Automate gpg decryption

[Art Hampton]

Philippe Lochon wrote:
> 
> Hi everyone
> 
> I'm trying to automate file decryption with the following command:
> 
> gpg --output file --decrypt file.gpg
[snip]
> Is there a way to do this with gpg, or do I have to use expect ?


Philippe,

Several months ago, I joined this list to ask much the same question. 
Here's what I ended up with:

gpg --passphrase-fd <filedescriptor> --batch

where <filedescriptor> is the file descriptor that you will use to send
the password.

I have a setuid script which opens up a file with the passphrase, then
drops the setuid.  Next, it creates three pipes, one for STDIN, one for
STDOUT, and then a third for the passphrase file descriptor.  I then
fork a new process, get all the pipes set up correctly, and then exec
GPG as described above.

For my application, I'm signing something.  I send it via STDIN, then
close the STDIN pipe.  Then I send the passphrase, and close the
passphrase pipe.  I then save whatever I get back from STDOUT until I
hit the EOF.  Then I of course wait for the exit status of the child.

There may be a better way to do this.  I remember Werner mentioning
something about a co-process example, but I never got around to
investigating that.

Hope this helps.