Alex, thank you for your answer, which did not helped me at all but committed me to verify once again this point.
Yes I looked before at auth.log, but since the auth.log tells the same thing whether I activate debug in pam.d/common-auth or not (see below), I decided to ask here.
log WITHOUT ACTIVATING debug when I auth via ssh :
May 12 14:35:49 wheezy-test sshd[20586]: Accepted password for root from 192.168.56.100 port 37456 ssh2
May 12 14:35:49 wheezy-test sshd[20586]: pam_unix(sshd:session): session opened for user root by (uid=0)
log WITH debug ACTIVATED
May 12 14:37:35 wheezy-test sshd[20637]: Accepted password for root from 192.168.56.100 port 37457 ssh2
May 12 14:37:35 wheezy-test sshd[20637]: pam_unix(sshd:session): session opened for user root by (uid=0)
So, the difference between debug activated or not in auth.log is not obviously different.
moreover, since I tried to catch every debug level message in /var/log/debug.log, I hoped that debug messages would go there too…
Did I still ask stupid question ? Please let me sleep tonight less stupid I woke up this morning.
Cedric