"Double-Spending" is an attack or failure which results in the same digital moneys being spent twice or more.

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Can a double spend attack be obfuscated by mimicking the transactions seen before the fork?

An attacker controls the majority of the hashing power and wants to double spend without arousing suspicion. If they mirror the public chain's activity while generating the private fork and the fork ...
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How is over-spending one block apart prevented?

So, let's say you have a wallet with 20BTC, and you have a flow like so: You send 15BTC to A in block 1 You send 15BTC to B in block 2 What happens if at block 1 there are two chains across ...
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How big is the chance of double spending for 0, 1, 2, 3, 4, 5 and 6 confirmations?

I'm messing around with bitcoins a bit, and I'm now looking at the confirmations. In several places I read that 6 is the "normal" figure of confirmations needed to say a transactions has actually and ...
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34 views

Double spend in transaction with multiple inputs

Lets assume we have transaction with 2 inputs, each worth 1 bitcoin, and we send these 2 bitcoins to one address. What happens if there is successful double spend attack on bitcoins in our first ...
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4answers
121 views

How much money is a double spend worth?

I keep hearing people say that 0 confirmations are safe at small dollar amounts because it would take more money to do a double spend hack. At what value does it start to become unsafe to trust a 0 ...
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Which clients offer a way to replace unconfirmed transactions?

On this site, we see a lot of questions from people who have sent a transaction which isn't being confirmed, often because they have neglected to include an adequate fee. The Satoshi client doesn't ...
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79 views

Double spend attempts examples on testnet?

Where can I find some double spend attempts examples on testnet? Can anyone refer me some TXIDs with double spends on testnet? EDIT: I need some example in which an outpoint was intended to be used ...
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How to ignore balance check to cancel unconfirmed transaction (double spending)

I need to cancel a transaction that was sent without fee (using Blockchain.info). It is currently "Unconfirmed". The only way I have found is to do a double spend of the whole wallet, specifying a ...
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160 views

How do people actually try double-spends?

I've occasionally seen people online claiming they were victims of a double-spend because they handed over something of value in exchange for a bitcoin payment with no confirmations. Although I ...
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352 views

Can someone explain this double spend on blockchain.info?

I'm confused by double-spend linked below, in which both transactions ended up in blocks mined by different miners. Specifically, I'm wondering if there's any way to tell what exactly happened based ...
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78 views

How strong is the bitcoin network?

Let's say an organization decides to attack bitcoin by launching a 50% attack. Can someone estimate how big the effort is? How expensive would such an attack be from scratch (buying enough ASICs)? ...
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28 views

Inbound connection merchant risk

I was recently looking at the safest way to set up an online bitcoin service. When looking for security concerns I ran across this post on stackexchange What are the risks of running an online store ...
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How are Bitcoin ATMs secured against double-spends?

Some bitcoin ATMs allow the exchange from bitcoins to paper money. Since waiting in front of an ATM for a confirmation is not feasible, they must defend themselves in another way against ...
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2k views

Sending from Bitcoin-Qt to Coinbase

I had the idea to move all of my existing bitcoins (10 BTC) from Bitcoin-Qt/paper wallet to Coinbase. I had imported the paper wallet to Bitcoin-Qt and the balance there was 10 BTC total. For some ...
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2answers
120 views

What precautions to take when working with only one or two confirmations?

I am providing a service that accepts Bitcoin. In order to speed things up a bit, I am planning to reduce the number of confirmations I require before I consider the money to have arrived. I have ...
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1answer
681 views

How do I detect a double-spend?

As a vendor, how do you detect a double-spend? I'm happy to tell end-users that I'm proceeding with a transaction on 0 confirms, simply assuming everything is fine. I'm selling physical goods and so I ...
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1answer
430 views

How often do double spends happen successfully?

Judging from this list, not very often: https://blockchain.info/double-spends Why is this the case? I would have guessed they would have been more frequent. Is it OK to always accept ...
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97 views

Disappearing transactions- a double spend attack?

A particular transaction occurred but it's tx hash can no longer be found in the blockchain. How can that be explained?
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1answer
185 views

Is it possible to execute a double-spending attack in Ripple during a network split?

From the spec (emphasis added): Under a net split, a ledger may be closed by the minority of network, this ledger may be discarded in the future. Does this mean it would be possible to spend ...
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519 views

What is the Ripple equivalent of the “51% attack?”

In Bitcoin, if a participant gains control of more than 50% of the hash power, they can execute a double-spending attack. The "attack" could come from within, say if the existing powerful miners ...
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790 views

What are the pros and cons of Ripple's consensus as compared with Bitcoin's proof-of-work?

One of the major differences between Bitcoin and Ripple is how they process transactions. I can think of three ways in which Ripple's "consensus" scheme is superior to Bitcoin's proof-of-work: Less ...
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224 views

Is there a service that alerts about attempted double spends for merchants accepting 0-confirmation transactions?

I often come across merchants accepting payment in bitcoin, who require several confirmations on transactions, but where the probability of a successful double spend doesn't justify the waiting time ...
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318 views

Can double-spends be automated by malicious custom bitcoin clients? [duplicate]

Say a malicious mobile bitcoin client is created, programmed to quickly send two or more double-send transactions whenever a payment is made. Is this an attack vector we should be concerned of? ...
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Can Ripple Unique Node Lists (of validators) be weighted?

My understanding is that in order to limit / remove the risk of collusion in the Ripple system the consensus mechanism relies on local Unique Node Lists (UNLs) specific to each node in the network ...
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Is there a risk of double-spending when making a face-to-face transaction?

