05 - nVersion

nVersion is the first parameter in the transaction pre-image. It's value indicates the version of the node client that should be used to evaluate this transaction (4 bytes)

nVersion can be evaluated by splitting the first 4 bytes of the pre-image and moving the value to the top of the stack. In this way, a user can pre-set a version flag and enforce its use in a future transaction that spends the output.

This functionality was originally available in script using the OP_VER, OP_VERIF and OP_VERNOTIF opcodes, however these have been disabled and are no-longer part of the scripting language.

In its original form a wallet could potentially construct a transaction that only a sub-set of nodes would try to put into a block. The functionality isn't used by miners today and so application of this technique is limited.

Example 1: Replace OP_VER

To replicate the funtionality of OP_VER, the following script can be used:

Stack	Script	Description
<tx_preimg>	...	Pre-image has been validated
<tx_preimg>	OP_4	add 4 to the stack
<tx_preimg> 4	OP_SPLIT	Split version from pre-image
<version> <r_tx_preimg>	OP_SWAP	Swap version to front, or drop if not needed
<r_tx_preimg> <version>	...	Script continues

Example 2: OP_VERIF / OP_VERNOTIF

To replicate the funtionality of OP_VERIF or OP_VERNOTIF, the following script outline can be used:

Stack	Script	Description
<tx_preimg>	...	Pre-image has been validated
<tx_preimg>	OP_4	add 4 to the stack
<r_tx_preimg> 0x04	OP_SPLIT	Split version from pre-image
<version> <r_tx_preimg>	OP_SWAP / OP_NIP	Swap version to front, or drop if not needed
<r_tx_preimg> <version>	<expected_version>	To test the version, add a constant to the stack.
<r_tx_preimg> <version> <expected_version>	OP_EQUAL	Equality test
<r_tx_preimg>	OP_IF / OP_NOTIF	Enter if statement path
<r_tx_preimg>	...	process
<r_tx_preimg>	OP_ELSE	or alternative path (optional)
<r_tx_preimg>	...	alternative process (optional)
<remainder_data_items>	OP_ENDIF	End process - If-structure script may leave one or more different data items on the stack. These are passed to the rest of the script.
<remainder_data_items>	...	rest of script