Distributed clearing a la blockchain. there is?

Question

Electronic commerce

Suppose there is a holder of cryptocurrencies A.
And there is a holder of cryptocurrencies B.
They want to exchange their cryptocurrencies for each other. We agreed that the first one changes X signs of currency A, and the second changes Y signs of currency B.
And let's speculate, but how to make it so that there is such an opportunity?
After all, any currencies support only a confirmed transfer from one hand to another of exactly the same currency, but what about when you need to transfer to each other, so that no one deceived? there are solutions?

Answer 1

If the question is how to make a payment and not be deceived, the solution has been known for a long time.
We need an independent intermediary who is trusted by both parties, and who will act as a guarantor of the transaction.
As a rule, this is a bank, or a state.

Answer 2

With most of the existing cryptocurrencies - no way. Otherwise, there would not be such popularity among exchanges and exchangers.
You need either a guarantor or a smart contract that can check the conditions on its own.
For example, a contract is written, which indicates wallets, exchange amounts and time for the execution of the contract. It is not a problem at all to make contracts with contracts inside one system, but with different systems, especially those that do not support such contracts, it is more difficult. But it can still be implemented. For example, if there are many contracts, then there may be those that can close each other partially or completely. As a result, each crypt spins independently, but transactions take place.
Both send funds to the contract, he redistributes them. If something breaks down or is not executed, then after a specified time, the frozen funds are returned to the senders.