<s>
In	O
cryptography	O
,	O
a	O
certificate	B-Architecture
authority	I-Architecture
or	O
certification	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
is	O
an	O
entity	O
that	O
stores	O
,	O
signs	O
,	O
and	O
issues	O
digital	O
certificates	O
.	O
</s>
<s>
A	O
digital	O
certificate	O
certifies	O
the	O
ownership	O
of	O
a	O
public	B-Application
key	I-Application
by	O
the	O
named	O
subject	O
of	O
the	O
certificate	O
.	O
</s>
<s>
This	O
allows	O
others	O
(	O
relying	O
parties	O
)	O
to	O
rely	O
upon	O
signatures	O
or	O
on	O
assertions	O
made	O
about	O
the	O
private	B-Application
key	I-Application
that	O
corresponds	O
to	O
the	O
certified	O
public	B-Application
key	I-Application
.	O
</s>
<s>
One	O
particularly	O
common	O
use	O
for	O
certificate	B-Architecture
authorities	I-Architecture
is	O
to	O
sign	O
certificates	O
used	O
in	O
HTTPS	B-Protocol
,	O
the	O
secure	O
browsing	O
protocol	O
for	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
Trusted	O
certificates	O
can	O
be	O
used	O
to	O
create	O
secure	B-Protocol
connections	I-Protocol
to	O
a	O
server	O
via	O
the	O
Internet	O
.	O
</s>
<s>
The	O
client	O
uses	O
the	O
CA	B-Architecture
certificate	I-Architecture
to	O
authenticate	O
the	O
CA	O
signature	O
on	O
the	O
server	O
certificate	O
,	O
as	O
part	O
of	O
the	O
authorizations	O
before	O
launching	O
a	O
secure	O
connection	O
.	O
</s>
<s>
Usually	O
,	O
client	O
software	O
—	O
for	O
example	O
,	O
browsers	B-Application
—	O
include	O
a	O
set	O
of	O
trusted	O
CA	B-Architecture
certificates	I-Architecture
.	O
</s>
<s>
Commercial	O
CAs	O
charge	O
money	O
to	O
issue	O
certificates	O
,	O
and	O
their	O
customers	O
anticipate	O
the	O
CA	O
's	O
certificate	O
to	O
be	O
contained	O
within	O
the	O
majority	O
of	O
web	B-Application
browsers	I-Application
,	O
so	O
that	O
safe	O
connections	O
to	O
the	O
certified	O
servers	O
work	O
efficiently	O
out-of-the-box	O
.	O
</s>
<s>
The	O
quantity	O
of	O
internet	B-Application
browsers	I-Application
,	O
other	O
devices	O
and	O
applications	O
which	O
trust	O
a	O
particular	O
certificate	B-Architecture
authority	I-Architecture
is	O
referred	O
to	O
as	O
ubiquity	O
.	O
</s>
<s>
Mozilla	B-Operating_System
,	O
which	O
is	O
a	O
non-profit	O
business	O
,	O
issues	O
several	O
commercial	O
CA	B-Architecture
certificates	I-Architecture
with	O
its	O
products	O
.	O
</s>
<s>
While	O
Mozilla	B-Operating_System
developed	O
their	O
own	O
policy	O
,	O
the	O
CA/Browser	O
Forum	O
developed	O
similar	O
guidelines	O
for	O
CA	O
trust	O
.	O
</s>
<s>
A	O
single	O
CA	B-Architecture
certificate	I-Architecture
may	O
be	O
shared	O
among	O
multiple	O
CAs	O
or	O
their	O
resellers	O
.	O
</s>
<s>
A	O
root	B-Protocol
CA	I-Protocol
certificate	O
may	O
be	O
the	O
base	O
to	O
issue	O
multiple	O
intermediate	O
CA	B-Architecture
certificates	I-Architecture
with	O
varying	O
validation	O
requirements	O
.	O
</s>
<s>
In	O
addition	O
to	O
commercial	O
CAs	O
,	O
some	O
non-profits	O
issue	O
publicly-trusted	O
digital	O
certificates	O
without	O
charge	O
,	O
for	O
example	O
Let	B-Application
's	I-Application
Encrypt	I-Application
.	O
</s>
<s>
Some	O
large	O
cloud	O
computing	O
and	O
web	O
hosting	O
companies	O
are	O
also	O
publicly-trusted	O
CAs	O
and	O
issue	O
certificates	O
to	O
services	O
hosted	O
on	O
their	O
infrastructure	O
,	O
for	O
