<s>
S/MIME	B-Protocol
(	O
Secure/Multipurpose	O
Internet	O
Mail	O
Extensions	O
)	O
is	O
a	O
standard	O
for	O
public-key	B-Application
encryption	I-Application
and	O
signing	O
of	O
MIME	B-Protocol
data	O
.	O
</s>
<s>
S/MIME	B-Protocol
is	O
on	O
an	O
IETF	O
standards	O
track	O
and	O
defined	O
in	O
a	O
number	O
of	O
documents	O
,	O
most	O
importantly	O
.	O
</s>
<s>
It	O
was	O
originally	O
developed	O
by	O
RSA	O
Data	O
Security	O
,	O
and	O
the	O
original	O
specification	O
used	O
the	O
IETF	O
MIME	B-Protocol
specification	O
with	O
the	O
de	O
facto	O
industry	O
standard	O
PKCS	O
#7	O
secure	O
message	O
format	O
.	O
</s>
<s>
Change	O
control	O
to	O
S/MIME	B-Protocol
has	O
since	O
been	O
vested	O
in	O
the	O
IETF	O
,	O
and	O
the	O
specification	O
is	O
now	O
layered	O
on	O
Cryptographic	B-Protocol
Message	I-Protocol
Syntax	I-Protocol
(	O
CMS	O
)	O
,	O
an	O
IETF	O
specification	O
that	O
is	O
identical	O
in	O
most	O
respects	O
with	O
PKCS	O
#7	O
.	O
</s>
<s>
S/MIME	B-Protocol
functionality	O
is	O
built	O
into	O
the	O
majority	O
of	O
modern	O
email	O
software	O
and	O
interoperates	O
between	O
them	O
.	O
</s>
<s>
Since	O
it	O
is	O
built	O
on	O
CMS	O
,	O
MIME	B-Protocol
can	O
also	O
hold	O
an	O
advanced	O
digital	O
signature	O
.	O
</s>
<s>
S/MIME	B-Protocol
provides	O
the	O
following	O
cryptographic	O
security	O
services	O
for	O
electronic	O
messaging	O
applications	O
:	O
</s>
<s>
S/MIME	B-Protocol
specifies	O
the	O
MIME	B-Protocol
type	O
application/pkcs7	O
-mime	O
(	O
smime-type	O
"	O
enveloped-data	O
"	O
)	O
for	O
data	O
enveloping	O
(	O
encrypting	O
)	O
where	O
the	O
whole	O
(	O
prepared	O
)	O
MIME	B-Protocol
entity	O
to	O
be	O
enveloped	O
is	O
encrypted	O
and	O
packed	O
into	O
an	O
object	O
which	O
subsequently	O
is	O
inserted	O
into	O
an	O
application/pkcs7	O
-mime	O
MIME	B-Protocol
entity	O
.	O
</s>
<s>
Before	O
S/MIME	B-Protocol
can	O
be	O
used	O
in	O
any	O
of	O
the	O
above	O
applications	O
,	O
one	O
must	O
obtain	O
and	O
install	O
an	O
individual	O
key/certificate	O
either	O
from	O
one	O
's	O
in-house	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
or	O
from	O
a	O
public	O
CA	O
.	O
</s>
<s>
The	O
accepted	O
best	O
practice	O
is	O
to	O
use	O
separate	O
private	B-Application
keys	I-Application
(	O
and	O
associated	O
certificates	O
)	O
for	O
signature	O
and	O
for	O
encryption	O
,	O
as	O
this	O
permits	O
escrow	O
of	O
the	O
encryption	O
key	O
without	O
compromise	O
to	O
the	O
non-repudiation	O
property	O
of	O
the	O
signature	O
key	O
.	O
</s>
<s>
While	O
it	O
is	O
technically	O
possible	O
to	O
send	O
a	O
message	O
encrypted	O
(	O
using	O
the	O
destination	O
party	O
certificate	O
)	O
without	O
having	O
one	O
's	O
own	O
certificate	O
to	O
digitally	O
sign	O
,	O
in	O
practice	O
,	O
the	O
S/MIME	B-Protocol
clients	O
will	O
require	O
the	O
user	O
to	O
install	O
their	O
own	O
certificate	O
before	O
they	O
allow	O
encrypting	O
to	O
others	O
.	O
</s>
<s>
The	O
latter	O
,	O
at	O
a	O
minimum	O
,	O
is	O
mandatory	O
to	O
uphold	O
the	O
integrity	O
of	O
the	O
public	B-Application
key	I-Application
infrastructure	O
.	O
</s>
<s>
In	O
2020	O
,	O
the	O
S/MIME	B-Protocol
Certificate	O
Working	O
Group	O
of	O
the	O
CA/Browser	O
Forum	O
was	O
chartered	O
to	O
create	O
a	O
baseline	O
requirement	O
applicable	O
to	O
CAs	O
that	O
issue	O
S/MIME	B-Protocol
certificates	O
used	O
to	O
sign	O
,	O
verify	O
,	O
encrypt	O
,	O
and	O
decrypt	O
email	O
.	O
</s>
<s>
Version	O
1	O
of	O
the	O
Baseline	O
Requirements	O
for	O
the	O
Issuance	O
and	O
Management	O
of	O
Publicly‐Trusted	O
S/MIME	B-Protocol
Certificates	O
was	O
published	O
on	O
January	O
1	O
,	O
2023	O
by	O
the	O
CA/Browser	O
Forum	O
.	O
</s>
<s>
It	O
defined	O
four	O
types	O
of	O
S/MIME	B-Protocol
certificate	O
standards	O
