Learning From The LND Bug On Lightning – Bitcoin Magazine

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This is an opinion editorial by Shinobi, a self-taught educator in the Bitcoin space and tech-oriented Bitcoin podcast host.

On October 9, 2022, Burak from Bitmatrix (a swap tool built on the Liquid Network) created and broadcast a transaction to the main Bitcoin network, spending a UTXO with a Tapscript multisig with a 998-of-999 threshold. This transaction had 998 individual signatures in the witness field, and was almost 0.1 MB in size, and kind of hilariously, reused the exact same public key for every one of the 999 participants in the multisig. This transaction caused a massive disruption for the Lightning Network by exposing a bug in LND and btcd (an alternative client for the Bitcoin network).

The entire purpose of making this transaction was to demonstrate the improved scalability of multisignature scripts that Taproot has enabled. Even without using Schnorr-signature based MuSig protocols, Taproot can enable much larger multisig participant sets than prior versions of Bitcoin Script. This can be a bit of a nuanced discussion in regards to the previous size limitation of multisig if you dive into all the possible ways you can construct multisig with Bitcoin Script, so for the sake of simplicity I am going to simply discuss the previous limits applying to Pay-to-script-hash (P2SH) and Pay-to-witness-script-hash (P2WSH) multisig constructions. When it comes to the standard way to do a P2SH multisig, the maximum size limit of participants is only 15, and in the case of the standard P2WSH multisig the maximum size is 20. These limits are because of how big a script is allowed to be using these different script ops, and limitations in how many processing operations are allowed to be done in the scope of a single script. Violating either of these limits renders a transaction invalid.



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