example	O
IBM	B-General_Concept
Cloud	I-General_Concept
,	O
Amazon	B-Application
Web	I-Application
Services	I-Application
,	O
Cloudflare	B-Application
,	O
and	O
Google	B-General_Concept
Cloud	I-General_Concept
Platform	I-General_Concept
.	O
</s>
<s>
Large	O
organizations	O
or	O
government	O
bodies	O
may	O
have	O
their	O
own	O
PKIs	O
(	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
)	O
,	O
each	O
containing	O
their	O
own	O
CAs	O
.	O
</s>
<s>
Any	O
site	O
using	O
self-signed	B-Protocol
certificates	I-Protocol
acts	O
as	O
its	O
own	O
CA	O
.	O
</s>
<s>
Commercial	O
banks	O
that	O
issue	O
EMV	O
payment	O
cards	O
are	O
governed	O
by	O
the	O
EMV	O
Certificate	B-Architecture
Authority	I-Architecture
,	O
payment	O
schemes	O
that	O
route	O
payment	O
transactions	O
initiated	O
at	O
Point	O
of	O
Sale	O
Terminals	O
(	O
POS	O
)	O
to	O
a	O
Card	O
Issuing	O
Bank	O
to	O
transfer	O
the	O
funds	O
from	O
the	O
card	O
holder	O
's	O
bank	O
account	O
to	O
the	O
payment	O
recipient	O
's	O
bank	O
account	O
.	O
</s>
<s>
The	O
Issuer	O
Certificate	O
is	O
signed	O
by	O
EMV	O
CA	B-Architecture
Certificate	I-Architecture
.	O
</s>
<s>
The	O
POS	O
retrieves	O
the	O
public	B-Application
key	I-Application
of	O
EMV	O
CA	O
from	O
its	O
storage	O
,	O
validates	O
the	O
Issuer	O
Certificate	O
and	O
authenticity	O
of	O
the	O
payment	O
card	O
before	O
sending	O
the	O
payment	O
request	O
to	O
the	O
payment	O
scheme	O
.	O
</s>
<s>
Browsers	B-Application
and	O
other	O
clients	O
of	O
sorts	O
characteristically	O
allow	O
users	O
to	O
add	O
or	O
do	O
away	O
with	O
CA	B-Architecture
certificates	I-Architecture
at	O
will	O
.	O
</s>
<s>
While	O
server	O
certificates	O
regularly	O
last	O
for	O
a	O
relatively	O
short	O
period	O
,	O
CA	B-Architecture
certificates	I-Architecture
are	O
further	O
extended	O
,	O
so	O
,	O
for	O
repeatedly	O
visited	O
servers	O
,	O
it	O
is	O
less	O
error-prone	O
importing	O
and	O
trusting	O
the	O
CA	O
issued	O
,	O
rather	O
than	O
confirm	O
a	O
security	O
exemption	O
each	O
time	O
the	O
server	O
's	O
certificate	O
is	O
renewed	O
.	O
</s>
<s>
CAs	O
dispense	O
end-user	O
certificates	O
too	O
,	O
which	O
can	O
be	O
used	O
with	O
S/MIME	B-Protocol
.	O
</s>
<s>
However	O
,	O
encryption	O
entails	O
the	O
receiver	O
's	O
public	B-Application
key	I-Application
and	O
,	O
since	O
authors	O
and	O
receivers	O
of	O
encrypted	O
messages	O
,	O
apparently	O
,	O
know	O
one	O
another	O
,	O
the	O
usefulness	O
of	O
a	O
trusted	O
third	O
party	O
remains	O
confined	O
to	O
the	O
signature	O
verification	O
of	O
messages	O
sent	O
to	O
public	O
mailing	O
lists	O
.	O
</s>
<s>
Worldwide	O
,	O
the	O
certificate	B-Architecture
authority	I-Architecture
business	O
is	O
fragmented	O
,	O
with	O
national	O
or	O
regional	O
providers	O
dominating	O
their	O
home	O
market	O
.	O
</s>
<s>
This	O
is	O
because	O
many	O
uses	O
of	O
digital	O
certificates	O
,	O
such	O
as	O
for	O
legally	O
binding	O
digital	O
signatures	O
,	O
are	O
linked	O
to	O
local	O
law	O
,	O
regulations	O
,	O
and	O
accreditation	O
schemes	O
for	O
certificate	B-Architecture
authorities	I-Architecture
.	O
</s>
<s>
However	O
,	O
the	O
market	O
for	O
globally	O
trusted	O
TLS/SSL	B-Protocol
server	O
certificates	O
is	O
largely	O
held	O
by	O
a	O
small	O
number	O
of	O
multinational	O
companies	O
.	O
</s>
<s>
While	O
not	O
