getPSSH the n00b's way

[moaBoBmArLeY]

Hello, the get_pssh(mpd_url) found in some script never worked for me, so this is n00b's way (noob in python) :

import urllib.request

mpd_url = input("\nInput mpd url: ")
print("")

try:
    mpd = urllib.request.urlopen(mpd_url)
    print('[+] Retrieving mpd file')
    print("")
    mpd_content = mpd.read().decode("utf-8")
    Balise_A, Balise_B = mpd_content.find('<cenc:pssh>'), mpd_content.find('</cenc:pssh>')
    pssh = mpd_content[Balise_A+11:Balise_B]
except Exception as e:
    print(f"[-] couldn't retrive mpd file.\n\terror: {e}")
    exit()

No need of parsing xml brows tree etc. ...
It presume's that all encoded streams have the same pssh.

Et voilà

[Obo]

Sorry to be a spoiler, but this method won't reliably work if the mpd contains PSSH's for multiple content protection systems like PlayReady.

There is a reason why you'd usually want to parse the mpd content. You should at least check that you got a Widevine PSSH...

The easiest probably is to search for

<cenc:pssh>AAAA

or (imo even simpler) use a regex like:

import re
m = re.search("<cenc:pssh>(?P<pssh>AAAA.+)</cenc:pssh>", mpd_content)
print(m['pssh'])

[Karoolus]

Sorry to be even spoilier (is that even a word?) but what if there is no PSSH in the MPD?

In that case you're better off getting the PSSH from the init file. There's a python code floating around this forum to get that done.

[Obo]

(Deepl means: yes, exists; but has a different meaning )

For the record, here is a slightly modified version of the script you mentioned:

import sys
from pywidevine import PSSH

def find_wv_pssh_offsets(raw: bytes) -> list:
    offsets = []
    offset = 0
    while True:
        offset = raw.find(b'pssh', offset)
        if offset == -1:
            break
        size = int.from_bytes(raw[offset-4:offset], byteorder='big')
        pssh_offset = offset - 4
        offsets.append(raw[pssh_offset:pssh_offset+size])
        offset += size
    return offsets

def to_pssh(content: bytes) -> list:
    wv_offsets = find_wv_pssh_offsets(content)
    return [base64.b64encode(wv_offset).decode() for wv_offset in wv_offsets]

def from_file(file_path: str) -> list:
    with open(file_path, 'rb') as f:
        buffer = f.read(1_048_576)
    return to_pssh(buffer)

pssh_list = from_file(sys.argv[1])
target_pssh = None
for item in pssh_list:
    pssh = PSSH(item)
    if pssh.system_id == PSSH.SystemId.Widevine:
        print(pssh)

[moaBoBmArLeY]

lol I told you that it is the n00b way, and it works for me for a simple mpd file with the same pssh for all encrypted streams.