cosmrs: better document BIP-32 `SigningKey` derivation

[0xForerunner]

As it stands there is very little documentation surrounding the SigningKey trait and the way you are supposed to use it with Bip32. I think a very common use case would be generating this from a mnemonic string, and as it stands there is no obvious way to go about that. An example of doing that would likely be very helpful for many.

[tony-iqlusion]

This is the relevant method: https://docs.rs/cosmrs/latest/cosmrs/crypto/secp256k1/struct.SigningKey.html#method.derive_from_path

[0xForerunner]

This is the relevant method: https://docs.rs/cosmrs/latest/cosmrs/crypto/secp256k1/struct.SigningKey.html#method.derive_from_path

Is this not the correct way to do it?

        let mnemonic = bip32::Mnemonic::new(chain.private_key.clone(), Language::English).unwrap();
        let seed = mnemonic.to_seed("password");
        let xprv = XPrv::new(&seed)?;
        let signing_key: SigningKey = xprv.into();

[tony-iqlusion]

@ewoolsey that's the correct way to get the seed, however you have no derivation path in your example.

You can pass the seed into derive_from_path, but you need a derivation path like m/44'/118'/0'/0

[0xForerunner]

Ahhh so something like this?

        let mnemonic = bip32::Mnemonic::new(chain.mnemonic.clone(), Language::English).unwrap();
        let seed = mnemonic.to_seed("password");
        let child_path = "m/0/2147483647'/1/2147483646'";
        let child_xprv = XPrv::derive_from_path(&seed, &child_path.parse()?)?;        
        let signing_key: SigningKey = child_xprv.into();

[tony-iqlusion]

The last two lines need to be:

let signing_key = cosmrs::crypto::secp256k1::SigningKey::derive_from_path(&seed, child_path.parse()?)?);

[0xForerunner]

Gotcha! Thanks for the help! Appreciate it!

[meekteek]

Is this the only place for one to be able to get this knowledge?
There is still no good docs for this and I had to come here..

[tony-iqlusion]

@meekteek this issue is still open to track updating the documentation because it hasn't been done yet.

Contributions to improve the documentation would be welcome.