bitcoin

so i haven't used bitcoin for like a year, and i see some things have changed. So i use blockchain wallet and some guy sent me money and before it got any confirmations the transaction just disappeared. So is it possible to reverse the transactions now?( i didn't give him any gold because i'm quite sure he tried to scam)

 

Double spending has always been a thing. That's why people wait for confirmations. You should also remove your signature because posting people's IP with spiteful intent is against Sythe rules.

 

it doesn't say which forums ^ ^, and i don't think mods mind about giving scums a lesson

 

How would you go about aquiring such an IP?

 

not anything malicious if that's what you're interested in

 

I've removed it from your sig - it's not allowed

 

bitcoin is non reversable , wait for confirmations

 

??
Efficiency improvements, attempts to address the finite nature of the currency, shifts in public opinion...nothing that should really effect you doing bitcoin for RSGP deals.

Not sure what you mean here.

Wouldn't recommend blockchain wallets (if you mean what I think you do). Get something that you store locally on your computer. Never have your money "in the cloud", aka in another person's pocket.

"before it got any confirmations" = if he inputted your address correctly, had sufficient funds, successfully unlocked his wallet with his password or other security measures (if applicable), had Internet connection (and [preferably] already synced his wallet to the blockchain), and the platform both of you use is functioning correctly (there does not exist a bug in the wallet software you're using), then waiting for confirmations is just a formality.

Bitcoin is not reversible. Once sent, there's nothing you can do. If you made a mistake, all you can do is contact the person (if you can even find that out) and ask for it to be returned to you (good luck with that).

"just disappeared" = highly unlikely; in the context of a GP for BTC trade I would even be willing to use the word impossible. What do you mean exactly?

Did he provide you with a blockchain.info link that had accurate information? Or did he just say "okay bro, sent it" or copy and paste something that looked good?

What is there to reverse?

He was sending you BTC for your GP right? There's nothing you can do, it's his BTC -- not your concern.

Can he reverse it?
No. BTC transactions are irreversible.



Let me know if you have any further questions.

 

That's simply not true at all. People double spend all the time as a scam, someone even tried it on me. I have a transaction in my wallet that's been pending for 2 months and is no longer in the blockchain.

 

What do you mean by "double spend"?

 

Go ahead and google it, friend. You might not know as much about bitcoin as you think you do.

 

Thank you for bringing this to my attention. In my haste, I now realize my choice of words in my original reply definitely constitutes misinformation. On top of that, I was not aware of that the technical term for this attack vector was "double spend". I thought this was just a term you had made up, but it turns out I was the fool here. I was -- to put it bluntly -- an idiot to reply without first reading other replies to the thread and making sure I knew what they meant. A quick google search would have defined the term for me:

"if you try and spend your bitcoins twice – once to a merchant and once back to another address under your control, then you will have a 50% chance roughly of regaining your money and getting the product for free...
This does depend though on which transaction gets propagated further into the network. For example if transaction A reaches ten mining nodes controlling 80% of the network hashing power first and the other transaction only reaches 20%, then transaction A will have an 80% chance of being included in the next block and be the confirmed transaction."

Somehow throughout four years of university I never heard this term, but was drilled with AAA, identity management, etc.

At any rate, allow me to apologize to both you and OP. Also, thank you for encouraging me to research this topic.






Corrections, in detail:

All good here. Seems as though double spend has been around since the inception of BTC, and appears to be a commonality across all digital currencies.

The emphasized portion of the quote is the problem sentence. Waiting for confirmations is certainly not only a formality. While it is true that attack vectors such as double spend (and the very similar Finney Attack, are extremely unlikely to be successful, if you wish to be 100% safe you need to wait until the coins actually appear in your wallet.

My second paragraph, is technically true. I would not call these attack vectors a "reversal", rather an invalidation -- a manipulation of the inner-workings of the proof-of-work protection offered by miners and the blockchain.

However, I'm sure some would call it a "reversal", so please be wary when discussing it. I would probably concede up to the point of calling it a "redirection", which would encompass the scenario in which the second transaction is to a wallet you control, therefore your options are

• Wallet A (your wallet, sender)
  
• Wallet B (who you're paying)
• Wallet C (another wallet in your possession)

To quickly get into the premise behind these attack vectors (what I wish your original reply would have included, for added clarification. I wonder if the OP already knew what the attack was, if he researched it, or, worse yet, just moved on without looking into it, as I originally did)

Normally,
Wallet A --> Wallet B
Broadcast transaction.

You can decide how many confirmations you'd like to see before you spend the currency you've just received (spendable vs unconfirmed, or similar terminology -- wallet-dependent)

Technically, you can spend it after it reaches the blockchain and receives 1 confirmation. Some wallets require waiting until there are 3 confirmations. The "secure standard" seems to be set at 6.

