<s>
Universal	B-Protocol
2nd	I-Protocol
Factor	I-Protocol
(	O
U2F	B-Protocol
)	O
is	O
an	O
open	O
standard	O
that	O
strengthens	O
and	O
simplifies	O
two-factor	O
authentication	O
(	O
2FA	O
)	O
using	O
specialized	O
Universal	B-Protocol
Serial	I-Protocol
Bus	I-Protocol
(	O
USB	B-Protocol
)	O
or	O
near-field	O
communication	O
(	O
NFC	O
)	O
devices	O
based	O
on	O
similar	O
security	O
technology	O
found	O
in	O
smart	O
cards	O
.	O
</s>
<s>
While	O
initially	O
developed	O
by	O
Google	B-Application
and	O
Yubico	O
,	O
with	O
contribution	O
from	O
NXP	O
Semiconductors	O
,	O
the	O
standard	O
is	O
now	O
hosted	O
by	O
the	O
FIDO	O
Alliance	O
.	O
</s>
<s>
6-digit	O
codes	O
generated	O
on	O
Google	B-Application
Authenticator	O
)	O
were	O
a	O
significant	O
improvement	O
over	O
SMS-based	O
security	O
codes	O
,	O
a	O
number	O
of	O
security	O
vulnerabilities	O
were	O
still	O
possible	O
to	O
exploit	O
,	O
which	O
U2F	B-Protocol
sought	O
to	O
improve	O
.	O
</s>
<s>
In	O
terms	O
of	O
disadvantages	O
,	O
one	O
significant	O
difference	O
and	O
potential	O
drawback	O
to	O
be	O
considered	O
regarding	O
hardware-based	O
U2F	B-Protocol
solutions	O
is	O
that	O
unlike	O
with	O
TOTP	O
shared	O
secret	O
methods	O
,	O
there	O
is	O
no	O
possibility	O
of	O
"	O
backing	O
up	O
"	O
recovery	O
codes	O
or	O
shared	O
secrets	O
.	O
</s>
<s>
If	O
a	O
hardware	O
duplicate	O
or	O
alternate	O
hardware	O
key	O
is	O
not	O
kept	O
and	O
the	O
original	O
U2F	B-Protocol
hardware	O
key	O
is	O
lost	O
,	O
no	O
recovery	O
of	O
the	O
key	O
is	O
possible	O
(	O
because	O
the	O
private	B-Application
key	I-Application
exists	O
only	O
in	O
hardware	O
)	O
.	O
</s>
<s>
Therefore	O
,	O
for	O
services	O
that	O
do	O
not	O
provide	O
any	O
alternate	O
account	O
recovery	O
method	O
,	O
the	O
use	O
of	O
U2F	B-Protocol
should	O
be	O
carefully	O
considered	O
.	O
</s>
<s>
The	O
USB	B-Protocol
devices	I-Protocol
communicate	O
with	O
the	O
host	O
computer	O
using	O
the	O
human	B-General_Concept
interface	I-General_Concept
device	I-General_Concept
(	O
HID	O
)	O
protocol	O
,	O
essentially	O
mimicking	O
a	O
keyboard	O
.	O
</s>
<s>
Once	O
communication	O
is	O
established	O
,	O
the	O
application	O
exercises	O
a	O
challenge	O
–	O
response	O
authentication	O
with	O
the	O
device	O
using	O
public-key	B-Application
cryptography	I-Application
methods	O
and	O
a	O
secret	O
unique	O
device	O
key	O
manufactured	O
into	O
the	O
device	O
.	O
</s>
<s>
In	O
2020	O
,	O
independent	O
security	O
researchers	O
found	O
a	O
method	O
to	O
extract	O
private	B-Application
keys	I-Application
from	O
Google	B-Application
Titan	O
Key	O
,	O
a	O
popular	O
U2F	B-Protocol
