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Edited ch07.asciidoc with Atlas code editor

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judymcconville@roadrunner.com 2017-05-01 10:52:47 -07:00
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@ -195,7 +195,7 @@ Note that because the redeem script is not presented to the network until you at
[[op_return]]
=== Data Recording Output (RETURN)
((("transactions", "advanced", "data recording output")))((("scripting", "data recording output")))((("RETURN operator")))((("data recording (nonpayment data)")))((("nonpayment data")))((("blockchain technology", "nonpayment data recording")))Bitcoin's distributed and timestamped ledger, the blockchain, has potential uses far beyond payments. Many developers have tried to use the transaction scripting language to take advantage of the security and resilience of the system for applications such as digital notary services, stock certificates, and smart contracts. Early attempts to use bitcoin's script language for these purposes involved creating transaction outputs that recorded data on the blockchain; for example, to record a digital fingerprint of a file in such a way that anyone could establish proof-of-existence of that file on a specific date by reference to that transaction.
((("transactions", "advanced", "data recording output")))((("scripting", "data recording output")))((("RETURN operator")))((("data recording (nonpayment data)")))((("nonpayment data")))((("blockchain technology", "nonpayment data recording")))((("digital notary services")))Bitcoin's distributed and timestamped ledger, the blockchain, has potential uses far beyond payments. Many developers have tried to use the transaction scripting language to take advantage of the security and resilience of the system for applications such as digital notary services, stock certificates, and smart contracts. Early attempts to use bitcoin's script language for these purposes involved creating transaction outputs that recorded data on the blockchain; for example, to record a digital fingerprint of a file in such a way that anyone could establish proof-of-existence of that file on a specific date by reference to that transaction.
((("blockchain bloat")))((("bloat")))((("unspent transaction outputs (UTXO)")))((("UTXO sets")))The use of bitcoin's blockchain to store data unrelated to bitcoin payments is a controversial subject. Many developers consider such use abusive and want to discourage it. Others view it as a demonstration of the powerful capabilities of blockchain technology and want to encourage such experimentation. Those who object to the inclusion of nonpayment data argue that it causes "blockchain bloat," burdening those running full bitcoin nodes with carrying the cost of disk storage for data that the blockchain was not intended to carry. Moreover, such transactions create UTXO that cannot be spent, using the destination bitcoin address as a freeform 20-byte field. Because the address is used for data, it doesn't correspond to a private key and the resulting UTXO can _never_ be spent; it's a fake payment. These transactions that can never be spent are therefore never removed from the UTXO set and cause the size of the UTXO database to forever increase, or "bloat."
@ -207,7 +207,7 @@ In version 0.9 of the Bitcoin Core client, a compromise was reached with the int
RETURN <data>
----
The data portion is limited to 80 bytes and most often represents a hash, such as the output from the SHA256 algorithm (32 bytes). Many applications put a prefix in front of the data to help identify the application. For example, the http://proofofexistence.com[Proof of Existence] digital notarization service uses the 8-byte prefix +DOCPROOF+, which is ASCII encoded as +44 4f 43 50 52 4f 4f 46+ in hexadecimal.
((("Proof of Existence")))((("DOCPROOF prefix")))The data portion is limited to 80 bytes and most often represents a hash, such as the output from the SHA256 algorithm (32 bytes). Many applications put a prefix in front of the data to help identify the application. For example, the http://proofofexistence.com[Proof of Existence] digital notarization service uses the 8-byte prefix +DOCPROOF+, which is ASCII encoded as +44 4f 43 50 52 4f 4f 46+ in hexadecimal.
Keep in mind that there is no "unlocking script" that corresponds to +RETURN+ that could possibly be used to "spend" a +RETURN+ output. The whole point of +RETURN+ is that you can't spend the money locked in that output, and therefore it does not need to be held in the UTXO set as potentially spendable—+RETURN+ is _provably unspendable_. +RETURN+ is usually an output with a zero bitcoin amount, because any bitcoin assigned to such an output is effectively lost forever. If a +RETURN+ is referenced as an input in a transaction, the script validation engine will halt the execution of the validation script and mark the transaction as invalid. The execution of +RETURN+ essentially causes the script to "RETURN" with a +FALSE+ and halt. Thus, if you accidentally reference a +RETURN+ output as an input in a transaction, that transaction is invalid.