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| 2323202020202020202020446973696e63656e74 | ## Disincent
| 6976697a696e6720446f75626c652d5370656e64 | ivizing Double-Spend
| 696e67206279204d616b696e6720697420556e70 | ing by Making it Unp
| 726f66697461626c652020202020202020202323 | rofitable ##
| 0a23232323232020202020202020202020202020 | #####
| 2020202020202020202020202020204572696368 | Erich
| 2020457273747520202020202020202020202020 | Erstu
| 2020202020202020202020202020202023232323 | ####
| 230a232323232323202020202020202020202020 | # ######
| 20202020202020202020205b436f696e67616d69 | [Coingami
| 6e672047726f75705d5b31375d20202020202020 | ng Group][17]
| 23230a2323232323232020202020202020202020 | ## ######
| 2020202020202020202020202020202020204a75 | Ju
| 6c792c2032303137202020202020202020202020 | ly, 2017
| 2020202020202020202020202020202020232323 | ###
| 2323230a23232320202020202020202020202020 | ### ###
| 2020202020202020202020202020202020202020 |
| 202323230a2d2d2d2d2d2d2d2d2d2d2d2d2d2d2d | ### ---------------
| 2d2d2d2d2d2d2d2d2d2d2d2d2d2d2d2d2d2d2d2d | --------------------
| 2d2d2d2d2d0a2323232020202020202020202020 | ----- ###
| 2020202323230a23232320202020202020202020 | ### ###
| 2020204162737472616374202020202020202020 | Abstract
| 202020202323230a0a41207369676e6966696361 | ### A significa
| 6e746c792072656475636564206c696b656c6968 | ntly reduced likelih
| 6f6f64206f6620646f75626c652d7370656e6469 | ood of double-spendi
| 6e672061747461636b73206f6e20302d636f6e66 | ng attacks on 0-conf
| 69726d6174696f6e0a7061796d656e747320776f | irmation payments wo
| 756c64206f70656e20612077686f6c65206e6577 | uld open a whole new
| 207265616c6d206f6620706f73736962696c6974 | realm of possibilit
| 69657320666f7220426974636f696e2062757369 | ies for Bitcoin busi
| 6e65737365732e0a446f75626c652d5370656e64 | nesses. Double-Spend
| 2050726f6f667320616e64205061796d656e7420 | Proofs and Payment
| 4368616e6e656c732070726f7669646520706172 | Channels provide par
| 74206f662074686520736f6c7574696f6e206275 | t of the solution bu
| 7420626f74680a6f66207468656d207265717569 | t both of them requi
| 726520636f6d706c6578206e65772066756e6374 | re complex new funct
| 696f6e616c69747920746f206265206275696c74 | ionality to be built
| 206f6e20746f70206f662074686520426974636f | on top of the Bitco
| 696e0a70726f746f636f6c2e2057652070726f70 | in protocol. We prop
| 6f7365206120666565206d61726b657420736f6c | ose a fee market sol
| 7574696f6e20746f2074686520646f75626c652d | ution to the double-
| 7370656e64696e672070726f626c656d20757369 | spending problem usi
| 6e670a6d696e657220677265656420746f20696d | ng miner greed to im
| 70616972207468652065636f6e6f6d6963207072 | pair the economic pr
| 6f66697473206f66207468652061647665727361 | ofits of the adversa
| 72792e204d696e65727320617265206472697665 | ry. Miners are drive
| 6e0a746f20696e63726561736520746865697220 | n to increase their
| 726576656e75652062792070726566657272696e | revenue by preferrin
| 6720746f20636f6e6669726d2074686520737562 | g to confirm the sub
| 736574206f6620616c6c20756e636f6e6669726d | set of all unconfirm
| 65640a7472616e73616374696f6e732028545873 | ed transactions (TXs
| 292074686174207061797320746865206d6f7374 | ) that pays the most
| 20696e20666565732e205468697320756e777269 | in fees. This unwri
| 7474656e2072756c652063616e20626520757365 | tten rule can be use
| 640a746f2064657465637420616e642063697263 | d to detect and circ
| 756d76656e7420636f6d706574696e6720545873 | umvent competing TXs
| 2e204173206c6f6e67206173206d696e65727320 | . As long as miners
| 72656d61696e206772656564792c207765206361 | remain greedy, we ca
| 6e0a616c77617973206f7574626964206120646f | n always outbid a do
| 75626c652d7370656e64696e6720545820627920 | uble-spending TX by
| 737061776e696e672061204368696c642d506179 | spawning a Child-Pay
| 732d666f722d506172656e742028435046502920 | s-for-Parent (CPFP)
| 54580a7769746820612068696768657220666565 | TX with a higher fee
| 2e205468652061647665727361727920636f756c | . The adversary coul
| 6420726573706f6e642077697468207468652073 | d respond with the s
| 616d6520627574207468617420776f756c64206c | ame but that would l
| 6561640a746f2074686520706f696e7420776865 | ead to the point whe
| 7265207468652077686f6c6520545820636f6e74 | re the whole TX cont
| 61696e73206f6e6c79206665657320666f722074 | ains only fees for t
| 6865206d696e6572732e20436f6e76656e69656e | he miners. Convenien
| 746c792c0a7375636820616e20616e74692d646f | tly, such an anti-do
| 75626c652d7370656e6420646566656e7365206d | uble-spend defense m
| 656368616e69736d20646f6573206e6f74207265 | echanism does not re
| 717569726520616e79206368616e676520746f20 | quire any change to
| 7468650a426974636f696e277320636f6e73656e | the Bitcoin's consen
| 7375732070726f746f636f6c20616e64206d696e | sus protocol and min
| 65727320776f756c642062652068617070792074 | ers would be happy t
| 6f206765742074686f7365206578747261206665 | o get those extra fe
| 65732e0a0a0a0a23232320496e74726f64756374 | es. ### Introduct
| 696f6e2020202020202020202020202020202020 | ion
| 202020202323230a0a416363657074696e672075 | ### Accepting u
| 6e636f6e6669726d656420426974636f696e2070 | nconfirmed Bitcoin p
| 61796d656e747320686173206265636f6d652061 | ayments has become a
| 20636f6d6d6f6e20686162697420696e20746865 | common habit in the
| 20426974636f696e0a696e6475737472792e2045 | Bitcoin industry. E
| 76656e2074686f756768205361746f736869204e | ven though Satoshi N
| 616b616d6f746f20736f6c766564207468652064 | akamoto solved the d
| 6f75626c652d7370656e64696e672070726f626c | ouble-spending probl
| 656d20666f720a636f6e6669726d656420545873 | em for confirmed TXs
| 5b5e315d2c20756e636f6e6669726d6564205458 | [^1], unconfirmed TX
| 7320636f756c64207374696c6c20626520656173 | s could still be eas
| 696c792072657665727365645b5e335d2e0a0a49 | ily reversed[^3]. I
| 6e20736563746f72732077686572652063757374 | n sectors where cust
| 6f6d657220736174697366616374696f6e206973 | omer satisfaction is
| 206f662075746d6f737420696d706f7274616e63 | of utmost importanc
| 65206f72206966206f63636173696f6e616c0a64 | e or if occasional d
| 6f75626c652d7370656e64696e6720646f657320 | ouble-spending does
| 6e6f7420706f736520616e206578697374656e74 | not pose an existent
| 69616c2074687265617420746f20746865206275 | ial threat to the bu
| 73696e657373206974206973206f6674656e0a72 | siness it is often r
| 6561736f6e61626c6520746f2070726f76696465 | easonable to provide
| 207365727669636520776974686f757420776169 | service without wai
| 74696e6720666f72207468652066697273742063 | ting for the first c
| 6f6e6669726d6174696f6e206f66207468650a42 | onfirmation of the B
| 6974636f696e206e6574776f726b2e2054686520 | itcoin network. The
| 627573696e657373657320746861742064656c69 | businesses that deli
| 766572207365727669636520666f7220302d636f | ver service for 0-co
| 6e6669726d6174696f6e20545873207365656b0a | nfirmation TXs seek
| 746f206d696e696d697a65207468656972206c6f | to minimize their lo
| 7373657320696e2063617365206f6620646f7562 | sses in case of doub
| 6c652d7370656e642066726175642e2050726565 | le-spend fraud. Pree
| 6d7074697665206d656173757265732073756368 | mptive measures such
| 0a617320726571756972696e6720746865206375 | as requiring the cu
| 73746f6d657220746f2070726f76696465206120 | stomer to provide a
| 636f6d70657469746976652066656520666f7220 | competitive fee for
| 746865207061796d656e74207461636b6c65730a | the payment tackles
| 646f75626c652d7370656e64696e672072617468 | double-spending rath
| 65722077656c6c2e20486f77657665722c207768 | er well. However, wh
| 656e2074686520616476657273617279206f7574 | en the adversary out
| 626964732074686520666565206f66207468650a | bids the fee of the
| 696e697469616c207061796d656e7420696e2074 | initial payment in t
| 6865697220646f75626c652d7370656e64696e67 | heir double-spending
| 205458207468652061747461636b206f6674656e | TX the attack often
| 2073756363656564732e2041206365727461696e | succeeds. A certain
| 0a70657263656e74616765206f66206672617564 | percentage of fraud
| 20686173206265656e2061636365707465642061 | has been accepted a
| 7320756e61766f696461626c652e0a0a57686174 | s unavoidable. What
| 206973206e65656465642069732061206d656368 | is needed is a mech
| 616e69736d20666f722064616d6167652d636f6e | anism for damage-con
| 74726f6c207768656e206120646f75626c652d73 | trol when a double-s
| 70656e642068617320616c72656164790a626565 | pend has already bee
| 6e2062726f61646361737420746f207468652042 | n broadcast to the B
| 6974636f696e206e6574776f726b20616e642069 | itcoin network and i
| 73206e6f772077616974696e6720666f72206120 | s now waiting for a
| 6d696e657220746f20696e636c7564650a697420 | miner to include it
| 696e20746865206e65787420626c6f636b2e2049 | in the next block. I
| 6e20746869732070617065722c2077652070726f | n this paper, we pro
| 706f7365206120736f6c7574696f6e20746f2074 | pose a solution to t
| 686520646f75626c652d7370656e640a70726f62 | he double-spend prob
| 6c656d207573696e67206120435046505b5e325d | lem using a CPFP[^2]
| 20545820746f206f757462696420746865206665 | TX to outbid the fe
| 65206f66206120636f6d706574696e6720545820 | e of a competing TX
| 706f737369626c7920756e74696c0a616c6c2066 | possibly until all f
| 756e6473206f662074686520696e697469616c20 | unds of the initial
| 54582068617665206265656e207370656e74206f | TX have been spent o
| 6e20666565732e205468657265666f72652c2074 | n fees. Therefore, t
| 6865206164766572736172790a6c6f7365732074 | he adversary loses t
| 68652065636f6e6f6d696320696e63656e746976 | he economic incentiv
| 6520746f20646f75626c652d7370656e6420696e | e to double-spend in
| 2074686520666972737420706c6163652e205468 | the first place. Th
| 6973206d656368616e69736d0a69732072656c69 | is mechanism is reli
| 61626c65206173206c6f6e672061732074686520 | able as long as the
| 616476657273617279206973206e6f742061206d | adversary is not a m
| 696e657220616e64206d696e65727320696e2067 | iner and miners in g
| 656e6572616c2072656d61696e0a677265656479 | eneral remain greedy
| 20666f7220545820666565732e0a0a496e207468 | for TX fees. In th
| 65206e6578742073656374696f6e207765207769 | e next section we wi
| 6c6c206578616d696e65206d6f726520636c6f73 | ll examine more clos
| 656c79207468652070726f626c656d7320746861 | ely the problems tha
| 74206172697365207768656e0a61636365707469 | t arise when accepti
| 6e6720302d636f6e6669726d6174696f6e207061 | ng 0-confirmation pa
| 796d656e74732e205765207468656e2064657363 | yments. We then desc
| 726962652074686520736f6c7574696f6e206174 | ribe the solution at
| 20746865206869676865720a6c6576656c20746f | the higher level to
| 206578706c61696e2074686520636f6e63657074 | explain the concept
| 20776974686f757420746f6f206d616e7920696e | without too many in
| 647573747269616c2064657461696c732e20466f | dustrial details. Fo
| 72207468650a746563686e6963616c6c7920696e | r the technically in
| 636c696e65642072656164657273207765206c61 | clined readers we la
| 74657220676976652074686520746563686e6963 | ter give the technic
| 616c2064657461696c732e2046696e616c6c792c | al details. Finally,
| 2077650a69746572617465206f76657220746865 | we iterate over the
| 20706f74656e7469616c20627573696e65737320 | potential business
| 62656e6566697473206f66207468652067697665 | benefits of the give
| 6e20736f6c7574696f6e20616e6420636f6e636c | n solution and concl
| 7564650a746865206d6174746572207769746820 | ude the matter with
| 612073686f72742073756d6d6172792e0a0a0a0a | a short summary.
