<s>
Decentralized	B-Protocol
Privacy-Preserving	I-Protocol
Proximity	I-Protocol
Tracing	I-Protocol
(	O
DP-3T	B-Protocol
,	O
stylized	O
as	O
dp3t	O
)	O
is	O
an	O
open	O
protocol	O
developed	O
in	O
response	O
to	O
the	O
COVID-19	O
pandemic	O
to	O
facilitate	O
digital	B-Protocol
contact	I-Protocol
tracing	I-Protocol
of	O
infected	O
participants	O
.	O
</s>
<s>
The	O
protocol	O
,	O
like	O
competing	O
protocol	O
Pan-European	B-Protocol
Privacy-Preserving	I-Protocol
Proximity	I-Protocol
Tracing	I-Protocol
(	O
PEPP-PT	B-Protocol
)	O
,	O
uses	O
Bluetooth	B-General_Concept
Low	I-General_Concept
Energy	I-General_Concept
to	O
track	O
and	O
log	O
encounters	O
with	O
other	O
users	O
.	O
</s>
<s>
The	O
protocols	O
differ	O
in	O
their	O
reporting	O
mechanism	O
,	O
with	O
PEPP-PT	B-Protocol
requiring	O
clients	O
to	O
upload	O
contact	O
logs	O
to	O
a	O
central	O
reporting	O
server	O
,	O
whereas	O
with	O
DP-3T	B-Protocol
,	O
the	O
central	O
reporting	O
server	O
never	O
has	O
access	O
to	O
contact	O
logs	O
nor	O
is	O
it	O
responsible	O
for	O
processing	O
and	O
informing	O
clients	O
of	O
contact	O
.	O
</s>
<s>
Because	O
contact	O
logs	O
are	O
never	O
transmitted	O
to	O
third	O
parties	O
,	O
it	O
has	O
major	O
privacy	O
benefits	O
over	O
the	O
PEPP-PT	B-Protocol
approach	O
;	O
however	O
,	O
this	O
comes	O
at	O
the	O
cost	O
of	O
requiring	O
more	O
computing	O
power	O
on	O
the	O
client	O
side	O
to	O
process	O
infection	O
reports	O
.	O
</s>
<s>
The	O
Apple/Google	O
Exposure	B-Application
Notification	I-Application
project	O
is	O
based	O
on	O
similar	O
principles	O
as	O
the	O
DP-3T	B-Protocol
protocol	O
,	O
and	O
supports	O
a	O
variant	O
of	O
it	O
since	O
May	O
2020	O
.	O
</s>
<s>
Huawei	O
added	O
a	O
similar	O
implementation	O
of	O
DP-3T	B-Protocol
to	O
its	O
Huawei	O
Mobile	O
Services	O
APIs	O
known	O
as	O
"	O
Contact	O
Shield	O
"	O
in	O
June	O
2020	O
.	O
</s>
<s>
The	O
DP-3T	B-Protocol
SDK	O
and	O
calibration	O
apps	O
intend	O
to	O
support	O
the	O
Apple/Google	O
API	O
as	O
soon	O
as	O
it	O
is	O
released	O
to	O
iOS	O
and	O
Android	O
devices	O
.	O
</s>
<s>
On	O
the	O
21	O
April	O
2020	O
,	O
the	O
Swiss	B-Protocol
Federal	O
Office	O
of	O
Public	O
Health	O
announced	O
that	O
the	O
Swiss	B-Protocol
national	O
coronavirus	O
contact	O
tracing	O
app	O
will	O
be	O
based	O
on	O
DP-3T	B-Protocol
.	O
</s>
<s>
On	O
the	O
22	O
April	O
2020	O
,	O
the	O
Austrian	O
Red	O
Cross	O
,	O
leading	O
on	O
the	O
national	O
digital	B-Protocol
contact	I-Protocol
tracing	I-Protocol
app	O
,	O
announced	O
its	O
migration	O
to	O
the	O
approach	O
of	O
DP-3T	B-Protocol
.	O
</s>
<s>
Estonia	O
also	O
confirmed	O
that	O
their	O
app	O
would	O
be	O
based	O
