crypto service implementation

src/Core/Abstractions/ICryptoFunctionService.cs

@@ -21,5 +21,6 @@ namespace Bit.Core.Abstractions
         Task<byte[]> RsaExtractPublicKeyAsync(byte[] privateKey);
         Task<Tuple<byte[], byte[]>> RsaGenerateKeyPairAsync(int length);
         Task<byte[]> RandomBytesAsync(int length);
+        Task<uint> RandomNumberAsync();
     }
 }

src/Core/Constants.cs

@@ -3,5 +3,6 @@
     public static class Constants
     {
         public static string LockOptionKey = "lockOption";
+        public static string PinProtectedKey = "pinProtectedKey";
     }
 }

src/Core/Enums/KdfType.cs

@@ -0,0 +1,7 @@
+namespace Bit.Core.Enums
+{
+    public enum KdfType : byte
+    {
+        PBKDF2_SHA256 = 0
+    }
+}

src/Core/Models/Domain/CipherString.cs

@@ -7,7 +7,7 @@ namespace Bit.Core.Models.Domain
     {
         private string _decryptedValue;
 
-        public CipherString(EncryptionType encryptionType, string data, string iv, string mac)
+        public CipherString(EncryptionType encryptionType, string data, string iv = null, string mac = null)
         {
             if(string.IsNullOrWhiteSpace(data))
             {

src/Core/Services/CryptoService.cs

@@ -5,6 +5,7 @@ using Bit.Core.Models.Response;
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Text;
 using System.Threading.Tasks;
 
 namespace Bit.Core.Services
@@ -134,11 +135,12 @@ namespace Bit.Core.Services
             var encKeyCipher = new CipherString(encKey);
             if(encKeyCipher.EncryptionType == EncryptionType.AesCbc256_B64)
             {
-                // TODO
+                decEncKey = await DecryptToBytesAsync(encKeyCipher, key);
             }
             else if(encKeyCipher.EncryptionType == EncryptionType.AesCbc256_HmacSha256_B64)
             {
-                // TODO
+                var newKey = await StretchKeyAsync(key);
+                decEncKey = await DecryptToBytesAsync(encKeyCipher, newKey);
             }
             else
             {
@@ -179,7 +181,7 @@ namespace Bit.Core.Services
             {
                 return null;
             }
-            // TODO
+            _privateKey = await DecryptToBytesAsync(new CipherString(encPrivateKey), null);
             return _privateKey;
         }
 
