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Questions tagged [security]

This tag should be used for questions regarding security issues with the design or implementation of Bitcoin and related cryptocurrencies.

129
votes
8answers
46k views

What can an attacker with 51% of hash power do?

Suppose organization X has 51% of the hash power for a period of 1 week. In this week, what exactly can and can't X do?
112
votes
11answers
54k views

Is it possible to brute force bitcoin address creation in order to steal money?

Bitcoin users frequently generate new addresses for each transaction they make, which greatly increases the number of bitcoin addresses being used to receive money. Would it be possible (and ...
50
votes
4answers
29k views

What keeps the average block time at 10 minutes?

I've been told "the network" sets the difficulty level such that there will be one new block mined every ten minutes. I imagine the rich would instead prefer to keep difficulty as high as possible to ...
106
votes
5answers
18k views

Why is 6 the number of confirms that is considered secure?

Why is 6 is the number of confirmations that is considered secure? I haven't found any mathematical explanation or otherwise that explains why it is 6 and not 5 or 7. Is there a historical reason for ...
36
votes
5answers
2k views

How do I accept bitcoin payments at a real world store?

From what I understand, there are some requirements for using bitcoins at a physical store: Both the store owner and the client must have Internet access The client must have a cellphone or a laptop ...
33
votes
3answers
7k views

What are checkpoints?

I often read that checkpoints protect the network from a 51% attack because an attacker cannot reverse transactions made before the last checkpoint. How exactly does this checkpoint mechanism work? ...
54
votes
8answers
38k views

What effects would a scalable Quantum Computer have on Bitcoin?

A scalable quantum computer is a quantum computer that is easy to extend - adding more (q)bits of memory is not a fundamentally hard problem, and will happen. Or, alternatively, that it follows Moore'...
44
votes
9answers
9k views

How can I keep my wallet secure?

The standard Bitcoin client does not encrypt wallet files, so any malicious user that gains access to this file will be able to transfer all the coins to their own wallet. Furthermore, if I lose the ...
41
votes
4answers
14k views

Why was the target block time chosen to be 10 minutes?

According to the wiki, 10 minutes was chosen as a 'tradeoff'. Why ten minutes specifically? It is a tradeoff chosen by Satoshi between propagation time of new blocks in large networks and the ...
32
votes
1answer
51k views

How can I restore my wallet?

I just purchased a new laptop and I want to decommission my old one. I have a back-up of my wallet.dat file copied to my new PC. What do I need to do to restore it to my Bitcoin (Windows) client? ...
24
votes
3answers
34k views

What is a double spend?

What is a double spend? As someone who uses Bitcoin, what do I need to know about how the Bitcoin system prevents double spends? Are there still circumstances where they can occur?
18
votes
2answers
3k views

How does a client decide which is the longest block chain if there is a fork?

Is it only the block height or is it the effort that went into the forks? In other words: does this decision account for different difficulties?
26
votes
5answers
2k views

How much would it cost to execute a 51% attack?

I know that it's commonly said that they're not really in the realm of possibility right now, but how much would it really cost to execute a 51% attack on the network? And are there any organizations ...
30
votes
2answers
12k views

What is a Finney attack?

What is a Finney attack? Extra points for explaining its purpose, the prerequisites for it to be possible, how the attack can be performed and the origin of the name "Finney attack".
20
votes
4answers
3k views

Can a 51% attack be detected and dealt with?

If someone got 51% hash power and started rewriting the blockchain from scratch (building a longer chain with a completely different history), could this be detected? Could "the honest users" revert ...
53
votes
7answers
42k views

Can bitcoins be counterfeited?

Traditional currencies have anti-counterfeiting features designed to make them hard to copy, but despite this attempt counterfeit money is a real threat. Since bitcoins are digital, it's trivial to ...
47
votes
9answers
5k views

What methods could a government use to shutdown Bitcoin?

Let's say that a government decided to shut down the Bitcoin network. What options would they have to achieve this? Would they need to convince other governments? All governments? Let's set as a ...
35
votes
5answers
6k views

What is a deterministic wallet?

It sounds as if it's a process that enables a wallet to be rebuilt from a passphrase, or from several fragments scattered about. Could someone provide a detailed technical explanation?
13
votes
4answers
460 views

Scenario: disaster splits the Internet into dozens unconnected fragments

Major US cities restore connectivity locally for a couple of days, but the internet islands are separated (or have very poor interconnectivity). Two days later, connectivity is restored resulting in, ...
54
votes
4answers
6k views

Have any cryptography experts vetted the bitcoin source code?

Theoretically, bitcoin's open source nature makes it more resistant to bugs and exploits. However, due to the specialized nature of the code, even many programmers don't fully understand the ...
31
votes
2answers
2k views

Do mining pools centralize the Bitcoin network and make it less secure?

A mining pool with a significant percentage of the hash rate could allow double spend attacks by the pool manager. There are a number of pools that approach this level. However, most people say that a ...
6
votes
2answers
1k views

Can you mine without transactions?

As far as I know, mining means verifying transactions and thus keeping the Bitcoin network alive. Today, at a local meetup, some people have informed me that mining doesn't need transactions. They ...
19
votes
1answer
2k views

What protection does Bitcoin have against Denial of Service (DoS) attacks?

What protection does Bitcoin have against DoS attacks? Could you bring down a node by flooding it with data? What happens if someone starts flooding the network with legitimate, yet nonsense, ...
16
votes
2answers
9k views

What are the safety guidelines for using the Sign Message feature?

bitcoin-qt's dialog box states, "Be careful not to sign anything vague, as phishing attacks may try to trick you into signing your identity over to them. Only sign fully-detailed statements you agree ...
12
votes
3answers
1k views

Can the Bitcoin network be used for cracking?

