Audio/Video compression
Security
Alain Bouffioux
December, 20, 2006
Agenda
What is cryptography
Symmetric & public-key cryptography
Why cryptography for DVB ?
Conditional access information in MPEG/DVB
Conditional access mechanism
Conditional access interfaces
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AV Compression / Alain Bouffioux
December, 20, 2006
What is cryptography (1/2)
Why cryptography ?
ALC
IE
BO B
M ESS AG E
–
–
–
–
CONFIDENTIALITY - The message is not listened
INTEGRITY - The message is not modified
AUTHENTICITY - The message has been sent by Alice
NON-REPUDIATION - Alice cannot falsely deny she has sent the
message
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AV Compression / Alain Bouffioux
December, 20, 2006
What is cryptography (2/2)
Basic terminology
E n c ry p toi n
P al ni te x t
or
c el a r te x t
D e c ryp toi n
C pi h e r te x t
K ey
O r gi ni a l
p al ni te x t
c ry p to g ra ph ci a gl o r ithm
o r c pi he r
K ey
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AV Compression / Alain Bouffioux
December, 20, 2006
Symmetric & public-key cryptography(1)
E n c ryp toi n
P al ni te x t
or
c el a r te x t
D e c ryp toi n
C yp he r te x t
O r gi ni a l
p al ni te x t
K ey 1
Symmetric cryptography
Key1 = Key2
K ey 2
Public-key cryptography
Key 1  Key 2
Public-key cryptography
One Public-key (known by everybody) : PK
One Private-key or Secret-key (kept secret) : SK
C = EKey1(M)
 M = DKey2(C) = DKey2(EKey1(M))
In public-key cryptography, key1 may be PK or SK and key2 is the other
key.
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AV Compression / Alain Bouffioux
December, 20, 2006
Symmetric & public-key cryptography(2)
Example of symmetric cryptography
P al ni et x t
C pi he r et x t
O rgi ni a lp al ni te x t
+
+
P seudo -random
num be r gene ra ot r
K ey
S e cu re
channe l
P seudo -random
num be r gene ra ot r
K ey
– Key stream as long as message
– Key stream = pseudo-random sequence (easy to break)
– Low security should be compensated by frequent change of keys 
necessity of secure channel
 2 channels : one for the message & one for the key
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AV Compression / Alain Bouffioux
December, 20, 2006
Symmetric & public-key cryptography(3)
Example of public-key cryptography
P ub lci K e ys
P ub lci ke y o fB ob
E n c ryp tedm e ssage
A lci e en c ryp tsm e ssage
u s ni g B ob s' pub lci ke y
B ob de c ryp tsm e ssage
u s ni g h si se c re t ke y
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AV Compression / Alain Bouffioux
December, 20, 2006
Symmetric & public-key cryptography(4)
Symmetric cryptography example : DES
Public-key cryptography example : RSA (1977)
Symmetric versus public-key cryptography
– Symmetric cryptography is faster (about 1000 times).
– Low security of symmetric cryptography (due to the necessity of key transport) is
improved by a frequent change of the key.
– In Public-key cryptography the secret-key may be kept secret. It is never
transported  High security.
– Different usage : In DVB, symmetric key algorithm for encrypting data, public-key
algorithm for key management (secure channel).
Hybrid cryptosystem
Example : DES for message and RSA for key encryption
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AV Compression / Alain Bouffioux
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Cryptography and DVB (1/2)
Cryptography may prevent unauthorised receiver from decoding the
program.
DVB compared with banking or military secret
– high information rate
– low information value
– decryption must be cheap
Cost of cracking the system should be higher than the benefits gained
from the cracking
Cryptography in DVB is a trade-off between cost/complexity versus
piracy-proof.
CA (Conditional Access) = very sensitive subject. Some service
providers want their own CA system.
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AV Compression / Alain Bouffioux
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Cryptography and DVB (2/2)
MPEG does not specify a conditional access (CA) system but defines a frame
to support CA.
DVB characterises some aspect left undefined by MPEG,
It defines a CA interface.
The broadcaster develops its CA system using a CA interface.
DVB is based on
– symmetric cryptography for audio-visual transmission
– frequent key change to increase security
– Public-key cryptography for key-exchange
DVB relies on
– stream of ECM’s (Entitlement Control Message)
– stream of EMM’s (Entitlement Management Message)
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AV Compression / Alain Bouffioux
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CA information in MPEG TS (1/2)
U n coded v di eo
S am p lni g
COM PR ESS O
IN
LAYER
V di eo
E n code r
S am p lni g
V di eo e elm en ta ry
s tream (E S
. .)
