Get some flexibility in the tag type detection

[smortex]

Currently an internal table helps guessing the type of tag based on its SAK & ATS.
But now phones start being able to emulate tags (CyanogenMod, Kitkat,...) while SAK & ATS are still not under control, so being different than expected.

How to get more flexibility?

Getting an environment variable to bypass entirely detection and assume that we've a tag of the right type requested by the application?

Or instead of guessing the tag from its SAK/ATS, test proactively if the tag is of a specific type?

E.g. ASSERT_MIFARE_DESFIRE(tag) would not check a preset flag but would test actively if the card is a DESFIRE by sending a GetInfo command.

Original issue reported on code.google.com by yob...@gmail.com on 3 Dec 2013 at 3:23

[smortex]

The best is probably to trust the developer. If he find a tag and manipulates it with the DESFire API, he is likely expecting a DESFire target and presenting another type of tag is likely to fail quickly.

I therefore propose to basically remove the target type detection code.

A bunch of new API functions may however be created to proactively check if a tag behaves as expected by the developper, e.g.

int mifare_desfire_taste (MifareTag tag);

int main (void) {
    // ...
    if (mifare_desfire_taste (tag) < 0)
        errx (EXIT_FAILURE, "Not a MIFARE DESFire tag");
    // proceed
}

[smortex]

@doegox, since you initially opened this issue, maybe you would like to comment on this pull request?

This idea is to make the xxxxxxxxxx_tag_new() functions public, and pass them a nfc_device * and a nfc_target. The new xxxxxxxxxx_taste() helper functions accepting the same arguments can be used to check if a given tag is likely to act as some type of tag, but there is no requirement to use this.

nfc_device *device;
nfc_target target;
// [...]
if (!mifare_ultralight_taste(device, target)) {
  if ('n' == prompt_user("This does not look like a Mifare Ultralight.  Try anyway?"))
    return;
}
FreefareTag tag = mifare_ultralight_tag_new (device, target);
// [...]
mifare_ultralightc_tag_free (tag);

[doegox]

I like the API you proposed and the removal of the static checks.
merge!