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Questions tagged [security]

Questions relating to application security, safety, trust, and attacks against the Ethereum software stack and blockchain system.

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152
votes
17answers
42k views

How can I securely generate a random number in my smart contract?

If I am writing a smart contract for some kind of (gambling) game, how can I generate a random number securely? What are the best practises and security trade-offs to be aware of?
56
votes
6answers
75k views

What number of confirmations is considered secure in Ethereum?

In Bitcoin, 6 confirmations are considered secure (very low probability of transaction/block reversal). What is the current number in Ethereum?
26
votes
3answers
16k views

How can I make new account by JSON-RPC?

I'd like to make new account by JSON-RPC.I've checked the wiki of Ethereum, however I could not find out how to do that. I've found call existed accounts, but it isn't what I want. eth_accounts ...
48
votes
3answers
21k views

How to reduce the chances of your Ethereum wallet getting hacked?

Summary The hack that occurred on May 12 2016 to steal 7,218 ethers from Patrick only affects miners where: Incoming RPC connections from the Internet on TCP port 8545 are forwarded to the machine ...
48
votes
3answers
8k views

What is a recursive calling vulnerability?

What is a recursive calling vulnerability exactly? When creating smart contracts, DAOs or DAPPs, what measures can I take to ensure I am not vulnerable?
21
votes
2answers
4k views

Is each Ethereum address shared by (theoretically) 2 ** 96 private keys?

Summary This is an extended question to How are ethereum addresses generated?. In Ethereum, a private key is 256-bit long, but an address is only 160-bit long. By "Pigeonhole Principle", it ...
47
votes
3answers
12k views

Why can't contracts make API calls?

The model that contracts can only accept and process data, instead of also being able to retrieve data from the Internet, seems limiting (even if not, it's less direct). If a concern is that data or ...
25
votes
3answers
6k views

Can a contract safely rely on block.timestamp?

How safe is it to use block.timestamp as contract expiration time? If a miner provides an incorrect timestamp in a block header, how much can it be off before it is rejected by other nodes? Is there a ...
25
votes
1answer
4k views

When can BLOCKHASH be safely used for a random number? When would it be unsafe?

I've seen more complicated ways for a contract to generate a random number. But the Ethereum Yellow Paper itself suggests a "trivial solution" using the BLOCKHASH opcode (see below, bold is mine). ...
22
votes
1answer
2k views

What are effective techniques to encrypt/decrypt data stored in a smart contract?

Rather than sending and storing plain text in a smart contract, the use case objective is to encrypt first and then subsequently access the data and decrypt at the point of use. What is the most ...
21
votes
3answers
4k views

TheDAO hack FAQ: How did the attack happen on 17 June 2016?

Can anyone explain that how the DAO attack happened? Vitalik Buterin explained here that it was performed by splitting a DAO from the main DAO but calling the split function recursively. We know that ...
47
votes
3answers
31k views

<address>.send vs <address>.transfer best practice usage?

I noticed that address's offer two member functions that perform the transfer of ether to the specified address. send returns false upon failure and transfer raises an exception. In the Solidity ...
17
votes
3answers
3k views

What are the limits to gas refunds?

Gas refunds are provided when clearing storage or calling SELFDESTRUCT on contracts. The Yellow Paper mentions that refunds are "being capped up to a maximum of half...". What exactly is this "half" ...
10
votes
1answer
3k views

Can I unlock an account using JSON-RPC?

The only way I can work with ETH is through --rpc mode. Normally I would prefer to use the Python interface. But I found myself in a situation where the RPC is very limited. For example, I can't ...
19
votes
3answers
4k views

Account uniqueness guaranteed?

When we create an account in the Ethereum main network, how does the protocol guarantee the unique address of an account? Since even if we are not connected to the network we can create an account, is ...
9
votes
2answers
5k views

Solidity functions - private visibility

Is the following function parameter (string name) only visible to the owner contract itself? function myFunction(string name) private returns (bool) { return true; } So nobody can read / see ...
12
votes
1answer
4k views

Why isn't fast sync the default?

