Planning a Jailbreak: Use Steganography

Transcription

1 August 11, 2007

2 The Prisoners Problem Why hiding? Ethical Positions Historic Stegosystems Terminology Attack Hiding information in LSB Covert Channels Security by Obscurity public-key stego-systems SSA-CA

3 The Prisoners Problem Why hiding? Ethical Positions The Prisoners Problem Simmons Prisoner s Problem: Alice and Bob in Jail, seperated cells

4 The Prisoners Problem Why hiding? Ethical Positions The Prisoners Problem Simmons Prisoner s Problem: Alice and Bob in Jail, seperated cells plan an escape plan

5 The Prisoners Problem Why hiding? Ethical Positions The Prisoners Problem Simmons Prisoner s Problem: Alice and Bob in Jail, seperated cells plan an escape plan send messages via couriers, who are spys set by warden Eve.

6 The Prisoners Problem Why hiding? Ethical Positions The Prisoners Problem Simmons Prisoner s Problem: Alice and Bob in Jail, seperated cells plan an escape plan send messages via couriers, who are spys set by warden Eve. Eve suspicious both sent to Solitary confinement

7 The Prisoners Problem Why hiding? Ethical Positions The Prisoners Problem Simmons Prisoner s Problem: Alice and Bob in Jail, seperated cells plan an escape plan send messages via couriers, who are spys set by warden Eve. Eve suspicious both sent to Solitary confinement they shared a secret keyword before going to prison (allthough this is not necessary in all cases)

8 The Prisoners Problem Why hiding? Ethical Positions The Warden Eve can be passive, restricted to eavesdropping or active and change messages / emit new ones.

9 The Prisoners Problem Why hiding? Ethical Positions Normal Cryptography does not suffice here! (But may help at some point.)

10 The Prisoners Problem Why hiding? Ethical Positions Normal Cryptography does not suffice here! (But may help at some point.) have to somehow embed so called stegotext in harmless communication channel, the covertext.

11 information hiding Outline The Prisoners Problem Why hiding? Ethical Positions steganography, greek for covered writing.

12 Why hide, not cipher? The Prisoners Problem Why hiding? Ethical Positions

13 The Prisoners Problem Why hiding? Ethical Positions Ethics in cryptography: good guys want to keep their privacy against evil eavesdroppers in steganography: little more complicated

14 The Prisoners Problem Why hiding? Ethical Positions Ethics in cryptography: good guys want to keep their privacy against evil eavesdroppers in steganography: little more complicated contact abuse helplines anonymously, vote in private

15 The Prisoners Problem Why hiding? Ethical Positions Ethics in cryptography: good guys want to keep their privacy against evil eavesdroppers in steganography: little more complicated contact abuse helplines anonymously, vote in private plan crimes, pass secrets

16 Historic Stegosystems Herodotus ( B.C.) tatooed a shaved slave s head

17 Historic Stegosystems Herodotus ( B.C.) tatooed a shaved slave s head Gaspar Schott( ): hide messages in music scores

18 Historic Stegosystems Herodotus ( B.C.) tatooed a shaved slave s head Gaspar Schott( ): hide messages in music scores Johannes Trithemus ( ) Ave Maria code: Replace each letter by a word

19 Historic Stegosystems The Art in information hiding - The Vexierbild

20 Historic Stegosystems Figure: unknown artist: Vexierbild from the 19th century

21 Terminology Outline Terminology Attack Hiding information in LSB Covert Channels embedded data: the message you want to send secretly

22 Terminology Attack Hiding information in LSB Covert Channels Terminology embedded data: the message you want to send secretly this is hidden in cover-text (-image/-audio)

23 Terminology Attack Hiding information in LSB Covert Channels Terminology embedded data: the message you want to send secretly this is hidden in cover-text (-image/-audio) producing a stego-text (-object)

24 Terminology Attack Hiding information in LSB Covert Channels Terminology embedded data: the message you want to send secretly this is hidden in cover-text (-image/-audio) producing a stego-text (-object) stego-key controls hiding process and restricts detection and recovery

25 Terminology Attack Hiding information in LSB Covert Channels successful attack := detecting existence of communication

26 Least Significant Bit Terminology Attack Hiding information in LSB Covert Channels Figure: LSB

27 LSB Outline Terminology Attack Hiding information in LSB Covert Channels Store information here Figure: LSB 1 1

28 Terminology Attack Hiding information in LSB Covert Channels Issues BUT: easy to break. gets lost/damaged on reencoding

29 Covert Channels Outline Terminology Attack Hiding information in LSB Covert Channels Communication paths not designed to transport information

30 Terminology Attack Hiding information in LSB Covert Channels Examples dunno hides information in latencies of the apache webserver hide information in video signal for CRTs

31 What s wrong? Outline Terminology Attack Hiding information in LSB Covert Channels we assumed the adversary to be unaware of the method we use to hide the information...

32 Security by Obscurity set by Auguste Kerckhoff in 1883 first principles of cryptographic engineering

33 Security by Obscurity assume the method used to cypher/hide information is known to the opponent

34 Security by Obscurity assume the method used to cypher/hide information is known to the opponent security by choice of key

35 Security by Obscurity Security by Obscurity The assumption, that the enemy will ignore the system you use

36 Bad idea Outline Security by Obscurity better rely on a key and provable security

37 Security by Obscurity Bad idea better rely on a key and provable security So: we need a definition of security

38 public-key stego-systems SSA-CA A public-key stego-system consists of: An algorithm SK resulting in a public and a private stegokey spk, ssk (using some security paramter k)

39 public-key stego-systems SSA-CA A public-key stego-system consists of: An algorithm SK resulting in a public and a private stegokey spk, ssk (using some security paramter k) An algorithm SE hiding the information in the coverobject using spk, it outputs a covertext

40 public-key stego-systems SSA-CA A public-key stego-system consists of: An algorithm SK resulting in a public and a private stegokey spk, ssk (using some security paramter k) An algorithm SE hiding the information in the coverobject using spk, it outputs a covertext An algorithm SD extracting the embedded data via ssk, or (e.g. if there is no embedded data) an errorsymbol

41 Correctness Outline public-key stego-systems SSA-CA for all (spk, ssk) output by SK and all messages it should work often. That means: (P r[sd(ssk, SE(spk, m))) m] is neglible in a security parameter k

42 The Game Outline public-key stego-systems SSA-CA Security in Cryptography is defined via Games. A challenger with a defined input/output behaviour against an arbitrary adversary.

43 Security Outline public-key stego-systems SSA-CA The system is secure iff the propability to crack it is negligible for all adversaries.

44 public-key stego-systems SSA-CA sampling oracle for C ssk SS-CCA Challenger spk (m, s) Adv. SA 1 c i SD(ssk, c i ) SO1 b R {0, 1} c SE(spk, m ) or c R C (c, s) Adv. SA 2 c i SO2 (c i c ) SD(ssk, c i ) b Figure: Schematic Sketch

45 public-key stego-systems SSA-CA Steganographic Security against Active Attacks Pr [SA SOcca 2 2 (spk, m, c, s) = b; (spk, ssk) SK, (m, s) SA SO 1 1, b R {0, 1}, if b = 0 then c SE(spk, m ) else c R C ] = ɛ(k)