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by Rychard
1458 days ago
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The messages include an incrementing counter ID, which has to be accounted for when calculating the checksum. The purpose of this is specifically to prevent "replay attacks". If the counter doesn't increment on subsequent messages, it raises an error. If you increment the counter but do not recalculate the checksum, it raises an error. |
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No need to understand the protocol or anything - just record a few minutes of data, find when a message repeats (indicating whatever counter mechanism is in use has rolled over), and replay that loop repeatedly.
The only time this technique doesn't work is when a challenge-response algorithm is in use, but car stuff doesn't tend to do that except for some lock/security/firmware update type functionality.