in response to the challenge response, where the first response is a valid response provided after the first response delay and the second response is an invalid response provided after the second response delay, where the second response delay is longer than the first response delay. By providing valid responses with short response times and invalid responses with long response times, compatibility with legacy transmitters is ensured, while at the same time ensuring that receivers lacking protection against stalls in the challenge-response procedure are reinitialized with the invalid responses.

Description

[Technical field]

[0001] The present invention relates to a receiver configured to receive protected content from a transmitter, the transmitter configured to send a challenge to the receiver and receive a response from the receiver, the receiver having a processor including a challenge response generator, a communication receiver for receiving a challenge from the transmitter, and a communication transmitter for returning a response to the transmitter, the challenge response generator configured to receive a challenge from the communication receiver, generate a response and transmit the response to the communication transmitter after a response delay time. [Background technology]

[0002] Such receivers are known from the Digital Content Protection LLC proprietary standard called HDCP 2.3 edition 2018, available at https: / / www.digital-cp.com / . Linked PDF file

[0003] A random number Rn is generated by the transmitter and sent to the receiver.

[0004] The receiver generates a response based on the challenge, which in the case of HDCP 2.3 is a modification of a random number Rn based on a shared secret previously shared by the transmitter and receiver. Thus, the receiver proves that it possesses the shared secret and that the response to the challenge actually originated from the same receiver with which the secret was shared. The random number Rn is later used in the establishment of a secure authenticated channel.

[0005] By imposing a time limit, a 2018 HDCP 2.3 compliant transmitter ensures that the receiver is local.

[0006] This locality check is in view of technological developments and the distribution of content over other channels that do not impose such requirements on the location of the receiver is no longer needed. Therefore, setting a maximum response delay time is no longer a useful requirement and has been dropped. Summary of the Invention [Problem to be solved by the invention]

[0007] However, these updated transmitters create the problem of losing compatibility with the large installed base of transmitters and receivers, as well as the installed base of legacy transmitters with which the updated receivers are based.

[0008] If modern receivers are no longer capable of providing responses to challenges in time to senders that still impose maximum response time requirements, the senders will determine that the locality check fails and will not provide the content, leading to customer frustration.

[0009] Also, the sender no longer has any requirements on when a response to a challenge it sent must be received, and as a result the system stops and does not recover. Thus, a sender without a time-based locality check must still require a maximum response delay time, after which the locality check is considered to have failed and a new locality check using a new Rn challenge can be initiated. However, some updated senders lack this time check and therefore never time out. This causes frustration for customers. [Means for solving the problem]

[0010] To overcome these problems, the processor is configured to select one of a first response and a second response in response to a challenge, the first response being a valid response to be provided after a first response delay and the second response being an invalid response to be provided after a second response delay, the second response delay being longer than the first response delay.

[0011] This selection for a receiver according to the invention between a valid response having a response delay that meets the timing requirements imposed by the legacy transmitter and an invalid response that does not meet the timing requirements imposed by the legacy transmitter allows the receiver according to the invention to operate properly with both legacy and updated transmitters, and the invalid response with a second, longer response delay ensures that the updated transmitter, lacking a timeout, is forced to retry the challenge-response procedure based solely on the received invalid response, thus avoiding stalls in the updated receiver.

[0012] For legacy transmitters, e.g., transmitters that comply with the 2018 HDCP 2.3 standard and therefore require a short response delay time, the first valid response is the relevant response time.

[0013] In the case of an updated transmitter that no longer uses a time requirement in the locality check and is at risk of stalling in the challenge-response procedure, a second invalid response ensures the continuation of the challenge-response procedure, while such a second response does not cause any problems for the legacy transmitter beyond the legacy transmitter having to retry the challenge-response procedure.

[0014] The receiver may alternate between the first and second responses as responses to successive challenges, or the receiver may send the second response if the protected data is not received from the updated transmitter, which may be caused by a stall of the updated transmitter in the challenge-response procedure.

[0015] In this way, the receiver functions properly without the risk of stalling with updated transmitters while maintaining compatibility with legacy transmitters.

[0016] In one embodiment, the second response delay exceeds a decision criterion for a legacy transmitter, but the first response delay does not exceed a decision criterion for a legacy transmitter.

