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A Copy-Protection Technique with Multi-Level Error Coding Chen-Yin - - PowerPoint PPT Presentation
A Copy-Protection Technique with Multi-Level Error Coding Chen-Yin - - PowerPoint PPT Presentation
A Copy-Protection Technique with Multi-Level Error Coding Chen-Yin Liao, Jen-Wei Yeh and Ming-Seng Kao Department of Communication Engineering National Chiao-Tung University Hsinchu, Taiwan 1 Contents Introduction Coding scheme
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Copy protection of optical disks
No method can absolutely prevent unauthorized
copying of optical disks.
Approach: increase the difficulty of disk copying.
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Idea
- X-bits :
special bits with reflectivity (R) within a specified range.
If the range of R is rather limited, it will be very
difficult to reproduce X-bits.
Challenge:
– limit R to a very small range. – guarantee successful play of authorized disks. – reject unauthorized copies definitely.
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X-bits
Normal bits:
with normalized reflectivity equal to 1 or 0.
X-bits:
have reflectivity 0.5- α < R < 0.5+α. (e.g. α=0.01, 0.49 < R < 0.51).
If α is small enough, it will be very difficult to
reproduce X-bits, even for professional pirates.
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Double-read process
X-bits appear in the lead-in sector of the disk
- nly.
X-bits can be detected via double-read process:
– if the outcomes of two reading processes are different, the corresponding bit is assumed to be an X-bit. – Otherwise, it is taken as a normal bit.
The extra time for double-read process is little.
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Error probability of X-bits
Pe is a function of αn= α/σn.
(σn
2 : noise variance). Pe is high, with Pe(min) = 0.5. X-bits: used as potential
errors in the disk. λ
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Design idea
X-bits :
sets a hardware obstacle to disk copying.
Multi-level error coding :
– limit αn to a very small range (αn < αc, ). – if αn is out of range, the disk content will be lost.
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Proposed scheme
The content is scrambled by an encryption key
before stored in the disk.
This key is encoded by the multi-level error
coding plus the X-bit coding.
The encoded key with X-bits is stored in the
lead-in sector of the disk.
On playing the disk, the key should be correctly
decoded so as to de-scramble the content .
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Goal of coding scheme
Achieve sharp discrimination of αn .
– if αn < αc , then Pkey→ 1. – if αn > αc , then Pkey→ 0 .
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Encoding process
K C
N1 1st-level FC’s 1st-level coding
NL
Lth-level FC’s Lth-level coding
Optical
disk
X-bit coding (n,k) RS-code
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Decoding process
K C
N1 1st-level FC’s 1st-level decoding NL Lth-level FC’s Lth-level decoding Optical disk X-bit decoding (n,k) RS-decoder
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Detection probability of codewords
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Discussion
The detection probability
varies with decoding level.
A sharp cutoff exists in
the detection probability
- f codewords after the 1st-
level decoding.
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Detection probability of the key
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Discussion
For authorized disks with αn < αc, successful
playing is guaranteed since Pkey→ 1.
For copied disks with αn > αc, it will have no
chance to be played since Pkey→ 0.
Different RS codes can be used to specify αc.
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Discussion
If αc is small enough, X-bits can only be made with
very-high precision machines.
If these machines are controlled by few disk
manufacturers, it will be very difficult and costly to access them.
If double-read process is a standard procedure in
disk players and those high-precision machines are well controlled, professional piracy will be effectively prevented.
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Conclusions
A new copy protection technique based on
multi-level error coding is proposed.
The multi-level coding leads to a sharp cutoff in
the detection probability of the encryption key.
This sharp cutoff effect is sued to set a hard
- bstacle for disk copying.
The proposed scheme is feasible to effectively
prevent professional piracy.
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