From e6aed8534a0eadec0cc12e46c4b6c38a686a0e4a Mon Sep 17 00:00:00 2001
From: Greyson Parrelli <greyson@signal.org>
Date: Mon, 14 Jul 2025 20:18:20 -0400
Subject: [PATCH] Use libsignal for deriving thumbnail transit keys.

---
 .../signalservice/api/backup/MediaRootBackupKey.kt             | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/libsignal-service/src/main/java/org/whispersystems/signalservice/api/backup/MediaRootBackupKey.kt b/libsignal-service/src/main/java/org/whispersystems/signalservice/api/backup/MediaRootBackupKey.kt
index 7cea37fcb6..cbd4391e19 100644
--- a/libsignal-service/src/main/java/org/whispersystems/signalservice/api/backup/MediaRootBackupKey.kt
+++ b/libsignal-service/src/main/java/org/whispersystems/signalservice/api/backup/MediaRootBackupKey.kt
@@ -6,7 +6,6 @@
 package org.whispersystems.signalservice.api.backup
 
 import org.signal.libsignal.protocol.ecc.ECPrivateKey
-import org.signal.libsignal.protocol.kdf.HKDF
 import org.whispersystems.signalservice.api.push.ServiceId.ACI
 import org.signal.libsignal.messagebackup.BackupKey as LibSignalBackupKey
 
@@ -37,7 +36,7 @@ class MediaRootBackupKey(override val value: ByteArray) : BackupKey {
   }
 
   fun deriveThumbnailTransitKey(thumbnailMediaName: MediaName): ByteArray {
-    return HKDF.deriveSecrets(value, deriveMediaId(thumbnailMediaName).value, "20240513_Signal_Backups_EncryptThumbnail".toByteArray(), 64)
+    return LibSignalBackupKey(value).deriveThumbnailTransitEncryptionKey(deriveMediaId(thumbnailMediaName).value)
   }
 
   private fun deriveMediaSecrets(mediaId: MediaId): MediaKeyMaterial {