feat(e2ee): secret fingerprint

app/src/main/kotlin/me/rhunk/snapenhance/e2ee/E2EEImplementation.kt

@@ -5,6 +5,7 @@ import me.rhunk.snapenhance.bridge.e2ee.E2eeInterface
 import me.rhunk.snapenhance.bridge.e2ee.EncryptionResult
 import org.bouncycastle.pqc.crypto.crystals.kyber.*
 import java.io.File
+import java.security.MessageDigest
 import java.security.SecureRandom
 import javax.crypto.Cipher
 import javax.crypto.spec.IvParameterSpec
@@ -115,6 +116,21 @@ class E2EEImplementation (
         return File(e2eeFolder, "$friendId.key").exists()
     }
 
+    override fun getSecretFingerprint(friendId: String): String? {
+        val sharedSecretKey = runCatching {
+            File(e2eeFolder, "$friendId.key").readBytes()
+        }.onFailure {
+            context.log.error("Failed to read shared secret key", it)
+            return null
+        }.getOrThrow()
+
+        return MessageDigest.getInstance("SHA-256")
+            .digest(sharedSecretKey)
+            .joinToString("") { "%02x".format(it) }
+            .chunked(5)
+            .joinToString(" ")
+    }
+
     override fun encryptMessage(friendId: String, message: ByteArray): EncryptionResult? {
         val encryptionKey = runCatching {
             File(e2eeFolder, "$friendId.key").readBytes()

core/src/main/aidl/me/rhunk/snapenhance/bridge/e2ee/E2eeInterface.aidl

@@ -26,8 +26,20 @@ interface E2eeInterface {
     */
     boolean acceptPairingResponse(String friendId, in byte[] encapsulatedSecret);
 
+    /**
+    * Check if a friend key exists
+    * @param friendId
+    * @return true if the friend key exists
+    */
     boolean friendKeyExists(String friendId);
 
+    /**
+    * Get the fingerprint of a secret key
+    * @param friendId
+    * @return the fingerprint of the secret key
+    */
+    @nullable String getSecretFingerprint(String friendId);
+
     @nullable EncryptionResult encryptMessage(String friendId, in byte[] message);
 
     @nullable byte[] decryptMessage(String friendId, in byte[] message, in byte[] iv);

core/src/main/kotlin/me/rhunk/snapenhance/features/impl/experiments/EndToEndEncryption.kt

@@ -153,21 +153,35 @@ class EndToEndEncryption : MessagingRuleFeature(
 
     private fun openManagementPopup() {
         val conversationId = context.feature(Messaging::class).openedConversationUUID?.toString() ?: return
+        val friendId = context.database.getDMOtherParticipant(conversationId)
 
-        if (context.database.getDMOtherParticipant(conversationId) == null) {
+        if (friendId == null) {
             context.shortToast("This menu is only available in direct messages.")
             return
         }
 
         val actions = listOf(
-            "Initiate a new shared secret"
+            "Initiate a new shared secret",
+            "Show shared key fingerprint"
         )
 
         ViewAppearanceHelper.newAlertDialogBuilder(context.mainActivity!!).apply {
             setTitle("End-to-end encryption")
             setItems(actions.toTypedArray()) { _, which ->
                 when (which) {
-                    0 -> askForKeys(conversationId)
+                    0 -> {
+                        warnKeyOverwrite(friendId) {
+                            askForKeys(conversationId)
+                        }
+                    }
+                    1 -> {
+                        val fingerprint = e2eeInterface.getSecretFingerprint(friendId)
+                        ViewAppearanceHelper.newAlertDialogBuilder(context).apply {
+                            setTitle("End-to-end encryption")
+                            setMessage("Your fingerprint is:\n\n$fingerprint\n\nMake sure to check if it matches your friend's fingerprint!")
+                            setPositiveButton("OK") { _, _ -> }
+                        }.show()
+                    }
                 }
             }
             setPositiveButton("OK") { _, _ -> }