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Diffusion and a Key-Recovery Attack on a WM Scheme by Li and Yuan (Hans) Georg Schaathun Department of Computing University of Surrey 22-23 September 2008 (Hans) Georg Schaathun Diffusion 22-23 September 2008 1 / 23 Do not reuse the key

  1. Diffusion and a Key-Recovery Attack on a WM Scheme by Li and Yuan (Hans) Georg Schaathun Department of Computing University of Surrey 22-23 September 2008 (Hans) Georg Schaathun Diffusion 22-23 September 2008 1 / 23

  2. Do not reuse the key Andrew Ker Keys are reused in cryptography The one-time pad is not practical The solution is diffusion Each key bit is spread widely across output Dependendy between key and output is too complex for analysis We shall see lack of diffusion later (stay awake) (Hans) Georg Schaathun Diffusion 22-23 September 2008 2 / 23

  3. Watermarking is not Cryptography Ingemar Cox If we don’t study watermarking as a cryptographic problem, how do you know that cryptanalysis cannot break it? If it can be cast as a cryptographic problem you have to use cryptology in the design, because your adversary may use it in the attack Cryptology is a methodology, not just a series of primitives Admittedly, Li-Yuan is better seen as a layered system We break the cryptological layer We do not touch the watermarking layer (embedding) i.e. Cox’ view may stand . . . for now (Hans) Georg Schaathun Diffusion 22-23 September 2008 3 / 23

  4. Watermarking is not Cryptography Ingemar Cox If we don’t study watermarking as a cryptographic problem, how do you know that cryptanalysis cannot break it? If it can be cast as a cryptographic problem you have to use cryptology in the design, because your adversary may use it in the attack Cryptology is a methodology, not just a series of primitives Admittedly, Li-Yuan is better seen as a layered system We break the cryptological layer We do not touch the watermarking layer (embedding) i.e. Cox’ view may stand . . . for now (Hans) Georg Schaathun Diffusion 22-23 September 2008 3 / 23

  5. Watermarking is not Cryptography Ingemar Cox If we don’t study watermarking as a cryptographic problem, how do you know that cryptanalysis cannot break it? If it can be cast as a cryptographic problem you have to use cryptology in the design, because your adversary may use it in the attack Cryptology is a methodology, not just a series of primitives Admittedly, Li-Yuan is better seen as a layered system We break the cryptological layer We do not touch the watermarking layer (embedding) i.e. Cox’ view may stand . . . for now (Hans) Georg Schaathun Diffusion 22-23 September 2008 3 / 23

  6. Authentication and Watermarking Outline Authentication and Watermarking 1 Li-Yuan Authentication WM 2 How to break it 3 How to fix it – maybe 4 Closure 5 (Hans) Georg Schaathun Diffusion 22-23 September 2008 4 / 23

  7. Authentication and Watermarking Digital Watermarking Message Recovered Extractor Host File Embedding Digital Watermarking ‘hides’ a message in another file (the host ) The watermarked image can replace the cover Perceptually Equivalent In fragile watermarking The host cannot be modified without destroying the hidden message In robust watermarking The hidden message cannot be modified or destroyed without destroying the host (Hans) Georg Schaathun Diffusion 22-23 September 2008 5 / 23

  8. Authentication and Watermarking Digital Watermarking Message Recovered Extractor Host File Embedding Digital Watermarking ‘hides’ a message in another file (the host ) The watermarked image can replace the cover Perceptually Equivalent In fragile watermarking The host cannot be modified without destroying the hidden message In robust watermarking The hidden message cannot be modified or destroyed without destroying the host (Hans) Georg Schaathun Diffusion 22-23 September 2008 5 / 23

  9. Authentication and Watermarking Digital Watermarking Message Recovered Extractor Host File Embedding Digital Watermarking ‘hides’ a message in another file (the host ) The watermarked image can replace the cover Perceptually Equivalent In fragile watermarking The host cannot be modified without destroying the hidden message In robust watermarking The hidden message cannot be modified or destroyed without destroying the host (Hans) Georg Schaathun Diffusion 22-23 September 2008 5 / 23

