Generate a linkingKey followed by LN concept

[jjhesk]

linkingKey derivation

linkingKey derivation for BIP-32 based wallets:

There exists a private hashingKey which is derived by user LN WALLET using m/138'/0 path. LN SERVICE full domain name is extracted from login LNURL and then hashed using hmacSha256(hashingKey, full service domain name). Full domain name here means FQDN with last full-stop (aka "point") omitted (Example: for https://x.y.z.com/... it would be x.y.z.com). First 16 bytes are taken from resulting hash and then turned into a sequence of 4 Long values which are in turn used to derive a service-specific linkingKey using m/138'/<long1>/<long2>/<long3>/<long4> path, a Scala example:

import fr.acinq.bitcoin.crypto
import fr.acinq.bitcoin.Protocol
import java.io.ByteArrayInputStream
import fr.acinq.bitcoin.DeterministicWallet._
val domainName = "site.com"
val hashingPrivKey = derivePrivateKey(walletMasterKey, hardened(138L) :: 0L :: Nil)
val derivationMaterial = hmac256(key = hashingPrivKey.toBin, message = domainName)
val stream = new ByteArrayInputStream(derivationMaterial.slice(0, 16).toArray)
val pathSuffix = Vector.fill(4)(Protocol.uint32(stream, ByteOrder.BIG_ENDIAN)) // each uint32 call consumes next 4 bytes
val linkingPrivKey = derivePrivateKey(walletMasterKey, hardened(138L) +: pathSuffix)
val linkingPubKey = linkingPrivKey.publicKey