email-print-mime-structure: Add --use-gpg-agent for decryption

In some cases, the user may want to try to use their own GnuPG secret
keys to decrypt encrypted parts of the message.

By default it is disabled so that we aren't accidentally triggering
the use of user secret key material.

Note that gpg(1) says:

    It is highly recommended to use [--batch] along with the options
    --status-fd and --with-colons for any unattended use of gpg.

I am deliberately choosing to not use either --status-fd or
--with-colons for email-print-mime-structure.

I'm not using --with-colons because there is no output from GnuPG that
we expect to be machine-readable -- we're just looking for the cleartext
of whatever ciphertext is in the message part.

I'm not using --status-fd because there is nothing actionable we can do
with GnuPG status messages, and asking for them would require switching
from subprocess.run to subprocess.Popen to take advantage of the
pass_fds argument, which in turn would make the script only work in a
POSIX environment (I believe, but have not tested, that the script can
currently be used on Windows).

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

1 files changed, 19 insertions, 5 deletions

diff --git a/email-print-mime-structure.1.pod b/email-print-mime-structure.1.pod
index b846d87..69b1cdc 100644
--- a/email-print-mime-structure.1.pod
+++ b/email-print-mime-structure.1.pod
@@ -29,6 +29,25 @@ standard input, this key will be tried for decryption.  May be used
 multiple times if you want to try decrypting with more than one secret
 key.
 
+OpenPGP secret keys listed in B<--pgpkey=> are used ephemerally, and
+do not interact with any local GnuPG keyring.
+
+=item B<--use-gpg-agent=>I<true>|I<false>
+
+If I<true>, and B<email-print-mime-structure> encounters a
+PGP/MIME-encrypted part, it will try to decrypt the part using the
+secret keys found in the local installation of GnuPG. (default:
+I<false>)
+
+If both B<--pgpkey=>I<KEYFILE> and B<--use-gpg-agent=true> are
+supplied, I<KEYFILE> arguments will be tried before falling back to
+GnuPG.
+
+If B<email-print-mime-structure> has been asked to decrypt parts with
+either B<--pgpkey=>I<KEYFILE> or with B<--use-gpg-agent=true>, and it
+is unable to decrypt an encrypted part, it will emit a warning to
+stderr.
+
 =item B<--help>, B<-h>
 
 Show usage instructions.
@@ -49,11 +68,6 @@ Show usage instructions.
 
 =head1 LIMITATIONS
 
-B<email-print-mime-structure> only decrypts encrypted e-mails using
-raw, non-password-protected OpenPGP secret keys (see B<--pgpkey>,
-above).  If it is unable to decrypt an encrypted part with the
-supplied keys, it will warn on stderr.
-
 B<email-print-mime-structure>'s output is not stable, and is not
 intended to be interpreted by machines, so please do not depend on it
 in scripts!