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Date: Sat, 25 Apr 2015 11:40:37 -0400
Message-ID: <CABHVRKS0EYV0CqKW1MVtUZC3u4KvSxMB=Uks9UrCUBQbozO9xQ@mail.gmail.com>
From: Stephen Morse <stephencalebmorse@gmail.com>
To: Gregory Maxwell <gmaxwell@gmail.com>
Content-Type: multipart/alternative; boundary=f46d0444812b92eb3805148e5573
Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] 75%/95% threshold for transaction versions
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Hi Gregory,

In particular not covering the ID allows for transaction replay which
> can result in monetary losses far more severe than any possible
> mishandling of malleability could result in. Byzantine attackers can
> costlessly replay your old transactions any time anyone reuses an
> address, even accidentally (which cannot be easily prevented since
> they can race).
>

With the SIGHASH_WITHOUT_PREV_VALUE flag, signatures have to explicitly
specify that they are to be signed without the previous UTXO's
value/amount. This means that, at worst, replay attacks can send the money
to the same place it was sent before (which in many cases is likely not be
a loss of funds), and only if the amount sent to the reused address is the
exact same as it was before. I don't think this is worse than an attacker
being able to mutate their transaction and extort a merchant who accepts
zero-conf transactions. Anyway, not signing the input ID wouldn't exactly
be the norm, there would be a defined set of flags for standard use cases.
Not signing the input TXID would only be used in specialized cases, such as
setting up micropayment channels.


> There are no free lunches;  the proposal linked to there is itself a
> game of wack-a-mole with assorted masking flags;


I agree that it is also a bit of wac-a-mole, but the defined space of
issues is possibly more limited here. There are only X number of things
that can be signed/not signed in a transaction, and the 'Build your own
nHashType' proposal enables you to fully specify which of those are being
signed. If you don't want to get burned by not fully signing your
transactions, then don't use the non-standard sighash flags.

many of which we have
> no notion of if they're useful for any particular application(s);


A few of the flags, indeed, may not ever be useful. But we can't predict
the future, and I think it's better to build in a more flexible solution
now than to wish we had more flexible nHashTypes later.

To the original point of this thread, hopefully the suggested proposal
won't be necessary as wallets will upgrade to use version 3 transactions
and the rules associated with them over time.

Best,
Stephen

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<div dir=3D"ltr">Hi Gregory,<br><div class=3D"gmail_extra"><br><div class=
=3D"gmail_quote"><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px =
0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-l=
eft-style:solid;padding-left:1ex">In particular not covering the ID allows =
for transaction replay which<br>
can result in monetary losses far more severe than any possible<br>
mishandling of malleability could result in. Byzantine attackers can<br>
costlessly replay your old transactions any time anyone reuses an<br>
address, even accidentally (which cannot be easily prevented since<br>
they can race).<br></blockquote><div><br></div>With the SIGHASH_WITHOUT_PRE=
V_VALUE flag, signatures have to explicitly specify that they are to be sig=
ned without the previous UTXO&#39;s value/amount. This means that, at worst=
, replay attacks can send the money to the same place it was sent before (w=
hich in many cases is likely not be a loss of funds), and only if the amoun=
t sent to the reused address is the exact same as it was before. I don&#39;=
t think this is worse than an attacker being able to mutate their transacti=
on and extort a merchant who accepts zero-conf transactions. Anyway, not si=
gning the input ID wouldn&#39;t exactly be the norm, there would be a defin=
ed set of flags for standard use cases. Not signing the input TXID would on=
ly be used in specialized cases, such as setting up micropayment channels.=
=C2=A0<div>=C2=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0p=
x 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);bo=
rder-left-style:solid;padding-left:1ex">There are no free lunches;=C2=A0 th=
e proposal linked to there is itself a<br>
game of wack-a-mole with assorted masking flags; </blockquote><div><br></di=
v><div>I agree that it is also a bit of wac-a-mole, but the defined space o=
f issues is possibly more limited here. There are only X number of things t=
hat can be signed/not signed in a transaction, and the &#39;Build your own =
nHashType&#39; proposal enables you to fully specify which of those are bei=
ng signed. If you don&#39;t want to get burned by not fully signing your tr=
ansactions, then don&#39;t use the non-standard sighash flags.</div><div><b=
r></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex=
;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style=
:solid;padding-left:1ex">many of which we have<br>
no notion of if they&#39;re useful for any particular application(s); </blo=
ckquote><div><br></div><div>A few of the flags, indeed, may not ever be use=
ful. But we can&#39;t predict the future, and I think it&#39;s better to bu=
ild in a more flexible solution now than to wish we had more flexible nHash=
Types later.</div><div><br></div><div>To the original point of this thread,=
 hopefully the suggested proposal won&#39;t be necessary as wallets will up=
grade to use version 3 transactions and the rules associated with them over=
 time.=C2=A0</div><div><br></div><div>Best,</div><div>Stephen</div></div></=
div></div>

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