Secure exchange

The Decred developers created a smart contract, using SCRIPT to allow an atomic swap between [ digital coins] DCR and LTC.

If I have a script, a mall program which can take a coin from A and wait for coin B, or take coin b and wait for coin A.  Then when the script has acquired both A and B, it just waits for the designated parties to take the coin as bearer cash or register the exchange on a blockchain.

Who guarantees the script?
The 'miners' do, they can verify when the protocol is in the stable condition with a CRC check and approve its conduct.  It works because the stable states become part of the consistent database during transitions. 

But miners do not need this on every swap, they can let the script run to completion without intermediate checks if their is high  security on the protocol machine itself. In other words, it the miner can be guaranteed honestly the underlying processor, then we need only one miner, but we can still use blockchain.  Eventually we will adopt this method, just have the processor guarantee the proper script sequence at each step, locally.