executing PGP-signed commands with procmail

-----BEGIN PGP SIGNED MESSAGE-----

Can I safely put the following rule in my .procmailrc so that it will
execute my own signed mail-order commands and nothing else? 

                # 1. check signed exec command with pgp and strip
                #    signature to prevent replay attacks
        :B fbw
        ^^-----BEGIN PGP SIGNED MESSAGE-----$$exec
        | pgp +batchmode -f +pubring=$HOME/.pgp/authorized.pgp
        :a fb   # 2. execute body with /bin/sh
        | sh
        :A fhw  # 3. generate return header
        | formail -r
        :a w    # 4. send result back
        ! -t

There seems to be no better pgp command syntax to have it check that
its input is indeed a message PGP signed by a specific user and have
it not let anything else pass through to stdout or exit(EX_OK=0).

I'd place a keyring with none but the authorized user's public key in
~/.pgp/pubring.pgp so that signature OKs from any other people's
signatures won't give any false positive.

Is this a safe method?

-----BEGIN PGP SIGNATURE-----
Version: 2.6.3i
Charset: latin1
Comment: http://home.pages.de/~oma/pgpkey.txt

iQCVAwUBND3kGvGfW7iHMpmVAQEnegP+KtcoVg3qzaLSeitCov9cOf8gO4LxSLwM
ye5OJoBo99kK75JY5+kV4nXBorndBe6byubk2xIN6E9LvXNVwg/hlMm/oOS9g6O9
xB9ytB1ETUuRPSKdduPtzhib77/0SlgOjBGInQyDrO00dxmJ0mCCT0Vg6RyB560L
N9VWw7uJbMM=
=pn8Q
-----END PGP SIGNATURE-----