legally	O
required	O
,	O
new	O
providers	O
may	O
choose	O
to	O
undergo	O
annual	O
security	O
audits	O
(	O
such	O
as	O
WebTrust	O
for	O
certificate	B-Architecture
authorities	I-Architecture
in	O
North	O
America	O
and	O
ETSI	O
in	O
Europe	O
)	O
to	O
be	O
included	O
as	O
a	O
trusted	O
root	O
by	O
a	O
web	B-Application
browser	I-Application
or	O
operating	O
system	O
.	O
</s>
<s>
,	O
147	O
root	B-Protocol
certificates	I-Protocol
,	O
representing	O
52	O
organizations	O
,	O
are	O
trusted	O
in	O
the	O
Mozilla	B-Application
Firefox	I-Application
web	B-Application
browser	I-Application
,	O
168	O
root	B-Protocol
certificates	I-Protocol
,	O
representing	O
60	O
organizations	O
,	O
are	O
trusted	O
by	O
macOS	B-Application
,	O
and	O
255	O
root	B-Protocol
certificates	I-Protocol
,	O
representing	O
101	O
organizations	O
,	O
are	O
trusted	O
by	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
On	O
November	O
18	O
,	O
2014	O
,	O
a	O
group	O
of	O
companies	O
and	O
nonprofit	O
organizations	O
,	O
including	O
the	O
Electronic	O
Frontier	O
Foundation	O
,	O
Mozilla	B-Operating_System
,	O
Cisco	O
,	O
and	O
Akamai	O
,	O
announced	O
Let	B-Application
's	I-Application
Encrypt	I-Application
,	O
a	O
nonprofit	O
certificate	B-Architecture
authority	I-Architecture
that	O
provides	O
free	O
domain	O
validated	O
X.509	O
certificates	O
as	O
well	O
as	O
software	O
to	O
enable	O
installation	O
and	O
maintenance	O
of	O
certificates	O
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
is	O
operated	O
by	O
the	O
newly	O
formed	O
Internet	O
Security	O
Research	O
Group	O
,	O
a	O
California	O
nonprofit	O
recognized	O
as	O
federally	O
tax-exempt	O
.	O
</s>
<s>
According	O
to	O
Netcraft	O
in	O
May	O
2015	O
,	O
the	O
industry	O
standard	O
for	O
monitoring	O
active	O
TLS	O
certificates	O
,	O
"	O
Although	O
the	O
global	O
 [ TLS ] 	O
ecosystem	O
is	O
competitive	O
,	O
it	O
is	O
dominated	O
by	O
a	O
handful	O
of	O
major	O
CAs	O
—	O
three	O
certificate	B-Architecture
authorities	I-Architecture
(	O
Symantec	O
,	O
Comodo	O
,	O
GoDaddy	O
)	O
account	O
for	O
three-quarters	O
of	O
all	O
issued	O
 [ TLS ] 	O
certificates	O
on	O
public-facing	O
web	O
servers	O
.	O
</s>
<s>
The	O
top	O
spot	O
has	O
been	O
held	O
by	O
Symantec	O
(	O
or	O
VeriSign	B-Application
before	O
it	O
was	O
purchased	O
by	O
Symantec	O
)	O
ever	O
since	O
 [ our ] 	O
survey	O
began	O
,	O
with	O
it	O
currently	O
accounting	O
for	O
just	O
under	O
a	O
third	O
of	O
all	O
certificates	O
.	O
</s>
<s>
In	O
2020	O
,	O
according	O
to	O
independent	O
survey	O
company	O
Netcraft	O
,	O
"	O
DigiCert	O
is	O
the	O
world	O
's	O
largest	O
high-assurance	O
certificate	B-Architecture
authority	I-Architecture
,	O
commanding	O
60%	O
of	O
the	O
Extended	B-Application
Validation	I-Application
Certificate	I-Application
market	O
,	O
and	O
96%	O
of	O
organization-validated	O
certificates	O
globally	O
.	O
</s>
<s>
the	O
survey	O
company	O
W3Techs	O
,	O
which	O
collects	O
statistics	O
on	O
certificate	B-Architecture
authority	I-Architecture
usage	O
among	O
the	O
Alexa	O
top	O
10	O
million	O
and	O
the	O
Tranco	O
top	O
1	O
million	O
websites	O
,	O
lists	O
the	O
six	O
largest	O
authorities	O
by	O
absolute	O
usage	O
share	O
as	O
below	O
.	O
</s>
<s>
The	O
commercial	O
CAs	O
that	O
issue	O
the	O
bulk	O
of	O
certificates	O
for	O
HTTPS	B-Protocol
