Let say!
I am a data scientist, I have a model that I want to train it, lets say some deep neural network, I want to train it on your data. right? so if wana train on your data, If there is 3rd party involve, say some central app, right? some central server, so three different people can steal from the each other right?
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If I am the AI scientist and I am trying to train on your data, I can steal your data, right? If I give you my model you could steal my model, If we use the third party, It will steal your data and my model right?
So What is way we can have this transaction? your data my model, I pay for your data, I train my model on your data and return it back without anyone having to trust each other.