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You need to enter a lobby, identified by a pseudo-secret name, to generate or restore wallets in a live ceremony.
Create a huge amount of wallets on the server and have the seed split into multiple parts. The seed can only be restored by a collaboration of a set amount of people providing their keys. You will get the wallet address and viewkey of a specific wallet (specified by its index) to check incoming transactions.
Access a wallet by uploading your key part to the server, allowing other users in your lobby to add their key.
This website is a donation-based open-source service that allows users to use multisignature wallets in Monero today, even though the protocol doesn't allow this yet and probably won't for quite some time. Please consider donating to keep this service running.
All participants of a multisignature wallet use this website to join the same lobby together and generate a multisignature wallet. During this generation ceremony, the server will create a secret key km for every participant and the seeds sn of hundreds of wallets. Those seeds are then XOR'd with a provided size of pairs of keys, so that for a wallet with a size of two it is kx ⊕ ky ⊕ sn. The result of those pairs are called pads pn and, if later combined with the keys, will XOR to the correct seed, like sn = kx ⊕ ky ⊕ pn. After the generation ceremony, every user can download a file that includes a unique session ID, his key and all of the pads he needs to restore every wallet.
The seeds are created on the server to prevent any party accessing it before a restoration ceremony took place. It needs to be known to at least one entity to create the address and viewkey without leaking the private spendkey. To reduce the impact of an attacker stealing the seed from memory, the wallets are created and always treated in batches of hundreds. Due to slow blockchain scanning in Monero and the huge amount of wallets that would need to be scanned regularily to track changes, this makes an attack highly unfeasible, especially for a low-budget donation-based service like this one. The most likely attack scenario is one of the participants, knowing the wallet that's been agreed upon, attempting to steal other users keys via malware and social engineering.