Previous | ToC | Next Labs: Cryptography. Part 1. Math Alive

## HBO Application

You just studied binary and parity addition because these operations are used in many applications, in cryptography and elsewhere. The encryption method that you will be learning here uses parity addition.

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:

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: