Hi Yacov, 

thanks for your reply, let me clarify the jargon here so more people can understand

pre-image: data itself and its salt

"Private data is disseminated in a point to point manner among peers even now.The peers that posses the private data, send the peers that don't (but are eligible of receiving it) the hash pre-images, and the receiving peers validate the hash pre-images indeed correspond to the hashes on the public block.

I don't see any technical obstacle that prevents you to add a salt per collection name for a given transaction, that will be concatenated to the computation of the hash of the key and the value for the said collection.The salt can be part of the data element that is generated at the time of chaincode invocation, and will be passed along with the private data itself."

first of all, I appreciate you agree that another point 2 point connection must be established between orgs to pass the salt and the image itself, anything on chain can be used to launch pre-image/dictionary attack

of course there is no technical obstacle to create salt, but the issue here is that it creates a false sense that data is private and can be validated. let me explain:

you try to argue that the salted hash on the public chain is a proof that some data is "valid". this itself is a terrible argument because hashes (unlike digital signature, homomorphic encryption) is not something that others can verify when the data (hash) it put on public chain.

here is an example: my national ID is "1234567", but I am a bad guy and want others to believe that my national ID number is "7654321". so I put the false hash(salt, "7654321") on chain, and then send pre-images (salt, "7654321")  to whoever I want to convince. Since nobody can verify the hash(salt, "7654321")  when the hash was put on chain without prior knowledge of the data, an adversary can use the claims about private data functionality to trick people to believe forged data.

my point is that the claims about private data mislead people to believe this feature will either help to orgs to protect data or validate a pre-existing data, but neither is true and can be easily used by an adversary to decode data (if there is no salt or salt is known) or to trick people believe in wrong data like the sample above.