I had to do a little digging into where it was pulling its crypto methods from and it looks like the have a bit of their own library, but it uses libsodium via javascript wrappers underneath:

https://github.com/TankerHQ/sdk-js/tree/master/packages/cryp...

https://github.com/jedisct1/libsodium.js

So, I guess take that as you will. I haven't read much of the actual source yet.

edit: I sort of expected there would be a move to the server since it looks like they built their library to run on the server, even if it's running all of the libsodium methods in javascript but it's definitely pulling the browser version and running it all in the client: https://github.com/TankerHQ/sdk-js/blob/master/packages/file...