NOTE: This post was imported from my previous blog – v3l0c1r4pt0r.tk. It was originally published on 6th November 2015.
Last year, I published a program for Microsoft Dreamspark’s SDC file decryption. Soon after that I wrote article about SDC file format and its analysis. Now it’s time to complete the description with newest data.
This article wouldn’t be written if not the contribution of GitHub’s user @halorhhr who spotted multi-file SDC container and let me know on project’s page. Thanks!
When writing that post year ago, I had no idea what multi-file container really looks like. Any suspicions could not then be confirmed, because it seemed that these files simply where not used in the wild. A days ago situation changed. I got a working sample of multi-file container so I was able to start its analysis.
Real container format
After quick analysis, I knew that I was wrong with my suspicions. Filename length and encrypted filename strings are not part of a file description. In fact they are placed after them and filename is concatenated string of all filenames (including trailing null-byte). So to sum up filename of n-th element starts at file[n].filename_offset and ends just like any other c-style string.
Whole header structure is like on the sample header on the right. Note that all headers beside 0xb3 one has been already decrypted for readability. In real header the only unencrypted field is header size at the very beginning of the file. 0xb3 sample has unencrypted header and header size is not present in a file. However file name is encrypted in some way, I haven’t figured out as of now. Encryption method is blowfish-compat (the difference between this and blowfish is ciphertext endiannes). Filenames are encrypted once again.
After header, all other data is XORed using key from EDV string and then deflated, so before reading them, you have to inflate and XOR again. Format of data in 0xb3 version is still unknown, however analysis of compressed and file size hints that it may be stored the same way. It is important to note that depending on file signature different configuration of deflater may be needed. It is now known that files older than 0xd1 header, which appears to be newest (because only this one supports files greater than 4 GiB) need to have deflater initialized with
This errata does not contain all information needed to support all variations of SDC files. Beside unknowns I mentioned above, there is another variation that uses 0xc4 signature and which I had no sample of. The only trace of its existence is condition in SDM code. Because of that I cannot write support for that type of file. There is also possibility of multi-file containers having 0xb5 or 0xb3 signatures existence. That type of files seems to appear only lately, but it is probable that it existed in the past. Because of having no samples of them I cannot verify that xSDM properly handles them.
So if you have sample of any of variations mentioned here, just send them to me at my email address: v3l0c1r4pt0r at gmail dot com or if you suppose it may be illegal in your country, just send me SDX link or any other hint for me how can I find them.
Few days ago, after I started writing this post Github user @adiantek let me know in issues that there is a method to obey SDM in Dreamspark download process. To download plain, unencrypted file you just have to replace ‘dfc=1‘ to ‘sdm=0‘ in a link Dreamspark provided in SDX file. If it true that it works in every file Dreamspark provides, my xSDM project would be obsolete now. However, because Microsoft’s intentions when creating this backdoor (it seems to be created just for debugging) are unknown, I will continue to support the project and fix any future bugs I will be aware of. But now it seems that this project will start to be just proof of concept for curious hackers and will start to slowly die.
Nevertheless, if you have something that might help me or anyone who may be interested in SDC format in future, just let me know somehow, so it will be available somewhere on the internet.