<s>
The	O
Signal	B-Protocol
Protocol	I-Protocol
(	O
formerly	O
known	O
as	O
the	O
TextSecure	B-Protocol
Protocol	I-Protocol
)	O
is	O
a	O
non-federated	O
cryptographic	B-Protocol
protocol	I-Protocol
that	O
can	O
be	O
used	O
to	O
provide	O
end-to-end	O
encryption	O
for	O
voice	O
calls	O
and	O
instant	B-Application
messaging	I-Application
conversations	B-Protocol
.	O
</s>
<s>
The	O
protocol	O
was	O
developed	O
by	O
Open	O
Whisper	O
Systems	O
in	O
2013	O
and	O
was	O
first	O
introduced	O
in	O
the	O
open-source	B-Application
TextSecure	B-Protocol
app	O
,	O
which	O
later	O
became	O
Signal	B-Application
.	O
</s>
<s>
Several	O
closed-source	B-Application
applications	O
have	O
implemented	O
the	O
protocol	O
,	O
such	O
as	O
WhatsApp	B-Application
,	O
which	O
is	O
said	O
to	O
encrypt	O
the	O
conversations	B-Protocol
of	O
"	O
more	O
than	O
a	O
billion	O
people	O
worldwide	O
"	O
or	O
Google	B-Application
who	O
provides	O
end-to-end	O
encryption	O
by	O
default	O
to	O
all	O
RCS-based	O
conversations	B-Protocol
between	O
users	O
of	O
their	O
Messages	B-Protocol
app	O
for	O
one-to-one	O
conversations	B-Protocol
.	O
</s>
<s>
Facebook	B-Protocol
Messenger	I-Protocol
also	O
say	O
they	O
offer	O
the	O
protocol	O
for	O
optional	O
Secret	O
Conversations	B-Protocol
,	O
as	O
does	O
Skype	B-Application
for	O
its	O
Private	O
Conversations	B-Protocol
.	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
a	O
triple	O
Elliptic-curve	B-Protocol
Diffie	I-Protocol
–	I-Protocol
Hellman	I-Protocol
(	O
3-DH	O
)	O
handshake	O
,	O
and	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
Signal	B-Protocol
Protocol	I-Protocol
's	O
development	O
was	O
started	O
by	O
Trevor	O
Perrin	O
and	O
Moxie	O
Marlinspike	O
(	O
Open	O
Whisper	O
Systems	O
)	O
in	O
2013	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
the	O
protocol	O
,	O
TextSecure	B-Protocol
v1	O
,	O
was	O
based	O
on	O
Off-the-Record	B-Protocol
Messaging	I-Protocol
(	O
OTR	B-Protocol
)	O
.	O
</s>
<s>
On	O
24	O
February	O
2014	O
,	O
Open	O
Whisper	O
Systems	O
introduced	O
TextSecure	B-Protocol
v2	O
,	O
which	O
migrated	O
to	O
the	O
Axolotl	B-Algorithm
Ratchet	I-Algorithm
.	O
</s>
<s>
The	O
design	O
of	O
the	O
Axolotl	B-Algorithm
Ratchet	I-Algorithm
is	O
based	O
on	O
the	O
ephemeral	O
key	O
exchange	O
that	O
was	O
introduced	O
by	O
OTR	B-Protocol
and	O
combines	O
it	O
with	O
a	O
symmetric-key	O
ratchet	O
modeled	O
after	O
the	O
Silent	B-Protocol
Circle	I-Protocol
Instant	I-Protocol
Messaging	I-Protocol
Protocol	I-Protocol
(	O
SCIMP	B-Protocol
)	O
.	O
</s>
<s>
It	O
brought	O
about	O
support	O
for	O
asynchronous	O
communication	O
(	O
"	O
offline	O
messages	B-Protocol
"	O
)	O
as	O
its	O
major	O
new	O
feature	O
,	O
as	O
well	O
as	O
better	O
resilience	O
with	O
distorted	O
order	O
of	O
messages	B-Protocol
and	O
simpler	O
support	O
for	O
conversations	B-Protocol
with	O
multiple	O
participants	O
.	O
</s>
<s>
The	O
Axolotl	B-Algorithm
Ratchet	I-Algorithm
was	O
named	O
after	O
the	O
critically	O
endangered	O
aquatic	O
salamander	O
Axolotl	B-Protocol
,	O
which	O
has	O
extraordinary	O
self-healing	O
capabilities	O
.	O
</s>
<s>
The	O
developers	O