@@ -194,9 +196,616 @@ namespace Bit.Core.Services
                 throw new Exception("No public key available.");
             }
             var keyFingerprint = await _cryptoFunctionService.HashAsync(publicKey, CryptoHashAlgorithm.Sha256);
-            // TODO
+            var userFingerprint = await HkdfExpandAsync(keyFingerprint, Encoding.UTF8.GetBytes(userId), 32);
+            return HashPhrase(userFingerprint);
+        }
+
+        public async Task<Dictionary<string, SymmetricCryptoKey>> GetOrgKeysAsync()
+        {
+            if(_orgKeys != null && _orgKeys.Count > 0)
+            {
+                return _orgKeys;
+            }
+            var encOrgKeys = await _storageService.GetAsync<Dictionary<string, string>>(Keys_EncOrgKeys);
+            if(encOrgKeys == null)
+            {
+                return null;
+            }
+            var orgKeys = new Dictionary<string, SymmetricCryptoKey>();
+            var setKey = false;
+            foreach(var org in encOrgKeys)
+            {
+                var decValue = await RsaDecryptAsync(org.Value);
+                orgKeys.Add(org.Key, new SymmetricCryptoKey(decValue));
+                setKey = true;
+            }
+
+            if(setKey)
+            {
+                _orgKeys = orgKeys;
+            }
+            return _orgKeys;
+        }
+
+        public async Task<SymmetricCryptoKey> GetOrgKeyAsync(string orgId)
+        {
+            if(string.IsNullOrWhiteSpace(orgId))
+            {
+                return null;
+            }
+            var orgKeys = await GetOrgKeysAsync();
+            if(orgKeys == null || !orgKeys.ContainsKey(orgId))
+            {
+                return null;
+            }
+            return orgKeys[orgId];
+        }
+
+        public async Task<bool> HasKeyAsync()
+        {
+            var key = await GetKeyAsync();
+            return key != null;
+        }
+
+        public async Task<bool> HasEncKeyAsync()
+        {
+            var encKey = await _storageService.GetAsync<string>(Keys_EncKey);
+            return encKey != null;
+        }
+
+        public async Task ClearKeyAsync()
+        {
+            _key = _legacyEtmKey = null;
+            await _secureStorageService.RemoveAsync(Keys_Key);
+        }
+
+        public async Task ClearKeyHashAsync()
+        {
+            _keyHash = null;
+            await _storageService.RemoveAsync(Keys_KeyHash);
+        }
+
+        public async Task ClearEncKeyAsync(bool memoryOnly = false)
+        {
+            _encKey = null;
+            if(!memoryOnly)
+            {
+                await _storageService.RemoveAsync(Keys_EncKey);
+            }
+        }
+
+        public async Task ClearKeyPairAsync(bool memoryOnly = false)
+        {
+            _publicKey = _privateKey = null;
+            if(!memoryOnly)
+            {
+                await _storageService.RemoveAsync(Keys_EncPrivateKey);
+            }
+        }
+
+        public async Task ClearOrgKeysAsync(bool memoryOnly = false)
+        {
+            _orgKeys = null;
+            if(!memoryOnly)
+            {
+                await _storageService.RemoveAsync(Keys_EncOrgKeys);
+            }
+        }
+
+        public async Task ClearPinProtectedKeyAsync()
+        {
+            await _storageService.RemoveAsync(Constants.PinProtectedKey);
+        }
+
+        public async Task ClearKeysAsync()
+        {
+            await Task.WhenAll(new Task[]
+            {
+                ClearKeyAsync(),
+                ClearKeyHashAsync(),
+                ClearOrgKeysAsync(),
+                ClearEncKeyAsync(),
+                ClearKeyPairAsync(),
+                ClearPinProtectedKeyAsync()
+            });
+        }
+
+        public async Task ToggleKeyAsync()
+        {
+            var key = await GetKeyAsync();
+            var option = await _storageService.GetAsync<int?>(Constants.LockOptionKey);
+            if(option != null || option == 0)
+            {
+                await ClearKeyAsync();
+                _key = key;
+                return;
+            }
+            await SetKeyAsync(key);
+        }
+
+        public async Task<SymmetricCryptoKey> MakeKeyAsync(string password, string salt,