[0017] By selecting a second response delay that exceeds the decision criterion of the legacy transmitter, it is ensured that the decision criterion of the legacy transmitter is based on the validity of the response and not on the late arrival of the response. Late arrival of a valid or invalid response is not used in the legacy transmitter since there is a predefined time limit that must be met by the timely arrival of a response. Since the updated transmitter does not have such a predefined time as a limit, the late arrival of an invalid response ensures that the updated transmitter retries (i.e., restarts) the challenge-response procedure, thus ending a stall situation that would otherwise lead to customer dissatisfaction.

[0018] In one embodiment, the second response delay is selected from a range of response delays that exceed all of the legacy transmitter's decision criteria.

[0019] Late arrival of a valid or invalid response is not used for legacy transmitters since there is a predefined time limit that must be met by the timely arrival of a response. Since the legacy transmitter has a fixed and strict predefined time limit, it does not matter how late the response arrives. Since the updated transmitter does not have such a predefined time as a limit, any invalid response arriving arbitrarily late still ensures that the updated transmitter retries (i.e., restarts) the challenge-response procedure. Since the function of this second response is to cause the updated transmitter to restart the challenge-response procedure, the second response delay can be selected from a range such that the second response delay exceeds the predefined time limit of the legacy transmitter.

[0020] In one embodiment, the legacy sender's decision criteria determine whether protected content is provided to the receiver.

[0021] The legacy transmitter uses a challenge-response procedure to determine, among other criteria, whether the protected content should be provided to the receiver. If the receiver of the present invention cannot meet this criteria, the protected content will not be provided by the legacy transmitter, leading to customer dissatisfaction.

[0022] The method according to the invention has similar advantages on the basis of the corresponding method steps. [Brief description of the drawings]

[0023] [Figure 1] FIG. 2 illustrates a legacy transmitter and a legacy receiver. [Diagram 2] FIG. 1 illustrates an updated transmitter and an updated receiver. [Diagram 3] Legacy locality check timing diagram. [Figure 4] Updated locality check timing diagram. [Diagram 5]1 shows a receiver according to the invention; [Figure 6] FIG. 4 is a timing diagram of a locality check between a legacy transmitter and receiver in accordance with the present invention. [Figure 7] FIG. 4 is a timing diagram of an updated transmitter-receiver locality check in accordance with the present invention; [Figure 8] 3 shows the steps of the receiver method according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] FIG. 1 shows a legacy transmitter and a legacy receiver.

[0025] The legacy receiver 2 is configured to receive protected content from the legacy sender 1, and the legacy sender 1 imposes a maximum response delay time between sending a challenge to the legacy receiver 2 and receiving a response from the legacy receiver 2.

[0026] To be able to provide a response to a challenge, the legacy receiver 2 has a processor 3. This processor may be a general purpose processor with associated circuitry for controlling the receiver, or it may be configured to control the challenge-response process together with any necessary external circuitry.

[0027] The processor comprises a challenge-response generator 4 which receives from the receiver's communication receiver circuit 5 a challenge transmitted by the transmitter 1 using the transmitter's communication transmitter circuit 7 which transmits a random number generated by a random number generator 8 included in the transmitter 1.

[0028] After receiving the challenge, the challenge response generator 4 calculates a response. This response can be, for example, a modification of the received challenge using a secret shared and known to both the transmitter 1 and the receiver 2. This calculation takes a certain amount of time, after which the challenge response generator provides the response to the receiver's transmit communication circuit 6, which transmits the response to the legacy transmitter 1. The legacy transmitter 1 receives the response via the transmitter's communication receive circuit 11. While the challenge was being transmitted to the legacy receiver 2, the legacy transmitter performed the same calculation as the challenge response unit 4 of the legacy receiver 2. The result of this local calculation performed in the legacy transmitter is provided to the locality verification unit 10, as well as the received response. This locality verification unit 10 performs two functions. It verifies that the locally calculated result is equal to the received response and that the received response was received within a predefined time. A timer 12 provides timing information to the locality verification unit 10. The timer is started when a challenge is sent by legacy sender 1 and is stopped when a response is received by legacy sender 1.

[0029] If both conditions are met, the locality verification unit 10 allows the legacy sender 1 to provide the protected content to the legacy receiver 2 .

[0030] To this end, the protected content is received by the transmitter 1 and encrypted by an encryptor 13. After encryption, the protected content is transmitted to the receiver using the transmitter's content transmitter circuit 14. It is received by the receiver's content receiver circuit 15, which provides the protected content to a decryptor 16, where the protected content is decrypted for further use.