servers	O
typically	O
use	O
a	O
technique	O
called	O
"	O
domain	B-Protocol
validation	I-Protocol
"	O
to	O
authenticate	O
the	O
recipient	O
of	O
the	O
certificate	O
.	O
</s>
<s>
The	O
techniques	O
used	O
for	O
domain	B-Protocol
validation	I-Protocol
vary	O
between	O
CAs	O
,	O
but	O
in	O
general	O
domain	B-Protocol
validation	I-Protocol
techniques	O
are	O
meant	O
to	O
prove	O
that	O
the	O
certificate	O
applicant	O
controls	O
a	O
given	O
domain	O
name	O
,	O
not	O
any	O
information	O
about	O
the	O
applicant	O
's	O
identity	O
.	O
</s>
<s>
Many	O
Certificate	B-Architecture
Authorities	I-Architecture
also	O
offer	O
Extended	B-Application
Validation	I-Application
(	O
EV	O
)	O
certificates	O
as	O
a	O
more	O
rigorous	O
alternative	O
to	O
domain	B-Protocol
validated	I-Protocol
certificates	I-Protocol
.	O
</s>
<s>
Extended	B-Application
validation	I-Application
is	O
intended	O
to	O
verify	O
not	O
only	O
control	O
of	O
a	O
domain	O
name	O
,	O
but	O
additional	O
identity	O
information	O
to	O
be	O
included	O
in	O
the	O
certificate	O
.	O
</s>
<s>
Some	O
browsers	B-Application
display	O
this	O
additional	O
identity	O
information	O
in	O
a	O
green	O
box	O
in	O
the	O
URL	O
bar	O
.	O
</s>
<s>
One	O
limitation	O
of	O
EV	O
as	O
a	O
solution	O
to	O
the	O
weaknesses	O
of	O
domain	B-Protocol
validation	I-Protocol
is	O
that	O
attackers	O
could	O
still	O
obtain	O
a	O
domain	B-Protocol
validated	I-Protocol
certificate	I-Protocol
for	O
the	O
victim	O
domain	O
,	O
and	O
deploy	O
it	O
during	O
an	O
attack	O
;	O
if	O
that	O
occurred	O
,	O
the	O
difference	O
observable	O
to	O
the	O
victim	O
user	O
would	O
be	O
the	O
absence	O
of	O
a	O
green	O
bar	O
with	O
the	O
company	O
name	O
.	O
</s>
<s>
There	O
is	O
some	O
question	O
as	O
to	O
whether	O
users	O
would	O
be	O
likely	O
to	O
recognise	O
this	O
absence	O
as	O
indicative	O
of	O
an	O
attack	O
being	O
in	O
progress	O
:	O
a	O
test	O
using	O
Internet	B-Application
Explorer	I-Application
7	I-Application
in	O
2009	O
showed	O
that	O
the	O
absence	O
of	O
IE7	B-Application
's	O
EV	O
warnings	O
were	O
not	O
noticed	O
by	O
users	O
,	O
however	O
Microsoft	O
's	O
current	O
browser	B-Application
,	O
Edge	B-Application
,	O
shows	O
a	O
significantly	O
greater	O
difference	O
between	O
EV	O
and	O
domain	B-Protocol
validated	I-Protocol
certificates	I-Protocol
,	O
with	O
domain	B-Protocol
validated	I-Protocol
certificates	I-Protocol
having	O
a	O
hollow	O
,	O
grey	O
lock	O
.	O
</s>
<s>
Domain	B-Protocol
validation	I-Protocol
suffers	O
from	O
certain	O
structural	O
security	O
limitations	O
.	O
</s>
<s>
In	O
particular	O
,	O
it	O
is	O
always	O
vulnerable	O
to	O
attacks	O
that	O
allow	O
an	O
adversary	O
to	O
observe	O
the	O
domain	B-Protocol
validation	I-Protocol
probes	O
that	O
CAs	O
send	O
.	O
</s>
<s>
These	O
can	O
include	O
attacks	O
against	O
the	O
DNS	O
,	O
TCP	O
,	O
or	O
BGP	O
protocols	O
(	O
which	O
lack	O
the	O
cryptographic	O
protections	O
of	O
TLS/SSL	B-Protocol
)	O
,	O
or	O
the	O
compromise	O
of	O
routers	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
common	O
domain	B-Protocol
validation	I-Protocol
techniques	O
involves	O
sending	O
an	O
email	O
containing	O
an	O
authentication	O
token	O
or	O
link	O
to	O
an	O
email	O
address	O
that	O
is	O
likely	O
to	O
be	O
administratively	O
responsible	O
for	O
the	O
domain	O