| 2323232050726f626c656d20446566696e697469 | ### Problem Definiti
| 6f6e202020202020202020202020202020202020 | on
| 0a0a5768656e65766572206120627573696e6573 | Whenever a busines
| 73206163636570747320302d636f6e6669726d61 | s accepts 0-confirma
| 74696f6e207061796d656e747320616e6420696d | tion payments and im
| 6d6564696174656c792070726f76696465732074 | mediately provides t
| 68650a637573746f6d6572207769746820612070 | he customer with a p
| 726f64756374206f722073657276696365206974 | roduct or service it
| 206372656174657320696e63656e746976652066 | creates incentive f
| 6f72206d616c6963696f7573206163746f727320 | or malicious actors
| 746f0a6578706c6f697420746865206661637420 | to exploit the fact
| 7468617420756e636f6e6669726d656420545873 | that unconfirmed TXs
| 20617265206e6f742066696e616c2e20496e2066 | are not final. In f
| 6163742c20756e636f6e6669726d656420545873 | act, unconfirmed TXs
| 0a63616e20656173696c79206265207265766572 | can easily be rever
| 736564206576656e206279206e6f6e2d6d696e65 | sed even by non-mine
| 72735b5e345d2e0a0a4f6e6c696e652063617369 | rs[^4]. Online casi
| 6e6f732074686174206163636570742042697463 | nos that accept Bitc
| 6f696e207061796d656e74732061726520706572 | oin payments are per
| 6861707320746865206d6f737420706f70756c61 | haps the most popula
| 7220746172676574730a666f7220646f75626c65 | r targets for double
| 2d7370656e64657273206265636175736520696e | -spenders because in
| 207468652067616d626c696e6720627573696e65 | the gambling busine
| 73732074686520636f6d7065746974696f6e2066 | ss the competition f
| 726f6d206f746865720a636173696e6f7320666f | rom other casinos fo
| 726365732065766572796f6e6520746f20616363 | rces everyone to acc
| 65707420756e636f6e6669726d6564206465706f | ept unconfirmed depo
| 736974205458732e20416674657220616c6c2c20 | sit TXs. After all,
| 74686520706c617965720a676f657320746f2074 | the player goes to t
| 686520636173696e6f20746861742070726f7669 | he casino that provi
| 6465732074686520626573742075736572206578 | des the best user ex
| 70657269656e63652e2049662061206d616c6963 | perience. If a malic
| 696f757320757365720a77696e73207468657920 | ious user wins they
| 77696c6c206e6f74206c61756e6368206120646f | will not launch a do
| 75626c652d7370656e642061747461636b20736f | uble-spend attack so
| 207468657920636f756c6420736166656c792077 | they could safely w
| 697468647261770a74686569722066756e64732e | ithdraw their funds.
| 20486f77657665722c2069662074686520637573 | However, if the cus
| 746f6d6572206c6f736573206265666f72652074 | tomer loses before t
| 686520696e697469616c20545820676574732074 | he initial TX gets t
| 68652066697273740a636f6e6669726d6174696f | he first confirmatio
| 6e207468656e2074686579206578656375746520 | n then they execute
| 74686520646f75626c652d7370656e64696e6720 | the double-spending
| 61747461636b20696e20686f7065206f66207265 | attack in hope of re
| 76657273696e670a7468656972206465706f7369 | versing their deposi
| 742054582e0a0a496e206f6e6c696e6520766964 | t TX. In online vid
| 656f2067616d6573207768657265207468657265 | eo games where there
| 2061726520696e2d67616d652070757263686173 | are in-game purchas
| 657320697420697320616c736f20657870656374 | es it is also expect
| 65642062790a74686520706c6179657273207468 | ed by the players th
| 6174207468656972207061796d656e7473206172 | at their payments ar
| 6520616363657074656420696d6d656469617465 | e accepted immediate
| 6c7920616e642074686174207468657920726563 | ly and that they rec
| 656976650a7468656972207669727475616c2067 | eive their virtual g
| 6f6f6473206f7220736572766963657320696e20 | oods or services in
| 7265616c2074696d652e20496e2074686520636f | real time. In the co
| 6e74657874206f66207375636820766964656f20 | ntext of such video
| 67616d65730a6974206d616b6573206576656e20 | games it makes even
| 6d6f72652073656e736520746f20616363657074 | more sense to accept
| 20302d636f6e6669726d6174696f6e207061796d | 0-confirmation paym
| 656e7473206265636175736520646f75626c652d | ents because double-
| 7370656e64730a636f756c64206e6f7420636175 | spends could not cau
| 7365207369676e69666963616e742065636f6e6f | se significant econo
| 6d6963206c6f7373657320746f20746865206275 | mic losses to the bu
| 73696e6573732e20496e2074686520776f727374 | siness. In the worst
| 2d636173650a7363656e6172696f207468652064 | -case scenario the d
| 6f75626c652d7370656e64657220776f756c6420 | ouble-spender would
| 6a75737420616e6e6f7920686f6e65737420706c | just annoy honest pl
| 61796572732e20486f77657665722c2073696e63 | ayers. However, sinc
| 65207468650a63757272656e74206d616a6f7269 | e the current majori
| 7479206f66206e6f6465732064656c6962657261 | ty of nodes delibera
| 74656c792072656675736520746f2072656c6179 | tely refuse to relay
| 20646f75626c652d7370656e64696e6720545873 | double-spending TXs
| 5b5e335d2c0a7468652073657276696365207072 | [^3], the service pr
| 6f766964657220776f756c64206d6f7374206c69 | ovider would most li
| 6b656c79206c6561726e2061626f757420746865 | kely learn about the
| 2061747461636b207768656e206d6f7374206f66 | attack when most of
| 207468650a64616d6167652068617320616c7265 | the damage has alre
| 616479206265656e20646f6e652e204966206974 | ady been done. If it
| 20776173206e6f74207468652063617365207468 | was not the case th
| 656e20746865206d616c6963696f757320706c61 | en the malicious pla
| 7965720a636f756c642062652062616e6e656420 | yer could be banned
| 66726f6d207468652067616d6520696d6d656469 | from the game immedi
| 6174656c792061667465722074686579206c6175 | ately after they lau
| 6e63686564207468652061747461636b2e0a0a4c | nched the attack. L
| 69766520766964656f2073747265616d696e6720 | ive video streaming
| 73657276696365732061726520616c736f206120 | services are also a
| 676f6f6420757365206361736520666f72206163 | good use case for ac
| 63657074696e6720756e636f6e6669726d65640a | cepting unconfirmed
| 426974636f696e207061796d656e747320647565 | Bitcoin payments due
| 20746f207468652066616374207468617420666f | to the fact that fo
| 722074686520736572766963652070726f766964 | r the service provid
| 657220746865207265736f757263652074686174 | er the resource that
| 0a63616e206265202a73746f6c656e2a20697320 | can be *stolen* is
| 6d6572656c792062616e6477696474682e205375 | merely bandwidth. Su
| 6368206120627573696e6573732077696c6c206d | ch a business will m
| 6f7374206c696b656c79206e6f7420676f0a6261 | ost likely not go ba
| 6e6b727570742062656361757365206f66206120 | nkrupt because of a
| 636f75706c65206f662073747265616d65727320 | couple of streamers
| 77686f2072656675736520746f207061792e2048 | who refuse to pay. H
| 6f77657665722c207468657920776f756c640a76 | owever, they would v
| 657279206d756368206c696b6520746f20637574 | ery much like to cut
| 20746865207363616d6d6572277320766964656f | the scammer's video
| 2073747265616d20696d6d6564696174656c7920 | stream immediately
| 616674657220646973636f766572696e670a7468 | after discovering th
| 6520646f75626c652d7370656e642054582e0a0a | e double-spend TX.
