md5 in pg_shadow?

Hi,
I was trying to write a function to validate the username and its password in
the pg_shadow table. The function should take in the username and password as
arguments and see if it matches with those in the pg_shadow table. But the md5
hashed password in pg_shadow doesnot match with what i get when i do
select 'md5' || md5('some_password');

Is there a function already present to do this or how can i get the same hashed
password that i get in pg_shadow so i can write my own function?


Please help me out on this.
Thanks,
Subhash.