You may have seen my reddit post of the same title a few days ago: http://www.reddit.com/r/Bitcoin/comments/239bj1/doublespending_unconfirmed_transactions_is_a_lot/

I've done some more experiments since, with good results. For instance here's a real-world double-spend of the gambling service Lucky Bit: Original: 7801c3b996716025dbac946ca7a123b7c1c5429341738e8a6286a389de51bd20 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 Double-spend: f4e8e930bdfa3666b4a46c67544e356876a72ec70060130b2c7078c4ce88582a 01000000012a14c8e6ce1e625513847b2ff271b3e6a1849f2a634c601b7f383ef710483f79000000006a473044022074f0c6912b482c6b51f1a91fb2bdca3f3dde3a3aed4fc54bd5ed563390011c2d02202719fe49578591edfbdd4b79ceeaa7f9550e4323748b3dbdd4135f38e70c476d012103a11c09c09874833eedc58a031d01d161ab4d2eba3874959537c5609ef5d5401fffffffff01d9c90f00000000001976a914d5245b64fcf8e873a9d1c0bfe2d258492bec6cc888ac00000000 The double-spend was mined by Eligius and made use of the fact that Eligius blacklists transactions to a number of addresses considered to be "spam" by the pool operators; affected transactions are not added to the Eligus mempool at all. Lucky Bit has a real-time display of bets as they are accepted; I simply watched that display to determine whether or not I had lost. With Eligius at 8% and the house edge at 1.75% the attack is profitable when automated. My replace-by-fee patch(1) was used, although as there are only a handful of such nodes running - none connected directly to Eligius from what I can determine - I submitted the double-spend transactions to Eligius directly via their pushtxn webform.(2) Of course, this is an especially difficult case, as you must send the double-spend after the original transaction - normally just sending a non-standard tx to Eligius first would suffice. Note how this defeats Andresen's double-spend-relay patch(3) as proposed since the double-spend is a non-standard transaction. In discussion with Lucky Bit they have added case-specific code to reject transactions with known blacklisted outputs; the above double-spend I preformed is no longer possible. Of course, if the (reused) Lucky Bit addresses are added to that blacklist, that approach isn't viable - I suggest they switch to a scheme where addresses are not reused. (per-customer? rotated?) They also have added code to keep track of double-spend occurances and trigger human intervention prior to unacceptable losses. Longer term as with most services (e.g. Just-Dice) they intend to move to off-chain transactions. They are also considering implementing replace-by-fee scorched earth(4) - in their case a single pool, such as Eligius, implementing it would be enough to make the attack unprofitable. It may also be enough security to allow users to use their deposits prior to the first confirmation in a Just-Dice style off-chain implementation. 1) https://github.com/petertodd/bitcoin/tree/replace-by-fee-v0.9.1 2) http://eligius.st/~wizkid057/newstats/pushtxn.php 3) https://github.com/bitcoin/bitcoin/pull/3354 and https://github.com/bitcoin/bitcoin/pull/3883 4) https://bitcointalk.org/index.php?topic=251233.msg2669189#msg2669189 -- 'peter'[:-1]@petertodd.org 000000000000000024abc60eebba42333d74b30635ca5fb0b7c776a579c307a8

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