.	O
</s>
<s>
This	O
could	O
be	O
the	O
technical	O
contact	O
email	O
address	O
listed	O
in	O
the	O
domain	O
's	O
WHOIS	B-Protocol
entry	O
,	O
or	O
an	O
administrative	O
email	O
like	O
,	O
,	O
,	O
or	O
the	O
domain	O
.	O
</s>
<s>
Some	O
Certificate	B-Architecture
Authorities	I-Architecture
may	O
accept	O
confirmation	O
using	O
,	O
,	O
or	O
in	O
the	O
domain	O
.	O
</s>
<s>
The	O
theory	O
behind	O
domain	B-Protocol
validation	I-Protocol
is	O
that	O
only	O
the	O
legitimate	O
owner	O
of	O
a	O
domain	O
would	O
be	O
able	O
to	O
read	O
emails	O
sent	O
to	O
these	O
administrative	O
addresses	O
.	O
</s>
<s>
Domain	B-Protocol
validation	I-Protocol
implementations	O
have	O
sometimes	O
been	O
a	O
source	O
of	O
security	O
vulnerabilities	O
.	O
</s>
<s>
Prior	O
to	O
2011	O
,	O
there	O
was	O
no	O
standard	O
list	O
of	O
email	O
addresses	O
that	O
could	O
be	O
used	O
for	O
domain	B-Protocol
validation	I-Protocol
,	O
so	O
it	O
was	O
not	O
clear	O
to	O
email	O
administrators	O
which	O
addresses	O
needed	O
to	O
be	O
reserved	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
the	O
CA/Browser	O
Forum	O
Baseline	O
Requirements	O
,	O
adopted	O
November	O
2011	O
,	O
specified	O
a	O
list	O
of	O
such	O
addresses	O
.	O
</s>
<s>
In	O
January	O
2015	O
,	O
a	O
Finnish	O
man	O
registered	O
the	O
username	O
"	O
hostmaster	O
"	O
at	O
the	O
Finnish	O
version	O
of	O
Microsoft	O
Live	O
and	O
was	O
able	O
to	O
obtain	O
a	O
domain-validated	B-Protocol
certificate	I-Protocol
for	O
live.fi,	O
despite	O
not	O
being	O
the	O
owner	O
of	O
the	O
domain	O
name	O
.	O
</s>
<s>
A	O
CA	O
issues	O
digital	O
certificates	O
that	O
contain	O
a	O
public	B-Application
key	I-Application
and	O
the	O
identity	O
of	O
the	O
owner	O
.	O
</s>
<s>
The	O
matching	O
private	B-Application
key	I-Application
is	O
not	O
made	O
available	O
publicly	O
,	O
but	O
kept	O
secret	O
by	O
the	O
end	O
user	O
who	O
generated	O
the	O
key	B-Application
pair	I-Application
.	O
</s>
<s>
The	O
certificate	O
is	O
also	O
a	O
confirmation	O
or	O
validation	O
by	O
the	O
CA	O
that	O
the	O
public	B-Application
key	I-Application
contained	O
in	O
the	O
certificate	O
belongs	O
to	O
the	O
person	O
,	O
organization	O
,	O
server	O
or	O
other	O
entity	O
noted	O
in	O
the	O
certificate	O
.	O
</s>
<s>
In	O
essence	O
,	O
the	O
certificate	B-Architecture
authority	I-Architecture
is	O
responsible	O
for	O
saying	O
"	O
yes	O
,	O
this	O
person	O
is	O
who	O
they	O
say	O
they	O
are	O
,	O
and	O
we	O
,	O
the	O
CA	O
,	O
certify	O
that	O
"	O
.	O
</s>
<s>
If	O
the	O
user	O
trusts	O
the	O
CA	O
and	O
can	O
verify	O
the	O
CA	O
's	O
signature	O
,	O
then	O
they	O
can	O
also	O
assume	O
that	O
a	O
certain	O
public	B-Application
key	I-Application
does	O
indeed	O
belong	O
to	O
whoever	O
is	O
identified	O
in	O
the	O
certificate	O
.	O
</s>
<s>
Public-key	B-Application
cryptography	I-Application
can	O
be	O
used	O
to	O
encrypt	O
data	O
communicated	O
between	O
two	O
parties	O
.	O
</s>
<s>
This	O
can	O
typically	O
happen	O
when	O
a	O
user	O
logs	O
on	O
to	O
any	O
site	O
that	O
implements	O
the	O
HTTP	B-Protocol
Secure	I-Protocol
protocol	O
.	O
</s>
<s>
When	O
the	O
user	O
opens	O
www.bank.example	O
homepage	O
,	O
they	O
receive	O
a	O
public	B-Application
key	I-Application
along	O
with	O
all	O
the	O
data	O
that	O
their	O
web-browser	B-Application
