<s>
Signal	O
is	O
an	O
encrypted	O
messaging	O
service	O
for	O
instant	B-Application
messaging	I-Application
,	O
voice	O
,	O
and	O
video	B-Application
calls	I-Application
.	O
</s>
<s>
The	O
instant	B-Application
messaging	I-Application
function	O
includes	O
sending	O
text	O
,	O
voice	O
notes	O
,	O
images	B-Algorithm
,	O
videos	O
,	O
and	O
other	B-Operating_System
files	I-Operating_System
.	O
</s>
<s>
The	O
application	O
uses	O
a	O
centralized	B-General_Concept
computing	I-General_Concept
architecture	O
,	O
and	O
is	O
cross-platform	B-Operating_System
software	I-Operating_System
.	O
</s>
<s>
It	O
is	O
developed	O
by	O
the	O
non-profit	O
Signal	O
Foundation	O
and	O
its	O
subsidiary	O
,	O
Signal	B-Application
Messenger	I-Application
LLC	O
.	O
</s>
<s>
Signal	O
's	O
software	O
is	O
free	B-License
and	I-License
open-source	I-License
.	O
</s>
<s>
Its	O
mobile	B-Application
clients	B-Protocol
are	O
published	O
under	O
the	O
GPL-3.0-only	B-License
license	B-License
,	O
while	O
the	O
desktop	B-Application
client	O
and	O
server	B-Application
are	O
published	O
under	O
the	O
AGPL-3.0-only	B-Application
license	B-License
.	O
</s>
<s>
The	O
official	O
Android	B-Application
app	B-Application
generally	O
uses	O
the	O
proprietary	B-Application
Google	B-Application
Play	I-Application
Services	I-Application
,	O
although	O
it	O
is	O
designed	O
to	O
work	O
without	O
them	O
.	O
</s>
<s>
Signal	O
is	O
also	O
distributed	O
for	O
iOS	B-Application
and	O
desktop	B-Application
programs	O
for	O
Windows	B-Application
,	O
macOS	B-Application
,	O
and	O
Linux	B-Application
.	O
</s>
<s>
Registration	O
for	O
desktop	B-Application
use	O
requires	O
an	O
iOS	B-Application
or	O
Android	B-Application
device	O
.	O
</s>
<s>
Signal	O
uses	O
mobile	B-Application
telephone	O
numbers	O
as	O
identifier	O
for	O
users	O
.	O
</s>
<s>
The	O
client	B-Protocol
software	I-Protocol
includes	O
mechanisms	O
by	O
which	O
users	O
can	O
independently	O
verify	O
the	O
identity	O
of	O
their	O
contacts	O
and	O
the	O
integrity	O
of	O
the	O
data	O
channel	O
.	O
</s>
<s>
The	O
Android	B-Application
version	O
also	O
optionally	O
functions	O
as	O
an	O
SMS	B-Device
app	B-Application
,	O
but	O
this	O
functionality	O
will	O
be	O
removed	O
in	O
2023	O
.	O
</s>
<s>
Signal	O
is	O
the	O
successor	O
of	O
the	O
RedPhone	O
encrypted	O
voice	O
calling	O
app	B-Application
and	O
the	O
TextSecure	B-Protocol
encrypted	O
texting	O
program	O
.	O
</s>
<s>
The	O
beta	O
versions	O
of	O
RedPhone	O
and	O
TextSecure	B-Protocol
were	O
first	O
launched	O
in	O
May	O
2010	O
by	O
Whisper	B-Application
Systems	I-Application
,	O
a	O
startup	O
company	O
co-founded	O
by	O
security	O
researcher	O
Moxie	O
Marlinspike	O
and	O
roboticist	O
Stuart	O
Anderson	O
.	O
</s>
<s>
Whisper	B-Application
Systems	I-Application
also	O
produced	O
a	O
firewall	O
and	O
tools	O
for	O
encrypting	O
other	O
forms	O
of	O
data	O
.	O
</s>
<s>
All	O
of	O
these	O
were	O
proprietary	B-Application
enterprise	O
mobile	B-Application
security	O
software	O
and	O
were	O
only	O
available	O
for	O
Android	B-Application
.	O
</s>
<s>
In	O
November	O
2011	O
,	O
Whisper	B-Application
Systems	I-Application
announced	O
that	O
it	O
had	O
been	O
acquired	O
by	O
Twitter	B-Application
.	O
</s>
<s>
Shortly	O
after	O
the	O
acquisition	O
,	O
Whisper	B-Application
Systems	I-Application
 '	O
RedPhone	O
service	O
was	O
made	O
unavailable	O
.	O
</s>
<s>
Twitter	B-Application
released	O
TextSecure	B-Protocol
as	O
free	B-License
and	I-License
open-source	I-License
software	I-License
under	O
the	O
GPLv3	B-License
license	B-License
in	O
December	O
2011	O
.	O
</s>
<s>
RedPhone	O
was	O
also	O
released	O
under	O
the	O
same	O
license	B-License
in	O
July	O
2012	O
.	O
</s>
<s>
Marlinspike	O
later	O
left	O
Twitter	B-Application
and	O
founded	O
Open	O
Whisper	B-Application
Systems	I-Application
as	O
a	O
collaborative	O
Open	B-Application
Source	I-Application
project	O
for	O
the	O
continued	O
development	O
of	O
TextSecure	B-Protocol
and	O
RedPhone	O
.	O
</s>
<s>
Open	O
Whisper	B-Application
Systems	I-Application
 '	O
website	O
was	O
launched	O
in	O
January	O
2013	O
.	O
</s>
<s>
In	O
February	O
2014	O
,	O
Open	O
Whisper	B-Application
Systems	I-Application
introduced	O
the	O
second	O
version	O
of	O
their	O
TextSecure	B-Protocol
Protocol	I-Protocol
(	O
now	O
Signal	B-Protocol
Protocol	I-Protocol
)	O
,	O
which	O
added	O
end-to-end	O
encrypted	O
group	O
chat	O
and	O
instant	B-Application
messaging	I-Application
capabilities	O
to	O
TextSecure	B-Protocol
.	O
</s>
<s>
Toward	O
the	O
end	O
of	O
July	O
2014	O
,	O
they	O
announced	O
plans	O
to	O
merge	O
the	O
RedPhone	O
and	O
TextSecure	B-Protocol
applications	O
as	O
Signal	O
.	O
</s>
<s>
This	O
announcement	O
coincided	O
with	O
the	O
initial	O
release	O
of	O
Signal	O
as	O
a	O
RedPhone	O
counterpart	O
for	O
iOS	B-Application
.	O
</s>
<s>
The	O
developers	O
said	O
that	O
their	O
next	O
steps	O
would	O
be	O
to	O
provide	O
TextSecure	B-Protocol
instant	B-Application
messaging	I-Application
capabilities	O
for	O
iOS	B-Application
,	O
unify	O
the	O
RedPhone	O
and	O
TextSecure	B-Protocol
applications	O
on	O
Android	B-Application
,	O
and	O
launch	O
a	O
web	B-Application
client	I-Application
.	O
</s>
<s>
Signal	O
was	O
the	O
first	O
iOS	B-Application
app	B-Application
to	O
enable	O
end-to-end	O
encrypted	O
voice	O
calls	O
for	O
free	O
.	O
</s>
<s>
TextSecure	B-Protocol
compatibility	O
was	O
added	O
to	O
the	O
iOS	B-Application
application	O
in	O
March	O
2015	O
.	O
</s>
<s>
From	O
its	O
launch	O
in	O
May	O
2010	O
until	O
March	O
2015	O
,	O
the	O
Android	B-Application
version	O
of	O
Signal	O
(	O
then	O
called	O
TextSecure	B-Protocol
)	O
included	O
support	O
for	O
encrypted	O
SMS/MMS	O
messaging	O
.	O
</s>
<s>
From	O
version	O
2.7.0	O
onward	O
,	O
the	O
Android	B-Application
application	I-Application
only	O
supported	O
sending	O
and	O
receiving	O
encrypted	O
messages	O
via	O
the	O
data	O
channel	O
.	O
</s>
<s>
Reasons	O
for	O
this	O
included	O
security	O
flaws	O
of	O
SMS/MMS	O
and	O
problems	O
with	O
the	O
key	O
exchange	O
.	O
</s>
<s>
Open	O
Whisper	B-Application
Systems	I-Application
 '	O
abandonment	O
of	O
SMS/MMS	O
encryption	O
prompted	O
some	O
users	O
to	O
create	O
a	O
fork	B-Application
named	O
Silence	O
(	O
initially	O
called	O
SMSSecure	O
)	O
that	O
is	O
meant	O
solely	O
for	O
the	O
exchange	O
of	O
encrypted	O
SMS	B-Device
and	O
MMS	O
messages	O
.	O
</s>
<s>
In	O
November	O
2015	O
,	O
the	O
TextSecure	B-Protocol
and	O
RedPhone	O
applications	O
on	O
Android	B-Application
were	O
merged	O
to	O
become	O
Signal	O
for	O
Android	B-Application
.	O
</s>
<s>
A	O
month	O
later	O
,	O
Open	O
Whisper	B-Application
Systems	I-Application
announced	O
Signal	O
Desktop	B-Application
,	O
a	O
Chrome	B-Operating_System
app	I-Operating_System
that	O
could	O
link	O
with	O
a	O
Signal	O
mobile	B-Application
client	O
.	O
</s>
<s>
At	O
launch	O
,	O
the	O
app	B-Application
could	O
only	O
be	O
linked	O
with	O
the	O
Android	B-Application
version	O
of	O
Signal	O
.	O
</s>
<s>
On	O
26	O
September	O
2016	O
,	O
Open	O
Whisper	B-Application
Systems	I-Application
announced	O
that	O
Signal	O
Desktop	B-Application
could	O
now	O
be	O
linked	O
with	O
the	O
iOS	B-Application
version	O
of	O
Signal	O
as	O
well	O
.	O
</s>
<s>
On	O
31	O
October	O
2017	O
,	O
Open	O
Whisper	B-Application
Systems	I-Application
announced	O
that	O
the	O
Chrome	B-Operating_System
app	I-Operating_System
was	O
deprecated	B-General_Concept
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
they	O
announced	O
the	O
release	O
of	O
a	O
standalone	O
desktop	B-Application
client	O
(	O
based	O
on	O
the	O
Electron	B-Operating_System
framework	I-Operating_System
)	O
for	O
Windows	B-Application
,	O
macOS	B-Application
and	O
certain	O
Linux	B-Application
distributions	I-Application
.	O
</s>
<s>
On	O
4	O
October	O
2016	O
,	O
the	O
American	O
Civil	O
Liberties	O
Union	O
(	O
ACLU	O
)	O
