It is worth taking a moment, having carefully defined and explored message integrity, to justify why it is a good thing, and why protecting message integrity is important enough to justify trumping the theoretical benefits some of the integrity attacks, such as annotation or ClearPlay's service, offer.
The logic is quite simple, flowing almost directly from the definition of free speech:
- the right to free speech
- The right to send any message in public, and the corresponding right to receive anybody's message in public, without being pressured, denied access, arrested, or otherwise punished by anyone, subject to somewhat fuzzy, but fairly well-understood exceptions.
This is straight from the definition of free speech, suitably updated for a message model rather then an expression model. The bold parts are the key point: If you can not know that your message is going to be accurately transmitted without someone else degrading its integrity, then you have lost the right to "send any message" in public; instead, you only have the right to "send a message subject to somebody else's modifications". This is not Free Speech.
On the flip side, the more-unusual case of externally-imposed integrity attacks (forcing everyone to use censorware for instance) means that you can not access anybody's message. You can only "receive a message subject to somebody else's modifications". This is not Free Speech.
You can not have Free Speech without guaranteeing integrity.