How U2F works

NIIBE Yutaka gniibe at
Tue Feb 28 00:28:21 CET 2017


Let me ask a question about U2F.  Or, more generally, possibility to
enhance GnuPG for web authentication.

While I maintain scdaemon of GnuPG and develop Gnuk (an OpenPGPcard
implementation), I sometimes am asked about U2F support, these days.
(I think that this is due to Yubikey.)

IIUC, major use case of U2F is web authentication.  It seems for me
that it doesn't fit directly to OpenPGPcard use case.

Anyhow, it would be possible for Gnuk to add U2F support (somehow
limited, because of available resource on board).  Also, it would be
possible for scdaemon (or other application) to emulate U2F protocol
(just like Scute does emulate PKCS#11).

Well, I have two concerns for U2F.

(1) Atterstation key

In the document of U2F:

It explains about Atterstation key.

If it were common for services to do this Atterstation key check, U2F
emulation or free U2F implementation will be no real use with no private
key of the vendor.   (It reminds me the old days when Apache couldn't
serve https because no certificate authority issued certificate for servers
with Apache.)  I wondor if Atterstation key check is common or not.

(2) JavaScript

It seems for me that there are special JavaScript(s) to offer access API
to U2F.  I don't quite understand how it works to the physical device.

I don't like nonfree JavaScript which may interfere user' control.

Is it easy for free script (as in freedom) to integrate a script for U2F
access?  Any such example scripts or any such services which do so?

Here, my concern is that if it is all for proprietary world, I am
reluctant to consider seriously about U2F.

And finally, if web authentication is important, I would like to use the
infrastructure of GnuPG to manage my own crypto computation and my own
private keys.  Currently, we can use GnuPG for SSH authentication by
its ssh-agent emulation.  I would like to extend this.

Any thoughts?  Thanks in advance.

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