08 - Outpoint

The outpoint for the input being signed is a 36 byte long data item made of the concatenation of the input TXID and Vout (32 + 4 bytes).

It can be used for a variety of purposes including:

• Source of unique token ID

• Input randomness

• Check input sources

We will now look at examples of each of these.

Example 1: Source of unique token ID

Tokens within the system require a unique 20 byte ID. This is achieved by hashing <outpoint> using OP_HASH160.

Stack	Script	Description
<r_tx_preimg>	...	nVersion, hashPrevouts and hashSequence have been removed
<r_tx_preimg>	0x24	add outpoint length to the stack
<r_tx_preimg> 0x24	OP_SPLIT	Split outpoint from pre-image
<outpoint> <rr_tx_preimg>	OP_SWAP	Swap outpoint to front
<rr_tx_preimg> <outpoint>	OP_HASH160	Calculate txid using OP_HASH160
<rr_tx_preimg> <token_id>	...	rest of script

The token_id can now be added to an output script and enforced forward. This will be covered later in this chapter.

Example 2: Input randomness

The TXID cannot be known until the transaction is final, making it ideal as a source of input randomness. This example uses the MODULO function to implement a Rock-Paper-Scissors game.

Stack	Script	Description
<r_tx_preimg>	...	Version, hash_prevouts and hash_nSequence have been removed
<r_tx_preimg>	0x24	add outpoint length to the stack
<r_tx_preimg> 0x24	OP_SPLIT	Split outpoint from pre-image
<outpoint> <r_tx_preimg>	OP_DROP	Drop remaining pre-image
<outpoint>	OP_SHA256	Generate random output
<random_value>	OP_3	Put 3 on the stack
<random_value> 0x03	OP_MOD	Calculate modulo 2 - 1 or 0 outcome
<rem>	OP_IFDUP	Duplicates the stack if not zero
[0x00, 0x01 0x01, 0x02 0x02]	OP_NOTIF	3 possible stack states after OP_IFDUP. If 0, enter if statement path
	<rock>	Zero is rock
[0x01 , 0x02]	OP_ELSE	If not zero, duplicated outcome remains.
[0x01 , 0x02]	OP_1	Test for 1
[0x01 , 0x02] 0x01	OP_EQUAL	Equality check
<result>	OP_IF	Enter if
	<paper>	1 is paper
	OP_ELSE	Else
	<scissors>	2 is scissors
<rock/paper/scissors>	OP_ENDIF	Endif
<rock/paper/scissors>	...	rest of script

Example 3: Check input sources

This UTXO requires that it be spent in a transaction with the two outputs below it in the previous transaction. It uses hash_prevouts which must be left on the stack in an earlier section of script.

Stack	Script	Description
<hash_prevouts> <r_tx_preimg>	...	Version and hash_nSequence have been removed and any needed conditions have been checked.
<hash_prevouts> <r_tx_preimg>	0x24	add outpoint length to the stack
<hash_prevouts> r_tx_preimg> 0x24	OP_SPLIT	Split outpoint from pre-image
<hash_prevouts> <outpoint> <r_tx_preimg>	OP_SWAP	Swap outpoint to front
<hash_prevouts> <r_tx_preimg> <outpoint>	OP_DUP
<hash_prevouts><r_tx_preimg> <outpoint> <outpoint>	0x20	add 32 to stack
<hash_prevouts><r_tx_preimg> <outpoint> <outpoint> 0x20	OP_SPLIT	Split the txid and vout from the outpoint
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout>	OP_1ADD	add 1 to the vout
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1>	OP_2DUP	Duplicate txid and vout+1
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1> <txid><vout+1>	OP_1ADD	add 1 to the vout
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1> <txid><vout+2>	OP_CAT	Create outpoint 3
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1> <outpoint3>	OP_CAT	Join to previous vout
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1&outpoint3>	OP_CAT	Join to txid
<hash_prevouts><r_tx_preimg> <outpoint> <outpoint2&3>	OP_CAT	Join all three outpoints together
<hash_prevouts><r_tx_preimg> <outpoint1&2&3>	OP_SHA256	hash outpoints
<hash_prevouts><r_tx_preimg> <hashoutpoints>	OP_ROT	Rotate hash_prevouts to top of stack
<r_tx_preimg> <hashoutpoints> <hash_prevouts>	OP_EQUALVERIFY	Check condition is met
<r_tx_preimg>	...	Rest of script