u2f seed

Fox, Kevin M Kevin.Fox at pnnl.gov
Sat Jan 4 03:15:16 AEDT 2020

How does a u2f website then authenticate the same user, with the same keyfob, on a different machine? If that actually works, then we should be able to use the same mechanism. Maybe it doesn't, and some people are going to be locked out of their account when their machine fails and they have to go to another one. portability was one of the selling points of u2f though I thought. Maybe I'll try and dig up the u2f spec and see if there is any detail in it.


From: openssh-unix-dev <openssh-unix-dev-bounces+kevin.fox=pnnl.gov at mindrot.org> on behalf of Christian Weisgerber <naddy at mips.inka.de>
Sent: Friday, January 3, 2020 5:01 AM
To: openssh-unix-dev at mindrot.org
Subject: Re: u2f seed

On 2020-01-02, "Fox, Kevin M" <Kevin.Fox at pnnl.gov> wrote:

> That sounds like the application param is still used as part of the process though? Would allowing the user to specify the application work in the Solokey case?

Let's cut this short without losing ourselves in details: Even if
you resend exactly the same U2F registration message, the token may
still create a different key pair.  Only a very minimal U2F token
without an on-board RNG might derive the key pair purely from the
parameters in the registration message; I don't know if any such
devices exist.

This actually made me curious and I checked the simple FIDO1 U2F
token I have here (HyperFIDO Titanium): It issues a different key
pair each time, even if the registration message is exactly the
same.  As would the Solokey.

Every time you run "ssh-keygen -t ecdsa-sk", the token will give
you a different key pair, and this is enforced by the token itself.

> What is stored in the private keyfile? The documentation says no private key is stored there. So is it just information used to reseed the public/private key?

The OpenSSH private key file stores the U2F key handle.  The key
handle is an opaque blob which you need to pass back to the token
so it can find the private key.

Christian "naddy" Weisgerber                          naddy at mips.inka.de
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