Extraction: Move non-stateful signature decryption functionality into yt_data_extract

2 files changed, 98 insertions, 1 deletions

diff --git a/youtube/yt_data_extract/__init__.py b/youtube/yt_data_extract/__init__.py
index f2f07c0..898141e 100644
--- a/youtube/yt_data_extract/__init__.py
+++ b/youtube/yt_data_extract/__init__.py
@@ -7,4 +7,5 @@ from .everything_else import (extract_channel_info, extract_search_info,
     extract_playlist_metadata, extract_playlist_info, extract_comments_info)
 
 from .watch_extraction import (extract_watch_info, get_caption_url,
-    update_with_age_restricted_info)
+    update_with_age_restricted_info, requires_decryption,
+    extract_decryption_function, decrypt_signatures)
diff --git a/youtube/yt_data_extract/watch_extraction.py b/youtube/yt_data_extract/watch_extraction.py
index ff39f62..09abbe3 100644
--- a/youtube/yt_data_extract/watch_extraction.py
+++ b/youtube/yt_data_extract/watch_extraction.py
@@ -7,6 +7,7 @@ from .common import (get, multi_get, deep_get, multi_deep_get,
 import json
 import urllib.parse
 import traceback
+import re
 
 # from https://github.com/ytdl-org/youtube-dl/blob/master/youtube_dl/extractor/youtube.py
 _formats = {
@@ -377,7 +378,11 @@ def extract_watch_info(polymer_json):
     info['base_js'] = deep_get(top_level, 'player', 'assets', 'js')
     if info['base_js']:
         info['base_js'] = normalize_url(info['base_js'])
+        info['player_name'] = get(info['base_js'].split('/'), -2)
+    else:
+        info['player_name'] = None
 
+    # extract stuff from visible parts of page
     mobile = 'singleColumnWatchNextResults' in deep_get(top_level, 'response', 'contents', default={})
     if mobile:
         info.update(_extract_watch_info_mobile(top_level))
@@ -447,3 +452,94 @@ def update_with_age_restricted_info(info, video_info_page):
 
     _extract_formats(info, player_response)
     _extract_playability_error(info, player_response, error_prefix=ERROR_PREFIX)
+
+def requires_decryption(info):
+    return ('formats' in info) and info['formats'] and info['formats'][0]['s']
+
+# adapted from youtube-dl and invidious:
+# https://github.com/omarroth/invidious/blob/master/src/invidious/helpers/signatures.cr
+decrypt_function_re = re.compile(r'function\(a\)\{(a=a\.split\(""\)[^\}]+)\}')
+op_with_arg_re = re.compile(r'[^\.]+\.([^\(]+)\(a,(\d+)\)')
+def extract_decryption_function(info, base_js):
+    '''Insert decryption function into info. Return error string if not successful.
+    Decryption function is a list of list[2] of numbers.
+    It is advisable to cache the decryption function (uniquely identified by info['player_name']) so base.js (1 MB) doesn't need to be redownloaded each time'''
+    info['decryption_function'] = None
+    decrypt_function_match = decrypt_function_re.search(base_js)
+    if decrypt_function_match is None:
+        return 'Could not find decryption function in base.js'
+
+    function_body = decrypt_function_match.group(1).split(';')[1:-1]
+    if not function_body:
+        return 'Empty decryption function body'
+
+    var_name = get(function_body[0].split('.'), 0)
+    if var_name is None:
+        return 'Could not find var_name'
+
+    var_body_match = re.search(r'var ' + re.escape(var_name) + r'=\{(.*?)\};', base_js, flags=re.DOTALL)
+    if var_body_match is None:
+        return 'Could not find var_body'
+
+    operations = var_body_match.group(1).replace('\n', '').split('},')
+    if not operations:
+        return 'Did not find any definitions in var_body'
+    operations[-1] = operations[-1][:-1]    # remove the trailing '}' since we split by '},' on the others
+    operation_definitions = {}
+    for op in operations:
+        colon_index = op.find(':')
+        opening_brace_index = op.find('{')
+
+        if colon_index == -1 or opening_brace_index == -1:
+            return 'Could not parse operation'
+        op_name = op[:colon_index]
+        op_body = op[opening_brace_index+1:]
+        if op_body == 'a.reverse()':
+            operation_definitions[op_name] = 0
+        elif op_body == 'a.splice(0,b)':
+            operation_definitions[op_name] = 1
+        elif op_body.startswith('var c=a[0]'):
+            operation_definitions[op_name] = 2
+        else:
+            return 'Unknown op_body: ' + op_body
+
+    decryption_function = []
+    for op_with_arg in function_body:
+        match = op_with_arg_re.fullmatch(op_with_arg)
+        if match is None:
+            return 'Could not parse operation with arg'
+        op_name = match.group(1)
+        if op_name not in operation_definitions:
+            return 'Unknown op_name: ' + op_name
+        op_argument = match.group(2)
+        decryption_function.append([operation_definitions[op_name], int(op_argument)])
+
+    info['decryption_function'] = decryption_function
+    return False
+
+def _operation_2(a, b):
+    c = a[0]
+    a[0] = a[b % len(a)]
+    a[b % len(a)] = c
+
+def decrypt_signatures(info):
+    '''Applies info['decryption_function'] to decrypt all the signatures. Return err.'''
+    if not info.get('decryption_function'):
+        return 'decryption_function not in info'
+    for format in info['formats']:
+        if not format['s'] or not format['sp'] or not format['url']:
+            print('Warning: s, sp, or url not in format')
+            continue
+
+        a = list(format['s'])
+        for op, argument in info['decryption_function']:
+            if op == 0:
+                a.reverse()
+            elif op == 1:
+                a = a[argument:]
+            else:
+                _operation_2(a, argument)
+
+        signature = ''.join(a)
+        format['url'] += '&' + format['sp'] + '=' + signature
+    return False