// fixing PNG headers by guessing is quite simple
// - the width and heght fields (2x 4 bytes) are XORed with a "random" key
// -> this key is an ascii representation of a hash.
// -> Thus it consists of 8 chars in the range of '0'..'9' or 'a'..'f'
// - first we assume dimensions < 65536 in each direction (sounds sane)
// -> this means we already have 4 of the 8 key bytes (the two high order bytes from each dimension)
// - now we walk through all possible key values "00" to "ff" for each of the lower order byte pairs
// -> this means 256 (for x dimension) x 256 (for y dimension) tries
// -> in each try, we check if the CRC if the IHDR chunk is now correct
// -> if it is correct then we found the remaining 2x2 bytes of the key
private byte[] FixPngHdr(byte[] hdr, ref string key, ICRCAlgorithm crcAlgo)
{
byte[] result = new byte[hdr.Length];
ulong target_crc = (ulong)hdr[hdr.Length - 4] << 24 |
(ulong)hdr[hdr.Length - 3] << 16 |
(ulong)hdr[hdr.Length - 2] << 8 |
(ulong)hdr[hdr.Length - 1];
ulong crc;
hdr.CopyTo(result, 0);
// first, set higher order positions to zero
result[4] = result[5] = 0;
result[8] = result[9] = 0;
for (int key1 = 0; key1 < 256; key1++)
{
// ascii representation of key1
string k1 = key1.ToString("x2").ToLower();
byte b1 = (byte)(k1[0]);
byte b2 = (byte)(k1[1]);
result[6] = (byte)(hdr[6] ^ b1);
result[7] = (byte)(hdr[7] ^ b2);
for (int key2 = 0; key2 < 256; key2++)
{
string k2 = key2.ToString("x2").ToLower();
byte b3 = (byte)(k2[0]);
byte b4 = (byte)(k2[1]);
result[10] = (byte)(hdr[10] ^ b3);
result[11] = (byte)(hdr[11] ^ b4);
crc = crcAlgo.CalculateCRC(result, result.Length - 4);
if (crc == target_crc)
{
key = Encoding.ASCII.GetString(new byte[] { hdr[4], hdr[5], b1, b2, hdr[8], hdr[9], b3, b4 });
return result;
}
}
}
key = "";
return null;
}
