I also suspect that 50% chance of being friendly is generous
This is rather relevant :)
Basically, I'm killing any AI that doesn't signal friendliness in some way. I currently have some prior that the AI is friendly, call it 1% chance of friendly. I've pre-commited to destroy AIs, so clearly I'll destroy any AI that has a posterior odd of 1% or less after I update on the message it sends. The exact threshold is unknown to me, because I'm human - so mild evidence of friendliness, say, 1.1%, might also still get it destroyed.
The AI gets one chance to update my probability that it's friendly (this is the original message). As you aptly demonstrated, hacking does not change my odds, so I retain my commitment to kill it.
The fact that I haven't changed my priors, that I haven't gained information, is, itself, something I can use as evidence. Same as how we conclude god doesn't exist because we would expect to see evidence if he did :)
(Ref: http://lesswrong.com/lw/ih/absence_of_evidence_is_evidence_of_absence/)
Eliezer proposed in a comment:
>More difficult version of AI-Box Experiment: Instead of having up to 2 hours, you can lose at any time if the other player types AI DESTROYED. The Gatekeeper player has told their friends that they will type this as soon as the Experiment starts. You can type up to one sentence in your IRC queue and hit return immediately, the other player cannot type anything before the game starts (so you can show at least one sentence up to IRC character limits before they can type AI DESTROYED). Do you think you can win?
This spawned a flurry of ideas on what the AI might say. I think there's a lot more ideas to be mined in that line of thought, and the discussion merits its own thread.
So, give your suggestion - what might an AI might say to save or free itself?
(The AI-box experiment is explained here)
EDIT: one caveat to the discussion: it should go without saying, but you probably shouldn't come out of this thinking, "Well, if we can just avoid X, Y, and Z, we're golden!" This should hopefully be a fun way to get us thinking about the broader issue of superinteligent AI in general. (Credit goes to Elizer, RichardKennaway, and others for the caveat)