| 54686573652077657265206a757374206120636f | These were just a co
| 75706c65206f66206578616d706c657320776865 | uple of examples whe
| 726520616363657074696e6720302d636f6e6669 | re accepting 0-confi
| 726d6174696f6e207061796d656e747320686173 | rmation payments has
| 0a6265636f6d6520746865206e6f726d2e205468 | become the norm. Th
| 6f736520627573696e6573736573206172652063 | ose businesses are c
| 757272656e746c7920666f7263656420746f2068 | urrently forced to h
| 616e646c6520646f75626c652d7370656e64696e | andle double-spendin
| 670a61747461636b7320617320616e20696e6576 | g attacks as an inev
| 697461626c65206c6f73732e20496e2074686520 | itable loss. In the
| 6e6578742073656374696f6e7320776520646573 | next sections we des
| 6372696265207768617420627573696e65737365 | cribe what businesse
| 730a63616e20646f20746f20646574657220616e | s can do to deter an
| 64206576656e2070656e616c697a6520646f7562 | d even penalize doub
| 6c652d7370656e64696e672e0a0a0a0a23232320 | le-spending. ###
| 48696768204c6576656c20536f6c7574696f6e20 | High Level Solution
| 202020202020202020202020202323230a0a5468 | ### Th
| 652063757272656e746c7920646f6d696e616e74 | e currently dominant
| 20302d636f6e6669726d6174696f6e20646f7562 | 0-confirmation doub
| 6c652d7370656e64696e672070726576656e7469 | le-spending preventi
| 6f6e206d656368616e69736d20697320746f0a6f | on mechanism is to o
| 6e6c792061636365707420746865206669727374 | nly accept the first
| 2d7365656e205458206173207468652076616c69 | -seen TX as the vali
| 64206f6e652e2053686f756c6420616e6f746865 | d one. Should anothe
| 72205458206c61746572206172726976652c0a63 | r TX later arrive, c
| 6f6e666c696374696e67207769746820616e2065 | onflicting with an e
| 78697374696e672054582c207468656e20746865 | xisting TX, then the
| 206e6577636f6d65722069732073696d706c7920 | newcomer is simply
| 72656a65637465645b5e385d2e205475726e730a | rejected[^8]. Turns
| 6f7574207468617420696e207072616374696365 | out that in practice
| 207375636820612073797374656d20646f657320 | such a system does
| 6e6f7420776f726b2061732077656c6c20617320 | not work as well as
| 6974732063726561746f7273206d696768740a68 | its creators might h
| 617665207769736865645b5e335d2c20736f206e | ave wished[^3], so n
| 61747572616c6c792070656f706c652068617665 | aturally people have
| 207374617274656420746f207365656b20666f72 | started to seek for
| 206f74686572206d6f726520706f74656e740a77 | other more potent w
| 61797320666f72207269736b206d697469676174 | ays for risk mitigat
| 696f6e2e0a0a54686520666972737420616e6420 | ion. The first and
| 6d6f7374206f6276696f7573207374617274696e | most obvious startin
| 6720706f696e7420666f7220616e792062757369 | g point for any busi
| 6e65737320746861742077697368657320746f20 | ness that wishes to
| 64657465720a646f75626c652d7370656e64696e | deter double-spendin
| 67206f6620756e636f6e6669726d656420706179 | g of unconfirmed pay
| 6d656e747320697320746f206465746563742061 | ments is to detect a
| 6e79207375636820617474656d70747320617320 | ny such attempts as
| 6561726c790a617320706f737369626c652e2054 | early as possible. T
| 68697320616c6c6f77732074686520627573696e | his allows the busin
| 65737320746f2073746f702070726f766964696e | ess to stop providin
| 67207365727669636520746f2074686520626164 | g service to the bad
| 206163746f720a696d6d6564696174656c792061 | actor immediately a
| 6e64207468657265666f72652069742068656c70 | nd therefore it help
| 7320746f206d696e696d697a65206c6f73736573 | s to minimize losses
| 2e0a0a4f6e652077617920746f20646574656374 | . One way to detect
| 20646f75626c652d7370656e642061747461636b | double-spend attack
| 7320697320746f2072656c79206f6e2061207468 | s is to rely on a th
| 6972642070617274792073657276696365207375 | ird party service su
| 63680a617320746865206f6e652070726f766964 | ch as the one provid
| 656420627920626c6f636b636861696e2e696e66 | ed by blockchain.inf
| 6f5b5e365d2e204974206973206e6f2077617920 | o[^6]. It is no way
| 61207265636f6d6d656e64656420736f6c757469 | a recommended soluti
| 6f6e0a62757420697420696c6c75737472617465 | on but it illustrate
| 7320746865206661637420746861742069742063 | s the fact that it c
| 616e20626520646f6e6520616e64207468617420 | an be done and that
| 646f75626c652d7370656e647320686176650a69 | double-spends have i
| 6e64656564206265636f6d652061206e6f726d20 | ndeed become a norm
| 6f6e2074686520426974636f696e206e6574776f | on the Bitcoin netwo
| 726b2e0a0a546865206f74686572206d6f726520 | rk. The other more
| 67656e6572616c20617070726f61636820697320 | general approach is
| 746f2063686f6f73652074686520726967687420 | to choose the right
| 6b696e64206f6620426974636f696e20736f6674 | kind of Bitcoin soft
| 776172652e0a466f72206578616d706c652c2042 | ware. For example, B
| 6974636f696e205854206861732061206275696c | itcoin XT has a buil
| 742d696e206d656368616e69736d20666f722064 | t-in mechanism for d
| 6f75626c652d7370656e6420646574656374696f | ouble-spend detectio
| 6e5b5e355d2e0a5768696c6520696e20736f6d65 | n[^5]. While in some
| 20636173657320746865206561726c7920646574 | cases the early det
| 656374696f6e206f662066726175642069732061 | ection of fraud is a
| 6c726561647920656e6f75676820746f20646973 | lready enough to dis
| 636f75726167650a646f75626c652d7370656e64 | courage double-spend
| 696e672c207468657265206973206e6f20726561 | ing, there is no rea
| 736f6e20746f2073746f7020686572652e205475 | son to stop here. Tu
| 726e73206f757420746861742069742069732070 | rns out that it is p
| 6f737369626c650a746f207275696e2074686520 | ossible to ruin the
| 646f75626c652d7370656e6420666f7220746865 | double-spend for the
| 20616476657273617279206173206c6f6e672061 | adversary as long a
| 732074686520636f6d706574696e672054582072 | s the competing TX r
| 656d61696e730a756e636f6e6669726d65642e0a | emains unconfirmed.
| 0a54686520617574686f7273206f662074686973 | The authors of this
| 20776f726b2062656c6965766520746861742073 | work believe that s
| 696e636520646f75626c652d7370656e64696e67 | ince double-spending
| 206f6620756e636f6e6669726d6564205458730a | of unconfirmed TXs
| 63616e6e6f74206265206566666563746976656c | cannot be effectivel
| 792070726576656e7465642c2069742073686f75 | y prevented, it shou
| 6c6420626520656d62726163656420616e642074 | ld be embraced and t
| 616b656e20616476616e74616765206f662e0a49 | aken advantage of. I
| 6e20746865206e6578742073656374696f6e2077 | n the next section w
| 65206578706c61696e20686f77206e6174757261 | e explain how natura
| 6c206d696e65722067726565642063616e206265 | l miner greed can be
| 207574696c697a656420666f72207468650a7075 | utilized for the pu
| 72706f7365206f6620646566656e64696e672061 | rpose of defending a
| 207061796d656e74206166746572206120646f75 | payment after a dou
| 626c652d7370656e642068617320617070656172 | ble-spend has appear
| 6564206f6e20746865206e6574776f726b2e0a57 | ed on the network. W
| 686174206973206d6f72652c2077652072657665 | hat is more, we reve
| 616c207468617420627920656d62726163696e67 | al that by embracing
| 20646f75626c652d7370656e64696e6720746865 | double-spending the
| 20636f6e74726f7665727369616c0a6665617475 | controversial featu
| 7265206b6e6f776e206173205265706c6163652d | re known as Replace-
| 42792d4665655b5e375d20285242462920776f75 | By-Fee[^7] (RBF) wou
| 6c64206e6f206c6f6e676572206265206a757374 | ld no longer be just
| 696669656420616e6420636f756c640a74687573 | ified and could thus
| 2062652072656d6f7665642e0a0a0a0a23232320 | be removed. ###
| 536f6c7574696f6e2044657461696c7320202020 | Solution Details
| 202020202020202020202020202323230a0a5768 | ### Wh
| 696c652074686520636f6e63657074206f66206f | ile the concept of o
| 757462696464696e6720636f6d706574696e6720 | utbidding competing
| 54587320627920696e6372656173696e67207468 | TXs by increasing th
| 6520666565207065722062797465206f660a7468 | e fee per byte of th
| 6520646566656e64696e67205458206973207261 | e defending TX is ra
| 746865722073696d706c652c20697420646f6573 | ther simple, it does
| 20636f6d652077697468206120636f75706c6520 | come with a couple
| 6f6620707265726571756973697465732e0a4669 | of prerequisites. Fi
| 7273746c792c20746865206d696e657273206172 | rstly, the miners ar
| 65206f7567687420746f206f7074696d697a6520 | e ought to optimize
| 74686569722054582073656c656374696f6e2061 | their TX selection a
| 6c676f726974686d20666f72207468650a686967 | lgorithm for the hig
| 6865737420706f737369626c6520726576656e75 | hest possible revenu
| 652e205365636f6e646c792c206e6574776f726b | e. Secondly, network
| 206e6f6465732073686f756c642072656c617920 | nodes should relay
| 646f75626c652d7370656e64696e670a54587320 | double-spending TXs
| 2a616c6d6f73742a206173206966207468657920 | *almost* as if they
| 77657265206e6f726d616c205458732e0a0a496e | were normal TXs. In
| 2074686520666f6c6c6f77696e67207375627365 | the following subse
| 6374696f6e732077652062726965666c79206469 | ctions we briefly di
| 73637573732074686520746563686e6963616c20 | scuss the technical
| 657373656e7469616c73206f66207468650a6e61 | essentials of the na
| 6d656420707265726571756973697465732e2049 | med prerequisites. I
| 6e2074686520656e642c2077652070726573656e | n the end, we presen
| 74207468652070726f706f73656420736f6c7574 | t the proposed solut
| 696f6e20746f2074686520646f75626c652d0a73 | ion to the double- s
| 70656e64696e672070726f626c656d2e20497420 | pending problem. It
| 77696c6c20626520646973637573736564206672 | will be discussed fr
| 6f6d2074686520656e67696e656572696e672070 | om the engineering p
| 6f696e74206f662076696577206275740a666f72 | oint of view but for
| 20746865206c6573732d746563686e6963616c20 | the less-technical
| 72656164657220776520616c736f2070726f7669 | reader we also provi
| 646520746865207068696c6f736f70686963616c | de the philosophical
| 206a757374696669636174696f6e206f660a7468 | justification of th
| 6520676976656e20617070726f6163682e0a0a0a | e given approach.
