<s>
Opportunistic	B-Protocol
encryption	I-Protocol
(	O
OE	O
)	O
refers	O
to	O
any	O
system	O
that	O
,	O
when	O
connecting	O
to	O
another	O
system	O
,	O
attempts	O
to	O
encrypt	O
communications	O
channels	O
,	O
otherwise	O
falling	O
back	O
to	O
unencrypted	O
communications	O
.	O
</s>
<s>
Opportunistic	B-Protocol
encryption	I-Protocol
can	O
be	O
used	O
to	O
combat	O
passive	O
wiretapping	O
.	O
</s>
<s>
Opportunistic	B-Protocol
encryption	I-Protocol
on	O
the	O
Internet	O
is	O
described	O
in	O
"	O
Opportunistic	B-Protocol
Encryption	I-Protocol
using	O
the	O
Internet	O
Key	O
Exchange	O
(	O
IKE	O
)	O
"	O
,	O
"	O
Opportunistic	O
Security	O
:	O
Some	O
Protection	O
Most	O
of	O
the	O
Time	O
"	O
,	O
and	O
in	O
"	O
Opportunistic	O
Security	O
for	O
HTTP/2	B-Protocol
"	O
.	O
</s>
<s>
The	O
FreeS/WAN	B-Protocol
project	O
was	O
one	O
of	O
the	O
early	O
proponents	O
of	O
OE	O
.	O
</s>
<s>
The	O
effort	O
is	O
continued	O
by	O
the	O
former	O
freeswan	O
developers	O
now	O
working	O
on	O
Libreswan	B-Application
.	O
</s>
<s>
Libreswan	B-Application
aims	O
to	O
support	O
different	O
authentication	O
hooks	O
for	O
Opportunistic	B-Protocol
Encryption	I-Protocol
with	O
IPsec	B-Protocol
.	O
</s>
<s>
Version	O
3.16	O
,	O
which	O
was	O
released	O
in	O
December	O
2015	O
,	O
had	O
support	O
for	O
Opportunistic	O
IPsec	B-Protocol
using	O
AUTH-NULL	O
which	O
is	O
based	O
on	O
.	O
</s>
<s>
The	O
Libreswan	B-Application
Project	O
is	O
currently	O
working	O
on	O
(	O
forward	O
)	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
Security	I-Protocol
Extensions	I-Protocol
(	O
DNSSEC	B-Protocol
)	O
and	O
Kerberos	B-Protocol
support	O
for	O
Opportunistic	O
IPsec	B-Protocol
.	O
</s>
<s>
Openswan	B-Protocol
has	O
also	O
been	O
ported	O
to	O
the	O
OpenWrt	B-Application
project	O
.	O
</s>
<s>
Openswan	B-Protocol
used	O
reverse	O
DNS	B-Protocol
records	O
to	O
facilitate	O
the	O
key	O
exchange	O
between	O
the	O
systems	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
use	O
OpenVPN	B-Language
and	O
networking	O
protocols	O
to	O
set	O
up	O
dynamic	O
VPN	O
links	O
which	O
act	O
similar	O
to	O
OE	O
for	O
specific	O
domains	O
.	O
</s>
<s>
The	O
FreeS/WAN	B-Protocol
and	O
forks	O
such	O
as	O
Openswan	B-Protocol
and	O
strongSwan	B-Protocol
offer	O
VPNs	O
which	O
can	O
also	O
operate	O
in	O
OE	O
mode	O
using	O
IPsec	B-Protocol
based	O
technology	O
.	O
</s>
<s>
Obfuscated	B-Protocol
TCP	I-Protocol
is	O
another	O
method	O
of	O
implementing	O
OE	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
platforms	O
have	O
an	O
implementation	O
of	O
OE	O
installed	O
by	O
default	O
.	O
</s>
<s>
This	O
method	O
uses	O
IPsec	B-Protocol
to	O
secure	O
the	O
traffic	O
and	O
is	O
a	O
simple	O
procedure	O
to	O
turn	O
on	O
.	O
</s>
<s>
It	O
is	O
accessed	O
via	O
the	O
MMC	B-Application
and	O
"	O
IP	B-Protocol
Security	I-Protocol
Policies	O
