Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Hello! 

I'm trying to put DRM (fairplay) with `HLS SAES` but for some reason is not working as expected

1. When I put the `constant_IV` it puts it in each fragment when it should only put it in the "header" and besides it is not constant in each video fragment it increases in 1. **(look IV in the actual m3u8)**
2. The encryption type of the `EXT-X-KEY:METHOD` is left as `AES-128` and I am expecting it to be `SAMPLE-AES`.
### Command
```bash
gpac -i 1080.mp4 -o res-hls/manifest.m3u8:segdur=10:hlsdrm=drm-hls.xml
```
### drm-hls.xml
```xml
<?xml version="1.0" encoding="UTF-8"?>
<GPACDRM type="HLS SAES">
    <DRMInfo type="pssh" version="1" cypherOffset="1">
        <!-- System ID (Fairplay)  -->
        <BS ID128="94CE86FB07FF4F43ADB893D2FA968CA2"/>
        <BS value="1" bits="32"/>
        <BS ID128="0x4c2xxxxxxxxxxxxxxxxxxxxxdb156f8cda"/>
        <BS ID128="0xe98xxxxxxxxxxxxxxxxxxxxxe3261e7e1d"/>
    </DRMInfo>
    <CrypTrack constant_IV="0x4c2xxxxxxxxxxxxxxxxxxxxxdb156f8cda">
        <key KID="0x4c2xxxxxxxxxxxxxxxxxxxxxdb156f8cda" value="0xe98xxxxxxxxxxxxxxxxxxxxx3261e7e1d" hlsInfo='URI="skd://fps.drm_provider.com/;4c2xxxxx-xxxx-xxxx-xxxx-xxdb156f8cda"'/>
    </CrypTrack>
</GPACDRM>

```

### Actual
```m3u
#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-VERSION:6
#EXT-X-MEDIA-SEQUENCE:1
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-MAP:URI="1080_dash_track1_init.mp4"
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDB
#EXTINF:14.9333,
1080_dash_track1_1.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDC
#EXTINF:5.2,
1080_dash_track1_2.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDD
#EXTINF:10.2667,
1080_dash_track1_3.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDE
#EXTINF:10.1333,
1080_dash_track1_4.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDF
#EXTINF:10.1333,
1080_dash_track1_5.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCD0
#EXTINF:9.8,
1080_dash_track1_6.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCD1
#EXTINF:10.2,
1080_dash_track1_7.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCD2
```
### Expected
```m3u8
#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-VERSION:6
#EXT-X-MEDIA-SEQUENCE:1
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-MAP:URI="1080_dash_track1_init.mp4"
#EXT-X-KEY:METHOD=SAMPLE-AES,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDA
#EXTINF:14.9333,
1080_dash_track1_1.m4s
#EXTINF:5.2,
1080_dash_track1_2.m4s
#EXTINF:10.2667,
1080_dash_track1_3.m4s
#EXTINF:10.1333,
1080_dash_track1_4.m4s
#EXTINF:10.1333,
1080_dash_track1_5.m4s
#EXTINF:9.8,
1080_dash_track1_6.m4s
#EXTINF:10.2,
1080_dash_track1_7.m4s
```

I don't know if maybe I'm missing something, but from all the documentation I've read, the command and the DRM config looks good 


DRM with fairplay and HLS SAES problms
This is a GitHub Issue
repo:gpac
owner:gpac
Title : DRM with fairplay and HLS SAES problms
Issue date:

--- start body ---
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Hello! 

I'm trying to put DRM (fairplay) with `HLS SAES` but for some reason is not working as expected

1. When I put the `constant_IV` it puts it in each fragment when it should only put it in the "header" and besides it is not constant in each video fragment it increases in 1. **(look IV in the actual m3u8)**
2. The encryption type of the `EXT-X-KEY:METHOD` is left as `AES-128` and I am expecting it to be `SAMPLE-AES`.
### Command
```bash
gpac -i 1080.mp4 -o res-hls/manifest.m3u8:segdur=10:hlsdrm=drm-hls.xml
```
### drm-hls.xml
```xml
<?xml version="1.0" encoding="UTF-8"?>
<GPACDRM type="HLS SAES">
    <DRMInfo type="pssh" version="1" cypherOffset="1">
        <!-- System ID (Fairplay)  -->
        <BS ID128="94CE86FB07FF4F43ADB893D2FA968CA2"/>
        <BS value="1" bits="32"/>
        <BS ID128="0x4c2xxxxxxxxxxxxxxxxxxxxxdb156f8cda"/>
        <BS ID128="0xe98xxxxxxxxxxxxxxxxxxxxxe3261e7e1d"/>
    </DRMInfo>
    <CrypTrack constant_IV="0x4c2xxxxxxxxxxxxxxxxxxxxxdb156f8cda">
        <key KID="0x4c2xxxxxxxxxxxxxxxxxxxxxdb156f8cda" value="0xe98xxxxxxxxxxxxxxxxxxxxx3261e7e1d" hlsInfo='URI="skd://fps.drm_provider.com/;4c2xxxxx-xxxx-xxxx-xxxx-xxdb156f8cda"'/>
    </CrypTrack>
</GPACDRM>

```

### Actual
```m3u
#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-VERSION:6
#EXT-X-MEDIA-SEQUENCE:1
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-MAP:URI="1080_dash_track1_init.mp4"
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDB
#EXTINF:14.9333,
1080_dash_track1_1.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDC
#EXTINF:5.2,
1080_dash_track1_2.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDD
#EXTINF:10.2667,
1080_dash_track1_3.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDE
#EXTINF:10.1333,
1080_dash_track1_4.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDF
#EXTINF:10.1333,
1080_dash_track1_5.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCD0
#EXTINF:9.8,
1080_dash_track1_6.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCD1
#EXTINF:10.2,
1080_dash_track1_7.m4s
#EXT-X-KEY:METHOD=AES-128,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCD2
```
### Expected
```m3u8
#EXTM3U
#EXT-X-TARGETDURATION:10
#EXT-X-VERSION:6
#EXT-X-MEDIA-SEQUENCE:1
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-MAP:URI="1080_dash_track1_init.mp4"
#EXT-X-KEY:METHOD=SAMPLE-AES,URI="skd://fps.drm_provider.com/;4c2XXXXX-7ab1-XXXX-XXXX-XXXXXXXXcda",IV=0x4C2XXXXX7AB1XXXXXXXXXXXXXXXXCDA
#EXTINF:14.9333,
1080_dash_track1_1.m4s
#EXTINF:5.2,
1080_dash_track1_2.m4s
#EXTINF:10.2667,
1080_dash_track1_3.m4s
#EXTINF:10.1333,
1080_dash_track1_4.m4s
#EXTINF:10.1333,
1080_dash_track1_5.m4s
#EXTINF:9.8,
1080_dash_track1_6.m4s
#EXTINF:10.2,
1080_dash_track1_7.m4s
```

I don't know if maybe I'm missing something, but from all the documentation I've read, the command and the DRM config looks good 


--- end body ---

The issue describes a problem with the implementation of DRM using Fairplay and HLS SAES in the GPAC library. The user reports that the encryption method is incorrectly set to AES-128 instead of SAMPLE-AES, and the constant IV is not being applied correctly, which could lead to security vulnerabilities in the DRM implementation. This could allow unauthorized access to the content being protected by DRM.