<s>
The	O
Online	B-Protocol
Certificate	I-Protocol
Status	I-Protocol
Protocol	I-Protocol
(	O
OCSP	B-Protocol
)	O
stapling	O
,	O
formally	O
known	O
as	O
the	O
TLS	B-Protocol
Certificate	I-Protocol
Status	I-Protocol
Request	I-Protocol
extension	I-Protocol
,	O
is	O
a	O
standard	O
for	O
checking	O
the	O
revocation	O
status	O
of	O
X.509	O
digital	O
certificates	O
.	O
</s>
<s>
It	O
allows	O
the	O
presenter	O
of	O
a	O
certificate	O
to	O
bear	O
the	O
resource	O
cost	O
involved	O
in	O
providing	O
Online	B-Protocol
Certificate	I-Protocol
Status	I-Protocol
Protocol	I-Protocol
(	O
OCSP	B-Protocol
)	O
responses	O
by	O
appending	O
(	O
"	O
stapling	O
"	O
)	O
a	O
time-stamped	O
OCSP	B-Protocol
response	O
signed	O
by	O
the	O
CA	O
to	O
the	O
initial	O
TLS	O
handshake	O
,	O
eliminating	O
the	O
need	O
for	O
clients	O
to	O
contact	O
the	O
CA	O
,	O
with	O
the	O
aim	O
of	O
improving	O
both	O
security	O
and	O
performance	O
.	O
</s>
<s>
The	O
original	O
OCSP	B-Protocol
implementation	O
has	O
a	O
number	O
of	O
issues	O
.	O
</s>
<s>
Firstly	O
,	O
it	O
can	O
introduce	O
a	O
significant	O
cost	O
for	O
the	O
certificate	B-Architecture
authorities	I-Architecture
(	O
CA	O
)	O
because	O
it	O
requires	O
them	O
to	O
provide	O
responses	O
to	O
every	O
client	O
of	O
a	O
given	O
certificate	O
in	O
real	O
time	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
a	O
certificate	O
is	O
issued	O
to	O
a	O
high	O
traffic	O
website	O
,	O
the	O
servers	O
of	O
CAs	O
are	O
likely	O
to	O
be	O
hit	O
by	O
enormous	O
volumes	O
of	O
OCSP	B-Protocol
requests	O
querying	O
the	O
validity	O
of	O
the	O
certificate	O
.	O
</s>
<s>
Also	O
,	O
OCSP	B-Protocol
checking	O
potentially	O
impairs	O
users	O
 '	O
privacy	O
and	O
slows	O
down	O
browsing	O
,	O
since	O
it	O
requires	O
the	O
client	O
to	O
contact	O
a	O
third	O
party	O
(	O
the	O
CA	O
)	O
to	O
confirm	O
the	O
validity	O
of	O
each	O
certificate	O
that	O
it	O
encounters	O
.	O
</s>
<s>
Moreover	O
,	O
if	O
the	O
client	O
fails	O
to	O
connect	O
to	O
the	O
CA	O
for	O
an	O
OCSP	B-Protocol
response	O
,	O
then	O
it	O
is	O
forced	O
to	O
decide	O
between	O
:	O
(	O
a	O
)	O
continuing	O
the	O
connection	O
anyway	O
;	O
defeating	O
the	O
purpose	O
of	O
OCSP	B-Protocol
or	O
(	O
b	O
)	O
terminating	O
the	O
connection	O
based	O
on	O
the	O
assumption	O
that	O
there	O
is	O
an	O
attack	O
;	O
but	O
which	O
could	O
result	O
in	O
excessive	O
false	O
warnings	O
and	O
blocks	O
.	O
</s>
<s>
OCSP	B-Protocol
stapling	I-Protocol
is	O
aimed	O
at	O
addressing	O
these	O
issues	O
with	O
the	O
original	O
OCSP	B-Protocol
implementation	O
.	O
</s>
<s>
OCSP	B-Protocol
stapling	I-Protocol
resolves	O
both	O
problems	O
in	O
a	O
fashion	O
reminiscent	O
of	O
the	O
Kerberos	B-Protocol
ticket	I-Protocol
.	O
</s>
<s>
In	O
a	O
stapling	O
scenario	O
,	O
the	O
certificate	O
holder	O
itself	O
queries	O
the	O
OCSP	B-Protocol
server	O
at	O
regular	O
intervals	O
,	O
obtaining	O
a	O
signed	O
time-stamped	O
OCSP	B-Protocol
response	O
.	O
</s>
<s>
When	O
the	O
site	O
's	O
visitors	O
attempt	O
to	O
connect	O
to	O
the	O
site	O
,	O
this	O
response	O
is	O
included	O
(	O
"	O
stapled	O
"	O
)	O
with	O
the	O
TLS/SSL	B-Protocol
handshake	I-Protocol
via	O
the	O
Certificate	B-Protocol
Status	I-Protocol
Request	I-Protocol
extension	O
