2

According to the first sidechain article (Link), the cross chain transaction process can be briefly describe as follows:

  1. Alice has a wallet on sidechain S1. She wants to send some coins to sidechain S2. So she makes a transaction containing some coins, send it to a certain locked-output on S1.

  2. Wait for confirmation period to end.

  3. After the transaction is confirmed, she submits a SPV proof to S2.

  4. Again, she has to wait for a contest period.

  5. After the contest period ends, an equivalent amount of coins is created and sent to her wallet on S2.

I assume that both the sidechains are secure, otherwise there is not much left to discuss. If they are secure, then after the confirmation period in step 2 above, the sidechain S2 already knows that the transaction is confirmed on sidechain S1. For example, if Alice purchases something using Bitcoins, step 3 is where she can get the product she purchased from a normal store.

In my opinion, after the confirmation period, the transaction is already in a stable part of S1. Thus, there cannot be any valid SPV proof that contradicts the SPV proof Alice sent, unless S1 does not satisfy the persistence property.

Therfore, I would like to ask, why is the contest period necessary? Is there something I am missing in my argument? Thank you.

0

she submits a SPV proof to S2.

The way this worked in the first implementation of the sidechains paper by the same authors (called Elements Alpha, now elements) is as follows: The genesis block of the sidechain S2 creates a bunch of outputs with a new opcode OP_WPV that can be spent with an SPV proof showing that some amount on S1 has been "sent to a certain-locked output". Note that S2 follows the UTXO model of Bitcoin and there's no global state that OP_WPV could look up. That means that it's possible to provide exactly the same SPV proof to another OP_WPV locked output and effectively transfer your coins from S1 twice. That's why there needs to be a contest period where honest nodes have the opportunity to provide a fraud proof and lock the coins back into the OP_WPV.

fraud proof

Now if there needs to be a contest period anyway, we can make the confirmation period shorter than what we would assume to be secure against reorgs. Instead, the contest period is used to provide a window for reorganization proofs.

You can find more information about how this mechanism worked in this article: https://github.com/jonasnick/elementsproject.github.io/blob/master/source/_posts/The-Federated-Peg-in-Elements-Alpha.md.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.