I dont know if this attack is even possible, it came to my mind and I will
try to explain it as good as possible.
Some transacions keep unconfirmed forever and finally they are purged by
Bitcoin nodes, mostly due to the lack of fees.
Example:
---------
Alice is selling a pizza to Bob, Bob is now making the payment with Bitcoin.
The main goal of this attack is to store a unconfirmed transaction send by
Bob for a few days (it will not be included in the blockchain because it
has no fee or due to other reason), Bob might resend the payment or might
just cancel the deal with Alice.
Bob forgets about that failed trade but a couple of days later, Alice, who
has stored the signed transacion, relays the transaction to the network (or
mines it directly with his own hashpower).
Bob does not know what is happening, he believed that that transaction was
"canceled forever", he even does not remember the failed pizza deal.
Alice has now the bitcoins and Bob does not know what happened with his
money.
---------
This might also work with the Payment Protocol because when using it Bob
does not relay the transaction to the network, its Alices job to do it,
Alice stores it and tells Bob to resend the payment, Bob creates another
transaction (If has the same inputs as the first TX this does not work)
(this one is relayed by Alice to the network).
Alice comes back a couple of days later and mines with his hashrate the
first transaction (the one she didnt relayed to the network).
Alice now has two payments, Bob does not know what happened.
-----------
I hope that I explained well this possible attack, I dont know if there is
already a fix for this problem or if it is simply impossible to execute
this kind of attack.
Thanks for your time.
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