Re: [bitcoin-dev] [Bitcoin-development] New BIP32 structure for P2SH multisig wallets [BIP-45]

[Thomas Kerin <thomas.kerin@gmail.com>]

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Hi Jean Pierre,

This is a problem I've considered before, though I have to say I prefer
your solution.

The problem is, how can a person who restores his wallet from just a
seed restore
all his multi-signature addresses with other parties?

Your proposal is nice because all participants are equal, and it
minimalizes the data
required for recovery because it's deterministic, and the (extended)
public key is the first
piece of metadata you'll ask for from others.. Let it be the only thing
we need!

Regards amending BIP45 - BIP's are not amended after the fact, however
bad it may be
in retrospect. It might be best to write a BIP specifying a
"pseudorandom & deterministic
path generation for HD/multi-signature accounts"

TK

On 04/10/15 16:18, Jean-Pierre Rupp via bitcoin-dev wrote:
> I have a possible solution:
>
> Take all public keys encoded in the purpose-specific extended public
> keys (m/45') of all cosigners and sort them lexicographically, according
> to BIP-45.  Serialize this information and calculate its HASH160
> (RIPEMD160 ∘ HASH256).  Split the output in five 32-bit chunks, setting
> the MSB on all of them to 0. Use these 32-bit chunks to build a
> derivation path from the purpose-specific extended public keys.  Treat
> this derivation path as if it was the purpose-specific extended public
> key in BIP-45.
>
> This scheme will avoid public key sharing, and as long as you share your
> purpose-specific extended public key only with your cosigners, it should
> be relatively hard for a passive observer to link activity between
> different cosigning accounts.
>
> On 03/10/15 13:42, Jean-Pierre Rupp via bitcoin-dev wrote:
>> Hello,
>>
>> I have been reviewing BIP-45 today.  There is a privacy problem with it
>> that should at least be mentioned in the document.
>>
>> When using the same extended public key for all multisig activity, and
>> dealing with different cosigners in separate multisig accounts, reuse of
>> the same set of public keys means that all cosigners from all accounts
>> will be able to monitor multisig activity from every other cosigner, in
>> every other account.
>>
>> Besides privacy considerations, HD wallet's non-reuse of public keys
>> provide some defence against wallets that do not implement deterministic
>> signing, and use poor entropy for signature nonces.
>>
>> Unless users are expected to establish a single cosigning account, this
>> scheme will result in reuse of public keys, and degradation of privacy.
>>
>> I understand that for convenience it is useful to have a single extended
>> public key that can be handed to every cosigner.  This makes setting up
>> accounts or recovering from data loss a easier.
>>
>> I suggest that privacy & potential security degradation due to increased
>> public key reuse in the case of users with multiple multisig accounts
>> should get a mention in the BIP-45 document.
>>
>> Greetings
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- -- 
My PGP key can be found here: <https://thomaskerin.io/me.pub.asc>
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