An Easy Method to Implement Audio Cryptography Schemes without Computing Devices

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An Easy Method to Implement Audio Cryptography
Schemes without Computing Devices

• Shin Yan Chiou and Chi Sung Laih
• Department of Electrical Engineering,
• Director, Computer and Communication Network
  Center,
• National Cheng Kung University, Tainan, TAIWAN
• Republic Of China

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Contents

• Introduction of ACS

Preview Works (DHQ and DLQ ACS)
Easy Audio Cryptography Scheme
Comparison
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Introduction of ACS

• A secret sharing scheme
• A method to hide a message into n sounds
• Perceive the message by ears by playing t of the
  n sounds simultaneously
• Similar to Visual Secret Sharing Scheme but use
  ears instead of eyes to perceive the secret

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Previous Works

• DHQ Audio Cryptography Scheme(Desmedt, Hou
  Quisquater, Asiacrypt98)
• DLQ Audio Cryptography Scheme(Desmedt, Le
  Quisquater, Proceedings of Info Hiding99)

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DHQ ACS

• Wave based (2, 2) audio crypto scheme.
• In phase to be secret 1 and out of phase to be
  secret 0
• A (2, n) scheme needs ?lon2n? different sounds

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A DHQ (2, 2) instance
M (0 0 1 1)
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Disadvantages of DHQ ACS

• Low contrast when secret perceiving
• ?lon2n? cover sounds are needed for (2, n) scheme
• Precise synchronization is needed
• When secret hiding, a computing device is needed

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DLQ ACS

• A sort of (2, 2) ACS
• Nonbinary Audio, such as human speech, can be
  perceived.
• To hide the secret by embedding the preshares
  into a covering signal to be shares
• To perceive the secret by both using a mixer to
  eliminate the covering signal and a amplifier to
  amplify the message synthesized by two shares (or
  preshares)

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Disadvantages of DLQ ACS

• ?lon2n? cover sounds are needed for (2, n) scheme
• Precisely synchronization is needed
• When secret hiding, a special computing device is
  needed
• When secret perceiving, some special devices such
  as a mixer and a amplifier are needed

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The proposed Easy Audio Cryptography Scheme

• A (2, n) ACS

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• Definition 1 and are n ?
  1 Matrices representing bit 0 and bit 1
  respectively, where the notation ?k, 1 ? k ? n,
  denotes the sound k which is the kth sound of
  the given n sounds.

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• Construction Assume the m-bit secret message M
  M1, M2, , Mm Mj ?0, 1. Let BL and BH be
  Matrices specified in Definition 1 and Sn?m
  sij B1B2Bm be an n ? m Matrices
  whereThen the n shares are constructed by
  recording the sound sij into the jth sound of
  the ith share for all i and j.

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• Reconstruction Let two shares be Si si1,
  si2, , sim and Sj sj1, sj2, , sjm i ? j.
  Then
• By playing any two of the constructed n shares
  simultaneously, then the kth bit would be bit 1
  if the two of the kth sound of the two shares are
  different and would be bit 0 if they are the same.

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Example 1 (Different Sounds)
????
Rhythm X
? ???
Rhythm Y
?
?
? ?
Rhythm XY
?
?
0 1 0 1
? Message
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Example 2 (General Sentences)
Rhythm X
My number is 4441234. -- Please call me. --
Rhythm Y
My number is 8885678. -- Please call me. --
1 0 1 0
? Message
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Comparison

• Note
• n?BW relationship between n and bandwidth
• IP inverse proportion