Really? I thought that it should start with uint8_t hmacKey[16] https://github.com/javimadgit/amiitool/blob/master/include/nfc3d/keygen.h and 7F 75 28 is the hardcoded type string "unfixed infos" I found @ pastbin
I know i though so too. finally i wrote a program to write every possible combination of key files using every single bit of data available from that pastebin file, and checked for the one which matched the md5 posted by others , and thats what it says in the start of my locked secret file.
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I should note that when with the two tags I made, 3DS gave a corrupted tag error but allowed me to reset the tag. After resetting it worked just fine. So there must be some small part of the encryption which is broken but as long as you don't care about the data already in the file it works fine.