<s>
TextSecure	B-Protocol
was	O
an	O
encrypted	O
messaging	O
application	O
for	O
Android	B-Application
that	O
was	O
developed	O
from	O
2010	O
to	O
2015	O
.	O
</s>
<s>
It	O
was	O
a	O
predecessor	O
to	O
Signal	B-Application
and	O
the	O
first	O
application	O
to	O
use	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
,	O
which	O
has	O
since	O
been	O
implemented	O
into	O
WhatsApp	B-Application
and	O
other	O
applications	O
.	O
</s>
<s>
TextSecure	B-Protocol
used	O
end-to-end	O
encryption	O
to	O
secure	O
the	O
transmission	O
of	O
text	O
messages	O
,	O
group	O
messages	O
,	O
attachments	O
and	O
media	O
messages	O
to	O
other	O
TextSecure	B-Protocol
users	O
.	O
</s>
<s>
TextSecure	B-Protocol
was	O
first	O
developed	O
by	O
Whisper	B-Application
Systems	I-Application
,	O
who	O
were	O
later	O
acqui-hired	O
by	O
Twitter	B-Application
.	O
</s>
<s>
The	O
application	O
's	O
source	O
code	O
was	O
then	O
released	O
under	O
a	O
free	B-License
and	I-License
open-source	I-License
software	I-License
license	O
.	O
</s>
<s>
In	O
2013	O
,	O
TextSecure	B-Protocol
's	O
development	O
was	O
picked	O
up	O
by	O
an	O
independent	O
group	O
called	O
Open	O
Whisper	B-Application
Systems	I-Application
,	O
who	O
merged	O
it	O
with	O
an	O
encrypted	O
voice	O
calling	O
application	O
called	O
RedPhone	O
and	O
renamed	O
the	O
product	O
as	O
Signal	B-Application
.	O
</s>
<s>
TextSecure	B-Protocol
started	O
as	O
an	O
application	O
for	O
sending	O
and	O
receiving	O
encrypted	O
SMS	O
messages	O
.	O
</s>
<s>
Its	O
beta	O
version	O
was	O
first	O
launched	O
on	O
May	O
25	O
,	O
2010	O
,	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>
In	O
addition	O
to	O
launching	O
TextSecure	B-Protocol
,	O
Whisper	B-Application
Systems	I-Application
produced	O
a	O
firewall	O
,	O
tools	O
for	O
encrypting	O
other	O
forms	O
of	O
data	O
,	O
and	O
RedPhone	O
,	O
an	O
application	O
that	O
provided	O
encrypted	O
voice	O
calls	O
.	O
</s>
<s>
All	O
of	O
these	O
were	O
proprietary	B-Application
enterprise	O
mobile	O
security	O
software	O
.	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	O
in	O
December	O
2011	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	O
Source	O
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>
Open	O
Whisper	B-Application
Systems	I-Application
started	O
working	O
to	O
bring	O
TextSecure	B-Protocol
to	O
iOS	B-Application
in	O
March	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
group	O
chat	O
and	O
push	O
messaging	O
capabilities	O
to	O
TextSecure	B-Protocol
.	O
</s>
<s>
Toward	O
the	O
end	O
of	O
July	O
2014	O
,	O
Open	O
Whisper	B-Application
Systems	I-Application
announced	O
plans	O
to	O
unify	O
its	O
RedPhone	O
and	O
TextSecure	B-Protocol
applications	O
as	O
Signal	B-Application
.	O
</s>
<s>
This	O
announcement	O
coincided	O
with	O
the	O
initial	O
release	O
of	O
Signal	B-Application
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	O
client	O
.	O
</s>
<s>
Signal	B-Application
was	O
the	O
first	O
iOS	B-Application
app	O
to	O
enable	O
easy	O
,	O
strongly	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>
Later	O
that	O
month	O
,	O
Open	O
Whisper	B-Application
Systems	I-Application
ended	O
support	O
for	O
sending	O
and	O
receiving	O
encrypted	O
SMS/MMS	O
messages	O
on	O
Android	B-Application
.	O
</s>
<s>
From	O
version	O
2.7.0	O
onward	O
,	O
TextSecure	B-Protocol
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>