As I understand, fast sync has all the security guarantees of a full sync, but is faster. Is that correct? If so, why isn't fast sync the default?
48
votes
7answers
7k views

How can I verify that a contract on the blockchain matches the source code?

Given the (Solidity) source code of a smart contract, is there a way to deterministically compile the code and compare it with the code on the blockchain? I want to verify that the contract does what ...
16
votes
3answers
5k views

How does the Ethereum ETH accounting system work and prevent double-spends?

In Bitcoin, the BTC accounting system is based on UTXOs (Unspent Transaction Outputs), which prevents the same BTC from being spent multiple times. I heard that Ethereum has a simpler account balance ...
18
votes
4answers
5k views

Is there any safe way to send money from a cold wallet, using an untrusted computer?

You have 2 computers. One is 100% offline and, thus, trusted. Other is online. Private keys are stored on the offline computer and the online computer has no access to it. Is there any way to sign a ...
14
votes
1answer
1k views

What if I had the private key that had the public address of a contract?

Could I spend the money a contract owns if I had the private key that goes to the public address that a contract resides at? (I realize that I'd have a better chance of being struck by lightning ...
9
votes
1answer
814 views

How many ethers have been drained through the recursive call attacks on The DAO?

The recursive call attacks on The DAO have now stopped as there are no more ethers in The DAO account. What are the amounts and percentages that can be attributed to the hostile and friendly attacks, ...
16
votes
2answers
2k views

During a 51% attack, What Can the Attacker Actually Accomplish?

During an attack on the network, where the attacker has more than 51% of hashing power (or the equivalent of Ether in PoS), what can the attacker do and not do to the chain and the contracts on it? ...
15
votes
5answers
2k views

How will regular users be able to use dApps without buying ether?

Requiring users to get ether before they can use a dApp, is too much of a barrier for any kind of mainstream adoption. If dApps offer free ether in a naive way, they can be attacked by Sybils where a ...
15
votes
1answer
687 views

Why were empty accounts allowed to be on the blockchain?

It's hard to see what the benefits of allowing empty accounts to be on the blockchain are. This was a known subtlety (see "note: there is a difference between zero-balance and nonexistent") and ...
8
votes
1answer
5k views

How to unlock accounts programmatically after the node has started?

Is there any way of just unlocking accounts without opening the console with geth? I'm writing some scripts to automate cluster generation and I wish I could just call geth to unlocking some accounts ...
48
votes
4answers
11k views

Is there anything stopping me from using the ethereum blockchain as data storage?

As I understand it, smart contracts are currently immortal by default. Does this mean that I could create a contract that happened to contain an encoded form of baby photos and have it stored forever? ...
8
votes
1answer
1k views

How many The DAO recursive call vulnerability attacks have occurred to date?

The first identified The DAO recursive call vulnerability attack occurred on 17 June 2016, with the accounts identified in Which accounts are involved in mounting the recursive call vulnerability ...
19
votes
2answers
3k views

Methodological security review of a smart contract

Is there a tool, checklist of methodology to help reviewing smart contract security? E.g. for each of function you review Is there re-entry possibility and how it behaves on re-entry How it behaves ...
16
votes
2answers
9k views

Is it possible to overflow uints?

The Ethereum token tutorial has a check for a uint256 overflow. function transfer(address _to, uint256 _value) { /* Check if sender has balance and for overflows */ if (balanceOf[msg.sender] ...
13
votes
4answers
8k views

What Hierarchical Deterministic (HD) wallets are available for Ethereum?

A Hierarchical Deterministic (HD) wallet has some nice features such as easier backup (instead of needing to backup multiple keystore files and the passwords for each of them). Are there any ...
11
votes
4answers
1k views

“potential rewrite attack” message on Geth

When I was sending Ether using Mist I got the following message on Geth. I0528 18:34:57.312997 core/blockchain.go:959] imported 1 block(s) (0 queued 0 ignored) including 3 txs in 19.344963ms. #...
9
votes
1answer
627 views

What data will zk-SNARKs protect?

