How to calculate the MasterChecksum and DataForkChecksum on a dmg file

Question

I try to change some information on a dmg files, It works well but when i tried to open the dmg i have the error message

checksum invalid

So i get the header of my dmg file, and i get all the information that i need . I have a DataForkChecksum and a MasterChecksum but I don't know how to calculate them .

does anyone knows how to do this ?

Answer 1

The master checksums is a checksum of checksums. It is computed as the CRC-32 checksum (polynomial=0xedb88320) computed on the concatenated binary values of all the checksums of the BLKX blocks present in the DMG. It's quite painful to compute.

libdmg contains GPL code that does this. In particular have a look at the calculateMasterChecksum() function in http://shanemcc.co.uk/libdmg/libdmg-hfsplus/dmg/dmglib.c

Answer 2

If you don’t know a lot about Checksums I recommend taking a quick look at the Wikipedia entry. Essentially they are used to check the integrity of a file to make sure that it has not been changed or interfered with in anyway. I believe this is especially important in the open source community, as the code uploaded to sharing websites could have been interfered with by someone other than it original provider.

(Also take a quick look at MD5 on Wikipedia, one read of this and you will quickly appreciate the difficulty of the problem and proposed solutions. )

By including a Checksum the provider is not guaranteeing the quality of the code (they may well do separately) but they are giving you the ability to ensure that what you are downloading is exactly what they provided. A change to a single byte will change the Checksum.

In your case by modifying the DMG you are changing the Checksum. Without knowing the specifics it’s hard to advise you how to get around it. If your setup is communicating with the original DMG provider in some way to compare the checksums then it will be very difficult to fix. You also have no way of knowing what their checksum is.

If it is comparing it with a locally stored file then you have a chance. The simplest way will be to get one of the free tools for creating Checksums and replace them both.

However all this brings up a question. Why are you modifying an externally provided DMG? If you want your computer to preform additional actions when you click on it I believe there are much simpler ways