System.Security.Cryptography.CryptoStream.WriteAsync C# (CSharp) Method

        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        {
            CheckWriteArguments(buffer, offset, count);
            return WriteAsyncInternal(buffer, offset, count, cancellationToken);
        }

 public async static Task<string> EncryptAsync(string plainText, string passPhrase)
 {
     var plainTextBytes = Encoding.UTF8.GetBytes(plainText);
     using (var password = new Rfc2898DeriveBytes(passPhrase, InitVectorBytes))
     {
         var keyBytes = password.GetBytes(Keysize / 8);
         using (var symmetricKey = new RijndaelManaged())
         {
             symmetricKey.Mode = CipherMode.CBC;
             symmetricKey.Padding = PaddingMode.PKCS7;
             using (var encryptor = symmetricKey.CreateEncryptor(keyBytes, InitVectorBytes))
             {
                 using (var memoryStream = new MemoryStream())
                 {
                     using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                     {
                         await cryptoStream.WriteAsync(plainTextBytes, 0, plainTextBytes.Length);
                         cryptoStream.FlushFinalBlock();
                         var cipherTextBytes = memoryStream.ToArray();
                         return Convert.ToBase64String(cipherTextBytes);
                     }
                 }
             }
         }
     }
 }