Compatibility	O
issues	O
with	O
iOS	B-Application
:	O
Not	O
possible	O
to	O
send	O
or	O
receive	O
encrypted	O
SMS/MMS	O
messages	O
on	O
iOS	B-Application
due	O
to	O
the	O
lack	O
of	O
APIs	B-Application
.	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
encryption	O
of	O
SMS	O
and	O
MMS	O
messages	O
.	O
</s>
<s>
In	O
November	O
2015	O
,	O
the	O
RedPhone	O
application	O
was	O
merged	O
into	O
TextSecure	B-Protocol
and	O
it	O
was	O
renamed	O
as	O
Signal	B-Application
for	O
Android	B-Application
.	O
</s>
<s>
TextSecure	B-Protocol
allowed	O
users	O
to	O
send	O
encrypted	O
text	O
messages	O
,	O
audio	O
messages	O
,	O
photos	O
,	O
videos	O
,	O
contact	O
information	O
,	O
and	O
a	O
wide	O
selection	O
of	O
emoticons	B-Protocol
over	O
a	O
data	O
connection	O
(	O
e.g.	O
</s>
<s>
Wi-Fi	O
,	O
3G	B-Architecture
or	O
4G	B-Architecture
)	O
to	O
other	O
TextSecure	B-Protocol
users	O
with	O
smartphones	B-Application
running	O
Android	B-Application
.	O
</s>
<s>
TextSecure	B-Protocol
also	O
allowed	O
users	O
to	O
exchange	O
unencrypted	O
SMS	O
and	O
MMS	O
messages	O
with	O
people	O
who	O
did	O
not	O
have	O
TextSecure	B-Protocol
.	O
</s>
<s>
Messages	O
sent	O
with	O
TextSecure	B-Protocol
to	O
other	O
TextSecure	B-Protocol
users	O
were	O
automatically	O
end-to-end	O
encrypted	O
,	O
which	O
meant	O
that	O
they	O
could	O
only	O
be	O
read	O
by	O
the	O
intended	O
recipients	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>
TextSecure	B-Protocol
had	O
a	O
built-in	O
function	O
for	O
verifying	O
that	O
the	O
user	O
was	O
communicating	O
with	O
the	O
right	O
person	O
and	O
that	O
no	O
man-in-the-middle	O
attack	O
had	O
occurred	O
.	O
</s>
<s>
TextSecure	B-Protocol
allowed	O
users	O
to	O
chat	O
with	O
more	O
than	O
one	O
person	O
at	O
a	O
time	O
.	O
</s>
<s>
Group	O
chats	O
were	O
automatically	O
end-to-end	O
encrypted	O
and	O
held	O
over	O
an	O
available	O
data	O
connection	O
if	O
all	O
participants	O
were	O
registered	O
TextSecure	B-Protocol
users	O
.	O
</s>
<s>
Users	O
could	O
create	O
groups	O
with	O
a	O
title	O
and	O
avatar	O
icon	O
,	O
add	O
their	O
friends	O
,	O
join	O
or	O
leave	O
groups	O
,	O
and	O
exchange	O
messages/media	O
,	O
all	O
with	O
the	O
same	O
encryption	O
properties	O
pairwise	O
TextSecure	B-Protocol
chats	O
provided	O
.	O
</s>
<s>
The	O
application	O
could	O
also	O
function	O
as	O
a	O
drop-in	O
replacement	O
for	O
Android	B-Application
's	O
native	O
messaging	O
application	O
as	O
it	O
could	O
fall	O
back	O
to	O
sending	O
unencrypted	O
SMS	O
and	O
MMS	O
messages	O
.	O
</s>
<s>
TextSecure	B-Protocol
required	O
that	O
the	O
user	O
had	O
a	O
phone	O
number	O
for	O
verification	O
.	O
</s>
<s>
The	O
official	O
TextSecure	B-Protocol
client	O
required	O
Google	B-Application
Play	I-Application
Services	I-Application
because	O
the	O
app	O
was	O
dependent	O
on	O
Google	O
's	O
GCM	O
push	O
messaging	O
framework	O
.	O
</s>
<s>
From	O
February	O
2014	O
to	O
March	O
2015	O
,	O
TextSecure	B-Protocol
used	O
GCM	O
as	O
the	O
transport	O
for	O
message	O
delivery	O
over	O
the	O
data	O
channel	O
.	O
</s>
<s>
From	O
March	O
2015	O
forward	O
,	O
TextSecure	B-Protocol
's	O
message	O
delivery	O
was	O
done	O
by	O
Open	O
Whisper	B-Application
Systems	I-Application
themselves	O
and	O
the	O
client	O
relied	O
on	O
GCM	O
only	O
for	O
a	O
wakeup	O
event	O
.	O
</s>
<s>
TextSecure	B-Protocol
was	O
the	O
