Questions tagged [security]

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

Filter by
Sorted by
Tagged with
148 votes
7 answers
55k 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?
user avatar
  • 26.4k
140 votes
11 answers
79k 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 ...
user avatar
  • 11.4k
29 votes
3 answers
14k views

Is each Bitcoin address unique?

I am wondering whether each bitcoin address is unique. I would assume yes. Then, how is this enforced given the distributed address generation? As I understand it, each time an address is generated, a ...
user avatar
  • 393
60 votes
4 answers
32k 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 ...
user avatar
  • 2,020
121 votes
5 answers
22k 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 ...
user avatar
  • 5,451
59 votes
8 answers
40k 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'...
user avatar
  • 26.4k
40 votes
5 answers
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 ...
user avatar
  • 11.4k
39 votes
3 answers
12k 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? ...
user avatar
  • 11.4k
44 votes
10 answers
11k 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 ...
user avatar
26 votes
4 answers
38k 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?
user avatar
  • 8,339
38 votes
2 answers
17k 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".
user avatar
  • 8,339
33 votes
1 answer
53k 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? ...
user avatar
  • 2,182
22 votes
2 answers
4k 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?
user avatar
  • 1,999
28 votes
5 answers
3k 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 ...
user avatar
  • 14.3k
53 votes
5 answers
30k 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 amount ...
user avatar
  • 2,445
57 votes
10 answers
9k 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 ...
user avatar
  • 3,434
56 votes
7 answers
53k 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 ...
user avatar
  • 14.4k
22 votes
4 answers
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 ...
user avatar
  • 26.4k
39 votes
5 answers
8k 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?
user avatar
  • 7,513
13 votes
4 answers
486 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, ...
user avatar
  • 131
57 votes
4 answers
7k 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 ...
user avatar
  • 5,372
46 votes
8 answers
74k 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, ...
user avatar
  • 2,247
17 votes
3 answers
7k 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 ...
user avatar
12 votes
3 answers
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 ...
user avatar
  • 26.4k
6 votes
2 answers
2k 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 ...
user avatar
  • 163
34 votes
2 answers
3k 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 ...
user avatar
15 votes
5 answers
5k 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 ...
user avatar
11 votes
2 answers
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?
user avatar
  • 303
17 votes
2 answers
10k 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 ...
user avatar
  • 5,451
13 votes
3 answers
2k 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 ...
user avatar
29 votes
7 answers
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?
user avatar
  • 26.4k
3 votes
4 answers
745 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?"). ...
user avatar
12 votes
4 answers
907 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? ...
user avatar
4 votes
3 answers
376 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 ...
user avatar
1 vote
2 answers
132 views

1 MB Block Size as a Anti-DoS Attack Measure

Wikipedia explains the rationale behind the 1 MB block size limit as follows: The one megabyte block size limit was added in 2010 by Satoshi Nakamoto as a temporary anti-DoS measure A SE post ...
user avatar
  • 399
13 votes
3 answers
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)....
user avatar
11 votes
1 answer
13k 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.
user avatar
  • 431
10 votes
2 answers
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 ...
user avatar
  • 42.8k
19 votes
1 answer
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, ...
user avatar
  • 5,287
11 votes
3 answers
757 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 ...
user avatar
  • 1,469
7 votes
4 answers
9k 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?
user avatar
9 votes
2 answers
2k views

Is there something about Bitcoin that prevents us from implementing the same privacy protocols of Monero and Zcash?

Certain coins like Monero and Zcash implement privacy protocols that make it nearly impossible to trace origin and destination of transactions - making them completely anonymous. I'm curious as to why ...
user avatar
  • 399
9 votes
2 answers
925 views

Can the blockchain be outpaced by a chain of low-difficulty blocks?

Let's say someone creates a blockchain fork starting from the genesis block, when the difficulty was absurdly low compared to today; then he starts mining new blocks from there up to the current block ...
user avatar
  • 1,048
10 votes
1 answer
3k 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 ...
user avatar
  • 203
9 votes
2 answers
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?
user avatar
  • 91
6 votes
1 answer
619 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 ...
user avatar
5 votes
2 answers
997 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 ...
user avatar
  • 28.8k
2 votes
1 answer
208 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?
user avatar
  • 647
2 votes
2 answers
960 views

Can a Trezor seed be used in any other HD wallet?

Trezor FAQ says If you do not use a passphrase, your recovery seed is all that is needed to access your bitcoins. If you do not use a passphrase, can the Trezor seed be used to recreate a ...
user avatar
  • 257
25 votes
8 answers
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 ...
user avatar
  • 7,513