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Update 5_4_Sending_a_Raw_Transaction_with_Data.md
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# 5.1: Sending a Raw Transaction with Data
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# 5.4: Sending a Raw Transaction with Data
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> **NOTE:** This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
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A P2PKH transaction is one of the simplest forms of Bitcoin transaction... except perhaps the OP_RETURN. That's because an OP_RETURN is basically a null: an invalid output. Why would you use one? To store data on the blockchain: up to 80 bytes.
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This opens up whole new possibilities for the blockchain, because you can embed data that proves that certain things happened at certain times. Though there is some controversy over using the Bitcoin blockchain in this way, various organizations have used OP_RETURNs for proof of existence, for copyright, to color coins, and [for other purposes](https://en.bitcoin.it/wiki/OP_RETURN). Though 80 bytes might not seem a lot, it can be quite effective if OP_RETURNs are used to store hashes of the actual data.
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The basic `bitcoin-cli` interface for sending raw transactions has a few other features, which start to veer away from the idea of just instantly sending out funds. One of those is the ability to send up to 80 bytes of data to the blockchain using an OP_RETURN code.
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## Create Your OP_RETURN Data
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The first thing you need to do is create the 80 bytes (or less) of data that you'll be recording in your OP_RETURN. This might be as simple as preparing a message or you might be hasing existing data. For example, `md5sum` produces 128 bits of data, which is 16 bytes, well under the limits:
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The first thing you need to do is create the 80 bytes (or less) of data that you'll be recording in your OP_RETURN. This might be as simple as preparing a message or you might be hasing existing data. For example, `sha256sum` produces 256 bits of data, which is 32 bytes, well under the limits:
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```
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$ md5sum mycontract.jpg
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3b110a164aa18d3a5ab064ba93fdce62 mycontract.jpg
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$ op_return_data="3b110a164aa18d3a5ab064ba93fdce62"
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$ sha256sum contract.jpg
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fe7f0a3b69f56ef2d055a78823ed3bd1422e46c3183658ea854253033ae0ccef contract.jpg
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$ op_return_data="fe7f0a3b69f56ef2d055a78823ed3bd1422e46c3183658ea854253033ae0ccef"
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```
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_What is an OP_RETURN?_ All Bitcoin transactions are built upon scripts that we'll meet in the next chapter. The OP_RETURN is a simple opcode that defines a transaction as invalid. Convention has caused it to be used to embed data on the blockchain.
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## Prepare Some Money
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Your purpose in creating an OP_RETURN isn't to send money to anyone, it's to put data into the blockchain. However, you _must_ send money to do so. You just need to use a change address as your _only_ recipient. Then you can identify a UTXO and send that to your change address, minus a transaction, while also using the same transaction to create an OP_RETURN.
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Your purpose in creating a data transaction isn't to send money to anyone, it's to put data into the blockchain. However, you _must_ send money to do so. You just need to use a change address as your _only_ recipient. Then you can identify a UTXO and send that to your change address, minus a transaction fee, while also using the same transaction to create an OP_RETURN.
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Here's that process:
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Here's our standard setup:
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```
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$ bitcoin-cli listunspent
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[
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{
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"txid": "0c198125f5a0e2e598ce3b7e4253a95dce780dec12601ed0a44c8544606782b2",
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"vout": 0,
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"address": "n3cogFw4y3A1LJgRz2G97otEGT7BK8ca3D",
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"account": "",
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"scriptPubKey": "76a914f26e11dc0fcc79fe76fca1d24d7588798922ca7488ac",
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"amount": 0.06600000,
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"confirmations": 11,
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"txid": "4460175e8276d5a1935f6136e36868a0a3561532d44ddffb09b7cb878f76f927",
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"vout": 1,
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"address": "my7UvPbPWDbqCxYpAcUZELDbK1X7w9Whmc",
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"scriptPubKey": "76a914c101d8c34de7b8d83b3f8d75416ffaea871d664988ac",
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"amount": 0.90000000,
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"confirmations": 682,
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"spendable": true,
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"solvable": true
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}
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]
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$ utxo_txid_4="0c198125f5a0e2e598ce3b7e4253a95dce780dec12601ed0a44c8544606782b2"
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$ utxo_vout_4="0"
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$ changeaddress_4=$(bitcoin-cli getrawchangeaddress)
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$ utxo_txid=$(bitcoin-cli listunspent | jq -r '.[0] | .txid')
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$ utxo_vout=$(bitcoin-cli listunspent | jq -r '.[0] | .vout')
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$ changeaddress=$(bitcoin-cli getrawchangeaddress)
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```
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## Write A Rawtransaction
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## Write A Raw Transaction
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You can now write a new rawtransaction with two outputs: one is your change address to get back (most of) your money, the other is a data address, which is bitcoin-cli's term for an OP_RETURN.
