Audio Cryptography

March 5th, 2014

1.     Make sure that your computer has a pair of speakers (the left and the right); if you are using a laptop, this requirement is often automatically met

2.     Listen to this music file ([about 38M bytes], called share 1 hereafter)

3.     Now, listen to this music file  ([about 38M bytes], called share 2 hereafter). Can you find any difference between it and the above one?

a.     They are normal songs, aren't they?

Now what?


We will try to play these two music files on two speakers simultaneously. How can we do that?

·         You do not need a general computer to do that. If you have two MP3 players, you can simply simultaneously play one music file on one player.

·         Of course, you can do parallel playing on a single computer and I will give the instructions below.


Before you jump into details, if you are way too eager, click this link to hear the parallel playing result. What have you heard?

·         What? You have not noticed anything in the music file? Need to listen again

·         Hints:

o   There is a “hidden” bit message in this simultaneous playing: five seconds of high volume is 1, five seconds of low volume is 0

o   What is the “hidden” bit message?


·         Common misconception: No, no, this is not a regular hiding. An attacker who knows every detail of the system (including the “hiding” method) but does not have both shares will not be able to recover the “hidden” bit message, no matter how fast his/her computer is

[Instructions to play the above music files simultaneously on a single computer] The following steps teach you how to play share 1 and share 2 simultaneously on two speakers. We are going to use Audacity, an open-source software to do this and to that end, an Audacity project has been created for your convenience.  (Later, you will create your own Audacity project from the above share 1 and share 2 files; see more instructions below.) But for now, let's try my Audacity project first to see/hear the effect of playing them simultaneously by following Steps 4~9.

4.   Download and install Audacity from here

5.   If you do not have 7ZIP, please download and install it from here

6.   Download Mozart.7z and unzip it with 7ZIP. You will get a subdirectory called mozart_data and a file called mozart.aup

a.   mozart.aup is an Audacity project that can play share 1 and share 2 simultaneously on your left/right speakers respectively

7.   Run Audacity and open mozart.aup with "File | Open"

8.   Click on the "Play" button to play

1)      Listen carefully. Listen multiple times if necessary. Can you feel its difference from listening shares 1 and 2 separately above? (This Play should have the same effect as playing this link, if you have done so.)

OK. How to play share 1 in Step 1 and share 2 in Step 2 simultaneously by yourself, without using my Audacity project? Here are the instructions: 1) First, download share 1 with the link in Step 1; 2) Second, download share 2 with the link in Step 2; 3) Third, run Audacity to open Share 1 (with "File | Open") but do not play it yet; 4) In the current window, import Share 2 file (by clicking "File | Import | Audio"); 5) Your current window has two audio files/tracks, the top is Share 1 and the bottom is Share 2. In the left panel of the top half/track, there is a slider called Pan Control (with "L" and "R" labels, do not pick it wrong) and drag it to the leftmost position. In the left panel of the bottom half/track, drag the Pan Control slider to the rightmost position. In this way, you will play Share 1 to the left speaker and Share 2 to the right speaker simultaneously; 5) Click the Play button.

How to combine share 1 and share 2 into a single file (as in this link) with the same effect as in Steps 4~9? Simply click "File | Export", choose "Save as type" as "WAV (Microsoft) signed 16 bit PCM", type in your file name, click "Save" and you are done. You will have an audio file similar to this this audio file [about 30M bytes]


1)      This demo is a re-implementation of the audio cryptography scheme developed in this paper

[DHQ98] Yvo Desmedt, Shuang Hou, and Jean-Jacques Quisquater. Audio and Optical Cryptography. In Proceedings of ASIACRYPT'98, LNCS 1514, pp. 392- 404, 1998.

2)      This re-implementation was developed by Wooyoung Chung

3)      The original audio file is from here and was performed by Idil Biret