| 2323232320477265656479205472616e73616374 | #### Greedy Transact
| 696f6e205072696f726974697a6174696f6e2020 | ion Prioritization
| 0a0a5765206b6e6f772074686174206d6f737420 | We know that most
| 6d696e6572732070726566657220746f20636f6e | miners prefer to con
| 6669726d2074686520736574206f662054587320 | firm the set of TXs
| 7468617420706179732074686520677265617465 | that pays the greate
| 73740a6665652070657220627974652e20576520 | st fee per byte. We
| 63616e20726566657220746f2073756368206265 | can refer to such be
| 686176696f72206173206865616c746879206772 | havior as healthy gr
| 65656420616e642077652063616e2072656c7920 | eed and we can rely
| 6f6e0a697420746f207072656469637420776869 | on it to predict whi
| 636820736574206f66205458732077696c6c206d | ch set of TXs will m
| 6f7374206c696b656c792067657420636f6e6669 | ost likely get confi
| 726d656420696e20746865206e65787420626c6f | rmed in the next blo
| 636b2e0a46696e64696e67207375636820612073 | ck. Finding such a s
| 6574206f66205458732063616e20626520766965 | et of TXs can be vie
| 77656420617320612054726176656c696e672053 | wed as a Traveling S
| 616c65736d616e2050726f626c656d2028545350 | alesman Problem (TSP
| 292e0a546865206d696e6572207365656b732074 | ). The miner seeks t
| 6f2066696e64206120737562736574206f662061 | o find a subset of a
| 6c6c20756e636f6e6669726d6564205458732074 | ll unconfirmed TXs t
| 68617420776f756c64206e6f7420657863656564 | hat would not exceed
| 20610a6365727461696e2073697a6520282a6375 | a certain size (*cu
| 7272656e746c792031204d422a29207768696c65 | rrently 1 MB*) while
| 20706179696e67206173206d75636820696e2066 | paying as much in f
| 65657320617320706f737369626c652e0a0a4966 | ees as possible. If
| 20746865726520776173206e6f7420666f722043 | there was not for C
| 5046502054587320696e2074686520736574206f | PFP TXs in the set o
| 6620616c6c20756e636f6e6669726d6564205458 | f all unconfirmed TX
| 73207468656e20736f7274696e67207468650a73 | s then sorting the s
| 6574206f6620756e636f6e6669726d6564205458 | et of unconfirmed TX
| 732064657363656e64696e676c79206279206665 | s descendingly by fe
| 6520706572206279746520776f756c6420626520 | e per byte would be
| 656e6f75676820746f20707265646963740a7768 | enough to predict wh
| 696368205458732077696c6c206d6f7374206c69 | ich TXs will most li
| 6b656c7920626520636f6e6669726d656420696e | kely be confirmed in
| 20746865206e65787420626c6f636b2e20486f77 | the next block. How
| 657665722c2043504650205458730a6361757365 | ever, CPFP TXs cause
| 20737563682073696d706c697374696320736f72 | such simplistic sor
| 74696e6720746f206265636f6d6520696e656666 | ting to become ineff
| 65637469766520696e2063617365732077686572 | ective in cases wher
| 6520756e636f6e6669726d65640a545820636861 | e unconfirmed TX cha
| 696e7320626567696e2077697468206d616e7920 | ins begin with many
| 6c6f772d666565205458732062757420656e6420 | low-fee TXs but end
| 77697468206120686967682d6665652054582e0a | with a high-fee TX.
| 0a416c74686f756768207468652070726f636573 | Although the proces
| 73206f66207072696f726974697a696e67205458 | s of prioritizing TX
| 7320696e20737563682061207761792063616e20 | s in such a way can
| 62652070726f6772616d6d61746963616c6c790a | be programmatically
| 6368616c6c656e67696e672073696d696c61726c | challenging similarl
| 7920746f20746865205453502c207765206b6e6f | y to the TSP, we kno
| 77206d6f7374206365727461696e6c7920746861 | w most certainly tha
| 742069742063616e20626520646f6e652e0a5768 | t it can be done. Wh
| 6174206973206d6f72652c207468652072657375 | at is more, the resu
| 6c747320776f756c642072656d61696e206d6f73 | lts would remain mos
| 746c7920756e69666f726d206163726f73732061 | tly uniform across a
| 6c6c206e6f6465732077686963680a616c69676e | ll nodes which align
| 732077656c6c2077697468207468652058747265 | s well with the Xtre
| 6d65205468696e626c6f636b735b5e395d207465 | me Thinblocks[^9] te
| 63686e6f6c6f677920646576656c6f7065642062 | chnology developed b
| 792074686520426974636f696e0a556e6c696d69 | y the Bitcoin Unlimi
| 746564207465616d2e0a0a0a232323232052656c | ted team. #### Rel
| 6179696e6720446f75626c652d5370656e64696e | aying Double-Spendin
| 67205472616e73616374696f6e73202020202020 | g Transactions
| 2020202020202020232323230a0a4576656e2074 | #### Even t
| 686f75676820646f75626c652d7370656e642054 | hough double-spend T
| 58732061726520616c72656164792072656c6179 | Xs are already relay
| 65642062792074686520426974636f696e205854 | ed by the Bitcoin XT
| 5b5e355d20617370656374206f660a7468652042 | [^5] aspect of the B
| 6974636f696e2070726f746f636f6c2c20746865 | itcoin protocol, the
| 726520697320612067726f77696e672074656e64 | re is a growing tend
| 656e637920666f72206f7468657220696d706c65 | ency for other imple
| 6d656e746174696f6e7320746f0a616c736f2061 | mentations to also a
| 646420737570706f727420666f722072656c6179 | dd support for relay
| 696e6720646f75626c652d7370656e64732e2053 | ing double-spends. S
| 75636820612074656e64656e6379206973206d61 | uch a tendency is ma
| 696e6c792064726976656e2062790a746865206d | inly driven by the m
| 61726b65742064656d616e642062656361757365 | arket demand because
| 206d6f726520616e64206d6f726520627573696e | more and more busin
| 65737365732072656c79206f6e20302d636f6e66 | esses rely on 0-conf
| 69726d6174696f6e205458730a77686963682065 | irmation TXs which e
| 766964656e746c79206578706f73657320746865 | vidently exposes the
| 6d20746f20646f75626c652d7370656e64696e67 | m to double-spending
| 2e0a0a546865206964656120746f2062616e2064 | . The idea to ban d
| 6f75626c652d7370656e647320697320736c6f77 | ouble-spends is slow
| 6c79206c6f73696e672072656c6576616e63792e | ly losing relevancy.
| 204576656e2074686520696e66616d6f75735b5e | Even the infamous[^
| 31315d0a426974636f696e20436f726520646576 | 11] Bitcoin Core dev
| 656c6f706d656e74207465616d2068617320696e | elopment team has in
| 6469726563746c7920656e646f7273656420646f | directly endorsed do
| 75626c652d7370656e64696e6720627920616464 | uble-spending by add
| 696e670a74686520636f6e74726f766572736961 | ing the controversia
| 6c206f70742d696e205242465b5e375d206b6c75 | l opt-in RBF[^7] klu
| 64676520746f2074686569722070617274696375 | dge to their particu
| 6c617220696d706c656d656e746174696f6e206f | lar implementation o
| 660a74686520426974636f696e206e6f64652e20 | f the Bitcoin node.
| 496e207468617420636173652061207370656369 | In that case a speci
| 616c20737562736574206f6620646f75626c652d | al subset of double-
| 7370656e64696e67205458732069732067697665 | spending TXs is give
| 6e0a7468652070726976696c65676520746f2062 | n the privilege to b
| 652072656c617965642062792074686520426974 | e relayed by the Bit
| 636f696e20436f7265206e6f6465732e204e616d | coin Core nodes. Nam
| 656c792c206f6e6c792074686520646f75626c65 | ely, only the double
| 2d0a7370656e64696e6720545873207468617420 | - spending TXs that
| 7061792061206869676865722066656520746861 | pay a higher fee tha
| 6e2074686569722070726576696f757320766572 | n their previous ver
| 73696f6e2c20776869636820646f206e6f740a63 | sion, which do not c
| 6f6e7461696e20616e79206e657720756e636f6e | ontain any new uncon
| 6669726d656420696e7075747320616e6420636f | firmed inputs and co
| 6d706c7920746f207365766572616c206f746865 | mply to several othe
| 722063656e7472616c6c7920706c616e6e65640a | r centrally planned
| 636f6e73747261696e7473206172652072656c61 | constraints are rela
| 7965645b5e31305d2e0a0a4f6e65206f66207468 | yed[^10]. One of th
| 65206361766561747320746f2072656c6179696e | e caveats to relayin
| 6720646f75626c652d7370656e64696e67205458 | g double-spending TX
| 7320697320746865206661637420746861742069 | s is the fact that i
| 7420636f756c640a6578706f736520746865206e | t could expose the n
| 6f646520746f20766172696f757320666f726d73 | ode to various forms
| 206f662044656e69616c2d6f662d536572766963 | of Denial-of-Servic
| 652028446f53292061747461636b732e20546f20 | e (DoS) attacks. To
| 737065636966792c0a7468652061647665727361 | specify, the adversa
| 727920636f756c6420666c6f6f6420746865206e | ry could flood the n
| 6574776f726b20776974682061206c6172676520 | etwork with a large
| 6e756d626572206f6620646f75626c652d737065 | number of double-spe
| 6e64696e67205458730a77686572652065616368 | nding TXs where each
| 206f66207468656d20696e636c75646573206120 | of them includes a
| 66656520646966666572656e742066726f6d2061 | fee different from a
| 6c6c20746865206f74686572732e204966207468 | ll the others. If th
| 65206c6f776573740a66656520696e2074686520 | e lowest fee in the
| 736574206f662073756368206d616c6963696f75 | set of such maliciou
| 7320646f75626c652d7370656e64696e67205458 | s double-spending TX
| 73206973207374696c6c20686967686572207468 | s is still higher th
| 616e207468650a68696768657374206665652069 | an the highest fee i
| 6e2074686520736574206f6620726567756c6172 | n the set of regular
| 20756e636f6e6669726d65642054587320746865 | unconfirmed TXs the
| 6e207468652061647665727361727920776f756c | n the adversary woul
| 6420686176650a6566666563746976656c79206b | d have effectively k
| 69636b6564206f757420616c6c20746865207265 | icked out all the re
| 67756c6172205458732066726f6d20746865202a | gular TXs from the *
| 6d656d706f6f6c2a206f66206576657279206e6f | mempool* of every no
| 64652e0a0a5768696c652074686973206d696768 | de. While this migh
| 7420736f756e64206c696b6520616e2061726775 | t sound like an argu
| 6d656e7420616761696e73742072656c6179696e | ment against relayin
| 732c2069740a6973207265616c6c79206d6f7265 | s, it is really more
| 206f6620616e20657863757365206f6620616e20 | of an excuse of an
| 696e636f6d706574656e7420736f667477617265 | incompetent software
| 20617263686974656374207468616e2061207661 | architect than a va
| 6c69640a636f756e7465722d617267756d656e74 | lid counter-argument
| 2e20416c6c20706f70756c61722066756c6c206e | . All popular full n
| 6f646520696d706c656d656e746174696f6e7320 | ode implementations
| 68617665207468656972202a6d656d706f6f6c73 | have their *mempools
| 2a0a6c696d6974656420746f2061207370656369 | * limited to a speci
| 66696320636170616369747920282a7479706963 | fic capacity (*typic
| 616c6c7920333030204d69422062792064656661 | ally 300 MiB by defa
| 756c745b5e31325d2a292e205468657920617265 | ult[^12]*). They are
| 0a616c726561647920707265706172656420666f | already prepared fo
| 72206465616c696e6720776974682061206d6173 | r dealing with a mas
| 7369766520666c6f6f64206f6620756e636f6e66 | sive flood of unconf
| 69726d6564205458732e204f6276696f75736c79 | irmed TXs. Obviously
| 2c0a746865206578697374696e6720616e74692d | , the existing anti-
| 666c6f6f64696e67206d656368616e69736d2063 | flooding mechanism c
| 6f756c6420626520616d656e64656420746f2064 | ould be amended to d
| 65616c2077697468207468652065786365737369 | eal with the excessi
| 76650a70726f7061676174696f6e206f6620646f | ve propagation of do
| 75626c652d7370656e64696e67205458732e0a0a | uble-spending TXs.