+            KdfType? kdf, int? kdfIterations)
+        {
+            byte[] key = null;
+            if(kdf == null || kdf == KdfType.PBKDF2_SHA256)
+            {
+                if(kdfIterations == null)
+                {
+                    kdfIterations = 5000;
+                }
+                if(kdfIterations < 5000)
+                {
+                    throw new Exception("PBKDF2 iteration minimum is 5000.");
+                }
+                key = await _cryptoFunctionService.Pbkdf2Async(password, salt,
+                    CryptoHashAlgorithm.Sha256, kdfIterations.Value);
+            }
+            else
+            {
+                throw new Exception("Unknown kdf.");
+            }
+            return new SymmetricCryptoKey(key);
+        }
+
+        public async Task<SymmetricCryptoKey> MakeKeyFromPinAsync(string pin, string salt,
+            KdfType kdf, int kdfIterations)
+        {
+            var pinProtectedKey = await _storageService.GetAsync<string>(Constants.PinProtectedKey);
+            if(pinProtectedKey == null)
+            {
+                throw new Exception("No PIN protected key found.");
+            }
+            var protectedKeyCs = new CipherString(pinProtectedKey);
+            var pinKey = await MakePinKeyAysnc(pin, salt, kdf, kdfIterations);
+            var decKey = await DecryptToBytesAsync(protectedKeyCs, pinKey);
+            return new SymmetricCryptoKey(decKey);
+        }
+
+        public async Task<Tuple<CipherString, SymmetricCryptoKey>> MakeShareKeyAsync()
+        {
+            var shareKey = await _cryptoFunctionService.RandomBytesAsync(64);
+            var publicKey = await GetPublicKeyAsync();
+            var encShareKey = await RsaEncryptAsync(shareKey, publicKey);
+            return new Tuple<CipherString, SymmetricCryptoKey>(encShareKey, new SymmetricCryptoKey(shareKey));
+        }
+
+        public async Task<Tuple<string, CipherString>> MakeKeyPairAsync(SymmetricCryptoKey key = null)
+        {
+            var keyPair = await _cryptoFunctionService.RsaGenerateKeyPairAsync(2048);
+            var publicB64 = Convert.ToBase64String(keyPair.Item1);
+            var privateEnc = await EncryptAsync(keyPair.Item2, key);
+            return new Tuple<string, CipherString>(publicB64, privateEnc);
+        }
+
+        public async Task<SymmetricCryptoKey> MakePinKeyAysnc(string pin, string salt, KdfType kdf, int kdfIterations)
+        {
+            var pinKey = await MakeKeyAsync(pin, salt, kdf, kdfIterations);
+            return await StretchKeyAsync(pinKey);
+        }
+
+        public async Task<string> HashPasswordAsync(string password, SymmetricCryptoKey key)
+        {
+            if(key == null)
+            {
+                key = await GetKeyAsync();
+            }
+            if(password == null || key == null)
+            {
+                throw new Exception("Invalid parameters.");
+            }
+            var hash = await _cryptoFunctionService.Pbkdf2Async(key.Key, password, CryptoHashAlgorithm.Sha256, 1);
+            return Convert.ToBase64String(hash);
+        }
+
+        public async Task<Tuple<SymmetricCryptoKey, CipherString>> MakeEncKeyAsync(SymmetricCryptoKey key)
+        {
+            var theKey = await GetKeyForEncryptionAsync(key);
+            var encKey = await _cryptoFunctionService.RandomBytesAsync(64);
+            return await BuildEncKeyAsync(theKey, encKey);
+        }
+
+        public async Task<Tuple<SymmetricCryptoKey, CipherString>> RemakeEncKeyAsync(SymmetricCryptoKey key)
+        {
+            var encKey = await GetEncKeyAsync();
+            return await BuildEncKeyAsync(key, encKey.Key);
+        }
+
+        public async Task<CipherString> EncryptAsync(string plainValue, SymmetricCryptoKey key = null)
+        {
+            if(plainValue == null)
+            {
+                return null;
+            }
+            return await EncryptAsync(Encoding.UTF8.GetBytes(plainValue), key);
+        }
+
+        public async Task<CipherString> EncryptAsync(byte[] plainValue, SymmetricCryptoKey key = null)
+        {
+            if(plainValue == null)
+            {