When exchanging funds or services face-to-face, is there a risk of a double-spending attack? If there is, how could it be technically done?
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Are there alternatives for proof of work?

Bitcoin uses proof of work to secure the network, Ripple uses a global consensus system and PPCoin uses proof of stake. Are there any known alternatives to the above methods?
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Possible to create a double spend attack by replacing a Tx in the merkle tree?

This question on crypto.SE describes how preimage attacks are avoided in the Bitcoin network. For a second preimage attack: hash(x) = hash(y) implies hash(hash(x)) = hash(hash(y)) So it ...
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How is it possible to detect system wide segmentation?

In this post, MoonShadow says it's easy to detect system wide segmentation. Yet a system wide segmentation is fairly easy to detect, the current client just doesn't do it. If code implementing ...
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How does Ripple solve the double-spend problem?

How does the Ripple system solve the double-spend problem? Does it also use some block-chain-like entity that officially dictates which transactions are confirmed and which are not, or does it use ...
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344 views

How does the Bitcoin client check for double spend?

When the Bitcoin client looks at the input of a transaction, it know the hash of the previous transaction from which the funds are coming, however then, how does it know that this output has not been ...
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What can happen if/when a node is surrounded by rogue nodes?

If a node (V) accepts incoming connections, is he vulnerable to becoming completely surrounded by corrupted nodes? Would an attacker be able to control which transactions are relayed to V and which ...
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126 views

Double spending from “green addresses” & mitigation

Suppose a user pays for a service (service1) using a "green address" (0 confirmations required) and service1 needs to immediately respend it using it's own green address How many blocks should the ...
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125 views

In a given contract, should the “LockTime” timestamp include an extra hour for confirmations to prevent a double spend?

Anytime a transaction has a timestamp attached to it, and there are signed but not submitted transactions at play, shouldn't the holder of the non-submitted tx have to take into account the ...
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Without being continuously connected to the network, how can I protect from Double Spend attacks?

I'm a vendor who is interested in protecting myself from double spend attacks. What software, configuration, connectivity, or timeout is required to mitigate this risk? Namely, how would I protect ...
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How to force bitcoind to include a specific transaction as an input of another transaction?

I would want to force bitcoind to include a specific transaction as an input of another transaction. This is so as to prevent people front wanting to launch a double-spend attack on the website. The ...
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994 views

Is performing a 51% attack / double spend on Bitcoin illegal?

Suppose I had an ultra large ASIC from the future, and could perform a double spend on the core Bitcoin network. At this point in Bitcoin's development, it is no longer a game - there's real money ...
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275 views

My wallet won't load because it says it contained a duplicate transaction. Are my coins lost forever or can i load it some other way?

Question pretty much says my dilemma. Wallet won't load due to a "duplicate transaction" which I thought could not happen. I use multibit btw, if that helps. Please tell me I can load my wallet ...
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628 views

What to do about a “stuck” transaction?

I started the standard client with --rescan instead of -rescan after switching wallets and before synchronizing with the latest blockchain, and made an going transaction. This transaction got ...
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534 views

What does a double spend look like?

What does a double spend look like? Can anyone link to an example on blockchain.info? Will invalid half of the double spend simply remain at 0-confirmations, or might it ever reach 1 or 2? Will the ...
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1answer
630 views

How to clear invalid transactions from one's wallet?

I've recently been testing double-spend attacks against myself using a couple wallets, and the standard client does not appear to clear conflicting transactions from its transaction list, even if the ...
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What is the best way to recover from an accidental double-spend?

Came across an issue where I was given the message: "Error: The transaction was rejected. This might happen if some of the coins in your wallet were already spent, such as if you used a copy of ...
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Does any pool accept higher-fee transactions of a double spend, instead of the earlier one?

Some pools use a modified bitcoind that uses different rules for choosing the transactions to include in a block. Are there any pools that knowing one transaction to be included in a block would ...
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Double spend attack by sending higher fee transaction directly to the pools possible?

The standard client prioritizes TXs by the time of receipt, e.g. a conflicting transaction will not be relayed. This is not enforced by the network, though. Is there anything that would prevent this ...
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How does blockchain.info detect double spends?

http://blockchain.info/tx-index/242749/36080ed09eeb0fd66ad5dc8a909a49e364f55ff860c12327c2ff9fbe4a9dde77 Warning! this transaction is a double spend. You should be extremely careful when trusting ...
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How does Open Transactions solve the double spend problem?

OT is based on digital cash. From the article: To tackle the double spending problem, the payee has to verify the coin with the bank at the point of sale in each of the transactions. This ...
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How to perform a double spend attack in websites that accept transactions with 0 confirmations?

To perform a Finney attack or a 51% attack I need to have lots of computational power, but in this case (a website that accepts transactions with 0 confirmations) it seems to me that I only need to ...
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Do no-fee transaction represent a greater risk if the product is delivered instantly?

Let's say I want to accept payments instantly on a site for digital downloads. So I publish an address and wait for the unconfirmed transaction, I see the transaction and release the digital ...
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Would a reduced block generation time make the Finney attack more difficult?

Related Question: What can be done to mitigate the risk of a Finney attack? The Finney attack is a form of double-spending attack where the attacker pre-mines a block containing a conflicting ...
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In a double spend attempt, how is the valid transaction chosen?

Suppose I publish the private key of an address that contains 1 bitcoin. If two people try to transfer this bitcoin to another address before a new block is found, who gets it? How is this decision ...
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What does the Bitcoin client do when it receives conflicting transactions?

I've been assuming that the second transaction will simply be discarded as invalid, while the first waits at 0/unconfirmed until it actually gets into a block (requiring a "-rescan" or similar if the ...