Cisco RF Gateway 1 Modules, Configuration Manual

Page: 195 / 286

Scrambling, Control Word, and Cryptoperiod
78-4025112-01 Rev H0 171
Scrambling, Control Word, and Cryptoperiod
At the transmission site of a CA System, services multiplexed into a Transport
Stream can be scrambled using a DVB common scrambling algorithm with a
scrambling/descrambling key called Control Word (CW). At the receiver site, the
scrambled services can be descrambled by an appropriate descrambling algorithm
using the same CW.
To increase the security of a CA System, the CW used to scramble and descramble
services changes periodically (typically every 10 seconds). The duration of
scrambling by one CW is called Crypto Period. CWs, typically 64 bits long, are
generated by a Control Word Generator (CWG) and requested by the Simulcrypt
Synchronizer (SCS).
CWs cannot be delivered from the transmitter to the receiver site in the clear. They
need to be encrypted. The algorithm used to encrypt the CW is unique to each CA
System and implemented in a secure device of the descrambler and uses the smart
card of the customer’s Conditional Access Module (CAM).