[Enigmail] On signatures

[Robert J. Hansen]

Jan Steffen wrote:
> Other example: A smoke detector can give a false alarm, but that doesn't
> mean that a smoke detector "possesses absolutely no informational value."

Let's fix this analogy, and see where it leads us.

I have a truth alarm in my home which uses alien technology.  I use it
to keep my home safe against fires.  My truth alarm does not detect
smoke; it does not detect fire; it simply makes assertions about the
truth of various claims.  I have programmed my truth alarm to test the
truth of the claim "my home is on fire".

My truth alarm will never make a Type I error.  It will sometimes make a
Type II error.  If it were to ever make a Type I error--which it
can't--my truth alarm would assert my home was not on fire even while it
was burning down.  Sometimes it will make Type II errors and conclude my
home is on fire even when it's not.

If my truth alarm ever tells me my home is on fire, should I conclude my
home is on fire?

Clearly, no.  I know the truth alarm is capable of making errors.  I
should instead only draw the conclusion "my home /may/ be on fire".
That is the exact same knowledge I would have if I had no truth alarm at
all.  If I had no truth alarm, I would never know (barring seeing the
flames or smelling the smoke) if my home was on fire at any given
moment.  It may be.  It may not be.  I just wouldn't know.

As long as the truth alarm is telling me "the assertion 'my home is on
fire' is false", I can rely on that one hundred percent and be confident
my home is not on fire.  As soon as the truth alarm tells me "the
assertion 'my home is on fire' is true", I need to find out for myself
whether my home is on fire--I can't trust the truth alarm's answer.



... Now, OpenPGP happens to be a perfect mathematical analogue to the
alien truth alarm.  It will examine a message and attempt to determine
the truth of the proposition "this message has been tampered with".  If
OpenPGP tells me "the proposition 'this message has been tampered with'
is false", then I can be assured the message is authentic.  If OpenPGP
tells me "the proposition 'this message has been tampered with' is
true", then I need to find out for myself what's happened.

Exactly the same as if OpenPGP had not given me any answer at all.