Made changes to ch05.asciidoc

ch05.asciidoc

@@ -450,30 +450,30 @@ When executed, this combined script will evaluate to TRUE if, and only if, the u
 [[op_return]]
 ==== Data Output (OP_RETURN)
 
-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.
+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.
 
-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 non-payment 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 free-form 20-byte field. Since 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. This practice causes the size of the in-memory UTXO set to increase and these transactions which can never be spent are therefore never removed, forcing bitcoin nodes to carry these forever in RAM which is far more expensive.
+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 non-payment 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 free-form 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. This practice causes the size of the in-memory UTXO set to increase and these transactions that can never be spent are therefore never removed, forcing bitcoin nodes to carry these forever in RAM, which is far more expensive.
 
-In version 0.9 of the bitcoin core client, a compromise was reached, with the introduction of the OP_RETURN operator. OP_RETURN allows developers to add 40 bytes of non-payment data to a transaction output. However, unlike the use of "fake" UTXO, the OP_RETURN operator creates an explicitly _provably un-spendable_ output, which does not need to be stored in the UTXO set. OP_RETURN outputs are recorded on the blockchain, so they consume disk space and contribute to the increase in the blockchain's size, but they are not stored in the UTXO set and therefore do not bloat the UTXO memory pool and burden full nodes with the cost of more expensive RAM. 
+In version 0.9 of the Bitcoin Core client, a compromise was reached, with the introduction of the +OP_RETURN+ operator. +OP_RETURN+ allows developers to add 40 bytes of nonpayment data to a transaction output. However, unlike the use of "fake" UTXO, the +OP_RETURN+ operator creates an explicitly _provably unspendable_ output, which does not need to be stored in the UTXO set. +OP_RETURN+ outputs are recorded on the blockchain, so they consume disk space and contribute to the increase in the blockchain's size, but they are not stored in the UTXO set and therefore do not bloat the UTXO memory pool and burden full nodes with the cost of more expensive RAM. 
 
-OP_RETURN scripts look like this:
++OP_RETURN+ scripts look like this:
 
 ----
 OP_RETURN <data>
 ----
 
-where the data portion is limited to 40 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 proofofexistence.com digital notarization service uses the 8-byte prefix "DOCPROOF" which is ASCII encoded as 44f4350524f4f46 in hexadecimal.
+where the data portion is limited to 40 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/[proofofexistence.com] digital notarization service uses the 8-byte prefix "DOCPROOF," which is ASCII encoded as 44f4350524f4f46 in hexadecimal.
 
-Keep in mind that there is no "unlocking script" that corresponds to OP_RETURN that could possibly be used to "spend" an OP_RETURN output. The whole point of OP_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 - OP_RETURN is _provably un-spendable_. OP_RETURN is usually an output with a zero bitcoin amount, since any bitcoin assigned to such an output is effectively lost forever. If an OP_RETURN is encountered by the script validation software, it results immediately in halting the execution of the validation script and marking the transaction as invalid. Thus, if you accidentally reference an OP_RETURN output as an input in a transaction, that transaction is invalid. 
+Keep in mind that there is no "unlocking script" that corresponds to +OP_RETURN+ that could possibly be used to "spend" an +OP_RETURN+ output. The whole point of +OP_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—+OP_RETURN+ is _provably un-spendable_. +OP_RETURN+ is usually an output with a zero bitcoin amount, because any bitcoin assigned to such an output is effectively lost forever. If an +OP_RETURN+ is encountered by the script validation software, it results immediately in halting the execution of the validation script and marking the transaction as invalid. Thus, if you accidentally reference an +OP_RETURN+ output as an input in a transaction, that transaction is invalid. 
 
-A standard transaction (one that conforms to the +isStandard()+ checks) can have only one OP_RETURN output. However, a single OP_RETURN output can be combined in a transaction with outputs of any other type. 
+A standard transaction (one that conforms to the +isStandard()+ checks) can have only one +OP_RETURN+ output. However, a single +OP_RETURN+ output can be combined in a transaction with outputs of any other type. 
 
