An anonymous reader writes "Bitcoin values dropped sharply over the weekend after the largest trading exchange, MtGox, revealed that an investigation into unusual trading activity turned up a flaw in the underlying Bitcoin software that allowed an attacker to double withdrawal a transaction" Not so fast according to database experts: the real problem is that Mt Gox (and other exchanges) are using a surrogate transaction id rather than a natural key in their databases: "The flaw isn't so much in Bitcoin as it is in exchange-systems. Many exchanges use the tx-id to uniquely identify transactions, but as it turns out, an attacker can change the tx-id without changing the actual transaction, rebroadcast the changed transaction (effectively creating a double-spend) and if his altered transaction gets accepted into a block instead of the legit transaction, the attacker receives his coins and can complain with the exchange that he didn't. The exchange will then check their db, fetch the tx-id from it, look it up in the blockchain and not find it. So they could conclude that the transaction indeed failed and credit the account with the coins. ... A simple workaround is to not use the tx-id to identify transactions on the exchange side, but the (amount, address, timestamp) instead."
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