ELI5: How is Multisig going to work in Monero?

Question

From what I understand Monero multisig will likely be a special Monero address, similar to something like a p2sh address to address specific use cases. Will it work like a traditional multisig system where you simply just distribute n number of public keys?

Answer 1

OK, here's my attempt at an ELI5 on this. Please help to correct any errors in the usual manner.

A multisig is a special requirement put on to a transaction so that it cannot be spent unless a certain number of signatures are present. This threshold is set something like "3 out of 5" or more generally "t of n".

Multisig transactions are useful because they prevent just one key having control over the funds which helps businesses to reduce theft (perhaps through multiple people signing) or multiple device ownership (2 factor authentication).

In Monero we have "Ring Signatures" where a transaction can be signed but the actual single signing key is unknown to preserve anonymity. We can extend this to introduce multiple signing keys and a threshold using "Ring Multisignature".

We'll gloss over the technical details of setting up the shared key except to say that everyone involved makes sure that all their communications are securely signed to prevent anyone messing with the component keys in transit and that the outcome is control of a particular agreed address. This is covered in the Ring Multisignature protocol laid out in Section 4.4 of MRL-005 (pdf).

This address looks no different to a normal Monero address because of the way the mathematics works when applying additional signing keys and a threshold. This helps with fungibility since there is nothing special about it to indicate it as different from anything else. Funds can be spent to the address in the usual manner.

When the time comes to spend the funds held in the address the owners of the signing keys collaborate to produce a number of signatures that satisfy the threshold.