From 8e0aa42f7c921593f8427e4153702bdc1fdc6e24 Mon Sep 17 00:00:00 2001
From: Martin Asprusten <martin_asprusten@hotmail.com>
Date: Tue, 27 Feb 2024 20:48:06 +0100
Subject: [PATCH] Made the screenlock authenticator try to use StrongBox and
 attestation if available, by try again without them if it doesn't work

---
 .../screenlock/ScreenLockCredentialStore.kt   | 43 ++++++++++++++++++-
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/play-services-fido/core/src/main/kotlin/org/microg/gms/fido/core/transport/screenlock/ScreenLockCredentialStore.kt b/play-services-fido/core/src/main/kotlin/org/microg/gms/fido/core/transport/screenlock/ScreenLockCredentialStore.kt
index 7a0f1cdb5d..b42f5fb8a8 100644
--- a/play-services-fido/core/src/main/kotlin/org/microg/gms/fido/core/transport/screenlock/ScreenLockCredentialStore.kt
+++ b/play-services-fido/core/src/main/kotlin/org/microg/gms/fido/core/transport/screenlock/ScreenLockCredentialStore.kt
@@ -6,12 +6,14 @@
 package org.microg.gms.fido.core.transport.screenlock
 
 import android.content.Context
+import android.content.pm.PackageManager
 import android.database.sqlite.SQLiteDatabase
 import android.database.sqlite.SQLiteOpenHelper
 import android.os.Build.VERSION.SDK_INT
 import android.security.keystore.KeyGenParameterSpec
 import android.security.keystore.KeyPermanentlyInvalidatedException
 import android.security.keystore.KeyProperties
+import android.security.keystore.StrongBoxUnavailableException
 import android.util.Base64
 import android.util.Log
 import androidx.annotation.RequiresApi
@@ -32,6 +34,8 @@ class ScreenLockCredentialStore(val context: Context) {
 
     @RequiresApi(23)
     fun createKey(rpId: String, challenge: ByteArray): ByteArray {
+        var useStrongbox = false
+        if (SDK_INT >= 28) useStrongbox = context.packageManager.hasSystemFeature(PackageManager.FEATURE_STRONGBOX_KEYSTORE)
         val keyId = Random.nextBytes(32)
         val identifier = getAlias(rpId, keyId)
         Log.d(TAG, "Creating key for $identifier")
@@ -40,9 +44,44 @@ class ScreenLockCredentialStore(val context: Context) {
             .setDigests(KeyProperties.DIGEST_SHA256)
             .setAlgorithmParameterSpec(ECGenParameterSpec("secp256r1"))
             .setUserAuthenticationRequired(true)
+        if (SDK_INT >= 28) builder.setIsStrongBoxBacked(useStrongbox)
         if (SDK_INT >= 24) builder.setAttestationChallenge(challenge)
-        generator.initialize(builder.build())
-        generator.generateKeyPair()
+
+        var generatedKeypair = false
+        val exceptionClassesCaught = HashSet<Class<Exception>>()
+        while (!generatedKeypair) {
+            try {
+                generator.initialize(builder.build())
+                generator.generateKeyPair()
+                generatedKeypair = true
+            } catch (e: Exception) {
+                // Catch each exception class at most once.
+                // If we've caught the exception before, tried to correct it, and still catch the
+                // same exception, then we can't fix it and the exception should be thrown further
+                if (exceptionClassesCaught.contains(e.javaClass)) {
+                    throw e
+                }
+                exceptionClassesCaught.add(e.javaClass)
+
+                if (SDK_INT >= 28 && e is StrongBoxUnavailableException) {
+                    Log.w(TAG, "Failed with StrongBox, retrying without it...")
+                    // Not all algorithms are backed by the Strongbox. If the Strongbox doesn't
+                    // support this keypair, fall back to TEE
+                    builder.setIsStrongBoxBacked(false)
+                } else if (SDK_INT >= 24 && e is ProviderException) {
+                    Log.w(TAG, "Failed with attestation challenge, retrying without it...")
+                    // This ProviderException is often thrown if the TEE or Strongbox doesn't have
+                    // a built-in key to attest the new key pair with. If this happens, remove the
+                    // attestation challenge and create an unattested key
+                    builder.setAttestationChallenge(null)
+                } else {
+                    // We don't know how to handle other errors, so they should be thrown up the
+                    // system
+                    throw e
+                }
+            }
+        }
+
         return keyId
     }