and	O
Open	O
Whisper	B-Application
Systems	I-Application
published	O
a	O
series	O
of	O
documents	O
revealing	O
that	O
OWS	O
had	O
received	O
a	O
subpoena	O
requiring	O
them	O
to	O
provide	O
information	O
associated	O
with	O
two	O
phone	O
numbers	O
for	O
a	O
federal	O
grand	O
jury	O
investigation	O
in	O
the	O
first	O
half	O
of	O
2016	O
.	O
</s>
<s>
In	O
March	O
2017	O
,	O
Open	O
Whisper	B-Application
Systems	I-Application
transitioned	O
Signal	O
's	O
calling	O
system	O
from	O
RedPhone	O
to	O
WebRTC	B-Protocol
,	O
also	O
adding	O
the	O
ability	O
to	O
make	O
video	B-Application
calls	I-Application
with	O
the	O
mobile	B-Application
apps	I-Application
.	O
</s>
<s>
On	O
21	O
February	O
2018	O
,	O
Moxie	O
Marlinspike	O
and	O
WhatsApp	B-Application
co-founder	O
Brian	O
Acton	O
announced	O
the	O
formation	O
of	O
the	O
Signal	O
Technology	O
Foundation	O
,	O
a	O
501(c )	O
(	O
3	O
)	O
nonprofit	O
organization	O
whose	O
mission	O
is	O
"	O
to	O
support	O
,	O
accelerate	O
,	O
and	O
broaden	O
Signal	O
's	O
mission	O
of	O
making	O
private	O
communication	O
accessible	O
and	O
ubiquitous	O
"	O
.	O
</s>
<s>
Acton	O
started	O
the	O
foundation	O
with	O
$50million	O
in	O
funding	O
and	O
became	O
the	O
foundation	O
's	O
executive	O
chairman	O
after	O
leaving	O
WhatsApp	B-Application
's	O
parent	O
company	O
Facebook	O
in	O
September	O
2017	O
.	O
</s>
<s>
Marlinspike	O
continued	O
as	O
Signal	B-Application
Messenger	I-Application
's	O
first	O
CEO	O
.	O
</s>
<s>
Between	O
November	O
2019	O
and	O
February	O
2020	O
,	O
Signal	O
added	O
iPad	B-Device
support	O
,	O
view-once	O
images	B-Algorithm
and	O
videos	O
,	O
stickers	O
,	O
and	O
reactions	O
.	O
</s>
<s>
During	O
the	O
first	O
week	O
of	O
June	O
,	O
the	O
encrypted	O
messaging	B-Application
app	I-Application
was	O
downloaded	O
over	O
five	O
times	O
more	O
than	O
it	O
had	O
been	O
during	O
the	O
week	O
prior	O
to	O
the	O
murder	O
of	O
George	O
Floyd	O
.	O
</s>
<s>
CNN	B-Architecture
and	O
MacRumors	O
linked	O
the	O
surge	O
with	O
a	O
WhatsApp	B-Application
privacy	O
policy	O
change	O
and	O
a	O
Signal	O
endorsement	O
by	O
Elon	O
Musk	O
and	O
Edward	O
Snowden	O
via	O
Twitter	B-Application
.	O
</s>
<s>
Between	O
12	O
and	O
14	O
January	O
2021	O
,	O
the	O
number	O
of	O
Signal	O
installations	O
listed	O
on	O
Google	B-Application
Play	O
increased	O
from	O
over	O
10	O
million	O
to	O
over	O
50	O
million	O
.	O
</s>
<s>
On	O
the	O
afternoon	O
of	O
16	O
January	O
,	O
Signal	O
announced	O
via	O
Twitter	B-Application
that	O
service	O
had	O
been	O
restored	O
.	O
</s>
<s>
On	O
10	O
January	O
2022	O
,	O
Moxie	O
Marlinspike	O
announced	O
that	O
he	O
was	O
stepping	O
down	O
from	O
his	O
role	O
as	O
CEO	O
of	O
Signal	B-Application
Messenger	I-Application
.	O
</s>
<s>
In	O
August	O
2022	O
,	O
Signal	O
notified	O
1900	O
users	O
that	O
their	O
data	O
had	O
been	O
affected	O
by	O
the	O
Twilio	B-General_Concept
breach	O
including	O
user	O
phone	O
numbers	O
and	O
SMS	B-Device
verification	O
codes	O
.	O
</s>
<s>
Signal	O
's	O
userbase	O
started	O
in	O
May	O
2010	O
,	O
when	O
its	O
predecessor	O
TextSecure	B-Protocol
was	O
launched	O
by	O
Whisper	B-Application
Systems	I-Application
.	O
</s>
<s>
According	O
to	O
App	B-Application
Annie	O
,	O
Signal	O
had	O
approximately	O
20	O
million	O
monthly	O
active	O
users	O
at	O
the	O
end	O
of	O
December	O
2020	O
.	O
</s>
<s>
The	O
development	O
of	O
Signal	O
and	O
its	O
predecessors	O
at	O
Open	O
Whisper	B-Application
Systems	I-Application
was	O
funded	O
by	O
a	O
combination	O
of	O
consulting	O
contracts	O
,	O
donations	O
and	O
grants	O
.	O
</s>
<s>
Signal	O
is	O
now	O
developed	O
by	O
Signal	B-Application
Messenger	I-Application
LLC	O
,	O
a	O
software	O
company	O
founded	O
by	O
Moxie	O
Marlinspike	O
and	O
Brian	O
Acton	O
in	O
2018	O
,	O
which	O
is	O
wholly	O
owned	O
by	O
a	O
tax-exempt	O
nonprofit	O
corporation	O
called	O
the	O
Signal	O
Technology	O
Foundation	O
,	O
also	O
created	O
by	O
them	O
in	O
2018	O
