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Merge pull request #34 from jonasnick/add-whitelist-md-bip32

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Add explanation about how BIP32 unhardened derivation can be used to …
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Andrew Poelstra 2018-12-04 16:15:14 +00:00 committed by GitHub
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src/modules/whitelist/whitelist.md
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@ -93,4 +93,13 @@ the remaining public keys are verified out-of-band when setting up the system,
so there is no direct benefit to this. We do it only to reduce fragility and
increase safety of unforeseen uses.
Having to access the offline key `Q_i` to compute the secret to the sum `W +
Q_i` for every authorization is onerous. Instead, if the whitelisted keys are
created using
[BIP32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki)
unhardened derivation, the sum can be computed on an online machine. In order
to achieve that, the offline key `Q_j` is set to the negated last hardened
BIP32 derived parent key (typically, the public key corresponding to the xpub).
As a result `W + Q_i = I_L*G` where `I_L` is the public tweak used
to derive `W` and can be easily computed online using the extended public key
and the derivation path.