hardware	O
security	O
token	O
.	O
</s>
<s>
U2F	B-Protocol
security	O
keys	O
are	O
supported	O
by	O
Google	B-Application
Chrome	I-Application
since	O
version	O
38	O
,	O
Firefox	B-Application
since	O
version	O
57	O
and	O
Opera	B-Application
since	O
version	O
40	O
.	O
</s>
<s>
U2F	B-Protocol
security	O
keys	O
can	O
be	O
used	O
as	O
an	O
additional	O
method	O
of	O
two-step	O
verification	O
on	O
online	O
services	O
that	O
support	O
the	O
U2F	B-Protocol
protocol	O
,	O
including	O
Google	B-Application
,	O
Azure	B-Application
,	O
Dropbox	B-Application
,	O
GitHub	B-Application
,	O
GitLab	B-Application
,	O
Bitbucket	B-Protocol
,	O
Nextcloud	B-General_Concept
,	O
Facebook	B-Application
,	O
and	O
others	O
.	O
</s>
<s>
Chrome	B-Application
,	O
Firefox	B-Application
,	O
and	O
Opera	B-Application
were	O
,	O
,	O
the	O
only	O
browsers	O
supporting	O
U2F	B-Protocol
natively	O
.	O
</s>
<s>
Microsoft	O
has	O
enabled	O
FIDO	O
2.0	O
support	O
for	O
Windows	B-Operating_System
10	I-Operating_System
's	O
Windows	O
Hello	O
login	O
platform	O
.	O
</s>
<s>
Microsoft	B-Application
Edge	I-Application
browser	O
gained	O
support	O
for	O
U2F	B-Protocol
in	O
the	O
October	O
2018	O
Windows	O
Update	O
.	O
</s>
<s>
Microsoft	O
accounts	O
,	O
including	O
Office	B-Operating_System
365	I-Operating_System
,	O
OneDrive	B-Application
,	O
and	O
other	O
Microsoft	O
services	O
,	O
do	O
not	O
yet	O
have	O
U2F	B-Protocol
support	O
.	O
</s>
<s>
Mozilla	B-Operating_System
has	O
integrated	O
it	O
into	O
Firefox	B-Application
57	O
,	O
and	O
enabled	O
it	O
by	O
default	O
in	O
Firefox	B-Application
60	O
and	O
Thunderbird	B-Application
60	O
.	O
</s>
<s>
Microsoft	B-Application
Edge	I-Application
starting	O
from	O
build	O
17723	O
support	O
FIDO2	O
.	O
</s>
<s>
As	O
of	O
iOS	O
and	O
iPadOS	O
13.3	O
Apple	O
now	O
supports	O
U2F	B-Protocol
in	O
the	O
Safari	O
browser	O
on	O
those	O
platforms	O
.	O
</s>
<s>
The	O
U2F	B-Protocol
standard	O
has	O
undergone	O
two	O
major	O
revisions	O
:	O
</s>
<s>
WebAuthn	O
and	O
CTAP	O
provide	O
a	O
complete	O
replacement	O
for	O
U2F	B-Protocol
,	O
which	O
has	O
been	O
renamed	O
"	O
CTAP1	O
"	O
in	O
the	O
latest	O
version	O
of	O
the	O
FIDO2	O
standard	O
.	O
</s>
<s>
The	O
WebAuthn	O
protocol	O
is	O
backward-compatible	O
(	O
via	O
the	O
AppID	O
extension	O
)	O
with	O
U2F-only	O
security	O
keys	O
but	O
the	O
U2F	B-Protocol
protocol	O
is	O
not	O
compatible	O
with	O
a	O
WebAuthn-only	O
authenticator	O
.	O
</s>
<s>
Some	O
authenticators	O
support	O
both	O
U2F	B-Protocol
and	O
WebAuthn	O
while	O
some	O
WebAuthn	O
clients	O
support	O
keys	O
created	O
via	O
the	O
legacy	O
U2F	B-Protocol
API	O
.	O
</s>