.	O
</s>
<s>
All	O
of	O
the	O
organization	O
's	O
products	O
are	O
published	O
as	O
free	B-License
and	I-License
open-source	I-License
software	I-License
.	O
</s>
<s>
Signal	O
allows	O
users	O
to	O
make	O
one-to-one	O
and	O
group	O
voice	O
and	O
video	B-Application
calls	I-Application
with	O
up	O
to	O
40	O
people	O
on	O
iOS	B-Application
,	O
Android	B-Application
,	O
and	O
desktop	B-Application
.	O
</s>
<s>
Signal	O
also	O
allows	O
users	O
to	O
send	O
text	O
messages	O
,	O
files	O
,	O
voice	O
notes	O
,	O
pictures	O
,	O
GIFs	O
,	O
and	O
video	O
messages	O
over	O
a	O
Wi-Fi	O
or	O
data	O
connection	O
to	O
other	O
Signal	O
users	O
on	O
iOS	B-Application
,	O
Android	B-Application
and	O
a	O
desktop	B-Application
program	O
.	O
</s>
<s>
To	O
verify	O
that	O
a	O
correspondent	O
is	O
really	O
the	O
person	O
that	O
they	O
claim	O
to	O
be	O
,	O
Signal	O
users	O
can	O
compare	O
key	B-Algorithm
fingerprints	I-Algorithm
(	O
or	O
scan	O
QR	O
codes	O
)	O
out-of-band	B-Protocol
.	O
</s>
<s>
On	O
Android	B-Application
,	O
users	O
can	O
opt	O
into	O
making	O
Signal	O
the	O
default	O
SMS/MMS	O
application	O
,	O
allowing	O
them	O
to	O
send	O
and	O
receive	O
unencrypted	O
SMS	B-Device
messages	O
in	O
addition	O
to	O
the	O
standard	O
end-to-end	O
encrypted	O
Signal	O
messages	O
.	O
</s>
<s>
As	O
of	O
October	O
2022	O
,	O
this	O
feature	O
has	O
been	O
deprecated	B-General_Concept
due	O
to	O
safety	O
and	O
security	O
concerns	O
,	O
and	O
will	O
be	O
removed	O
in	O
the	O
near	O
future	O
.	O
</s>
<s>
TextSecure	B-Protocol
allowed	O
the	O
user	O
to	O
set	O
a	O
passphrase	O
that	O
encrypted	O
the	O
local	O
message	O
database	O
and	O
the	O
user	O
's	O
encryption	O
keys	O
.	O
</s>
<s>
The	O
Signal	O
applications	O
on	O
Android	B-Application
and	O
iOS	B-Application
can	O
be	O
locked	O
with	O
the	O
phone	O
's	O
pin	O
,	O
passphrase	O
,	O
or	O
biometric	O
authentication	B-Algorithm
.	O
</s>
<s>
Signal	O
also	O
includes	O
a	O
cryptocurrency	B-Protocol
wallet	I-Protocol
functionality	O
that	O
allows	O
users	O
to	O
store	O
,	O
send	O
and	O
receive	O
in-app	O
payments	O
.	O
</s>
<s>
Apart	O
from	O
certain	O
regions	O
and	O
countries	O
,	O
the	O
feature	O
was	O
enabled	O
globally	O
in	O
November	O
2021	O
.	O
,	O
the	O
only	O
supported	O
payment	O
method	O
is	O
MobileCoin	B-Protocol
.	O
</s>
<s>
A	O
number	O
can	O
only	O
be	O
registered	O
on	O
one	O
mobile	B-Application
device	O
at	O
a	O
time	O
.	O
</s>
<s>
Account	O
registration	O
requires	O
an	O
iOS	B-Application
or	O
Android	B-Application
device	O
.	O
</s>
<s>
This	O
mandatory	O
connection	O
to	O
a	O
phone	O
number	O
(	O
a	O
feature	O
Signal	O
shares	O
with	O
WhatsApp	B-Application
,	O
KakaoTalk	B-Protocol
,	O
and	O
others	O
)	O
has	O
been	O
criticized	O
as	O
a	O
"	O
major	O
issue	O
"	O
for	O
privacy-conscious	O
users	O
who	O
are	O
not	O
comfortable	O
with	O
giving	O
out	O
their	O
private	O
phone	O
number	O
.	O
</s>
<s>
A	O
similar	O
vulnerability	O
was	O
used	O
to	O
attack	O
at	O
least	O
one	O
user	O
in	O
August	O
2022	O
,	O
though	O
the	O
attack	O
was	O
performed	O
via	O
the	O
provider	O
of	O
Signal	O
's	O
SMS	B-Device
services	O
,	O
not	O
any	O
user	O
's	O
provider	O
.	O
</s>
<s>
When	O
linking	O
Signal	O
Desktop	B-Application
to	O
a	O
mobile	B-Application
device	O
,	O
the	O
conversations	O
history	O
will	O
not	O
be	O
synced	O
;	O
only	O
the	O
new	O
messages	O
will	O
be	O
shown	O
on	O
Signal	O
Desktop	B-Application
.	O
</s>
<s>
The	O
study	O
concluded	O
that	O
21	O
out	O
of	O
28	O
participants	O
failed	O
to	O
correctly	O
compare	O
public	B-Algorithm
key	I-Algorithm
fingerprints	I-Algorithm
in	O
order	O
to	O
verify	O
the	O
identity	O
of	O
other	O
Signal	O
users	O
,	O
and	O
that	O
most	O
of	O
these	O
users	O
believed	O
they	O
had	O
succeeded	O
,	O
while	O
they	O
had	O
actually	O
failed	O
.	O
</s>
<s>
Signal	O
messages	O
are	O
encrypted	O
with	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
(	O
formerly	O
known	O
as	O