response	O
(	O
note	O
:	O
the	O
TLS	O
client	O
must	O
explicitly	O
include	O
a	O
Certificate	B-Protocol
Status	I-Protocol
Request	I-Protocol
extension	O
in	O
its	O
ClientHello	O
TLS/SSL	B-Protocol
handshake	I-Protocol
message	O
)	O
.	O
</s>
<s>
While	O
it	O
may	O
appear	O
that	O
allowing	O
the	O
site	O
operator	O
to	O
control	O
verification	O
responses	O
would	O
allow	O
a	O
fraudulent	O
site	O
to	O
issue	O
false	O
verification	O
for	O
a	O
revoked	O
certificate	O
,	O
the	O
stapled	O
responses	O
ca	O
n't	O
be	O
forged	O
as	O
they	O
need	O
to	O
be	O
directly	O
signed	O
by	O
the	O
certificate	B-Architecture
authority	I-Architecture
,	O
not	O
the	O
server	O
.	O
</s>
<s>
If	O
the	O
client	O
does	O
not	O
receive	O
a	O
stapled	O
response	O
,	O
it	O
will	O
just	O
contact	O
the	O
OCSP	B-Protocol
server	O
by	O
itself	O
.	O
</s>
<s>
The	O
only	O
increased	O
risk	O
of	O
OCSP	B-Protocol
stapling	I-Protocol
is	O
that	O
the	O
notification	O
of	O
revocation	O
for	O
a	O
certificate	O
may	O
be	O
delayed	O
until	O
the	O
last-signed	O
OCSP	B-Protocol
response	O
expires	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
clients	O
continue	O
to	O
have	O
verifiable	O
assurance	O
from	O
the	O
certificate	B-Architecture
authority	I-Architecture
that	O
the	O
certificate	O
is	O
presently	O
valid	O
(	O
or	O
was	O
quite	O
recently	O
)	O
,	O
but	O
no	O
longer	O
need	O
to	O
individually	O
contact	O
the	O
OCSP	B-Protocol
server	O
.	O
</s>
<s>
Overall	O
performance	O
is	O
also	O
improved	O
:	O
When	O
the	O
client	O
fetches	O
the	O
OCSP	B-Protocol
response	O
directly	O
from	O
the	O
CA	O
,	O
it	O
usually	O
involves	O
the	O
lookup	O
of	O
the	O
domain	O
name	O
of	O
the	O
CA	O
's	O
OCSP	B-Protocol
server	O
in	O
the	O
DNS	O
as	O
well	O
as	O
establishing	O
a	O
connection	O
to	O
the	O
OCSP	B-Protocol
server	O
.	O
</s>
<s>
When	O
OCSP	B-Protocol
stapling	I-Protocol
is	O
used	O
,	O
the	O
certificate	O
status	O
information	O
is	O
delivered	O
to	O
the	O
client	O
through	O
an	O
already	O
established	O
channel	O
,	O
reducing	O
overhead	O
and	O
improving	O
performance	O
.	O
</s>
<s>
The	O
TLS	B-Protocol
Certificate	I-Protocol
Status	I-Protocol
Request	I-Protocol
extension	I-Protocol
is	O
specified	O
in	O
,	O
Section	O
8	O
.	O
</s>
<s>
defines	O
a	O
Multiple	O
Certificate	B-Protocol
Status	I-Protocol
Request	I-Protocol
extension	O
,	O
which	O
allows	O
a	O
server	O
to	O
send	O
multiple	O
OCSP	B-Protocol
responses	O
in	O
the	O
TLS	O
handshake	O
.	O
</s>
<s>
A	O
draft	O
proposal	O
for	O
an	O
X509v3	O
extension	O
field	O
,	O
which	O
expired	O
in	O
April	O
2013	O
,	O
specified	O
that	O
a	O
compliant	O
server	O
presenting	O
a	O
certificate	O
carrying	O
the	O
extension	O
must	O
return	O
a	O
valid	O
OCSP	B-Protocol
token	O
in	O
its	O
response	O
if	O
the	O
status_request	O
extension	O
is	O
specified	O
in	O
the	O
TLS	O
client	O
hello	O
.	O
</s>
<s>
TLS	O
developer	O
Adam	O
Langley	O
discussed	O
the	O
extension	O
in	O
an	O
April	O
2014	O
article	O
following	O
the	O
repair	O
of	O
the	O
Heartbleed	B-Error_Name
OpenSSL	B-Language
bug	O
.	O
</s>
<s>
OCSP	B-Protocol
stapling	I-Protocol
support	O
is	O
being	O
progressively	O
implemented	O
.	O
</s>
<s>
The	O
OpenSSL	B-Language
project	O
included	O
support	O
in	O
their	O
