A vehicle may receive an encrypted software update responsive to an update request to an update server; identify a key offset into a long key stored to the vehicle according to a timestamp of the update request; and decrypt the encrypted software update according to the long key indexed to the key offset. An update server may receive, from a vehicle, an update request including a timestamp and a vehicle identifier; retrieve a long key according to the vehicle identifier; identify a key offset into the long key according to a timestamp of the update request; and encrypt a software update into an encrypted software update to send to the vehicle according to the long key indexed to the key offset.