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In-Reply-To: <20190628094937.u5zisdhdyfc6g5ts@erisian.com.au>
From: "Russell O'Connor" <roconnor@blockstream.io>
Date: Fri, 28 Jun 2019 07:16:46 -0400
Message-ID: <CAMZUoK==mXiJQjcJO+KgxtCSf9qBan4tgsGzfPKLjP4CHcZ_iA@mail.gmail.com>
To: Anthony Towns <aj@erisian.com.au>
Content-Type: multipart/alternative; boundary="000000000000a5a0c7058c606a42"
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Taproot proposal
Precedence: list

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Hmm? If I'm following what you mean, that's not the P2P rules, it's the
> Unserialize code, in particular:
>
>   compat/assumptions.h:52:static_assert(sizeof(int) == 4, "32-bit int
> assumed");
>
>   serialize.h:289:uint64_t ReadCompactSize(Stream& is)
>
>   serialize.h-679-template<typename Stream, unsigned int N, typename T,
> typename V>
>   serialize.h-680-void Unserialize_impl(Stream& is, prevector<N, T>& v,
> const V&)
>   serialize.h-681-{
>   serialize.h-682-    v.clear();
>   serialize.h:683:    unsigned int nSize = ReadCompactSize(is);
>
>   (and other Unserialize_impl implementations)
>
> However, ReadCompactSize throws "size too large" if the return value is
> greater than MAX_SIZE == 0x02000000 =~ 33.5M, which prior to the implicit
> cast to 32 bits in Unserialize_impl. And it looks like that check's been
> there since Satoshi...
>
> So as far as I can see, that encoding's just unsupported/invalid, rather
> than equivalent/non-canonical?
>

Thanks for this correction!  I totally missed that MAX_SIZE == 0x02000000.
I think I mistook it for SIZE_MAX when reviewing this, or just didn't
notice it at all.

Cheers,
> aj
>
>

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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote">=
<blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-=
left:1px solid rgb(204,204,204);padding-left:1ex">
Hmm? If I&#39;m following what you mean, that&#39;s not the P2P rules, it&#=
39;s the<br>
Unserialize code, in particular:<br>
<br>
=C2=A0 compat/assumptions.h:52:static_assert(sizeof(int) =3D=3D 4, &quot;32=
-bit int assumed&quot;);<br>
<br>
=C2=A0 serialize.h:289:uint64_t ReadCompactSize(Stream&amp; is)<br>
<br>
=C2=A0 serialize.h-679-template&lt;typename Stream, unsigned int N, typenam=
e T, typename V&gt;<br>
=C2=A0 serialize.h-680-void Unserialize_impl(Stream&amp; is, prevector&lt;N=
, T&gt;&amp; v, const V&amp;)<br>
=C2=A0 serialize.h-681-{<br>
=C2=A0 serialize.h-682-=C2=A0 =C2=A0 v.clear();<br>
=C2=A0 serialize.h:683:=C2=A0 =C2=A0 unsigned int nSize =3D ReadCompactSize=
(is);<br>
<br>
=C2=A0 (and other Unserialize_impl implementations)<br>
<br>
However, ReadCompactSize throws &quot;size too large&quot; if the return va=
lue is<br>
greater than MAX_SIZE =3D=3D 0x02000000 =3D~ 33.5M, which prior to the impl=
icit<br>
cast to 32 bits in Unserialize_impl. And it looks like that check&#39;s bee=
n<br>
there since Satoshi...<br>
<br>
So as far as I can see, that encoding&#39;s just unsupported/invalid, rathe=
r<br>
than equivalent/non-canonical?<br></blockquote><br></div><div class=3D"gmai=
l_quote">Thanks for this correction!=C2=A0 I totally missed that MAX_SIZE =
=3D=3D 0x02000000.=C2=A0 I think I mistook it for SIZE_MAX when reviewing t=
his, or just didn&#39;t notice it at all.</div><div class=3D"gmail_quote"><=
br><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;bord=
er-left:1px solid rgb(204,204,204);padding-left:1ex">
Cheers,<br>
aj<br>
<br>
</blockquote></div></div>

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