4 ms·

That's how one-time password tokens like RSA SecurID and YubiKey works, and hardware/battery/cost wise, they seem to be well within scope for a car key.

ethbro 9y ago
How do you ensure the clocks are sync'd? Or is there a one-channel method that doesn't require synchronized times? E.g. window-based Or is it that cheap to have ~5-10 yr accurate clocks implemented in hardware now? (Not that the margin on keyfobs seems exactly slim...)
- mseebach 9y ago
  A combination of good, accurate clocks (remember that a $10 digital Casio drifts less than a second per day) and some functionality on the server side to detect and correct for drift. When the key window is 30 seconds, the key won't drift more than 10 keys per year, which should be easy to correct for without compromising security (and if in doubt, it's easy to "ask" for a second code - just fail the attempt, the user will try again).
- dfox 9y ago
  The timestamps used in OTP tokens are usually quite coarse (30 - 60 second resolution is common), also it's common to use window of few accepted timestamps around the valid one. (IIRC SecurID has this window in default configuration surprisingly large, on the order of tens of minutes) RTC with common 32768Hz xtal is probably accuratte enough for this application given reasonably stable temperature (ie. key stored in room temperature or near human body) with few minute acceptance window. The problem with battery life for such keyfob is that it also has relatively high powered RF transmitter and combining low permanent current consumption with high current bursts is not something that cheap primary lithium cells are optimized for (you have to pick one or the other) Edit: also given the fact that there is only one receiver and that it usually also has second bidirectional channel (immobilizer/keyless start) you can compensate for some limited clock drift in software or even periodically resynchronize the clock. Manual for Fabia III even describes procedure to "resynchronize" the keyless fob by using it as if it were ignition key, which probably does exactly this. But who knows, almost all post 2000 VWs have similar procedure for central locking remote which probably simply overwrites the counter in car with whatever next value from the keyfob as there cannot be any bidirectional communication (the old procedure involves lock in driver's door, no NFC antennas there and the imobilizer NFC chip in keyfob is completely separate from the RF part)

ethbro 9y ago

How do you ensure the clocks are sync'd? Or is there a one-channel method that doesn't require synchronized times? E.g. window-based Or is it that cheap to have ~5-10 yr accurate clocks implemented in hardware now? (Not that the margin on keyfobs seems exactly slim...)

mseebach 9y ago
A combination of good, accurate clocks (remember that a $10 digital Casio drifts less than a second per day) and some functionality on the server side to detect and correct for drift. When the key window is 30 seconds, the key won't drift more than 10 keys per year, which should be easy to correct for without compromising security (and if in doubt, it's easy to "ask" for a second code - just fail the attempt, the user will try again).
dfox 9y ago
The timestamps used in OTP tokens are usually quite coarse (30 - 60 second resolution is common), also it's common to use window of few accepted timestamps around the valid one. (IIRC SecurID has this window in default configuration surprisingly large, on the order of tens of minutes) RTC with common 32768Hz xtal is probably accuratte enough for this application given reasonably stable temperature (ie. key stored in room temperature or near human body) with few minute acceptance window. The problem with battery life for such keyfob is that it also has relatively high powered RF transmitter and combining low permanent current consumption with high current bursts is not something that cheap primary lithium cells are optimized for (you have to pick one or the other) Edit: also given the fact that there is only one receiver and that it usually also has second bidirectional channel (immobilizer/keyless start) you can compensate for some limited clock drift in software or even periodically resynchronize the clock. Manual for Fabia III even describes procedure to "resynchronize" the keyless fob by using it as if it were ignition key, which probably does exactly this. But who knows, almost all post 2000 VWs have similar procedure for central locking remote which probably simply overwrites the counter in car with whatever next value from the keyfob as there cannot be any bidirectional communication (the old procedure involves lock in driver's door, no NFC antennas there and the imobilizer NFC chip in keyfob is completely separate from the RF part)