I got the same problem with DBMS_OBFUSCATION_TOOLKIT.MD5... There are 2 functions : one for VARCHAR2 and the other for RAW, but Oracle can't make the difference between the two signatures !

The solution is to use named parameters, but this is not supported in SQL : you have to call it from PL/SQL :

Code:
DECLARE
    strTmp VARCHAR2(100);
BEGIN
    strTmp  := DBMS_OBFUSCATION_TOOLKIT.DESGETKEY ( seed_string => 'XXXX' );
    DBMS_OUTPUT.PUT_LINE(strTmp);
END;
/
BTW according to the documentation , seed_string must be at least 80 characters long, so 'XXXX' won't work (I've tested with more than 80 "X" and it works ).

HTH & Regards,

rbaraer