Background information:

With GnuPG 2.1, my webmailer does no longer work.

In principle, I use the following procedure e. g. for signing an e-mail:

1. My GnuPG 2.0 is compiled with the option

--with-pinentry-pgm=/path/to/pinentrywrapper

2. The user enters text and passphrase into the HTML form.

3. I encrypt the passphrase with symmetric cryptography

4. I set the environment variable PINENTRY_USER_DATA to the encrypted password

(see also T799)

5. I set the environment variable GPG_TTY to "PINENTRY/pinentry-permail"

6. I also set the environment variables HOME and GNUPGHOME.

7. I launch /path/to/gpg-agent --daemon --sh --no-allow-mark-trusted

8. I parse the output GPG_AGENT_INFO=/path/to/socket:process_number:version_number

9. Then I sign, encrypt, decrypt, verify or whatever the user wants by
   • putting GPG_AGENT_INFO and all other needed variables into the environment
   • starting /path/to/gpgsm with all needed options for the respective transaction

10. Then gpgsm contacts the just started gpg-agent which calls my

/path/to/pinentrywrapper which detects the "magic" GPG_TTY setting and does not
try to start a dialog on the (non-existent) terminal or desktop, but simply
responds with the decrypted content of PINENTRY_USER_DATA whenever a passphrase
input is requested.

11. Finally I kill the gpg-agent using the process_number extracted above.

This procedure does no longer work with GnuPG 2.1 because I cannot start a new
agent for every transaction: gpg-agent of 2.1 uses the default socket, not a new
one, and does not write its process_number into GPG_AGENT_INFO, and, most
important, gpgsm disregards GPG_AGENT_INFO so that I cannot tell gpgsm which
running gpg-agent to contact. (There can be multiple transactions at the same
time; I trust in gpg-agent to properly lock files where necessary.)

As long as there is no way of passing the entered passphrase from my webmailer
to gpg-agent in any other way than by writing it into the environment when
starting gpg-agent and using a special pinentry that reads this environment, I
have to start a new gpg-agent for every transaction because different
transactions may need different passphrases.

That, of course, is only an ugly, ugly circumvention of a limitation of gpgsm.

gpg2 knows options --pinentry-mode loopback --passphrase-fd file_number, and
gpg-agent offers all support for using these options. Only gpgsm does not
support it.

If gpgsm would also offer these options, the whole hack with a magic GPG_TTY,
with the encrypted PINENTRY_USER_DATA, with using a pinentry wrapper, and with
using special options when compiling GnuPG 2.0 would be completely unnecessary.

So please please please copy the code that implements --pinentry-mode loopback
--passphrase-fd file_number from gpg2 to gpgsm.

Thank you very much!

Okay that can be done. It won't be in 2.1., though.