gnupg: GPGSM DECRYPT

 
 5.6.2 Decrypting a message
 --------------------------
 
 Input and output FDs are set the same way as in encryption, but 'INPUT'
 refers to the ciphertext and 'OUTPUT' to the plaintext.  There is no
 need to set recipients.  'GPGSM' automatically strips any S/MIME headers
 from the input, so it is valid to pass an entire MIME part to the INPUT
 pipe.
 
    The decryption is done by using the command
 
        DECRYPT
 
    It performs the decrypt operation after doing some check on the
 internal state (e.g.  that all needed data has been set).  Because it
 utilizes the GPG-Agent for the session key decryption, there is no need
 to ask the client for a protecting passphrase - GpgAgent takes care of
 this by requesting this from the user.