0.9.8g	O
release	O
with	O
the	O
assistance	O
of	O
a	O
grant	O
from	O
the	B-Operating_System
Mozilla	I-Operating_System
Foundation	I-Operating_System
.	O
</s>
<s>
Apache	B-Application
HTTP	I-Application
Server	I-Application
supports	O
OCSP	B-Protocol
stapling	I-Protocol
since	O
version	O
2.3.3	O
,	O
the	O
nginx	B-Operating_System
web	O
server	O
since	O
version	O
1.3.7	O
,	O
LiteSpeed	B-Protocol
Web	I-Protocol
Server	I-Protocol
since	O
version	O
4.2.4	O
,	O
Microsoft	O
's	O
IIS	B-Application
since	O
Windows	B-Device
Server	I-Device
2008	I-Device
,	O
HAProxy	B-Language
since	O
version	O
1.5.0	O
,	O
F5	O
Networks	O
BIG-IP	O
since	O
version	O
11.6.0	O
,	O
KEMP	O
LoadMasters	O
since	O
Version	O
7.2.37.1	O
,	O
and	O
lighttpd	B-Protocol
since	O
version	O
1.4.56	O
.	O
</s>
<s>
While	O
many	O
web	O
servers	O
advertise	O
support	O
for	O
OCSP	B-Protocol
stapling	I-Protocol
,	O
implementations	O
are	O
not	O
always	O
reliable	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
Apache	B-Application
queries	O
the	O
OCSP	B-Protocol
server	O
,	O
in	O
the	O
event	O
of	O
a	O
temporary	O
failure	O
,	O
it	O
will	O
discard	O
the	O
cached	O
good	O
response	O
from	O
the	O
previous	O
request	O
,	O
and	O
start	O
serving	O
bad	O
response	O
.	O
</s>
<s>
Nginx	B-Operating_System
performs	O
lazy	B-General_Concept
loading	I-General_Concept
of	O
OCSP	B-Protocol
responses	O
,	O
which	O
means	O
that	O
for	O
the	O
first	O
few	O
web	O
requests	O
it	O
is	O
unable	O
to	O
add	O
the	O
OCSP	B-Protocol
response	O
.	O
</s>
<s>
On	O
the	O
browser	O
side	O
,	O
OCSP	B-Protocol
stapling	I-Protocol
was	O
implemented	O
in	O
Firefox	B-Application
26	O
,	O
in	O
Internet	B-Application
Explorer	I-Application
since	O
Windows	B-Application
Vista	I-Application
,	O
and	O
Google	O
Chrome	B-Operating_System
in	O
Linux	O
,	O
ChromeOS	B-Operating_System
,	O
and	O
Windows	O
since	O
Vista	B-Application
.	O
</s>
<s>
OCSP	B-Protocol
stapling	I-Protocol
is	O
designed	O
to	O
reduce	O
the	O
cost	O
of	O
an	O
OCSP	B-Protocol
validation	O
,	O
both	O
for	O
the	O
client	O
and	O
the	O
OCSP	B-Protocol
responder	O
,	O
especially	O
for	O
large	O
sites	O
serving	O
many	O
simultaneous	O
users	O
.	O
</s>
<s>
However	O
,	O
OCSP	B-Protocol
stapling	I-Protocol
supports	O
only	O
one	O
OCSP	B-Protocol
response	O
at	O
a	O
time	O
,	O
which	O
is	O
insufficient	O
for	O
certificate	O
chains	O
with	O
intermediate	O
CA	O
certs	O
.	O
</s>
<s>
This	O
limitation	O
has	O
been	O
addressed	O
by	O
Multiple	O
Certificate	B-Protocol
Status	I-Protocol
Request	I-Protocol
Extension	O
,	O
specified	O
in	O
.	O
</s>
<s>
It	O
adds	O
the	O
support	O
for	O
sending	O
multiple	O
OCSP	B-Protocol
responses	O
.	O
</s>
<s>
TLSv1.3	O
automatically	O
removes	O
this	O
limitation	O
,	O
making	O
browser	O
support	O
for	O
moot	O
,	O
as	O
more	O
and	O
more	O
web	O
servers	O
drop	O
support	O
for	O
TLS	O
1.2	O
.	O
</s>
<s>
Under	O
TLS	O
1.2	O
only	O
one	O
stapled	O
response	O
can	O
be	O
sent	O
by	O
a	O
server	O
,	O
the	O
OCSP	B-Protocol
response	O
associated	O
with	O
the	O
end-certificate	O
.	O
</s>
<s>
Under	O
TLS	O
1.3	O
a	O
server	O
can	O
send	O
multiple	O
OCSP	B-Protocol
responses	O
,	O
typically	O
one	O
for	O
each	O
certificate	O
in	O
the	O
certificate	O
chain	O
.	O
</s>