refer	O
to	O
the	O
algorithm	O
as	O
self-healing	O
because	O
it	O
automatically	O
disables	O
an	O
attacker	O
from	O
accessing	O
the	O
cleartext	O
of	O
later	O
messages	B-Protocol
after	O
having	O
compromised	O
a	O
session	O
key	O
.	O
</s>
<s>
The	O
third	O
version	O
of	O
the	O
protocol	O
,	O
TextSecure	B-Protocol
v3	O
,	O
made	O
some	O
changes	O
to	O
the	O
cryptographic	O
primitives	O
and	O
the	O
wire	B-Application
protocol	O
.	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
TextSecure	B-Protocol
v3	O
.	O
</s>
<s>
In	O
March	O
2016	O
,	O
the	O
developers	O
renamed	O
the	O
protocol	O
as	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
They	O
also	O
renamed	O
the	O
Axolotl	B-Algorithm
Ratchet	I-Algorithm
as	O
the	O
Double	B-Algorithm
Ratchet	I-Algorithm
algorithm	I-Algorithm
to	O
better	O
differentiate	O
between	O
the	O
ratchet	O
and	O
the	O
full	O
protocol	O
because	O
some	O
had	O
used	O
the	O
name	O
Axolotl	B-Protocol
when	O
referring	O
to	O
the	O
full	O
protocol	O
.	O
</s>
<s>
,	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
is	O
based	O
on	O
TextSecure	B-Protocol
v3	O
,	O
but	O
with	O
additional	O
cryptographic	O
changes	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
post-compromise	O
security	O
(	O
aka	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
and	O
requires	O
servers	O
for	O
the	O
relaying	O
of	O
messages	B-Protocol
and	O
storing	O
of	O
public	O
key	O
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>
For	O
authentication	B-Algorithm
,	O
users	O
can	O
manually	O
compare	O
public	B-Algorithm
key	I-Algorithm
fingerprints	I-Algorithm
through	O
an	O
outside	O
channel	O
.	O
</s>
<s>
The	O
Signal	B-Protocol
Protocol	I-Protocol
does	O
not	O
prevent	O
a	O
company	O
from	O
retaining	O
information	O
about	O
when	O
and	O
with	O
whom	O
users	O
communicate	O
.	O
</s>
<s>
Signal	B-Application
's	O
privacy	O
policy	O
states	O
that	O
recipients	O
 '	O
identifiers	O
are	O
only	O
kept	O
on	O
the	O
Signal	B-Application
servers	O
as	O
long	O
as	O
necessary	O
in	O
order	O
to	O
transmit	O
each	O
message	O
.	O
</s>
<s>
In	O
June	O
2016	O
,	O
Moxie	O
Marlinspike	O
told	O
The	O
Intercept	O
:	O
"	O
the	O
closest	O
piece	O
of	O
information	O
to	O
metadata	O
that	O
the	O
Signal	B-Application
server	O
stores	O
is	O
the	O
last	O
time	O
each	O
user	O
connected	O
to	O
the	O
server	O
,	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
</s>
<s>
In	O
October	O
2018	O
,	O
Signal	B-Application
Messenger	I-Application
announced	O
that	O
they	O
had	O
implemented	O
a	O
"	O
sealed	O
sender	O
"	O
feature	O
into	O
Signal	B-Application
,	O
which	O
reduces	O
the	O
amount	O
of	O
metadata	O
that	O
the	O
Signal	B-Application
servers	O
have	O
access	O
to	O
by	O
concealing	O
the	O
sender	O
's	O
identifier	O
.	O
</s>
<s>
This	O
is	O
done	O
automatically	O
if	O
the	O
sender	O
is	O
in	O
the	O
recipient	O
's	O
contacts	O
or	O
has	O
access	O
to	O
their	O
Signal	B-Application
Profile	O
.	O
</s>
<s>
Users	O
can	O
also	O
enable	O
an	O
option	O
to	O
receive	O
"	O
sealed	O
sender	O
"	O
messages	B-Protocol
from	O
non-contacts	O
and	O
people	O
who	O
do	O
not	O
