I think these two prompts could be combined:

84 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.
Need to get 71.4MB/72.2MB of archives.
After unpacking 488kB of additional disk space will be used.
Do you want to continue? [Y/n] y
WARNING: The following packages cannot be securely authenticated!
  debconf-english debconf base-config debconf-doc debconf-utils debhelper
Abort? [Y/n] n

It could look something like this:

84 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.
Need to get 71.4MB/72.2MB of archives.
After unpacking 488kB of additional disk space will be used.
WARNING: The following packages cannot be securely authenticated!
  debconf-english debconf base-config debconf-doc debconf-utils debhelper
Do you want to continue? [y/N]

Of course the do you want to continue would change its default if there were
unverifiable packages to be upgraded.

Hmm, interesting.  The reason I did it the way I did was to preserve the
meaning of APT::Get::Assume-Yes (-y).  I did _not_ want this flag to bypass
the authentication check.  apt-get(1):

              Automatic yes to prompts; assume "yes" as answer to all  prompts
              and  run non-interactively. If an undesirable situation, such as
              changing a held package or removing an essential package  occurs
              then  apt-get will abort.  Configuration Item: APT::Get::Assume-
              Yes.

In this case, these two sentences would be in conflict, since the answer
which would avoid the undesirable situation would be "no".