 [[p2sh]]
-==== Pay to Script Hash (P2SH)
+==== Pay-to-Script-Hash (P2SH)
 
 Pay-to-Script-Hash (P2SH) was introduced in the winter of 2012 as a powerful new type of transaction that greatly simplifies the use of complex transaction scripts. To explain the need for P2SH, let's look at a practical example.
 
-In <<ch01_intro_what_is_bitcoin>> we introduced Mohammed, an electronics importer based in Dubai. Mohammed's company uses bitcoin's multi-signature feature extensively for its corporate accounts. Multi-signature scripts are one of the most common uses of bitcoin's advanced scripting capabilities and are a very powerful feature. Mohammed's company uses a multi-signature script for all customer payments, known in accounting terms as "accounts receivable" or AR. With the multi-signature scheme, any payments made by customers are locked in such a way that they require at least two signatures to release, from Mohammed and one of his partners or from his attorney who has a backup key. A multi-signature scheme like that offers corporate governance controls and protects against theft, embezzlement or loss. 
+In <<ch01_intro_what_is_bitcoin>> we introduced Mohammed, an electronics importer based in Dubai. Mohammed's company uses bitcoin's multi-signature feature extensively for its corporate accounts. Multi-signature scripts are one of the most common uses of bitcoin's advanced scripting capabilities and are a very powerful feature. Mohammed's company uses a multi-signature script for all customer payments, known in accounting terms as "accounts receivable," or AR. With the multi-signature scheme, any payments made by customers are locked in such a way that they require at least two signatures to release, from Mohammed and one of his partners or from his attorney who has a backup key. A multi-signature scheme like that offers corporate governance controls and protects against theft, embezzlement, or loss. 
 
 The resulting script is quite long and looks like this:
 
@@ -482,30 +482,30 @@ The resulting script is quite long and looks like this:
 <Partner3 Public Key> <Attorney Public Key> 5 OP_CHECKMULTISIG
 ----
     
-While multi-signature scripts are a powerful feature, they are cumbersome to use. Given the script above, Mohammed would have to communicate this script to every customer prior to payment. Each customer would have to use special bitcoin wallet software with the ability to create custom transaction scripts and each customer would have to understand how to create a transaction using custom scripts. Furthermore, the resulting transaction would be about five times larger than a simple payment transaction, as this script contains very long public keys. The burden of that extra-large transaction would be borne by the customer in the form of fees. Finally, a large transaction script like this would be carried in the UTXO set in RAM in every full node, until it was spent. All of these issues make using complex output scripts difficult in practice. 
+Although multi-signature scripts are a powerful feature, they are cumbersome to use. Given the preceding script, Mohammed would have to communicate this script to every customer prior to payment. Each customer would have to use special bitcoin wallet software with the ability to create custom transaction scripts, and each customer would have to understand how to create a transaction using custom scripts. Furthermore, the resulting transaction would be about five times larger than a simple payment transaction, because this script contains very long public keys. The burden of that extra-large transaction would be borne by the customer in the form of fees. Finally, a large transaction script like this would be carried in the UTXO set in RAM in every full node, until it was spent. All of these issues make using complex output scripts difficult in practice. 
 
-Pay-to-Script-Hash (P2SH) was developed to resolve these practical difficulties and to make the use of complex scripts as easy as a payment to a bitcoin address. With P2SH payments, the complex locking script is replaced with its digital fingerprint, a cryptographic hash. When a transaction attempting to spend the UTXO is presented later, it must contain the script that matches the hash, in addition to the unlocking script. In simple terms, P2SH means "pay to a script matching this hash, a script which will be presented later when this output is spent". 
+Pay-to-Script-Hash (P2SH) was developed to resolve these practical difficulties and to make the use of complex scripts as easy as a payment to a bitcoin address. With P2SH payments, the complex locking script is replaced with its digital fingerprint, a cryptographic hash. When a transaction attempting to spend the UTXO is presented later, it must contain the script that matches the hash, in addition to the unlocking script. In simple terms, P2SH means "pay to a script matching this hash, a script that will be presented later when this output is spent." 
 
-In P2SH transactions, the locking script that is replaced by a hash is referred to as the _redeem script_ because it is presented to the system at redemption time rather than as a locking script. 
+In P2SH transactions, the locking script that is replaced by a hash is referred to as the _redeem script_ because it is presented to the system at redemption time rather than as a locking script. <<without_p2sh>> shows the script without P2SH and <<with_p2sh>> shows the same script encoded with P2SH.
 