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```
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$ rawtxhex4=$(bitcoin-cli createrawtransaction '''[ { "txid": "'$utxo_txid_4'", "vout": '$utxo_vout_4' } ]''' '''{ "data": "'$op_return_data'", "'$changeaddress_4'": 0.0655 }''')
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$ rawtxhex=$(bitcoin-cli -named createrawtransaction transactions='''[ { "txid": "'$utxo_txid'", "vout": '$utxo_vout' } ]''' outputs='''{ "data": "'$op_return_data'", "'$changeaddress'": 0.8995 }''')
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```
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Here's what that transaction actually looks like:
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```
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$ bitcoin-cli decoderawtransaction $rawtxhex4
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$ bitcoin-cli -named decoderawtransaction hexstring=$rawtxhex
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{
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"txid": "58a75a85833c396082e6603a0a2537c7e8c207d188656791679df7da7fe0879a",
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"hash": "58a75a85833c396082e6603a0a2537c7e8c207d188656791679df7da7fe0879a",
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"size": 112,
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"vsize": 112,
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"version": 1,
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"txid": "531660f9bcb2571b2b790ebe6e7b6ed40618f608f3f6722a511275257942790b",
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"hash": "531660f9bcb2571b2b790ebe6e7b6ed40618f608f3f6722a511275257942790b",
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"size": 128,
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"vsize": 128,
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"version": 2,
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"locktime": 0,
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"vin": [
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{
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"txid": "0c198125f5a0e2e598ce3b7e4253a95dce780dec12601ed0a44c8544606782b2",
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"vout": 0,
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"txid": "4460175e8276d5a1935f6136e36868a0a3561532d44ddffb09b7cb878f76f927",
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"vout": 1,
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"scriptSig": {
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"asm": "",
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"hex": ""
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@ -74,42 +72,42 @@ $ bitcoin-cli decoderawtransaction $rawtxhex4
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"value": 0.00000000,
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"n": 0,
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"scriptPubKey": {
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"asm": "OP_RETURN 3b110a164aa18d3a5ab064ba93fdce62",
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"hex": "6a103b110a164aa18d3a5ab064ba93fdce62",
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"asm": "OP_RETURN fe7f0a3b69f56ef2d055a78823ed3bd1422e46c3183658ea854253033ae0ccef",
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"hex": "6a20fe7f0a3b69f56ef2d055a78823ed3bd1422e46c3183658ea854253033ae0ccef",
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"type": "nulldata"
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}
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},
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{
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"value": 0.06550000,
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"value": 0.89950000,
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"n": 1,
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"scriptPubKey": {
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"asm": "OP_DUP OP_HASH160 effa6f1be9433e051ddbc33d7021259f5e1333c7 OP_EQUALVERIFY OP_CHECKSIG",
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"hex": "76a914effa6f1be9433e051ddbc33d7021259f5e1333c788ac",
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"asm": "OP_DUP OP_HASH160 166692bda9f25ced145267bb44286e8ee3963d26 OP_EQUALVERIFY OP_CHECKSIG",
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"hex": "76a914166692bda9f25ced145267bb44286e8ee3963d2688ac",
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"reqSigs": 1,
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"type": "pubkeyhash",
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"addresses": [
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"n3PqoMxnQQmafdup2hAmV6SzanJGo4FG8f"
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"mhZQ3Bih6wi7jP1tpFZrCcyr4NsfCapiZP"
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]
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}
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}
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]
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}
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```
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As you can see, this sends the majority of the money straight back to the change address (n3PqoMxnQQmafdup2hAmV6SzanJGo4FG8f) minus that standard transaction fee we've been using of 0.0005 BTC. _Do_ note that these transactions get larger as they add extra inputs and outputs — and now 16 bytes of data. More importantly, the first output shows an OP_RETURN with the data (3b110a164aa18d3a5ab064ba93fdce62) right after it.
