masking

I need to mask a substring of a column.

example the account_id is the first 3 letters of my last name (name_id) and the last 4 of my cc_number. 

I need to mask this account_id column that is located EVERYWHERE.

I need to create a foreign key constraint on name_id (VARCHAR2(3)) and substr(account_id,1,3) so the masking process can do its magic.

I was able to get the key in a view but the view wasn't updateable!

I indexed the column on the first 3 characters of the account_id but couldn't get the foreign key on index working.