Some information:
- fingerprint is 5 bytes, and is at position 0 on device. There's no fingerprint on user file.
- a token is 128 bytes
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on device, fseek to position 133 (i.e. after the fingerprint + token). Write the fingerprint + the old token. If power is cut during the writing, or after, the next login will be fine. (successful login if there's a failure = old token at pos 5 on device + old token at pos 0 on user file)
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once the old token is written, fseek back to position 5 (right after the fingerprint), and write the new token. If power is cut during the writing or after, the next login will fail, look for a fingerprint, if there is, compare with the token after. Since it's the old one, the login will be fine. (successful login if there's a failure = old token at pos 138 on device + old token at position 0 on user file)
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on user file, fseek to position 128 (right after the token), write fingerprint + old token. If it fails, same scenario as the previous line. (successful login if there's a failure = old token at pos 138 on device + old token at pos 0 on user file)
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once that's done, fseek to position 0 and write the new token. If power is cut during this time, the login will fail, look for fingerprint on device at position 133, find the old token, login will fail again, look for the fingerprint on user file at position 128, find the old token, and login will work. (successful login if there's a failure = old token at pos 138 on device + old token at pos 128 on user file)
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at this point, fill with zeroes the redundant fingerprints and old tokens from both the device and the user file. If this fails, the next login will work right away, and the process will be repeated.