on	O
DP-3T	B-Protocol
.	O
</s>
<s>
On	O
April	O
28	O
,	O
2020	O
,	O
it	O
was	O
announced	O
that	O
Finland	O
was	O
piloting	O
a	O
version	O
of	O
DP-3T	B-Protocol
called	O
"	O
Ketju	O
"	O
.	O
</s>
<s>
In	O
Germany	O
,	O
a	O
national	O
app	O
is	O
being	O
built	O
upon	O
DP-3T	B-Protocol
by	O
SAP	B-Application
SE	I-Application
and	O
Deutsche	O
Telekom	O
alongside	O
CISPA	O
,	O
one	O
of	O
the	O
organisations	O
that	O
authored	O
the	O
protocol	O
.	O
</s>
<s>
As	O
of	O
September	O
30	O
,	O
2020	O
,	O
contact	O
tracing	O
apps	O
using	O
DP-3T	B-Protocol
are	O
available	O
in	O
Austria	O
,	O
Belgium	O
,	O
Croatia	O
,	O
Germany	O
,	O
Ireland	O
,	O
Italy	O
,	O
the	O
Netherlands	O
,	O
Portugal	O
and	O
Switzerland	B-Protocol
.	O
</s>
<s>
The	O
DP-3T	B-Protocol
protocol	O
works	O
off	O
the	O
basis	O
of	O
Ephemeral	O
IDs	O
(	O
EphID	O
)	O
,	O
semi-random	O
rotating	O
strings	O
that	O
uniquely	O
identify	O
clients	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
competing	O
protocols	O
like	O
PEPP-PT	B-Protocol
,	O
where	O
the	O
central	O
reporting	O
server	O
receives	O
and	O
processes	O
client	O
contact	O
logs	O
.	O
</s>
<s>
Similar	O
to	O
the	O
TCN	B-Protocol
Protocol	I-Protocol
and	O
its	O
Temporary	B-Protocol
Contact	I-Protocol
Numbers	I-Protocol
,	O
the	O
DP-3T	B-Protocol
protocol	O
makes	O
use	O
of	O
16	O
byte	O
Ephemeral	O
IDs	O
(	O
EphID	O
)	O
to	O
uniquely	O
identify	O
devices	O
in	O
the	O
proximity	O
of	O
a	O
client	O
.	O
</s>
<s>
To	O
generate	O
an	O
EphID	O
,	O
first	O
a	O
client	O
generates	O
a	O
secret	O
key	O
that	O
rotates	O
daily	O
(	O
)	O
by	O
computing	O
,	O
where	O
is	O
a	O
cryptographic	O
hash	O
function	O
such	O
as	O
SHA-256	B-Algorithm
.	O
</s>
<s>
Given	O
the	O
secret	O
day	O
key	O
,	O
each	O
device	O
computes	O
,	O
where	O
is	O
a	O
global	O
fixed	O
string	O
,	O
is	O
a	O
pseudo-random	O
function	O
like	O
HMAC-SHA256	B-Algorithm
,	O
and	O
is	O
a	O
stream	O
cipher	O
producing	O
bytes	O
.	O
</s>
<s>
The	O
DP-3T	B-Protocol
protocol	O
is	O
made	O
up	O
of	O
two	O
separate	O
responsibilities	O
,	O
tracking	O
and	O
logging	O
close	O
range	O
encounters	O
with	O
other	O
users	O
(	O
device	O
handshake	O
)	O
,	O
and	O
the	O
reporting	O
of	O
those	O
encounters	O
such	O
that	O
other	O
clients	O
can	O
determine	O
if	O
they	O
have	O
been	O
in	O
contact	O
with	O
an	O
infected	O
patient	O
(	O
infection	O
reporting	O
)	O
.	O
</s>
<s>
Like	O
most	O
digital	B-Protocol
contact	I-Protocol
tracing	I-Protocol
protocols	O
,	O
the	O
device	O
handshake	O
uses	O
Bluetooth	B-General_Concept
Low	I-General_Concept
Energy	I-General_Concept
to	O
find	O
and	O
exchange	O
details	O
with	O
local	O
clients	O
,	O
and	O
the	O