displays	O
.	O
</s>
<s>
The	O
public	B-Application
key	I-Application
could	O
be	O
used	O
to	O
encrypt	O
data	O
from	O
the	O
client	O
to	O
the	O
server	O
but	O
the	O
safe	O
procedure	O
is	O
to	O
use	O
it	O
in	O
a	O
protocol	O
that	O
determines	O
a	O
temporary	O
shared	O
symmetric	O
encryption	O
key	O
;	O
messages	O
in	O
such	O
a	O
key	O
exchange	O
protocol	O
can	O
be	O
enciphered	O
with	O
the	O
bank	O
's	O
public	B-Application
key	I-Application
in	O
such	O
a	O
way	O
that	O
only	O
the	O
bank	O
server	O
has	O
the	O
private	B-Application
key	I-Application
to	O
read	O
them	O
.	O
</s>
<s>
The	O
rest	O
of	O
the	O
communication	O
then	O
proceeds	O
using	O
the	O
new	O
(	O
disposable	O
)	O
symmetric	O
key	O
,	O
so	O
when	O
the	O
user	O
enters	O
some	O
information	O
to	O
the	O
bank	O
's	O
page	O
and	O
submits	O
the	O
page	O
(	O
sends	O
the	O
information	O
back	O
to	O
the	O
bank	O
)	O
then	O
the	O
data	O
the	O
user	O
has	O
entered	O
to	O
the	O
page	O
will	O
be	O
encrypted	O
by	O
their	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
This	O
mechanism	O
is	O
only	O
safe	O
if	O
the	O
user	O
can	O
be	O
sure	O
that	O
it	O
is	O
the	O
bank	O
that	O
they	O
see	O
in	O
their	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
If	O
the	O
user	O
types	O
in	O
www.bank.example	O
,	O
but	O
their	O
communication	O
is	O
hijacked	O
and	O
a	O
fake	O
website	O
(	O
that	O
pretends	O
to	O
be	O
the	O
bank	O
website	O
)	O
sends	O
the	O
page	O
information	O
back	O
to	O
the	O
user	O
's	O
browser	B-Application
,	O
the	O
fake	O
web-page	O
can	O
send	O
a	O
fake	O
public	B-Application
key	I-Application
to	O
the	O
user	O
(	O
for	O
which	O
the	O
fake	O
site	O
owns	O
a	O
matching	O
private	B-Application
key	I-Application
)	O
.	O
</s>
<s>
This	O
is	O
what	O
the	O
certificate	B-Architecture
authority	I-Architecture
mechanism	O
is	O
intended	O
to	O
prevent	O
.	O
</s>
<s>
A	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
is	O
an	O
organization	O
that	O
stores	O
public	B-Application
keys	I-Application
and	O
their	O
owners	O
,	O
and	O
every	O
party	O
in	O
a	O
communication	O
trusts	O
this	O
organization	O
(	O
and	O
knows	O
its	O
public	B-Application
key	I-Application
)	O
.	O
</s>
<s>
When	O
the	O
user	O
's	O
web	B-Application
browser	I-Application
receives	O
the	O
public	B-Application
key	I-Application
from	O
www.bank.example	O
it	O
also	O
receives	O
a	O
digital	O
signature	O
of	O
the	O
key	O
(	O
with	O
some	O
more	O
information	O
,	O
in	O
a	O
so-called	O
X.509	O
certificate	O
)	O
.	O
</s>
<s>
The	O
browser	B-Application
already	O
possesses	O
the	O
public	B-Application
key	I-Application
of	O
the	O
CA	O
and	O
consequently	O
can	O
verify	O
the	O
signature	O
,	O
trust	O
the	O
certificate	O
and	O
the	O
public	B-Application
key	I-Application
in	O
it	O
:	O
since	O
www.bank.example	O
uses	O
a	O
public	B-Application
key	I-Application
that	O
the	O
certification	B-Architecture
authority	I-Architecture
certifies	O
,	O
a	O
fake	O
www.bank.example	O
can	O
only	O
use	O
the	O
same	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Since	O
the	O
fake	O
www.bank.example	O
does	O
not	O
know	O
the	O
corresponding	O
private	B-Application
key	I-Application
,	O
it	O
cannot	O
create	O
the	O
signature	O
needed	O
to	O
verify	O
its	O
authenticity	O
.	O
</s>
<s>
In	O
some	O
enterprise	O