on	O
Local	O
Computer	O
"	O
and	O
then	O
editing	O
the	O
properties	O
to	O
assign	O
the	O
"	O
(	O
Request	O
Security	O
)	O
"	O
policy	O
.	O
</s>
<s>
This	O
will	O
turn	O
on	O
optional	O
IPsec	B-Protocol
in	O
a	O
Kerberos	B-Protocol
environment	O
.	O
</s>
<s>
Many	O
systems	O
also	O
have	O
problems	O
when	O
either	O
side	O
is	O
behind	O
a	O
NAT	B-Protocol
.	O
</s>
<s>
This	O
problem	O
is	O
addressed	O
by	O
NAT	B-Protocol
Traversal	I-Protocol
(	O
NAT-T	B-Protocol
)	O
and	O
is	O
accomplished	O
by	O
editing	O
a	O
registry	B-Protocol
item	O
.	O
</s>
<s>
Using	O
the	O
filtering	O
options	O
provided	O
in	O
MMC	B-Application
,	O
it	O
is	O
possible	O
to	O
tailor	O
the	O
networking	O
to	O
require	O
,	O
request	O
or	O
permit	O
traffic	O
to	O
various	O
domains	O
and	O
protocols	O
to	O
use	O
encryption	O
.	O
</s>
<s>
Opportunistic	B-Protocol
Encryption	I-Protocol
can	O
also	O
be	O
used	O
for	O
specific	O
traffic	O
like	O
e-mail	O
using	O
the	O
SMTP	B-Protocol
STARTTLS	B-Protocol
extension	O
for	O
relaying	O
messages	O
across	O
the	O
Internet	O
,	O
or	O
the	O
Internet	B-Protocol
Message	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
IMAP	B-Protocol
)	O
STARTTLS	B-Protocol
extension	O
for	O
reading	O
e-mail	O
.	O
</s>
<s>
With	O
this	O
implementation	O
,	O
it	O
is	O
not	O
necessary	O
to	O
obtain	O
a	O
certificate	O
from	O
a	O
certificate	B-Architecture
authority	I-Architecture
,	O
as	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
can	O
be	O
used	O
.	O
</s>
<s>
PGP	B-Application
,	O
p≡p	O
,	O
Hushmail	B-Application
,	O
and	O
Ciphire	O
,	O
among	O
others	O
can	O
all	O
be	O
set	O
up	O
to	O
work	O
in	O
this	O
mode	O
.	O
</s>
<s>
In	O
practice	O
,	O
STARTTLS	B-Protocol
in	O
SMTP	B-Protocol
is	O
often	O
deployed	O
with	O
self-signed	B-Protocol
certificates	I-Protocol
,	O
which	O
represents	O
a	O
minimal	O
one-time	O
task	O
for	O
a	O
system	O
administrator	O
,	O
and	O
results	O
in	O
most	O
email	O
traffic	O
being	O
opportunistically	O
encrypted	O
.	O
</s>
<s>
Some	O
Voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
solutions	O
provide	O
for	O
painless	O
encryption	O
of	O
voice	O
traffic	O
when	O
possible	O
.	O
</s>
<s>
Some	O
versions	O
of	O
the	O
Sipura	O
Technology	O
and	O
Linksys	O
lines	O
of	O
analog	B-Protocol
telephony	I-Protocol
adapters	I-Protocol
(	O
ATA	O
)	O
include	O
a	O
hardware	O
implementation	O
of	O
SRTP	B-Protocol
with	O
the	O
installation	O
of	O
a	O
certificate	O
from	O
Voxilla	O
,	O
a	O
VoIP	B-Application
information	O
site	O
.	O
</s>
<s>
When	O
the	O
call	O
is	O
placed	O
an	O
attempt	O
is	O
made	O
to	O
use	O
SRTP	B-Protocol
,	O
if	O
successful	O
a	O
series	O
of	O
tones	O
are	O
played	O
into	O
the	O
handset	O
,	O
if	O
not	O
the	O
call	O
proceeds	O
without	O
using	O
encryption	O
.	O
</s>
<s>
Skype	B-Application
and	O
Amicima	O
use	O
only	O
secure	O
connections	O