+                return null;
+            }
+            var encObj = await AesEncryptAsync(plainValue, key);
+            var iv = Convert.ToBase64String(encObj.Iv);
+            var data = Convert.ToBase64String(encObj.Data);
+            var mac = encObj.Mac != null ? Convert.ToBase64String(encObj.Mac) : null;
+            return new CipherString(encObj.Key.EncType, data, iv, mac);
+        }
+
+        public async Task<byte[]> EncryptToBytesAsync(byte[] plainValue, SymmetricCryptoKey key = null)
+        {
+            var encValue = await AesEncryptAsync(plainValue, key);
+            var macLen = 0;
+            if(encValue.Mac != null)
+            {
+                macLen = encValue.Mac.Length;
+            }
+            var encBytes = new byte[1 + encValue.Iv.Length + macLen + encValue.Data.Length];
+            Buffer.BlockCopy(new byte[] { (byte)encValue.Key.EncType }, 0, encBytes, 0, 1);
+            Buffer.BlockCopy(encValue.Iv, 0, encBytes, 1, encValue.Iv.Length);
+            if(encValue.Mac != null)
+            {
+                Buffer.BlockCopy(encValue.Mac, 0, encBytes, 1 + encValue.Iv.Length, encValue.Mac.Length);
+            }
+            Buffer.BlockCopy(encValue.Data, 0, encBytes, 1 + encValue.Iv.Length + macLen, encValue.Data.Length);
+            return encBytes;
+        }
+
+        public async Task<CipherString> RsaEncryptAsync(byte[] data, byte[] publicKey = null)
+        {
+            if(publicKey == null)
+            {
+                publicKey = await GetPublicKeyAsync();
+            }
+            if(publicKey == null)
+            {
+                throw new Exception("Public key unavailable.");
+            }
+            var encBytes = await _cryptoFunctionService.RsaEncryptAsync(data, publicKey, CryptoHashAlgorithm.Sha1);
+            return new CipherString(EncryptionType.Rsa2048_OaepSha1_B64, Convert.ToBase64String(encBytes));
+        }
+
+        public async Task<byte[]> DecryptToBytesAsync(CipherString cipherString, SymmetricCryptoKey key = null)
+        {
+            var iv = Convert.FromBase64String(cipherString.Iv);
+            var data = Convert.FromBase64String(cipherString.Data);
+            var mac = !string.IsNullOrWhiteSpace(cipherString.Mac) ? Convert.FromBase64String(cipherString.Mac) : null;
+            return await AesDecryptToBytesAsync(cipherString.EncryptionType, data, iv, mac, key);
+        }
+
+        public async Task<string> DecryptToUtf8Async(CipherString cipherString, SymmetricCryptoKey key = null)
+        {
+            return await AesDecryptToUtf8Async(cipherString.EncryptionType, cipherString.Data,
+                cipherString.Iv, cipherString.Mac, key);
+        }
+
+        public async Task<byte[]> DecryptFromBytesAsync(byte[] encBytes, SymmetricCryptoKey key)
+        {
+            if(encBytes == null)
+            {
+                throw new Exception("no encBytes.");
+            }
+
+            var encType = (EncryptionType)encBytes[0];
+            byte[] ctBytes = null;
+            byte[] ivBytes = null;
+            byte[] macBytes = null;
+
+            switch(encType)
+            {
+                case EncryptionType.AesCbc128_HmacSha256_B64:
+                case EncryptionType.AesCbc256_HmacSha256_B64:
+                    if(encBytes.Length < 49) // 1 + 16 + 32 + ctLength
+                    {
+                        return null;
+                    }
+                    ivBytes = new ArraySegment<byte>(encBytes, 1, 16).ToArray();
+                    macBytes = new ArraySegment<byte>(encBytes, 17, 32).ToArray();
+                    ctBytes = new ArraySegment<byte>(encBytes, 49, encBytes.Length - 49).ToArray();
+                    break;
+                case EncryptionType.AesCbc256_B64:
+                    if(encBytes.Length < 17) // 1 + 16 + ctLength
+                    {
+                        return null;
+                    }
+                    ivBytes = new ArraySegment<byte>(encBytes, 1, 16).ToArray();
+                    ctBytes = new ArraySegment<byte>(encBytes, 17, encBytes.Length - 17).ToArray();
+                    break;
+                default:
                     return null;
             }
 
