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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.

How does it work?

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.

How safe is it?

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.

Credits

Cryptographic JS Library by xmr.llcoins.net
Explaination Avatars by UserInsights (CC BY 3.0)

Introduction: The Trade

Alice Money
This is Alice. She wants to buy a bible online and be confident she doesn't get scammed when sending money online.
Bible Bob
This is Bob. He sells bibles and accepts Monero as a payment method. He only delivers if he can be confident to get the money.

The Ugly: Escrow

Alice Money
Alice sends her money to Claire's wallet and tells her when the bible arrived safely.
Money Claire
This is Claire. She creates a wallet that Alice pays into and forwards it to Bob if the bible arrives. However, she can steal the money at any point!
Bible Bob
Bob sends the bible as soon as he gets the confirmation of payment. He will get the money from Claire after delivery.

The Good: Multisignature Wallets

Key Alice Money
Alice sends her money to the multisignature wallet so that nobody can access it on their own. If the bible arrives, she allows Claire to help Bob get the money. Otherwise Claire helps herself, Alice, get the money back.
Key Claire
Claire gets a part of a key for the multisignature wallet. She can help Alice or Bob access the money, but can not do it on her own.
Bible Bob Key
Bob can see himself when the money is deposited. To access it, he needs the keys from Alice or Claire.