Fix and expand channel keypath (#1147)

* Fix funding pubkey to channel key path computation

Channel key path is generated from 8 bytes computed from our funding pubkey, but we extracted 4 uint32 values instead of 2 (last 2 were always 0). We now use 128 bits to derive channel key paths.

* Add a channel key path compatibility test

This test will fail if we change the way we compute channel key paths, which would break existing channels.

eclair-core/src/main/scala/fr/acinq/eclair/crypto/KeyManager.scala

@@ -112,7 +112,7 @@ object KeyManager {
     * @return a BIP32 path
     */
   def channelKeyPath(fundingPubKey: PublicKey) : DeterministicWallet.KeyPath = {
-    val buffer = fundingPubKey.hash160.take(8)
+    val buffer = fundingPubKey.hash160.take(16)
     val bis = new ByteArrayInputStream(buffer.toArray)
     DeterministicWallet.KeyPath(Seq(Protocol.uint32(bis, ByteOrder.BIG_ENDIAN), Protocol.uint32(bis, ByteOrder.BIG_ENDIAN), Protocol.uint32(bis, ByteOrder.BIG_ENDIAN), Protocol.uint32(bis, ByteOrder.BIG_ENDIAN)))
   }

eclair-core/src/test/scala/fr/acinq/eclair/crypto/LocalKeyManagerSpec.scala

@@ -16,9 +16,9 @@
 
 package fr.acinq.eclair.crypto
 
-import fr.acinq.bitcoin.{Block, ByteVector32, DeterministicWallet}
-import fr.acinq.bitcoin.Crypto.PublicKey
+import fr.acinq.bitcoin.Crypto.{PrivateKey, PublicKey}
 import fr.acinq.bitcoin.DeterministicWallet.KeyPath
+import fr.acinq.bitcoin.{Block, ByteVector32, DeterministicWallet}
 import org.scalatest.FunSuite
 import scodec.bits._
 
@@ -57,4 +57,12 @@ class LocalKeyManagerSpec extends FunSuite {
     assert(keyManager1.fundingPublicKey(keyPath) != keyManager2.fundingPublicKey(keyPath))
     assert(keyManager1.commitmentPoint(keyPath, 1) != keyManager2.commitmentPoint(keyPath, 1))
   }
+
+  test("compute channel key path from funding keys") {
+    // if this test fails it means that we don't generate the same channel key path from the same funding pubkey, which
+    // will break existing channels !
+    val pub = PrivateKey(ByteVector32.fromValidHex("01" * 32)).publicKey
+    val keyPath = KeyManager.channelKeyPath(pub)
+    assert(keyPath.toString() == "m/2041577608/1982247572/689197082'/1288840885")
+  }
 }