Jens Lechtenboerger wrote:
> Hi there,

> the calls to gnus-completing-read in smime-sign-buffer and
> smime-decrypt-buffer do not provide completion support.

> If I change gnus-completing-read-function from
> gnus-emacs-completing-read to completing-read, everything works:

> (setq gnus-completing-read-function 'completing-read)

> A related problem is reported there:
> https://bugs.launchpad.net/ubuntu/+source/emacs24/+bug/1096449

> Best wishes
> Jens

This has been fixed at 2013-05-01 (bug#14304), so it will work
properly in the forthcoming Emacs 24.4.  Here's a patch for 24.3: