<s>
Off-the-Record	B-Protocol
Messaging	I-Protocol
(	O
OTR	O
)	O
is	O
a	O
cryptographic	B-Protocol
protocol	I-Protocol
that	O
provides	O
encryption	O
for	O
instant	B-Application
messaging	I-Application
conversations	B-Protocol
.	O
</s>
<s>
OTR	O
uses	O
a	O
combination	O
of	O
AES	B-Algorithm
symmetric-key	B-Algorithm
algorithm	I-Algorithm
with	O
128	O
bits	O
key	O
length	O
,	O
the	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
with	O
1536	O
bits	O
group	O
size	O
,	O
and	O
the	O
SHA-1	B-Algorithm
hash	I-Algorithm
function	O
.	O
</s>
<s>
The	O
primary	O
motivation	O
behind	O
the	O
protocol	O
was	O
providing	O
deniable	B-Protocol
authentication	I-Protocol
for	O
the	O
conversation	O
participants	O
while	O
keeping	O
conversations	B-Protocol
confidential	O
,	O
like	O
a	O
private	O
conversation	O
in	O
real	O
life	O
,	O
or	O
off	O
the	O
record	O
in	O
journalism	O
sourcing	O
.	O
</s>
<s>
The	O
initial	O
introductory	O
paper	O
was	O
named	O
"	O
Off-the-Record	O
Communication	O
,	O
or	O
,	O
Why	O
Not	O
To	O
Use	O
PGP	B-Application
"	O
.	O
</s>
<s>
They	O
provide	O
a	O
client	O
library	B-Library
to	O
facilitate	O
support	O
for	O
instant	B-Application
messaging	I-Application
client	O
developers	O
who	O
want	O
to	O
implement	O
the	O
protocol	O
.	O
</s>
<s>
A	O
Pidgin	B-Protocol
and	O
Kopete	B-Language
plugin	O
exists	O
that	O
allows	O
OTR	O
to	O
be	O
used	O
over	O
any	O
IM	O
protocol	O
supported	O
by	O
Pidgin	B-Protocol
or	O
Kopete	B-Language
,	O
offering	O
an	O
auto-detection	B-Protocol
feature	O
that	O
starts	O
the	O
OTR	O
session	O
with	O
the	O
buddies	O
that	O
have	O
it	O
enabled	O
,	O
without	O
interfering	O
with	O
regular	O
,	O
unencrypted	O
conversations	B-Protocol
.	O
</s>
<s>
This	O
version	O
aims	O
to	O
provide	O
online	O
and	O
offline	O
deniability	O
,	O
to	O
update	O
the	O
cryptographic	O
primitives	O
,	O
and	O
to	O
support	O
out-of-order	B-Protocol
delivery	I-Protocol
and	O
asynchronous	O
communication	O
.	O
</s>
<s>
Brewer	O
as	O
an	O
improvement	O
over	O
the	O
OpenPGP	O
and	O
the	O
S/MIME	B-Protocol
system	O
at	O
the	O
"	O
Workshop	O
on	O
Privacy	O
in	O
the	O
Electronic	O
Society	O
"	O
(	O
WPES	O
)	O
.	O
</s>
<s>
In	O
2007	O
Olivier	O
Goffart	O
published	O
mod_otr	O
for	O
ejabberd	B-Protocol
,	O
making	O
it	O
possible	O
to	O
perform	O
man-in-the-middle	O
attacks	O
on	O
OTR	O
users	O
who	O
do	O
n't	O
check	O
key	B-Algorithm
fingerprints	I-Algorithm
.	O
</s>
<s>
OTR	O
developers	O
countered	O
this	O
attack	O
by	O
introducing	O
a	O
socialist	O
millionaire	O
protocol	O
implementation	O
in	O
libotr	B-Protocol
.	O
</s>
<s>
Several	O
solutions	O
have	O
been	O
proposed	O
for	O
supporting	O
conversations	B-Protocol
with	O
multiple	O
participants	O
.	O
</s>
<s>
The	O
method	O
called	O
"	O
Multi-party	O
Off-the-Record	B-Protocol
Messaging	I-Protocol
"	O
(	O
mpOTR	O
)	O
which	O
was	O
published	O
in	O
2009	O
works	O
without	O
a	O
central	O
management	O
host	O
and	O
was	O
introduced	O
in	O
Cryptocat	B-Protocol
by	O
Ian	O
Goldberg	O
et	O
al	O
.	O
</s>
<s>
In	O
2013	O
,	O
the	O
Signal	B-Protocol
Protocol	I-Protocol
was	O
introduced	O
,	O
which	O
is	O
based	O
on	O
OTR	B-Protocol
Messaging	I-Protocol
and	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	O
"	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	O
and	O
simpler	O
support	O
for	O
conversations	B-Protocol
with	O
multiple	O
participants	O
.	O
</s>
<s>
OMEMO	B-Application
,	O
introduced	O
in	O
an	O
Android	B-Application
XMPP	B-Protocol
client	O
called	O
Conversations	B-Protocol
in	O
2015	O
,	O
integrates	O
the	O
Double	B-Algorithm
Ratchet	I-Algorithm
Algorithm	I-Algorithm
