void ProcessIntent(Intent intent)
{
IParcelable[] rawMsg = intent.GetParcelableArrayExtra(NfcAdapter.ExtraNdefMessages);
NdefMessage msg = (NdefMessage)rawMsg[0];
string datagram = Encoding.UTF8.GetString(msg.GetRecords()[0].GetPayload());
contactName = datagram.Split(',')[0];
Padbook friendBook = pm.GetPadbookForUsername(contactName);
friendBook.AppendPads(datagram.Split(',')[1].Split('#')); // keys are seperated with a hash
}
/// <summary>
/// Retrieves a message from the server, and if it exists decrypts and displays it
/// </summary>
private string RetrieveMessage()
{
// endpoint: GET /messages/:to/:from; in this case, we want to find a message to me, from the user we are viewing
string url = "http://safe-inlet-16663.herokuapp.com/messages/" + Identity.Username + "/" + contactName;
string msg = wc.DownloadString(url);
// Debug:
//Toast.MakeText(ApplicationContext, url, ToastLength.Long).Show();
// Decrypt message by using pad from padbook
// If no pad exists, returns empty string
Padbook pb = pm.GetPadbookForUsername(contactName);
long pbSize = pb.BytesLeft();
AesManaged aes = new AesManaged();
KeySizes[] ks = aes.LegalKeySizes;
string decryptedMsg = "";
if (pbSize >= (aes.LegalKeySizes[0].MaxSize + aes.LegalBlockSizes[0].MaxSize / 8) * 2)
{
string pad = pb.GetNextPad();
if (pad != null && pad.Length > 0)
{
decryptedMsg = Encoding.UTF8.GetString(Crypto.XorStrings(msg, pad)); // Decrypt message using pad
}
}
else // use AES
{
List <byte> key = new List <byte>();
List <byte> IV = new List <byte>();
List <string> pads = new List <string>();
while (key.Count * sizeof(byte) < 128)
{
pads.Add(pb.GetNextPad());
byte[] f = Encoding.UTF8.GetBytes(pads.Last());
foreach (byte b in f)
{
key.Add(b);
}
}
while (IV.Count * sizeof(byte) < 128 / 8)
{
pads.Add(pb.GetNextPad());
byte[] f = Encoding.UTF8.GetBytes(pads.Last());
foreach (byte b in f)
{
IV.Add(b);
}
}
//resave the pads so AES can be used again
pb.AppendPads(pads.ToArray());
aes.Key = key.ToArray();
aes.IV = IV.ToArray();
ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
using (MemoryStream msDecrypt = new MemoryStream(Encoding.UTF8.GetBytes(msg)))
{
using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
{
using (StreamReader srDecrypt = new StreamReader(csDecrypt))
{
// Read the decrypted bytes from the decrypting stream
// and place them in a string.
decryptedMsg = srDecrypt.ReadToEnd();
}
}
}
}
return(decryptedMsg);
}
