Which hook to use for auto sign in message mode?

[XeCycle]

Hello, I want to set automatic signing with OpenPGP when
composing new messages.  I've added this to my .gnus.el:

          (add-hook 'message-setup-hook
          'mml-secure-message-sign-pgpmime)

But when I decide to reply someone, The <#secure> line is placed
below any quoted text, and on sending, the signing process is
ignored.  I guess I've chosen a wrong hook for that; so, which
hook is run after the original message is inserted?

Thank you.

-- 
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

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