It's a peculiarity of Oracle PL/SQL that stored procedure parameters and function return types cannot be limited. That is, we cannot have a procedure with a signature like this:
SQL> create or replace procedure my_proc (p1 in varchar2(30))
2 is
3 begin
4 null;
5 end;
6 /
Warning: Procedure created with compilation errors.
SQL> show error
Errors for PROCEDURE MY_PROC:
LINE/COL ERROR
-------- -----------------------------------------------------------------
1/34 PLS-00103: Encountered the symbol "(" when expecting one of the
following:
:= . ) , @ % default character
The symbol ":=" was substituted for "(" to continue.
SQL> create or replace procedure my_proc (p1 in varchar2)
2 is
3 begin
4 null;
5 end;
6 /
Procedure created.
SQL>
Sure we can define the procedure's parameter using a SUBTYPE but Oracle will ignore it. Same goes for function return types...
SQL> create or replace package my_subtypes as
2 subtype ltd_string is varchar2(30);
3 end;
4 /
Package created.
SQL> create or replace function my_func return my_subtypes.ltd_string
2 is
3 begin
4 return lpad('a', 4000, 'a');
5 end;
6 /
Function created.
SQL> select length(my_func) from dual
2 /
LENGTH(MY_FUNC)
---------------
4000
SQL>
The only way of limiting parameters and return types is to declare variables using subtypes within the stored procedure. Use the variables within the package, and assign them to the OUT paramters (or RETURN the variable for functions).
Which is a long-winded way of saying, you can use DBMS_OBFUSCATION_TOOLKIT.VARCHAR2_CHECKSUM
in your code confident that it won't prevent your function returning 32 characters.
However, it will confuse developers who will lookup the SUBTYPE declaration. In the worst case these people will use the subtype to declare their own working variables with the following tragic result:
SQL> declare
2 v my_subtypes.ltd_string;
3 begin
4 v := my_func;
5 end;
6 /
declare
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at line 4
SQL>
So, it is better not to use an inappropriate subtype. Instead declare your own.