| 53696e6365206974206d616b65732061206c6f74 | Since it makes a lot
| 206f662073656e736520746f2074726561742062 | of sense to treat b
| 616e6477696474682061732061207265736f7572 | andwidth as a resour
| 6365206a757374206c696b65206d656d6f72792c | ce just like memory,
| 0a66756c6c206e6f6465732073686f756c64206c | full nodes should l
| 696d697420746865205458207468726f75676870 | imit the TX throughp
| 7574206f6620746865697220696d6d6564696174 | ut of their immediat
| 65206e65696768626f7273206f6e207468650a6e | e neighbors on the n
| 6574776f726b2e2053686f756c6420616e79206f | etwork. Should any o
| 6620746865206e65696768626f72732065786365 | f the neighbors exce
| 656420746865206c696d6974207468657920636f | ed the limit they co
| 756c64206265206465616c7420776974680a7265 | uld be dealt with re
| 73706563746976656c792e2054686520626f7474 | spectively. The bott
| 6f6d206c696e6520697320746861742066726f6d | om line is that from
| 207468652066726565206d61726b657427732073 | the free market's s
| 74616e64706f696e742072656c6179696e670a64 | tandpoint relaying d
| 6f75626c652d7370656e64696e67205458732069 | ouble-spending TXs i
| 7320616e20696e7472696e736963207472616974 | s an intrinsic trait
| 2e204120676f6f6420616e616c6f677920696e20 | . A good analogy in
| 7468697320636f6e74657874206973207468650a | this context is the
| 666163742074686174206c617720646f6573206e | fact that law does n
| 6f742070726576656e74206372696d696e616c73 | ot prevent criminals
| 2066726f6d207573696e67206669726561726d73 | from using firearms
| 2e20496e20666163742c2073747269637465720a | . In fact, stricter
| 67756e20636f6e74726f6c206769766573207468 | gun control gives th
| 65206372696d696e616c7320616e20616476616e | e criminals an advan
| 7461676520696e2066726f6e74206f66206c6177 | tage in front of law
| 2d61626964696e6720636974697a656e732e0a4d | -abiding citizens. M
| 616b696e6720646f75626c652d7370656e64696e | aking double-spendin
| 6720617661696c61626c6520666f722065766572 | g available for ever
| 796f6e6520776f756c6420616c736f206d616b65 | yone would also make
| 207468652077686f6c6520426974636f696e0a6e | the whole Bitcoin n
| 6574776f726b206d6f7265206661697220666f72 | etwork more fair for
| 2065766572796f6e652e0a0a0a23232323204f75 | everyone. #### Ou
| 7462696464696e6720436f6d706574696e672054 | tbidding Competing T
| 72616e73616374696f6e73202020202020202020 | ransactions
| 202020202020202020232323230a0a5468652073 | #### The s
| 6f6c7574696f6e2070726f706f73656420696e20 | olution proposed in
| 7468697320706170657220696d706c6965732074 | this paper implies t
| 68617420646f75626c652d7370656e64696e6720 | hat double-spending
| 5458732061726520667265656c790a72656c6179 | TXs are freely relay
| 6564206f6e2074686520426974636f696e206e65 | ed on the Bitcoin ne
| 74776f726b20736f207468617420746865207669 | twork so that the vi
| 6374696d20636f756c6420717569636b6c79206c | ctim could quickly l
| 6561726e2061626f7574207468650a636f6d7065 | earn about the compe
| 74696e6720545820616e64206974732064657461 | ting TX and its deta
| 696c732e2049662074686520636f6d706574696e | ils. If the competin
| 6720545820686173206120677265617465722066 | g TX has a greater f
| 65652070657220627974650a7468616e20746865 | ee per byte than the
| 206f726967696e616c205458207468656e207468 | original TX then th
| 65207265636569766572206f6620746865206f72 | e receiver of the or
| 6967696e616c20545820636f756c642073706177 | iginal TX could spaw
| 6e2061206368696c640a54582074686174207370 | n a child TX that sp
| 656e6473206576656e206d6f7265206f66206974 | ends even more of it
| 7320696e7075742066756e6473206f6e20746865 | s input funds on the
| 20545820666565207468616e20746865206d616c | TX fee than the mal
| 6963696f75730a646f75626c652d7370656e642e | icious double-spend.
| 204173206120726573756c742c20776869636865 | As a result, whiche
| 76657220666f726b206f662074686520756e636f | ver fork of the unco
| 6e6669726d656420545820636861696e20756c74 | nfirmed TX chain ult
| 696d6174656c790a706179732074686520677265 | imately pays the gre
| 6174657374206665652070657220627974652077 | atest fee per byte w
| 6f756c6420626520636f6e6669726d65642e0a0a | ould be confirmed.
| 4966207468652061647665727361727920636f6e | If the adversary con
| 74726f6c732061207369676e69666963616e7420 | trols a significant
| 706f7274696f6e206f662074686520676c6f6261 | portion of the globa
| 6c2068617368696e6720706f776572207468656e | l hashing power then
| 0a6f6276696f75736c7920746865206465736372 | obviously the descr
| 6962656420646566656e7365206d656368616e69 | ibed defense mechani
| 736d20776f756c6420626520696e656666656374 | sm would be ineffect
| 6976652e20486f77657665722c20696e20746869 | ive. However, in thi
| 730a706170657220776520617373756d65207468 | s paper we assume th
| 6174207468652061647665727361727920646f65 | at the adversary doe
| 73206e6f7420636f6e74726f6c20737563682072 | s not control such r
| 65736f757263657320626563617573652069660a | esources because if
| 7468657920646964207468656e206576656e2031 | they did then even 1
| 2d636f6e6669726d6174696f6e2054587320776f | -confirmation TXs wo
| 756c6420626520756e7361666520746f20616363 | uld be unsafe to acc
| 6570742e205468652070726f706f7365640a736f | ept. The proposed so
| 6c7574696f6e20697320696e74656e6465642066 | lution is intended f
| 6f72206361736573207768657265207468652074 | or cases where the t
| 72616e736d697474656420616d6f756e74206f66 | ransmitted amount of
| 2066756e647320697320766173746c790a6c6573 | funds is vastly les
| 73207468616e2074686520656666656374697665 | s than the effective
| 20626c6f636b207265776172642e0a0a546f206d | block reward. To m
| 696e696d697a6520746865207269736b206f6620 | inimize the risk of
| 6672617564206576656e206d6f72652c20746865 | fraud even more, the
| 20706179656520636f756c64206f6e6c79207072 | payee could only pr
| 6f7669646520676f6f6473206f720a7365727669 | ovide goods or servi
| 63657320666f7220302d636f6e6669726d617469 | ces for 0-confirmati
| 6f6e207061796d656e74732074686174206f7269 | on payments that ori
| 67696e6174652066726f6d20616c726561647920 | ginate from already
| 636f6e6669726d6564205458732e0a4c6f6e6720 | confirmed TXs. Long
| 636861696e73206f6620756e636f6e6669726d65 | chains of unconfirme
| 64205458732073686f756c642062652074726561 | d TXs should be trea
| 7465642077697468206361726520616e64206e6f | ted with care and no
| 742062652061636365707465640a617320706179 | t be accepted as pay
| 6d656e7473206265636175736520737061776e69 | ments because spawni
| 6e67206120435046502054582061742074686520 | ng a CPFP TX at the
| 656e64206f662073756368206120636861696e20 | end of such a chain
| 636f756c64206661696c206475650a746f20696d | could fail due to im
| 706c656d656e746174696f6e20636f6e73747261 | plementation constra
| 696e7473206f66207468652066756c6c206e6f64 | ints of the full nod
| 6520736f6674776172652e20466f72206578616d | e software. For exam
| 706c652c207468652067726170680a6469616d65 | ple, the graph diame
| 746572206f6620616e20756e636f6e6669726d65 | ter of an unconfirme
| 6420545820636861696e2063616e6e6f74206578 | d TX chain cannot ex
| 6365656420323520696e20426974636f696e2043 | ceed 25 in Bitcoin C
| 6f72652e0a0a416c74686f756768206e6f746869 | ore. Although nothi
| 6e672073746f7073207468652061647665727361 | ng stops the adversa
| 72792066726f6d20636f6e74696e756f75736c79 | ry from continuously
