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
Contents
Introduction of ACS
Preview Works (DHQ and DLQ ACS)
Easy Audio Cryptography Scheme
Comparison
2
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
3
Previous Works

DHQ Audio Cryptography Scheme
(Desmedt, Hou & Quisquater, Asiacrypt’98)

DLQ Audio Cryptography Scheme
(Desmedt, Le & Quisquater, Proceedings of Info
Hiding’99)
4
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
5
A DHQ (2, 2) instance
M = (0 0 1 1)
6
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
7
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)
8
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
9
The proposed
Easy Audio Cryptography Scheme
 A (2, n) ACS

 1 
 K 




2

K
Definition 1: BL 
and BH 
 
  
 n 
 K 
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.
11

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] =
[B1|B2|…|Bm] be an n  m Matrices where
[BL ]P , if M j  0
Bj  
P
[
B
]
, if M j  1
H

Then the n shares are constructed by recording the
“sound sij” into the jth sound of the ith share for all i
and j.
12

Reconstruction: Let two shares be Si = [si1, si2, …,
sim] and Sj = [sj1, sj2, …, sjm] i  j. Then
0, if sik  s jk

Mk  
, k  1,2,..., m
1, if sik  s jk

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.
13
Example 1 (Different Sounds)
Rhythm X
…………
… ………


Rhythm X+Y …
…… …


 Message 0 … 1 … 0 … 1 …
Rhythm Y
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Example 2 (General Sentences)
Rhythm X
Hello! --
This is Linda. --
Rhythm Y Hey! --
This is Linda. --
Rhythm X
My number is 4441234. --
Please call me. --
Rhythm Y My number is 8885678. -Hello! --
Please call me. -This is Linda. –
(Hey! --)
(This is Linda. --)
My number is 4441234. --
Please call me. –
Rhythm X+Y
(My number is 8885678. --) (Please call me. --)
 Message
1
… 0… 1 … 0…
15
Comparison
Item
Contrast
Band
Width
DHQ
Low
Low
DLQ
-
High
Proposed
High
Middle
Method
Cover
Sound
Precisely
Synchronize
Computing
Device
Cost
lon2n   
Yes
Yes
Middle
lon2n   
Yes
Yes
High
No
No
Low
0
nBW
indep
 Note:
• nBW: relationship between n and bandwidth
• IP: inverse proportion
16