user84941: I found the following Github Repo that makes it possible to build your own Yubikey with an Arduino Leonardo device.
https://github.com/pagong/arduino-yksim
Pretty cool. However I am wondering if using this is legal at all?
YubiCo is a company and they sell sticks with the YubiKey OTP feature. The verification process happens through their servers. Somehow I can't quite imagine that the company YubiCo tolerates that their processes and products could be easily replaced by open source products? Maybe they have any patents or utility models? And I was not able to find any licensing information which makes sense since official firmware seems to be closed source. So I guess it makes sense that no licensing information is available since the yksim project is a reverse engineered thing?! But I'll admit quite frankly: I have no idea of all this.
I would also like to use this library and possibly also for my customer projects, but I need to know whether this is really allowed.
Hope someone can help me here and I know that the internet is no source for a 100% sure legal advice. But maybe someone has a clue or a hint for me.

user10794: According to https://www.yubico.com/blog/secure-hardware-vs-open-source/

The YubiKey hardware with its integral firmware has never been open sourced, whereas almost all of the supporting applications are open source.

I am not a lawyer, however it would seem that providing you aren't copying their (non open source) hardware, but are merely replicating the functionality, then this would appear to be OK. Disclaimer: I may be wrong.
Having said that, I agree with the comment under the question, you should probably ask at https://law.stackexchange.com/
Questions about the legality of an Arduino application really are legal questions, not technical ones.

84941: <p>I found the following Github Repo that makes it possible to build your own Yubikey with an Arduino Leonardo device.</p>
<p><a href="https://github.com/pagong/arduino-yksim" rel="nofollow noreferrer">https://github.com/pagong/arduino-yksim</a></p>
<p>Pretty cool. However I am wondering if using this is legal at all?</p>
<p>YubiCo is a company and they sell sticks with the YubiKey OTP feature. The verification process happens through their servers. Somehow I can't quite imagine that the company YubiCo tolerates that their processes and products could be easily replaced by open source products? Maybe they have any patents or utility models? And I was not able to find any licensing information which makes sense since official firmware seems to be closed source. So I guess it makes sense that no licensing information is available since the yksim project is a reverse engineered thing?! But I'll admit quite frankly: I have no idea of all this.</p>
<p>I would also like to use this library and possibly also for my customer projects, but I need to know whether this is really allowed.</p>
<p>Hope someone can help me here and I know that the internet is no source for a 100% sure legal advice. But maybe someone has a clue or a hint for me.</p>
 10794: <p>According to <a href="https://www.yubico.com/blog/secure-hardware-vs-open-source/" rel="nofollow noreferrer">https://www.yubico.com/blog/secure-hardware-vs-open-source/</a></p>
<blockquote>
<p>The YubiKey hardware with its integral firmware has never been open sourced, whereas almost all of the supporting applications are open source.</p>
</blockquote>
<p>I am not a lawyer, however it would seem that providing you aren't copying their (non open source) hardware, but are merely replicating the functionality, then this would appear to be OK. <strong>Disclaimer</strong>: I may be wrong.</p>
<p>Having said that, I agree with the comment under the question, you should probably ask at <a href="https://law.stackexchange.com/">https://law.stackexchange.com/</a></p>
<p>Questions about the legality of an Arduino application really are legal questions, not technical ones.</p>