first	O
application	O
to	O
use	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
(	O
then	O
called	O
the	O
TextSecure	B-Protocol
Protocol	I-Protocol
)	O
,	O
which	O
has	O
since	O
been	O
implemented	O
into	O
WhatsApp	B-Application
,	O
Facebook	B-Protocol
Messenger	I-Protocol
,	O
and	O
Google	B-Protocol
Allo	I-Protocol
,	O
encrypting	O
the	O
conversations	O
of	O
"	O
more	O
than	O
a	O
billion	O
people	O
worldwide	O
"	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
a	O
3-DH	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
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>
All	O
client-server	O
communications	O
were	O
protected	O
by	O
TLS	B-Protocol
.	O
</s>
<s>
Open	O
Whisper	B-Application
Systems	I-Application
asserted	O
that	O
their	O
servers	O
did	O
not	O
keep	O
this	O
metadata	O
.	O
</s>
<s>
In	O
order	O
to	O
determine	O
which	O
contacts	O
were	O
also	O
TextSecure	B-Protocol
users	O
,	O
cryptographic	B-Algorithm
hashes	I-Algorithm
of	O
the	O
user	O
's	O
contact	O
numbers	O
were	O
periodically	O
transmitted	O
to	O
the	O
server	O
.	O
</s>
<s>
The	O
server	O
then	O
checked	O
to	O
see	O
if	O
those	O
matched	O
any	O
of	O
the	O
SHA256	O
hashes	B-Algorithm
of	O
registered	O
users	O
and	O
told	O
the	O
client	O
if	O
any	O
matches	O
were	O
found	O
.	O
</s>
<s>
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
"	O
practical	O
privacy	O
preserving	O
contact	O
discovery	O
remains	O
an	O
unsolved	O
problem	O
"	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
that	O
was	O
used	O
by	O
TextSecure	B-Protocol
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>
From	O
CyanogenMod	B-Application
11.0	O
onward	O
,	O
the	O
client	O
logic	O
was	O
contained	O
in	O
a	O
system	O
app	O
called	O
WhisperPush	O
.	O
</s>
<s>
According	O
to	O
Open	O
Whisper	B-Application
Systems	I-Application
,	O
the	O
Cyanogen	O
team	O
ran	O
their	O
own	O
TextSecure	B-Protocol
server	O
for	O
WhisperPush	O
clients	O
,	O
which	O
federated	O
with	O
Open	O
Whisper	B-Application
Systems	I-Application
 '	O
TextSecure	B-Protocol
server	O
,	O
so	O
that	O
both	O
clients	O
could	O
exchange	O
messages	O
with	O
each-other	O
seamlessly	O
.	O
</s>
<s>
The	O
CyanogenMod	B-Application
team	O
discontinued	O
WhisperPush	O
in	O
February	O
2016	O
,	O
and	O
recommended	O
that	O
its	O
users	O
switch	O
to	O
Signal	B-Application
.	O
</s>
<s>
The	O
complete	O
source	O
code	O
of	O
TextSecure	B-Protocol
was	O
available	O
on	O
GitHub	B-Application
under	O
a	O
free	B-License
software	I-License
license	I-License
.	O
</s>
<s>
The	O
software	O
that	O
handled	O
message	O
routing	O
for	O
the	O
TextSecure	B-Protocol
data	O
channel	O
was	O
also	O
open	O
source	O
.	O
</s>
<s>
TextSecure	B-Protocol
was	O
officially	O
distributed	O
only	O
through	O
Google	B-Application
Play	I-Application
.	O
</s>
<s>
In	O
October	O
2015	O
,	O
TextSecure	B-Protocol
had	O
been	O
installed	O
over	O
1000000	O
times	O
through	O
Google	B-Application
Play	I-Application
.	O
</s>
<s>
TextSecure	B-Protocol
was	O
briefly	O
included	O
in	O
the	O
F-Droid	B-Application
software	O
repository	O
in	O
2012	O
,	O
but	O
was	O
removed	O
at	O
the	O
developer	O
's	O
request	O
because	O
it	O
was	O
an	O
unverified	O
build	O
and	O
exceptionally	O
out	O
of	O
date	O
.	O
</s>
<s>
Open	O
Whisper	B-Application
Systems	I-Application
have	O
subsequently	O
said	O
that	O
they	O
will	O
not	O
support	O
