Migrate from Xamarin.Essentials secure storage to .NET MAUI secure storage

Xamarin.Essentials and .NET Multi-platform App UI (.NET MAUI) both have a SecureStorage class that helps you securely store simple key/value pairs. However, there are implementation differences between the SecureStorage class in Xamarin.Essentials and .NET MAUI:

For more information about the SecureStorage class in Xamarin.Essentials, see Xamarin.Essentials: Secure storage. For more information about the SecureStorage class in .NET MAUI, see Secure storage.

When migrating a Xamarin.Forms app that uses the SecureStorage class to .NET MAUI, you must deal with these implementation differences to provide users with a smooth upgrade experience. This article describes how you can use the the LegacySecureStorage class and helper classes to deal with the implementation differences. The LegacySecureStorage class enables your .NET MAUI app on Android and iOS to read secure storage data that was created with a previous Xamarin.Forms version of your app.

Access legacy secure storage data

The following code shows the LegacySecureStorage class, which provides the secure storage implementation from Xamarin.Essentials:

#nullable enable
#if ANDROID || IOS

namespace MigrationHelpers;

public class LegacySecureStorage
{
    internal static readonly string Alias = $"{AppInfo.PackageName}.xamarinessentials";

    public static Task<string> GetAsync(string key)
    {
        if (string.IsNullOrWhiteSpace(key))
            throw new ArgumentNullException(nameof(key));

        string result = string.Empty;

#if ANDROID
        object locker = new object();
        string? encVal = Preferences.Get(key, null, Alias);

        if (!string.IsNullOrEmpty(encVal))
        {
            byte[] encData = Convert.FromBase64String(encVal);
            lock (locker)
            {
                AndroidKeyStore keyStore = new AndroidKeyStore(Platform.AppContext, Alias, false);
                result = keyStore.Decrypt(encData);
            }
        }
#elif IOS
        KeyChain keyChain = new KeyChain();
        result = keyChain.ValueForKey(key, Alias);
#endif
        return Task.FromResult(result);
    }

    public static bool Remove(string key)
    {
        bool result = false;

#if ANDROID
        Preferences.Remove(key, Alias);
        result = true;
#elif IOS
        KeyChain keyChain = new KeyChain();
        result = keyChain.Remove(key, Alias);
#endif
        return result;
    }

    public static void RemoveAll()
    {
#if ANDROID
        Preferences.Clear(Alias);
#elif IOS
        KeyChain keyChain = new KeyChain();
        keyChain.RemoveAll(Alias);
#endif
    }
}
#endif

Android

On Android, the LegacySecureStorage class uses the AndroidKeyStore class to store the cipher key used to encrypt a value before it's saved into a shared preferences object with a name of {your-app-package-id}.xamarinessentials. The following code shows the AndroidKeyStore class:

using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Security;
using Android.Security.Keystore;
using Java.Security;
using Javax.Crypto;
using Javax.Crypto.Spec;
using System.Text;

namespace MigrationHelpers;

class AndroidKeyStore
{
    const string androidKeyStore = "AndroidKeyStore"; // this is an Android const value
    const string aesAlgorithm = "AES";
    const string cipherTransformationAsymmetric = "RSA/ECB/PKCS1Padding";
    const string cipherTransformationSymmetric = "AES/GCM/NoPadding";
    const string prefsMasterKey = "SecureStorageKey";
    const int initializationVectorLen = 12; // Android supports an IV of 12 for AES/GCM

    internal AndroidKeyStore(Context context, string keystoreAlias, bool alwaysUseAsymmetricKeyStorage)
    {
        alwaysUseAsymmetricKey = alwaysUseAsymmetricKeyStorage;
        appContext = context;
        alias = keystoreAlias;

        keyStore = KeyStore.GetInstance(androidKeyStore);
        keyStore.Load(null);
    }

    readonly Context appContext;
    readonly string alias;
    readonly bool alwaysUseAsymmetricKey;
    readonly string useSymmetricPreferenceKey = "essentials_use_symmetric";