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As you can see, this sends the majority of the money straight back to the change address (mhZQ3Bih6wi7jP1tpFZrCcyr4NsfCapiZP) minus that standard transaction fee we've been using of 0.0005 BTC. _Do_ note that these transactions get larger as they add extra inputs and outputs — and now 16 bytes of data. More importantly, the first output shows an OP_RETURN with the data (fe7f0a3b69f56ef2d055a78823ed3bd1422e46c3183658ea854253033ae0ccef) right after it.
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Sign it and send it, and soon that OP_RETURN will be embedded in the blockchain!
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## Check Your OP_RETURN
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Again, remember that you can look at this transaction using a blockchain explorer: [https://live.blockcypher.com/btc-testnet/tx/ed445dc970bb40b17c207109e19a37b2be301acb474ccd30680c431cb681bce2/](https://live.blockcypher.com/btc-testnet/tx/ed445dc970bb40b17c207109e19a37b2be301acb474ccd30680c431cb681bce2/)
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Again, remember that you can look at this transaction using a blockchain explorer: [https://live.blockcypher.com/btc-testnet/tx/3a62b396afb8d8a59ebe7b9e52d6aa2485f1082a1d3fc6ece61fb8b55373823d/](https://live.blockcypher.com/btc-testnet/tx/3a62b396afb8d8a59ebe7b9e52d6aa2485f1082a1d3fc6ece61fb8b55373823d/)
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You may note a warning about the data being in an "unknown protocol". If you were designing some regular use of OP_RETURN data, you'd probably mark it with a special prefix, to mark that protocol. Then, the actual OP_RETURN data might be something like "CONTRACTS3b110a164aa18d3a5ab064ba93fdce62". This example didn't use a prefix to avoid muddying the data space.
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[Coinsecrets](http://coinsecrets.org/) offers another interesting way to look at OP_RETURN data. It does its best to keep abreast of protocols, so that it can tell you who is doing what in the blockchain. Here's thisour transaction there: [https://www.blocktrail.com/tBTC/tx/ed445dc970bb40b17c207109e19a37b2be301acb474ccd30680c431cb681bce2](https://www.blocktrail.com/tBTC/tx/ed445dc970bb40b17c207109e19a37b2be301acb474ccd30680c431cb681bce2)
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[Coinsecrets](http://coinsecrets.org/) offers another interesting way to look at OP_RETURN data. It does its best to keep abreast of protocols, so that it can tell you who is doing what in the blockchain. Here's this transaction there: [https://www.blocktrail.com/tBTC/tx/3a62b396afb8d8a59ebe7b9e52d6aa2485f1082a1d3fc6ece61fb8b55373823d](https://www.blocktrail.com/tBTC/tx/3a62b396afb8d8a59ebe7b9e52d6aa2485f1082a1d3fc6ece61fb8b55373823d)
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## Summary: Sending a Raw Transaction with an OP_RETURN
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You can use an OP_RETURN output to store up to 80 bytes of data on the blockchain. You do this with the 'data' codeword for a 'vout'. You still have to send money along too, but you just send it back to a change address, minus a transaction fee.
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## Summary: Sending a Raw Transaction with Data
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You can use an OP_RETURN opcode to store up to 80 bytes of data on the blockchain. You do this with the 'data' codeword for a 'vout'. You still have to send money along too, but you just send it back to a change address, minus a transaction fee.
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_What is the Power of OP_RETURN?_ The OP_RETURN opens up whole new possibilities for the blockchain, because you can embed data that proves that certain things happened at certain times. Various organizations have used OP_RETURNs for proof of existence, for copyright, to color coins, and [for other purposes](https://en.bitcoin.it/wiki/OP_RETURN). Though 80 bytes might not seem a lot, it can be quite effective if OP_RETURNs are used to store hashes of the actual data. Note that there is some controversy over using the Bitcoin blockchain in this way,
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