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From: Mike Hearn <mike@plan99.net>
To: Jeremy Spilman <jeremy@taplink.co>
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Optional "wallet-linkable" address format
 - Payment Protocol
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Agree with Jeremy and once the payment protocol work is further along I'd
like to see us define an extension that lets you send payment requests
containing public keys+chain codes, so further payments can be made
push-style with no recipient interaction (e.g. for repeated billing). How
apps choose to arrange their chains internally seems like an area for
experimentation. I definitely want to implement HD wallets in bitcoinj to
allow this and if that means not using the same tree structure as in the
BIP then so be it.


On Thu, Jun 20, 2013 at 5:54 AM, Jeremy Spilman <jeremy@taplink.co> wrote:

> > BIP 32 already specifies how to use the first three tree levels:
>  M/i/j/k,
> > i~wallet, j~Internal/External, k~address.  The first level is actually
> > type-1 derived, and thus we cannot create an arbitrary number of them
> > without pre-computing them from the offline wallet.  So it's not "free"
> to
> > create new wallets unless we redefine how the levels work.
>
> Initially I was thinking that you would share the public key and chain co=
de
> from [m/i'/0] so that you can receive payments at [m/i'/0/k], for a uniqu=
e
> value of 'i' for each receive chain.
>
> For the case of generating new receive chains from a *watch-only* wallet,
> as
> you say, the options are to either keep a cache of PubKey/ChainCode for
> unused [m/i'] or simply increment 'j' past 1 for an existing [m/i'/j] --
> the
> concept of 'internal/'external' and change addresses at Depth=3D2 don't m=
ake
> sense for handing out receive chains to lots of people anyway, and
> certainly
> BIP32 doesn't *require* 0 <=3D j <=3D 1.  So I think incrementing 'j' is =
the
> way
> to go here...
>
> The "default" layout of BIP32 does NOT mean that implementations should n=
ot
> check for transactions with j > 1. That would be a useless constraint and
> obviously self-limiting. It might be helpful to add to the 'Compatibility=
'
> section some minimum expectations about how a wallet should be 'probed'
> when
> imported. If you don't feel completely free to monotonically increment 'j=
'
> to your hearts content to achieve major usability benefits, then I say
> BIP32
> could use some clarifying.
>
> BTW - the spec calls for addition not multiplication now, so we should ca=
ll
> it the 'Addend' not the 'Multiplier' :-)
>
> > Do these extra wallet chains behave as different wallets, or sub-wallet=
s?
>
> They could, but they certainly don't need to!  A single-wallet
> implementation treats this merely as an address-generation algorithm, and
> does not expose any hierarchy to the user interface.  The user just
> =E2=80=9Cmagically=E2=80=9D gets the ability to send multiple payments to=
 their contacts
> without immediately sacrificing their privacy
> (http://www.wired.com/wiredenterprise/2013/06/bitcoin_retai/). Everything
> goes into the same ledger, balance, coin pool, etc. Most of the code base
> is
> unaware BIP32 is even in use.
>
> While it is *possible* to support separate ledgers, balances, etc. it is
> certainly not required, and you get all the benefits either way.
>
> I think, since your proposal generates and receives payments into
> BIP32-style addresses, we both need similar underlying wallet code. The
> only
> difference is that you are passing the Kpar for [m/i'/0/k] and the *resul=
t*
> of CKD'((Kpar, cpar), k), and instead I proposed passing Kpar and cpar, a=
nd
> leaving 'k' out of it, letting the receive choose 'k'.
>
> > For instance, maybe there's a benefit to using the same parent pubkey
> > across multiple services, as a form of identity.   If I don't want that=
,
> I
> > use your method.  If I do want that, I use my method.
