Received-SPF: pass (sog-mx-4.v43.ch3.sourceforge.com: domain of gmail.com
	designates 209.85.220.47 as permitted sender)
	client-ip=209.85.220.47; envelope-from=elombrozo@gmail.com;
	helo=mail-pa0-f47.google.com; 
From: Eric Lombrozo <elombrozo@gmail.com>
Content-Type: multipart/signed;
	boundary="Apple-Mail=_449E2016-C8B9-49D1-B3D2-D63D90C89667";
	protocol="application/pgp-signature"; micalg=pgp-sha1
Message-Id: <B837A283-4B5B-4753-A4A2-1AA27F1A2175@gmail.com>
Date: Sat, 1 Mar 2014 04:29:48 -0800
To: bitcoin-development@lists.sourceforge.net
Mime-Version: 1.0 (Mac OS X Mail 6.6 \(1510\))
Subject: [Bitcoin-development] Making the H in HD keychains useful
Precedence: list


--Apple-Mail=_449E2016-C8B9-49D1-B3D2-D63D90C89667
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain;
	charset=us-ascii

I've been trying to find ways to make HD keychain wallets (BIP0032) =
really usable from an application development perspective. I think we =
all know a number of solid use cases and possible applications for the D =
in HD, but nobody seems to have really found a way to make use of the H =
in a way that is actually manageable from a usability standpoint.

After pondering it a bit more, I think I've stumbled upon at least a =
couple issues that seem to give hints as to how we can change this.

Hierarchical organizations do not generally tend to be designed up =
front, cast in stone. In the real world, hierarchies tend to evolve =
organically, growing new branches as entities differentiate themselves =
to different purposes. Organizations grow over time. Sometimes branches =
merge, sometimes branches die. This means that for HD keychains to be =
truly useful, they too need to be sufficiently flexible to adapt to the =
needs of a growing and evolving organization. It needs to be simple to =
create and move branches around as the need for them arises without =
having to plan the structure a priori.

A significant problem I'm runnign into in trying to build applications =
around the BIP0032 standard is the lack of a clear separation between =
signing keys and hierarchical nodes. That's to say, a child of a node =
can either be used as a signing key or as a parent for new branches to =
the tree. =46rom a usability standpoint, what this means is that one =
must be very careful in how one allocates keys from the very beginning - =
if one mixes signing keys with new branching nodes in the same =
generation, the whole thing becomes a horrendous mess. Moreover, it is =
impossible to generally distinguish these two fundamentally different =
types of objects (at least from a use model perspective) just from the =
extended key representation, something that is certain to create =
significant confusion as we try to design applications that can share =
these types of objects.

An organization might begin as a single individual who just wants to =
generate signing keys for him/herself. Later on, this individual might =
bring on another individual or two and create new branches for them. =
With the current HD keychain structure, unless this individual made sure =
to set aside these new branches from the start, the individual is now =
forced to mix the new branches in at the same level of the hierarchy as =
the signing keys. Instead, it should be possible to branch off any node =
without having to worry at all about whether or not that node has been =
used to generate signing keys at all.

A possible workaround to this issue is to always allocate a specific =
child for hierarchical derivation and the rest of the children for =
signing keys. Then to create subbranches, the specific child would be =
used as the new parent, effectively alternating generations between =
signing keys and organizational nodes. However, this solution seems =
pretty ugly.

A better solution, IMO, is to only use BIP0032 for organizational =
hierarchy and have a different mechanism for generating a sequence of =
signing keys from a given node. This different mechanism could be used =
standalone by those not needing the full set of hierarchical features. =
For those who do want to use the hierarchical features, it could be =
seeded by the keys in the BIP0032 hierarchy. These individual signing =
keys would NEVER be represented in the same format as the organizational =
hierarchy nodes, thus ensuring applications can share these structures =
without risk of confusion.

Until we make this clear distinction between organizational hierarchy =
(which parallels real-world organizations) and signing keys (which are =
merely cryptographic primitives, preferably never even shown directly to =
most endusers), I think we'll fail to find good ways to make the H in HD =
keychains useful.

-Eric Lombrozo

--Apple-Mail=_449E2016-C8B9-49D1-B3D2-D63D90C89667
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment;
	filename=signature.asc
Content-Type: application/pgp-signature;
	name=signature.asc
Content-Description: Message signed with OpenPGP using GPGMail

-----BEGIN PGP SIGNATURE-----
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=glfk
-----END PGP SIGNATURE-----

--Apple-Mail=_449E2016-C8B9-49D1-B3D2-D63D90C89667--