the	O
TextSecure	B-Protocol
Protocol	I-Protocol
)	O
.	O
</s>
<s>
The	O
protocol	O
combines	O
the	O
Double	B-Algorithm
Ratchet	I-Algorithm
Algorithm	I-Algorithm
,	O
prekeys	O
,	O
and	O
an	O
Extended	O
Triple	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
(	O
X3DH	B-Protocol
)	O
handshake	O
.	O
</s>
<s>
It	O
uses	O
Curve25519	O
,	O
AES-256	B-Algorithm
,	O
and	O
HMAC-SHA256	B-Algorithm
as	O
primitives	O
.	O
</s>
<s>
The	O
protocol	O
provides	O
confidentiality	O
,	O
integrity	O
,	O
authentication	B-Algorithm
,	O
participant	O
consistency	O
,	O
destination	O
validation	O
,	O
forward	O
secrecy	O
,	O
backward	O
secrecy	O
(	O
future	O
secrecy	O
)	O
,	O
causality	O
preservation	O
,	O
message	O
unlinkability	O
,	O
message	B-Protocol
repudiation	I-Protocol
,	O
participation	O
repudiation	B-Protocol
,	O
and	O
asynchronicity	O
.	O
</s>
<s>
It	O
does	O
not	O
provide	O
anonymity	O
preservation	O
,	O
and	O
requires	O
servers	O
for	O
the	O
relaying	O
of	O
messages	O
and	O
storing	O
of	O
public	B-Application
key	I-Application
material	O
.	O
</s>
<s>
The	O
Signal	B-Protocol
Protocol	I-Protocol
also	O
supports	O
end-to-end	O
encrypted	O
group	O
chats	O
.	O
</s>
<s>
The	O
group	O
chat	O
protocol	O
is	O
a	O
combination	O
of	O
a	O
pairwise	O
double	B-Algorithm
ratchet	I-Algorithm
and	O
multicast	B-Protocol
encryption	I-Protocol
.	O
</s>
<s>
In	O
October	O
2014	O
,	O
researchers	O
from	O
Ruhr	O
University	O
Bochum	O
published	O
an	O
analysis	O
of	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
In	O
July	O
2017	O
,	O
researchers	O
from	O
Ruhr	O
University	O
Bochum	O
found	O
during	O
another	O
analysis	O
of	O
group	O
messengers	B-Protocol
a	O
purely	O
theoretic	O
attack	O
against	O
the	O
group	O
protocol	O
of	O
Signal	O
:	O
A	O
user	O
who	O
knows	O
the	O
secret	O
group	O
ID	O
of	O
a	O
group	O
(	O
due	O
to	O
having	O
been	O
a	O
group	O
member	O
previously	O
or	O
stealing	O
it	O
from	O
a	O
member	O
's	O
device	O
)	O
can	O
become	O
a	O
member	O
of	O
the	O
group	O
.	O
</s>
<s>
,	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
has	O
been	O
implemented	O
into	O
WhatsApp	B-Application
,	O
Facebook	B-Protocol
Messenger	I-Protocol
,	O
Skype	B-Application
,	O
and	O
Google	B-Protocol
Allo	I-Protocol
,	O
making	O
it	O
possible	O
for	O
the	O
conversations	O
of	O
"	O
more	O
than	O
a	O
billion	O
people	O
worldwide	O
"	O
to	O
be	O
end-to-end	O
encrypted	O
.	O
</s>
<s>
In	O
Google	B-Protocol
Allo	I-Protocol
,	O
Skype	B-Application
and	O
Facebook	B-Protocol
Messenger	I-Protocol
,	O
conversations	O
are	O
not	O
encrypted	O
with	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
by	O
default	O
;	O
they	O
only	O
offer	O
end-to-end	O
encryption	O
in	O
an	O
optional	O
mode	O
.	O
</s>
<s>
Up	O
until	O
March	O
2017	O
,	O
Signal	O
's	O
voice	O
calls	O
were	O
encrypted	O
with	O
SRTP	B-Protocol
and	O
the	O
ZRTP	B-Protocol
key-agreement	O
protocol	O
,	O
which	O
was	O
developed	O
by	O
Phil	O
Zimmermann	O
.	O
</s>
<s>
In	O
March	O
2017	O
,	O
Signal	O
transitioned	O
to	O
a	O
new	O
WebRTC-based	O
calling	O
system	O
that	O
introduced	O
the	O
ability	O
to	O
make	O
video	B-Application
calls	I-Application
.	O
</s>
<s>
Signal	O
's	O
voice	O
and	O
video	B-Application
calling	I-Application
functionalities	O
use	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
channel	O
for	O
authentication	B-Algorithm
instead	O
of	O
ZRTP	B-Protocol
.	O
</s>
<s>
To	O
verify	O
that	O
a	O
correspondent	O
is	O
really	O
the	O
person	O
that	O
they	O
claim	O
to	O
be	O
,	O
Signal	O
users	O
can	O
compare	O
key	B-Algorithm
fingerprints	I-Algorithm
(	O
or	O
scan	O
QR	O
codes	O
)	O
out-of-band	B-Protocol
.	O
</s>
<s>
After	O
receiving	O
and	O
decrypting	O
messages	O
,	O
the	O
application	O
stored	O
them	O
locally	O
on	O
each	O
device	O
in	O
a	O
SQLite	B-Language
database	O
that	O
is	O
encrypted	O
with	O
SQLCipher	O
.	O
</s>
<s>
In	O
December	O
2020	O
,	O
Cellebrite	O
published	O
a	O
blog	O
post	O
announcing	O