 [[without_p2sh]]
-.Complex Script without P2SH
+.Complex script without P2SH
 |=======
 | Locking Script | 2 PubKey1 PubKey2 PubKey3 PubKey4 PubKey5 5 OP_CHECKMULTISIG
 | Unlocking Script | Sig1 Sig2   
 |=======
 
 [[with_p2sh]]
-.Complex Script as P2SH
+.Complex script as P2SH
 |=======
 | Redeem Script | 2 PubKey1 PubKey2 PubKey3 PubKey4 PubKey5 5 OP_CHECKMULTISIG
 | Locking Script | OP_HASH160 <20-byte hash of redeem script> OP_EQUAL
 | Unlocking Script | Sig1 Sig2 redeem script 
 |=======
 
-As you can see from the tables above, with P2SH the complex script that details the conditions for spending the output (redeem script) is not presented in the locking script. Instead, only a hash of it is in the locking script and the redeem script itself is presented later, as part of the unlocking script when the output is spent. This shifts the burden in fees and complexity from the sender to the recipient (spender) of the transaction. 
+As you can see from the tables, with P2SH the complex script that details the conditions for spending the output (redeem script) is not presented in the locking script. Instead, only a hash of it is in the locking script and the redeem script itself is presented later, as part of the unlocking script when the output is spent. This shifts the burden in fees and complexity from the sender to the recipient (spender) of the transaction. 
 
-Let's look at Mohammed's company, their complex multi-signature script and the resulting P2SH scripts. 
+Let's look at Mohammed's company, the complex multi-signature script, and the resulting P2SH scripts. 
 
 First, the multi-signature script that Mohammed's company uses for all incoming payments from customers:
 ----
@@ -513,7 +513,7 @@ First, the multi-signature script that Mohammed's company uses for all incoming
 <Partner3 Public Key> <Attorney Public Key> 5 OP_CHECKMULTISIG
 ----
 
-If the placeholders above are replaced by actual public keys (shown below as 520 bit numbers starting with 04) you can see that this script becomes very long:
+If the placeholders are replaced by actual public keys (shown here as 520-bit numbers starting with 04) you can see that this script becomes very long:
 ----
 2 
 04C16B8698A9ABF84250A7C3EA7EEDEF9897D1C8C6ADF47F06CF73370\
@@ -530,7 +530,7 @@ AABF1FDEEC78A6A45E394BA29A1EDF518C022DD618DA774D207D137AA\
 B59E0B000EB7ED238F4D800 5 OP_CHECKMULTISIG
 ----
 
-The entire script above can instead be represented by a 20-byte cryptographic hash, by first applying the SHA256 hashing algorithm and then applying the RIPEMD160 algorithm on the result. The 20-byte hash of the above script is:
+This entire script can instead be represented by a 20-byte cryptographic hash, by first applying the SHA256 hashing algorithm and then applying the RIPEMD160 algorithm on the result. The 20-byte hash of the preceding script is:
 
 ----
 54c557e07dde5bb6cb791c7a540e0a4796f5e97e
@@ -540,7 +540,7 @@ A P2SH transaction locks the output to this hash instead of the longer script, u
 ----
 OP_HASH160 54c557e07dde5bb6cb791c7a540e0a4796f5e97e OP_EQUAL
 ----
-which, as you can see is much shorter. Instead of "pay to this 5-key multi-signature script", the P2SH equivalent transaction is "pay to a script with this hash". A customer making a payment to Mohammed's company need only include this much shorter locking script in their payment. When Mohammed wants to spend this UTXO, they must present the original redeem script (the one whose hash locked the UTXO) and the signatures necessary to unlock it, like this:
+which, as you can see, is much shorter. Instead of "pay to this 5-key multi-signature script," the P2SH equivalent transaction is "pay to a script with this hash." A customer making a payment to Mohammed's company need only include this much shorter locking script in his payment. When Mohammed wants to spend this UTXO, they must present the original redeem script (the one whose hash locked the UTXO) and the signatures necessary to unlock it, like this:
 