    KeyStore keyStore;
    bool useSymmetric = false;

    ISecretKey GetKey()
    {
        // check to see if we need to get our key from past-versions or newer versions.
        // we want to use symmetric if we are >= 23 or we didn't set it previously.
        var hasApiLevel = Build.VERSION.SdkInt >= BuildVersionCodes.M;

        useSymmetric = Preferences.Get(useSymmetricPreferenceKey, hasApiLevel, alias);

        // If >= API 23 we can use the KeyStore's symmetric key
        if (useSymmetric && !alwaysUseAsymmetricKey)
            return GetSymmetricKey();

        // NOTE: KeyStore in < API 23 can only store asymmetric keys
        // specifically, only RSA/ECB/PKCS1Padding
        // So we will wrap our symmetric AES key we just generated
        // with this and save the encrypted/wrapped key out to
        // preferences for future use.
        // ECB should be fine in this case as the AES key should be
        // contained in one block.

        // Get the asymmetric key pair
        var keyPair = GetAsymmetricKeyPair();

        var existingKeyStr = Preferences.Get(prefsMasterKey, null, alias);

        if (!string.IsNullOrEmpty(existingKeyStr))
        {
            try
            {
                var wrappedKey = Convert.FromBase64String(existingKeyStr);

                var unwrappedKey = UnwrapKey(wrappedKey, keyPair.Private);
                var kp = unwrappedKey.JavaCast<ISecretKey>();

                return kp;
            }
            catch (InvalidKeyException ikEx)
            {
                System.Diagnostics.Debug.WriteLine($"Unable to unwrap key: Invalid Key. This may be caused by system backup or upgrades. All secure storage items will now be removed. {ikEx.Message}");
            }
            catch (IllegalBlockSizeException ibsEx)
            {
                System.Diagnostics.Debug.WriteLine($"Unable to unwrap key: Illegal Block Size. This may be caused by system backup or upgrades. All secure storage items will now be removed. {ibsEx.Message}");
            }
            catch (BadPaddingException paddingEx)
            {
                System.Diagnostics.Debug.WriteLine($"Unable to unwrap key: Bad Padding. This may be caused by system backup or upgrades. All secure storage items will now be removed. {paddingEx.Message}");
            }
            LegacySecureStorage.RemoveAll();
        }

        var keyGenerator = KeyGenerator.GetInstance(aesAlgorithm);
        var defSymmetricKey = keyGenerator.GenerateKey();

        var newWrappedKey = WrapKey(defSymmetricKey, keyPair.Public);

        Preferences.Set(prefsMasterKey, Convert.ToBase64String(newWrappedKey), alias);

        return defSymmetricKey;
    }

    // API 23+ Only
#pragma warning disable CA1416
    ISecretKey GetSymmetricKey()
    {
        Preferences.Set(useSymmetricPreferenceKey, true, alias);

        var existingKey = keyStore.GetKey(alias, null);

        if (existingKey != null)
        {
            var existingSecretKey = existingKey.JavaCast<ISecretKey>();
            return existingSecretKey;
        }

        var keyGenerator = KeyGenerator.GetInstance(KeyProperties.KeyAlgorithmAes, androidKeyStore);
        var builder = new KeyGenParameterSpec.Builder(alias, KeyStorePurpose.Encrypt | KeyStorePurpose.Decrypt)
            .SetBlockModes(KeyProperties.BlockModeGcm)
            .SetEncryptionPaddings(KeyProperties.EncryptionPaddingNone)
            .SetRandomizedEncryptionRequired(false);

        keyGenerator.Init(builder.Build());

        return keyGenerator.GenerateKey();
    }
#pragma warning restore CA1416

    KeyPair GetAsymmetricKeyPair()
    {
        // set that we generated keys on pre-m device.
        Preferences.Set(useSymmetricPreferenceKey, false, alias);

        var asymmetricAlias = $"{alias}.asymmetric";

        var privateKey = keyStore.GetKey(asymmetricAlias, null)?.JavaCast<IPrivateKey>();
        var publicKey = keyStore.GetCertificate(asymmetricAlias)?.PublicKey;