used	O
in	O
Signal	O
into	O
the	O
instant	B-Application
messaging	I-Application
protocol	O
XMPP	B-Protocol
(	O
"	O
Jabber	B-Protocol
"	O
)	O
and	O
also	O
enables	O
encryption	O
of	O
file	B-Protocol
transfers	I-Protocol
.	O
</s>
<s>
In	O
the	O
autumn	O
of	O
2015	O
it	O
was	O
submitted	O
to	O
the	O
XMPP	B-Protocol
Standards	I-Protocol
Foundation	I-Protocol
for	O
standardisation	O
.	O
</s>
<s>
In	O
addition	O
to	O
providing	O
encryption	O
and	O
authentication	O
—	O
features	O
also	O
provided	O
by	O
typical	O
public-key	O
cryptography	O
suites	O
,	O
such	O
as	O
PGP	B-Application
,	O
GnuPG	B-Application
,	O
and	O
X.509	O
(	O
S/MIME	B-Protocol
)	O
—	O
OTR	O
also	O
offers	O
some	O
less	O
common	O
features	O
:	O
</s>
<s>
Forward	O
secrecy	O
:	O
Messages	O
are	O
only	O
encrypted	O
with	O
temporary	O
per-message	O
AES	B-Algorithm
keys	I-Algorithm
,	O
negotiated	O
using	O
the	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
protocol	O
.	O
</s>
<s>
The	O
compromise	O
of	O
any	O
long-lived	O
cryptographic	O
keys	O
does	O
not	O
compromise	O
any	O
previous	O
conversations	B-Protocol
,	O
even	O
if	O
an	O
attacker	O
is	O
in	O
possession	O
of	O
ciphertexts	O
.	O
</s>
<s>
Deniable	B-Protocol
authentication	I-Protocol
:	O
Messages	O
in	O
a	O
conversation	O
do	O
not	O
have	O
digital	O
signatures	O
,	O
and	O
after	O
a	O
conversation	O
is	O
complete	O
,	O
anyone	O
is	O
able	O
to	O
forge	O
a	O
message	O
to	O
appear	O
to	O
have	O
come	O
from	O
one	O
of	O
the	O
participants	O
in	O
the	O
conversation	O
,	O
assuring	O
that	O
it	O
is	O
impossible	O
to	O
prove	O
that	O
a	O
specific	O
message	O
came	O
from	O
a	O
specific	O
person	O
.	O
</s>
<s>
This	O
feature	O
makes	O
it	O
possible	O
for	O
users	O
to	O
verify	O
the	O
identity	O
of	O
the	O
remote	O
party	O
and	O
avoid	O
a	O
man-in-the-middle	O
attack	O
without	O
the	O
inconvenience	O
of	O
manually	O
comparing	O
public	B-Algorithm
key	I-Algorithm
fingerprints	I-Algorithm
through	O
an	O
outside	O
channel	O
.	O
</s>
<s>
As	O
of	O
version	O
3	O
of	O
the	O
protocol	O
specification	O
,	O
an	O
extra	O
symmetric	B-Algorithm
key	I-Algorithm
is	O
derived	O
during	O
authenticated	O
key	O
exchanges	O
that	O
can	O
be	O
used	O
for	O
secure	O
communication	O
(	O
e.g.	O
,	O
encrypted	O
file	B-Protocol
transfers	I-Protocol
)	O
over	O
a	O
different	O
channel	O
.	O
</s>
<s>
(	O
SRTP	B-Protocol
with	O
ZRTP	B-Protocol
exists	O
for	O
that	O
purpose	O
.	O
)	O
</s>
<s>
A	O
project	O
to	O
produce	O
a	O
protocol	O
for	O
multi-party	O
off-the-record	B-Protocol
messaging	I-Protocol
(	O
mpOTR	O
)	O
has	O
been	O
organized	O
by	O
Cryptocat	B-Protocol
,	O
eQualitie	O
,	O
and	O
other	O
contributors	O
including	O
Ian	O
Goldberg	O
.	O
</s>
<s>
Since	O
OTR	O
protocol	O
v3	O
(	O
libotr	B-Protocol
4.0.0	O
)	O
the	O
plugin	O
supports	O
multiple	O
OTR	O
conversations	B-Protocol
with	O
the	O
same	O
buddy	O
who	O
is	O
logged	O
in	O
at	O
multiple	O
locations	O
.	O
</s>
<s>
These	O
clients	O
support	O
Off-the-Record	B-Protocol
Messaging	I-Protocol
out	O
of	O
the	O
box	O
.	O
</s>
<s>
The	O
following	O
clients	O
require	O
a	O
plug-in	O
to	O
use	O
Off-the-Record	B-Protocol
Messaging	I-Protocol
.	O
</s>
<s>
Although	O
Gmail	O
's	O
Google	B-Application
Talk	I-Application
uses	O
the	O
term	O
"	O
off	O
the	O
record	O
"	O
,	O
the	O
feature	O
has	O
no	O
connection	O
to	O
the	O
Off-the-Record	B-Protocol
Messaging	I-Protocol
protocol	O
described	O
in	O
this	O
article	O
,	O
its	O
chats	O
are	O
not	O
encrypted	O
in	O
the	O
way	O
described	O
above	O
—	O
and	O
could	O
be	O
logged	O
internally	O
by	O
Google	O
even	O
if	O
not	O
accessible	O
by	O
end-users	O
.	O
</s>