>
> I think it's a interesting idea using static public keys as a means for
> persistent identity and hence security from MitM. If you want a shared
> public key across multiple services we could just combine both ideas and
> get
> all the benefits, by making the data structure { ParentPubKey, Addend,
> ChainCode }:
>
>    ParentPubKey: Public key of m/i' -- 33 bytes
>    Addend: I[L]*G from CDK'(m/i', j) -- 33 bytes
>    ChainCode: I[R] from CDK'(m/i', j) -- 32 bytes
>
> All that remains secret is the ChainCode from [m/i'] -- and of course the
> private keys.  The ParentPubKey is a common value across multiple service=
s,
> corresponding to user's identity rooted in [m/i'].  Each service gets the=
ir
> own 'j'.  ParentPubKey + Addend gives you the PubKey of [m/i'/j].  With t=
he
> ChainCode, the receiver then can generate [m/i'/j/k] for monotonically
> increasing 'k'. Again, from the user perspective all transactions under
> [m/i'] can be presented in a single ledger, or not.
>
> Anyway, fundamentally my feedback is if you are designing for persistent
> long-term relationships, you could build in a mechanism for generating
> address chains so you don't need any further communication after the
> initial
> exchange, and it need not complicate the wallet.
>
> Thanks,
> --Jeremy
>
>
>
>
> -------------------------------------------------------------------------=
-----
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>

--001a11c30992712a9804df90f274
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<div dir=3D"ltr">Agree with Jeremy and once the payment protocol work is fu=
rther along I&#39;d like to see us define an extension that lets you send p=
ayment requests containing public keys+chain codes, so further payments can=
 be made push-style with no recipient interaction (e.g. for repeated billin=
g). How apps choose to arrange their chains internally seems like an area f=
or experimentation. I definitely want to implement HD wallets in bitcoinj t=
o allow this and if that means not using the same tree structure as in the =
BIP then so be it.</div>
<div class=3D"gmail_extra"><br><br><div class=3D"gmail_quote">On Thu, Jun 2=
0, 2013 at 5:54 AM, Jeremy Spilman <span dir=3D"ltr">&lt;<a href=3D"mailto:=
jeremy@taplink.co" target=3D"_blank">jeremy@taplink.co</a>&gt;</span> wrote=
:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-le=
ft:1px #ccc solid;padding-left:1ex">
<div class=3D"im">&gt; BIP 32 already specifies how to use the first three =
tree levels: =C2=A0M/i/j/k,<br>
&gt; i~wallet, j~Internal/External, k~address. =C2=A0The first level is act=
ually<br>
&gt; type-1 derived, and thus we cannot create an arbitrary number of them<=
br>
&gt; without pre-computing them from the offline wallet. =C2=A0So it&#39;s =
not &quot;free&quot; to<br>
&gt; create new wallets unless we redefine how the levels work.<br>
<br>
</div>Initially I was thinking that you would share the public key and chai=
n code<br>
from [m/i&#39;/0] so that you can receive payments at [m/i&#39;/0/k], for a=
 unique<br>
value of &#39;i&#39; for each receive chain.<br>
<br>
For the case of generating new receive chains from a *watch-only* wallet, a=
s<br>
you say, the options are to either keep a cache of PubKey/ChainCode for<br>
unused [m/i&#39;] or simply increment &#39;j&#39; past 1 for an existing [m=
/i&#39;/j] -- the<br>
concept of &#39;internal/&#39;external&#39; and change addresses at Depth=
=3D2 don&#39;t make<br>
sense for handing out receive chains to lots of people anyway, and certainl=
y<br>
BIP32 doesn&#39;t *require* 0 &lt;=3D j &lt;=3D 1. =C2=A0So I think increme=
nting &#39;j&#39; is the way<br>
to go here...<br>
<br>
The &quot;default&quot; layout of BIP32 does NOT mean that implementations =
should not<br>
check for transactions with j &gt; 1. That would be a useless constraint an=
d<br>
