A method performed by a computing system includes receiving from a publishing entity a message and a first piece of evidence that the message was sent by the publishing entity, time-stamping the first piece of evidence, storing the time-stamped first piece of evidence, sending the message to a first subscribing entity, receiving from the first subscribing entity a second piece of evidence that the message was received by the first subscribing entity, time-stamping the second piece of evidence, and storing the time-stamped second piece of evidence.