Occasionally streams are very slow to download (`n`-sig deciphering)

[Tyrrrz]

Version

v6.2.13 / master

Details

When using the TVHTML5_SIMPLY_EMBEDDED_PLAYER client (or any other JS-enabled client, but this is the one we're using), occasionally YouTube may return a stream that has the n parameter. This parameter is an encrypted signature, similar to sig, but is responsible for rate throttling.

It also incorporates much more complicated cipher functions, so the existing deciphering code will not be enough to tackle it.

Currently, these kinds of streams are not too common, but I assume they will become more prolific as time goes on. It's not a huge issue now as the TVHTML5_SIMPLY_EMBEDDED_PLAYER client is only used for age-restricted videos.

Steps to reproduce

• Use the Demo.Gui project
• Resolve stream manifest for any age-restricted video:

  YoutubeExplode/YoutubeExplode.Tests/TestData/VideoIds.cs

  Lines 11 to 13 in a8318c0

  	public const string AgeRestrictedViolent = "rXMX4YJ7Lks";
  	public const string AgeRestrictedSexual = "SkRSXFQerZs";
  	public const string AgeRestrictedEmbedRestricted = "hySoCSoH-g8";

• Try to download any stream and observe very low download speed
• If the issue doesn't happen, retry from the beginning again (this can take several attempts)

Updated, much more consistent steps to reproduce that require a VPN:

• Use the Demo.Gui project
• VPN to US
• Resolve stream manifest for https://www.youtube.com/watch?v=BJhIIOyUd9k (or any other "free with ads movie" video)
• Try to download any stream and observe very low download speed

[xBaank]

I wonder if using the base.js functions could be a solution for the TVHTML5_SIMPLY_EMBEDDED_PLAYER. Maybe this PR could be used as start point #601 but it would be needed to add a js interpreter

[Mrgaton]

i have same issue :C

[sakib1361]

As Tyrrrz suspected, this is causing problems for more and more videos. I am using the library for Realtime playback (WInui 3) and on some videos, I can't even play smoothly. Link generated by ytdlp seems to be working though.