How to calculate CRC of a WinRAR file header?

Question

Talking about this: http://www.win-rar.com/index.php?id=24&kb_article_id=162

I'm able to calculate the correct CRC of an archive header (MAIN_HEAD) by doing:

$crc = crc32(mb_substr($data, $blockOffset + 2, 11, '8bit'));
$crc = dechex($crc);
$crc = substr($crc, -4, 2) . substr($crc, -2, 2);
$crc = hexdec($crc);

The first line will read "CRC of fields HEAD_TYPE to RESERVED2" as states in the documentation. As I noted, it works fine for the archive header.

When I try to calculate the CRC of a file header it always spits out the wrong CRC for unknown reason. I did as the documentation says - "CRC of fields from HEAD_TYPE to FILEATTR" but it simply doesn't work. I've also tried different read-length variations in case the documentation is wrong and it might actually be from HEAD_TYPE to FILE_NAME. Everything without success.

Anyone can give me a hint? I've also checked the unrar source code but it doesn't make me smarter, probably because I don't know C language at all...

Answer 1

I wrote some code that does the same thing. Here is it with some additional snippets for a better understanding:

$this->fh = $fileHandle;
$this->startOffset = ftell($fileHandle); // current location in the file

// reading basic 7 byte header block
$array = unpack('vheaderCrc/CblockType/vflags/vheaderSize', fread($this->fh, 7));
$this->headerCrc = $array['headerCrc'];
$this->blockType = $array['blockType'];
$this->flags = $array['flags'];
$this->hsize = $array['headerSize'];
$this->addSize = 0; // size of data after the header


// -- check CRC of block header --
$offset = ftell($this->fh);
fseek($this->fh, $this->startOffset + 2, SEEK_SET);
$crcData = fread($this->fh, $this->hsize - 2);
// only the 4 lower order bytes are used
$crc = crc32($crcData) & 0xffff;
// igonore blocks with no CRC set (same as twice the blockType)
if ($crc !== $this->headerCrc && $this->headerCrc !== 0x6969 // SRR Header
            && $this->headerCrc !== 0x6a6a // SRR Stored File
            && $this->headerCrc !== 0x7171 // SRR RAR block
            && $this->blockType !== 0x72 // RAR marker block (fixed: magic number)
) {
    array_push($warnings, 'Invalid block header CRC found: header is corrupt.');
}
// set offset back to where we started from
fseek($this->fh, $offset, SEEK_SET);

I tested it on a couple of SRR files and it works as expected. I started with reading the basic 7 byte header. The size of the header can be found there. I used this to grab the correct amount of data for the crc32 function. I noticed that when you convert it to hexadecimal, you can get false positives when comparing: '0f00' != 'f00'. You would need to pad it with zeros. This is why I kept the decimal representations of crc32() and unpack() for the comparison. Also, the number of fields of a file block can vary if some header flags are set: it is possible you took a wrong size.