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Universal PDF MD5 collision

This project enables one to generate virtually any single-page PDF MD5 collisions without any additional collision computation, such as this text-only MD5 quine which displays its own MD5 checksum.

How to use this to build my PDF collision files?

The process to generate the desired PDF file(s) will vary depending on your specific use case. The detailed instruction for creating the above-mentioned MD5 quine is available in the example/ directory. This can serve as a helpful blueprint for constructing your own collision file.

How does it work?

A PDF file mainly consists of a hierarchy of objects. A page in a PDF file is represented by a page object, and its content is specified by the Contents key in the page object, which normally references a content stream (which is also a type of object). There are some useful properties that make precomputing a multi-way PDF collision possible:

  • The Contents can be an array containing multiple content streams, and it is equivalent to concatenating all the content streams in the array together to form a single stream.
  • The format of a reference (specifically, an indirect reference) in the Contents array is [object-id] [generation] R, where [generation] can be fixed to 0 in our use case.
  • The references can be separated by spaces or newlines.
  • PDF files can contain comments, which starts with a % character and ends with a new line (CR or LF) or a NUL byte.
  • The order of objects in a file is arbitrary.

Thus, we can construct a PDF file where the target page object is the first page object. Thanks to HashClash's identical-prefix attack that produces a single-bit difference at the LSB of the 10-th byte, a collision like this can be precomputed and placed in the Contents array:

#byte |0123456789abcdef
file 1|      1000 0 R %[a bunch of collision bytes]
file 2|      1001 0 R %[a bunch of collision bytes]

If the collision bytes does not contain the three characters that ends a comment, this will be a valid reference. This will give us a MD5 collision pair where the one file having the contents of object 1000 and the other having the contents of object 1001. We can repeat this process on objects (1002, 1003), (1004, 1005) and so on to construct a multi-way collision.

After precomputing the collision blocks, constructing arbitrary PDF collision is easy: just put the content that we wish to collide in the content streams with the corresponding object ID, and finally reconstruct a valid PDF footer (specifically, the cross-reference stream).

With N pairs of collision blocks, up to 2N different files with the same MD5 digest can be constructed, although it depends heavily on the use case. For example, to construct a MD5 quine that displays its MD5 hash in hexadecimal format, we need 16x32=512 pairs of collision pairs; if instead the hash should be displayed in binary format, only 128 collision pairs are needed.

In this project, a total of 4000 collision blocks were precomputed, which should be sufficient for all common use cases. The collisions were precomputed in 38 days using a server with two Xeon® E5-2620 v4 CPUs (a total of 32 threads).

Running the precomputation

The scripts to run the precomputation is in gen-coll/. It can be used to generate collision pairs for different PDF versions or to generate more collision pairs beyond the existing 4000 pairs.

  1. Download and build HashClash.
  2. cd to gen-coll/ and create a prefix file table-prefix.
    • The file size should be a multiple of 64 bytes. An example is table-prefix.example.
  3. Run python gen-coll.py -d [path-of-HashClash] [start] [end].
    • [start] and [end] denote the range of the generated object IDs. The number of pairs will be (end-start)/2.
  4. The generated collision pairs will be stored in collisions-1 and collisions-2.

On average, a collision pair will take 10-15 minutes on a machine with 32 threads, though the variance will be large: some collision pair might take up to 2-3 hours to generate because of forbidden characters.

In case the script is interrupted, you can resume the process of generating the remaining pairs by renaming the collisions-1 file to table-prefix and then restarting the script with the appropriate [start] value.