[Enigmail] On signatures

[Jan Steffen]

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Robert J. Hansen wrote:
> So far a lot of people have been hotly contesting my claim that you
> cannot conclude anything from a bad (syntactically incorrect) signature.
> 
> I'm going to prove that you can't.  
[cut]

> Clearly, there are at least six different things that can cause a
> message to go haywire.  Each will be discussed in turn.
> 
> 1.  YOU'RE USING THE WRONG SIGNATURE ALGORITHM.
> 2.  YOU'RE USING A CORRUPT OR INCORRECT CERTIFICATE.
> 3.  YOU'RE USING THE WRONG HASH.
> 4.  THE MESSAGE HAS BEEN ALTERED.
> 5.  EQUALITY DOESN'T MEAN EQUALS.
> 6.  THE SIGNATURE ITSELF HAS BEEN ALTERED.
>
> ... There are six elements in that mathematical equation that tell us
> whether a signature is good or bad.  Any of the six elements can fail.
> One of those six elements is phenomenally unlikely.  The others are all
> within the realm of possibility.
> 

Let me reduce this to two causes: 1. message alteration or 2. technical
problems.

> If you want to assert that a bad signature means the message has been
> tampered with, there are three ways to go about it.
> 
> 1.  YOU CAN PROVE THE MESSAGE HAS BEEN ALTERED WITHOUT USING THE SIG.
> 2.  YOU CAN ELIMINATE POSSIBILITIES 1-3 AND 6.
> 3.  YOU CAN SIMPLY DECLARE #4 TO BE THE CASE.

Message alteration is indeed the most uncommon case and technical
problems are indeed quite common: incompatible mail agents, wrong
settings here and there, untrusted certificate/key and many more.
But these things can usually be overcome:
Just tell the sender about it and ask him to send the message again
(modify settings if necessary). Repeat that until the message comes
through correctly.

But here is your main flaw:
> By itself, a bad signature tells you absolutely nothing about whether a
> message was tampered with in transit.  By itself, a bad signature
> possesses absolutely no informational value.  

This conclusion is just wrong.
I don't *need* a 100% prove that the message was tampered with. Just the
possibility of alteration is enough reason to do some checking.
If I communicate with someone regularly and his messages are always
correctly signed and than suddenly one message has a "BAD SIGNATURE"
this can be s strong sign of message alteration (if neither he or me
changed any settings).
The bad signature is a sign that something *might* be wrong.

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

Greetings, Jan
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