        // Return the existing key if found
        if (privateKey != null && publicKey != null)
            return new KeyPair(publicKey, privateKey);

        var originalLocale = Java.Util.Locale.Default;
        try
        {
            // Force to english for known bug in date parsing:
            // https://issuetracker.google.com/issues/37095309
            SetLocale(Java.Util.Locale.English);

            // Otherwise we create a new key
#pragma warning disable CA1416
            var generator = KeyPairGenerator.GetInstance(KeyProperties.KeyAlgorithmRsa, androidKeyStore);
#pragma warning restore CA1416

            var end = DateTime.UtcNow.AddYears(20);
            var startDate = new Java.Util.Date();
#pragma warning disable CS0618 // Type or member is obsolete
            var endDate = new Java.Util.Date(end.Year, end.Month, end.Day);
#pragma warning restore CS0618 // Type or member is obsolete

#pragma warning disable CS0618
            var builder = new KeyPairGeneratorSpec.Builder(Platform.AppContext)
                .SetAlias(asymmetricAlias)
                .SetSerialNumber(Java.Math.BigInteger.One)
                .SetSubject(new Javax.Security.Auth.X500.X500Principal($"CN={asymmetricAlias} CA Certificate"))
                .SetStartDate(startDate)
                .SetEndDate(endDate);

            generator.Initialize(builder.Build());
#pragma warning restore CS0618

            return generator.GenerateKeyPair();
        }
        finally
        {
            SetLocale(originalLocale);
        }
    }

    byte[] WrapKey(IKey keyToWrap, IKey withKey)
    {
        var cipher = Cipher.GetInstance(cipherTransformationAsymmetric);
        cipher.Init(CipherMode.WrapMode, withKey);
        return cipher.Wrap(keyToWrap);
    }

#pragma warning disable CA1416
    IKey UnwrapKey(byte[] wrappedData, IKey withKey)
    {
        var cipher = Cipher.GetInstance(cipherTransformationAsymmetric);
        cipher.Init(CipherMode.UnwrapMode, withKey);
        var unwrapped = cipher.Unwrap(wrappedData, KeyProperties.KeyAlgorithmAes, KeyType.SecretKey);
        return unwrapped;
    }
#pragma warning restore CA1416

    internal string Decrypt(byte[] data)
    {
        if (data.Length < initializationVectorLen)
            return null;

        var key = GetKey();

        // IV will be the first 16 bytes of the encrypted data
        var iv = new byte[initializationVectorLen];
        Buffer.BlockCopy(data, 0, iv, 0, initializationVectorLen);

        Cipher cipher;

        // Attempt to use GCMParameterSpec by default
        try
        {
            cipher = Cipher.GetInstance(cipherTransformationSymmetric);
            cipher.Init(CipherMode.DecryptMode, key, new GCMParameterSpec(128, iv));
        }
        catch (InvalidAlgorithmParameterException)
        {
            // If we encounter this error, it's likely an old bouncycastle provider version
            // is being used which does not recognize GCMParameterSpec, but should work
            // with IvParameterSpec, however we only do this as a last effort since other
            // implementations will error if you use IvParameterSpec when GCMParameterSpec
            // is recognized and expected.
            cipher = Cipher.GetInstance(cipherTransformationSymmetric);
            cipher.Init(CipherMode.DecryptMode, key, new IvParameterSpec(iv));
        }

