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The details are very complex, but the core concept is fairly simple. Ripple solves the double-spend problem by consensus.
The analogy I use is an "agreement room". To walk into the room, you have to agree with everyone who is already in there. If you want to disagree, you have to leave and form your own room. Everyone who is honest wants to get into the ...
19
The big advantage on the Bitcoin side is that its technology is now well-proven. Ripple's consensus system is the newcomer. Ripple was designed by people who had the benefit of seeing exactly what Bitcoin was doing and its strengths and weaknesses.
Ripple's consensus process can validate a transaction such that irreversibility is reasonably assured much ...
13
Essentially, there is no such thing as a "suspicous transaction" and bitcoin is already set up to work in the way that you want.
Double spending is handled like this -- if two transactions spending the same input(s) happen in the same block, both will be rejected. If one transaction makes it into a block before the other, the first one will be accepted and ...
13
Every full Bitcoin node maintains a database of which unspent outputs are left.
When verifying a transaction, all its inputs are fetched from the database. If one is missing, validation fails. Among the data retrieved is the value of those unspent outputs, and their script (od address), which define the conditions under which the output can be spent. This ...
11
Ripple solves the double spend problem by consensus. Everyone who wants to run a server on the Ripple network picks a set of validators and tries to reach a consensus with them on which transactions are valid.
The Ripple equivalent of Bitcoin's 51% attack would be if some group obtained control over enough validators that the consensus process failed. ...
10
Double-spending has nothing to do with double clicking the send button.
It involves a malicious user, who will not use the standard client. It will hack one to avoid any client-side limits, sending two transactions to a different side of a blockchain fork, so both sides don't know about the transaction sent to the counterpart and includes the transaction in ...
10
This is definitely a concern, and is the reason why Bitcoin users are encouraged to wait for several confirmations before accepting a transaction and delivering goods.
It's not quite as easy as you suggest, though. When you made your transaction at the cafe, it was, as you say, broadcast across the network. Barring connectivity problems, every node on the ...
10
Each full node must maintain a list of all unspent outputs. Currently there are 15 million unspent outputs with a total size of 525 MB. To check whether a new transaction is a double-spend, Bitcoin sees whether the outputs being spent are in this unspent transaction output (UTXO) database. If they are, the new transaction is not a double-spend, and if the ...
9
My question then is: why doesn't bitcoin specify a maximum duration of time and/or a maximum number of confirmations, after which a competing/forking block is rejected even if it's backed by a longer chain of (secretly premined) child blocks?
Because you can't prove that to nodes that weren't on the network at the time of the attack. Which means that either:...
9
There are multiple definitions of the term "double spending" at play here. First, there is the actual definition of double spending: to spend the same money multiple times. A simple example of this is the coin on a string in a vending machine. You put tie a string to a coin, when you put the coin in the machine, it thinks you paid, you get whatever you want ...
8
On March 11th, 2013 there was an unplanned hard fork and as a result there was at least one double spend attack performed in which a large amount of funds (bitcoins worth $10K USD) were double spent.
http://bitcointalk.org/index.php?topic=152348
http://www.bitcoinmoney.com/post/47048259653
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First, a quick clarification: assuming two chains both have valid blocks, it's the chain with the most proof of work that wins, not necessarily the chain with the most blocks.
Second, thanks for the psuedocode. It's always nice answering a question written in clear code.
The answer is that we want nodes to be able to agree on the best block chain based ...
8
How to Double Spend
You can double spend using Electrum quite easily. Even without duplicating your wallet, be it on the same or on 2 different machines. Btw.: Duplicating wallets is super easy with Electrum.
Just go to Tools → Preferences → Transactions and check View transaction before signing.
Close the settings, go to the "Send" tab, and enter the ...
7
At the moment there's very little orphaning of blocks, and next to no occurrence of two-block side chains. Unless you're dealing with large amounts (larger than two block rewards worth, probably), your chances of attack are incredibly incredibly low. If you're dealing with amounts less than a few BTC, I'd personally be happy accepting one-confirmation ...
7
The other two answers explain what has to be done to discover doublespends. I'll try to explain how it works.
