Question

I am using the following:

 RijndaelManaged rm = new RijndaelManaged();
 encryptor = rm.CreateEncryptor(key, vector);
 decryptor = rm.CreateDecryptor(key, vector);
 rm.Padding = PaddingMode.None;

This works good for me. However when I try to intentionally use an incorrect key I get an exception when decoding here:

public byte[] Decrypt(byte[] buffer) {
 MemoryStream decryptStream = new MemoryStream();
 using (CryptoStream cs = new CryptoStream(decryptStream, decryptor, CryptoStreamMode.Write)) {
  cs.Write(buffer, 0, buffer.Length);
 }
 return decryptStream.ToArray();
}

System.Security.Cryptography.CryptographicException was unhandled by user code
  Message=Padding is invalid and cannot be removed.

Is there some way I can cleanly manage this and is this to be expected if I on purpose use the WRONG key?

Now I changed this to the following based on a suggestion below:

public byte[] Decrypt(byte[] buffer)
    {
        try {
            MemoryStream decryptStream = new MemoryStream();
            using (CryptoStream cs = new CryptoStream(decryptStream, decryptor, CryptoStreamMode.Write))
            {
                cs.Write(buffer, 0, buffer.Length);
            }
            return decryptStream.ToArray();
        } catch(CryptographicException e){
            //... do something with it ...
            return null;
        }
    }

How can I pass up the exception to the next method which is:

public string DecryptFromUrl(string encrypted)
{
    return Decrypt(HttpUtility.UrlDecode(encrypted));
}

Answer 1

In any case you should be try { … } catching() the error and handling it from there. You could return a specific error, generic error, log it etc depending on your requirements.

try
{
... code ...
}
catch(CryptographicException e)
{
... do something with it ...
}