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authorPieter Wuille <pieter.wuille@gmail.com>2015-02-01 11:00:39 -0400
committerbitcoindev <bitcoindev@gnusha.org>2015-02-01 15:00:46 +0000
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Re: [Bitcoin-development] var_int ambiguous serialization consequences
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+References: <D89EBAA3-ED78-4D9C-B693-FBBF27501938@bitsofproof.com>
+Date: Sun, 1 Feb 2015 11:00:39 -0400
+Message-ID: <CAPg+sBhmB9tZkOVdUstRYd4DAhNpFoirK-p7-yxVarbQzTqCkQ@mail.gmail.com>
+From: Pieter Wuille <pieter.wuille@gmail.com>
+To: Tamas Blummer <tamas@bitsofproof.com>
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+Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
+Subject: Re: [Bitcoin-development] var_int ambiguous serialization
+ consequences
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+--001a1141f186d60419050e08197e
+Content-Type: text/plain; charset=ISO-8859-1
+
+Hashes are always computed by reserializing data structures, never by
+hashing wire data directly. This has been the case in every version of the
+reference client's code that I know of.
+
+This even meant that for example a block of 999999 bytes with non-shortest
+length for the transaction count could be over the mazimum block size, but
+still be valid.
+
+As Wladimir says, more recently we switched to just failing to deserialize
+(by throwing an exception) whenever a non-shortest form is used.
+On Feb 1, 2015 1:34 AM, "Tamas Blummer" <tamas@bitsofproof.com> wrote:
+
+> I wonder of consequences if var_int is used in its longer than necessary
+> forms (e.g encoding 1 as 0xfd0100 instead of 0x01)
+>
+> This is already of interest if applying size limit to a block, since
+> transaction count is var_int but is not part of the hashed header or the
+> merkle tree.
+>
+> It could also be used to create variants of the same transaction message
+> by altered representation of txIn and txout counts, that would remain valid
+> provided signatures validate with the shortest form, as that is created
+> while re-serializing for signature hashing. An implementation that holds
+> mempool by raw message hashes could be tricked to believe that a modified
+> encoded version of the same transaction is a real double spend. One could
+> also mine a valid block with transactions that have a different hash if
+> regularly parsed and re-serialized. An SPV client could be confused by such
+> a transaction as it was present in the merkle tree proof with a different
+> hash than it gets for the tx with its own serialization or from the raw
+> message.
+>
+> Tamas Blummer
+> Bits of Proof
+>
+>
+>
+> ------------------------------------------------------------------------------
+> Dive into the World of Parallel Programming. The Go Parallel Website,
+> sponsored by Intel and developed in partnership with Slashdot Media, is
+> your
+> hub for all things parallel software development, from weekly thought
+> leadership blogs to news, videos, case studies, tutorials and more. Take a
+> look and join the conversation now. http://goparallel.sourceforge.net/
+> _______________________________________________
+> Bitcoin-development mailing list
+> Bitcoin-development@lists.sourceforge.net
+> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
+>
+>
+
+--001a1141f186d60419050e08197e
+Content-Type: text/html; charset=ISO-8859-1
+Content-Transfer-Encoding: quoted-printable
+
+<p dir=3D"ltr">Hashes are always computed by reserializing data structures,=
+ never by hashing wire data directly. This has been the case in every versi=
+on of the reference client&#39;s code that I know of.</p>
+<p dir=3D"ltr">This even meant that for example a block of 999999 bytes wit=
+h non-shortest length for the transaction count could be over the mazimum b=
+lock size, but still be valid.</p>
+<p dir=3D"ltr">As Wladimir says, more recently we switched to just failing =
+to deserialize (by throwing an exception) whenever a non-shortest form is u=
+sed.</p>
+<div class=3D"gmail_quote">On Feb 1, 2015 1:34 AM, &quot;Tamas Blummer&quot=
+; &lt;<a href=3D"mailto:tamas@bitsofproof.com">tamas@bitsofproof.com</a>&gt=
+; wrote:<br type=3D"attribution"><blockquote class=3D"gmail_quote" style=3D=
+"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style=
+=3D"word-wrap:break-word"><div>I wonder of consequences if var_int is used =
+in its longer than necessary forms (e.g encoding 1 as 0xfd0100 instead of 0=
+x01)</div><div><br></div><div>This is already of interest if applying size =
+limit to a block, since transaction count is var_int but is not part of the=
+ hashed header or the merkle tree.</div><div><br></div><div>It could also b=
+e used to create variants of the same transaction message by altered repres=
+entation of txIn and txout counts, that would remain valid provided signatu=
+res validate with the shortest form, as that is created while re-serializin=
+g for signature hashing. An implementation that holds mempool by raw messag=
+e hashes could be tricked to believe that a modified encoded version of the=
+ same transaction is a real double spend. One could also mine a valid block=
+ with transactions that have a different hash if regularly parsed and re-se=
+rialized. An SPV client could be confused by such a transaction as it was p=
+resent in the merkle tree proof with a different hash than it gets for the =
+tx with its own serialization or from the raw message.</div><br><div>
+<div style=3D"color:rgb(0,0,0);font-family:Helvetica;font-size:12px;font-st=
+yle:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;lin=
+e-height:normal;text-align:start;text-indent:0px;text-transform:none;white-=
+space:normal;word-spacing:0px">Tamas Blummer</div><div style=3D"color:rgb(0=
+,0,0);font-family:Helvetica;font-size:12px;font-style:normal;font-variant:n=
+ormal;font-weight:normal;letter-spacing:normal;line-height:normal;text-alig=
+n:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing=
+:0px">Bits of Proof</div><div style=3D"color:rgb(0,0,0);font-family:Helveti=
+ca;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;=
+letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;t=
+ext-transform:none;white-space:normal;word-spacing:0px"><br></div></div></d=
+iv><br>--------------------------------------------------------------------=
+----------<br>
+Dive into the World of Parallel Programming. The Go Parallel Website,<br>
+sponsored by Intel and developed in partnership with Slashdot Media, is you=
+r<br>
+hub for all things parallel software development, from weekly thought<br>
+leadership blogs to news, videos, case studies, tutorials and more. Take a<=
+br>
+look and join the conversation now. <a href=3D"http://goparallel.sourceforg=
+e.net/" target=3D"_blank">http://goparallel.sourceforge.net/</a><br>_______=
+________________________________________<br>
+Bitcoin-development mailing list<br>
+<a href=3D"mailto:Bitcoin-development@lists.sourceforge.net">Bitcoin-develo=
+pment@lists.sourceforge.net</a><br>
+<a href=3D"https://lists.sourceforge.net/lists/listinfo/bitcoin-development=
+" target=3D"_blank">https://lists.sourceforge.net/lists/listinfo/bitcoin-de=
+velopment</a><br>
+<br></blockquote></div>
+
+--001a1141f186d60419050e08197e--
+
+