Random values for generateLocalAlias()

eclair-core/src/main/scala/fr/acinq/eclair/ShortChannelId.scala

@@ -58,7 +58,25 @@ object ShortChannelId {
 
   def toShortId(blockHeight: Int, txIndex: Int, outputIndex: Int): Long = ((blockHeight & 0xFFFFFFL) << 40) | ((txIndex & 0xFFFFFFL) << 16) | (outputIndex & 0xFFFFL)
 
-  def generateLocalAlias(): Alias = Alias(System.nanoTime()) // TODO: fixme (duplicate, etc.)
+  /**
+   * At block height 350 000 LN didn't exist, so all real scids less than that will never be used. This results in
+   * more than `2^58` values.
+   *
+   * The expected number of values before we have a collision can be approximated by (*):
+   * `Q(H) ~= sqrt( Pi * H / 2) = sqrt(Pi * 2^57) = 673 000 000`
+   *
+   * We don't expect to have anywhere close to that many channels at any given time on our node, so we don't need to
+   * check for duplicate values.
+   *
+   * (*) https://en.wikipedia.org/wiki/Birthday_attack
+   */
+  private val aliasUpperBound = 2^58
+
+  def generateLocalAlias(): Alias = {
+    // modulo won't skew the distribution because 2^64 is a multiple of 2^58
+    val l = Math.abs(randomLong() % aliasUpperBound)
+    Alias(l)
+  }
 
   @inline
   def blockHeight(shortChannelId: ShortChannelId): BlockHeight = BlockHeight((shortChannelId.toLong >> 40) & 0xFFFFFF)

eclair-core/src/main/scala/fr/acinq/eclair/channel/Register.scala

@@ -52,6 +52,12 @@ class Register extends Actor with ActorLogging {
       // We map all known scids (real or alias) to the channel_id. The relayer is in charge of deciding whether a real
       // scid can be used or not for routing (see option_scid_alias), but the register is neutral.
       val m = (scidAssigned.shortIds.real.toOption.toSeq :+ scidAssigned.shortIds.localAlias).map(_ -> scidAssigned.channelId).toMap
+      // duplicate check for aliases (we use a random value in a large enough space that there should never be collisions)
+      shortIds.get(scidAssigned.shortIds.localAlias) match {
+        case Some(channelId) if channelId != scidAssigned.channelId =>
+          log.error("duplicate alias={} for channelIds={},{} this should never happen!", scidAssigned.shortIds.localAlias, channelId, scidAssigned.channelId)
+        case _ => ()
+      }
       context become main(channels, shortIds ++ m, channelsTo)
 
     case Terminated(actor) if channels.values.toSet.contains(actor) =>

eclair-core/src/test/scala/fr/acinq/eclair/ShortChannelIdSpec.scala

@@ -70,4 +70,11 @@ class ShortChannelIdSpec extends AnyFunSuite {
     Seq(-561L, 0xffffffffffffffffL, 0x2affffffffffffffL).foreach(id => assert(Alias(id) == UnspecifiedShortChannelId(id)))
   }
 
+  test("basic check on random alias generation") {
+    for(_ <- 0 until 100) {
+      val alias = ShortChannelId.generateLocalAlias()
+      assert(alias.toLong >= 0 && alias.toLong <= 384_829_069_721_665_536L)
+    }
+  }
+
 }