diff --git a/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt b/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt
index 3f6846792..7efa7fb8d 100644
--- a/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt
@@ -3,6 +3,7 @@ package com.bitchat.android.mesh
 import android.util.Log
 import com.bitchat.android.crypto.EncryptionService
 import com.bitchat.android.protocol.BitchatPacket
+import com.bitchat.android.sync.PacketIdUtil
 import com.bitchat.android.protocol.MessageType
 import com.bitchat.android.model.RoutedPacket
 import com.bitchat.android.noise.AuthenticatedNoiseSession
@@ -241,18 +242,28 @@ class SecurityManager(private val encryptionService: EncryptionService, private
     /**
      * Generate message ID for duplicate detection
      */
+    /**
+     * Identity used for replay and duplicate detection.
+     *
+     * This was a 32-bit `contentHashCode()` over at most the first 64 bytes of
+     * the payload. Two packets from the same peer in the same millisecond that
+     * agreed on that prefix collided, and a collision here is a *dropped
+     * message* — the second packet is discarded as a duplicate and there is no
+     * signal that it happened.
+     *
+     * `PacketIdUtil` is the identity the rest of the stack already uses for
+     * exactly this question (gossip sync membership, message IDs in
+     * `MessageHandler`), and iOS derives it the same way: the first 16 bytes of
+     * SHA-256 over type, senderID, timestamp and the **whole** payload. Using
+     * it here makes the security path agree with the sync path instead of
+     * carrying a weaker private notion of "same packet".
+     *
+     * Peer scoping is kept: `PacketIdUtil` covers the packet's own senderID,
+     * while this key is scoped by the peer the packet was received from, and
+     * those are not the same thing for a relayed packet.
+     */
     private fun generateMessageID(packet: BitchatPacket, peerID: String): String {
-        return when (MessageType.fromValue(packet.type)) {
-            MessageType.FRAGMENT -> {
-                // For fragments, include the payload hash to distinguish different fragments
-                "${packet.timestamp}-$peerID-${packet.type}-${packet.payload.contentHashCode()}"
-            }
-            else -> {
-                // For other messages, use a truncated payload hash
-                val payloadHash = packet.payload.sliceArray(0 until minOf(64, packet.payload.size)).contentHashCode()
-                "${packet.timestamp}-$peerID-$payloadHash"
-            }
-        }
+        return "$peerID-${PacketIdUtil.computeIdHex(packet)}"
     }
     
     /**
diff --git a/app/src/test/kotlin/com/bitchat/android/mesh/SecurityManagerTest.kt b/app/src/test/kotlin/com/bitchat/android/mesh/SecurityManagerTest.kt
index 2a56e708b..e43e11b59 100644
--- a/app/src/test/kotlin/com/bitchat/android/mesh/SecurityManagerTest.kt
+++ b/app/src/test/kotlin/com/bitchat/android/mesh/SecurityManagerTest.kt
@@ -598,4 +598,65 @@ class SecurityManagerTest {
 
     private fun String.hexToBytes(): ByteArray =
         chunked(2).map { it.toInt(16).toByte() }.toByteArray()
+
+    // Duplicate-detection identity.
+
+    /**
+     * Two distinct packets that agree on their first 64 bytes and share a
+     * timestamp. The old key hashed only that prefix, with a 32-bit
+     * `contentHashCode`, so these were "the same packet" and the second was
+     * silently dropped.
+     */
+    private fun prefixSharingPair(): Pair<BitchatPacket, BitchatPacket> {
+        val shared = ByteArray(64) { 0x7 }
+        val first = BitchatPacket(
+            version = 1u,
+            type = MessageType.NOISE_ENCRYPTED.value,
+            senderID = otherPeerID.hexToByteArrayForTest(),
+            recipientID = myPeerID.hexToByteArrayForTest(),
+            timestamp = 1_700_000_000_000uL,
+            payload = shared + byteArrayOf(0x01, 0x02, 0x03),
+            ttl = 7u
+        )
+        val second = first.copy(payload = shared + byteArrayOf(0x0A, 0x0B, 0x0C))
+        return first to second
+    }
+
+    @Test
+    fun `packets differing only past the first 64 bytes are not treated as duplicates`() {
+        val (first, second) = prefixSharingPair()
+
+        assertTrue(securityManager.validatePacket(first, otherPeerID))
+        assertTrue(
+            "A distinct packet must not be dropped as a duplicate",
+            securityManager.validatePacket(second, otherPeerID)
+        )
+    }
+
+    @Test
+    fun `a genuine replay of the same packet is still rejected`() {
+        // The other half: strengthening the identity must not weaken replay
+        // protection, which is the reason this cache exists.
+        val (first, _) = prefixSharingPair()
+
+        assertTrue(securityManager.validatePacket(first, otherPeerID))
+        assertFalse(
+            "The identical packet must still be caught",
+            securityManager.validatePacket(first, otherPeerID)
+        )
+    }
+
+    @Test
+    fun `the same packet from two different peers is tracked separately`() {
+        // Peer scoping is deliberately kept: PacketIdUtil covers the packet's
+        // own senderID, which is not the peer it was received from once a
+        // packet has been relayed.
+        val (first, _) = prefixSharingPair()
+
+        assertTrue(securityManager.validatePacket(first, otherPeerID))
+        assertTrue(securityManager.validatePacket(first, unknownPeerID))
+    }
+
+    private fun String.hexToByteArrayForTest(): ByteArray =
+        chunked(2).map { it.toInt(16).toByte() }.toByteArray()
 }