systems	O
,	O
local	O
forms	O
of	O
authentication	O
such	O
as	O
Kerberos	B-Protocol
can	O
be	O
used	O
to	O
obtain	O
a	O
certificate	O
which	O
can	O
in	O
turn	O
be	O
used	O
by	O
external	O
relying	O
parties	O
.	O
</s>
<s>
To	O
minimize	O
this	O
hazard	O
,	O
the	O
certificate	B-Protocol
transparency	I-Protocol
initiative	O
proposes	O
auditing	O
all	O
certificates	O
in	O
a	O
public	O
unforgeable	O
log	O
,	O
which	O
could	O
help	O
in	O
the	O
prevention	O
of	O
phishing	O
.	O
</s>
<s>
In	O
large-scale	O
deployments	O
,	O
Alice	O
may	O
not	O
be	O
familiar	O
with	O
Bob	O
's	O
certificate	B-Architecture
authority	I-Architecture
(	O
perhaps	O
they	O
each	O
have	O
a	O
different	O
CA	O
server	O
)	O
,	O
so	O
Bob	O
's	O
certificate	O
may	O
also	O
include	O
his	O
CA	O
's	O
public	B-Application
key	I-Application
signed	O
by	O
a	O
different	O
CA2	O
,	O
which	O
is	O
presumably	O
recognizable	O
by	O
Alice	O
.	O
</s>
<s>
This	O
process	O
typically	O
leads	O
to	O
a	O
hierarchy	O
or	O
mesh	O
of	O
CAs	O
and	O
CA	B-Architecture
certificates	I-Architecture
.	O
</s>
<s>
Hence	O
,	O
revocation	O
is	O
an	O
important	O
part	O
of	O
a	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
.	O
</s>
<s>
Due	O
to	O
the	O
cost	O
of	O
revocation	O
checks	O
and	O
the	O
availability	O
impact	O
from	O
potentially-unreliable	O
remote	O
services	O
,	O
Web	B-Application
browsers	I-Application
limit	O
the	O
revocation	O
checks	O
they	O
will	O
perform	O
,	O
and	O
will	O
fail-soft	O
where	O
they	O
do	O
.	O
</s>
<s>
Certificate	B-Protocol
revocation	I-Protocol
lists	I-Protocol
are	O
too	O
bandwidth-costly	O
for	O
routine	O
use	O
,	O
and	O
the	O
Online	B-Protocol
Certificate	I-Protocol
Status	I-Protocol
Protocol	I-Protocol
presents	O
connection	O
latency	O
and	O
privacy	O
issues	O
.	O
</s>
<s>
Certificate	B-Architecture
Authority	I-Architecture
Security	O
Council	O
(	O
CASC	O
)	O
–	O
In	O
February	O
2013	O
,	O
the	O
CASC	O
was	O
founded	O
as	O
an	O
industry	O
advocacy	O
organization	O
dedicated	O
to	O
addressing	O
industry	O
issues	O
and	O
educating	O
the	O
public	O
on	O
internet	O
security	O
.	O
</s>
<s>
The	O
founding	O
members	O
are	O
the	O
seven	O
largest	O
Certificate	B-Architecture
Authorities	I-Architecture
.	O
</s>
<s>
CA/Browser	O
Forum	O
–	O
In	O
2005	O
,	O
a	O
new	O
consortium	O
of	O
Certificate	B-Architecture
Authorities	I-Architecture
and	O
web	B-Application
browser	I-Application
vendors	O
was	O
formed	O
to	O
promote	O
industry	O
standards	O
and	O
baseline	O
requirements	O
for	O
internet	O
security	O
.	O
</s>
<s>
Comodo	O
Group	O
CEO	O
Melih	O
Abdulhayoğlu	O
organized	O
the	O
first	O
meeting	O
and	O
is	O
considered	O
the	O
founder	O
of	O
the	O
CA/Browser	O
Forum	O
.	O
</s>
<s>
The	O
CA/Browser	O
Forum	O
publishes	O
the	O
Baseline	O
Requirements	O
,	O
a	O
list	O
of	O
policies	O
and	O
technical	O
requirements	O
for	O
CAs	O
to	O
follow	O
.	O
</s>
<s>
These	O
are	O
a	O
requirement	O
for	O
inclusion	O
in	O
the	O
certificate	O
stores	O
of	O
Firefox	B-Application
and	O
Safari	O
.	O
</s>
<s>
Eve	O
,	O
however	O
,	O
would	O
have	O
the	O
all-important	O
private	B-Application
key	I-Application
associated	O
with	O
the	O
certificate	O
.	O
</s>
<s>
Bob	O
might	O
even	O
respond	O
with	O
encrypted	O
email	O
,	O
believing	O
that	O
it	O
could	O
only	O
be	O
read	O
by	O
Alice	O
