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CH02::Change update for illustration, add detail

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- Update text to refer to new illustration

- Introduce the term "change output" in addition to existing term
  "change address"

- Add information about the privacy advantages of unique change
  addresses
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David A. Harding 2023-02-04 12:25:58 -10:00
parent b05a5ee6f8
commit a4327fa4fe
1 changed files with 22 additions and 11 deletions
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ch02.asciidoc
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@ -253,22 +253,33 @@ that's what the protocol itself uses.
==== Making Change ==== Making Change
((("change, making")))((("change addresses")))((("addresses", "change ((("change, making")))((("change addresses")))((("addresses", "change
addresses")))Many bitcoin transactions will include outputs that addresses")))In addition one or more outputs that pay the receiver of
reference both an address of the new owner and an address of the current bitcoins, many tranactions will also include an output that pays the
owner, called the _change_ address. This is because transaction inputs, spender of the bitcoins, called a _change_ output.
This is because transaction inputs,
like currency notes, cannot be divided. If you purchase a $5 US dollar like currency notes, cannot be divided. If you purchase a $5 US dollar
item in a store but use a $20 US dollar bill to pay for the item, you item in a store but use a $20 dollar bill to pay for the item, you
expect to receive $15 US dollars in change. The same concept applies to expect to receive $15 dollars in change. The same concept applies to
bitcoin transaction inputs. If you purchased an item that costs 5 bitcoin transaction inputs. If you purchased an item that costs 5
bitcoin but only had a 20 bitcoin input to use, you would send one
output of 5 bitcoin to the store owner and one output of 15 bitcoin back
to yourself as change (less any applicable transaction fee).
Importantly, the change address does not have to be the same address as
that of the input and for privacy reasons is often a new address from
the owner's wallet.
Different wallets may use different strategies when aggregating inputs Different wallets may use different strategies when aggregating inputs
to make a payment requested by the user. They might aggregate many small to make a payment requested by the user. They might aggregate many small
bitcoins but only had an input worth 20 bitcoins to use, you would send one
output of 5 bitcoins to the store owner and one output of 15 bitcoins back
to yourself as change (not counting your transaction fee).
At the level of the Bitcoin protocol, there is no difference between a
change output (and the address it pays, called a _change address_) and a
payment output.
Importantly, the change address does not have to be the
same address as that of the input and for privacy reasons is often a new
address from the owner's wallet. In ideal circumstances, the two
different uses of outputs both use never-before-been addresses and
otherwise look identical, preventing any third party from determining
which outputs are change and which are payments. However, for
illustration purposes, we've added shading to the change outputs in
<<transaction-chain>>.
inputs, or use one that is equal to or larger than the desired payment. inputs, or use one that is equal to or larger than the desired payment.
Unless the wallet can aggregate inputs in such a way to exactly match Unless the wallet can aggregate inputs in such a way to exactly match
the desired payment plus transaction fees, the wallet will need to the desired payment plus transaction fees, the wallet will need to