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HACKING.md

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Hello hackers!

General remarks about contributing

Contributions to the libfreefare are welcome! Here are some directions to get you started:

Install Cutter

Cutter is a cool unit test framework for C code. You will need it to run the regression tests suite (make check) and ensure that your code does not break other features.

To cover all tests you'll need several blank cards:

  • MIFARE Classic 4k with default keys (FFFFFFFFFF)
  • MIFARE DESFire EV1 4k with default PICC key (0000000000000000)
  • MIFARE Ultralight
  • MIFARE Ultralight C with default key (BREAKMEIFYOUCAN!)

After "make check", you can run sub-sets of tests directly with cutter:

$ cutter -n /ultralight/ test
$ cutter -n /classic/ test
$ cutter -n /desfire/ test

Follow style conventions

The source code of the library trend to follow some conventions so that it is consistent in style and thus easier to read. Basically, it follows FreeBSD's style(9); adding 4-space indentation and 8-space tabs (which you should configure in your editor, e.g. :set sw=4 ts=8 in vim). You are also advised to :set cinoptions=t0(0:0 so that you don't have to care about indentation anymore. For more details, please read the style(9) man page from FreeBSD's website.

Write tests

I already told you cutter is lovely, so you really should use it! If you want to contribute code, write test: only code with appropriate tests will be considered. And remember that TDD (Test Driven Development) is cool and writing all tests at the end deeply depressing, so test early, test often!

Adding support for a new type of card

Adding a new supported card to the libfreefare requires a few modification in multiple places. Here is a list of the things to do in order to have the infrastructure ready for hacking the new card support:

  • Edit libfreefare/freefare.h:
    • Add your tag to the freefare_tag_type enum;
    • Add a <tag>_connect() and a <tag>_disconnect() function prototype;
  • Edit libfreefare/freefare.3:
    • Add your tag to the `freefare_tag_type' enum documentation;
  • Edit libfreefare/freefare_internal.h:
    • Add a new <tag>_tag struct. It's very first member SHALL be struct freefare_tag __tag;
    • Add a <tag>_tag_new() and a <tag>_tag_free() function prototype;
    • Add a ASSERT_<TAG>() macro to check the tag's type;
    • Add a <TAG>() macro to cast a generic tag to your type.
  • Edit libfreefare/freefare.c:
    • Add your tag type to the supported_tags array;
    • Edit the freefare_get_tags() function so that it calls <tag>_tag_new() when it finds your tag;
    • Edit the freefare_free_tags() function so that it calls <tag>_tag_free() to free your tags;
  • Create libfreefare/<tag>.c and implement all that's missing:
    • <tag>_tag_new() MUST allocate all data-structure the tag may need to use during it's lifetime. We do not want to have any function to fail later because the running system is out of resources. Buffers for cryptographic operations on random amount of data MAY however be (re)allocated on demand, in such case refrain from shrinking unnecessarily the buffer size.
    • <tag>_connect() SHOULD initialise data allocated by <tag>_tag_new(). Keep in mind that a single tag may be disconnected from and connected to again, without being freed in the meantime. Since all memory allocations are done in <tag>_tag_new(), your code SHOULD only care about initialising these data structures;
    • <tag>_disconnect() MAY do more that just send a disconnect command to the tag. At time of writing I have no idea what it could be but who knows...
    • <tag>_tag_free() SHALL free all resources allocated for the tag (surprising, isn't it?)

Various guidelines

  • If a given card has different cryptographic modes, you SHOULD use switch/cases to handle specific branches of code, even when applicable to only one cypher. The idea is that if you don't provide support for all cryptographic schemes, or if an evolution of the card provides more cryptographic possibilities, when adding support for a new cypher, the compiler can warn the developer about unhandled values in switch statements. Please refer to the MIFARE DESFire code for an example.