How does companies like Storj handle the possible situation of farmers who can change the files that they store?

Say I upload to the blockchain cloud network the file "cat.jpg". Now a contract added to the blockchain, with details of : "cat.jpg is stored at:X,Y,Z and the hash of this file is 0x123".

Now, what happen if all the farmers who stores my cat.jpg file (X Y and Z in this exmaple) change this file? when I want to return this file from this blockchain cloud, I would see in the contract that X Y and Z have my files, and all of them change the file (by comparing the hash value of the file in the contract, and the hash value of the file after they changed it), and now what? Companies like Storj can't just say to the costumer that his file is corrupted.. what is the point of this costumer to use this cloud , if there's a possible chance that the farmers (clients that store my files) can easily (or not, but there's a possible ways) change my files?

So how do companies like Storj prevent this situation and promise that the file that I store on their blockchain cloud, will return to me without any corruption?

Thanks in advance

closed as off-topic by Tjaden Hess, Badr Bellaj, Rob Hitchens B9lab, niksmac, Afri Feb 10 '17 at 11:22

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up vote 3 down vote accepted

You refer the files by hashes. You check this hash when you get the file back. If the file is edited the hash changes. Mathematically it is not possible (layman terms) or very unlikely (mathematical terms) that one can edit a file and still retain the same hash. Furthermore if the file is damaged its hash changes.

Farmers are incentivized to store and retrieved files correctly through the economic model of the network. Otherwise they will lose future revenues.

From Storj whitepaper

We instead designate the client as the ultimate decision maker in the net- work. After all, it is the client who is paying for the storage space on the network. Using the blockchain as an irrefutable source, even though the client is the ultimate decision maker, it cannot cheat the farmer. Thanks to the au- diting algorithm, it does not matter how many dishonest or colluding nodes are on the network. Malicious nodes will not be able to pass audits and unique, encrypted redundant copies will ensure that colluding nodes and Sybil attacks 13 are not possible.

  • Ohtamma , I know that. this is what I wrote. My question was: what happen if the file change? I wouldn't get my file ever again, so what is the point of using this kind of cloud service instead centerllized service such as Dropbox of google drive? How companies like Storj prevent the fact that anyone can change the file that I stored? – Roy Kuper Feb 9 '17 at 20:26

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