user5960: Bitcoin's main innovation is the blockchain, responsible for making sure that the same bitcoin cannot be spent twice.
Mastercoin rides on top of bitcoin, but does not have it's own blockchain.
How does Mastercoin avoid double-spend attacks?

user892: Mastercoin adds a layer on top of Bitcoin.
It uses small transaction in the Bitcoin blockchain to transfer Mastercoins.
Thus the Bitcoin blockchain is used to avoid double spending the same way it is used for Bitcoins.

user78: The answer is double-layered:

Mastercoin uses Bitcoin for timestamping. It's impossible to revert the transaction at the Bitcoin level without doing a Bitcoin double spend (super costly).
Mastercoin clients track the state of the system including how much is stored at each address. If a client double spends by sending two contradicting Mastercoin transactions, the second one will be discarded as an invalid spend by any client that parses the transaction stream.


5960: <p>Bitcoin's main innovation is the blockchain, responsible for making sure that the same bitcoin cannot be spent twice.</p>

<p>Mastercoin rides on top of bitcoin, but does not have it's own blockchain.</p>

<p>How does Mastercoin avoid double-spend attacks?</p>
 892: <p>Mastercoin adds a layer on top of Bitcoin.<br/>
It uses small transaction in the Bitcoin blockchain to transfer Mastercoins.<br/>
Thus the Bitcoin blockchain is used to avoid double spending the same way it is used for Bitcoins.</p>
 78: <p>The answer is double-layered:</p>

<ol>
<li>Mastercoin uses Bitcoin for timestamping. It's impossible to revert the transaction at the Bitcoin level without doing a Bitcoin double spend (super costly).</li>
<li>Mastercoin clients track the state of the system including how much is stored at each address. If a client double spends by sending two contradicting Mastercoin transactions, the second one will be discarded as an invalid spend by any client that parses the transaction stream.</li>
</ol>
