I have tried this on
Fedora and on
kshitiz@kshitiz:~$su su: Authentication failure kshitiz@kshitiz:~$sudo su root@kshitiz:/home/kshitiz#
I am entering the same password in both the cases.
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su requires the password of the account whose privileges you are trying to assume (apparently
root in this case).
sudo requires the password of the current user - that is, the password for user
sudo su, you are effectively becoming
root, then running
su to get a root shell - that is, your privileges are already elevated to
root before the call to
su is executed, which is why you don't get prompted for the root password again.