and	O
Gizmo5	B-Application
attempts	O
a	O
secure	O
connection	O
between	O
its	O
clients	O
.	O
</s>
<s>
Phil	O
Zimmermann	O
,	O
Alan	O
Johnston	O
,	O
and	O
Jon	O
Callas	O
have	O
proposed	O
a	O
new	O
VoIP	B-Application
encryption	O
protocol	O
called	O
ZRTP	B-Protocol
.	O
</s>
<s>
They	O
have	O
an	O
implementation	O
of	O
it	O
called	O
Zfone	B-Application
whose	O
source	O
and	O
compiled	O
binaries	O
are	O
available	O
.	O
</s>
<s>
For	O
encrypting	O
WWW/HTTP	O
connections	O
,	O
HTTPS	B-Protocol
is	O
typically	O
used	O
,	O
which	O
requires	O
strict	O
encryption	O
and	O
has	O
significant	O
administrative	O
costs	O
,	O
both	O
in	O
terms	O
of	O
initial	O
setup	O
and	O
continued	O
maintenance	O
costs	O
for	O
the	O
website	O
operator	O
.	O
</s>
<s>
Most	O
browsers	B-Application
verify	O
the	O
webserver	B-Application
's	O
identity	O
to	O
make	O
sure	O
that	O
an	O
SSL	O
certificate	O
is	O
signed	O
by	O
a	O
trusted	O
certificate	B-Architecture
authority	I-Architecture
and	O
has	O
not	O
expired	O
,	O
usually	O
requiring	O
the	O
website	O
operator	O
to	O
manually	O
change	O
the	O
certificate	O
every	O
one	O
or	O
two	O
years	O
.	O
</s>
<s>
The	O
easiest	O
way	O
to	O
enable	O
some	O
sort	O
of	O
opportunistic	O
website	O
encryption	O
is	O
by	O
using	O
self-signed	B-Protocol
certificates	I-Protocol
,	O
but	O
this	O
causes	O
browsers	B-Application
to	O
display	O
a	O
warning	O
each	O
time	O
the	O
website	O
is	O
visited	O
unless	O
the	O
user	O
manually	O
marks	O
the	O
website	O
's	O
certificate	O
as	O
trusted	O
.	O
</s>
<s>
Because	O
unencrypted	O
websites	O
do	O
not	O
currently	O
display	O
any	O
such	O
warnings	O
,	O
the	O
use	O
of	O
self-signed	B-Protocol
certificates	I-Protocol
is	O
not	O
well	O
received	O
.	O
</s>
<s>
In	O
2015	O
,	O
Mozilla	B-Operating_System
started	O
to	O
roll	O
out	O
opportunistic	B-Protocol
encryption	I-Protocol
in	O
Firefox	B-Application
version	O
37	O
.	O
</s>
<s>
Browser	B-Application
extensions	O
like	O
HTTPS	B-Operating_System
Everywhere	I-Operating_System
and	O
HTTPSfinder	O
find	O
and	O
automatically	O
switch	O
the	O
connection	O
to	O
HTTPS	B-Protocol
when	O
possible	O
.	O
</s>
<s>
Several	O
proposals	O
were	O
available	O
for	O
true	O
,	O
seamless	O
opportunistic	B-Protocol
encryption	I-Protocol
of	O
HTTP/2	B-Protocol
protocol	O
.	O
</s>
<s>
Poul-Henning	O
Kamp	O
,	O
lead	O
developer	O
of	O
Varnish	B-Language
and	O
a	O
senior	O
FreeBSD	B-Operating_System
kernel	O
developer	O
,	O
has	O
criticized	O
the	O
IETF	O
for	O
following	O
a	O
particular	O
political	O
agenda	O
with	O
HTTP/2	B-Protocol
for	O
not	O
implementing	O
opportunistic	B-Protocol
encryption	I-Protocol
in	O
the	O
standard	O
.	O
</s>
<s>
STARTTLS	B-Protocol
implementations	O
often	O
used	O
with	O
SMTP	B-Protocol
are	O
vulnerable	O
to	O
STRIPTLS	O
attacks	O
when	O
subject	O
to	O
active	O
wiretapping	O
.	O
</s>