        // Decrypt starting after the first 16 bytes from the IV
        var decryptedData = cipher.DoFinal(data, initializationVectorLen, data.Length - initializationVectorLen);

        return Encoding.UTF8.GetString(decryptedData);
    }

    internal void SetLocale(Java.Util.Locale locale)
    {
        Java.Util.Locale.Default = locale;
        var resources = appContext.Resources;
        var config = resources.Configuration;

        if (Build.VERSION.SdkInt >= BuildVersionCodes.N)
            config.SetLocale(locale);
        else
#pragma warning disable CS0618 // Type or member is obsolete
            config.Locale = locale;
#pragma warning restore CS0618 // Type or member is obsolete

#pragma warning disable CS0618 // Type or member is obsolete
        resources.UpdateConfiguration(config, resources.DisplayMetrics);
#pragma warning restore CS0618 // Type or member is obsolete
    }
}

The Android KeyStore is used to store the cipher key used to encrypt the value before it's saved into a Shared Preferencesfile with a name of {your-app-package-id}.xamarinessentials. The key (not a cryptographic key, the key to the value) used in the shared preferences file is an MD5 Hash of the key passed into the SecureStorage APIs.

On API 23+, an AES key is obtained from the Android KeyStore and used with an AES/GCM/NoPadding cipher to encrypt the value before it's stored in the shared preferences file. On API 22 and lower, the Android KeyStore only supports storing RSA keys, which is used with an RSA/ECB/PKCS1Padding cipher to encrypt an AES key (randomly generated at runtime) and stored in the shared preferences file under the key SecureStorageKey, if one hasn't already been generated.

iOS

On iOS, the LegacySecureStorage class uses the KeyChain class to store values securely. The SecRecord used to store values has a Service value set to {your-app-package-id}.xamarinessentials. The following code shows the KeyChain class:

using Foundation;
using Security;

namespace MigrationHelpers;

class KeyChain
{
    SecRecord ExistingRecordForKey(string key, string service)
    {
        return new SecRecord(SecKind.GenericPassword)
        {
            Account = key,
            Service = service
        };
    }

    internal string ValueForKey(string key, string service)
    {
        using (var record = ExistingRecordForKey(key, service))
        using (var match = SecKeyChain.QueryAsRecord(record, out var resultCode))
        {
            if (resultCode == SecStatusCode.Success)
                return NSString.FromData(match.ValueData, NSStringEncoding.UTF8);
            else
                return null;
        }
    }

    internal bool Remove(string key, string service)
    {
        using (var record = ExistingRecordForKey(key, service))
        using (var match = SecKeyChain.QueryAsRecord(record, out var resultCode))
        {
            if (resultCode == SecStatusCode.Success)
            {
                RemoveRecord(record);
                return true;
            }
        }
        return false;
    }

    internal void RemoveAll(string service)
    {
        using (var query = new SecRecord(SecKind.GenericPassword) { Service = service })
        {
            SecKeyChain.Remove(query);
        }
    }

    bool RemoveRecord(SecRecord record)
    {
        var result = SecKeyChain.Remove(record);
        if (result != SecStatusCode.Success && result != SecStatusCode.ItemNotFound)
            throw new Exception($"Error removing record: {result}");

        return true;
    }
}

To use this code, you must have an Entitlements.plist file for your iOS app with the Keychain entitlement set:

<key>keychain-access-groups</key>
<array>
  <string>$(AppIdentifierPrefix)$(CFBundleIdentifier)</string>
</array>

You must also ensure that the Entitlements.plist file is set as the Custom Entitlements field in the Bundle Signing settings for your app. For more information, see iOS Entitlements.

Consume legacy secure storage data

The LegacySecureStorage class can be used to consume legacy secure storage data, on Android and iOS, that was created with a previous Xamarin.Forms version of your app:

#if ANDROID || IOS
using MigrationHelpers;
...

string username = await LegacySecureStorage.GetAsync("username");
bool result = LegacySecureStorage.Remove("username");
await SecureStorage.SetAsync("username", username);
#endif

The example shows using the LegacySecureStorage class to read and remove a value from legacy secure storage, and then write the value to .NET MAUI secure storage.