 ----
 <Sig1> <Sig2> <2 PK1 PK2 PK3 PK4 PK5 5 OP_CHECKMULTISIG>
@@ -550,14 +550,14 @@ The two scripts are combined in two stages. First, the redeem script is checked
 ----
 <2 PK1 PK2 PK3 PK4 PK5 5 OP_CHECKMULTISIG> OP_HASH160 <redeem scriptHash> OP_EQUAL
 ----
-If the redeem script hash matches, then the unlocking script is executed on its own, to unlock the redeem script:
+If the redeem script hash matches, the unlocking script is executed on its own, to unlock the redeem script:
 ----
 <Sig1> <Sig2> 2 PK1 PK2 PK3 PK4 PK5 5 OP_CHECKMULTISIG
 ----
 
-===== Pay-to-Script-Hash Addresses
+===== Pay-to-Script-Hash addresses
 
-Another important part of the P2SH feature is the ability to encode a script hash as an address, as defined in BIP0013. P2SH addresses are Base58Check encodings of the 20-byte hash of a script, just like bitcoin addresses are Base58Check encodings of the 20-byte hash of a public key. P2SH addresses use the version prefix "5", which results in Base58Check encoded addresses that start with a "3". For example, Mohammed's complex script, hashed and Base58Check encoded as P2SH address becomes +39RF6JqABiHdYHkfChV6USGMe6Nsr66Gzw+. Now, Mohammed can give this "address" to his customers and they can use almost any bitcoin wallet to make a simple payment, as if it were a bitcoin address. The 3 prefix gives them a hint that this is a special type of address, one corresponding to a script instead of a public key, but otherwise it works in exactly the same way as a payment to a bitcoin address. 
+Another important part of the P2SH feature is the ability to encode a script hash as an address, as defined in BIP0013. P2SH addresses are Base58Check encodings of the 20-byte hash of a script, just like bitcoin addresses are Base58Check encodings of the 20-byte hash of a public key. P2SH addresses use the version prefix "5", which results in Base58Check encoded addresses that start with a "3". For example, Mohammed's complex script, hashed and Base58Check-encoded as P2SH address becomes +39RF6JqABiHdYHkfChV6USGMe6Nsr66Gzw+. Now, Mohammed can give this "address" to his customers and they can use almost any bitcoin wallet to make a simple payment, as if it were a bitcoin address. The 3 prefix gives them a hint that this is a special type of address, one corresponding to a script instead of a public key, but otherwise it works in exactly the same way as a payment to a bitcoin address. 
 
 P2SH addresses hide all of the complexity, so that the person making a payment does not see the script. 
 
@@ -565,12 +565,12 @@ P2SH addresses hide all of the complexity, so that the person making a payment d
 
 The Pay-to-Script-Hash feature offers the following benefits compared to the direct use of complex scripts in locking outputs:
 
-* Complex scripts are replaced by shorter fingerprint in the transaction output, making the transaction smaller
-* Scripts can be coded as an address, so the sender and the sender's wallet don't need complex engineering to implement P2SH
-* P2SH shifts the burden of constructing the script to the recipient not the sender
-* P2SH shifts the burden in data storage for the long script from the output (which is in the UTXO set and therefore impacts memory) to the input (only stored on the blockchain)
-* P2SH shifts the burden in data storage for the long script from the present time (payment) to a future time (when it is spent)
-* P2SH shifts the transaction fee cost of a long script from the sender to the recipient who has to include the long redeem script to spend it
+* Complex scripts are replaced by shorter fingerprints in the transaction output, making the transaction smaller.
+* Scripts can be coded as an address, so the sender and the sender's wallet don't need complex engineering to implement P2SH.
+* P2SH shifts the burden of constructing the script to the recipient, not the sender.
+* P2SH shifts the burden in data storage for the long script from the output (which is in the UTXO set and therefore impacts memory) to the input (only stored on the blockchain).
+* P2SH shifts the burden in data storage for the long script from the present time (payment) to a future time (when it is spent).
+* P2SH shifts the transaction fee cost of a long script from the sender to the recipient who has to include the long redeem script to spend it.
 
 ===== Redeem Script and isStandard Validation