that	O
one	O
of	O
their	O
products	O
could	O
now	O
access	O
this	O
key	O
and	O
use	O
it	O
to	O
"	O
decrypt	O
the	O
Signal	B-Application
app	I-Application
"	O
.	O
</s>
<s>
Technology	O
reporters	O
later	O
published	O
articles	O
about	O
how	O
Cellebrite	O
had	O
claimed	O
to	O
have	O
the	O
ability	O
to	O
"	O
break	O
into	O
the	O
Signal	B-Application
app	I-Application
"	O
and	O
"	O
crack	O
Signal	O
's	O
encryption	O
"	O
.	O
</s>
<s>
This	O
latter	O
interpretation	O
was	O
rejected	O
by	O
several	O
experts	O
,	O
as	O
well	O
as	O
representatives	O
from	O
Signal	O
,	O
who	O
said	O
the	O
original	O
post	O
by	O
Cellebrite	O
had	O
been	O
about	O
accessing	O
data	O
on	O
"	O
an	O
unlocked	O
Android	B-Application
phone	I-Application
in	O
their	O
physical	O
possession	O
"	O
and	O
that	O
they	O
"	O
could	O
have	O
just	O
opened	O
the	O
app	B-Application
to	O
look	O
at	O
the	O
messages	O
"	O
.	O
</s>
<s>
Similar	O
extraction	O
tools	O
also	O
exist	O
for	O
iOS	B-Application
devices	O
and	O
Signal	O
Desktop	B-Application
.	O
</s>
<s>
Signal	O
relies	O
on	O
centralized	B-General_Concept
servers	O
that	O
are	O
maintained	O
by	O
Signal	B-Application
Messenger	I-Application
.	O
</s>
<s>
In	O
addition	O
to	O
routing	O
Signal	O
's	O
messages	O
,	O
the	O
servers	O
also	O
facilitate	O
the	O
discovery	O
of	O
contacts	O
who	O
are	O
also	O
registered	O
Signal	O
users	O
and	O
the	O
automatic	O
exchange	O
of	O
users	O
 '	O
public	B-Application
keys	I-Application
.	O
</s>
<s>
By	O
default	O
,	O
Signal	O
's	O
voice	O
and	O
video	B-Application
calls	I-Application
are	O
peer-to-peer	O
.	O
</s>
<s>
If	O
the	O
caller	O
is	O
not	O
in	O
the	O
receiver	O
's	O
address	O
book	O
,	O
the	O
call	O
is	O
routed	O
through	O
a	O
server	B-Application
in	O
order	O
to	O
hide	O
the	O
users	O
 '	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
The	O
servers	O
store	O
registered	O
users	O
 '	O
phone	O
numbers	O
,	O
public	B-Application
key	I-Application
material	O
and	O
push	O
tokens	O
which	O
are	O
necessary	O
for	O
setting	O
up	O
calls	O
and	O
transmitting	O
messages	O
.	O
</s>
<s>
In	O
order	O
to	O
determine	O
which	O
contacts	O
are	O
also	O
Signal	O
users	O
,	O
cryptographic	B-Algorithm
hashes	I-Algorithm
of	O
the	O
user	O
's	O
contact	O
numbers	O
are	O
periodically	O
transmitted	O
to	O
the	O
server	B-Application
.	O
</s>
<s>
The	O
server	B-Application
then	O
checks	O
to	O
see	O
if	O
those	O
match	O
any	O
of	O
the	O
SHA256	O
hashes	B-Algorithm
of	O
registered	O
users	O
and	O
tells	O
the	O
client	O
if	O
any	O
matches	O
are	O
found	O
.	O
</s>
<s>
The	O
hashed	O
numbers	O
are	O
thereafter	O
discarded	O
from	O
the	O
server	B-Application
.	O
</s>
<s>
In	O
2014	O
,	O
Moxie	O
Marlinspike	O
wrote	O
that	O
it	O
is	O
easy	O
to	O
calculate	O
a	O
map	O
of	O
all	O
possible	O
hash	B-Algorithm
inputs	O
to	O
hash	B-Algorithm
outputs	O
and	O
reverse	O
the	O
mapping	O
because	O
of	O
the	O
limited	O
preimage	O
space	O
(	O
the	O
set	O
of	O
all	O
possible	O
hash	B-Algorithm
inputs	O
)	O
of	O
phone	O
numbers	O
,	O
and	O
that	O
a	O
"	O
practical	O
privacy	O
preserving	O
contact	O
discovery	O
remains	O
an	O
unsolved	O
problem.	O
"	O
</s>
<s>
In	O
September	O
2017	O
,	O
Signal	O
's	O
developers	O
announced	O
that	O
they	O
were	O
working	O
on	O
a	O
way	O
for	O
the	O
Signal	O
client	B-Protocol
applications	I-Protocol
to	O
"	O
efficiently	O
and	O
scalably	O
determine	O
whether	O
the	O
contacts	O
in	O
their	O
address	O
book	O
are	O
Signal	O
users	O
without	O
revealing	O
the	O
contacts	O
in	O
their	O
address	O
book	O
to	O
the	O
Signal	O
service.	O
"	O
</s>
<s>
All	O
client-server	O
communications	O
are	O
protected	O
by	O
TLS	B-Protocol
.	O
</s>
<s>
In	O
June	O
2016	O
,	O
Marlinspike	O
told	O
The	O
Intercept	O
that	O
"	O
the	O
closest	O
piece	O
of	O
information	O
to	O
metadata	O
that	O
the	O
Signal	O
server	B-Application
stores	O
is	O
the	O
last	O
time	O
each	O
user	O
connected	O
to	O
the	O
server	B-Application
,	O
and	O
