if you have a simple strategy to solve a task the program comes easily i find

my strategy for this would be :-
monitor the serial input for 0x07 chr [the start of a transmission pkt]
when you get an 07 read in the pkt
(a pkt when received can be decoded [eg '070D0000313B' the 5th byte is the key value data the sixth is a simple xor checksum])
check if the 5th byte is bel [(0x08) the go button]
if it is not
place it in a password length sized ring buffer and continue monitoring the serial input
if it is
check if ring buffer contains the password key code sequence
if it does
then its a winner
else
object to the failed attempt
keep reading in data