<s>
The	O
Temporary	B-Protocol
Contact	I-Protocol
Numbers	I-Protocol
Protocol	O
,	O
or	O
TCN	B-Protocol
Protocol	I-Protocol
,	O
is	O
an	O
open	B-License
source	I-License
,	O
decentralized	O
,	O
anonymous	O
exposure	B-Application
alert	I-Application
protocol	O
developed	O
by	O
Covid	B-Application
Watch	I-Application
in	O
response	O
to	O
the	O
COVID-19	O
pandemic	O
.	O
</s>
<s>
The	O
Covid	B-Application
Watch	I-Application
team	O
,	O
started	O
as	O
an	O
independent	O
research	O
collaboration	O
between	O
Stanford	O
University	O
and	O
the	O
University	O
of	O
Waterloo	O
was	O
the	O
first	O
in	O
the	O
world	O
to	O
publish	O
a	O
white	O
paper	O
,	O
develop	O
,	O
and	O
open	B-License
source	I-License
fully	O
anonymous	O
Bluetooth	O
exposure	B-Application
alert	I-Application
technology	O
in	O
collaboration	O
with	O
CoEpi	O
after	O
writing	O
a	O
blog	O
post	O
on	O
the	O
topic	O
in	O
early	O
March	O
.	O
</s>
<s>
Covid	B-Application
Watch	I-Application
's	O
TCN	B-Protocol
Protocol	I-Protocol
received	O
significant	O
news	O
coverage	O
and	O
was	O
followed	O
by	O
similar	O
decentralized	O
protocols	O
in	O
early	O
April	O
2020	O
like	O
DP-3T	B-Protocol
,	O
PACT	O
,	O
and	O
Google/Apple	O
Exposure	B-Application
Notification	I-Application
framework	B-Architecture
.	O
</s>
<s>
Covid	B-Application
Watch	I-Application
then	O
helped	O
other	O
groups	O
like	O
the	O
TCN	O
Coalition	O
and	O
MIT	O
SafePaths	O
implement	O
the	O
TCN	B-Protocol
Protocol	I-Protocol
within	O
their	O
open	B-License
source	I-License
projects	O
to	O
further	O
the	O
development	O
of	O
decentralized	O
technology	O
and	O
foster	O
global	O
interoperability	O
of	O
contact	O
tracing	O
and	O
exposure	O
alerting	O
apps	O
,	O
a	O
key	O
aspect	O
of	O
achieving	O
widespread	O
adoption	O
.	O
</s>
<s>
Covid	B-Application
Watch	I-Application
volunteers	O
and	O
nonprofit	O
staff	O
also	O
built	O
a	O
fully	O
open	B-License
source	I-License
mobile	O
app	O
for	O
sending	O
anonymous	O
exposure	O
alerts	O
first	O
using	O
the	O
TCN	B-Protocol
Protocol	I-Protocol
and	O
later	O
using	O
the	O
very	O
similar	O
Google/Apple	O
Exposure	B-Application
Notification	I-Application
Framework	B-Architecture
(	O
ENF	O
)	O
.	O
</s>
<s>
The	O
protocol	O
,	O
like	O
BlueTrace	B-Protocol
and	O
the	O
Google	B-Application
/	I-Application
Apple	I-Application
contact	I-Application
tracing	I-Application
project	I-Application
,	O
use	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
major	O
distinction	O
between	O
TCN	O
and	O
protocols	O
like	O
BlueTrace	B-Protocol
is	O
the	O
fact	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
approaches	O
like	O
the	O
one	O
used	O
in	O
BlueTrace	B-Protocol
.	O
</s>
<s>
The	O
TCN	B-Protocol
protocol	I-Protocol
received	O
notoriety	O
as	O
one	O
of	O
the	O
first	O
widely	O
released	O
digital	O
contact	O
tracing	O
protocols	O
alongside	O
BlueTrace	B-Protocol
,	O
the	O
Exposure	B-Application
Notification	I-Application
framework	B-Architecture
,	O
and	O