  10. Authentication and Watermarking Digital Watermarking Message Key Recovered Extractor Host File Embedding Digital Watermarking ‘hides’ a message in another file (the host ) The watermarked image can replace the cover Perceptually Equivalent In fragile watermarking The host cannot be modified without destroying the hidden message In robust watermarking The hidden message cannot be modified or destroyed without destroying the host (Hans) Georg Schaathun Diffusion 22-23 September 2008 5 / 23

  11. Authentication and Watermarking The Authentication Problem Alice sends a message to Bob Bob wants to assure that it is authentic (Hans) Georg Schaathun Diffusion 22-23 September 2008 6 / 23

  12. Authentication and Watermarking The Authentication Problem Alice sends a message to Bob Bob wants to assure that it is authentic Eve wants to modify the message and fool Bob (Hans) Georg Schaathun Diffusion 22-23 September 2008 6 / 23

  13. Authentication and Watermarking Authentication Techniques Cryptograhic solutions Message Authentication Code (MAC) – Secret Key Digital Signatures – Public Key Watermarking embeds Authentication Information in the file no appended signature to handle everything fits into the host file format Creating and attacking the authentication information remains a cryptological prolem layered system (here Cox and I agree) It does not matter if the designer agrees I, as an attacker, can use cryptology anyway (Hans) Georg Schaathun Diffusion 22-23 September 2008 7 / 23

  14. Authentication and Watermarking Authentication Techniques Cryptograhic solutions Message Authentication Code (MAC) – Secret Key Digital Signatures – Public Key Watermarking embeds Authentication Information in the file no appended signature to handle everything fits into the host file format Creating and attacking the authentication information remains a cryptological prolem layered system (here Cox and I agree) It does not matter if the designer agrees I, as an attacker, can use cryptology anyway (Hans) Georg Schaathun Diffusion 22-23 September 2008 7 / 23

  15. Authentication and Watermarking Authentication Techniques Cryptograhic solutions Message Authentication Code (MAC) – Secret Key Digital Signatures – Public Key Watermarking embeds Authentication Information in the file no appended signature to handle everything fits into the host file format Creating and attacking the authentication information remains a cryptological prolem layered system (here Cox and I agree) It does not matter if the designer agrees I, as an attacker, can use cryptology anyway (Hans) Georg Schaathun Diffusion 22-23 September 2008 7 / 23

  16. Authentication and Watermarking Cryptography Authentication Techniques Cryptograhic solutions Message Authentication Code (MAC) – Secret Key Digital Signatures – Public Key Certificate of Authenticity (Signature or MAC) ... appended to the message does not fit into standard file formats Only Alice can produce a valid certificate well-studied and trusted technology mathematical security (Hans) Georg Schaathun Diffusion 22-23 September 2008 8 / 23

  17. Authentication and Watermarking Cryptography Authentication Techniques Cryptograhic solutions Message Authentication Code (MAC) – Secret Key Digital Signatures – Public Key Certificate of Authenticity (Signature or MAC) ... appended to the message does not fit into standard file formats Only Alice can produce a valid certificate well-studied and trusted technology mathematical security (Hans) Georg Schaathun Diffusion 22-23 September 2008 8 / 23

  18. Authentication and Watermarking Cryptography Authentication Techniques Cryptograhic solutions Message Authentication Code (MAC) – Secret Key Digital Signatures – Public Key Certificate of Authenticity (Signature or MAC) ... appended to the message does not fit into standard file formats Only Alice can produce a valid certificate well-studied and trusted technology mathematical security (Hans) Georg Schaathun Diffusion 22-23 September 2008 8 / 23

  19. Authentication and Watermarking Cryptography Authentication Techniques Cryptograhic solutions Message Authentication Code (MAC) – Secret Key Digital Signatures – Public Key Certificate of Authenticity (Signature or MAC) ... appended to the message does not fit into standard file formats Only Alice can produce a valid certificate well-studied and trusted technology mathematical security (Hans) Georg Schaathun Diffusion 22-23 September 2008 8 / 23

  20. Authentication and Watermarking Authentication Watermarking Authentication information is embedded in the file no appended signature to handle everything fits into the host file format Some watermarking systems offer extra advantages localisation of changes/errors further analysis of modification processes Creating and attacking the authentication information remains a cryptological prolem layered system (here Cox and I agree) (Hans) Georg Schaathun Diffusion 22-23 September 2008 9 / 23

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