A method for validating data received by a first party from a second party, the data comprising an aggregate sum of units of data recorded by one or more metering devices, the method comprising: receiving, at the first party, the aggregate sum of units of data from the second party and an encrypted aggregate sum of the units of data from a message aggregator to which each metering device has reported its readings, the aggregate sum being encrypted using an encryption key associated with the second party, encrypting the sum of the units of the data received from the second party using the encryption key; and comparing the result of encrypting the sum of the units of data received from the second party with the encrypted aggregate sum received from the message aggregator.