PL/pgGRESQL, SHA, BYTEA - Creating SHA1 hash for Bytea Value in stored procedure

Hi!

I have the following scenario:
    Table:
        Fil_no    BigSerial
        Fil_Name    varchar
        Fil_Data    Bytea
        Fil_HASH    varchar

    Trigger:
        Before Insert  (in Pseudo Code)
            ...
            vchSHA = SHA(Fil_DATA)
            NEW.Fil_HASH = vchSHA
            ...

I tried to find a function that calculates the SHA (preference is sha-256),
but could not find anything for PostGRE. However, I want to implement the
calculation within the DB, for several reasons.

Has anyone an idea on how or any pointer into the right direction to
accomplish the above task?

Thanks already in advance!

-Enrico