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jamesmontemagnojamesmontemagno
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GH-380 Use Internal Preferences for Secure Storage consistency. (#386)
* Use Internal Preferences for Secure Storage consistency. Save if we created key pre-M so we always use pre-M if device upgrades. * Fix logic for pre-m key check The logic was slightly off, I think this fixes it, but would be good to have another set of eyes... 1. We check to see if the device is pre-M (if it does _not_ have `M`, or if we already set the fact it's pre-M in the preference - aka from a previous install before an upgrade of the OS) 2. If we aren't pre-M, we can't use Symmetric Key from Keystore 3. If we make it down to using Asymmetric Key, we set the pre-M preference to `true` to persist the value for future invocations, which will make it 'stick' in the event of a pre-M to M+ OS upgrade. * Address feedback on key naming. * Added test for secure storage to simulate upgrade From API < 23 to API >= 23 after storing data with an asymmetric key and then moving to a platform supporting symmetric keys. * Ensure we always set flags when using specified keygen
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-59
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‎DeviceTests/DeviceTests.Shared/SecureStorage_Tests.cs‎

Lines changed: 25 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -85,5 +85,30 @@ public async Task Remove_All_Keys(bool emulatePreApi23, string[] keys)
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foreach (var key in keys)
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Assert.Null(await SecureStorage.GetAsync(key));
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}
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#if __ANDROID__
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[Fact]
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public async Task Asymmetric_to_Symmetric_API_Upgrade()
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{
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var key = "asym_to_sym_upgrade";
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var expected = "this is the value";
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SecureStorage.RemoveAll();
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// Emulate pre api 23
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SecureStorage.AlwaysUseAsymmetricKeyStorage = true;
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await SecureStorage.SetAsync(key, expected);
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// Simulate Upgrading to API23+
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SecureStorage.AlwaysUseAsymmetricKeyStorage = false;
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var v = await SecureStorage.GetAsync(key);
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SecureStorage.RemoveAll();
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Assert.Equal(expected, v);
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}
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#endif
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}
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}

‎Xamarin.Essentials/SecureStorage/SecureStorage.android.cs‎

Lines changed: 36 additions & 59 deletions
Original file line numberDiff line numberDiff line change
@@ -18,9 +18,8 @@ static Task<string> PlatformGetAsync(string key)
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{
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var context = Platform.AppContext;
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string encStr;
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using (var prefs = context.GetSharedPreferences(Alias, FileCreationMode.Private))
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encStr = prefs.GetString(Utils.Md5Hash(key), null);
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string defaultEncStr = null;
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var encStr = Preferences.Get(Utils.Md5Hash(key), defaultEncStr, Alias);
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string decryptedData = null;
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if (!string.IsNullOrEmpty(encStr))
@@ -40,13 +39,8 @@ static Task PlatformSetAsync(string key, string data)
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var ks = new AndroidKeyStore(context, Alias, AlwaysUseAsymmetricKeyStorage);
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var encryptedData = ks.Encrypt(data);
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using (var prefs = context.GetSharedPreferences(Alias, FileCreationMode.Private))
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using (var prefsEditor = prefs.Edit())
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{
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var encStr = Convert.ToBase64String(encryptedData);
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prefsEditor.PutString(Utils.Md5Hash(key), encStr);
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prefsEditor.Commit();
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}
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var encStr = Convert.ToBase64String(encryptedData);
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Preferences.Set(Utils.Md5Hash(key), encStr, Alias);
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return Task.CompletedTask;
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}
@@ -56,36 +50,13 @@ static bool PlatformRemove(string key)
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var context = Platform.AppContext;
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key = Utils.Md5Hash(key);
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using (var prefs = context.GetSharedPreferences(Alias, FileCreationMode.Private))
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{
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if (prefs.Contains(key))
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{
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using (var prefsEditor = prefs.Edit())
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{
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prefsEditor.Remove(key);
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prefsEditor.Commit();
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return true;
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}
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}
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}
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Preferences.Remove(key, Alias);
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return false;
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}
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static void PlatformRemoveAll()
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{
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var context = Platform.AppContext;
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using (var prefs = context.GetSharedPreferences(Alias, FileCreationMode.Private))
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using (var prefsEditor = prefs.Edit())
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{
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foreach (var key in prefs.All.Keys)
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prefsEditor.Remove(key);
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prefsEditor.Commit();
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}
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}
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static void PlatformRemoveAll() =>
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Preferences.Clear(Alias);
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internal static bool AlwaysUseAsymmetricKeyStorage { get; set; } = false;
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}
@@ -114,10 +85,18 @@ internal AndroidKeyStore(Context context, string keystoreAlias, bool alwaysUseAs
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KeyStore keyStore;
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bool alwaysUseAsymmetricKey;
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bool useSymmetric = false;
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string useSymmetricPreferenceKey = "essentials_use_symmetric";
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ISecretKey GetKey()
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{
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// check to see if we need to get our key from past-versions or newer versions.
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// we want to use symmetric if we are >= 23 or we didn't set it previously.
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useSymmetric = Preferences.Get(useSymmetricPreferenceKey, Platform.HasApiLevel(BuildVersionCodes.M), SecureStorage.Alias);
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// If >= API 23 we can use the KeyStore's symmetric key
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if (Platform.HasApiLevel(BuildVersionCodes.M) && !alwaysUseAsymmetricKey)
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if (useSymmetric && !alwaysUseAsymmetricKey)
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return GetSymmetricKey();
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// NOTE: KeyStore in < API 23 can only store asymmetric keys
@@ -131,40 +110,35 @@ ISecretKey GetKey()
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// Get the asymmetric key pair
132111
var keyPair = GetAsymmetricKeyPair();
133112

