# 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 `` using `OP_HASH160`. | Stack | Script | Description | | ------------------------------ | ----------- | --------------------------------------------------------- | | \ | ... | nVersion, hashPrevouts and hashSequence have been removed | | \ | 0x24 | add outpoint length to the stack | | \ 0x24 | OP\_SPLIT | Split outpoint from pre-image | | \ \ | OP\_SWAP | Swap outpoint to front | | \ \ | OP\_HASH160 | Calculate txid using OP\_HASH160 | | \ \ | ... | 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 | | ----------------------------- | ----------- | ---------------------------------------------------------------------- | | \ | ... | Version, hash\_prevouts and hash\_nSequence have been removed | | \ | 0x24 | add outpoint length to the stack | | \ 0x24 | OP\_SPLIT | Split outpoint from pre-image | | \ \ | OP\_DROP | Drop remaining pre-image | | \ | OP\_SHA256 | Generate random output | | \ | OP\_3 | Put 3 on the stack | | \ 0x03 | OP\_MOD | Calculate modulo 2 - 1 or 0 outcome | | \ | 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 | | | \ | 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 | | \ | OP\_IF | Enter if | | | \ | 1 is paper | | | OP\_ELSE | Else | | | \ | 2 is scissors | | \ | OP\_ENDIF | Endif | | \ | ... | 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.
StackScriptDescription
<hash_prevouts> <r_tx_preimg>...Version and hash_nSequence have been removed and any needed conditions have been checked.
<hash_prevouts> <r_tx_preimg>0x24add outpoint length to the stack
<hash_prevouts> r_tx_preimg> 0x24OP_SPLITSplit outpoint from pre-image
<hash_prevouts> <outpoint> <r_tx_preimg>OP_SWAPSwap outpoint to front
<hash_prevouts> <r_tx_preimg> <outpoint>OP_DUP
<hash_prevouts><r_tx_preimg> <outpoint> <outpoint>0x20add 32 to stack
<hash_prevouts><r_tx_preimg> <outpoint> <outpoint> 0x20OP_SPLITSplit the txid and vout from the outpoint
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout>OP_1ADDadd 1 to the vout
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1>OP_2DUPDuplicate txid and vout+1
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1> <txid><vout+1>OP_1ADDadd 1 to the vout
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1> <txid><vout+2>OP_CATCreate outpoint 3
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1> <outpoint3>OP_CATJoin to previous vout
<hash_prevouts><r_tx_preimg> <outpoint> <txid><vout+1&outpoint3>OP_CATJoin to txid
<hash_prevouts><r_tx_preimg> <outpoint> <outpoint2&3>OP_CATJoin all three outpoints together
<hash_prevouts><r_tx_preimg> <outpoint1&2&3>OP_SHA256hash outpoints
<hash_prevouts><r_tx_preimg> <hashoutpoints>OP_ROTRotate hash_prevouts to top of stack
<r_tx_preimg> <hashoutpoints> <hash_prevouts>OP_EQUALVERIFYCheck condition is met
<r_tx_preimg>...Rest of script
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