their	O
applications	O
being	O
distributed	O
through	O
F-Droid	B-Application
because	O
it	O
does	O
not	O
provide	O
timely	O
software	O
updates	O
,	O
relies	O
on	O
a	O
centralized	O
trust	O
model	O
and	O
necessitates	O
allowing	O
the	O
installation	O
of	O
apps	O
from	O
unknown	O
sources	O
which	O
harms	O
Android	B-Application
's	O
security	O
for	O
average	O
users	O
.	O
</s>
<s>
In	O
October	O
2013	O
,	O
iSEC	O
Partners	O
published	O
a	O
blog	O
post	O
in	O
which	O
they	O
said	O
that	O
they	O
had	O
audited	O
several	O
of	O
the	O
projects	O
supported	O
by	O
the	O
Open	O
Technology	O
Fund	O
over	O
the	O
past	O
year	O
,	O
including	O
TextSecure	B-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
TextSecure	B-Protocol
encryption	O
protocol	O
.	O
</s>
<s>
Former	O
NSA	O
contractor	O
Edward	O
Snowden	O
endorsed	O
TextSecure	B-Protocol
on	O
multiple	O
occasions	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
TextSecure	B-Protocol
for	O
its	O
ease-of-use	O
.	O
</s>
<s>
During	O
an	O
interview	O
with	O
The	O
New	O
Yorker	O
in	O
October	O
2014	O
,	O
he	O
recommended	O
using	O
"	O
anything	O
from	O
Moxie	O
Marlinspike	O
and	O
Open	O
Whisper	B-Application
Systems	I-Application
"	O
.	O
</s>
<s>
Asked	O
about	O
encrypted	O
messaging	B-Application
apps	I-Application
during	O
a	O
Reddit	B-Application
AMA	O
in	O
May	O
2015	O
,	O
he	O
recommended	O
TextSecure	B-Protocol
.	O
</s>
<s>
In	O
October	O
2014	O
,	O
the	O
Electronic	O
Frontier	O
Foundation	O
(	O
EFF	O
)	O
included	O
TextSecure	B-Protocol
in	O
their	O
updated	O
Surveillance	O
Self-Defense	O
guide	O
.	O
</s>
<s>
In	O
November	O
2014	O
,	O
TextSecure	B-Protocol
received	O
a	O
perfect	O
score	O
on	O
the	O
EFF	O
's	O
Secure	O
Messaging	O
Scorecard	O
.	O
</s>
<s>
TextSecure	B-Protocol
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
providers	O
do	O
n't	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
correspondent	O
's	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
their	O
code	O
open	O
to	O
independent	O
review	O
(	O
open-source	B-Application
)	O
,	O
having	O
their	O
security	O
designs	O
well-documented	O
,	O
and	O
having	O
recent	O
independent	O
security	O
audits	O
.	O
</s>
<s>
At	O
the	O
time	O
,	O
"	O
ChatSecure	B-Protocol
+	O
Orbot	B-Language
"	O
,	O
Cryptocat	B-Protocol
,	O
"	O
Signal	B-Application
/	O
RedPhone	O
"	O
,	O
Pidgin	B-Protocol
(	O
with	O
OTR	B-Protocol
)	O
,	O
Silent	O
Phone	O
,	O
Silent	O
Text	O
,	O
and	O
Telegram	B-Application
's	O
optional	O
secret	O
chats	O
also	O
received	O
seven	O
out	O
of	O
seven	O
points	O
on	O
the	O
scorecard	O
.	O
</s>
<s>
TextSecure	B-Protocol
was	O
developed	O
by	O
a	O
nonprofit	O
software	O
group	O
called	O
Open	O
Whisper	B-Application
Systems	I-Application
.	O
</s>
<s>
The	O
group	O
is	O
funded	O
by	O
a	O
combination	O
of	O
donations	O
and	O
grants	O
,	O
and	O
all	O
of	O
its	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>
Open	O
Whisper	B-Application
Systems	I-Application
has	O
received	O
grants	O
from	O
the	O
Knight	O
Foundation	O
,	O
the	O
Shuttleworth	O
Foundation	O
,	O
and	O
the	O
Open	O
Technology	O
Fund	O
,	O
a	O
U.S.	O
government	O
funded	O
program	O
that	O
has	O
also	O
supported	O
other	O
privacy	O
projects	O
like	O
the	O
anonymity	O
software	O
Tor	B-Application
and	O
the	O
encrypted	O
instant	B-Application
messaging	I-Application
app	O
Cryptocat	B-Protocol
.	O
</s>
