[Bitcoin-development] BIP32 Index Randomisation

[Gregory Maxwell]

This seems overly complicated to me, unless I'm missing something.

Instead, I think you should just give the server the master pubkey P
only without the chaincode.


Then when you transact you generate the address in whatever manner you
like and tell the server the scalar value iL which the user computes
as

iL = HMAC-SHA512(Key = cpar, Data = serP(Kpar) || ser32(i))[first 32
byes],  (per BIP 32).

and the server computes P + iL*G  and checks agreement with the address.

It would be inaccurate to call this private, as the server still
learns this particular relation. (and really users should _not_ be
using the same chaincode with different parties... as it exacerbates
the private key leak risk), but its certainly more private than giving
people the chain code.

The approach I suggest is also not gratuitously incompatible with
hardened derivation, which is what parties should be doing when they
don't actually need a third party to generate future addresses for
them without their cooperation (as appears to be the case here).










On Fri, Mar 13, 2015 at 3:48 AM, Matias Alejo Garcia <matias at bitpay.com> wrote:
>
> Hello everyone,
>
> We are working on bitcore-wallet-server (BWS), a HD multisig wallet
> 'facilitator'. We have a couple of questions regarding BIP32 path usage, and
> we would love to have feedback from you before moving forward.
>
> Currently the BWS instances hold the set of extended public keys of the
> wallet's peers to be able to derive  addresses.
>
> Since this is a problem from the privacy point of view, we thought using
> pseudo-random  BIP32 paths, with a seed only known be the peers, so the
> server will be able to verify that addresses  submitted by peers belong to
> the wallet, but will not be able to derive future wallet addresses.
>
> The workflow would be something like:
>
> ```
> Peer >   getCurrentIndex
>
> < Server [index]
>
> Peer:
>   pathSeed = PRNG(seed, index);
>
> Peer > createAddress(index, pathSeed);
>
> Server:
>   derives the address and add it to the wallet.
>
> < Server  new address
>
> Peer: Verifies the address and inform it the user.
> ```
>
> This way, accessing server data won't reveal future wallet addresses. The
> seed (only known by the peers) could
> be derived from hashes of their xprivs, so wallet funds can still be recover
> with:
>   1) The complete set of xprivs
>   2) The quorum of xprivs + the complete set of xpubs + the address seed.
>
> Thanks a lot in advance for any comment on this schema.
>
> matías
>
> --
> BitPay.com
>
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