# Transferring credence without transferring evidence?

Agent X has evidence for proposition P that agent Y does not have, and X wants to convince Y that P is true (or more precisely, increase Y's credence in P as well). However, there are the following unfortunate conditions:  
1) Y doesn't trust X, meaning that X just telling Y that they have evidence will do little to convince Y. (In particular, they don't have [common knowledge of their credences](https://www.lesswrong.com/tag/aumann-s-agreement-theorem).)  
2) X wants to reveal as little as possible about the evidence they have.  
Is there anything clever X and Y can do here? Or is there a reason why this is impossible? Can one prove a tradeoff between the update on Y's credence and the amount of details about the evidence revealed? If this is too vague/does not make formal sense for some reason, as well as for illustration, here are some particular examples of what I have in mind:  
  
Example 1: X has a picture of the Moon and wants to convince Y that the Moon exists. The picture is a selfie taken by Adolf Hitler living on the far side of the Moon; X doesn't want Y to know the various other things of which this picture or X's possession of the picture would be evidence (e.g. because X wants to maintain their edge in the "space Nazi invasion of Earth by 2030" prediction market), so just handing the picture to Y is not a good option for X.  
  
Example 2: The State Department wants to convince the public that Russia is in the process of making a specific disinformation video while revealing as little as possible about how they found out that this is the case. See [https://www.youtube.com/watch?v=8G3WtjU9XA4](https://www.youtube.com/watch?v=8G3WtjU9XA4) and [https://www.youtube.com/watch?v=rU1_BHMHsI8](https://www.youtube.com/watch?v=rU1_BHMHsI8) (but let's ignore the question of whether they actually, in reality, in this particular case, have information or not; this is just supposed to be a hypothetical example for the purpose of this discussion).  
  
  
My observations thus far & more refined versions of the previous questions:  
This seems obviously close to [zero-knowledge proofs](https://en.wikipedia.org/wiki/Zero-knowledge_proof), but I haven't been able to flesh out a correspondence to any satisfactory degree. Should X be trying to give something like the evidential analogue of a zero-knowledge proof of P?  Or should X be trying to give something like a zero-knowledge proof of "I have evidence that P"? Maybe these are equivalent, but one is a better way to think about it than the other? What would such a thing look like in the above examples? Is the best X can do to just generally become more trustworthy, or maybe to generally provide evidence they are trustworthy? Perhaps the reason some googling did not provide an answer is that I'm missing the right vocabulary; if so, I'd appreciate any pointers.  
  
Edit with some further thoughts: I think one might have a chance to make the correspondence with zero-knowledge proofs work if "being a picture that, if handed to me by some particular person, would increase my credence that the Moon exists" were an NP complete problem in some unclear sense. Then maybe one could reduce it to a graph coloring problem and give a zero-knowledge proof of having a coloring of the graph (I recall at least the second step being explained well here [https://www.youtube.com/watch?v=5ovdoxnfFVc](https://www.youtube.com/watch?v=5ovdoxnfFVc) by Avi Widgerson). The obvious issue is that I have no idea how to write down a polynomial time verifier for something like "being a picture that would increase my credence that the Moon exists". How can agents X and Y agree on some graph to color which should have a coloring if and only if X has evidence that the Moon exists? That would be cool, but I have no idea how to do this. If we both knew a priori that the Moon should have some hard to compute graph coloring drawn on its surface, i.e. we both knew exactly what the hard graph to color that happens to be on the moon is, but Y did not know the coloring, then you could convince me you have a picture of the moon by giving a zero-knowledge proof that you know the coloring. Is there something physical about the Moon which is sufficiently similar to such a thing? Locations of craters are easy to verify to be legit but it's hard to construct legit arrangements of craters? Giving a list of details about the Moon more generally that is computationally hard to come up with but easy to check if it makes sense? By the way, in such cases, I think X providing more details of a situation can actually increase the credence Y has in X being truthful, which interacts interestingly with [https://www.lesswrong.com/posts/Yq6aA4M3JKWaQepPJ/burdensome-details](https://www.lesswrong.com/posts/Yq6aA4M3JKWaQepPJ/burdensome-details) (there is of course no contradiction here; I think it's just that being able to provide additional descriptive details about something in such cases is itself evidence for the describeratum, even as the describeratum becomes more detailed, and it seems possible for the balance to be in the direction of higher credence).