infection	O
reporting	O
stage	O
uses	O
HTTPS	B-Protocol
to	O
upload	O
a	O
report	O
to	O
a	O
central	O
reporting	O
server	O
.	O
</s>
<s>
In	O
order	O
to	O
find	O
and	O
communicate	O
with	O
clients	O
in	O
proximity	O
of	O
a	O
device	O
,	O
the	O
protocol	O
makes	O
use	O
of	O
both	O
the	O
server	O
and	O
client	O
modes	O
of	O
Bluetooth	B-General_Concept
LE	I-General_Concept
,	O
switching	O
between	O
the	O
two	O
frequently	O
.	O
</s>
<s>
When	O
a	O
user	O
installs	O
a	O
DP-3T	B-Protocol
app	O
,	O
they	O
are	O
asked	O
if	O
they	O
want	O
to	O
opt	O
in	O
to	O
sharing	O
data	O
with	O
epidemiologists	O
.	O
</s>
<s>
To	O
facilitate	O
compatibility	O
between	O
DP-3T	B-Protocol
apps	O
administered	O
by	O
separate	O
health	O
authorities	O
,	O
apps	O
maintain	O
a	O
local	O
list	O
of	O
the	O
regions	O
a	O
user	O
has	O
visited	O
.	O
</s>
<s>
Cryptography	O
and	O
security	O
scholar	O
Serge	O
Vaudenay	O
,	O
analyzing	O
the	O
security	O
of	O
DP-3T	B-Protocol
argued	O
that	O
:	O
</s>
<s>
Vaudenay	O
's	O
work	O
presents	O
several	O
attacks	O
against	O
DP-3T	B-Protocol
and	O
similar	O
systems	O
.	O
</s>
<s>
In	O
response	O
,	O
the	O
DP-3T	B-Protocol
group	O
claim	O
that	O
out	O
of	O
twelve	O
risks	O
Vaudenay	O
presents	O
,	O
eight	O
are	O
also	O
present	O
in	O
centralized	O
systems	O
,	O
three	O
do	O
not	O
work	O
,	O
and	O
one	O
,	O
which	O
involves	O
physical	O
access	O
to	O
the	O
phone	O
,	O
works	O
but	O
can	O
be	O
mitigated	O
.	O
</s>
<s>
Tang	O
surveys	O
the	O
major	O
digital	B-Protocol
contact	I-Protocol
tracing	I-Protocol
systems	O
and	O
shows	O
that	O
DP-3T	B-Protocol
is	O
subject	O
to	O
what	O
he	O
calls	O
"	O
targeted	O
identification	O
attacks	O
"	O
.	O
</s>
<s>
Theoretical	O
attacks	O
on	O
DP-3T	B-Protocol
have	O
been	O
simulated	O
showing	O
that	O
persistent	O
tracking	O
of	O
users	O
of	O
the	O
first	O
version	O
of	O
the	O
DP-3T	B-Protocol
system	O
who	O
have	O
voluntarily	O
uploaded	O
their	O
identifiers	O
can	O
be	O
made	O
easy	O
to	O
any	O
3rd	O
party	O
who	O
can	O
install	O
a	O
large	O
fleet	O
of	O
Bluetooth	B-General_Concept
Low	I-General_Concept
Energy	I-General_Concept
devices	O
.	O
</s>
<s>
This	O
attack	O
leverages	O
the	O
linkability	O
of	O
a	O
user	O
during	O
a	O
day	O
,	O
and	O
therefore	O
is	O
possible	O
on	O
within	O
a	O
day	O
on	O
all	O
users	O
of	O
some	O
centralized	O
systems	O
such	O
as	O
the	O
system	O
proposed	O
in	O
the	O
United	O
Kingdom	O
,	O
but	O
does	O
not	O
function	O
on	O
'	O
unlinkable	O
 '	O
versions	O
of	O
DP-3T	B-Protocol
where	O
infected	O
users	O
 '	O
identifiers	O
are	O
not	O
transmitted	O
using	O
a	O
compact	O
representation	O
such	O
as	O
a	O
key	O
or	O
seed	O
.	O
</s>
