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Sjors / libwally-swift

Swift wrapper for LibWally, a collection of useful primitives for cryptocurrency wallets
MIT License
40 stars 18 forks source link

Non extended private key derivation? #20

Closed Fonta1n3 closed 4 years ago

Fonta1n3 commented 4 years ago

Is it possible to derive a normal private key instead of only extended private keys?

Sjors commented 4 years ago

I have a fork which adds a privKey method (you'll have to make it public though): https://github.com/Sjors/libwally-swift/blob/dev/LibWally/BIP32.swift#L253

Note: that fork is still in flux and not very well tested.

Fonta1n3 commented 4 years ago

Thanks a lot, that did the trick :)

Sjors commented 4 years ago

Update: this is in master now. Feel free to open a PR here to make it public. I'm a bit reluctant to expose it though; what are you trying to do with it?

Fonta1n3 commented 4 years ago

Thanks for that. I am using it in this project: https://github.com/BlockchainCommons/Bitcoin-Standup/tree/master/iOS/StandUp-Remote

The app basically connects to your node over tor, then creates seeds locally on your device with LibWally-Swift. For the single signature architecture we store the seed locally keeping the node cold. I use BIP32 derivations to see what index private keys I need to sign a transaction with, fetch the WIF locally using LibWally then pass to the node for signing with signrawtransactionwithkey.

Sjors commented 4 years ago

That makes sense, but it's risky to let the key off the device. If you use an xpub without hardened derivation after it, then revealing one private key lets you spend the entire xpub.

A safer approach would be to create a PSBT on the node and sign that on the device.

Fonta1n3 commented 4 years ago

Will certainly try and accomplish that.

Fonta1n3 commented 4 years ago

This library is fantastic. I got the offline signing working first go, really impressive stuff, thank you!