Replaying a batch of secure commands in a secure channel

Communication method comprising, on a third-party device, the following steps:

• receiving, from a master device, a batch of commands to be sent to a slave device, the batch of commands including a command, called first command, indicating the establishment, with the slave device, of a communication channel secured by at least one session key dependent on a current value of a dynamic variable shared between the master device and the slave device, and at least one command, called second command, secured by a message authentication code computed using the session key;
• playing the batch of commands so as to send, to the slave device, the commands of the batch sequentially, command by command, as long as no error is detected;
• characterized in that it further comprises, on the third-party device, the following step:
• replaying the batch of commands as long as the third-party device does not receive, from the slave device, an indication of an update of the current value of the dynamic variable by the slave device.

Processing device comprising a non-volatile memory storing a current value of a dynamic variable, and a processor configured to:

• receive, from a third-party device, at least one command, called first command, indicating the establishment of a communication channel secured by at least one session key;
• compute an update value of the dynamic variable from the current value stored in non-volatile memory;
• generate the session key from the computed update value;
• receive, from the third-party device, at least one command, called second command, secured by a message authentication code;
• and
• check the message authentication code using the session key before executing said second command;
• characterized in that the processing device further comprises a test counter, and the processor is further configured to:
• increment the test counter on reception of the first command indicating the establishment of a secure communication channel;
• and
• overwrite the current value of the dynamic variable in non-volatile memory with the computed update value when the test counter reaches a threshold value.

Processing device comprising a processor configured to:

• receive, from a master device, a batch of commands to be sent to a slave device, the batch of commands including a command, called first command, indicating the establishment, with the slave device, of a communication channel secured by at least one session key dependent on a current value of a dynamic variable shared between the master device and the slave device, and at least one command, called second command, secured by a message authentication code computed using the session key;
• play the batch of commands so as to send, to the slave device, said commands of the batch sequentially, command by command, as long as no error is detected;
• characterized in that the processor is further configured to:
• replay the batch of commands as long as the processing device does not receive, from the slave device, an indication of an update of the current value of the dynamic variable by the slave device.