How to decrypt the encrypted audio file using mp4box with eme

Question

I'm getting trouble with encrypt/decrypt file using mp4box and EME. I don't know how to do step by step.

https://simpl.info/eme/clearkey/

http://www.gpac-licensing.com/2014/03/06/free-slides-mpeg-cenc-html-eme-leveraging-content-protection-web-distribution/

Now I can encrypt the audio file, but I don't know how to decrypt it. I used this key to decrypt like the example in simpl.info:

    <key KID="0x279926496a7f5d25da69f2b3b2799a7f" value="0xccc0f2b3b279926496a7f5d25da692f6"/>

---

function generateLicense(message) {
  // Parse the clearkey license request.
  var request = JSON.parse(new TextDecoder().decode(message));
  // We only know one key, so there should only be one key ID.
  // A real license server could easily serve multiple keys.
  console.assert(request.kids.length === 1);

  var keyObj = {
    kty: 'oct',
    alg: 'A128KW',
    kid: '0x279926496a7f5d25da69f2b3b2799a7f',//request.kids[0],
    k: '0xccc0f2b3b279926496a7f5d25da692f6'//toBase64(KEY)
  };
  return new TextEncoder().encode(JSON.stringify({
    keys: [keyObj]
  }));
}

Here is my key_file to encrypt:

<?xml version="1.0" encoding="UTF-8"?>
<GPACDRM type="CENC AES-CTR">
<!-- example for GPAC 'clear' DRM System - keys are listed after the content and UL follows -->
<DRMInfo type="pssh" version="1" cypherOffset="9" cypherKey="0x6770616363656E6364726D746F6F6C31" cypherIV="0x00000000000000000000000000000001">
<BS ID128="6770616363656E6364726D746F6F6C31"/>
<BS value="2" bits="32"/>
<BS ID128="0x279926496a7f5d25da69f2b3b2799a7f"/>
<BS ID128="0x676cb88f302d10227992649885984045"/>
<BS bits="8" string="CID=Toto"/>
<BS ID128="0xccc0f2b3b279926496a7f5d25da692f6"/>
<BS ID128="0xccc0f2b3b279926496a7f5d25da692d6"/>
</DRMInfo>
<CrypTrack trackID="1" IsEncrypted="1" IV_size="16" first_IV="0x0a610676cb88f302d10ac8bc66e039ed" saiSavedBox="senc">
<key KID="0x279926496a7f5d25da69f2b3b2799a7f" value="0xccc0f2b3b279926496a7f5d25da692f6"/>
</CrypTrack>
</GPACDRM>

Anyone can help me? Thanks!