I am designing an cross-chain atomic swap in a custom blockchain - Protoblock. I think it can be done as long as Protoblock knows how to parse p2pkh bitcoin transactions. The critical part is being able to "reverse" things when it sees a double spend.

  • Alice has 1 Bitcoin
  • Bob has 100 Fantasybit
  • Alice will swap 1 BTC for Bobs 100 FB

Step1) Bob signs a Protoblock "Swap" transaction(TX1a) that contains an unsigned bitcoin transaction(TX1b) that must have 1. a ScriptSig input with p2pkh from Alice(bitcoin-address) and 2. a p2pkh output to Bob(bitcoin-address)

Step2) 100 FB of Bobs is locked for 24 hours or until...

Step3) a Protoblock transaction(TX2a) is seen that contains the signature for TX1b and TXID(TX1a)

Step4) 100 FB is transferred to Alice and is locked for 24 hours or until...

Step5) a Protoblock transaction(TX3a) is seen that contains a signed bitcoin transaction(TX2b) that has 1. the same ScriptSig input(s) with p2pkh from Alice(bitcoin-address) as TX1b and 2. an output that is NOT a p2pkh output to bob(bitcoin-address)

Step6) 100 FB is sent back to Bob, the same state as before Step1

The reason for Step5 is that, a signed bitcoin transaction is not enough to prove that Bob received the bitcoin from Alice, because Alice could just sign the transaction, but not spend it to bitcoin, and just double spend it.

However, Bob will see that Alice spent the input and will create TX3a that gives him back his 100 Fantasybit. In fact, anyone can be watching bitcoin and create TX3a as a service for Bob.

Question: is there a way that Alice can spend the same inputs from TX1b, that will be undetectable with above algo?

Note: the bitcoin transaction with the outputs for the TX1b inputs, will be created by the wallet software doing the atomic swap. So Alice will first have to send her 1 Bitcoin to herself to make sure its a simple p2pkh. Here is the Protoblock code that creates the p2pkh outputs and bitcoin tx:



1 Answer 1


No Atomic Swaps are by definition atomic meaning either there's a swap and both parties redeem the coins or no . I suggest you take a look at the Decred repository to understand the mechanics of the swap .

  • the term "atomic" swap is used, even though its not atomic. i bet decred is just as "atomic", I will take a look... anyways but the question i'm asking is in relation to a double spend, and if there can be proof of a spent input.
    – jaybny
    Commented Aug 9, 2018 at 17:06
  • decred uses the standard timeOut technique, which (i believe) requires one party to move the coins again - or else the swap does not go through
    – jaybny
    Commented Aug 9, 2018 at 17:20

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