[0031] The challenge can include a random number, which can be used in encrypting and decrypting the protected content.

[0032] FIG. 2 shows an updated transmitter and an updated receiver.

[0033] The updated receiver 22 is configured to receive protected content from the updated sender 21, and the updated sender 21 imposes a maximum response delay time between sending a challenge to the updated receiver 22 and receiving a response from the updated receiver 22.

[0034] To be able to provide a response to a challenge, the updated receiver 22 comprises a processor 3. This processor may be a general purpose processor with associated circuitry for controlling the receiver, or it may be arranged to control the challenge-response process together with any necessary external circuitry.

[0035] The processor comprises a challenge-response generator 24 which receives a challenge from the receiver's communication-receiver circuit 25 transmitted by the transmitter 21 using the transmitter's communication-transmit circuit 27 which transmits a random number generated by a random number generator 28 included in the transmitter 21.

[0036] After receiving the challenge, the challenge response generator 4 calculates a response. This response can be, for example, a modification of the received challenge using a secret shared and known by both the transmitter 21 and the receiver 22. This calculation takes a certain time, after which the challenge response generator provides the response to the receiver's transmit communication circuit 26, which transmits the response to the updated transmitter 21. The updated transmitter 21 receives the response via the transmitter's communication receive circuit 11. While the challenge is being transmitted to the updated receiver 22, the updated transmitter performs the same calculation as the challenge response unit 24 of the updated receiver 22. The result of this local calculation performed in the updated transmitter is the locally generated response and is provided to the locality verification unit 30 in the same way as the received response. This locality verification unit 30 performs a single function: it verifies that the locally calculated response is equal to the received response, and does not check that the received response was received within a certain time. Thus, the locality verification unit does not time out. Operation of the transmitter 21 will cease in this condition and the transmitter will have to be reset, leading to frustration by the user.

[0037] If a correct response is received, the locality verification unit 30 enables the updated sender 21 to provide the protected content to the updated receiver 22 .

[0038] To this end, the protected content is received by the transmitter 21 and encrypted by an encryptor 33. After encryption, the protected content is transmitted to the receiver using the transmitter's content transmitter circuitry 34. It is received by the receiver's content receiver circuitry 35, which provides the protected content to a decryptor 36, where the protected content is decrypted for further use.

[0039] However, if the updated sender 21 stalls for any reason during the challenge-response procedure, the protected content will not be provided to the updated receiver 22 .

[0040] FIG. 3 shows a timing diagram of the legacy locality check.

[0041] Figure 3 shows the behavior of a legacy transmitter on the left and a legacy receiver on the right.

[0042] The transmitter first generates a challenge, e.g. a random number Rn, and transmits this challenge to the receiver at time T1, e.g. using the command LC_INIT containing the random number Rn. This challenge is received by the receiver at time T3, and the receiver's challenge-response generator starts to calculate a response. This response can be, e.g., a modification of the random number Rn using a secret pre-shared between the transmitter and the receiver. In parallel, the transmitter generates a local response by performing the same calculation as the receiver's challenge-response generator. Once the receiver's challenge-response generator has calculated the response, this response is transmitted to the transmitter, as shown at time T4 in Fig. 3. After the transmitter receives the response at time T2, e.g. via LC_Send_Lprime, it compares the received response with the locally generated response. In addition, the locality verification unit checks whether the response is received within a predefined time limit, i.e., whether T2-T1<predefined time limit. If the locally generated response and the received response are identical and the response is received within a predefined time limit, the sender continues and provides the protected content to the receiver.

[0043] If the locally generated and received responses are not identical and / or exceed a predefined time, the transmitter retries the locality check by generating a new Rn and sending it as a new challenge to the receiver, without providing the receiver with any protected content in this case. If no response is received, the system times out based on a predefined time and a new challenge is sent to the receiver. For a complete description of such legacy systems, the document "High bandwidth Digital Content Protection System, Mapping HDCP to HDMI, Revision 2.3 Dated 28 February 2018, section 2.3 Locality check on pages 16 and 17" is included as a reference.

[0044] As a result, the legacy transmitter does not stall because a response has not been received.

[0045] A new challenge-response procedure is initiated after the expiration of the predefined time limit, so that late-received responses are ignored. Legacy senders perform 1024 retries before finally failing. Thus, a small number of invalid responses received after the expiration of the predefined time limit do not significantly delay the delivery of content to the receiver.

