<s>
Email	B-Protocol
encryption	I-Protocol
is	O
encryption	O
of	O
email	O
messages	O
to	O
protect	O
the	O
content	O
from	O
being	O
read	O
by	O
entities	O
other	O
than	O
the	O
intended	O
recipients	O
.	O
</s>
<s>
Email	B-Protocol
encryption	I-Protocol
may	O
also	O
include	O
authentication	O
.	O
</s>
<s>
By	O
default	O
,	O
popular	O
email	O
services	O
such	O
as	O
Gmail	B-Application
and	O
Outlook	O
do	O
not	O
enable	O
end-to-end	O
encryption	O
.	O
</s>
<s>
Email	B-Protocol
encryption	I-Protocol
can	O
rely	O
on	O
public-key	B-Application
cryptography	I-Application
,	O
in	O
which	O
users	O
can	O
each	O
publish	O
a	O
public	B-Application
key	I-Application
that	O
others	O
can	O
use	O
to	O
encrypt	O
messages	O
to	O
them	O
,	O
while	O
keeping	O
secret	O
a	O
private	B-Application
key	I-Application
they	O
can	O
use	O
to	O
decrypt	O
such	O
messages	O
or	O
to	O
digitally	O
encrypt	O
and	O
sign	O
messages	O
they	O
send	O
.	O
</s>
<s>
With	O
the	O
original	O
design	O
of	O
email	B-Protocol
protocol	I-Protocol
,	O
the	O
communication	O
between	O
email	O
servers	O
was	O
in	O
plain	O
text	O
,	O
which	O
posed	O
a	O
huge	O
security	O
risk	O
.	O
</s>
<s>
Transport	B-Protocol
layer	I-Protocol
encryption	I-Protocol
is	O
often	O
easier	O
to	O
set	O
up	O
and	O
use	O
;	O
end-to-end	O
encryption	O
provides	O
stronger	O
defenses	O
,	O
but	O
can	O
be	O
more	O
difficult	O
to	O
set	O
up	O
and	O
use	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
commonly	O
used	O
email	B-Protocol
encryption	I-Protocol
extensions	O
is	O
STARTTLS	B-Protocol
.	O
</s>
<s>
Assuming	O
that	O
the	O
email	O
servers	O
on	O
both	O
the	O
sender	O
and	O
the	O
recipient	O
side	O
support	O
encrypted	O
communication	O
,	O
an	O
eavesdropper	O
snooping	O
on	O
the	O
communication	O
between	O
the	O
mail	O
servers	O
cannot	O
use	O
a	O
sniffer	B-Protocol
to	O
see	O
the	O
email	O
contents	O
.	O
</s>
<s>
Similar	O
STARTTLS	B-Protocol
extensions	O
exist	O
for	O
the	O
communication	O
between	O
an	O
email	O
client	O
and	O
the	O
email	O
server	O
(	O
see	O
IMAP4	B-Protocol
and	O
POP3	B-Protocol
,	O
as	O
stated	O
by	O
RFC	O
2595	O
)	O
.	O
</s>
<s>
STARTTLS	B-Protocol
may	O
be	O
used	O
regardless	O
of	O
whether	O
the	O
email	O
's	O
contents	O
are	O
encrypted	O
using	O
another	O
protocol	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
the	O
encryption	O
takes	O
place	O
between	O
individual	O
SMTP	B-Protocol
relays	O
,	O
not	O
between	O
the	O
sender	O
and	O
the	O
recipient	O
.	O
</s>
<s>
A	O
key	O
positive	O
trait	O
of	O
transport	B-Protocol
layer	I-Protocol
encryption	I-Protocol
is	O
that	O
users	O
do	O
not	O
need	O
to	O
do	O
or	O
change	O
anything	O
;	O
the	O
encryption	O
automatically	O
occurs	O
when	O
they	O
send	O
email	O
.	O
</s>
<s>
The	O
Electronic	O
Frontier	O
Foundation	O
encourages	O
the	O
use	O
of	O
STARTTLS	B-Protocol
,	O
and	O
has	O
launched	O
the	O
'	O
STARTTLS	B-Protocol
Everywhere	O
 '	O
initiative	O
to	O
"	O
make	O
it	O
simple	O
and	O
easy	O
for	O
everyone	O
to	O
help	O
ensure	O
their	O
communications	O
(	O
over	O
email	O
)	O
are	O
n’t	O
vulnerable	O
to	O
mass	O
surveillance.	O
"	O
</s>
<s>
Support	O
for	O
STARTTLS	B-Protocol
has	O
become	O
quite	O
common	O
;	O
Google	O
reports	O
that	O
on	O
Gmail	B-Application
,	O
90%	O
of	O
incoming	O
email	O
and	O
90%	O
of	O
outgoing	O
email	O
was	O
encrypted	O
using	O
STARTTLS	B-Protocol
by	O
July	O
24	O
,	O
2018	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
most	O
email	O
that	O
is	O
delivered	O
over	O
TLS	O
uses	O
only	O
opportunistic	B-Protocol
encryption	I-Protocol
.	O
</s>
<s>
DANE	B-Protocol
is	O
a	O
proposed	O
standard	O
that	O
makes	O
an	O
incremental	O
transition	O
to	O
verified	O
encryption	O
for	O
Internet	O
mail	O
delivery	O
possible	O
.	O
</s>
<s>
The	O
STARTTLS	B-Protocol
Everywhere	O
project	O
uses	O
an	O
alternative	O
approach	O
:	O
they	O
support	O
a	O
“	O
preload	O
list	O
”	O
of	O
email	O
servers	O
that	O
have	O
