Dex exchange between bitcoin and altcoin blockchain

[OmegaHodler]

What is the difficult part in creating a decentralized platform that exchange between different cryptocurrency blockchains? or at least similar blockchain such as Bitcoin hard forks.

My question is what is the hardest part. We already have the ability to decentralize between tokens of the same blockchain.

[d5000]

A major problem of atomic swaps is that they include optionality. Basically, a standard atomic swap (like in Tier Nolan's model) is much more similar to a call option than to a normal buy-sell operation, because one party can abandon the trade without being punished, leaving the other party with paid transaction fees.*

However, if the coin's script language supports a certain contract model it seems to be solvable (so e.g. an Ethereum-ETC atomic swap should be possible without problems).

See a description of the problem and possible solutions in the following paper by Runchao Han et al: https://eprint.iacr.org/2019/896.pdf

For a practical example of a DEX supporting several blockchains (including Bitcoin) see Atomic DEX: https://github.com/KomodoPlatform/atomicDEX-Desktop

*This problem can also be solved "the other way around" converting the swaps in "fair" call options where the party which benefits from optionality pays a premium so the incentives are balanced again. See this thread, in particular @aliashraf's solution.