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I see that many new Bitcoin 2.0 apps are making use of the additional OP_RETURN parameter for attaching data. I see that max limit is 40 bytes or roughly 40 characters? My question now regarding OP_RETURN is:

  • So is it one OP_RETURN parameter for each bitcoin transaction or per each input/output address? If its each transaction, so its safe to say its about .0001 btc to send 40 byte message?

  • Which desktop / mobile wallets support adding a custom OP_RETURN message? (ex. attach a short url link)

  • Aside from OP_RETURN message attachment, what other methods are there to attach custom data?

  • Since the time this was first posted, the limit has been increased to 80 bytes – Stephen Gornick May 6 at 23:07
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So is it one OP_RETURN parameter for each bitcoin transaction or per each input/output address?

One OP_RETURN output per transaction regardless of the number of inputs and outputs is standard for 0.9.x bitcoin core clients.

If its each transaction, so its safe to say its about .0001 btc to send 40 byte message?

Yes, but you should not do it without significant reasons.

Aside from OP_RETURN message attachment, what other methods are there to attach custom data?

There are different methods. You can use fake public keys in bare multisig outputs. Or encode some data with values or addresses of other outputs.

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Aside from OP_RETURN message attachment, what other methods are there to attach custom data?

IP Transactions can do that. BIP0070 will probably be able to do that.

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Which desktop / mobile wallets support adding a custom OP_RETURN message? (ex. attach a short url link)

Coinb.in

New -> Transaction -> Advanced Options

Null Data (80 byte limit, 40 bytes recommended)

Allow data to be sent within the transaction and stored in the blockchain by using OP_RETURN.

When using this option you may enter a hex string or address into the address field on the output tab.

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