134-
using (var prefs = appContext.GetSharedPreferences(alias, FileCreationMode.Private))
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{
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var existingKeyStr = prefs.GetString(prefsMasterKey, null);
113+
var existingKeyStr = Preferences.Get(prefsMasterKey, null, alias);
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138-
if (!string.IsNullOrEmpty(existingKeyStr))
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{
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var wrappedKey = Convert.FromBase64String(existingKeyStr);
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if (!string.IsNullOrEmpty(existingKeyStr))
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{
117+
var wrappedKey = Convert.FromBase64String(existingKeyStr);
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142-
var unwrappedKey = UnwrapKey(wrappedKey, keyPair.Private);
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var kp = unwrappedKey.JavaCast<ISecretKey>();
119+
var unwrappedKey = UnwrapKey(wrappedKey, keyPair.Private);
120+
var kp = unwrappedKey.JavaCast<ISecretKey>();
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145-
return kp;
146-
}
147-
else
148-
{
149-
var keyGenerator = KeyGenerator.GetInstance(aesAlgorithm);
150-
var defSymmetricKey = keyGenerator.GenerateKey();
122+
return kp;
123+
}
124+
else
125+
{
126+
var keyGenerator = KeyGenerator.GetInstance(aesAlgorithm);
127+
var defSymmetricKey = keyGenerator.GenerateKey();
151128

152-
var wrappedKey = WrapKey(defSymmetricKey, keyPair.Public);
129+
var wrappedKey = WrapKey(defSymmetricKey, keyPair.Public);
153130

154-
using (var prefsEditor = prefs.Edit())
155-
{
156-
prefsEditor.PutString(prefsMasterKey, Convert.ToBase64String(wrappedKey));
157-
prefsEditor.Commit();
158-
}
131+
Preferences.Set(prefsMasterKey, Convert.ToBase64String(wrappedKey), alias);
159132

160-
return defSymmetricKey;
161-
}
133+
return defSymmetricKey;
162134
}
163135
}
164136

165137
// API 23+ Only
166138
ISecretKey GetSymmetricKey()
167139
{
140+
Preferences.Set(useSymmetricPreferenceKey, true, SecureStorage.Alias);
141+
168142
var existingKey = keyStore.GetKey(alias, null);
169143

170144
if (existingKey != null)
@@ -186,6 +160,9 @@ ISecretKey GetSymmetricKey()
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187161
KeyPair GetAsymmetricKeyPair()
188162
{
163+
// set that we generated keys on pre-m device.
164+
Preferences.Set(useSymmetricPreferenceKey, false, SecureStorage.Alias);
165+
189166
var asymmetricAlias = $"{alias}.asymmetric";
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191168
var privateKey = keyStore.GetKey(asymmetricAlias, null)?.JavaCast<IPrivateKey>();

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