<s>
The	O
Online	B-Protocol
Certificate	I-Protocol
Status	I-Protocol
Protocol	I-Protocol
(	O
OCSP	B-Protocol
)	O
is	O
an	O
Internet	O
protocol	O
used	O
for	O
obtaining	O
the	O
revocation	O
status	O
of	O
an	O
X.509	O
digital	O
certificate	O
.	O
</s>
<s>
It	O
was	O
created	O
as	O
an	O
alternative	O
to	O
certificate	B-Protocol
revocation	I-Protocol
lists	I-Protocol
(	O
CRL	O
)	O
,	O
specifically	O
addressing	O
certain	O
problems	O
associated	O
with	O
using	O
CRLs	O
in	O
a	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
(	O
PKI	O
)	O
.	O
</s>
<s>
Messages	O
communicated	O
via	O
OCSP	B-Protocol
are	O
encoded	O
in	O
ASN.1	B-Protocol
and	O
are	O
usually	O
communicated	O
over	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
The	O
"	O
request/response	O
"	O
nature	O
of	O
these	O
messages	O
leads	O
to	O
OCSP	B-Protocol
servers	B-Application
being	O
termed	O
OCSP	B-Protocol
responders	O
.	O
</s>
<s>
Some	O
web	B-Application
browsers	I-Application
(	O
Firefox	B-Application
)	O
use	O
OCSP	B-Protocol
to	O
validate	O
HTTPS	B-Protocol
certificates	O
,	O
while	O
others	O
have	O
disabled	O
it	O
.	O
</s>
<s>
Most	O
OCSP	B-Protocol
revocation	O
statuses	O
on	O
the	O
Internet	O
disappear	O
soon	O
after	O
certificate	O
expiration	O
.	O
</s>
<s>
Since	O
an	O
OCSP	B-Protocol
response	O
contains	O
less	O
data	O
than	O
a	O
typical	O
certificate	B-Protocol
revocation	I-Protocol
list	I-Protocol
(	O
CRL	O
)	O
,	O
it	O
puts	O
less	O
burden	O
on	O
network	B-Architecture
and	O
client	B-Protocol
resources	O
.	O
</s>
<s>
Since	O
an	O
OCSP	B-Protocol
response	O
has	O
less	O
data	O
to	O
parse	B-Language
,	O
the	O
client-side	O
libraries	B-Library
that	O
handle	O
it	O
can	O
be	O
less	O
complex	O
than	O
those	O
that	O
handle	O
CRLs	O
.	O
</s>
<s>
OCSP	B-Protocol
discloses	O
to	O
the	O
responder	O
that	O
a	O
particular	O
network	B-Architecture
host	O
used	O
a	O
particular	O
certificate	O
at	O
a	O
particular	O
time	O
.	O
</s>
<s>
OCSP	B-Protocol
does	O
not	O
mandate	O
encryption	O
,	O
so	O
other	O
parties	O
may	O
intercept	O
this	O
information	O
.	O
</s>
<s>
Alice	O
and	O
Bob	O
have	O
public	O
key	O
certificates	O
issued	O
by	O
Carol	O
,	O
the	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
.	O
</s>
<s>
Bob	O
,	O
concerned	O
that	O
Alice	O
's	O
private	O
key	O
may	O
have	O
been	O
compromised	O
,	O
creates	O
an	O
'	O
OCSP	B-Protocol
request	O
 '	O
that	O
contains	O
Alice	O
's	O
certificate	O
serial	O
number	O
and	O
sends	O
it	O
to	O
Carol	O
.	O
</s>
<s>
Carol	O
's	O
OCSP	B-Protocol
responder	O
reads	O
the	O
certificate	O
serial	O
number	O
from	O
Bob	O
's	O
request	O
.	O
</s>
<s>
The	O
OCSP	B-Protocol
responder	O
uses	O
the	O
certificate	O
serial	O
number	O
to	O
look	O
up	O
the	O
revocation	O
status	O
of	O
Alice	O
's	O
certificate	O
.	O
</s>
<s>
The	O
OCSP	B-Protocol
responder	O
looks	O
in	O
a	O
CA	O
database	O
that	O
Carol	O
maintains	O
.	O
</s>
<s>
Carol	O
's	O
OCSP	B-Protocol
responder	O
confirms	O
that	O
Alice	O
's	O
certificate	O
is	O
still	O
OK	O
,	O
and	O
returns	O
a	O
signed	O
,	O
successful	O
'	O
OCSP	B-Protocol
response	O
 '	O
to	O
Bob	O
.	O
</s>
<s>
An	O
OCSP	B-Protocol
responder	O
(	O
a	O
server	B-Application
typically	O
run	O
by	O
the	O
certificate	O
issuer	O
)	O
may	O
return	O
a	O
signed	O
response	O
signifying	O
that	O
