From 59eb6d10c12b150b1f9ca61f916e9d0d3c1bad04 Mon Sep 17 00:00:00 2001
From: Jon Chambers <jon@signal.org>
Date: Fri, 15 Jan 2021 17:48:09 -0500
Subject: [PATCH] Gate based on destination rather than random.

---
 .../textsecuregcm/auth/AmbiguousIdentifier.java           | 8 ++++++++
 .../textsecuregcm/controllers/MessageController.java      | 8 +++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/auth/AmbiguousIdentifier.java b/service/src/main/java/org/whispersystems/textsecuregcm/auth/AmbiguousIdentifier.java
index b06ce03c5..ffe6c81ab 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/auth/AmbiguousIdentifier.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/auth/AmbiguousIdentifier.java
@@ -37,4 +37,12 @@ public class AmbiguousIdentifier {
   public boolean hasNumber() {
     return number != null;
   }
+
+  public int sendingGateHash() {
+    if (uuid != null) {
+      return (int)(uuid.getLeastSignificantBits() & 0xff);
+    } else {
+      return number.hashCode() & 0xff;
+    }
+  }
 }
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/MessageController.java b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/MessageController.java
index ecb9dd7d1..9ef2ad33d 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/MessageController.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/MessageController.java
@@ -124,7 +124,7 @@ public class MessageController {
                                          @Valid                                    IncomingMessageList messages)
       throws RateLimitExceededException
   {
-    if (random.nextDouble() <= getSuccessPercentage()) {
+    if (shouldSend(destinationName)) {
       if (!source.isPresent() && !accessKey.isPresent()) {
         throw new WebApplicationException(Response.Status.UNAUTHORIZED);
       }
@@ -199,6 +199,12 @@ public class MessageController {
     }
   }
 
+  private boolean shouldSend(final AmbiguousIdentifier destination) {
+    final double hash = destination.sendingGateHash();
+
+    return (hash / 255.0) <= getSuccessPercentage();
+  }
+
   private double getSuccessPercentage() {
     if (featureFlagsManager.isFeatureFlagActive("SEND_MESSAGE_1_PERCENT")) {
       return 0.01;