We discuss a toy example here: in order to be able to transmit some movie channels to their customers, local cable companies subscribing to the movie service must have a password to unscramble the signal sent to them by the television station. As explained in class, the password is broadcast itself, encrypted differently for each subscriber. Each subscriber has a key, consisting of a binary string of (say) 20 digits. The password itself is also a binary string with 20 entries. The television station sends out an enormous string that contains, in successive 20-digit strings, the XOR addition of the password and every acceptable key, in random order.

Therefore the concatenated string may look like this:

(password key3) (password key6) (password key1) (password key7) (password key5) (password key4)

Parity addition has the neat property that if you parity add the (password key) to the key, then you will get back the password.

For example:

password:	10110110111010
your key:	01011011110011
password your key:	11101101001001

password your key :	11101101001001
your key :	01011011110011
(password your key) your key	10110110111010

NOTE: password = (password key) key

You can exploit this as follows: parity add your 20-digit keyword to every one of the 20-digit pieces in the long string sent out by the station. One of them will be the password. We denote password by pwd.

password key2	password key4	password key7	password your key	password key1
pwd key2 your key	pwd key4 your key	pwd key7 your key	pwd your key your key	pwd key1 your key
junk	junk	junk	password	junk