| 20696e6372656173696e67207468652066656520 | increasing the fee
| 6f66207468650a646f75626c652d7370656e6469 | of the double-spendi
| 6e672054582c20697420776f756c64206f6e6c79 | ng TX, it would only
| 206c65616420746f2074686520706f696e742077 | lead to the point w
| 686572652074686520545820666565206265636f | here the TX fee beco
| 6d657320736f0a6c61726765207468617420646f | mes so large that do
| 75626c652d7370656e64696e67206e6f206c6f6e | uble-spending no lon
| 6765722070617973206f66662e20546865206175 | ger pays off. The au
| 74686f7273206f66207468697320776f726b2062 | thors of this work b
| 656c696576650a74686174207468652074687265 | elieve that the thre
| 6174206f6620646f75626c652d7370656e64696e | at of double-spendin
| 67206973207369676e69666963616e746c792072 | g is significantly r
| 656475636564206966207468657265206973206e | educed if there is n
| 6f7468696e670a746f2077696e2066726f6d2069 | othing to win from i
| 742e20496e20612077617920697420776f726b73 | t. In a way it works
| 206f7574206a757374206c696b65204d75747561 | out just like Mutua
| 6c2041737375726564204465737472756374696f | l Assured Destructio
| 6e5b5e31335d2e0a0a0a23232323204120426574 | n[^13]. #### A Bet
| 74657220416c7465726e617469766520746f2052 | ter Alternative to R
| 65706c6163652d42792d46656520202020202020 | eplace-By-Fee
| 202020202020232323230a0a5768656e20746865 | #### When the
| 20626c6f636b2073697a65206c696d6974206f66 | block size limit of
| 2031204d4220776173206d65742064756520746f | 1 MB was met due to
| 2074686520696e6372656173696e67206d61726b | the increasing mark
| 65742064656d616e640a666f7220626c6f636b20 | et demand for block
| 737061636520696e20323031362c205458206665 | space in 2016, TX fe
| 6573207374617274656420746f20726973652064 | es started to rise d
| 72616d61746963616c6c792e2053696e6365206d | ramatically. Since m
| 6f7265205458730a77657265206265696e67206d | ore TXs were being m
| 616465207468616e20426974636f696e206d696e | ade than Bitcoin min
| 65727320636f756c6420636f6e6669726d2c2074 | ers could confirm, t
| 6865206261636b6c6f67206f6620756e636f6e66 | he backlog of unconf
| 69726d65640a54587320686164206e6f20686f70 | irmed TXs had no hop
| 65206f66206576657220636c656172696e67206f | e of ever clearing o
| 75742e204173206120726573756c742c2070656f | ut. As a result, peo
| 706c65207374617274656420746f207574696c69 | ple started to utili
| 7a650a766172696f75732077617973206f662054 | ze various ways of T
| 5820636f6e6669726d6174696f6e20616363656c | X confirmation accel
| 65726174696f6e5b5e31355d2e0a0a4f6e65206f | eration[^15]. One o
| 6620746865207761797320746f20616363656c65 | f the ways to accele
| 7261746520545820636f6e6669726d6174696f6e | rate TX confirmation
| 20697320746f20696e6372656173652069747320 | is to increase its
| 6665652e205768696c6520435046500a6d657468 | fee. While CPFP meth
| 6f642069732075736566756c20666f7220746865 | od is useful for the
| 20545820726563656976657220746f2061636365 | TX receiver to acce
| 6c65726174652069747320636f6e6669726d6174 | lerate its confirmat
| 696f6e2c20746865206f746865720a6c6573732d | ion, the other less-
| 706f70756c61722077617920697320746f207574 | popular way is to ut
| 696c697a65205242462e20546865206c61747465 | ilize RBF. The latte
| 722069732061207370656369616c206b696e6420 | r is a special kind
| 6f6620646f75626c652d7370656e640a74686174 | of double-spend that
| 206973202a686172642d636f6465642a20746f20 | is *hard-coded* to
| 62652072656c6179656420627920746865204269 | be relayed by the Bi
| 74636f696e20436f7265206e6f6465732e204563 | tcoin Core nodes. Ec
| 6f6e6f6d6963616c6c792069742069730a6d6f72 | onomically it is mor
| 6520776173746566756c20746f20737061776e20 | e wasteful to spawn
| 4350465020545873207468616e20746f20646f75 | CPFP TXs than to dou
| 626c652d7370656e6420696e206f726465722074 | ble-spend in order t
| 6f20696e637265617365207468652054580a6665 | o increase the TX fe
| 652e205468617420697320626563617573652061 | e. That is because a
| 204350465020545820697473656c662077696c6c | CPFP TX itself will
| 20636f6e73756d6520626c6f636b207370616365 | consume block space
| 20616e6420746875732068617320746f0a696e63 | and thus has to inc
| 6c756465206120666565206f6620697473206f77 | lude a fee of its ow
| 6e20776865726561732061205242462d7374796c | n whereas a RBF-styl
| 6520646f75626c652d7370656e6420646f657320 | e double-spend does
| 6e6f7420696e74726f6475636520616e790a7375 | not introduce any su
| 6368206f766572686561642e0a0a417320776520 | ch overhead. As we
| 63616e207365652c207468657265206973206563 | can see, there is ec
| 6f6e6f6d696320696e63656e7469766520626568 | onomic incentive beh
| 696e64206d616b696e6720697420706f73736962 | ind making it possib
| 6c6520746f207265706c6163650a54587320746f | le to replace TXs to
| 20737065656420757020746865697220636f6e66 | speed up their conf
| 69726d6174696f6e2074696d652e20496e746572 | irmation time. Inter
| 657374696e676c792c206c6f6e67206265666f72 | estingly, long befor
| 652074686520626c6f636b730a61637475616c6c | e the blocks actuall
| 7920676f742066756c6c20426974636f696e2043 | y got full Bitcoin C
| 6f726520696e636f72706f726174656420524246 | ore incorporated RBF
| 20746f2070726f7669646520612077617920666f | to provide a way fo
| 72206974732075736572730a746f20696e637265 | r its users to incre
| 6173652074686520666565206f66206120545820 | ase the fee of a TX
| 746861742068617320616c726561647920626565 | that has already bee
| 6e2062726f61646361737420746f20746865206e | n broadcast to the n
| 6574776f726b5b5e375d2e0a5768657468657220 | etwork[^7]. Whether
| 746869732068696e747320666f72207468652069 | this hints for the i
| 6e74656e74696f6e616c20707265706172617469 | ntentional preparati
| 6f6e20666f722066756c6c20626c6f636b732061 | on for full blocks a
| 6e6420686967682054580a666565732077697468 | nd high TX fees with
| 2074686520756c746572696f72206d6f74697665 | the ulterior motive
| 206f6620666f7263696e6720426974636f696e20 | of forcing Bitcoin
| 54587320696e746f20707269766174656c79206f | TXs into privately o
| 776e656420616e640a63656e7472616c697a6564 | wned and centralized
| 207061796d656e74206875627320697320757020 | payment hubs is up
| 746f207468652072656164657220746f20646563 | to the reader to dec
| 6964655b5e31345d2e0a0a496e20736f66747761 | ide[^14]. In softwa
| 726520656e67696e656572696e67207468652061 | re engineering the a
| 72636869746563747572616c2062656175747920 | rchitectural beauty
| 6f6620616e206170706c69636174696f6e206c69 | of an application li
| 657320696e206974730a73696d706c6963697479 | es in its simplicity
| 2e20496620646f75626c652d7370656e64696e67 | . If double-spending
| 2054587320776572652072656c61796564206279 | TXs were relayed by
| 20616c6c206e6f646573207468656e2074686520 | all nodes then the
| 524246206b6c756467650a776f756c64206e6f74 | RBF kludge would not
| 206265206e656365737361727920617420616c6c | be necessary at all
| 2e20546865207265737065637469766520636f64 | . The respective cod
| 6520636f756c64207468656e2062652064656c65 | e could then be dele
| 7465642066726f6d0a74686520636f6465626173 | ted from the codebas
| 6520616e642074687573206120736d616c6c2070 | e and thus a small p
| 617274206f662074686520656e6f726d6f757320 | art of the enormous
| 746563686e6963616c20646562745b5e31365d20 | technical debt[^16]
| 426974636f696e0a63757272656e746c79206861 | Bitcoin currently ha
| 7320776f756c642062652070616964206f66662e | s would be paid off.
