A data center has a plurality of secure processing units; a plurality of data stores holding encrypted data records; and a network connecting the secure processing units and the data stores. The secure processing units comprise computing functionality configured to execute a data processing operation in parallel on the secure processing units by being configured to read encrypted records from the stores, process one or more of the encrypted records within the secure processing units, send one or more of the encrypted records to the stores. The data center is configured to carry out a secret shuffle of the data records to protect the privacy of data processed in the data center from an observer observing any one or more of: the reading of the records, the sending of the records, the writing of the records; the secret shuffle comprising a random permutation of the records hidden from the observer.