This requires a quick look at how BTC transactions work. "Confirmed" really means that your transaction's block has been appended to the blockchain -- placed at the end. The blockchain is only meant to grow longer, never lose blocks from the end, nor be rearranged. However, this does happen, albeit very rarely.

Think: at any given point, there are many new transactions occurring. Additionally, the computational power of the network is very widespread.

The last block in the chain is dropped off several times per day -- due to things such as double spend attack attempts, but many things could cause this.

However, the genius of bitcoin is that the likelihood of this occurring gets exponentially lower as the number of blocks appended increases. Meaning:

Chance of the last 2, 3, or 4 blocks being dropped: Less than 1000 occurrences since Bitcoins inception.

Chance of the last 5 blocks being dropped: has only occurred once in history

But, in theory, your transaction is never really "100% confirmed". You can only be 99.9999...% positive. After all, we're just dealing with 1s and 0s here


Back on topic, attack vectors like double spend take advantage of the fact that the longest block will be the one to be appended, and if another block is able to surpass the originally added block, the blockchain will be reorganized meaning the "faulty" (weaker) block will be dropped, and the block with more confidence will take its place.

This brings me back to the word reversible. After bitcoins leave Wallet A, they cannot return to Wallet A (I even strengthened this statement by detailing several conditions to check for -- Internet connection, sync with blockchain, ask for blockchain link to transaction, use legitimate wallet software, etc.).

However, if you're dealing with a nefarious party, they could attempt to broadcast the same transaction twice at the same time or close to the same time.

From here, they are relying on their transaction from Wallet A --> Wallet C is appended, and that you notice you have an incoming transaction from Wallet A and give the person the product.

This can happen for many reasons, including but not limited to:

• Wallet A --> Wallet C was broadcasted slightly before A --> C
• The nefarious party has a % of the networks computational power

The first bullet is unlikely because of how efficient Bitcoin is at catching things like this. Usually within minutes, but with 99%+ certainty within an hour, the blockchain has selected a transaction to be appended, and has discarded the duplicate.

Quote from Satoshi: "Only 6 blocks or 1 hour is enough to make reversal computationally impractical [whichever is greater]."
note: the number 6 appears again


As for the second bullet, consider the following chart:
P < 0.001
q=0.10 z=5
q=0.15 z=8
q=0.20 z=11
q=0.25 z=15
q=0.30 z=24
q=0.35 z=41
q=0.40 z=89
q=0.45 z=340


P = Probability of attack success = 99.9% sure it will fail
z = number of blocks = to put the likelihood at p as q scales
q = computational power



To summarize the above, basically, the chance of someone having a combination of enough time and enough computational power to catch up to and replace the "correct" transaction is extremely low. On top of that, while Bitcoin is decentralized, obviously there are many very wealthy, powerful people invested and constantly watching it, people have their career field in something BTC-related, there is a massive userbase, and the value is partially (I would say: primarily) dictated by the opinions of the users of the currency. Therefore, if someone began seriously attempting to pull such attacks off, the world would take notice and serious events would begin to unfold.

The more extravagant the attempt, such as the 51% attack, in which a single party has the majority of the computational power, meaning that given enough time (note: time scales; 51% (attacker) vs 49% (world) = you will be dead before you get that money, whereas 90% (you) vs 10% (world) would occur much more quickly, but would also be much more noticeable)

Sources/additional reading:
Why is 6 the number of confirms that is considered secure?

How long does it take for a Bitcoin transaction to be confirmed? | Coin Center

https://bitcoin.org/bitcoin.pdf

bips/bip-0050.mediawiki at master · bitcoin/bips · GitHub

Has there ever been a successful double spend attack on the Bitcoin network? If not, is it really necessary to wait for confirmations?

Is Double Spending Unconfirmed Transactions a Concern for Bitcoin? | CoinDesk

What is a double spend?

​

tl;dr:

The money is not in your wallet. It isn't even displaying as "incoming" or "unconfirmed".

A merchant using Paypal would not trade over their gold until they saw the money in their account.

Why would you trade over your gold if you don't even have evidence the other party attempted to send you any money?

If it was there, and is now no longer there either:
a) wallet is glitching -- check the blockchain to confirm he sent it correctly. resync your wallet

or

b) buyer attempted to scam you with an attack vector that is mitigated by waiting a period of time; the time period scaling with how confident you'd like to be (the less time you wait, the higher chance of success the attacker has)

but the bottom line is definitely: you don't have the money. you don't have evidence the money is headed your way. don't give him the product.













& lastly, one final thank you to @PureAddiction . This was a lot of fun to research. I learned a lot and I apologize for my incorrect information earlier.

If you see any errors in what I wrote, please let me know.