have	O
access	O
to	O
their	O
Signal	B-Application
Profile	O
.	O
</s>
<s>
A	O
contemporaneous	O
wiretap	O
of	O
the	O
user	O
's	O
device	O
and/or	O
the	O
Signal	B-Application
servers	O
may	O
still	O
reveal	O
that	O
the	O
device	O
's	O
IP	O
address	O
accessed	O
a	O
Signal	B-Application
server	O
to	O
send	O
or	O
receive	O
messages	B-Protocol
at	O
certain	O
times	O
.	O
</s>
<s>
Open	O
Whisper	O
Systems	O
first	O
introduced	O
the	O
protocol	O
in	O
their	O
TextSecure	B-Protocol
app	O
.	O
</s>
<s>
They	O
later	O
merged	O
an	O
encrypted	O
voice	O
calling	O
application	O
called	O
RedPhone	O
into	O
the	O
TextSecure	B-Protocol
app	O
and	O
renamed	O
it	O
as	O
Signal	B-Application
.	O
</s>
<s>
In	O
November	O
2014	O
,	O
Open	O
Whisper	O
Systems	O
announced	O
a	O
partnership	O
with	O
WhatsApp	B-Application
to	O
provide	O
end-to-end	O
encryption	O
by	O
incorporating	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
into	O
each	O
WhatsApp	B-Application
client	O
platform	O
.	O
</s>
<s>
Open	O
Whisper	O
Systems	O
said	O
that	O
they	O
had	O
already	O
incorporated	O
the	O
protocol	O
into	O
the	O
latest	O
WhatsApp	B-Application
client	O
for	O
Android	B-Application
and	O
that	O
support	O
for	O
other	O
clients	O
,	O
group/media	O
messages	B-Protocol
,	O
and	O
key	O
verification	O
would	O
be	O
coming	O
soon	O
after	O
.	O
</s>
<s>
On	O
April	O
5	O
,	O
2016	O
,	O
WhatsApp	B-Application
and	O
Open	O
Whisper	O
Systems	O
announced	O
that	O
they	O
had	O
finished	O
adding	O
end-to-end	O
encryption	O
to	O
"	O
every	O
form	O
of	O
communication	O
"	O
on	O
WhatsApp	B-Application
,	O
and	O
that	O
users	O
could	O
now	O
verify	O
each	O
other	O
's	O
keys	O
.	O
</s>
<s>
In	O
February	O
2017	O
,	O
WhatsApp	B-Application
announced	O
a	O
new	O
feature	O
,	O
WhatsApp	B-Application
Status	O
,	O
which	O
uses	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
to	O
secure	O
its	O
contents	O
.	O
</s>
<s>
In	O
October	O
2016	O
,	O
WhatsApp	B-Application
's	O
parent	O
company	O
Facebook	B-Application
also	O
deployed	O
an	O
optional	O
mode	O
called	O
Secret	O
Conversations	B-Protocol
in	O
Facebook	B-Protocol
Messenger	I-Protocol
which	O
provides	O
end-to-end	O
encryption	O
using	O
an	O
implementation	O
of	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
In	O
September	O
2015	O
,	O
G	B-Application
Data	I-Application
Software	I-Application
launched	O
a	O
new	O
messaging	B-Application
app	I-Application
called	O
Secure	O
Chat	O
which	O
used	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
G	B-Application
Data	I-Application
discontinued	O
the	O
service	O
in	O
May	O
2018	O
.	O
</s>
<s>
In	O
September	O
2016	O
,	O
Google	B-Application
launched	O
a	O
new	O
messaging	B-Application
app	I-Application
called	O
Allo	B-Protocol
,	O
which	O
featured	O
an	O
optional	O
Incognito	O
Mode	O
that	O
used	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
for	O
end-to-end	O
encryption	O
.	O
</s>
<s>
In	O
March	O
2019	O
,	O
Google	B-Application
discontinued	O
Allo	B-Protocol
in	O
favor	O
of	O
their	O
Messages	B-Protocol
app	O
on	O
Android	B-Application
.	O
</s>
<s>
In	O
November	O
2020	O
,	O
Google	B-Application