obviously self-limiting. It might be helpful to add to the &#39;Compatibili=
ty&#39;<br>
section some minimum expectations about how a wallet should be &#39;probed&=
#39; when<br>
imported. If you don&#39;t feel completely free to monotonically increment =
&#39;j&#39;<br>
to your hearts content to achieve major usability benefits, then I say BIP3=
2<br>
could use some clarifying.<br>
<br>
BTW - the spec calls for addition not multiplication now, so we should call=
<br>
it the &#39;Addend&#39; not the &#39;Multiplier&#39; :-)<br>
<div class=3D"im"><br>
&gt; Do these extra wallet chains behave as different wallets, or sub-walle=
ts?<br>
<br>
</div>They could, but they certainly don&#39;t need to! =C2=A0A single-wall=
et<br>
implementation treats this merely as an address-generation algorithm, and<b=
r>
does not expose any hierarchy to the user interface. =C2=A0The user just<br=
>
=E2=80=9Cmagically=E2=80=9D gets the ability to send multiple payments to t=
heir contacts<br>
without immediately sacrificing their privacy<br>
(<a href=3D"http://www.wired.com/wiredenterprise/2013/06/bitcoin_retai/" ta=
rget=3D"_blank">http://www.wired.com/wiredenterprise/2013/06/bitcoin_retai/=
</a>). Everything<br>
goes into the same ledger, balance, coin pool, etc. Most of the code base i=
s<br>
unaware BIP32 is even in use.<br>
<br>
While it is *possible* to support separate ledgers, balances, etc. it is<br=
>
certainly not required, and you get all the benefits either way.<br>
<br>
I think, since your proposal generates and receives payments into<br>
BIP32-style addresses, we both need similar underlying wallet code. The onl=
y<br>
difference is that you are passing the Kpar for [m/i&#39;/0/k] and the *res=
ult*<br>
of CKD&#39;((Kpar, cpar), k), and instead I proposed passing Kpar and cpar,=
 and<br>
leaving &#39;k&#39; out of it, letting the receive choose &#39;k&#39;.<br>
<div class=3D"im"><br>
&gt; For instance, maybe there&#39;s a benefit to using the same parent pub=
key<br>
</div>&gt; across multiple services, as a form of identity. =C2=A0 If I don=
&#39;t want that, I<br>
<div class=3D"im">&gt; use your method. =C2=A0If I do want that, I use my m=
ethod.<br>
<br>
</div>I think it&#39;s a interesting idea using static public keys as a mea=
ns for<br>
persistent identity and hence security from MitM. If you want a shared<br>
public key across multiple services we could just combine both ideas and ge=
t<br>
all the benefits, by making the data structure { ParentPubKey, Addend,<br>
ChainCode }:<br>
<br>
=C2=A0 =C2=A0ParentPubKey: Public key of m/i&#39; -- 33 bytes<br>
=C2=A0 =C2=A0Addend: I[L]*G from CDK&#39;(m/i&#39;, j) -- 33 bytes<br>
=C2=A0 =C2=A0ChainCode: I[R] from CDK&#39;(m/i&#39;, j) -- 32 bytes<br>
<br>
All that remains secret is the ChainCode from [m/i&#39;] -- and of course t=
he<br>
private keys. =C2=A0The ParentPubKey is a common value across multiple serv=
ices,<br>
corresponding to user&#39;s identity rooted in [m/i&#39;]. =C2=A0Each servi=
ce gets their<br>
own &#39;j&#39;. =C2=A0ParentPubKey + Addend gives you the PubKey of [m/i&#=
39;/j]. =C2=A0With the<br>
ChainCode, the receiver then can generate [m/i&#39;/j/k] for monotonically<=
br>
increasing &#39;k&#39;. Again, from the user perspective all transactions u=
nder<br>
[m/i&#39;] can be presented in a single ledger, or not.<br>
<br>
Anyway, fundamentally my feedback is if you are designing for persistent<br=
>
long-term relationships, you could build in a mechanism for generating<br>
address chains so you don&#39;t need any further communication after the in=
itial<br>
exchange, and it need not complicate the wallet.<br>
<div class=3D"HOEnZb"><div class=3D"h5"><br>
Thanks,<br>
--Jeremy<br>
<br>
<br>
<br>
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</div></div></blockquote></div><br></div>

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