the	O
precision	O
of	O
this	O
information	O
is	O
reduced	O
to	O
the	O
day	O
,	O
rather	O
than	O
the	O
hour	O
,	O
minute	O
,	O
and	O
second	O
"	O
.	O
</s>
<s>
Instead	O
,	O
the	O
creation	O
,	O
updating	O
,	O
joining	O
,	O
and	O
leaving	O
of	O
groups	O
is	O
done	O
by	O
the	O
clients	B-Protocol
,	O
which	O
deliver	O
pairwise	O
messages	O
to	O
the	O
participants	O
in	O
the	O
same	O
way	O
that	O
one-to-one	O
messages	O
are	O
delivered	O
.	O
</s>
<s>
Signal	O
's	O
server	B-Application
architecture	O
was	O
federated	O
between	O
December	O
2013	O
and	O
February	O
2016	O
.	O
</s>
<s>
In	O
December	O
2013	O
,	O
it	O
was	O
announced	O
that	O
the	O
messaging	O
protocol	O
Signal	O
uses	O
had	O
successfully	O
been	O
integrated	O
into	O
the	O
Android-based	O
open-source	B-Application
operating	O
system	O
CyanogenMod	B-Application
.	O
</s>
<s>
Since	O
CyanogenMod	B-Application
11.0	O
,	O
the	O
client	O
logic	O
was	O
contained	O
in	O
a	O
system	O
app	B-Application
called	O
WhisperPush	O
.	O
</s>
<s>
According	O
to	O
Signal	O
's	O
developers	O
,	O
the	O
Cyanogen	O
team	O
ran	O
their	O
own	O
Signal	O
messaging	O
server	B-Application
for	O
WhisperPush	O
clients	B-Protocol
,	O
which	O
federated	O
with	O
the	O
main	O
server	B-Application
,	O
so	O
that	O
both	O
clients	B-Protocol
could	O
exchange	O
messages	O
with	O
each	O
other	O
.	O
</s>
<s>
The	O
WhisperPush	O
source	O
code	O
was	O
available	O
under	O
the	O
GPLv3	B-License
license	B-License
.	O
</s>
<s>
In	O
February	O
2016	O
,	O
the	O
CyanogenMod	B-Application
team	O
discontinued	O
WhisperPush	O
and	O
recommended	O
that	O
its	O
users	O
switch	O
to	O
Signal	O
.	O
</s>
<s>
In	O
May	O
2016	O
,	O
Moxie	O
Marlinspike	O
wrote	O
that	O
federation	O
with	O
the	O
CyanogenMod	B-Application
servers	O
had	O
degraded	O
the	O
user	O
experience	O
and	O
held	O
back	O
development	O
,	O
and	O
that	O
their	O
servers	O
will	O
probably	O
not	O
federate	O
with	O
other	O
servers	O
again	O
.	O
</s>
<s>
The	O
complete	O
source	O
code	O
of	O
the	O
Signal	O
clients	B-Protocol
for	O
Android	B-Application
,	O
iOS	B-Application
and	O
desktop	B-Application
is	O
available	O
on	O
GitHub	B-Application
under	O
a	O
free	B-License
software	I-License
license	I-License
.	O
</s>
<s>
It	O
also	O
allows	O
advanced	O
users	O
to	O
compile	O
their	O
own	O
copies	O
of	O
the	O
applications	O
and	O
compare	O
them	O
with	O
the	O
versions	O
that	O
are	O
distributed	O
by	O
Signal	B-Application
Messenger	I-Application
.	O
</s>
<s>
In	O
March	O
2016	O
,	O
Moxie	O
Marlinspike	O
wrote	O
that	O
,	O
apart	O
from	O
some	O
shared	O
libraries	O
that	O
are	O
not	O
compiled	O
with	O
the	O
project	O
build	O
due	O
to	O
a	O
lack	O
of	O
GradleNDK	O
support	O
,	O
Signal	O
for	O
Android	B-Application
is	O
reproducible	B-Application
.	O
</s>
<s>
Signal	O
's	O
servers	O
are	O
partially	O
open	B-Application
source	I-Application
,	O
but	O
the	O
server	B-Application
software	I-Application
's	O
anti-spam	O
component	O
is	O
proprietary	B-Application
and	O
closed	B-Application
source	I-Application
due	O
to	O
security	O
concerns	O
.	O
</s>
<s>
In	O
November	O
2014	O
,	O
Signal	O
received	O
a	O
perfect	O
score	O
on	O
the	O
EFF	O
's	O
secure	O
messaging	O
scorecard	O
;	O
it	O
received	O
points	O
for	O
having	O
communications	O
encrypted	O
in	O
transit	O
,	O
having	O
communications	O
encrypted	O
with	O
keys	O
the	O
provider	O
does	O
not	O
have	O
access	O
to	O
(	O
end-to-end	O
encryption	O
)	O
,	O
making	O
it	O
possible	O
for	O
users	O
to	O
independently	O
verify	O
their	O
correspondents	O
 '	O
identities	O
,	O
having	O
past	O
communications	O
secure	O
if	O
the	O
keys	O
are	O
stolen	O
(	O
forward	O
secrecy	O
)	O
,	O
having	O
the	O
code	O
open	O
to	O
independent	O
review	O
(	O
open	B-Application
source	I-Application
)	O
,	O
having	O
the	O
security	O
designs	O
well-documented	O
,	O
and	O
having	O
a	O
recent	O
independent	O
security	O
audit	O
.	O
</s>
<s>
At	O
the	O
time	O
,	O
"	O
