From df4ec3501b0b5ed2137dd8f3eab3bfb3b8c62014 Mon Sep 17 00:00:00 2001
From: darken <darken@darken.eu>
Date: Tue, 3 Jun 2025 16:13:30 +0200
Subject: [PATCH] Add resolvable private address check

This allows verifying the identity of a device if its Identity Resolving Key (IRK) is known.
The IRK can be optionally configured.
---
 .../darken/capod/monitor/core/PodMonitor.kt   | 21 ++++++++++-
 .../darken/capod/monitor/core/RPAChecker.kt   | 37 +++++++++++++++++++
 2 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 app-common/src/main/java/eu/darken/capod/monitor/core/RPAChecker.kt

diff --git a/app-common/src/main/java/eu/darken/capod/monitor/core/PodMonitor.kt b/app-common/src/main/java/eu/darken/capod/monitor/core/PodMonitor.kt
index ad80aa74..9b8dae91 100644
--- a/app-common/src/main/java/eu/darken/capod/monitor/core/PodMonitor.kt
+++ b/app-common/src/main/java/eu/darken/capod/monitor/core/PodMonitor.kt
@@ -22,9 +22,17 @@ import eu.darken.capod.pods.core.PodFactory
 import eu.darken.capod.pods.core.apple.protocol.ProximityPairing
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.delay
-import kotlinx.coroutines.flow.*
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.combine
+import kotlinx.coroutines.flow.firstOrNull
+import kotlinx.coroutines.flow.flatMapLatest
+import kotlinx.coroutines.flow.flowOf
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.onStart
+import kotlinx.coroutines.flow.retryWhen
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
+import okio.ByteString.Companion.toByteString
 import java.time.Duration
 import java.time.Instant
 import javax.inject.Inject
@@ -40,6 +48,7 @@ class PodMonitor @Inject constructor(
     private val debugSettings: DebugSettings,
     private val podDeviceCache: PodDeviceCache,
     private val permissionTool: PermissionTool,
+    private val rpaChecker: RPAChecker,
 ) {
 
     private val deviceCache = mutableMapOf<PodDevice.Id, PodDevice>()
@@ -121,6 +130,7 @@ class PodMonitor @Inject constructor(
                     log(TAG, WARN) { "Using unfiltered scan mode" }
                     setOf(ScanFilter.Builder().build())
                 }
+
                 else -> ProximityPairing.getBleScanFilter()
             }
 
@@ -189,8 +199,15 @@ class PodMonitor @Inject constructor(
     }
 
     private fun determineMainDevice(pods: List<PodDevice>): PodDevice? {
-        val mainDeviceModel = generalSettings.mainDeviceModel.value
+        generalSettings.mainDeviceIdentityKey.value
+            .also { log(TAG) { "Identity-Resolving-Key (IRK): ${it?.toByteString()}" } }
+            ?.let { irkKey -> pods.firstOrNull { pod -> rpaChecker.verify(pod, irkKey) } }
+            ?.let {
+                log(TAG) { "Main device determined via IRK: $it" }
+                return it
+            }
 
+        val mainDeviceModel = generalSettings.mainDeviceModel.value
         val presorted = sortPodsToInterest(pods).sortedByDescending {
             it.model == mainDeviceModel && it.model != PodDevice.Model.UNKNOWN
         }
diff --git a/app-common/src/main/java/eu/darken/capod/monitor/core/RPAChecker.kt b/app-common/src/main/java/eu/darken/capod/monitor/core/RPAChecker.kt
new file mode 100644
index 00000000..22c8de2a
--- /dev/null
+++ b/app-common/src/main/java/eu/darken/capod/monitor/core/RPAChecker.kt
@@ -0,0 +1,37 @@
+package eu.darken.capod.monitor.core
+
+import android.annotation.SuppressLint
+import eu.darken.capod.pods.core.PodDevice
+import javax.crypto.Cipher
+import javax.crypto.spec.SecretKeySpec
+import javax.inject.Inject
+
+class RPAChecker @Inject constructor() {
+
+    // Resolvable-Private-Address
+    fun verify(dev: PodDevice, irk: ByteArray): Boolean {
+        val rpa = dev.address.split(":").map { it.toInt(16).toByte() }.reversed().toByteArray()
+        val prand = rpa.copyOfRange(3, 6)
+        val hash = rpa.copyOfRange(0, 3)
+        val computedHash = ah(irk, prand)
+        return hash.contentEquals(computedHash)
+    }
+
+    // E function (Encryption function):
+    @SuppressLint("GetInstance")
+    private fun e(key: ByteArray, data: ByteArray): ByteArray {
+        val cipher = Cipher.getInstance("AES/ECB/NoPadding").apply {
+            val secretKey = SecretKeySpec(key.reversedArray(), "AES")
+            init(Cipher.ENCRYPT_MODE, secretKey)
+        }
+        return cipher.doFinal(data.reversedArray()).reversedArray()
+    }
+
+    // AH function (Address Hashing function):
+    private fun ah(k: ByteArray, r: ByteArray): ByteArray {
+        val rPadded = ByteArray(16).apply {
+            r.copyInto(this, 0, 0, 3)
+        }
+        return e(k, rPadded).copyOfRange(0, 3)
+    }
+}
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