HD wallet support [WIP]

[axic]

HD wallet support based on bitcoinjs-lib. It has been imported and made to work with secp256k1-node. It would be better having the code shared between this and bitcoinjs-lib, but that might take a bit of work.

Tests also came from bitcoinjs-lib and they all pass (bar two which use a mock BigInteger test).

Edit: found in the pile of issues over at bitcoinjs-lib, that there's on going discussion about this at bitcoinjs/bitcoinjs-lib#508

[axic]

@fanatid would you mind having a look at this?

There are a couple of things which could go into secp256k1:

• check if a public key is compressed
• more thorough validation (see below)
• have the zero check and curve order check separately available
• check if point (pubkey) is at infinity

For example this part I'm not sure secp256k1 does it?

// Verify that the X coordinate in the public point corresponds to a point on the curve.
// If not, the extended public key is invalid.
curve.validate(Q)

[fanatid]

You should throw error by yourself. publicKeyVerify return true/false

[fanatid]

@axic actions with secp256k1 looks good, except one nit.
You can check publicKeyVerify here: https://github.com/cryptocoinjs/secp256k1-node/blob/880a9ebb9e972f24803c1d86eaf851701c60ebd9/lib/js/ecpoint.js#L27

[axic]

Thanks @fanatid, dropping this in the favour of #4