ChatSecure	B-Protocol
+	O
Orbot	B-Language
"	O
,	O
Pidgin	B-Protocol
(	O
with	O
OTR	B-Protocol
)	O
,	O
Silent	O
Phone	O
,	O
and	O
Telegram	B-Application
's	O
optional	O
"	O
secret	O
chats	O
"	O
also	O
received	O
seven	O
out	O
of	O
seven	O
points	O
on	O
the	O
scorecard	O
.	O
</s>
<s>
In	O
his	O
keynote	O
speech	O
at	O
SXSW	O
in	O
March	O
2014	O
,	O
he	O
praised	O
Signal	O
's	O
predecessors	O
(	O
TextSecure	B-Protocol
and	O
RedPhone	O
)	O
for	O
their	O
ease	O
of	O
use	O
.	O
</s>
<s>
In	O
December	O
2014	O
,	O
Der	O
Spiegel	O
leaked	O
slides	O
from	O
an	O
internal	O
NSA	O
presentation	O
dating	O
to	O
June	O
2012	O
in	O
which	O
the	O
NSA	O
deemed	O
Signal	O
's	O
encrypted	O
voice	O
calling	O
component	O
(	O
RedPhone	O
)	O
on	O
its	O
own	O
as	O
a	O
"	O
major	O
threat	O
"	O
to	O
its	O
mission	O
of	O
accessing	O
users	O
 '	O
private	O
data	O
,	O
and	O
when	O
used	O
in	O
conjunction	O
with	O
other	O
privacy	O
tools	O
such	O
as	O
Cspace	O
,	O
Tor	B-Application
,	O
Tails	B-Application
,	O
and	O
TrueCrypt	B-Application
was	O
ranked	O
as	O
"	O
catastrophic	O
"	O
and	O
led	O
to	O
a	O
"	O
near-total	O
loss/lack	O
of	O
insight	O
to	O
target	O
communications	O
 [ and ] 	O
presence	O
"	O
.	O
</s>
<s>
On	O
27	O
September	O
2019	O
,	O
Natalie	O
Silvanovich	O
,	O
a	O
security	O
engineer	O
working	O
in	O
Google	B-Application
's	I-Application
vulnerability	O
research	O
team	O
at	O
Project	B-Application
Zero	I-Application
,	O
disclosed	O
how	O
a	O
bug	O
in	O
the	O
Android	B-Application
Signal	O
client	O
could	O
let	O
an	O
attacker	O
spy	O
on	O
a	O
user	O
without	O
their	O
knowledge	O
.	O
</s>
<s>
The	O
bug	O
was	O
fixed	O
the	O
same	O
day	O
that	O
it	O
was	O
reported	O
and	O
patched	O
in	O
release	O
4.47.7	O
of	O
the	O
app	B-Application
for	O
Android	B-Application
.	O
</s>
<s>
In	O
July	O
2020	O
,	O
Signal	O
became	O
the	O
most	O
downloaded	O
app	B-Application
in	O
Hong	O
Kong	O
on	O
both	O
the	O
Apple	O
App	B-Application
Store	O
and	O
the	B-Application
Google	I-Application
Play	O
Store	O
after	O
the	O
passage	O
of	O
the	O
Hong	O
Kong	O
national	O
security	O
law	O
.	O
</s>
<s>
Candiru	O
claims	O
the	O
ability	O
to	O
capture	O
data	O
from	O
Signal	B-Application
Private	I-Application
Messenger	I-Application
with	O
their	O
spyware	O
,	O
at	O
a	O
fee	O
of	O
€500.000	O
.	O
</s>
<s>
In	O
April	O
2021	O
,	O
Signal	O
announced	O
the	O
addition	O
of	O
a	O
cryptocurrency	B-Protocol
wallet	I-Protocol
feature	O
that	O
would	O
allow	O
users	O
to	O
send	O
and	O
receive	O
payments	O
in	O
MobileCoin	B-Protocol
.	O
</s>
<s>
Signal	O
's	O
domain	O
fronting	O
feature	O
relies	O
on	O
the	B-Application
Google	I-Application
App	B-Application
Engine	I-Application
(	O
GAE	O
)	O
service	O
.	O
</s>
<s>
This	O
does	O
not	O
work	O
in	O
Iran	O
because	O
Google	B-Application
has	O
blocked	O
Iranian	O
access	O
to	O
GAE	O
in	O
order	O
to	O
comply	O
with	O
U.S.	O
sanctions	O
.	O
</s>
<s>
In	O
early	O
2018	O
,	O
Google	B-Application
App	I-Application
Engine	I-Application
made	O
an	O
internal	O
change	O
to	O
stop	O
domain	O
fronting	O
for	O
all	O
countries	O
.	O
</s>
<s>
However	O
,	O
AWS	B-Application
also	O
announced	O
that	O
they	O
would	O
be	O
making	O
changes	O
to	O
their	O
service	O
to	O
prevent	O
domain	O
fronting	O
.	O
</s>
<s>
Signal	O
switched	O
from	O
AWS	B-Application
back	O
to	O
Google	B-Application
in	O
April	O
2019	O
.	O
</s>
<s>
In	O
January	O
2021	O
,	O
Iran	O
removed	O
the	O
app	B-Application
from	O
app	B-Application
stores	O
,	O
and	O
blocked	O
Signal	O
.	O
</s>
<s>
In	O
March	O
2021	O
,	O
the	O
United	O
Nations	O
recommended	O
Myanmar	O
residents	O
use	O
Signal	O
and	O
ProtonMail	B-Application
to	O
pass	O
and	O
preserve	O
evidence	O
of	O
human	O
rights	O
violations	O
committed	O
during	O
the	O
2021	O
coup	O
.	O
</s>
<s>
In	O
2016	O
,	O
authorities	O
in	O
India	O
arrested	O
members	O
of	O
a	O
suspected	O
ISIS-affiliated	O
terrorist	O
cell	O
that	O
communicated	O
via	O
Signal	O
.	O
</s>
