https://www.techdirt.com/articles/20160515/02094934446/bad-news-two-factor-authentication-pioneer-yubikey-drops-open-source-pgp-proprietary-version.shtml [techdirt.com]

Bad News: Two-Factor Authentication Pioneer YubiKey Drops Open Source PGP For Proprietary Version

While I'm sure that Yubico's intentions were good here, this has raised a lot of concerns and has led to other former fans of YubiKeys withdrawing their endorsements of the devices. Encryption is tricky. There are almost always vulnerabilities and bugs -- a point we've been making a lot lately. But the best way to fix those tends to be getting as many knowledgeable eyes on the code as possible. And that's not possible when it's closed source.