I'm assuming that the Bitcoin economy is rapidly building a computing cluster capable of unprecedented amount of hash searching. Would it be possible for an attacker to leverage this to crack other ...
11
votes
4answers
3k views

How safe is a brain wallet?

Edit: The answer is that they are unsafe and have been attacked. Please don't use them! Brain wallets can be created with a (minimum 15 character) pass phrase at https://www.bitaddress.org There are ...
3
votes
4answers
701 views

Would bitcoin be destroyed if a wealthy organization obtained a CPU majority for a sustained period?

Inspired by this question ("Could bitcoin be destroyed by an entity cornering the market?") and the top comment on this answer to this question ("How much would it cost to execute a 51% attack?"). ...
12
votes
4answers
824 views

What software and skills do I need?

Never having owned bitcoins I don't know how bitcoin wallets work and I'm wondering: if I buy bitcoins, what will I need to know about the software involved, and where should I keep my bitcoin wallet? ...
4
votes
3answers
343 views

Egoistic miners combined with large transactions destroy PoW?

EDIT There is a clearer description of some parts of the problem in the link questions. If there is a transaction value much higher than the payment for mining 10 new blocks, how can you trust this ...
36
votes
7answers
46k views

How long would it take a large computer to crack a private key?

I am doing a presentation on Bitcoins and I was looking for some calculations to make people feel safe about the private key encryption. Please first answer, how long in bytes the private key is, ...
14
votes
3answers
4k views

Which Two-factor authentication methods are available at which exchanges?

I see that Mt. Gox now supports the Google Authenticator app (on Android, iOS - iPhone, iPad, iPod Touch, and on Blackberry) for 2-factor authentication (in addition to support for Yubikey, of course)....
11
votes
1answer
9k views

How to download bitaddress.org to use offline?

The site does not have a download option. I can save the page using Google Crome by CTR-S, but the version I have saved in this way does not work.
11
votes
3answers
661 views

What can be done to mitigate the risk of a Finney-attack?

Related Question: Would a reduced block generation time make the Finney attack more difficult? I understand that the Finney-attack is the main reason why accepting a zero-confirmation transaction ...
10
votes
2answers
2k views

How secure are various models of Bitcoin clients?

As there was some disagreement on security of various models of Bitcoin clients in comments to this answer, I'd like to address the problem. How secure are various models of Bitcoin clients? Those ...
7
votes
4answers
8k views

How can one download the initial Blockchain data files securely?

I see several sources for binary downloads of the Bitcoin blockchain. Can any of these be used securely?
28
votes
7answers
3k views

How can one download the bitcoin client securely?

What is the correct procedure to download the official bitcoin client, and validate it was signed by the developers, and not infected with any malware by a man in the middle?
10
votes
2answers
2k views

Which blocks get to be checkpoints?

Which blocks get to be checkpoints, and why is one block chosen to be a checkpoint, rather than another block? And where can I find a list of checkpoint blocks?
10
votes
1answer
2k views

Can I trust the peers I am connected to?

From my understanding of Bitcoin, it operates on a P2P network. Having just downloaded a wallet app, I see it is connected to 6 peers. Does the Bitcoin protocol rely on trusting these peers? What does ...
7
votes
1answer
1k views

What happens if my wallet gets “hacked” and my BTC is stolen?

What if my wallet gets hacked and the hacker transfers all BTC to his account? Is there is any way to get it back, or to open a dispute to specific address or anything else?
5
votes
1answer
373 views

Where can I learn more about BIP30; namely the exploit and the background discussion?

I'm having trouble understanding the bug that was fixed in BIP030, and how it would be exploited. Is the premise of this attack that there would have to be a netsplit and the chain was forked, then ...
5
votes
2answers
801 views

How could a time warp attack be prevented?

jarpian explains what a time warp attack (AKA zeitgeist attack) is: This is a 51% attack where the attacker sets the block timestamps artificially to lower the difficulty, allowing him to profit ...
2
votes
1answer
204 views

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?
25
votes
8answers
3k views

How can a single person operation keep a collection of online wallets secure?

While this question is similar to others about securing wallets, I think it merits a question of it's own. One thing that has dissuaded me in the past from running an online wallet service of some ...
11
votes
2answers
2k views

How do you derive the private key from two signatures that share the same k value?

I wrote my own ECDSA signature algorithm just for the purpose of creating unit tests. With it I created two signatures which went into transaction 56ec7ca7df..., sending from 1GXFXm3es.... These ...
21
votes
4answers
1k views

Are there any fundamental security vulnerabilities in Bitcoin?

Although Bitcoin does its best to prevent double-spending, could it still happen in practice? Is it just very unlikely that fraud could happen, or is there a mathematical proof that it could never ...
17
votes
3answers
4k views

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?
21
votes
2answers
5k views

Could the EC2 infrastructure be used to efficiently launch a 51% attack?

This excellent answer from Serith states that one would need 31150 ATI 5870 graphics cards to launch a 51% attack. Amazon EC2 offers computers for rent for 2.10$ per hour. These instances have 2 x ...
12
votes
3answers
2k views

Can someone with 51% computing power earn more than he deserves?

See also What can an attacker with 51% of hash power do? This may have been discussed elsewhere, but I'm not sure what the answer is - if someone owns 51% of the hash power, can't he just ignore the ...
3
votes
1answer
300 views

How is network time enforced?

I've seen how nodes gather network time data, but how is it enforced in the blockchain? Are all of the timestamps digitally signed and included in the block as proof?
15
votes
2answers
10k views

Are the Thin clients Electrum and Multibit safe and secure?

How do Electrum and Multibit compare for being safe and secure. Both are open source. Are there any advantages, security wise, to use one or the other? I'm new to bitcoin and the only reason for ...