<s>
Pan-European	B-Protocol
Privacy-Preserving	I-Protocol
Proximity	I-Protocol
Tracing	I-Protocol
(	O
PEPP-PT/PEPP	O
)	O
is	O
a	O
full-stack	B-Application
open	O
protocol	O
designed	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
the	O
competing	O
Decentralized	B-Protocol
Privacy-Preserving	I-Protocol
Proximity	I-Protocol
Tracing	I-Protocol
(	O
DP-3T	B-Protocol
)	O
protocol	O
,	O
makes	O
use	O
of	O
Bluetooth	B-General_Concept
LE	I-General_Concept
to	O
discover	O
and	O
locally	O
log	O
clients	O
near	O
a	O
user	O
.	O
</s>
<s>
However	O
,	O
unlike	O
DP-3T	B-Protocol
,	O
it	O
uses	O
a	O
centralized	O
reporting	O
server	O
to	O
process	O
contact	O
logs	O
and	O
individually	O
notify	O
clients	O
of	O
potential	O
contact	O
with	O
an	O
infected	O
patient	O
.	O
</s>
<s>
It	O
has	O
also	O
been	O
put	O
forward	O
that	O
the	O
distinction	O
between	O
centralized/decentralized	O
systems	O
is	O
mostly	O
technical	O
and	O
PEPP-PT	B-Protocol
is	O
equally	O
able	O
to	O
preserve	O
privacy	O
.	O
</s>
<s>
The	O
suggested	O
proof-of-work	O
algorithm	O
is	O
scrypt	B-Algorithm
as	O
defined	O
in	O
RFC7914	O
,	O
popularized	O
in	O
various	O
blockchain	B-Application
systems	O
such	O
as	O
Dogecoin	O
and	O
Litecoin	B-Protocol
.	O
</s>
<s>
Scrypt	B-Algorithm
was	O
chosen	O
because	O
it	O
is	O
memory	O
bound	O
rather	O
than	O
CPU	O
bound	O
.	O
</s>
<s>
Once	O
a	O
user	O
registers	O
with	O
the	O
app	O
,	O
they	O
are	O
issued	O
a	O
unique	O
128	O
bit	O
pseudo-random	B-Error_Name
identifier	O
(	O
PUID	O
)	O
by	O
the	O
server	O
.	O
</s>
<s>
From	O
this	O
an	O
Ephemeral	O
Bluetooth	O
ID	O
(	O
EBID	O
)	O
is	O
calculated	O
for	O
each	O
user	O
with	O
the	O
algorithm	O
where	O
is	O
the	O
AES	B-Algorithm
encryption	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
Clients	O
then	O
constantly	O
broadcast	O
their	O
EBID	O
under	O
the	O
PEPP-PT	B-Protocol
Bluetooth	O
service	O
identifier	O
,	O
while	O
also	O
scanning	O
for	O
other	O
clients	O
.	O
</s>
<s>
The	O
user	O
then	O
transmits	O
the	O
contact	O
log	O
over	O
HTTPS	B-Protocol
to	O
the	O
reporting	O
server	O
to	O
be	O
processed	O
.	O
</s>
<s>
The	O
Helmholtz	O
Center	O
for	O
Information	O
Security	O
(	O
CISPA	O
)	O
confirmed	O
in	O
a	O
press	O
release	O
on	O
April	O
20	O
,	O
2020	O
that	O
it	O
was	O
withdrawing	O
from	O
the	O
consortium	O
,	O
citing	O
a	O
'	O
lack	O
of	O
transparency	O
and	O
clear	O
governance	O
 '	O
as	O
well	O
as	O
data	O
protection	O
concerns	O
around	O
the	O
PEPP-PT	B-Protocol
design	O
.	O
</s>
<s>
This	O
group	O
was	O
also	O
responsible	O
for	O
the	O
development	O
of	O
the	O
competing	O
Decentralized	B-Protocol
Privacy-Preserving	I-Protocol
Proximity	I-Protocol
Tracing	I-Protocol
protocol	I-Protocol
.	O
</s>
<s>
On	O
20	O
April	O
2020	O
,	O
an	O
open	O
letter	O
was	O
released	O
signed	O
by	O
over	O
300	O
security	O
and	O
privacy	O
academics	O
from	O
26	O
countries	O
criticising	O
the	O
approach	O
taken	O
by	O
PEPP-PT	B-Protocol
,	O
stating	O
that	O
'	O
solutions	O
which	O
allow	O
reconstructing	O
invasive	O
information	O
about	O
the	O
population	O
should	O
be	O
rejected	O
without	O
further	O
discussion	O
 '	O
.	O
</s>
