Twitter/Youtube video downloader app
<p>I had this idea of developing a mobile app that allows users to download videos from twitter and youtube to be able to access them offline.. is this considered illegal? (sorry if my question is in the wrong place)</p>

<p>It is not illegal to write such a program. It <em>is</em> illegal to use such a program, since the <a href="https://www.youtube.com/static?gl=GB&amp;template=terms" rel="nofollow noreferrer">TOS Sect. 5.1(C)</a> only allows using their technology:</p>

<blockquote>
  <p>you agree not to access Content through any technology or means other
  than the video playback pages of the Website itself, the YouTube
  Player, or such other means as YouTube may explicitly designate for
  this purpose</p>
</blockquote>

<p>Google Play would also not allow you to disseminate the program. Then the question is whether you might still be liable for helping others infringe on copyright, i.e. <a href="https://www.law.cornell.edu/wex/vicarious_infringement" rel="nofollow noreferrer">vicarious infringement</a>. <a href="https://www.law.cornell.edu/supct/html/04-480.ZS.html" rel="nofollow noreferrer">MGM v. Grokster</a>, 545 U.S. 913 would be a relevant case to consider. The courts held that</p>

<blockquote>
  <p>One who distributes a device with the object of promoting its use to
  infringe copyright, as shown by clear expression or other affirmative
  steps taken to foster infringement, going beyond mere distribution
  with knowledge of third-party action, is liable for the resulting acts
  of infringement by third parties using the device, regardless of the
  device’s lawful uses</p>
</blockquote>

<p>This raises a technical question, as to whether there are any non-infringing uses of the app, that is, is this a general program that can be directed at all sorts of online videos (where there are legal uses), or is this aimed just at Twitter &amp; Youtube, anthis only has infringing uses. In contrast, in <a href="https://www.law.cornell.edu/supremecourt/text/464/417" rel="nofollow noreferrer">Sony v. Universal City Studios</a>, 464 U.S. 417 it was found that there were significant non-infringing uses to the VCR, thus Sony would not be vicariously liable. It would be hard to argue that distributing such a program did not have the goal of promoting its use to infringe copyright.</p>

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