Lots of good stuff here but we still seem to be missing the critical
starting point: who do we trust and what is at risk.
Consider: do you trust the telephone company ? If so, then CNID/ANI
is sufficient (and actually stronger than call-back but not much, I have
two telephone lines at home.) Even more compelling is that CNID/ANI can
be completely tranparent to the user. Keep in mind that as telecommuting
increases and unmetered phone service becomes less available, call-back
may become desirable *but not for security*.
If on the other hand you are concerned that the information at risk is worth
the adversary subverting the telco switch or your trunk lines, then "more"
is needed (I would put that point around $100,000/loss of your "Get Out Of
Jail Free" card). "More" being full end to end encryption. As the cost of
"good enough (C)" encryption comes down, then move the point downward
accordingly but today, figure the cost of an "outside" attack on POTS is
Two factors: first the equipment/expertise required for a phone line attack
is not trivial. Second there is an element of risk since the tap/recording
device/forwarding mechanism is a physical device or code change that
can be detected if the right person looks.
On the Internet, the cost of an attack is much lower - so low that they
are often done for free, cellular telephone communications have the same
risk as the fraud statistics show. In addition, the risk of detection is
almost non-existant. No extra equipment is necessary.
Of course it is probably easier today for an attacker to get a job at an
ISP than a Telco but as yet that level of attack has not been necessary.
- Or has it ? For years I have been saying is that the major difference
between attacks by professionals and attacks by amateurs is that the
professional does not get caught primarily because you will never know
they were there.
End to end encryption answers this and many other questions by eliminating
the need to trust anything in the middle - if you can communicate, you
are authorized. This was the paradigm in use when the Internet began
in 1969. Like passwords which began in Roman times being changed daily
and have only gone downhill from there, the 'net started with something
that worked but people decided that was "too hard" and now are complaining
about the "easy" way being unsafe.