P TS
S TC
U n coded aud oi
P a cke t si ni g
P TS
A ud oi
E n code r
A ud oi e elm en ta ry
s tream (E S
. .)
E LEM EN TAR Y
S TR EAM
P a cke t si ni g
PES S ub al ye r
T ran spo rtS ub al ye r
EM M s'
PES
ECM s'
PES
C om pu te
PCR
V di eo
PES
P rog ram S pe c if ci
In fo m
r a toi n P
( S IS
, I)
A ud oi
PES
PAC KE T S
I ED
E LEM EN TAR Y
S TR EAM
M u ltpi el x ni g
SYS TEM
LAYER
TR AN SPO R T
S TR EAM
M PEG 2 T ran spo r tS tream
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AV Compression / Alain Bouffioux
December, 20, 2006
CA information in MPEG TS (2/2)
P rog ram A sso c ai toi n T ab el (PA T ) (P D
I =0 )
P rog ram N um be r
1
2
......
PM T -P D
I
x
y
......
H eade r
4 b y te s
M PEG TS
M u ltpi el x
P a y ol ad
184 b y te s
A ud oi
PD
I
V di eo
P rog ram M ap T ab el (PM T ) (P D
I =x)
S tream - type
A ud oi
V di eo
PC R s'
ECM s'
S tream P
- D
I
aa
bb
cc
dd
PC R
ECM s'
C ond itoi na lA cce ss T ab el C
( A T ) (P D
I =1 )
EMM s'
EMM s'
mm
T ab el
S om e po ss bi el a
t b el s
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AV Compression / Alain Bouffioux
December, 20, 2006
The CA mechanism : illustration
Encrypted AV data
Clear AV Data
Decryption
CW’s
SMARTCARD
Access control parameters
ECM’s
Decryption
(Program related)
SK
Entitlement
SK
EMM’s
Decryption
PDK
(CA system
related)
PDK1
PDK2
IK
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AV Compression / Alain Bouffioux
December, 20, 2006
The CA mechanism (1/2)
MPEG TS
(cela r)
Sc arm bel r
CW C( on tor W
l o dr )
Enc ryp toin
AV s tream s
AV s tream s
ECM s'
ECM s'
MPEG TS
MPEG TS
Desc arm bel r (cela r)
CW
Dec ryp toin
ACP
SK
SK S( e rv cieKey )
BaseKey
Enc ryp toin
EMM s'
EMM s'
ACP =AccessCon tor l
Pa arm e et rs .
Dec ryp toin
KI (Issue rKey )
En titem
l en t,
P
D
K
,
SK
En titem
l en t
PDK S, K
PDK P( or g armm e r
D si trbiu toinKey )
AV streams are scrambled with Control Words (CW) using symmetric
cryptography
CW are encrypted using Service Keys (SK), are placed in ECM’s and
are securely transmitted to the receiver
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AV Compression / Alain Bouffioux
December, 20, 2006
The CA mechanism (2/2)
SK are encrypted using public-key cryptography Keys are IK (unique key internal to the smartcard) or PDK (transmitted via
EMM’s in order to define user’s group)
ECM’s carries (informations related to a single program  PID of ECM’s in
PMT)
– enciphered CW
– access parameters
ECM’s are decoded to CW if the receiver contains the required entitlements
EMM’s carries (information related to a conditional access system  PID of
EMM’s in CAT)
– New entitlements, SK’s (Service Keys)
– Programmer distribution key
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AV Compression / Alain Bouffioux
December, 20, 2006
About DVB scrambling
Encryption occurs after compression (at the location in the stream
where the redundancy is at its lowest value) in order to have a robust
encryption system.
Encryption may occur at PES level or at TS level.
DVB scrambling is transparent (a valid TS remains valid after
scrambling)  facilitates transport and manipulation.
Synchronisation based on PCR’s  constant time required for
scrambling/descrambling.
Security device should authenticate EMM’s origin.
CA is only one aspects of cryptography usage in DVB. An other may
be copy protection by (watermarking) and authentication (by
signature).
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AV Compression / Alain Bouffioux
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AV Compression / Alain Bouffioux
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