bitwarden-android/src/Android/Services/KeyStoreBackedStorageService.cs

using System.IO;
using Java.Security;
using Javax.Crypto;
using Android.OS;
using Bit.App.Abstractions;
using System;
using Android.Security;
using Javax.Security.Auth.X500;
using Java.Math;
using Android.Security.Keystore;
using Android.App;
using Plugin.Settings.Abstractions;
using Java.Util;
using Javax.Crypto.Spec;

namespace Bit.Android.Services
{
    public class KeyStoreBackedStorageService : ISecureStorageService
    {
        private const string AndroidKeyStore = "AndroidKeyStore";
        private const string KeyAlias = "bitwardenKey";
        private const string SettingsFormat = "ksSecured:{0}";
        private const string AesKey = "ksSecured:aesKeyForService";

        private readonly string _rsaMode;
        private readonly bool _oldAndroid;
        private readonly ISettings _settings;
        private readonly KeyStore _keyStore;
        private readonly ISecureStorageService _oldKeyStorageService;

        public KeyStoreBackedStorageService(ISettings settings)
        {
            _oldAndroid = Build.VERSION.SdkInt < BuildVersionCodes.M;
            _rsaMode = _oldAndroid ? "RSA/ECB/PKCS1Padding" : "RSA/ECB/OAEPWithSHA-1AndMGF1Padding";

            _oldKeyStorageService = new KeyStoreStorageService(new char[] { });
            _settings = settings;

            _keyStore = KeyStore.GetInstance(AndroidKeyStore);
            _keyStore.Load(null);

            GenerateRsaKey();
            GenerateAesKey();
        }

        public bool Contains(string key)
        {
            return _settings.Contains(string.Format(SettingsFormat, key)) || _oldKeyStorageService.Contains(key);
        }

        public void Delete(string key)
        {
            CleanupOldKeyStore(key);
            _settings.Remove(string.Format(SettingsFormat, key));
        }

        public byte[] Retrieve(string key)
        {
            var formattedKey = string.Format(SettingsFormat, key);
            if(!_settings.Contains(formattedKey))
            {
                return TryGetAndMigrateFromOldKeyStore(key);
            }

            var cs = _settings.GetValueOrDefault<string>(formattedKey);
            if(string.IsNullOrWhiteSpace(cs))
            {
                return null;
            }

            var aesKey = GetAesKey();
            if(aesKey == null)
            {
                return null;
            }

            try
            {
                return App.Utilities.Crypto.AesCbcDecrypt(new App.Models.CipherString(cs), aesKey);
            }
            catch
            {
                Console.WriteLine("Failed to decrypt from secure storage.");
                _settings.Remove(formattedKey);
                return null;
            }
        }

        public void Store(string key, byte[] dataBytes)
        {
            var formattedKey = string.Format(SettingsFormat, key);
            CleanupOldKeyStore(key);
            if(dataBytes == null)
            {
                _settings.Remove(formattedKey);
                return;
            }

            var aesKey = GetAesKey();
            if(aesKey == null)
            {
                return;
            }

            try
            {
                var cipherString = App.Utilities.Crypto.AesCbcEncrypt(dataBytes, aesKey);
                _settings.AddOrUpdateValue(formattedKey, cipherString.EncryptedString);
            }
            catch
            {
                Console.WriteLine("Failed to encrypt to secure storage.");
            }
        }

        private void GenerateRsaKey()
        {
            if(_keyStore.ContainsAlias(KeyAlias))
            {
                return;
            }

            var gen = KeyPairGenerator.GetInstance(KeyProperties.KeyAlgorithmRsa, AndroidKeyStore);
            var start = Calendar.Instance;
            var end = Calendar.Instance;
            end.Add(CalendarField.Year, 30);
            var subject = new X500Principal($"CN={KeyAlias}");

            if(_oldAndroid)
            {
                var spec = new KeyPairGeneratorSpec.Builder(Application.Context)
                    .SetAlias(KeyAlias)
                    .SetSubject(subject)
                    .SetSerialNumber(BigInteger.Ten)
                    .SetStartDate(start.Time)
                    .SetEndDate(end.Time)
                    .Build();

                gen.Initialize(spec);
            }
            else
            {
                var spec = new KeyGenParameterSpec.Builder(KeyAlias, KeyStorePurpose.Decrypt | KeyStorePurpose.Encrypt)
                    .SetCertificateSubject(subject)
                    .SetDigests(KeyProperties.DigestSha1)
                    .SetEncryptionPaddings(KeyProperties.EncryptionPaddingRsaOaep)
                    .Build();

                gen.Initialize(spec);
            }

            gen.GenerateKeyPair();
        }

        private KeyStore.PrivateKeyEntry GetRsaKeyEntry()
        {
            return _keyStore.GetEntry(KeyAlias, null) as KeyStore.PrivateKeyEntry;
        }

        private void GenerateAesKey()
        {
            if(_settings.Contains(AesKey))
            {
                return;
            }

            var key = App.Utilities.Crypto.RandomBytes(512 / 8);
            var encKey = RsaEncrypt(key);
            _settings.AddOrUpdateValue(AesKey, Convert.ToBase64String(encKey));
        }

        private App.Models.SymmetricCryptoKey GetAesKey()
        {
            try
            {
                var encKey = _settings.GetValueOrDefault<string>(AesKey);
                if(encKey == null)
                {
                    return null;
                }

                var encKeyBytes = Convert.FromBase64String(encKey);
                var key = RsaDecrypt(encKeyBytes);
                return new App.Models.SymmetricCryptoKey(key);
            }
            catch
            {
                Console.WriteLine("Cannot get AesKey.");
                _keyStore.DeleteEntry(KeyAlias);
                _settings.Remove(AesKey);
                return null;
            }
        }

        private byte[] RsaEncrypt(byte[] data)
        {
            using(var entry = GetRsaKeyEntry())
            using(var cipher = Cipher.GetInstance(_rsaMode))
            {
                cipher.Init(CipherMode.EncryptMode, entry.Certificate.PublicKey);
                var cipherText = cipher.DoFinal(data);
                return cipherText;
            }
        }

        private byte[] RsaDecrypt(byte[] encData)
        {
            using(var entry = GetRsaKeyEntry())
            using(var cipher = Cipher.GetInstance(_rsaMode))
            {
                cipher.Init(CipherMode.DecryptMode, entry.PrivateKey, OAEPParameterSpec.Default);
                var plainText = cipher.DoFinal(encData);
                return plainText;
            }
        }

        private byte[] TryGetAndMigrateFromOldKeyStore(string key)
        {
            if(_oldKeyStorageService.Contains(key))
            {
                var value = _oldKeyStorageService.Retrieve(key);
                Store(key, value);
                _oldKeyStorageService.Delete(key);
                return value;
            }

            return null;
        }

        private void CleanupOldKeyStore(string key)
        {
            if(_oldKeyStorageService.Contains(key))
            {
                _oldKeyStorageService.Delete(key);
            }
        }
    }
}