From f76d08c19adb9d7a2445321a451d6fb8b09b2a82 Mon Sep 17 00:00:00 2001
From: akwizgran <michael@briarproject.org>
Date: Thu, 9 Jan 2020 15:18:58 +0000
Subject: [PATCH] Use StrongBox on API 28+ if available.

---
 .../bramble/api/crypto/KeyStoreConfig.java    |  7 ++++-
 .../bramble/crypto/CryptoComponentImpl.java   | 22 +++++++++++----
 .../briar/android/AndroidKeyStoreConfig.java  | 28 +++++++++++++++----
 3 files changed, 45 insertions(+), 12 deletions(-)

diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/crypto/KeyStoreConfig.java b/bramble-api/src/main/java/org/briarproject/bramble/api/crypto/KeyStoreConfig.java
index 39f545f2f..5533e994d 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/crypto/KeyStoreConfig.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/crypto/KeyStoreConfig.java
@@ -3,6 +3,7 @@ package org.briarproject.bramble.api.crypto;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.security.spec.AlgorithmParameterSpec;
+import java.util.List;
 
 /**
  * Configures the use of a stored key to strengthen password-based encryption.
@@ -23,5 +24,9 @@ public interface KeyStoreConfig {
 
 	String getMacAlgorithmName();
 
-	AlgorithmParameterSpec getParameterSpec();
+	/**
+	 * Returns a list of {@link AlgorithmParameterSpec AlgorithmParameterSpecs}
+	 * to use for key generation, in order of preference.
+	 */
+	List<AlgorithmParameterSpec> getParameterSpecs();
 }
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoComponentImpl.java b/bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoComponentImpl.java
index 0ae209b1b..a634a0c8b 100644
--- a/bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoComponentImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoComponentImpl.java
@@ -34,6 +34,7 @@ import java.security.NoSuchAlgorithmException;
 import java.security.Provider;
 import java.security.SecureRandom;
 import java.security.Security;
+import java.security.spec.AlgorithmParameterSpec;
 import java.util.logging.Logger;
 
 import javax.annotation.Nullable;
@@ -382,17 +383,28 @@ class CryptoComponentImpl implements CryptoComponent {
 			KeyStore ks = KeyStore.getInstance(config.getKeyStoreType());
 			ks.load(null);
 			// Load or generate the stored key
-			javax.crypto.SecretKey storedKey;
+			javax.crypto.SecretKey storedKey = null;
 			Entry e = ks.getEntry(config.getKeyAlias(), null);
 			if (e == null) {
 				if (!generateIfMissing) {
 					LOG.warning("Key not found in keystore");
 					return null;
 				}
-				KeyGenerator kg = KeyGenerator.getInstance(
-						config.getMacAlgorithmName(), config.getProviderName());
-				kg.init(config.getParameterSpec());
-				storedKey = kg.generateKey();
+				// Try the parameter specs in order of preference
+				for (AlgorithmParameterSpec spec : config.getParameterSpecs()) {
+					try {
+						KeyGenerator kg = KeyGenerator.getInstance(
+								config.getMacAlgorithmName(),
+								config.getProviderName());
+						kg.init(spec);
+						storedKey = kg.generateKey();
+					} catch (GeneralSecurityException e1) {
+						if (LOG.isLoggable(INFO))
+							LOG.info("Could not generate key: " + e1);
+						// Fall back to next spec
+					}
+				}
+				if (storedKey == null) throw new IllegalArgumentException();
 				LOG.info("Stored key in keystore");
 			} else {
 				if (!(e instanceof SecretKeyEntry))
diff --git a/briar-android/src/main/java/org/briarproject/briar/android/AndroidKeyStoreConfig.java b/briar-android/src/main/java/org/briarproject/briar/android/AndroidKeyStoreConfig.java
index e6aff2dad..5f9b4b1e8 100644
--- a/briar-android/src/main/java/org/briarproject/briar/android/AndroidKeyStoreConfig.java
+++ b/briar-android/src/main/java/org/briarproject/briar/android/AndroidKeyStoreConfig.java
@@ -6,21 +6,37 @@ import org.briarproject.bramble.api.crypto.KeyStoreConfig;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.security.spec.AlgorithmParameterSpec;
+import java.util.List;
 
 import androidx.annotation.RequiresApi;
 
+import static android.os.Build.VERSION.SDK_INT;
 import static android.security.keystore.KeyProperties.PURPOSE_SIGN;
+import static java.util.Arrays.asList;
+import static java.util.Collections.singletonList;
 
 @RequiresApi(23)
 @NotNullByDefault
 class AndroidKeyStoreConfig implements KeyStoreConfig {
 
-	private final KeyGenParameterSpec spec;
+	private final List<AlgorithmParameterSpec> specs;
 
 	AndroidKeyStoreConfig() {
-		spec = new KeyGenParameterSpec.Builder("db", PURPOSE_SIGN)
-				.setKeySize(256)
-				.build();
+		KeyGenParameterSpec noStrongBox =
+				new KeyGenParameterSpec.Builder("db", PURPOSE_SIGN)
+						.setKeySize(256)
+						.build();
+		if (SDK_INT >= 28) {
+			// Prefer StrongBox if available
+			KeyGenParameterSpec strongBox =
+					new KeyGenParameterSpec.Builder("db", PURPOSE_SIGN)
+							.setIsStrongBoxBacked(true)
+							.setKeySize(256)
+							.build();
+			specs = asList(strongBox, noStrongBox);
+		} else {
+			specs = singletonList(noStrongBox);
+		}
 	}
 
 	@Override
@@ -44,7 +60,7 @@ class AndroidKeyStoreConfig implements KeyStoreConfig {
 	}
 
 	@Override
-	public AlgorithmParameterSpec getParameterSpec() {
-		return spec;
+	public List<AlgorithmParameterSpec> getParameterSpecs() {
+		return specs;
 	}
 }