Evaluation of a monitoring function

A method of operating a server to evaluate a monitoring function to determine if a trigger condition is satisfied, the server storing a current monitoring state Ss of the server or an encrypted current monitoring state S of the monitoring function, the current monitoring state Ss of the server relating to the current monitoring state S of the monitoring function that is based on an evaluation of one or more previous events;

• wherein the method comprises:
• receiving an indication of a first event from a first client node;
• evaluating the monitoring function to determine if the first event satisfies the trigger condition, wherein the evaluation is performed using a privacy-preserving computation, PPC;
• wherein the server provides the current monitoring state Ss of the server as a first private input to the PPC or the encrypted current monitoring state S of the monitoring function as a first input to the PPC, and the PPC comprises the first event or an encryption thereof as a private input to the PPC of the first client node, wherein the current monitoring state is kept secret from the client and the first event is kept secret from the server,
• wherein the evaluation of the monitoring function provides an encrypted updated monitoring state S′ of the monitoring function or an updated monitoring state Ss′ of the server as an output of the monitoring function and an indication of whether the first event satisfies the trigger condition.

A first client node for use in evaluating a monitoring function to determine if a trigger condition is satisfied, the first client node comprising:

• a processing unit configured to:
• send an indication of a first event to a server,
• evaluate the monitoring function to determine if the first event satisfies the trigger condition, wherein the evaluation is performed using a privacy-preserving computation, PPC, with the server,
• wherein the PPC comprises a current monitoring state Ss of the server or an encrypted current monitoring state S of the monitoring function as a first private input to the PPC of the server and the first client node is configured to provides (i) a current monitoring state Sc of the first client node or an encryption of the current monitoring state S of the monitoring function;
• and (ii) the first event or an encryption thereof, as second private inputs to the PPC, wherein the current monitoring state is kept secret from the client and the first event is kept secret from the server,
• wherein the evaluation of the monitoring function provides an encrypted updated monitoring state S′ of the monitoring function or an updated monitoring state Sc′ of the first client node as an output of the monitoring function and an indication of whether the first event satisfies the trigger condition.

A method of operating a first client node to evaluate a monitoring function to determine if a trigger condition is satisfied, the method comprising:

• sending an indication of a first event to a server,
• evaluating the monitoring function to determine if the first event satisfies the trigger condition, wherein the evaluation is performed using a privacy-preserving computation, PPC, with the server,
• wherein the PPC comprises a current monitoring state Ss of the server or an encrypted current monitoring state S of the monitoring function as a first private input to the PPC of the server and the first client node provides (i) a current monitoring state Sc of the first client node or an encryption of the current monitoring state S of the monitoring function;
• and (ii) the first event or an encryption thereof, as second private inputs to the PPC, wherein the current monitoring state is kept secret from the client and the first event is kept secret from the server,
• wherein the evaluation of the monitoring function provides an encrypted updated monitoring state S′ of the monitoring function or an updated monitoring state Sc′ of the first client node as an output of the monitoring function and an indication of whether the first event satisfies the trigger condition.