The account.search and ids.search APIs allow searching using "contains" on unencrypted text (string) fields. The "contains" operand is not case sensitive, but if you want to perform a search on an encrypted field, you must enter a full string. In addition, you cannot use the order by clause on encrypted fields.
Encrypted fields include any text fields you defined as encrypted in your schema and also the fields listed here:
Once a field is saved as encrypted, the process cannot be reversed.