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Author Topic: What would a fake transaction look like?  (Read 938 times)
panda9228
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January 24, 2014, 10:51:00 PM
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What I mean is, I've heard it's possible to pretend to send bitcoins so it shows up on the blockchain as unconfirmed.  How easy is to do this?  What would it look like on the blockchain... just a transaction that never gets confirmed or what?

I'm just wondering if it means anything when I buy a bitcoin from someone and it "shows up" in my wallet immediately without confirmations.  Is that any evidence that they actually sent real bitcoins or could those easily be faked?
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byt411
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January 24, 2014, 10:53:35 PM
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All transactions "show up" in your wallet as unconfirmed. This does not mean it was a fake payment. All transactions start out as unconfirmed ones...
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January 24, 2014, 10:58:41 PM
Last edit: January 25, 2014, 12:21:09 AM by yogi
 #3

What I mean is, I've heard it's possible to pretend to send bitcoins so it shows up on the blockchain as unconfirmed.  How easy is to do this?  What would it look like on the blockchain... just a transaction that never gets confirmed or what?

I'm just wondering if it means anything when I buy a bitcoin from someone and it "shows up" in my wallet immediately without confirmations.  Is that any evidence that they actually sent real bitcoins or could those easily be faked?

Essentially you need to send the same bitcoins twice, once to the address you want to create a fake transaction to, and the other to another address of yours. The time between the transaction must be short and you must make the real transaction more desirable to the miners than the fake one, this is achieved by giving the real transaction a high transaction fee. But as said, the window of opportunity is small.

Take a look at;

https://en.bitcoin.it/wiki/Double-spending

Edit/

You could also use block withholding, if you had some computational grunt at your disposal. But the window of opportunity is still small.

Edit/ Edit/

Sending a transaction without a fee will work sometimes especially if it's a large number of bytes and/or low priority. The transaction will never be included in a block and will read as unconfirmed until the memory pool is flushed.

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January 24, 2014, 11:16:02 PM
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What I mean is, I've heard it's possible to pretend to send bitcoins so it shows up on the blockchain as unconfirmed.  How easy is to do this?  What would it look like on the blockchain... just a transaction that never gets confirmed or what?

I'm just wondering if it means anything when I buy a bitcoin from someone and it "shows up" in my wallet immediately without confirmations.  Is that any evidence that they actually sent real bitcoins or could those easily be faked?

Essentially you need to send the same bitcoins twice, once to the address you want to create a fake transaction to, and the other to another address of yours. The time between the transaction must be short and you must make the real transaction more desirable to the miners than the fake one, this is achieved by giving the real transaction a high transaction fee. But as said, the window of opportunity is small.

Take a look at;

https://en.bitcoin.it/wiki/Double-spending

Edit/

You could also use block withholding, if you had some computational grunt at your disposal. But the window of opportunity is still small.

He meant double spending? I thought he meant that Unconfirmed Transactions were fake, LOL.
Phinnaeus Gage
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January 24, 2014, 11:24:22 PM
 #5

What I mean is, I've heard it's possible to pretend to send bitcoins so it shows up on the blockchain as unconfirmed.  How easy is to do this?  What would it look like on the blockchain... just a transaction that never gets confirmed or what?

I'm just wondering if it means anything when I buy a bitcoin from someone and it "shows up" in my wallet immediately without confirmations.  Is that any evidence that they actually sent real bitcoins or could those easily be faked?

Essentially you need to send the same bitcoins twice, once to the address you want to create a fake transaction to, and the other to another address of yours. The time between the transaction must be short and you must make the real transaction more desirable to the miners than the fake one, this is achieved by giving the real transaction a high transaction fee. But as said, the window of opportunity is small.

Take a look at;

https://en.bitcoin.it/wiki/Double-spending

Edit/

You could also use block withholding, if you had some computational grunt at your disposal. But the window of opportunity is still small.