announced	O
that	O
they	O
would	O
be	O
using	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
to	O
provide	O
end-to-end	O
encryption	O
by	O
default	O
to	O
all	O
RCS-based	O
conversations	B-Protocol
between	O
users	O
of	O
their	O
Messages	B-Protocol
app	O
,	O
starting	O
with	O
one-to-one	O
conversations	B-Protocol
.	O
</s>
<s>
In	O
January	O
2018	O
,	O
Open	O
Whisper	O
Systems	O
and	O
Microsoft	O
announced	O
the	O
addition	O
of	O
Signal	B-Protocol
Protocol	I-Protocol
support	O
to	O
an	O
optional	O
Skype	B-Application
mode	O
called	O
Private	O
Conversations	B-Protocol
.	O
</s>
<s>
The	O
Signal	B-Protocol
Protocol	I-Protocol
has	O
had	O
an	O
influence	O
on	O
other	O
cryptographic	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
In	O
May	O
2016	O
,	O
Viber	B-Application
said	O
that	O
their	O
encryption	B-Protocol
protocol	I-Protocol
is	O
a	O
custom	O
implementation	O
that	O
"	O
uses	O
the	O
same	O
concepts	O
"	O
as	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
Forsta	O
's	O
developers	O
have	O
said	O
that	O
their	O
app	O
uses	O
a	O
custom	O
implementation	O
of	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
The	O
Double	B-Algorithm
Ratchet	I-Algorithm
algorithm	I-Algorithm
that	O
was	O
introduced	O
as	O
part	O
of	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
has	O
also	O
been	O
adopted	O
by	O
other	O
protocols	O
.	O
</s>
<s>
OMEMO	B-Application
is	O
an	O
XMPP	O
Extension	O
Protocol	O
(	O
XEP	O
)	O
that	O
was	O
introduced	O
in	O
the	O
Conversations	B-Protocol
messaging	B-Application
app	I-Application
and	O
approved	O
by	O
the	O
XMPP	B-Protocol
Standards	I-Protocol
Foundation	I-Protocol
(	O
XSF	O
)	O
in	O
December	O
2016	O
as	O
XEP-0384	O
.	O
</s>
<s>
Matrix	B-Protocol
is	O
an	O
open	O
communications	O
protocol	O
that	O
includes	O
Olm	O
,	O
a	O
library	O
that	O
provides	O
optional	O
end-to-end	O
encryption	O
on	O
a	O
room-by-room	O
basis	O
via	O
a	O
Double	B-Algorithm
Ratchet	I-Algorithm
algorithm	I-Algorithm
implementation	O
.	O
</s>
<s>
The	O
developers	O
of	O
Wire	B-Application
have	O
said	O
that	O
their	O
app	O
uses	O
a	O
custom	O
implementation	O
of	O
the	O
Double	B-Algorithm
Ratchet	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
Messaging	B-Application
Layer	I-Application
Security	I-Application
,	O
an	O
IETF	O
proposal	O
,	O
uses	O
Asynchronous	O
ratcheting	O
trees	O
to	O
efficiently	O
improve	O
upon	O
security	O
guarantees	O
over	O
Signal	B-Application
's	O
Double	B-Algorithm
Ratchet	I-Algorithm
.	O
</s>
<s>
Signal	B-Application
Messenger	I-Application
maintains	O
the	O
following	O
Signal	B-Protocol
Protocol	I-Protocol
libraries	B-Library
under	O
the	O
GPLv3	O
license	O
on	O
GitHub	B-Application
:	O
</s>
<s>
:	O
A	O
library	O
written	O
in	O
C	B-Language
with	O
additional	O
licensing	O
permissions	O
for	O
Apple	B-Application
's	I-Application
App	I-Application
Store	I-Application
.	O
</s>
<s>
:	O
A	O
library	O
written	O
in	O
Java	B-Language
.	O
</s>
<s>
There	O
also	O
exist	O
alternative	O
libraries	B-Library
written	O
by	O
third-parties	O
in	O
other	O
languages	O
,	O
such	O
as	O
TypeScript	B-Language
.	O
</s>
