The storage table contains the results of repeatable expression masking between sessions. A storage table row contains the source column and a masked value pair. The storage table for expression masking is a separate table from the storage table for substitution masking.
Each time the Data Masking transformation masks a value with a repeatable expression, it searches the storage table by dictionary name, locale, column name and input value. If it finds a row in the storage table, it returns the masked value from the storage table. If the Data Masking transformation does not find a row, it generates a masked value from the expression for the column.
You need to encrypt storage tables for expression masking when you have unencrypted data in the storage and use the same dictionary name as key.