the	O
certificate	O
specified	O
in	O
the	O
request	O
is	O
'	O
good	O
 '	O
,	O
'	O
revoked	O
 '	O
,	O
or	O
'	O
unknown	O
 '	O
.	O
</s>
<s>
The	O
OCSP	B-Protocol
request	O
format	O
supports	O
additional	O
extensions	O
.	O
</s>
<s>
OCSP	B-Protocol
can	O
be	O
vulnerable	O
to	O
replay	O
attacks	O
,	O
where	O
a	O
signed	O
,	O
'	O
good	O
 '	O
response	O
is	O
captured	O
by	O
a	O
malicious	O
intermediary	O
and	O
replayed	O
to	O
the	O
client	B-Protocol
at	O
a	O
later	O
date	O
after	O
the	O
subject	O
certificate	O
may	O
have	O
been	O
revoked	O
.	O
</s>
<s>
OCSP	B-Protocol
allows	O
a	O
nonce	O
to	O
be	O
included	O
in	O
the	O
request	O
that	O
may	O
be	O
included	O
in	O
the	O
corresponding	O
response	O
.	O
</s>
<s>
Because	O
of	O
high	O
load	O
,	O
most	O
OCSP	B-Protocol
responders	O
do	O
not	O
use	O
the	O
nonce	O
extension	O
to	O
create	O
a	O
different	O
response	O
for	O
each	O
request	O
,	O
instead	O
using	O
presigned	O
responses	O
with	O
a	O
validity	O
period	O
of	O
multiple	O
days	O
.	O
</s>
<s>
OCSP	B-Protocol
can	O
support	O
more	O
than	O
one	O
level	O
of	O
CA	O
.	O
</s>
<s>
OCSP	B-Protocol
requests	O
may	O
be	O
chained	O
between	O
peer	O
responders	O
to	O
query	O
the	O
issuing	O
CA	O
appropriate	O
for	O
the	O
subject	O
certificate	O
,	O
with	O
responders	O
validating	O
each	O
other	O
's	O
responses	O
against	O
the	O
root	O
CA	O
using	O
their	O
own	O
OCSP	B-Protocol
requests	O
.	O
</s>
<s>
An	O
OCSP	B-Protocol
responder	O
may	O
be	O
queried	O
for	O
revocation	O
information	O
by	O
delegated	B-Protocol
path	I-Protocol
validation	I-Protocol
(	O
DPV	O
)	O
servers	B-Application
.	O
</s>
<s>
OCSP	B-Protocol
does	O
not	O
,	O
by	O
itself	O
,	O
perform	O
any	O
DPV	O
of	O
supplied	O
certificates	O
.	O
</s>
<s>
The	O
certificate	O
's	O
issuer	O
may	O
delegate	O
another	O
authority	O
to	O
be	O
the	O
OCSP	B-Protocol
responder	O
.	O
</s>
<s>
OCSP	B-Protocol
checking	O
creates	O
a	O
privacy	O
concern	O
for	O
some	O
users	O
,	O
since	O
it	O
requires	O
the	O
client	B-Protocol
to	O
contact	O
a	O
third	O
party	O
(	O
albeit	O
a	O
party	O
trusted	O
by	O
the	O
client	B-Protocol
software	I-Protocol
vendor	O
)	O
to	O
confirm	O
certificate	O
validity	O
.	O
</s>
<s>
OCSP	B-Protocol
stapling	I-Protocol
is	O
a	O
way	O
to	O
verify	O
validity	O
without	O
disclosing	O
browsing	O
behavior	O
to	O
the	O
CA	O
.	O
</s>
<s>
OCSP-based	O
revocation	O
is	O
not	O
an	O
effective	O
technique	O
to	O
mitigate	O
against	O
the	O
compromise	O
of	O
an	O
HTTPS	B-Protocol
server	B-Application
's	O
private	O
key	O
.	O
</s>
<s>
An	O
attacker	O
who	O
has	O
compromised	O
a	O
server	B-Application
's	O
private	O
key	O
typically	O
needs	O
to	O
be	O
in	O
a	O
man-in-the-middle	O
position	O
on	O
the	O
network	B-Architecture
to	O
abuse	O
that	O
private	O
key	O
and	O
impersonate	O
a	O
server	B-Application
.	O
</s>
<s>
An	O
attacker	O
in	O
such	O
a	O
position	O
is	O
also	O
typically	O
in	O
a	O
position	O
to	O
interfere	O
with	O
the	O
client	B-Protocol
's	O
OCSP	B-Protocol
queries	O
.	O
</s>
<s>
Because	O
most	O
clients	O
will	O
silently	O
ignore	O
OCSP	B-Protocol
if	O
the	O
query	O
times	O
out	O
,	O
OCSP	B-Protocol
is	O
not	O
a	O
reliable	O
means	O
of	O
mitigating	O
HTTPS	B-Protocol
server	B-Application
key	O
compromise	O
.	O
</s>
<s>
The	O
MustStaple	O
TLS	O
extension	O
in	O
a	O