,	O
when	O
Eve	O
is	O
actually	O
able	O
to	O
decrypt	O
it	O
using	O
the	O
private	B-Application
key	I-Application
.	O
</s>
<s>
A	O
notable	O
case	O
of	O
CA	O
subversion	O
like	O
this	O
occurred	O
in	O
2001	O
,	O
when	O
the	O
certificate	B-Architecture
authority	I-Architecture
VeriSign	B-Application
issued	O
two	O
certificates	O
to	O
a	O
person	O
claiming	O
to	O
represent	O
Microsoft	O
.	O
</s>
<s>
Microsoft	O
and	O
VeriSign	B-Application
took	O
steps	O
to	O
limit	O
the	O
impact	O
of	O
the	O
problem	O
.	O
</s>
<s>
In	O
2008	O
,	O
Comodo	O
reseller	O
Certstar	O
sold	O
a	O
certificate	O
for	O
mozilla.com	O
to	O
Eddy	O
Nigg	O
,	O
who	O
had	O
no	O
authority	O
to	O
represent	O
Mozilla	B-Operating_System
.	O
</s>
<s>
In	O
2012	O
,	O
it	O
became	O
known	O
that	O
Trustwave	O
issued	O
a	O
subordinate	O
root	B-Protocol
certificate	I-Protocol
that	O
was	O
used	O
for	O
transparent	O
traffic	O
management	O
(	O
man-in-the-middle	O
)	O
which	O
effectively	O
permitted	O
an	O
enterprise	O
to	O
sniff	O
SSL	O
internal	O
network	O
traffic	O
using	O
the	O
subordinate	O
certificate	O
.	O
</s>
<s>
An	O
attacker	O
who	O
steals	O
a	O
certificate	B-Architecture
authority	I-Architecture
's	O
private	B-Application
keys	I-Application
is	O
able	O
to	O
forge	O
certificates	O
as	O
if	O
they	O
were	O
CA	O
,	O
without	O
needed	O
ongoing	O
access	O
to	O
the	O
CA	O
's	O
systems	O
.	O
</s>
<s>
Key	O
theft	O
is	O
therefore	O
one	O
of	O
the	O
main	O
risks	O
certificate	B-Architecture
authorities	I-Architecture
defend	O
against	O
.	O
</s>
<s>
CAs	O
typically	O
take	O
the	O
further	O
precaution	O
of	O
keeping	O
the	O
key	O
for	O
their	O
long-term	O
root	B-Protocol
certificates	I-Protocol
in	O
an	O
HSM	O
that	O
is	O
kept	O
offline	O
,	O
except	O
when	O
it	O
is	O
needed	O
to	O
sign	O
shorter-lived	O
intermediate	O
certificates	O
.	O
</s>
<s>
This	O
is	O
a	O
serious	O
shortcoming	O
given	O
that	O
the	O
most	O
commonly	O
encountered	O
technology	O
employing	O
X.509	O
and	O
trusted	O
third	O
parties	O
is	O
the	O
HTTPS	B-Protocol
protocol	O
.	O
</s>
<s>
As	O
all	O
major	O
web	B-Application
browsers	I-Application
are	O
distributed	O
to	O
their	O
end-users	O
pre-configured	O
with	O
a	O
list	O
of	O
trusted	O
CAs	O
that	O
numbers	O
in	O
the	O
dozens	O
this	O
means	O
that	O
any	O
one	O
of	O
these	O
pre-approved	O
trusted	O
CAs	O
can	O
issue	O
a	O
valid	O
certificate	O
for	O
any	O
domain	O
whatsoever	O
.	O
</s>
<s>
Given	O
that	O
the	O
contents	O
of	O
a	O
browser	B-Application
's	O
pre-configured	O
trusted	O
CA	O
list	O
is	O
determined	O
independently	O
by	O
the	O
party	O
that	O
is	O
distributing	O
or	O
causing	O
to	O
be	O
installed	O
the	O
browser	B-Application
application	O
there	O
is	O
really	O
nothing	O
that	O
the	O
CAs	O
themselves	O
can	O
do	O
.	O
</s>
<s>
This	O
issue	O
is	O
the	O
driving	O
impetus	O
behind	O
the	O
development	O
of	O
the	O
DNS-based	B-Protocol
Authentication	I-Protocol
of	I-Protocol
Named	I-Protocol
Entities	I-Protocol
(	O
DANE	O
)	O
protocol	O
.	O
</s>
<s>
If	O
adopted	O
in	O
conjunction	O
with	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
Security	I-Protocol
Extensions	I-Protocol
(	O
DNSSEC	B-Protocol
)	O
DANE	O
will	O
greatly	O
reduce	O
if	O
not	O
eliminate	O
the	O
role	O
of	O
trusted	O
third	O
parties	O
in	O
a	O
domain	O
's	O
PKI	O
.	O
</s>