There are no "balances", there are "coins"
A common misunderstanding is that there is a register of balances for addresses, and when you spend bitcoins, some of your balance gets deducted from your address. Actually, that's not a good description of ...
7
Double spending is not about doubling, but about using the same UTXO twice. So, you won't gain anything by double spending coins to yourself.
Let me explain you, with a simple example, how double-spending works:
Lets Alice be an attacker trying to double spend some coins. Alice goes to Bob's store to buy some goods. In order to pay for the goods, she ...
7
So are bitcoins, or the smallest allowable bitcoin fractions, distinctly labeled entities?
They are unspent amounts (outputs of prior transactions) associated with addresses. Unspent transaction outputs are abbreviated UTXO.
Bitcoins or fractions (millibits, Satoshis etc) don't exist as entities in computers running Bitcoin software. The computers just ...
6
You should also know that the task of finding an SHA256 collision (same hash for two different byte arrays) is a pretty complex one. Miners are spending terahashes per second to find a hash which is smaller than a target. An exact match would be very hard to find.
Also, the attacker in this task is required to be a miner with enough power to solve and ...
6
A really quick answer is this: The proof of work system is a solution to the distributed synchronization issue; in another guise it is called the Byzantine Generals' Problem. Thus, any solution to this problem is an acceptable alternative, however the proof-of-work solution is particularly suited to distributed systems.
You can read Satoshi Nakamoto's ...
6
Both Bitcoin and Ripple are systems made of many components. One such component is a method to synchronize transactions and secure against double-spending; Bitcoin uses mining based on proof of work for that, while Ripple uses a consensus mechanism. Another component is the initial issuing of coins - Bitcoin uses mining for that as well, while Ripple uses ...
6
From
http://www.cryptocoinsnews.com/news/the-mathematically-secure-way-to-accept-zero-confirmation-transactions/2014/02/13, here is what services like MyCelium and BitPay may be doing:
More specifically, with every additional second a larger percentage of active Bitcoin nodes will have heard the original transaction and everyone viewing the blockchain can ...
6
When a node hears about a new transaction, they try to accept it to their in-memory pool of transactions. If a node hears about a transaction that conflicts with one of the transactions that is already in their mem-pool, then they simply won't accept it to their mem-pool. The default client also won't forward the transaction on to any other nodes, either.
...
6
Occasionally, minor alternate chains emerge if multiple blocks are found for a given blockheight. Usually, these alternate chains only last for a single blocked, and are quickly dropped once another block has been found, allowing one chain to become longer (and thus have more work).
More rarely, these chains might last for a couple of blocks.
Block 525890 ...
6
First of all, what is the goal? The Bitcoin consensus rules already completely eliminate the possibility of an "double spend" within any individual version of history ("chain"). The concern is about individual wallets accepting a transaction as a fullfilled payment, before it is certain that that transaction will end up in the winning ...
5
Because of the way the Bitcoin protocol with the distributed and decentralized P2P network is architected, monitoring the way you describe would do more harm than good.
If a merchant is expecting to learn of double spends, then the monitoring service needs to know of each transaction. A race attack double spend attempt might, however, communicate one ...
5
Another alternative is proof of burn.
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@David's answer essentially doesn't talk about an attack scenario where the room is full of dishonest nodes. This is essentially what the proof-of-work was designed to fix.
If you get a hold of a lot of IPs (nodes in the Ripple network) you could become a majority in the room. The difference is that it's really easy to get an IP, but extremely hard to prove ...
5
The easiest way to do a double spend against someone who accepts zero-confirmation transactions is with a Finney attack. To perform a Finney attack, you must first mine a block and be willing to risk losing that block. So the answer is that if the value of the transaction is small relative to the risk of losing a block reward, you don't have to worry about ...
5
The original design assumes a common behavior of processing the incoming transaction on FIFO basis. That means the first transaction received by a miner wins and all conflicting transactions received after it will be ignored (no matter if the first one is already confirmed or not).
While this standard behavior is beneficial for the Network as a whole, some "...
5
Every single Bitcoin node in the network will reject and ignore the block as invalid if there is a double spend in it. The block won't even spread very far, because the peers that first receive it will ban the miner who sent it and not forward on the block to any other nodes.
It is not a big disruption as the check for this is fast. Other mining pools don't ...
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