+            return await AesDecryptToBytesAsync(encType, ctBytes, ivBytes, macBytes, key);
+        }
+
+        public async Task<int> RandomNumberAsync(int min, int max)
+        {
+            // Make max inclusive
+            max = max + 1;
+
+            var diff = (long)max - min;
+            var upperBound = uint.MaxValue / diff * diff;
+            uint ui;
+            do
+            {
+                ui = await _cryptoFunctionService.RandomNumberAsync();
+            } while(ui >= upperBound);
+            return (int)(min + (ui % diff));
+        }
+
+        // Helpers
+
+        private async Task<EncryptedObject> AesEncryptAsync(byte[] data, SymmetricCryptoKey key)
+        {
+            var obj = new EncryptedObject
+            {
+                Key = await GetKeyForEncryptionAsync(key),
+                Iv = await _cryptoFunctionService.RandomBytesAsync(16)
+            };
+            obj.Data = await _cryptoFunctionService.AesEncryptAsync(data, obj.Iv, obj.Key.EncKey);
+            if(obj.Key.MacKey != null)
+            {
+                var macData = new byte[obj.Iv.Length + obj.Data.Length];
+                Buffer.BlockCopy(obj.Iv, 0, macData, 0, obj.Iv.Length);
+                Buffer.BlockCopy(obj.Data, 0, macData, obj.Iv.Length, obj.Data.Length);
+                obj.Mac = await _cryptoFunctionService.HmacAsync(macData, obj.Key.MacKey, CryptoHashAlgorithm.Sha256);
+            }
+            return obj;
+        }
+
+        private async Task<string> AesDecryptToUtf8Async(EncryptionType encType, string data, string iv, string mac,
+            SymmetricCryptoKey key)
+        {
+            var keyForEnc = await GetKeyForEncryptionAsync(key);
+            var theKey = ResolveLegacyKey(encType, keyForEnc);
+            if(theKey.MacKey != null && mac == null)
+            {
+                // Mac required.
+                return null;
+            }
+            if(theKey.EncType != encType)
+            {
+                // encType unavailable.
+                return null;
+            }
+
+            // "Fast params" conversion
+            var encKey = theKey.EncKey;
+            var dataBytes = Convert.FromBase64String(data);
+            var ivBytes = Convert.FromBase64String(iv);
+
+            var macDataBytes = new byte[ivBytes.Length + dataBytes.Length];
+            Buffer.BlockCopy(ivBytes, 0, macDataBytes, 0, ivBytes.Length);
+            Buffer.BlockCopy(dataBytes, 0, macDataBytes, ivBytes.Length, dataBytes.Length);
+
+            byte[] macKey = null;
+            if(key.MacKey != null)
+            {
+                macKey = key.MacKey;
+            }
+            byte[] macBytes = null;
+            if(mac != null)
+            {
+                macBytes = Convert.FromBase64String(mac);
+            }
+
+            // Compute mac
+            if(macKey != null && macBytes != null)
+            {
+                var computedMac = await _cryptoFunctionService.HmacAsync(macDataBytes, macKey,
+                    CryptoHashAlgorithm.Sha256);
+                var macsEqual = await _cryptoFunctionService.CompareAsync(macBytes, computedMac);
+                if(!macsEqual)
+                {
+                    // Mac failed
+                    return null;
+                }
+            }
+
+            var decBytes = await _cryptoFunctionService.AesDecryptAsync(dataBytes, ivBytes, encKey);
+            return Encoding.UTF8.GetString(decBytes);
+        }
+
+        private async Task<byte[]> AesDecryptToBytesAsync(EncryptionType encType, byte[] data, byte[] iv, byte[] mac,
+            SymmetricCryptoKey key)
+        {
+
+            var keyForEnc = await GetKeyForEncryptionAsync(key);
+            var theKey = ResolveLegacyKey(encType, keyForEnc);
+            if(theKey.MacKey != null && mac == null)
+            {
+                // Mac required.
+                return null;
+            }
+            if(theKey.EncType != encType)
+            {
+                // encType unavailable.
+                return null;
+            }
+
+            // Compute mac
+            if(theKey.MacKey != null && mac != null)
+            {
+                var macData = new byte[iv.Length + data.Length];
+                Buffer.BlockCopy(iv, 0, macData, 0, iv.Length);
+                Buffer.BlockCopy(data, 0, macData, iv.Length, data.Length);
+
+                var computedMac = await _cryptoFunctionService.HmacAsync(macData, theKey.MacKey,
+                    CryptoHashAlgorithm.Sha256);
+                if(computedMac == null)
+                {
+                    return null;
+                }
+                var macsMatch = await _cryptoFunctionService.CompareAsync(mac, computedMac);
+                if(!macsMatch)
+                {
+                    // Mac failed
+                    return null;
+                }
+            }
+
+            return await _cryptoFunctionService.AesDecryptAsync(data, iv, theKey.EncKey);
+        }
+
+        private async Task<byte[]> RsaDecryptAsync(string encValue)
+        {
+            var headerPieces = encValue.Split('.');
+            EncryptionType? encType = null;
+            string[] encPieces = null;
+
+            if(headerPieces.Length == 1)
+            {
+                encType = EncryptionType.Rsa2048_OaepSha256_B64;
+                encPieces = new string[] { headerPieces[0] };
+            }
+            else if(headerPieces.Length == 2 && Enum.TryParse(headerPieces[0], out EncryptionType type))
+            {
+                encType = type;
+                encPieces = headerPieces[1].Split('|');
+            }
+
+            if(!encType.HasValue)
+            {
+                throw new Exception("encType unavailable.");
+            }
+            if(encPieces == null || encPieces.Length == 0)