| 0a0a0a232323232050656e616c697a696e672041 | #### Penalizing A
| 6e7469736f6369616c204265686176696f722020 | ntisocial Behavior
| 2020202020202020202020202020202020202023 | #
| 2323230a0a4f6e65206d69676874206172677565 | ### One might argue
| 207468617420736f6d6574696d65732074686520 | that sometimes the
| 616273656e6365206f662065636f6e6f6d696320 | absence of economic
| 696e63656e74697665206973202a6e6f742a2065 | incentive is *not* e
| 6e6f7567680a746f2064656d6f74697661746520 | nough to demotivate
| 6120646f75626c652d7370656e6465722e205468 | a double-spender. Th
| 697320697320636f72726563742e2041206d616c | is is correct. A mal
| 65766f6c656e7420637573746f6d6572206d6967 | evolent customer mig
| 6874206e6f740a6361726520666f722074686520 | ht not care for the
| 65636f6e6f6d6963206761696e20616e6420636f | economic gain and co
| 756c64207374696c6c20656e6761676520696e20 | uld still engage in
| 646f75626c652d7370656e64696e672e20576865 | double-spending. Whe
| 746865722069740a69732073696d706c7920666f | ther it is simply fo
| 722066756e206f722064756520746f2061207065 | r fun or due to a pe
| 72736f6e616c2067727564676520616761696e73 | rsonal grudge agains
| 7420746865206d65726368616e742c2073756368 | t the merchant, such
| 20616e206163740a697320646566696e6974656c | an act is definitel
| 7920706f737369626c652e0a0a53696e63652074 | y possible. Since t
| 6865206f6666656e6465722068617320616c7265 | he offender has alre
| 6164792072656365697665642073657276696365 | ady received service
| 20666f722074686520302d636f6e6669726d6174 | for the 0-confirmat
| 696f6e207061796d656e742c0a74686579206d6f | ion payment, they mo
| 7374206c696b656c792068617665206e6f746869 | st likely have nothi
| 6e6720656c736520746f206c6f73652e20537572 | ng else to lose. Sur
| 652c2074686520736f6c7574696f6e2070726f70 | e, the solution prop
| 6f73656420696e20746869730a70617065722077 | osed in this paper w
| 6f756c64206d616b6520697420696d706f737369 | ould make it impossi
| 626c6520666f7220746865206164766572736172 | ble for the adversar
| 7920746f20726576657273652074686520706179 | y to reverse the pay
| 6d656e742062757420746865790a6d6967687420 | ment but they might
| 7374696c6c20696e697469617465206120646f75 | still initiate a dou
| 626c652d7370656e642061747461636b20746f20 | ble-spend attack to
| 7275696e20746865206465616c20666f72207468 | ruin the deal for th
| 65206d65726368616e742e0a0a4c75636b696c79 | e merchant. Luckily
| 2c207375636820616e20616e7469736f6369616c | , such an antisocial
| 206265686176696f722063616e206265206d6164 | behavior can be mad
| 6520736f20636f73746c7920666f722074686520 | e so costly for the
| 6f6666656e64657220746861740a746865792077 | offender that they w
| 696c6c206d6f7374206c696b656c79206e6f7420 | ill most likely not
| 77616e7420746f20696e69746961746520612064 | want to initiate a d
| 6f75626c652d7370656e64696e6720636f6d7065 | ouble-spending compe
| 746974696f6e20696e207468650a666972737420 | tition in the first
| 706c6163652e2054686520747269636b20686572 | place. The trick her
| 6520666f7220746865206d65726368616e742069 | e for the merchant i
| 7320746f2061736b20666f72206120706c656467 | s to ask for a pledg
| 6520696e206164646974696f6e0a746f20746865 | e in addition to the
| 206e6f726d616c207072696365206f6620746865 | normal price of the
| 2070726f647563742e2055706f6e206120737563 | product. Upon a suc
| 6365737366756c207472616e73616374696f6e20 | cessful transaction
| 74686520706c656467652069730a72657475726e | the pledge is return
| 656420746f2074686520637573746f6d65722062 | ed to the customer b
| 757420696e2063617365206f66206120646f7562 | ut in case of a doub
| 6c652d7370656e642061747461636b2069742077 | le-spend attack it w
| 696c6c20626520757365642075700a696e207468 | ill be used up in th
| 652070726f63657373206f66206f757462696464 | e process of outbidd
| 696e672074686520636f6d706574696e67205458 | ing the competing TX
| 732e0a0a4f6e6c696e6520636173696e6f732061 | s. Online casinos a
| 6e642063757272656e63792065786368616e6765 | nd currency exchange
| 7320636f756c64207574696c697a652074686973 | s could utilize this
| 207365637572697479207061747465726e206279 | security pattern by
| 0a616c6c6f77696e67206f6e6c79206120667261 | allowing only a fra
| 6374696f6e206f662074686520302d636f6e6669 | ction of the 0-confi
| 726d6174696f6e206465706f73697420746f2062 | rmation deposit to b
| 6520696d6d6564696174656c7920757365642075 | e immediately used u
| 700a62792074686520637573746f6d65722e2053 | p by the customer. S
| 686f756c642074686174206465706f736974206c | hould that deposit l
| 617465722067657420646f75626c652d7370656e | ater get double-spen
| 742074686520626164206163746f7220776f756c | t the bad actor woul
| 640a6c6f7365206d756368206d6f726520746861 | d lose much more tha
| 6e2077686174207468657920776572652061626c | n what they were abl
| 6520746f2075736520757020696e207468652063 | e to use up in the c
| 6f6e74657874206f66207468652070726f766964 | ontext of the provid
| 65640a736572766963652e204e61747572616c6c | ed service. Naturall
| 792c207468652073697a65206f66207468652070 | y, the size of the p
| 6c656467652063616e2062652063616c69627261 | ledge can be calibra
| 746564206279206561636820627573696e657373 | ted by each business
| 0a696e646976696475616c6c7920746f20666974 | individually to fit
| 20746865206e65656473206f6620616e79207269 | the needs of any ri
| 736b2070726f66696c652e0a0a0a0a2323232042 | sk profile. ### B
| 7573696e6573732042656e656669747320202020 | usiness Benefits
| 2020202020202020202020202323230a0a497420 | ### It
| 697320666169726c79207472697669616c20746f | is fairly trivial to
| 20736565207468617420696620646f75626c652d | see that if double-
| 7370656e64696e67206f6620302d636f6e666972 | spending of 0-confir
| 6d6174696f6e2054587320626563616d650a6563 | mation TXs became ec
| 6f6e6f6d6963616c6c7920756e70726f66697461 | onomically unprofita
| 626c65207468656e206d6f726520627573696e65 | ble then more busine
| 7373657320636f756c6420737461727420616363 | sses could start acc
| 657074696e6720756e636f6e6669726d65640a54 | epting unconfirmed T
| 5873206173207061796d656e74732e2054686973 | Xs as payments. This
| 20776f756c64206e61747572616c6c79206c6561 | would naturally lea
| 6420746f20677265617465722070726f66697473 | d to greater profits
| 20666f722074686520627573696e65737365730a | for the businesses
| 616e6420736d616c6c6572207072696365732066 | and smaller prices f
| 6f722074686520636f6e73756d65722e20416674 | or the consumer. Aft
| 657220616c6c2c2069742069732074686520686f | er all, it is the ho
| 6e65737420637573746f6d6572732077686f0a61 | nest customers who a
| 726520696e6469726563746c79206d6164652072 | re indirectly made r
| 6573706f6e7369626c6520666f7220636f6d7065 | esponsible for compe
| 6e736174696e672074686520726576656e756520 | nsating the revenue
| 6c6f737420696e2066726175642e0a0a0a0a2323 | lost in fraud. ##
| 232053756d6d6172792020202020202020202020 | # Summary
| 2020202020202020202020202020202323230a0a | ###
| 496e20746869732070617065722077652070726f | In this paper we pro
| 706f7365642061206e6f76656c20617070726f61 | posed a novel approa
| 636820746f206d697469676174696e6720746865 | ch to mitigating the
| 207269736b206f66206672617564756c656e740a | risk of fraudulent
| 302d636f6e6669726d6174696f6e20646f75626c | 0-confirmation doubl
| 652d7370656e64696e67207061796d656e747320 | e-spending payments
| 6f6e2074686520736f66742070726f746f636f6c | on the soft protocol
| 206c6576656c2e20576520737461727465640a77 | level. We started w
| 6974682074686520746563686e6963616c207072 | ith the technical pr
| 6572657175697369746573207375636820617320 | erequisites such as
| 646f75626c652d7370656e642072656c6179696e | double-spend relayin
| 6720616e6420666565206772656564792054580a | g and fee greedy TX
| 7072696f726974697a6174696f6e2e2057652074 | prioritization. We t
| 68656e20696e74726f6475636564207468652063 | hen introduced the c
| 6f6e63657074206f66206f757462696464696e67 | oncept of outbidding
| 20636f6d706574696e67205458732062790a696e | competing TXs by in
| 6372656173696e672074686520666565206f6620 | creasing the fee of
| 746865206f726967696e616c207061796d656e74 | the original payment
| 2e20546865206c617474657220697320706f7373 | . The latter is poss
| 69626c65206f6e6c792077697468207468650a68 | ible only with the h
| 656c70206f662061204368696c642d506179732d | elp of a Child-Pays-
| 666f722d506172656e742054582e2053696e6365 | for-Parent TX. Since
| 2073756368206120747269636b20636f756c6420 | such a trick could
| 626520726570656174656420756e74696c0a7468 | be repeated until th
| 652077686f6c65207061796d656e742069732073 | e whole payment is s
| 70656e74206f6e20545820666565732c20697420 | pent on TX fees, it
| 6973206561737920746f20736565207468617420 | is easy to see that
| 65636f6e6f6d6963616c6c792074686572650a69 | economically there i
| 73206e6f7468696e6720666f7220746865206174 | s nothing for the at
| 7461636b657220746f206761696e2066726f6d20 | tacker to gain from
| 646f75626c652d7370656e64696e672e20546861 | double-spending. Tha
| 7420736169642c20776520686176650a65666665 | t said, we have effe
| 63746976656c7920646973696e63656e74697669 | ctively disincentivi
| 7a6564206672617564756c656e7420646f75626c | zed fraudulent doubl
| 652d7370656e64696e67206f6620302d636f6e66 | e-spending of 0-conf
| 69726d6174696f6e205458732e0a0a0a0a232323 | irmation TXs. ###
| 2043616c6c20746f20416374696f6e2020202020 | Call to Action
| 20202020202020202020202020202323230a0a53 | ### S
| 696e6365207468657265206973206e6f20736369 | ince there is no sci
| 656e7469666963616c6c79207269676f726f7573 | entifically rigorous
| 2077617920746f20646574656374206120667261 | way to detect a fra
| 7564756c656e7420756e636f6e6669726d65640a | udulent unconfirmed
| 545820776974686f7574206b6e6f77696e672061 | TX without knowing a
| 6e797468696e672061626f75742074686520636f | nything about the co
| 6e74657874206f662074686520545820616c6c20 | ntext of the TX all
| 5458732073686f756c6420626520766965776564 | TXs should be viewed
| 0a617320657175616c6c792076616c696420616e | as equally valid an
| 642072656c617965642072657370656374697665 | d relayed respective
| 6c792e20556c74696d6174656c792c2069742077 | ly. Ultimately, it w
| 696c6c20626520746865206d696e657273207768 | ill be the miners wh
| 6f0a646563696465207768696368206f6e65206f | o decide which one o
| 662074686520636f6e666c696374696e67205458 | f the conflicting TX
| 73206765747320696e636c7564656420696e2074 | s gets included in t
| 6865206e65787420626c6f636b2e20546875732c | he next block. Thus,
| 0a616c6c2066756c6c206e6f6465732073686f75 | all full nodes shou
| 6c6420766965772074686520736574206f662075 | ld view the set of u
| 6e636f6e6669726d6564205458732066726f6d20 | nconfirmed TXs from
| 746865206d696e6572277320706f696e74206f66 | the miner's point of
| 0a766965772e2054686520617574686f7273206f | view. The authors o
| 66207468697320776f726b207572676520746865 | f this work urge the
| 2066756c6c206e6f6465206f70657261746f7273 | full node operators
| 20746f207374617274207365656b696e6720666f | to start seeking fo
| 720a726561736f6e61626c65207761797320746f | r reasonable ways to
| 2072656c6179696e6720646f75626c652d737065 | relaying double-spe
| 6e64696e67205458732e20417320666f72207468 | nding TXs. As for th
| 6520426974636f696e206d696e6572732c206974 | e Bitcoin miners, it
| 0a697320686967686c79207265636f6d6d656e64 | is highly recommend
| 656420746f206f7074696d697a65207468652054 | ed to optimize the T
| 58207072696f726974697a6174696f6e20616c67 | X prioritization alg
| 6f726974686d20666f7220746865206d6f73740a | orithm for the most
| 70726f66697461626c65206d6f64656c20706f73 | profitable model pos
| 7369626c652e0a0a0a0a23232320526566657265 | sible. ### Refere
| 6e63657320202020202020202020202020202020 | nces
| 202020202020202323230a0a5b5e315d3a205361 | ### [^1]: Sa
| 746f736869204e616b616d6f746f2c205b426974 | toshi Nakamoto, [Bit
| 636f696e3a204120506565722d746f2d50656572 | coin: A Peer-to-Peer
| 20456c656374726f6e6963204361736820537973 | Electronic Cash Sys
| 74656d5d5b315d2c20323030380a0a5b5e325d3a | tem][1], 2008 [^2]:
| 20426974636f696e20436f726520302e31332e30 | Bitcoin Core 0.13.0
| 2c205b4d6f726520696e74656c6c6967656e7420 | , [More intelligent
| 7472616e73616374696f6e2073656c656374696f | transaction selectio
| 6e20666f72206d696e696e675d5b325d2c203230 | n for mining][2], 20
| 31360a0a5b5e335d3a204d696b6520486561726e | 16 [^3]: Mike Hearn
| 2c205b446f75626c65207370656e64696e672069 | , [Double spending i
| 6e20426974636f696e5d5b335d2c20323031350a | n Bitcoin][3], 2015
| 0a5b5e345d3a20476c6173732048756e742c205b | [^4]: Glass Hunt, [
| 446f75626c65205370656e6420426974636f696e | Double Spend Bitcoin
| 5d5b345d0a0a5b5e355d3a20426974636f696e20 | ][4] [^5]: Bitcoin
| 58542c205b4e6f7461626c652066656174757265 | XT, [Notable feature
| 7320696e2058543a20446f75626c65207370656e | s in XT: Double spen
| 642072656c6179696e675d5b355d0a0a5b5e365d | d relaying][5] [^6]
| 3a20426c6f636b636861696e2e696e666f2c205b | : Blockchain.info, [
| 446f75626c65205370656e64735d5b365d0a0a5b | Double Spends][6] [
| 5e375d3a20426974636f696e20436f726520302e | ^7]: Bitcoin Core 0.
