One can get tx history of non his address
If you know someones address, yopu can just make curl query to backends api to get its transactions. Well that is not good.

To solve this, viewkey will be used. But we dont want to store in in mysql. Instead we can store a hash of the viewkey. Then any tx related api request will need to check if hash of viewkey provided matches to what we have in database for that address. 