[0046] Figure 4 shows the updated locality check timing diagram.

[0047] Figure 4 shows the behavior of an updated transmitter on the left and an updated receiver on the right.

[0048] The transmitter first generates a challenge, e.g., a random number Rn, and transmits this challenge to the receiver at time T1, e.g., using the command LC_INIT containing the random number Rn. This challenge is received by the receiver at time T3, and the receiver's challenge response generator starts to calculate a response. This response can be, e.g., a modification of the random number Rn using a secret pre-shared between the transmitter and the receiver. In parallel, the transmitter generates a local response by performing the same calculation as the receiver's challenge response generator. Once the receiver's challenge response generator has calculated the response, this response is transmitted to the transmitter, as shown at time T4 in FIG. 4. When the transmitter receives the response, it compares the received response with the locally generated response. If the locally generated response and the received response are identical, the transmitter continues and provides the protected content to the receiver. If the locally generated response and the received response are not identical, the transmitter retries the locality check by generating a new Rn and transmitting it as a new challenge to the receiver. In this case, it does not provide the protected content to the receiver. If no response is received, the system stalls since the predetermined time is no longer checked. In this configuration, the time T2 of receipt of the response by the transmitter is no longer important, allowing relaxed processing times for computing the response from the challenge, but creating the problem that the system may stall waiting for a response from the receiver, since there is no internal timeout.

[0049] FIG. 5 shows a receiver according to the invention.

[0050] A receiver 52 according to the present invention is configured to receive protected content from an updated or legacy transmitter (not shown), where some transmitters impose a maximum response delay time between sending a challenge to a receiver 52 according to the present invention and receiving a response from the receiver 52 according to the present invention, while other transmitters do not impose such a predetermined time limit.

[0051] To enable it to provide a response to a challenge, the receiver 52 comprises a processor 53. This processor may be a general purpose processor with associated circuitry for controlling the receiver, or it may be arranged to control the challenge-response process together with any necessary external circuitry.

[0052] The processor 53 comprises a challenge-response generator 54 which receives the challenge sent by the transmitter, for example a challenge comprising a random number, from a communications receiver circuit 55 of the receiver.

[0053] After receiving the challenge, the challenge response generator 54 calculates a response. This response can be, for example, a modification of the received challenge using a secret shared and known to both the transmitter and the receiver 52. However, the processor 53 can also cause the challenge response generator 54 to generate an invalid response, for example by providing an incorrect challenge instead of the received challenge or by replacing the correct response generated by the challenge response generator 54 by an incorrect response, for example a fixed value. The valid and invalid responses are then each fed to a response delay control unit 59. This response delay control unit selects a delay from a range of delays. In the case of a valid response, the response delay control unit 59 selects a first response delay and in the case of an invalid response, the response delay control unit 59 selects a second response delay. The second response delay can be selected, for example, to exceed the decision criteria of the legacy transmitter, while the first response delay is selected so as not to exceed the decision criteria of the legacy transmitter. The second response delay can advantageously be selected from a range of response delays that exceed all the decision criteria of the legacy transmitter. The decision criteria of the legacy sender determine whether protected content is provided to the receiver.

[0054] The response delay control unit 59 then provides the response to the receiver's transmit communication circuitry 56, which transmits the response to the transmitter. The transmitter receives the response via the transmitter's receive communication circuitry. While the challenge is being transmitted to the receiver 52, the updated transmitter is performing the same calculations as the challenge response unit 54 of the receiver 52. The result of this local calculation performed in the transmitter is a locally generated response, which is provided to the locality verification unit as the received response. This locality verification unit either verifies that the locally calculated response is equal to the received response and does not check that the received response was received within a predefined time, or verifies that the locally calculated response is equal to the received response and also checks that the received response was received within a predefined time. Based on the verification performed by the locality verification unit, the protected content is provided by the transmitter to the receiver 52, the content is received by the receiver's content receiver 57, and the content receiver provides the protected content to the decryptor 58, where the protected content is decrypted for further use.

[0055] FIG. 6 shows a timing diagram of a locality check between a legacy transmitter and receiver in accordance with the present invention.

[0056] FIG. 6 shows the behavior of a legacy transmitter on the left and a receiver according to the invention on the right.

