On Apr 26, 2014 6:43 AM, "Mike Hearn" <mike@plan99.net> wrote:
>
> I'm not sure I understand why you need any special structure for this at
all. The way I'd do it is just use regular HD wallets for everyone, of the
regular form, and then swap the watching keys. Why do people need to be
given a cosigner index at all, given that they all have unique root keys
anyway?

I tried to explain that here:

The reason for using separate branches for each cosigner is we don't want
two of them generating the same address and receiving simultaneous payments
to it. The ideal case is that each address receives at most one payment,
requested by the corresponding cosigner.

To clarify, the problem the cosigner_index is trying to solve is race
conditions when receiving payments. Remember that we can't assume all
cosigners to be online at all times. Let's assume we use one shared branch
for everyone. Then two cosigners could need a new receiving address at the
same time, and get the next unused address on that branch. They then each
pass the same address to their payers, and we can get two payments to the
same address. Monitoring balances is not enough in this case because a
cosigner can never know when the others are generating a new address.
Separating branches and having each cosigner only use one branch makes this
problem go away.