I'm not a programmer, but I envision a relatively simple program on some handheld device, i.e. smartphone, that would activate when a real transaction takes place at some physical location, i.e. M&B, sending two transactions semi-simultaneously to two different addresses, with the first one with the higher transfer fee (pre-programmed in) winning the race, thus the merchant losing.

Thoughts?

~TMIBTCITW
yogi
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January 24, 2014, 11:25:25 PM
 #6

Thinking about it some more, sending a transaction without a fee will work sometimes especially if it's a large number of bytes and/or low priority. The transaction will never be included in a block and will read as unconfirmed until the memory pool is flushed.

yogi
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January 24, 2014, 11:32:09 PM
 #7

I'm not a programmer, but I envision a relatively simple program on some handheld device, i.e. smartphone, that would activate when a real transaction takes place at some physical location, i.e. M&B, sending two transactions semi-simultaneously to two different addresses, with the first one with the higher transfer fee (pre-programmed in) winning the race, thus the merchant losing.

Thoughts?

~TMIBTCITW

Possibly, but it would only work for merchants that process orders instantly with zero conformations and I doubt there are many of those. Also, it would be much simpler to use the method posted above, but then I guess the merchant could check that the transaction has a sufficient fee.

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January 24, 2014, 11:42:49 PM
 #8

I'm not a programmer, but I envision a relatively simple program on some handheld device, i.e. smartphone, that would activate when a real transaction takes place at some physical location, i.e. M&B, sending two transactions semi-simultaneously to two different addresses, with the first one with the higher transfer fee (pre-programmed in) winning the race, thus the merchant losing.

Thoughts?

~TMIBTCITW

Possibly, but it would only work for merchants that process orders instantly with zero conformations and I doubt there are many of those. Also, it would be much simpler to use the method posted above, but then I guess the merchant could check that the transaction has a sufficient fee.

Ergo, document and exploit the merchants that process orders instantly with zero conformations, albeit there might not be many of those, but then again...
yogi
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January 24, 2014, 11:51:16 PM
 #9

but then again...

Satoshi Dice

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January 25, 2014, 12:05:53 AM
 #10

I've heard it's possible to pretend to send bitcoins so it shows up on the blockchain as unconfirmed.

that why the rule of 6 confirmations ... is a rule.
in less than 1h30min now (from 0,2 BTC to 10BTC...)

Since 24/01/14, the network react strange ... (confirmation is more like "4 in less 10min" after 1h of waiting time with 0 confirmations).
But the all 6 confirmations time remainly the same. Cool
panda9228
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January 25, 2014, 02:50:13 AM
 #11

Yeah I apologize for my initial post being a bit unclear.  I understand all transactions show up as unconfirmed at first.  I'm just wondering, in practice, how much trust I can put into simply seeing the transaction show up.

Say I'm buying 1 btc and I have reason to trust the person, but only up to 99.5% confidence because you never know.  Does it make sense for me to send half the $$'s, have him send me 0.5 btc, then send him the rest of the $$'s after seeing the initial unconfirmed transfer?  This all occurring within a couple minutes before any confirmations.

I'm just wondering how difficult it would be for an individual to double-spend like that.  If it were super simple, then I shouldn't put much extra weight in seeing the unconfirmed transaction.  But if it were extremely difficult and required a lot of resources, then it might make sense for me to take my chances after seeing the transaction posted.
yogi
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January 25, 2014, 03:15:30 AM
 #12

It's easy, don't accept zero conformation transactions.

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January 25, 2014, 12:26:23 PM
 #13


Since 24/01/14, the network react strange ... (confirmation is more like "4 in less 10min" after 1h of waiting time with 0 confirmations).
But the all 6 confirmations time remainly the same. Cool

Difficulty adjustment happended on 24Jan, and the difficulty went up by 22%.

By the way, block finding is a random process.
"Several blocks in a short period of time / No block in a long period of time" happened from time to time, and is perfectly normal.
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