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

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

3 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

4 Previous Works DHQ Audio Cryptography Scheme (Desmedt, Hou & Quisquater, Asiacrypt’98) DLQ Audio Cryptography Scheme (Desmedt, Le & Quisquater, Proceedings of Info Hiding’99)

5 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  lon 2 n  different sounds

6 A DHQ (2, 2) instance M = ( )

7 Disadvantages of DHQ ACS Low contrast when secret perceiving  lon 2 n  cover sounds are needed for (2, n) scheme Precise synchronization is needed When secret hiding, a computing device is needed

8 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)

9 Disadvantages of DLQ ACS  lon 2 n  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

The proposed Easy Audio Cryptography Scheme  A (2, n) ACS

11 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.

12 Construction: Assume the m-bit secret message M = [M 1, M 2, …, M m ] M j  {0, 1}. Let B L and B H be Matrices specified in Definition 1 and S n  m = [s ij ] = [B 1 |B 2 |…|B m ] be an n  m Matrices where Then the n shares are constructed by recording the “sound s ij ” into the jth sound of the ith share for all i and j.

13 Reconstruction: Let two shares be S i = [s i1, s i2, …, s im ] and S j = [s j1, s j2, …, s jm ] 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.

14 Example 1 (Different Sounds) Rhythm X Rhythm Y Rhythm X+Y ……………………  …  …  … …  … …  … …     Message 0 … 1 … 0 … 1 …

15 Example 2 (General Sentences) Rhythm X Rhythm Y Hello! -- This is Linda. -- Hey! -- This is Linda. -- Rhythm X+Y Hello! -- This is Linda. – (Hey! --) (This is Linda. --) My number is Please call me. – (My number is ) (Please call me. --) Rhythm X Rhythm Y My number is Please call me. -- My number is Please call me. --  Message 1 … 0 … 1 … 0 …

16 Comparison Item Method Contrast Band Width Cover Sound n  BW Precisely Synchronize Computing Device Cost DHQ Low  lon 2 n      Yes Middle DLQ -High  lon 2 n      Yes High Proposed HighMiddle0indepNo Low  Note: n  BW: relationship between n and bandwidth IP: inverse proportion