Youtube generates revenue from user ad views, and it’s logical for the platform to implement restrictions to prevent people from downloading videos or even watching them on an unofficial client like YouTube Vanced. In this article, I will explain the technical details of these security mechanisms and how it’s possible to bypass them.

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Since mid-2021, YouTube has included the query parameter n in the majority of file URLs. This parameter needs to be transformed using a JavaScript algorithm located in the file base.js, which is distributed with the web page. YouTube utilizes this parameter as a challenge to verify that the download originates from an “official” client. If the challenge is not resolved and n is not transformed correctly, YouTube will silently apply throttling to the video download.

The JavaScript algorithm is obfuscated and changes frequently, so it’s not practical to attempt reverse engineering to understand it. The solution is simply to download the JavaScript file, extract the algorithm code, and execute it by passing the n parameter to it.

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Many projects currently use these techniques to circumvent the limitations put in place by YouTube in order to prevent video downloads. The most popular one is yt-dlp (a fork of youtube-dl) programmed in Python, but it includes its own custom JavaScript interpreter to transform the n parameter.