this validation process is called mining. As other people on the network hear that message, each adds it to a queue of pending transactions that theyve been told about, but which havent yet been approved by the network. What puzzle does David need to solve? One thing to note about the input is that theres nothing explicitly specifying how many bitcoins from the previous transaction should be spent in this transaction. Bitcoin is, instead, perhaps the most open and transparent financial instrument the world has ever seen.
The block chain is a marvellous target for these techniques. Of course, this is not a rigorous security analysis showing that Alice cannot double spend. This changes the nature of the currency considerably. Changelly is another good site for selling bitcoin, but only in exchange for other cryptocurrencies, so it does not work to convert to cash.
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All you do is kick back and wait. If she can slow communication even a little that makes her task of double spending much easier. One apparent oddity in this description is that although each output has a Bitcoin value associated to it, the inputs do not. The reward is used so that people on the network will try to help validate transactions, even though thats now been made a computationally costly process. After all, if Alice sends the message first to Bob, then Bob can verify the message, and tell everyone else in the network (including Charlie) to update their block chain. LocalBitcoins, it is hard to go wrong with this site. The short answer to that question is, Yes, you can sell bitcoin for cash. This is where the terms of the exchange can be spelled out clearly and any questions can be answered (like what will happen if the user is unable to deposit payment in time). This can happen, for instance, if by chance two miners happen to validate a block of transactions near-simultaneously both broadcast their newly-validated block out to the network, and some people update their block chain one way, and others update their block chain the other way.