Questions tagged [security]

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

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132
votes
8answers
48k 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?
119
votes
11answers
58k 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 ...
109
votes
5answers
19k 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 ...
56
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'...
55
votes
4answers
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 ...
54
votes
7answers
44k 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 ...
52
votes
4answers
30k 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 ...
50
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 ...
44
votes
4answers
16k 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 ...
43
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 ...
38
votes
7answers
50k 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, ...
37
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 ...
36
votes
3answers
8k 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? ...
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?
32
votes
2answers
13k 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".
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? ...
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 ...
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?
27
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 ...
26
votes
1answer
16k views

How secure is blockchain.info?

How secure is using a wallet on blockchain.info? How do you evaluate the different sites that provide wallets?
26
votes
1answer
12k views

Signing Bitcoin transactions

There is a part of the Bitcoin protocol that I don't quite understand. When you make a transaction, and spend the output of a previous transaction for say 1 BTC, you provide a signature using the ...
25
votes
3answers
36k 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?
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 ...
24
votes
3answers
5k views

Can one safely buy vanity addresses from a third party without risking one's coins from being stolen?

One can easily generate vanity addresses for Bitcoin, but they can take quite awhile to generate. Outsourcing that process to someone with a strong mining rig is an option, but then one runs into the ...
22
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 ...
21
votes
3answers
10k views

Is Each Bitcoin Address Unique?

I am wondering whether each bitcoin address is unique. I would assume yes. Then how is this enforced (distributed address generation)? As I understand it each time, an address is generated a new key-...
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 ...
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 ...
20
votes
1answer
778 views

Gaming the “off-by-one” bug (difficulty re-target based on 2015 instead of 2016 block time span)?

In this forum post from Gavin Andresen he writes: Second: I've written 'discourage blocks' infrastructure: https://github.com/gavinandresen/bitcoin-git/tree/discourageblocks (code reviews ...
20
votes
6answers
2k views

How can I send Bitcoins via SMS?

I am considering working on giving Africans access to trade using Bitcoins. In many ways, parts of Africa is more developed when it comes to mobile payments than the west. The success of M-PESA ...
19
votes
6answers
7k views

51% attack - apparently very easy? refering to CZ's “rollback btc chain” - How to make sure such corruptible scenario can never happen so easily?

I was shocked to see binance CZ comment to literally "roll back" the bitcoin chain by just "calling" in some favors from "friendly" Asian miners . This ONE person could effectively do it??? I mean ...
19
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?
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, ...
18
votes
2answers
2k views

What security weaknesses result from using off chain solutions?

What type of security weaknesses are potentially created by using off chain solutions (such as the lightning network) for a portion of Bitcoin network traffic?
17
votes
2answers
6k views

How does the Bitcoin client encrypt the wallet.dat file?

What is the algorithm used for encrypting the wallet.dat file?
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?
17
votes
1answer
2k views

Was a timejacking attack ever performed?

The timejacking attack appears to be a pretty straightforward attack to pull off successfully. I was wondering, whether it was ever performed and documented somewhere, or was it just theoretically ...
16
votes
5answers
2k views

Is Bitcoin future-proof?

As I understand things, the security and integrity of Bitcoin rely on the assumption that its hashing is "hard" in some sense. However, in the past, processing power has increased exponentially. ...
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 ...
16
votes
7answers
2k views

What is the safest medium on which to store my wallet/private keys?

What is the safest medium on which to store my wallet/private keys? I avoid eWallets like the plague. I have my wallet on my computer at the moment with the Bitcoin passphrase encryption and I have ...
16
votes
5answers
2k views

What can xxxCoin do to stop the impending 51% attack?

Within the coming weeks both Namecoin and SolidCoin have been threatened with the same attack. What can be done by the users, pool operators and developers of these forks to either; A. Minimise ...
16
votes
1answer
3k views

Should an end-user download the whole blockchain? Or is a “lightweight” client sufficient?

The average user normally doesn’t want to host a 3+GB file on his machine and wait for hours for it to download. What are the security risks of running a lightweight client? And how does this affect ...
15
votes
2answers
7k views

What is a block withholding attack?

How can a block withholding attack be performed, what would be its purpose and who is it a danger to? Edit: when I posted this I was thinking about withholding a block you mined in a pool. But ...
15
votes
1answer
2k views

What is the “zeitgeist attack”? Does it affect all blockchain technologies?

What exactly is this attack? It would be nice to have a good explanation of how it works and which chains can be effected by this. Does it apply to the bitcoin main chain, or only the newer and ...
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 ...
14
votes
2answers
3k 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 ...
14
votes
1answer
2k views

What is the origin of insecure 64-bit nonces in signatures in the Bitcoin chain?

In Biased Nonce Sense: Lattice Attacks against Weak ECDSA Signatures in Cryptocurrencies Joachim Breitner and Nadia Heninger use a lattice based algorithm to recover private keys from signatures in ...
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)....
14
votes
2answers
1k views

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 ...
13
votes
6answers
7k views

How can I stop websites running an integrated Bitcoin miner without disabling javascript altogether?

"Bitcoin Plus" is one example of a javascript bitcoin mining client that can CPU mine bitcoins through a browser when viewing a website. Here is a screenshot from 4bitcoins dot com which automatically ...