certificate	O
can	O
require	O
that	O
the	O
certificate	O
be	O
verified	O
by	O
a	O
stapled	B-Protocol
OCSP	I-Protocol
response	O
,	O
mitigating	O
this	O
problem	O
.	O
</s>
<s>
OCSP	B-Protocol
also	O
remains	O
a	O
valid	O
defense	O
against	O
situations	O
where	O
the	O
attacker	O
is	O
not	O
a	O
"	O
man-in-the-middle	O
"	O
(	O
code-signing	O
or	O
certificates	O
issued	O
in	O
error	O
)	O
.	O
</s>
<s>
The	O
OCSP	B-Protocol
protocol	O
assumes	O
the	O
requester	O
has	O
network	B-Architecture
access	O
to	O
connect	O
to	O
an	O
appropriate	O
OCSP	B-Protocol
responder	O
.	O
</s>
<s>
Some	O
requesters	O
may	O
not	O
be	O
able	O
to	O
connect	O
because	O
their	O
local	O
network	B-Architecture
prohibits	O
direct	O
Internet	O
access	O
(	O
a	O
common	O
practice	O
for	O
internal	O
nodes	O
in	O
a	O
data	O
center	O
)	O
.	O
</s>
<s>
Forcing	O
internal	O
servers	B-Application
to	O
connect	O
to	O
the	O
Internet	O
in	O
order	O
to	O
use	O
OCSP	B-Protocol
contributes	O
to	O
the	O
de-perimeterisation	O
trend	O
.	O
</s>
<s>
The	O
OCSP	B-Protocol
stapling	I-Protocol
protocol	O
is	O
an	O
alternative	O
that	O
allows	O
servers	B-Application
to	O
cache	O
OCSP	B-Protocol
responses	O
,	O
which	O
removes	O
the	O
need	O
for	O
the	O
requestor	O
to	O
directly	O
contact	O
the	O
OCSP	B-Protocol
responder	O
.	O
</s>
<s>
There	O
is	O
wide	O
support	O
for	O
OCSP	B-Protocol
amongst	O
most	O
major	O
browsers	B-Application
:	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
is	O
built	O
on	O
the	O
CryptoAPI	O
of	O
Windows	B-Application
and	O
thus	O
starting	O
with	O
version	B-Application
7	I-Application
on	O
Windows	B-Application
Vista	I-Application
(	O
not	O
XP	B-Application
)	O
supports	O
OCSP	B-Protocol
checking	O
.	O
</s>
<s>
All	O
versions	O
of	O
Mozilla	B-Application
Firefox	I-Application
support	O
OCSP	B-Protocol
checking	O
.	O
</s>
<s>
Firefox	B-Language
3	I-Language
enables	O
OCSP	B-Protocol
checking	O
by	O
default	O
.	O
</s>
<s>
Safari	B-Application
on	O
macOS	O
supports	O
OCSP	B-Protocol
checking	O
.	O
</s>
<s>
Versions	O
of	O
Opera	B-Application
from	O
8.0	O
to	O
the	O
current	O
version	O
support	O
OCSP	B-Protocol
checking	O
.	O
</s>
<s>
However	O
,	O
Google	B-Application
Chrome	I-Application
is	O
an	O
outlier	O
.	O
</s>
<s>
Google	O
disabled	O
OCSP	B-Protocol
checks	O
by	O
default	O
in	O
2012	O
,	O
citing	O
latency	O
and	O
privacy	O
issues	O
and	O
instead	O
uses	O
their	O
own	O
update	O
mechanism	O
to	O
send	O
revoked	O
certificates	O
to	O
the	O
browser	B-Application
.	O
</s>
<s>
Several	O
open	O
source	O
and	O
proprietary	B-Application
OCSP	B-Protocol
implementations	O
exist	O
,	O
including	O
fully	O
featured	O
servers	B-Application
and	O
libraries	B-Library
for	O
building	O
custom	O
applications	O
.	O
</s>
<s>
OCSP	B-Protocol
client	B-Protocol
support	O
is	O
built	O
into	O
many	O
operating	B-General_Concept
systems	I-General_Concept
,	O
web	B-Application
browsers	I-Application
,	O
and	O
other	O
network	B-Architecture
software	I-Architecture
due	O
to	O
the	O
popularity	O
of	O
HTTPS	B-Protocol
and	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
DogTag	O
,	O
Open	O
source	O
certificate	B-Architecture
authority	I-Architecture
CA	O
,	O
CRL	O
and	O
OCSP	B-Protocol
responder	O
.	O
</s>
<s>
XiPKI	O
,	O
CA	O
and	O
OCSP	B-Protocol
responder	O
.	O
</s>
