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

        public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        {
            CheckReadArguments(buffer, offset, count);
            return ReadAsyncInternal(buffer, offset, count, cancellationToken);
        }

 public async static Task<string> DecryptAsync(string cipherText, string passPhrase)
 {
     var cipherTextBytes = Convert.FromBase64String(cipherText);
     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 decryptor = symmetricKey.CreateDecryptor(keyBytes, InitVectorBytes))
             {
                 using (var memoryStream = new MemoryStream(cipherTextBytes))
                 {
                     using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                     {
                         var plainTextBytes = new byte[cipherTextBytes.Length];
                         var decryptedByteCount = await cryptoStream.ReadAsync(plainTextBytes, 0, plainTextBytes.Length);
                         return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
                     }
                 }
             }
         }
     }
 }