[0057] The transmitter first generates a challenge, for example a random number Rn, and transmits this challenge to the receiver according to the invention at time T1, for example using a command LC_INIT containing the random number Rn. This challenge is received by the receiver at time T3, and the receiver's challenge-response generator starts to calculate a response. This response can for example be a modification of the random number Rn using a secret previously shared between the transmitter and the receiver. In parallel, the transmitter generates a local response by performing the same calculation as the receiver's challenge-response generator. The receiver's challenge-response generator calculates the response. However, in comparison with the previous example, the receiver according to the invention can select two responses and associated response delay times. A valid response with a first short response delay time is sent back to the transmitter at T4, resulting in a response delay time T2-T1 measured by the transmitter that is shorter than a predefined time limit. However, the receiver according to the invention can also select an invalid response with an associated second longer response delay time. This second response delay time preferably exceeds the predefined time limit of the legacy transmitter so as to ensure that an invalid response, if sent at time T4', arrives at time T2' after the expiration of the predefined time limit at the transmitter, thus allowing the legacy transmitter to resume the challenge response based on the expiration of this predefined time limit rather than based on an invalid response. An invalid response arriving late at the legacy transmitter will cause the legacy transmitter to retry by sending another challenge, thus not stalling the transmitter. A valid response with a shorter response delay time will arrive at the legacy transmitter at time T2, so as to satisfy the predefined time limit imposed by HDCP 2.3, and the protected content can be provided. Having the legacy transmitter retry is not an issue, as it introduces only a minimal delay in the start of the provision of the protected content. When the transmitter receives a response, for example using the command LC_Send_Lprime, it compares the received response with the locally generated response.If the locally generated and received responses are identical, the sender continues and provides the protected content to the receiver. If the locally generated and received responses are not identical and / or the predefined time has expired, the sender retries the locality check by generating a new Rn and sending it as a new challenge to the receiver, without providing the protected content to the receiver in this case.

[0058] FIG. 7 shows a timing diagram of a locality check between an updated transmitter and a receiver according to the invention.

[0059] FIG. 7 shows the behavior of the updated transmitter on the left and the receiver according to the invention on the right.

[0060] The transmitter first generates a challenge, e.g. a random number Rn, and transmits this challenge to the receiver at time T1, e.g. using the command LC_INIT containing the random number Rn. This challenge is received by the receiver at time T3, and the receiver's challenge-response generator starts to calculate a response. This response can be, e.g., a modification of the random number Rn using a secret pre-shared between the transmitter and the receiver. In parallel, the transmitter generates a local response by performing the same calculation as the receiver's challenge-response generator. The receiver's challenge-response generator calculates the response. Compared to the previous example, the updated transmitter remains waiting for a response, effectively stalling the challenge-response procedure. Now, if the updated transmitter does not recognize a response, such as the unrecognized response sent at T4 and shown in grey in FIG. 7, or if no response is received from the receiver at all, the stall continues as no response is received and the transmitter does not reinitialize the new challenge-response procedure by itself as the legacy transmitter does.

[0061] To prevent this situation, the receiver according to the invention can send a second invalid response at time T4' to ensure that the updated transmitter reinitializes the challenge-response procedure and a new challenge is sent so that the receiver can calculate and provide a suitable correct response. The response delay control is used to delay the response if the response provided is an invalid response. If a valid response is provided, no delay or a short delay is used.

[0062] When the updated sender receives a response, it compares the received response with the locally generated response. If the locally generated and received responses are identical, the sender continues and provides the protected content to the receiver. If the locally generated and received responses are not identical, the sender reinitializes the locality check by generating a new Rn and sending it as a new challenge to the receiver. In this case, it does not provide the protected content to the receiver until a valid response is received. Because the updated sender does not impose a predetermined time limit, a later received response still allows the updated sender to provide the protected content.

[0063] FIG. 8 shows the steps of a receiver method according to the invention.

[0064] FIG. 8 illustrates a method for receiving protected content from a sender, where the sender imposes a maximum response delay time between sending a challenge and receiving a response, the method comprising: - receiving (101) a challenge from a transmitter, - a step (102) of generating a response; - transmitting a response to the transmitter after a response delay time (105); The method further comprises: - a step (103, 104) of controlling a response delay time, The method further comprises: - selecting (102) one of a first response and a second response in response to the challenge, The first response is a valid response that is to be provided after a first response delay, and the second response is an invalid response that is to be provided after a second response delay, the second response delay being longer than the first response delay.