promised	O
to	O
support	O
STARTTLS	B-Protocol
,	O
which	O
can	O
help	O
detect	O
and	O
prevent	O
downgrade	B-Protocol
attacks	I-Protocol
.	O
</s>
<s>
In	O
other	O
words	O
,	O
an	O
email	O
sent	O
with	O
end-to-end	O
encryption	O
would	O
be	O
encrypted	O
at	O
the	O
source	O
,	O
unreadable	O
to	O
service	O
providers	O
like	O
Gmail	B-Application
in	O
transit	O
,	O
and	O
then	O
decrypted	O
at	O
its	O
endpoint	O
.	O
</s>
<s>
Crucially	O
,	O
the	O
email	O
would	O
only	O
be	O
decrypted	O
for	O
the	O
end	O
user	O
on	O
their	O
computer	O
and	O
would	O
remain	O
in	O
encrypted	O
,	O
unreadable	O
form	O
to	O
an	O
email	O
service	O
like	O
Gmail	B-Application
,	O
which	O
would	O
n't	O
have	O
the	O
keys	O
available	O
to	O
decrypt	O
it	O
.	O
</s>
<s>
Notable	O
protocols	O
for	O
end-to-end	O
email	B-Protocol
encryption	I-Protocol
include	O
:	O
</s>
<s>
There	O
are	O
various	O
software	O
and	O
email-client	O
plugins	O
that	O
allow	O
users	O
to	O
encrypt	O
the	O
message	O
using	O
the	O
recipient	O
's	O
public	B-Application
key	I-Application
before	O
sending	O
it	O
.	O
</s>
<s>
At	O
its	O
core	O
,	O
OpenPGP	O
uses	O
a	O
Public	B-Application
Key	I-Application
Cryptography	I-Application
scheme	O
where	O
each	O
email	O
address	O
is	O
associated	O
with	O
a	O
public/private	O
key	O
pair	O
.	O
</s>
<s>
However	O
,	O
there	O
are	O
usability	O
issues	O
with	O
OpenPGP	O
—	O
it	O
requires	O
users	O
to	O
set	O
up	O
public/private	O
key	O
pairs	O
and	O
make	O
the	O
public	B-Application
keys	I-Application
available	O
widely	O
.	O
</s>
<s>
The	O
Signed	O
and	O
Encrypted	B-Protocol
Email	I-Protocol
Over	O
The	O
Internet	O
demonstration	O
has	O
shown	O
that	O
organizations	O
can	O
collaborate	O
effectively	O
using	O
secure	B-Protocol
email	I-Protocol
.	O
</s>
<s>
Previous	O
barriers	O
to	O
adoption	O
were	O
overcome	O
,	O
including	O
the	O
use	O
of	O
a	O
PKI	O
bridge	O
to	O
provide	O
a	O
scalable	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
(	O
PKI	O
)	O
and	O
the	O
use	O
of	O
network	O
security	O
guards	O
checking	O
encrypted	O
content	O
passing	O
in	O
and	O
out	O
of	O
corporate	O
network	O
boundaries	O
to	O
avoid	O
encryption	O
being	O
used	O
to	O
hide	O
malware	O
introduction	O
and	O
information	O
leakage	O
.	O
</s>
<s>
Transport	B-Protocol
layer	I-Protocol
encryption	I-Protocol
using	O
STARTTLS	B-Protocol
must	O
be	O
set	O
up	O
by	O
the	O
receiving	O
organization	O
.	O
</s>
<s>
This	O
is	O
typically	O
straightforward	O
;	O
a	O
valid	O
certificate	O
must	O
be	O
obtained	O
and	O
STARTTLS	B-Protocol
must	O
be	O
enabled	O
on	O
the	O
receiving	O
organization	O
's	O
email	O
server	O
.	O
</s>
<s>
Most	O
full-featured	O
email	O
clients	O
provide	O
native	O
support	O
for	O
S/MIME	B-Protocol
secure	B-Protocol
email	I-Protocol
(	O
digital	O
signing	O
and	O
message	O
encryption	O
using	O
certificates	O
)	O
.	O
</s>
<s>
Other	O
encryption	O
options	O
include	O
PGP	O
and	O
GNU	B-Application
Privacy	I-Application
Guard	I-Application
(	O
GnuPG	B-Application
)	O
.	O
</s>
<s>
Eight	O
years	O
later	O
,	O
another	O
group	O
of	O
Carnegie	O
Mellon	O
researchers	O
published	O
a	O
follow-up	O
paper	O
saying	O
that	O
,	O
although	O
a	O
newer	O
version	O
of	O
PGP	O
made	O
it	O
easy	O
to	O
decrypt	O
messages	O
,	O
most	O
people	O
still	O
struggled	O
with	O
encrypting	O
and	O
signing	O
messages	O
,	O
finding	O
and	O
verifying	O
other	O
people	O
's	O
public	B-Application
encryption	I-Application
keys	I-Application
,	O
and	O
sharing	O
their	O
own	O
keys	O
.	O
</s>
<s>
Recipients	O
who	O
are	O
not	O
behind	O
an	O
encryption	O
gateway	O
then	O
need	O
to	O
take	O
an	O
extra	O
step	O
,	O
either	O
procuring	O
the	O
public	B-Application
key	I-Application
,	O
or	O
logging	O
into	O
an	O
online	O
portal	O
to	O
retrieve	O
the	O
message	O
.	O
</s>
<s>
Since	O
2000	O
,	O
the	O
number	O
of	O
available	O
encrypted	B-Protocol
email	I-Protocol
providers	O
has	O
increased	O
significantly	O
.	O
</s>
