Are you sure it's DES with 56 bit key?
BISS mode 1 is similar to RAS in that it uses a fixed control word to encrypt the data in the transport stream. Unlike RAS, the cramblingalgorithm is non-proprietary, using the DVB Common ScramblingAlgorithm to allow interoperability with other manufacturers’encoding/scrambling equipment.
Scroll down to Menu #4.1 and edit the RAS mode (FIXED KEY MODE or DSNG KEYMODE) and the DSNG key (7-digit number)
7 digit number are not 56 bit!
10^7 are way less than 2^56
What the manual describe is the action of imputing keys into A RAS RECEIVER,it doesn't not mean the control word to decrypt the video is 7 digits.
(it could be,but is not saying that).
This 7 digits we have no idea how they are handled inside the ird.
I have the impression,and is only my gut feeling that this 7 digits are expanded into something else to produce a sort of control word that is either 56 or 64 bits,I doubt they are using a chipset in their receivers that don't do CSA or DES.
The only way to analyze this is starting with known clean video packets to see what's in there,then try to reverse encrypted packets encryption.
I doubt there are simple xor's.
See here
*************In SNG Key mode, a seven digit key is entered via the front panel on the
Encoder. In Fixed Link mode, an 8 digit key is assigned by TANDBERG
Television and pre-programmed into the Encoder and IRD. Scrambling can
be switched on and off under user control**********
Looks like fixed mode it uses an 8 digit key
8 digits key is 32 bits (half of 64)
7 digits is already 28 bits (half of 56)
Let's just imagine they add an LSB or an MSB to each digit,what does this become?
I mean as a way/exercise of expansion.
That's the first thing I would try to decrypt packets.