Re: [bitcoin-dev] Proposal: Bitcoin Secure Multisig Setup

[Hugo Nguyen <hugo@nunchuk.io>]

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*BIP39 seed words list.

On Thu, Feb 11, 2021 at 11:11 AM Hugo Nguyen <hugo@nunchuk.io> wrote:

> Hi Pavol,
>
> On Thu, Feb 11, 2021 at 8:25 AM Dmitry Petukhov via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> В Thu, 11 Feb 2021 05:45:33 -0800
>> Hugo Nguyen via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
>> wrote:
>>
>> > > > ENCRYPTION_KEY = SHA256(SHA256(TOKEN))
>> > >
>> > > This scheme might be vulnerable to rainbow table attack.
>> > >
>> >
>> > Thank you for pointing this out! Incidentally, Dmitry Petukhov also
>> > told me the same privately.
>>
>> My thought was that if TOKEN has the characteristics of a password
>> (short ASCII string), then it would be better to use key derivation
>> function designed for passwords, like PBKDF2.
>>
>> The counter-argument to this is that this adds another code dependency
>> for vendors, if the device firmware does not already have the required
>> key derivation function.
>>
>> Maybe this could be solved by going into opposite direction - make the
>> "token" even longer, use the mnemoic.
>>
>> The issue is that entering long data of the shared key into the device
>> manually is difficult UX-wise.
>>
>> Hww vendors that allow to enter custom keys into their device already
>> have to face this issue, and those who allow to enter custom keys via
>> mnemonic probably tackled this somehow.
>>
>> Maybe the shared key for multisig setup can be entered in the same way
>> ? (with maybe additional visual check via some fingerprint).
>>
>
> You just gave me a great idea! We can reuse the BIP32 seed words list!
> Perhaps the encryption key can just be 6 words, but it'll be derived the
> same way. BIP39 also uses PBKDF2 as a key derivation function, so it
> matches with what you described here.
>
> And all HWW should have this functionality already.
>
> Best,
> Hugo
>
>
>>
>> Although we would then have another issue of potential confusion
>> between two procedures (entering the main key and entering the shared
>> key for multisig setup), and the measures has to be taken to prevent
>> such confusion.
>>
>> The approaches can be combined - specify a key derivation function
>> suitable for passwords; via secure channel, share a password and/or the
>> derived key. If hww supports derivation function, it can derive the key
>> from password. If hww supports only keys, the key can be entered raw or
>> via mnemonic.
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>

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