Scrambling is a technique used to randomize a data stream to eliminate long '0'-only and '1'-only sequences and to assure energy dispersal. Long '0'-only and '1'-only sequences create difficulties for timing recovery circuit. Scramblers and descramblers are usually based on linear-feedback shift registers.

A scrambler randomizes the data stream to be transmitted. A descrambler restores the original stream from the scrambled one.

Scrambling shouldn't be confused with encryption, since it doesn't protect information from intruders.


BISS Scrambling (Basic Interoperable Scrambling System)

Basic Interoperable Scrambling System, usually known as BISS, is a satellite signal scrambling system developed by the European Broadcasting Union and a consortium of hardware manufacturers.

There are mainly two different types of BISS encryption used:

BISS-1 transmissions are protected by a 12 digit hexadecimal "session key" that is agreed by the transmitting and receiving parties prior to transmission. The key is entered into both the encoder and decoder, this key then forms part of the encryption of the digital TV signal and any receiver with BISS-support with the correct key will decrypt the signal.

BISS-E (E for encrypted) is a variation where the decoder has stored one secret BISS-key entered by for example a rightsholder. This is unknown to the user of the decoder. The user is then sent a 16-digit hexadecimal code, which is entered as a "session key". This session key is then mathematically combined internally to calculate a BISS-1 key that can decrypt the signal.

Only a decoder with the correct secret BISS-key will be able to decrypt a BISS-E feed. This gives rightsholder control as to exactly which decoder can be used to decrypt/decode a specific feed. Any BISS-E encrypted feed will have a corresponding BISS-1 key that will unlock it.