I know Ethereum is more advanced, and allows for more interesting things than associating data to a transaction. But let's say I have a legacy system that uses Bitcoin's OP_RETURN. What options do I have to move from the Bitcoin blockchain, to the Ethereum blockchain?
To put it another way, I need to add data, and then be able to look up that data by some unique id. (like Bitcoin's tx id). Also I need to be able to verify who placed the data. Can I just associate data to a tx? Or should I build a contract with a key-value field for the data, and add some way to verify authorship?