Decrypt returning junk data

Question

I got my encryption algorithm working fine and left it alone for a few months. Today I needed to use it again and somehow it broke. I have poked at it for a while and have been unable to spot what the problem is. There are no errors, it just returns junk data.

The Setup: A PHP script(that has worked in production for a long time) encrypts some string using:

function hexstr($hexstr) 
{
  // return pack('H*', $hexstr); also works but it's much harder to understand.
  $return = '';
  for ($i = 0; $i < strlen($hexstr); $i+=2) {
    $return .= chr(hexdec($hexstr[$i] . $hexstr[$i+1]));
  }
  return $return;
}

function encrypt($str, $key)
{
    $key = hexstr($key);
    $size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
    $iv = mcrypt_create_iv($size, MCRYPT_RAND);
    return $iv . mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $str, MCRYPT_MODE_CBC,$iv);
}

If I decrypt this on the php side it works fine.

Then the string is base64 encoded:

$encoded = base64_encode($encrypted);

Finally the C# program gets a hold of it and does the following:

....
byte[] decoded = Convert.FromBase64String(myWebText);
Decrypt(decoded)
.....


public static byte[] Decrypt(byte[] p)
{
    RijndaelManaged aes128 = new RijndaelManaged();
    aes128.KeySize = 128;
    //aes128.BlockSize = 
    aes128.Padding = PaddingMode.Zeros;
    aes128.Mode = CipherMode.CBC;
    aes128.Key = StringToByteArray("SOMEHEXSTRING");
    //pull the iv off the front of the byte array
    if (p.Length <= 16)
    {
        Utils.ReportError("byte array too short");
        return null;
    }
    byte[] iv = new byte[16];
    Array.Copy(p, 0, iv, 0, 16);
    aes128.IV = iv;
    ICryptoTransform transform = aes128.CreateDecryptor();
    byte[] result = transform.TransformFinalBlock(p, 16, p.Length - 16);

    Debug.Log("If this encrypted stuff was text it would be:"+System.Text.Encoding.UTF8.GetString(result));
    return result;
}


public static byte[] StringToByteArray(string hex)
{
    if (hex.Length % 2 == 1)
        throw new Exception("The binary key cannot have an odd number of digits");

    byte[] arr = new byte[hex.Length >> 1];

    for (int i = 0; i < hex.Length >> 1; ++i)
    {
        arr[i] = (byte)((GetHexVal(hex[i << 1]) << 4) + (GetHexVal(hex[(i << 1) + 1])));
    }

    return arr;
}

Does anyone know what this might not be working? It must be very close because I am sure it worked at one point. Update: I did a binary compare of when i C# encode\decode and php encode decode. The php encoded Hello World is not the same as the c# Hello world when using the same IV. is this possible or does this indicate they are not using the same configuration somehow.

Could you post an example encrypted string and key? I don't typically use php, but I might be able to try decrypting an already encrypted string for you to see if I can see any potential problems. — LVBen, May 21 '14 at 22:25
Ok sure, here is some key and Hello World encoded in (after encrypting its base64 encoded in this example too) "Data:ewp/sWeooBQd7xUCNxtmwTsTy5KdZkOglkPo7PPPuY0=" Key is "BAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB" don't forget the IV leading 16 bytes is in there too — user2292539, May 21 '14 at 22:58
So far, your C# code seems to be working fine on my end. I haven't tried decrypting your string yet, though. — LVBen, May 21 '14 at 23:57
I have edited your title. Please see, "[Should questions include “tags” in their titles?](http://meta.stackoverflow.com/questions/19190/)", where the consensus is "no, they should not". — John Saunders, May 22 '14 at 02:29

score 1 · Accepted Answer · answered May 22 '14 at 21:02

1

Turns out I had changed the key to have non upper case letters and my GetHexVal function shown there only took upper case letters... Ouch

answered May 22 '14 at 21:02

user2292539

245
1
11

score 0 · Answer 2 · answered May 22 '14 at 02:51

0

IDEA: The decryption key has changed.

Very nasty when it happens. Are you using a config file with decryption key? If not What is the decryptionKey used ? Has it changed?

If using localApp.config or ROOT.config and the key was changed, you can go hunting around all backups to get the key.

 <machineKey decryptionKey="SomeHexValueHere" validationKey="SomeveryLongHexvalueHere" />

But if your PHP version is working, the key must be "known"

answered May 22 '14 at 02:51

phil soady

11,043
5
50
95

Yes. I have the key. I tried another solution i found on here. http://stackoverflow.com/questions/165808/simple-two-way-encryption-for-c-sharp. I can encrypt and decrypt on the c# side but I get the same issue when going from php to c#. Just junk characters as the output. I tried adding the padding and using PKCS7 but I get:CryptographicException: Bad PKCS7 padding. Invalid length 197. I am handing the crypto stuff exactly 16 bytes so I am not sure where that number is coming from. – user2292539 May 22 '14 at 05:12

Decrypt returning junk data

2 Answers2