Quote from: DeathAndTaxes on February 17, 2014, 11:10:39 PM

Quote from: PrintCoins on February 17, 2014, 11:06:02 PM

Ok, now I finally grasped the attack I think on gox.

No the attack didn't involve tx from the user or deposits as both would require confirmations and any breakdown due to malleability would have not resulted in extra payments.

Simple version:
User has x BTC on his MtGox account.
User created a withdraw request for x BTC.
MtGox creates a withdraw transfer with tx id "A".
MtGox's wallet is creating "broken" tx anyways which make legitimate issues for tx propagation.
User takes tx id "A", modifies the mutable parts and this produced tx id "B".
Tx "B" ends up in a block and user has received x BTC.
User notifies MtGox they haven't received their withdraw.
MtGox looks in blockchain and there is no tx "A" and thus assumes (incorrectly) this means user has not been paid.
MtGox creates a new tx ("C") for x BTC to the address specified by user and has now paid the user twice.

User could not either deposit the double bitcoins and pull the con again or modified tx "C" so it is tx "D" and report a second failure ("MtGox tx C didn't confirm either please pay me again, I mean pay me) to continue to the theft.

MtGox has had legitimate issues with withdraws not being confirmed (due to their own errors) for more than a month. How many times did an attacker pull the attack described above? How many bitcoins were double paid?

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Or if you want to be optimistic, gox could have kept 90% of their funds in a cold wallet and all is safe.

Or they saw all these requests as valid normal outflows and when the hot wallet got low, moved funds from one or more cold wallets to replenish the hot wallet so the could continue to double, triple, quadruple pay withdraw requests.

Only MtGox knows how much they overpaid and they simply are not talking.

Ok, this make so much more sense. Thank you DaT.

So if I set up a trading system, and
* someone makes a withdraw
* I have a log of the transaction id A of that withdraw
* They make another duplicate transaction B which has a different transaction id of A
* I see that A didn't make it into the blockchain (though untracked transaction B did make it in)
* Upon the user's request I make another transaction C to remedy that A failed.
* Rinse and repeat.

Wow, that is a problem. I now have more sympathy for gox and others as this could easily happen, though they should have had some basic human powered fail-safes to prevent a run-away situation.

So as a provider, what information should I keep as the unique identifier for the transaction that would prevent this issue. I guess it would be something easily searchable in the blockchain.