+            {
+                throw new Exception("encPieces unavailable.");
+            }
+
+            var data = Convert.FromBase64String(encPieces[0]);
+            var privateKey = await GetPrivateKeyAsync();
+            if(privateKey == null)
+            {
+                throw new Exception("No private key.");
+            }
+
+            var alg = CryptoHashAlgorithm.Sha1;
+            switch(encType.Value)
+            {
+                case EncryptionType.Rsa2048_OaepSha256_B64:
+                case EncryptionType.Rsa2048_OaepSha256_HmacSha256_B64:
+                    alg = CryptoHashAlgorithm.Sha256;
+                    break;
+                case EncryptionType.Rsa2048_OaepSha1_B64:
+                case EncryptionType.Rsa2048_OaepSha1_HmacSha256_B64:
+                    break;
+                default:
+                    throw new Exception("encType unavailable.");
+            }
+
+            return await _cryptoFunctionService.RsaDecryptAsync(data, privateKey, alg);
+        }
+
+        private async Task<SymmetricCryptoKey> GetKeyForEncryptionAsync(SymmetricCryptoKey key = null)
+        {
+            if(key != null)
+            {
+                return key;
+            }
+            var encKey = await GetEncKeyAsync();
+            if(encKey != null)
+            {
+                return encKey;
+            }
+            return await GetKeyAsync();
+        }
+
+        private SymmetricCryptoKey ResolveLegacyKey(EncryptionType encKey, SymmetricCryptoKey key)
+        {
+            if(encKey == EncryptionType.AesCbc128_HmacSha256_B64 && key.EncType == EncryptionType.AesCbc256_B64)
+            {
+                // Old encrypt-then-mac scheme, make a new key
+                if(_legacyEtmKey == null)
+                {
+                    _legacyEtmKey = new SymmetricCryptoKey(key.Key, EncryptionType.AesCbc128_HmacSha256_B64);
+                }
+                return _legacyEtmKey;
+            }
+            return key;
+        }
+
+        private async Task<SymmetricCryptoKey> StretchKeyAsync(SymmetricCryptoKey key)
+        {
+            var newKey = new byte[64];
+            var enc = await HkdfExpandAsync(key.Key, Encoding.UTF8.GetBytes("enc"), 32);
+            Buffer.BlockCopy(enc, 0, newKey, 0, 32);
+            var mac = await HkdfExpandAsync(key.Key, Encoding.UTF8.GetBytes("mac"), 32);
+            Buffer.BlockCopy(mac, 0, newKey, 32, 32);
+            return new SymmetricCryptoKey(newKey);
+        }
+
+        // ref: https://tools.ietf.org/html/rfc5869
+        private async Task<byte[]> HkdfExpandAsync(byte[] prk, byte[] info, int size)
+        {
+            var hashLen = 32; // sha256
+            var okm = new byte[size];
+            var previousT = new byte[0];
+            var n = (int)Math.Ceiling((double)size / hashLen);
+            for(int i = 0; i < n; i++)
+            {
+                var t = new byte[previousT.Length + info.Length + 1];
+                previousT.CopyTo(t, 0);
+                info.CopyTo(t, previousT.Length);
+                t[t.Length - 1] = (byte)(i + 1);
+                previousT = await _cryptoFunctionService.HmacAsync(t, prk, CryptoHashAlgorithm.Sha256);
+                previousT.CopyTo(okm, i * hashLen);
+            }
+            return okm;
+        }
+
+        private List<string> HashPhrase(byte[] hash, int minimumEntropy = 64)
+        {
+            // TODO: word list
+            var EEFLongWordList = new string[] { };
+
+            var entropyPerWord = Math.Log(EEFLongWordList.Length) / Math.Log(2);
+            var numWords = (int)Math.Ceiling(minimumEntropy / entropyPerWord);
+
+            var entropyAvailable = hash.Length * 4;
+            if(numWords * entropyPerWord > entropyAvailable)
+            {
+                throw new Exception("Output entropy of hash function is too small");
+            }
+
+            var phrase = new List<string>();
+            // TODO: big int from array
+            var hashNumber = 123; // bigInt.fromArray(hash, 256);
+            while(numWords-- > 0)
+            {
+                // var remainder = hashNumber.mod(EEFLongWordList.Length);
+                // hashNumber = hashNumber.divide(EEFLongWordList.Length);
+                // phrase.Add(EEFLongWordList[remainder]);
+            }
+            return phrase;
+        }
+
+        private async Task<Tuple<SymmetricCryptoKey, CipherString>> BuildEncKeyAsync(SymmetricCryptoKey key,
+            byte[] encKey)
+        {
+            CipherString encKeyEnc = null;
+            if(key.Key.Length == 32)
+            {
+                var newKey = await StretchKeyAsync(key);
+                encKeyEnc = await EncryptAsync(encKey, newKey);
+            }
+            else if(key.Key.Length == 64)
+            {
+                encKeyEnc = await EncryptAsync(encKey, key);
+            }
+            else
+            {
+                throw new Exception("Invalid key size.");
+            }
+            return new Tuple<SymmetricCryptoKey, CipherString>(new SymmetricCryptoKey(encKey), encKeyEnc);
+        }
+
+        private class EncryptedObject
+        {
+            public byte[] Iv { get; set; }
+            public byte[] Data { get; set; }
+            public byte[] Mac { get; set; }
+            public SymmetricCryptoKey Key { get; set; }
+        }
     }
 }

src/Core/Services/PclCryptoFunctionService.cs

@@ -142,6 +142,11 @@ namespace Bit.Core.Services
             return Task.FromResult(CryptographicBuffer.GenerateRandom(length));
         }
 
+        public Task<uint> RandomNumberAsync()
+        {
+            return Task.FromResult(CryptographicBuffer.GenerateRandomNumber());
+        }
+
         private HashAlgorithm ToHashAlgorithm(CryptoHashAlgorithm algorithm)
         {
             switch(algorithm)