For PCI/DSS it required to store SHA1 keys in a separate server. We can propose Cassandra integration as a remote HA storage holding secret keys.

Cassandra store in a column family a pre generated dictionary of keys.

Custom column data type store encrypted values and in a header the key of Cassandra column family.

On write in the column the Cassandra key should be randomly changed using a function hold in cache for period of time. We re generate the function reading some informations in the status table that should give a level of randomness to avoid dictionary attacks