Hello, I'm bothered by the behaviour that Gnus always asks me to
decrypt my .authinfo.gpg, though none of the servers need
authentication.

I had a look at the source and found the bug here:

At nntp.el, around line 1197, at beginning of defun
nntp-send-authinfo, a call to auth-source-search is made.  This
function parses entries in auth-sources, which should be
nntp-authinfo-file, as suggested by the manual; but we didn't set
it.  Therefore it's trying to visit the default auth source
files, and decrypts that.

I guess the solution is to add this in the let*, before that
call:

(auth-sources (list nntp-authinfo-file))

Can you add it and try?

PS. I don't know why specifying (nntp-authinfo-function my-no-op)
doesn't work, where my-no-op is a function that accepts any
parameter and does nothing; it's never called.  Perhaps another
bug out there.

-- 
Carl Lei (XeCycle)
Department of Physics, Shanghai Jiao Tong University
OpenPGP public key: 7795E591
Fingerprint: 1FB6 7F1F D45D F681 C845 27F7 8D71 8EC4 7795 E591