Multiparty Hash Puzzles¶
In a hash puzzle contract, the spender has to provide a preimage x
that hashes to a predefined value y
to unlock a UTXO. It can be
extended to multiple parties so that multiple preimages have to be provided such that y1 = H(x1)
, y2 = H(x2)
, …, yN = H(xN)
[1]. Below shows an examples of three parties.
contract MultiPartyHashPuzzles {
Sha256 hash1; // hash1 = b'136523B9FEA2B7321817B28E254A81A683D319D715CEE2360D051360A272DD4C'
Sha256 hash2; // hash2 = b'E222E30CF5C982E5F6251D755B0B16F608ACE631EB3BA9BDAF624FF1651ABF98'
Sha256 hash3; // hash3 = b'2A79F5D9F8B3770A59F91E0E9B4C379F7C7A32353AA6450065E43A8616EF5722'
// preimage1: e.g., "bsv" -> b'627376'
// preimage2: e.g., "sCrypt" -> b'734372797074'
// preimage3: e.g., "IDE" -> b'494445'
public function unlock(bytes preimage1, bytes preimage2, bytes preimage3) {
require(sha256(preimage1) == this.hash1);
require(sha256(preimage2) == this.hash2);
require(sha256(preimage3) == this.hash3);
}
}
The above solution is problematic when N
is large since all N
hashes have to be included in the locking script, bloating the transaction.
Instead, we can combine all y
’s into a single y such that y = H(H(y1 || y2) || y3)
[2] as shown below.
contract MultiPartyHashPuzzlesCompact {
// only 1 hash needs to go into the locking script, saving space
Sha256 combinedHash; // combinedHash = b'C9392767AB23CEFF09D207B9223C0C26F01A7F81F8C187A821A4266F8020064D'
// preimage1: e.g., "bsv" -> b'627376'
// preimage2: e.g., "sCrypt" -> b'734372797074'
// preimage3: e.g., "IDE" -> b'494445'
public function unlock(bytes preimage1, bytes preimage2, bytes preimage3) {
Sha256 hash1 = sha256(preimage1);
Sha256 hash2 = sha256(preimage2);
Sha256 hash3 = sha256(preimage3);
Sha256 hash12 = sha256(hash1 + hash2);
Sha256 hash123 = sha256(hash12 + hash3);
require(hash123 == this.combinedHash);
}
}
[1] | H is a hash function. An online hash calculator can be found here. |
[2] | || is concatenation. |