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From: Riccardo Casatta <riccardo.casatta@gmail.com>
Date: Mon, 27 Apr 2020 22:11:43 +0200
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To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: [bitcoin-dev] PSBT in QR codes
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Hi all,

there is some discussion happening [1] about how to encode a PSBT in QR
codes.

According to the specification (page 15 [2]) a version 40 QR code could
contain up to 3706 bytes of data, however practical limitation are much
lower and a PSBT could grow bigger anyway. so the issue is that a PSBT does
not fit in 1 QR code.

There are proposals suggesting animated QR codes but I don't think it's a
good idea for the following reasons:
* they are not easy to print
* it's not clear, by a human look, how much data it's being transferred,
thus allowing more space for attacks
* old hardware may have resource constraint and not being able to scan

There are proposals suggesting alphanumeric mode for QR codes and a header
(like message 1 of n) to allow data reconstruction. Main argument for this
choices are:
* use of built-in standard scanner
* data is copypasteable
* not a big loose in efficiency comparing to binary with a proper encoding
* industrial QR code scanner put a \r at the end of transmission (making
binary mode difficult to handle with timeouts or similar)

I don't think alphanumeric with custom headers it's a good idea and I think
we should use binary encoding and using the already available mode in QR
code specification called "structured append" (page 55 [2]). Corresponding
counter-points are:
* since data need to be reconstructed, I would avoid built-in scanner and
manual appending of strings anyway.
* we can keep the already used base64 for copypaste
* the best of the encoding we already have, bech32, is 10% less efficient
than binary and if we want to be more efficient we need to introduce a new
specific encoding
* I don't have a strong counter-point on industrial scanner, however if
they use \r to signal end of transmission they don't support well binary at
all, why they don't send how many bytes they read?

There are some doubts about support of structured append in QR code
libraries which is not widely supported. While this is true I verified the
widely diffused zxing library on Android and Luca Vaccaro verified the
Apple built-in scanner, and both this libraries let's you access to the
scanned raw bytes, allowing to parse the structured append header.
For reference, structured append allows to chain up to 16 qr codes, and
contains 1 byte of parity.

[1] https://github.com/cryptoadvance/specter-diy/issues/57
[2]
https://www.swisseduc.ch/informatik/theoretische_informatik/qr_codes/docs/qr_standard.pdf


--
Riccardo Casatta - @RCasatta

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Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr">Hi all,<br><br>there is some discussion happening [1] abou=
t how to encode a PSBT in QR codes.<br><br>According to the specification (=
page 15 [2]) a version 40 QR code could contain up to 3706 bytes of data, h=
owever practical limitation are much lower and a PSBT could grow bigger any=
way. so the issue is that a PSBT does not fit in 1 QR code.<br><br>There ar=
e proposals suggesting animated QR codes but I don&#39;t think it&#39;s a g=
ood idea for the following reasons: <br>* they are not easy to print<br>* i=
t&#39;s not clear, by a human look, how much data it&#39;s being transferre=
d, thus allowing more space for attacks<br>* old hardware may have resource=
 constraint and not being able to scan<br><br>There are proposals suggestin=
g alphanumeric mode for QR codes and a header<span class=3D"gmail_default" =
style=3D"font-size:small"> (like message 1 of n)</span> to allow data recon=
struction. Main argument for this choices are:<div><span class=3D"gmail_def=
ault">* </span>use of <span class=3D"gmail_default">built-in </span>standar=
d scanner<br>* data is copypasteable<div>* not a big loose in efficiency co=
mparing to binary with a proper encoding<br>* industrial <span class=3D"gma=
il_default" style=3D"font-size:small">QR</span>=C2=A0code scanner put a \r =
at the end of transmission (making binary mode difficult<span class=3D"gmai=
l_default" style=3D"font-size:small"> to handle with timeouts or similar</s=
pan>)</div><div><br>I don&#39;t think<span class=3D"gmail_default" style=3D=
"font-size:small"> alphanumeric with custom headers</span> it&#39;s a good =
idea and I think we should use binary encoding and using the already availa=
ble mode in QR code specification called <span class=3D"gmail_default" styl=
e=3D"font-size:small">&quot;</span>structured append<span class=3D"gmail_de=
fault" style=3D"font-size:small">&quot;</span> (page 55 [2])<span class=3D"=
gmail_default" style=3D"font-size:small">. Corresponding counter-points are=
:</span><div><span class=3D"gmail_default" style=3D"font-size:small"></span=
></div><div><span class=3D"gmail_default" style=3D"font-size:small">* since=
 data need to be=C2=A0reconstructed, I would avoid </span><span class=3D"gm=
ail_default" style=3D"font-size:small">built-in scanner and manual appendin=
g of strings anyway.</span></div><div>* <span class=3D"gmail_default" style=
=3D"font-size:small">we can keep the already used base64 for copypaste</spa=
n></div><div><span class=3D"gmail_default" style=3D"font-size:small">* the =
best of the encoding we already have, bech32, is 10% less efficient than bi=
nary and if we want to be more efficient we need to introduce a new specifi=
c encoding</span></div><div><div class=3D"gmail_default" style=3D"font-size=
:small">* I don&#39;t have a strong counter-point on industrial scanner, ho=
wever if they use \r to signal end of transmission they don&#39;t support w=
ell binary at all, why they don&#39;t send how many bytes they read?</div><=
br><div><div class=3D"gmail_default" style=3D"font-size:small">There are so=
me doubts about support of structured append in QR code libraries which is =
not widely supported. While this is true I verified the widely diffused zxi=
ng library on Android and Luca Vaccaro verified the Apple built-in scanner,=
 and both this libraries=C2=A0let&#39;s you access to the scanned raw bytes=
, allowing to parse the structured append header.</div><div class=3D"gmail_=
default" style=3D"font-size:small">For reference, structured append allows =
to chain up to 16 qr codes, and contains 1 byte of parity.</div><br>[1] <a =
href=3D"https://github.com/cryptoadvance/specter-diy/issues/57">https://git=
hub.com/cryptoadvance/specter-diy/issues/57</a><br>[2]<span class=3D"gmail_=
default" style=3D"font-size:small"> </span><a href=3D"https://www.swisseduc=
.ch/informatik/theoretische_informatik/qr_codes/docs/qr_standard.pdf">https=
://www.swisseduc.ch/informatik/theoretische_informatik/qr_codes/docs/qr_sta=
ndard.pdf</a><br><br><br>--<br>Riccardo Casatta - @RCasatta<br></div></div>=
</div></div></div>

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