Implementing zk-SNARKs is planned in Ethereum roadmap, what data will be obfuscated? transaction sender and receiver? data in transactions? data stored in contracts? variables and methods in ...
6
votes
1answer
2k views

How does a fallback function rejecting ether work?

A contract in Solidity can have a fallback function like: // This contract rejects any Ether sent to it. It is good // practise to include such a function for every contract // in order not to loose ...
14
votes
1answer
3k views

What happens if transaction runs out of gas before transaction completes?

As gas cost can only be estimated until the transaction is executed against the actual contract state at the time of execution on the blockchain, what happens if transaction runs out of gas before the ...
25
votes
6answers
37k views

Is there a way to generate ethereum paper wallets?

Is it possible to store ether on paper wallets or generate printable private keys offline somehow?
9
votes
1answer
3k views

How to steal eth using delegatecall and selfdestruct

This documentation says Even if a contract’s code does not contain a call to selfdestruct, it can still perform that operation using delegatecall or callcode. This worries me. I've created ...
12
votes
1answer
2k views

How does the stack depth attack make a send() silently fail?

From a blogpost on security from Christian Reitwiessner: Because of the maximal stack depth of 1024 the new bidder can always increase the stack size to 1023 and then call bid() which will cause the ...
16
votes
2answers
4k views

Is it safe to reuse Ether addresses?

I believe it's viewed as unsafe in Bitcoin because of the way transactions are signed. Is this also true for Ether addresses?
5
votes
2answers
2k views

Honest lottery winner generation - pseudo random number obtaining

Suppose I would like to organize a honest lottery and prevent cheating when we need to find a winner. I known problem of random number in the known network arises. I would like to try the following ...
3
votes
1answer
281 views

Which split proposal was used to mount the recursive call vulnerability attack on The DAO?

I'm assuming that a split proposal was used to mount the recursive call vulnerability attack on The DAO. Which of the splits below was used to mount this attack? Here are the current list of splits: ...
3
votes
1answer
303 views

Which accounts are involved in mounting the recursive call vulnerability attacks on The DAO?

The first transaction 0x0ec3f2488a93839524add10ea229e773f6bc891b4eb4794c3337d4495263790b was initiated from account 0x969837498944ae1dc0dcac2d0c65634c88729b2d to the contract at ...
20
votes
3answers
5k views

How does Oraclize handle the TLSnotary secret?

Oraclize claims to offer a provably honest secure retrieval of a webpage by taking advantage of TLSnotary (a service that allows an auditor to verify if a specific web page was accurately retrieved) ...
13
votes
2answers
5k views

How to validate a private key?

I could not find any library that, given an hex value, returns true if the given string is a valid ether private key. Any hints?
8
votes
1answer
6k views

Security measures to overcome 51% attack

I was reading During a 51% attack, What Can the Attacker Actually Accomplish? from there, i understand the after-effects are something serious, so what are the security measures which ethereum have ...
2
votes
2answers
886 views

Issue with New version of Metamask: Remix cannot detect the Metamask address

I have just installed Metamask version: 6.3.1 Now in Remix, the address of my account in metamask is not detected. The account box in remix remains always blank now (see picture below). Note that I ...
7
votes
2answers
2k views

What are the best practices to keep your ethereum smart contract code from being copied and reused?

I'm new to smart contract development. I have built a couple of sample projects and deployed those to my private network. So now I am doing some research on best practices in regards to smart contract ...
3
votes
2answers
5k views

How do I “Add Data” to a send transaction in Solidity

In myetherwallet, there is an +Advanced: Add Data Section where I can add data to the transaction. How can I add this same data in a transaction I make in Solidity?
7
votes
7answers
3k views

Security analysis tool for Ethereum smart contracts

I'm looking for the best value affordable code checkers to find vulnerabilities in solidity code. I came across on Mythril[1] but I wonder which other products are used. [1] https://github.com/b-...
25
votes
3answers
3k views

Is Ethereum secure against an attack by quantum computers?

Does Ethereum implement Post-quantum cryptography? Or will the network break once a malicious actor gains control of an effective quantum computer?