.	O
</s>
<s>
S/MIME	B-Protocol
is	O
sometimes	O
considered	O
not	O
properly	O
suited	O
for	O
use	O
via	O
webmail	B-Protocol
clients	O
.	O
</s>
<s>
Though	O
support	O
can	O
be	O
hacked	O
into	O
a	O
browser	O
,	O
some	O
security	O
practices	O
require	O
the	O
private	B-Application
key	I-Application
to	O
be	O
kept	O
accessible	O
to	O
the	O
user	O
but	O
inaccessible	O
from	O
the	O
webmail	B-Protocol
server	O
,	O
complicating	O
the	O
key	O
advantage	O
of	O
webmail	B-Protocol
:	O
providing	O
ubiquitous	O
accessibility	O
.	O
</s>
<s>
This	O
issue	O
is	O
not	O
fully	O
specific	O
to	O
S/MIME	B-Protocol
:	O
other	O
secure	O
methods	O
of	O
signing	O
webmail	B-Protocol
may	O
also	O
require	O
a	O
browser	O
to	O
execute	O
code	O
to	O
produce	O
the	O
signature	O
;	O
exceptions	O
are	O
PGP	B-Application
Desktop	I-Application
and	O
versions	O
of	O
GnuPG	B-Application
,	O
which	O
will	O
grab	O
the	O
data	O
out	O
of	O
the	O
webmail	B-Protocol
,	O
sign	O
it	O
by	O
means	O
of	O
a	O
clipboard	O
,	O
and	O
put	O
the	O
signed	O
data	O
back	O
into	O
the	O
webmail	B-Protocol
page	O
.	O
</s>
<s>
S/MIME	B-Protocol
is	O
tailored	O
for	O
end-to-end	O
security	O
.	O
</s>
<s>
Solutions	O
which	O
store	O
private	B-Application
keys	I-Application
on	O
the	O
gateway	O
server	O
so	O
decryption	O
can	O
occur	O
prior	O
to	O
the	O
gateway	O
malware	O
scan	O
.	O
</s>
<s>
Solutions	O
which	O
store	O
private	B-Application
keys	I-Application
on	O
malware	O
scanners	O
so	O
that	O
it	O
can	O
inspect	O
messages	O
content	O
,	O
the	O
encrypted	O
message	O
is	O
then	O
relayed	O
to	O
its	O
destination	O
.	O
</s>
<s>
Due	O
to	O
the	O
requirement	O
of	O
a	O
certificate	O
for	O
implementation	O
,	O
not	O
all	O
users	O
can	O
take	O
advantage	O
of	O
S/MIME	B-Protocol
,	O
as	O
some	O
may	O
wish	O
to	O
encrypt	O
a	O
message	O
without	O
the	O
involvement	O
or	O
administrative	O
overhead	O
of	O
certificates	O
,	O
for	O
example	O
by	O
encrypting	O
the	O
message	O
with	O
a	O
public/private	O
key	O
pair	O
instead	O
.	O
</s>
<s>
Any	O
message	O
that	O
an	O
S/MIME	B-Protocol
email	O
client	O
stores	O
encrypted	O
cannot	O
be	O
decrypted	O
if	O
the	O
applicable	O
key	B-Application
pair	I-Application
's	O
private	B-Application
key	I-Application
is	O
unavailable	O
or	O
otherwise	O
unusable	O
(	O
e.g.	O
,	O
the	O
certificate	O
has	O
been	O
deleted	O
or	O
lost	O
or	O
the	O
private	B-Application
key	I-Application
's	O
password	O
has	O
been	O
forgotten	O
)	O
.	O
</s>
<s>
Neither	O
of	O
these	O
potential	O
dilemmas	O
is	O
specific	O
to	O
S/MIME	B-Protocol
but	O
rather	O
cipher	O
text	O
in	O
general	O
and	O
do	O
not	O
apply	O
to	O
S/MIME	B-Protocol
messages	O
that	O
are	O
only	O
signed	O
and	O
not	O
encrypted	O
.	O
</s>
<s>
S/MIME	B-Protocol
signatures	O
are	O
usually	O
"	O
detached	O
signatures	O
"	O
:	O
the	O
signature	O
information	O
is	O
separate	O
from	O
the	O
text	O
being	O
signed	O
.	O
</s>
<s>
The	O
MIME	B-Protocol
type	O
for	O
this	O
is	O
multipart/signed	O
with	O
the	O
second	O
part	O
having	O
a	O
MIME	B-Protocol
subtype	O
of	O
application/	O
( x	O
-	O
)	O
pkcs7-signature	O
.	O
</s>
<s>
Mailing	O
list	O
software	O
is	O
notorious	O
for	O
changing	O
the	O
textual	O
part	O
of	O
a	O
message	O
and	O
thereby	O
invalidating	O
the	O
signature	O
;	O
however	O
,	O
this	O
problem	O
is	O
not	O
specific	O
to	O
S/MIME	B-Protocol
,	O
and	O
a	O
digital	O
signature	O
only	O
reveals	O
that	O
the	O
signed	O
content	O
has	O
been	O
changed	O
.	O
</s>
<s>
On	O
May	O
13	O
,	O
2018	O
,	O
the	O
Electronic	O
Frontier	O
Foundation	O
(	O
EFF	O
)	O
announced	O
critical	O
vulnerabilities	O
in	O
S/MIME	B-Protocol
,	O
together	O
with	O
an	O
obsolete	O
form	O
of	O
PGP	O
that	O
is	O
still	O
used	O
,	O
in	O
many	O
email	O
clients	O
.	O
</s>
