gpg: add --passphrase-env VARNAME to read passphrase from environmentdkg/passphrase-env

* g10/keydb.h: declare set_passphrase_from_environment_variable()
* g10/passphrase.c: set_passphrase_from_environment_variable() new
function
* g10/gpg.c: add new --passphrase-env argument, handle it.

--

There are problems or difficulties (to varying degrees) with all of
the techniques available for sending a passphrase directly to the
GnuPG process when --pinentry-mode=loopback:

 * Passphrases on the command line often leak into the process table.

 * Passphrases in a file often leak into the disk.

 * Using an extra file descriptor to send a passphrase works well on
   platforms that make it easy to allocate and use extra file
   descriptors, but is pretty awkward on platforms that don't
   facilitate this.

So this patch adds a new form of passphrase-passing, using an
environment variable.  In POSIX shell, this looks like (for example):

    mypass="IUuKctdEhH8' gpg --batch --pinentry-mode=loopback\
      --passphrase-env=mypass --decrypt < message.txt

Hopefully, this is easier to use than --passphrase-fd on platforms or
language toolkits that don't facilitate file descriptor manipulation.

Signed-off-by: Daniel Kahn Gillmor <[email protected]>

1 files changed, 10 insertions, 1 deletions

diff --git a/doc/gpg.texi b/doc/gpg.texi
index 7f55cc7e3..55e31d2e7 100644
--- a/doc/gpg.texi
+++ b/doc/gpg.texi
@@ -670,7 +670,8 @@ used for no expiration date.
 If this command is used with @option{--batch},
 @option{--pinentry-mode} has been set to @code{loopback}, and one of
 the passphrase options (@option{--passphrase},
-@option{--passphrase-fd}, or @option{passphrase-file}) is used, the
+@option{--passphrase-fd}, @option{--passphrase-env}, or
+@option{passphrase-file}) is used, the
 supplied passphrase is used for the new key and the agent does not ask
 for it.  To create a key without any protection @code{--passphrase ''}
 may be used.
@@ -3172,6 +3173,14 @@ Note that since Version 2.0 this passphrase is only used if the
 option @option{--batch} has also been given. Since Version 2.1
 the @option{--pinentry-mode} also needs to be set to @code{loopback}.
 
+@item --passphrase-env @var{string}
+@opindex passphrase-env
+Use the value of the environment variable @var{string} as the passphrase.
+This can only be used if only one passphrase is supplied.
+
+This passphrase is only used if the option @option{--batch} has also
+been given, and if @option{--pinentry-mode} is set to @code{loopback}.
+
 @item --pinentry-mode @var{mode}
 @opindex pinentry-mode
 Set the pinentry mode to @var{mode}.  Allowed values for @var{mode}