| 31322e302c205b4f70742d696e20524246204641 | 12.0, [Opt-in RBF FA
| 515d5b375d2c20323031360a0a5b5e385d3a2042 | Q][7], 2016 [^8]: B
| 6974636f696e20436c61737369632c205b446f75 | itcoin Classic, [Dou
| 626c65205370656e642050726f6f66735d5b385d | ble Spend Proofs][8]
| 0a0a5b5e395d3a20506574657220547363686970 | [^9]: Peter Tschip
| 7065722c205b425549503031303a20587472656d | per, [BUIP010: Xtrem
| 65205468696e626c6f636b735d5b395d2c203230 | e Thinblocks][9], 20
| 31360a0a5b5e31305d3a20446176696420412e20 | 16 [^10]: David A.
| 48617264696e672c20506574657220546f646420 | Harding, Peter Todd
| 5b4f70742d696e2046756c6c205265706c616365 | [Opt-in Full Replace
| 2d62792d466565205369676e616c696e675d5b31 | -by-Fee Signaling][1
| 305d2c20323031350a0a5b5e31315d3a204a6fc3 | 0], 2015 [^11]: Jo
| ab6c2056616c656e7a75656c61205b5365637265 | l Valenzuela [Secre
| 7420426974636f696e2054726f6c6c2041726d79 | t Bitcoin Troll Army
| 2050757368657320666f72205365675769742041 | Pushes for SegWit A
| 646f7074696f6e3a20456d696e2047756e205369 | doption: Emin Gun Si
| 7265725d5b31315d2c20323031370a0a5b5e3132 | rer][11], 2017 [^12
| 5d3a20426974636f696e20436f726520302e3132 | ]: Bitcoin Core 0.12
| 2e302c205b4d656d6f727920706f6f6c206c696d | .0, [Memory pool lim
| 6974696e675d5b31325d2c20323031360a0a5b5e | iting][12], 2016 [^
| 31335d3a204e75636c6561722041676520506561 | 13]: Nuclear Age Pea
| 636520466f756e646174696f6e2c205b4d757475 | ce Foundation, [Mutu
| 616c204173737572656420446573747275637469 | al Assured Destructi
| 6f6e5d5b31335d2c20323031370a0a5b5e31345d | on][13], 2017 [^14]
| 3a204a6f686e20426c6f636b652c205b41202862 | : John Blocke, [A (b
| 7269656620616e6420696e636f6d706c65746529 | rief and incomplete)
| 20686973746f7279206f662063656e736f727368 | history of censorsh
| 697020696e202f722f426974636f696e5d5b3134 | ip in /r/Bitcoin][14
| 5d2c20323031360a0a5b5e31355d3a206e6f7061 | ], 2016 [^15]: nopa
| 726137332c205b412050726163746963616c2047 | ra73, [A Practical G
| 7569646520546f204163636964656e74616c204c | uide To Accidental L
| 6f7720466565205472616e73616374696f6e735d | ow Fee Transactions]
| 5b31355d2c20323031370a0a5b5e31365d3a2042 | [15], 2017 [^16]: B
| 6974636f696e20436c61737369632c205b486f77 | itcoin Classic, [How
| 20646f2053656757697420616e6420466c657854 | do SegWit and FlexT
| 72616e7320636f6d706172653f5d5b31365d2c20 | rans compare?][16],
| 323031370a0a5b315d3a2068747470733a2f2f62 | 2017 [1]: https://b
| 6974636f696e2e636f6d2f626974636f696e2e70 | itcoin.com/bitcoin.p
| 64660a5b325d3a2068747470733a2f2f62697463 | df [2]: https://bitc
| 6f696e636f72652e6f72672f656e2f323031362f | oincore.org/en/2016/
| 30382f32332f72656c656173652d302e31332e30 | 08/23/release-0.13.0
| 2f236d6f72652d696e74656c6c6967656e742d74 | /#more-intelligent-t
| 72616e73616374696f6e2d73656c656374696f6e | ransaction-selection
| 2d666f722d6d696e696e670a5b335d3a20687474 | -for-mining [3]: htt
| 70733a2f2f6d656469756d2e636f6d2f406f6374 | ps://medium.com/@oct
| 736b79776172642f646f75626c652d7370656e64 | skyward/double-spend
| 696e672d696e2d626974636f696e2d6265306631 | ing-in-bitcoin-be0f1
| 643165383030380a5b345d3a2068747470733a2f | d1e8008 [4]: https:/
| 2f7777772e676c61737368756e742e636f2f6578 | /www.glasshunt.co/ex
| 706c6f6974732f626974636f696e2d646f75626c | ploits/bitcoin-doubl
| 652d7370656e642f0a5b355d3a2068747470733a | e-spend/ [5]: https:
| 2f2f626974636f696e78742e736f667477617265 | //bitcoinxt.software
| 2f706174636865732e68746d6c0a5b365d3a2068 | /patches.html [6]: h
| 747470733a2f2f626c6f636b636861696e2e696e | ttps://blockchain.in
| 666f2f646f75626c652d7370656e64730a5b375d | fo/double-spends [7]
| 3a2068747470733a2f2f626974636f696e636f72 | : https://bitcoincor
| 652e6f72672f656e2f6661712f6f7074696e5f72 | e.org/en/faq/optin_r
| 62662f0a5b385d3a2068747470733a2f2f626974 | bf/ [8]: https://bit
| 636f696e636c61737369632e636f6d2f64657665 | coinclassic.com/deve
| 6c2f446f75626c652532305370656e6425323050 | l/Double%20Spend%20P
| 726f6f66732e68746d6c0a5b395d3a2068747470 | roofs.html [9]: http
| 733a2f2f626974636f2e696e2f666f72756d2f74 | s://bitco.in/forum/t
| 6872656164732f627569703031302d7061737365 | hreads/buip010-passe
| 642d787472656d652d7468696e626c6f636b732e | d-xtreme-thinblocks.
| 3737342f0a5b31305d3a2068747470733a2f2f67 | 774/ [10]: https://g
| 69746875622e636f6d2f626974636f696e2f6269 | ithub.com/bitcoin/bi
| 70732f626c6f622f6d61737465722f6269702d30 | ps/blob/master/bip-0
| 3132352e6d6564696177696b690a5b31315d3a20 | 125.mediawiki [11]:
| 68747470733a2f2f636f696e74656c6567726170 | https://cointelegrap
| 682e636f6d2f6e6577732f7365637265742d6269 | h.com/news/secret-bi
| 74636f696e2d74726f6c6c2d61726d792d707573 | tcoin-troll-army-pus
| 6865732d666f722d7365677769742d61646f7074 | hes-for-segwit-adopt
| 696f6e2d656d696e2d67756e2d73697265720a5b | ion-emin-gun-sirer [
| 31325d3a2068747470733a2f2f626974636f696e | 12]: https://bitcoin
| 2e6f72672f656e2f72656c656173652f76302e31 | .org/en/release/v0.1
| 322e30236d656d6f72792d706f6f6c2d6c696d69 | 2.0#memory-pool-limi
| 74696e670a5b31335d3a20687474703a2f2f7777 | ting [13]: http://ww
| 772e6e75636c65617266696c65732e6f72672f6d | w.nuclearfiles.org/m
| 656e752f6b65792d6973737565732f6e75636c65 | enu/key-issues/nucle
| 61722d776561706f6e732f686973746f72792f63 | ar-weapons/history/c
| 6f6c642d7761722f73747261746567792f737472 | old-war/strategy/str
| 61746567792d6d757475616c2d61737375726564 | ategy-mutual-assured
| 2d6465737472756374696f6e2e68746d0a5b3134 | -destruction.htm [14
| 5d3a2068747470733a2f2f6d656469756d2e636f | ]: https://medium.co
| 6d2f406a6f686e626c6f636b652f612d62726965 | m/@johnblocke/a-brie
| 662d616e642d696e636f6d706c6574652d686973 | f-and-incomplete-his
| 746f72792d6f662d63656e736f72736869702d69 | tory-of-censorship-i
| 6e2d722d626974636f696e2d6338356132393066 | n-r-bitcoin-c85a290f
| 6534330a5b31355d3a2068747470733a2f2f6861 | e43 [15]: https://ha
| 636b65726e6f6f6e2e636f6d2f686f6c792d636f | ckernoon.com/holy-co
| 772d692d73656e742d612d626974636f696e2d74 | w-i-sent-a-bitcoin-t
| 72616e73616374696f6e2d776974682d746f6f2d | ransaction-with-too-
| 6c6f772d666565732d6172652d6d792d636f696e | low-fees-are-my-coin
| 732d6c6f73742d666f72657665722d3761383635 | s-lost-forever-7a865
| 653265343562610a5b31365d3a2068747470733a | e2e45ba [16]: https:
| 2f2f626974636f696e636c61737369632e636f6d | //bitcoinclassic.com
| 2f646576656c2f466c65785472616e732d76732d | /devel/FlexTrans-vs-
| 5365675769742e68746d6c0a5b31375d3a206874 | SegWit.html [17]: ht
| 74703a2f2f7777772e636f696e67616d696e672e | tp://www.coingaming.
| 417574686f72277320746970206a61723a203145 | Author's tip jar: 1E
| 72696368315955646b55417039796e6634526677 | rich1YUdkUAp9ynf4Rfw
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