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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?

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In most of the cases I can think of, what can be achieved with OP_RETURN should be achieved with contracts.

For example what Open Assets does can be done via a contract - either a giant contract with all assets or a simple contract managing each separate asset. For the second option see "token contracts" (read more here and here)

If you really need or insist to copy how OP_RETURN works, that is possible also. Each transaction has (among other fields):

  • recipient
  • value
  • data / bytecode

This data contains the contract bytecode when creating a new contract, otherwise contains the message to be passed when executing a contract.

There is nothing stopping you using the data field when making a transaction between external accounts (i.e. addresses which are not contracts). You'll need to pay appropriate gas as the basic 21000 will not cover you.

Important: this only works between external accounts. For contracts, that field will be interpreted as a command and it is very likely to fail.

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