encrypting a query string

Hi all,

   Is it possible to take a string (ie: a user's password) and have 
postgres encrypt the string before performing the query?

   At the moment, I am using postgresql + postfix for email. I need to 
save the passwords in clear text in the DB and I don't feel safe doing 
that. I'd like to save the passwords as an SHA1 hash. Then when postfix 
checks the password it uses a query that converts the passed password 
into an SHA1 hash then performs the comparison.

   So a pseudo code of what I'd like is:

SELECT foo FROM table WHERE passwd=sha1($password);

   I did a search on postgres's website and the only reference to SHA1 I 
saw was in the connection to psql. I hope this doesn't mean it's not 
possible. I'm much less familiar with postfix and am hoping to avoid 
mucking around with it. :P

Thanks!!

Madison