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

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: https://github.com/Tyrrrz/YoutubeExplode/blob/a8318c0d304c1cac1465b1b92ac3de99af6e246c/YoutubeExplode.Tests/TestData/VideoIds.cs#L11-L13
• 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

Start by reproducing the slow download in Demo.Gui using an age-restricted video, then inspect the resolved stream manifest for the n parameter and the existing deciphering path. Test with the video IDs in TestData/VideoIds.cs and confirm that streams containing n download without severe throttling.