U2F Zero is an open source U2F token for 2 factor authentication. It is implemented securely. It works with Google accounts, Github, Duo, OpenSSH, and anything else supporting U2F.

You can easily build your own. You just need to order the 8 SMT parts, $4.6 PCBs, and programmer. It ends up being $35 for programmer and $4.5/board. The token is durable enough to survive on a key chain for years, even after going through the wash.

Check out the wiki for more on how to build your own.

The security level is about the same as a modern car key. Any secret information cannot be read or duplicated. A true random number generator is used to create unpredictable keys.

However, side channel leakage is an unsolved problem in industry and academia. So for well equipped adversaries that can make targetted attacks and get physical access, secret information leakage is possible. Any other hardware token that claims it's "impenetrable" or otherwise totally secure is still vulnerable to physical side channels and it's important to acknowledge. However, most people don't worry about targeted attacks from well equipped adversaries.

For more information about U2F Zero's secure implementation and the problem of side channels, check out the wiki.

Everything is open source and licensed under the Simplified BSD License.