A New Graphical Method for Encryption of Computer Data by Charles Schwartz and Cryptanalysis on Schw

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A New Graphical Method for Encryption of
Computer Data by Charles SchwartzandCryptanaly
sis on Schwartz Graphical Encryption Method by
Yuan-Chung Chin, PeCheng Wang and Jing-Jang Hwang

• Steven Montgomery

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Overview

• The Algorithm
• Discovery
• Implementation
• Estimated Security
• Demonstration
• Cryptanalysis
• Known Plaintext Attack
• Chosen Plaintext Attack

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The Algorithm

• Graphical operations on a MAC
• Private symmetric key system
• Use computers pseudo-random number generator -
  pair of numbers
• Graphical pen draws line
• Repeat 2000 times

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The Code

• int mess_it()
• int k, x, y
• PenSize(8,8)
• PenMode(10) // XOR(inversion) mode
• randSeedcodenum
• MoveTo(0,0)
• for(k0 klt2000 k)
• x256Random()/100
• y180Random()/150
• LineTo(x,y)

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Security

• No distinguishable pattern
• Brute Force
• Random num generator has cycle of 109
• Good guess will unscramble partially
• About 106 guesses
• Encrypt again to bring it up

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Demonstration
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Cryptanalysis with Plaintext/Ciphertext Pair

• Just confusion, No diffusion
• Encryption S(PT,k) PT xor L1 xor ldots xor Ln
  CT
• Decryption S(CT,k) CT xor L1 xor ldots xor Ln
  PT
• where
• S Schwartz encryption algorithm
• Li Graphical line

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Cryptanalysis with Plaintext/Ciphertext
Pair(cont.)

• Let E be an empty plaintext(all-zeros)
• PT xor L1 xor ... xor Ln PT xor E xor L1 xor
  ... xor Ln
• E xor L1 xor ... xor Ln can be obtained by
  S(E,k). Call this mask, Mk
• Encryption S(PT,k) PT xor Mk CT
• Decryption S(CT, k) CT xor Mk PT

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Cryptanalysis with Plaintext/Ciphertext
Pair(cont.)

• Dont need key, just mask!
• PT xor CT Mk
• Intercept CT
• PT CT xor Mk

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Cryptanalysis with Chosen Plaintext Attack

• Simply encrypt an empty plaintext, E to get mask
• Mk S(E, k)
• Or, encrypt any plaintext to get the
  corresponding ciphertext
• Mk PT xor CT

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Conclusions

• Easy to implement, errors dont propagate and
  size of CT is equal to PT
• Mask approach very successful since encryption is
  plaintext-independent
• Suggest including a plaintext-dependent process
  to resist attack
• Would cause errors to propagate

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Questions?