1. Audio/Video compression Security Alain Bouffioux December, 20, 2006

2. AV Compression / Alain Bouffioux 2 Agenda What is cryptography Symmetric & public-key cryptography Why cryptography for DVB ? Conditional access information in MPEG/DVB Conditional access mechanism Conditional access interfaces

3. December, 20, 2006 AV Compression / Alain Bouffioux 3 Why cryptography ? –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 What is cryptography (1/2)

4. December, 20, 2006 AV Compression / Alain Bouffioux 4 What is cryptography (2/2) Basic terminology

5. December, 20, 2006 AV Compression / Alain Bouffioux 5 Symmetric cryptography Public-key cryptography Key1 = Key2Key 1  Key 2 Public-key cryptography One Public-key (known by everybody) : PK One Private-key or Secret-key (kept secret) : SK C = E Key1 (M)  M = D Key2 (C) = D Key2 (E Key1 (M)) In public-key cryptography, key1 may be PK or SK and key2 is the other key. Symmetric & public-key cryptography(1)

6. December, 20, 2006 AV Compression / Alain Bouffioux 6 Symmetric & public-key cryptography(2) Example of symmetric cryptography –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

7. December, 20, 2006 AV Compression / Alain Bouffioux 7 Symmetric & public-key cryptography(3) Example of public-key cryptography

8. December, 20, 2006 AV Compression / Alain Bouffioux 8 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

9. December, 20, 2006 AV Compression / Alain Bouffioux 9 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.

10. December, 20, 2006 AV Compression / Alain Bouffioux 10 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)

11. December, 20, 2006 AV Compression / Alain Bouffioux 11 CA information in MPEG TS (1/2)

12. December, 20, 2006 AV Compression / Alain Bouffioux 12 CA information in MPEG TS (2/2)

13. December, 20, 2006 AV Compression / Alain Bouffioux 13 The CA mechanism : illustration Decryption Encrypted AV dataClear AV Data SMARTCARD Decryption ECM’s (Program related) EMM’s (CA system related) IK Entitlement SK Access control parameters SK CW’s PDK1PDK2 PDK

14. December, 20, 2006 AV Compression / Alain Bouffioux 14 The CA mechanism (1/2) 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

15. December, 20, 2006 AV Compression / Alain Bouffioux 15 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

16. December, 20, 2006 AV Compression / Alain Bouffioux 16 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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