[0065] In a step of controlling a response delay time (103, 104), the first valid response is associated with a first response delay time and the second invalid response is associated with a second response delay time, the first and second response delay times being different from each other.

[0066] In step 102, either the first valid response or the second invalid response is selected. This selection may be based on a predetermined or random pattern, or based on detection of a stalled transmitter condition by the receiver of the present invention, for example by detecting that after providing a response, no protected content is being provided, or that other parts of the procedure necessary to enable the provision of protected content are not being performed by the transmitter.

[0067] After selecting either the first valid response or the second invalid response, a response delay time is selected. If the first valid response is selected in step 102, a short response delay (or no response delay) is introduced in delay step 103. However, if the second invalid response is selected in step 102, a long response delay is introduced in delay step 104. The advantages of this are detailed in the receiver description and will not be repeated here for the sake of brevity.

[0068] After an appropriate delay is introduced in step 103 or 104, a (valid or invalid) response is sent to the transmitter (105). If the transmitter is stalled, the second invalid response will cause the transmitter to reinitialize the challenge-response procedure. If the second response is received by a legacy transmitter, the legacy transmitter will or has reinitialized the challenge-response and the response is ignored.

[0069] It should be noted that when the present application uses the terms first and second responses, this is used to refer simply to the type of response, not to the order and succession of the responses. The responses may be transmitted in any sequence. However, both updated and legacy transmitters enjoy compatibility with a receiver according to the present invention, at the expense of accidental failure of the challenge-response procedure.

[0070] Definition of Terms As used herein, the following terms have the following meanings: Legacy Transmitter: A transmitter that complies with earlier standards. Legacy Receiver: A receiver that complies with earlier standards. Updated Transmitter: A transmitter that conforms to a later version of a previous standard or conforms to an errata of such previous standard. Updated Receiver: A receiver which conforms to a later version of an earlier standard or conforms to an errata of such earlier standard.

[0071] It should further be noted that the receiver according to the present invention has been described using the HDCP standard because that standard is well understood by those skilled in the art and is publicly available. However, this does not mean that the invention is limited to this standard. Other data transmission standards have locality checks that impose time constraints on the response to a challenge, and the present invention can also be used with receivers of these standards.

Claims

1. 1. A receiver configured to receive protected content from a transmitter, the transmitter sending a challenge to the receiver and receiving a response from the receiver, the receiver comprising: a processor configured to execute part or all of a challenge-response generator; a communication receiver configured to receive a challenge from the transmitter; a communication transmitter configured to return a response to the transmitter; having the challenge response generator is configured to receive the challenge from the communication receiver, generate a response in response to the challenge, and transmit the response to the communication transmitter for return to the transmitter after a response delay; the processor is configured to select one of a first response and a second response in response to the challenge; the first response being a valid response to be provided after a first response delay; the second response being an invalid response to be provided after a second response delay; The second response delay is longer than the first response delay.

2. 2. The receiver of claim 1, wherein the second response delay exceeds a maximum response delay time of the legacy transmitter, and the first response delay does not exceed a maximum response delay time of the legacy transmitter.

3. The receiver of claim 2 , wherein the second response delay is selected from a range of response delays that exceeds all of the maximum response delay times of the legacy transmitters.

4. 4. The receiver of claim 2 or 3, wherein the maximum response delay time of the legacy sender determines whether protected content should be provided to the receiver.

5. 1. A method of receiving protected content from a transmitter, the transmitter imposing a maximum response delay between sending a challenge and receiving a response, the method comprising: receiving a challenge from the transmitter; generating a response in response to the challenge; sending the response to a transmitter after a response delay time; having The method further comprises controlling the response delay time; The method further comprises selecting one of a first response and a second response in response to the challenge; the first response being a valid response to be provided after a first response delay; the second response being an invalid response to be provided after a second response delay; The second response delay is longer than the first response delay.

6. The method of claim 5 , wherein the second response delay exceeds a maximum response delay time of the legacy transmitter and the first response delay does not exceed a maximum response delay time of the legacy transmitter.

7. The method of claim 6 , wherein the second response delay is selected from a range of response delays that exceeds a maximum response delay time of all of the legacy transmitters.

8. The method of claim 6 , wherein a maximum response delay time of the legacy sender determines whether protected content should be provided by the legacy sender.

9. A computer readable medium comprising instructions for carrying out the method of any one of claims 5 to 8.

10. A computer program for carrying out the steps of the method according to any one of claims 5 to 8.

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