02 - Pay to Hash Puzzle

A Pay to Hash Puzzle script is a simple script that uses a hash function as the checking function, requiring the spending party to provide a valid solution to the puzzle to spend the output.

When processing hash puzzle scripts, the validation engine takes one or more input argument and process them against one or more hash/algebraic functions to create a script that checks that the spending party has knowledge of a piece of hidden information. These scripts have very low overhead, but reduced security. Care should be taken when creating outputs with these functions as some hash function opcodes (e.g. OP_SHA1) use hashing functions that are computationally insecure.

A pay to hash script can be defined in many ways:

Example 1: OP_SHA256 <sha256_hash> OP_EQUAL

Example 2: OP_HASH256 <double_sha256_hash> OP_EQUAL

Example 3: OP_SHA256 OP_RIPEMD160 <ripemd160_hash> OP_EQUAL

To spend an output that is locked with the script in example 3, the following solution is provided:

<pre-hash_value>

The validation engine will evaluate the full script of Example 3 as follows:

<pre-hash_value> OP_SHA256 OP_RIPEMD160 <ripemd160_hash> OP_EQUAL

A breakdown of the script evaluation process is shown below:

Stack	Script	Description
Empty.	<pre-hash_value> | | OP_SHA256 OP_RIPEMD160 <ripemd160_hash> OP_EQUAL	scriptSig and scriptPubKey are combined.
<pre-hash_value>	OP_SHA256 OP_RIPEMD160 <ripemd160_hash> OP_EQUAL	Pre-hash value is added to the stack.
<sha256_hash>	OP_RIPEMD160 <ripemd160_hash> OP_EQUAL	Pre-hash value is hashed using SHA256 hashing algorithm
<ripemd160_hash>	<ripemd160_hash> OP_EQUAL	SHA256 hash of pre-hash value is hashed with RIPEMD160 hash algorithm
<ripemd160_hash> <ripemd160_hash>	OP_EQUAL	Expected RIPEMD160 hash is added to the stack
true	Empty.	Hash value is checked against the expected hash value

As shown above, the pre-hash value is pushed onto the stack, before being double hashed, first with the SHA256 hashing algorithm and subsequently with the RIPEMD160 hashing algorithm. If the correct pre-hash value is used, the double hash value will match the expected hash outcome, and the input can be spent in the transaction.

Pay to Script Hash (P2SH)

Pay to Script Hash (P2SH) was a so-called 'soft fork' change introduced to Bitcoin in 2012 ostensibly to allow more complex scripts to be used with smaller transactions. The scheme uses protocol hacks to bypass the expected behaviour of the network, and was disallowed following the 2020 Genesis upgrade on the BitcoinSV network.

Invalid P2SH outputs are defined using the following template:

OP_HASH160 <script_hash> OP_EQUAL

This script performs the same process outlined in Example 3 above, but uses the OP_HASH160 opcode to perform the double hash with a single instruction.

Because P2SH introduces functionality that is not compatible with the Bitcoin protocol, nodes on the BSV network will evaluate any transaction sent to the network that creates a P2SH output as invalid, and will reject it with the 'Error: Pay to Script Hash' error code.