the	O
Pan-European	B-Protocol
Privacy-Preserving	I-Protocol
Proximity	I-Protocol
Tracing	I-Protocol
(	O
PEPP-PT	B-Protocol
)	O
project	O
.	O
</s>
<s>
It	O
also	O
stood	O
out	O
for	O
its	O
incorporation	O
of	O
blockchain	B-Application
technology	I-Application
,	O
and	O
its	O
influence	O
over	O
the	O
Google/Apple	O
project	O
.	O
</s>
<s>
The	O
TCN	B-Protocol
protocol	I-Protocol
works	O
off	O
the	O
basis	O
of	O
Temporary	B-Protocol
Contact	I-Protocol
Numbers	I-Protocol
(	O
TCN	O
)	O
,	O
semi-random	O
identifiers	O
derived	O
from	O
a	O
seed	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
competing	O
protocols	O
like	O
BlueTrace	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>
The	O
entire	O
protocol	O
is	O
based	O
on	O
the	O
principle	O
of	O
temporary	B-Protocol
contact	I-Protocol
numbers	I-Protocol
(	O
TCN	O
)	O
,	O
a	O
unique	O
and	O
anonymous	O
128-bit	O
identifier	O
generated	O
deterministically	O
from	O
a	O
seed	O
value	O
on	O
a	O
client	O
device	O
.	O
</s>
<s>
Then	O
,	O
using	O
the	O
RAK	O
an	O
initial	O
temporary	O
contact	O
key	O
(	O
TCK	O
)	O
is	O
generated	O
using	O
the	O
algorithm	O
,	O
where	O
is	O
the	O
SHA-256	B-Algorithm
hash	B-Error_Name
function	I-Error_Name
as	O
.	O
</s>
<s>
A	O
128	O
bit	O
TCN	O
is	O
then	O
generated	O
from	O
a	O
given	O
TCK	O
using	O
the	O
algorithm	O
,	O
where	O
formats	O
a	O
supplied	O
number	O
as	O
a	O
little	O
endian	O
unsigned	O
2	O
byte	O
integer	O
,	O
and	O
is	O
the	O
SHA-256	B-Algorithm
hash	B-Error_Name
function	I-Error_Name
as	O
.	O
</s>
<s>
The	O
encounter	O
handshake	O
runs	O
on	O
Bluetooth	B-General_Concept
LE	I-General_Concept
and	O
defines	O
how	O
two	O
devices	O
acknowledge	O
each	O
other	O
's	O
presence	O
.	O
</s>
<s>
The	O
infection	O
reporting	O
is	O
built	O
on	O
HTTPS	B-Protocol
and	O
defines	O
how	O
infection	O
notices	O
are	O
distributed	O
among	O
clients	O
.	O
</s>
<s>
On	O
5	O
April	O
2020	O
,	O
the	O
global	O
TCN	O
Coalition	O
was	O
founded	O
by	O
Covid	B-Application
Watch	I-Application
and	O
other	O
groups	O
that	O
had	O
coalesced	O
around	O
what	O
was	O
essentially	O
the	O
same	O
approach	O
and	O
largely	O
overlapping	O
protocols	O
,	O
with	O
the	O
goal	O
to	O
reduce	O
fragmentation	O
,	O
and	O
enable	O
global	O
interoperability	O
of	O
tracing	O
and	O
alerting	O
apps	O
,	O
a	O
key	O
aspect	O
of	O
achieving	O
widespread	O
adoption	O
.	O
</s>
<s>
The	O
TCN	O
Coalition	O
also	O
helped	O
establish	O
the	O
Data	O
Rights	O
for	O
Digital	O
Contact	O
Tracing	O
and	O
Alerting	O
framework	B-Architecture
,	O
which	O
functions	O
as	O
a	O
bill	O
of	O
rights	O
for	O
users	O
of	O
such	O
apps	O
.	O
</s>
<s>
Currently	O
the	O
protocol	O
is	O
used	O
by	O
TCN	O
Coalition	O
members	O
CoEpi	O
and	O
Covid	B-Application
Watch	I-Application
,	O
and	O
was	O
likely	O
a	O
source	O
of	O
inspiration	O
for	O
the	O
similar	O
Google	B-Application
/	I-Application
Apple	I-Application
contact	I-Application
tracing	I-Application
project	I-Application
.	O
</s>
