I see the cause of our disagreement now.

You actually want to share a single BIP32 tree across different
currency types, but do it in a way that guarantees that they never use
the same keys.

I would have expected that different chains would use independent
chains, and have serializations encode which chain they belong to.

Let me offer an alternative suggestion, which is compatible with the
original default BIP32 structure:
* You can use one seed across different chains, but the master nodes
are separate.
* To derive the master node from the seed, the key string "Bitcoin
seed" is replaced by something chain-specific.
* Every encoded node (including master nodes) has a chain-specific
serialization magic.

This is in practice almost the same as your suggestion, except that
the m/cointype' in m/cointype'/account'/change/n is replaced by
different masters. The only disadvantage I see is that you do not have
a way to encode the "super master" that is the parent of all
chain-specific masters. You can - and with the same security
properties - encode the seed, though.

--
Pieter


On Tue, Apr 8, 2014 at 3:43 PM, slush <slush@centrum.cz> wrote:

tl;dr;

It is dangerous to expect that other seed than "xprv" does not contain
bitcoins or that "xprv" contains only bitcoins, because technically are both
situations possible. It is still safer to do the lookup; the magic itself is
ambiguous.

Marek

On Tue, Apr 8, 2014 at 3:40 PM, slush <slush@centrum.cz> wrote:

Serialization magic of bip32 seed is in my opinion completely unnecessary.
Most of software does not care about it anyway; You can use xprv/xpub pair
for main net, testnet, litecoin, dogecoin, whatevercoin.

Instead using the same seed (xprv) and then separate the chains *inside*
the bip32 path seems more useful to me.

Marek

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