<s>
A	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
(	O
PKI	O
)	O
is	O
a	O
set	O
of	O
roles	O
,	O
policies	O
,	O
hardware	O
,	O
software	O
and	O
procedures	O
needed	O
to	O
create	O
,	O
manage	O
,	O
distribute	O
,	O
use	O
,	O
store	O
and	O
revoke	O
digital	O
certificates	O
and	O
manage	O
public-key	B-Application
encryption	I-Application
.	O
</s>
<s>
In	O
cryptography	O
,	O
a	O
PKI	O
is	O
an	O
arrangement	O
that	O
binds	O
public	B-Application
keys	I-Application
with	O
respective	O
identities	O
of	O
entities	O
(	O
like	O
people	O
and	O
organizations	O
)	O
.	O
</s>
<s>
The	O
binding	O
is	O
established	O
through	O
a	O
process	O
of	O
registration	O
and	O
issuance	O
of	O
certificates	O
at	O
and	O
by	O
a	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
.	O
</s>
<s>
When	O
done	O
over	O
a	O
network	O
,	O
this	O
requires	O
using	O
a	O
secure	O
certificate	O
enrollment	O
or	O
certificate	B-Protocol
management	I-Protocol
protocol	I-Protocol
such	O
as	O
CMP	B-Protocol
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
's	O
RFC	O
3647	O
defines	O
an	O
RA	O
as	O
"	O
An	O
entity	O
that	O
is	O
responsible	O
for	O
one	O
or	O
more	O
of	O
the	O
following	O
functions	O
:	O
the	O
identification	O
and	O
authentication	O
of	O
certificate	O
applicants	O
,	O
the	O
approval	O
or	O
rejection	O
of	O
certificate	O
applications	O
,	O
initiating	O
certificate	B-Protocol
revocations	I-Protocol
or	O
suspensions	O
under	O
certain	O
circumstances	O
,	O
processing	O
subscriber	O
requests	O
to	O
revoke	O
or	O
suspend	O
their	O
certificates	O
,	O
and	O
approving	O
or	O
rejecting	O
requests	O
by	O
subscribers	O
to	O
renew	O
or	O
re-key	O
their	O
certificates	O
.	O
</s>
<s>
So	O
in	O
the	O
Microsoft	O
PKI	O
case	O
,	O
the	O
RA	O
functionality	O
is	O
provided	O
either	O
by	O
the	O
Microsoft	O
Certificate	O
Services	O
web	O
site	O
or	O
through	O
Active	B-Protocol
Directory	I-Protocol
Certificate	O
Services	O
which	O
enforces	O
Microsoft	O
Enterprise	O
CA	O
and	O
certificate	B-Protocol
policy	I-Protocol
through	O
certificate	O
templates	O
and	O
manages	O
certificate	O
enrollment	O
(	O
manual	O
or	O
auto-enrollment	O
)	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
Microsoft	O
Standalone	O
CAs	O
,	O
the	O
function	O
of	O
RA	O
does	O
not	O
exist	O
since	O
all	O
of	O
the	O
procedures	O
controlling	O
the	O
CA	O
are	O
based	O
on	O
the	O
administration	O
and	O
access	O
procedure	O
associated	O
with	O
the	O
system	O
hosting	O
the	O
CA	O
and	O
the	O
CA	O
itself	O
rather	O
than	O
Active	B-Protocol
Directory	I-Protocol
.	O
</s>
<s>
A	O
third-party	O
validation	B-Protocol
authority	I-Protocol
(	O
VA	O
)	O
can	O
provide	O
this	O
entity	O
information	O
on	O
behalf	O
of	O
the	O
CA	O
.	O
</s>
<s>
The	O
X.509	O
standard	O
defines	O
the	O
most	O
commonly	O
used	O
format	O
for	O
public	B-Application
key	I-Application
certificates	O
.	O
</s>
<s>
Perhaps	O
the	O
most	O
common	O
use	O
of	O
PKI	O
for	O
confidentiality	O
purposes	O
is	O
in	O
the	O
context	O
of	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	B-Protocol
)	O
.	O
</s>
<s>
TLS	B-Protocol
is	O
a	O
capability	O
underpinning	O
the	O
security	O
of	O
data	O
in	O
transit	O
,	O
i.e.	O
</s>
<s>
A	O
classic	O
example	O
of	O
TLS	B-Protocol
for	O
confidentiality	O
is	O
when	O
using	O
an	O
internet	B-Application
browser	I-Application
to	O
log	O
on	O
to	O
a	O
service	O
hosted	O
on	O
an	O
internet	O
based	O
web	O
site	O
by	O
entering	O
a	O
password	O
.	O
</s>
<s>
The	O
latter	O
is	O
termed	O
client-side	O
authentication	O
-	O
sometimes	O
used	O
when	O
authenticating	O
using	O
a	O
smart	O
card	O
(	O
hosting	O
a	O
digital	O
certificate	O
and	O
private	B-Application
key	I-Application
)	O
.	O
</s>
<s>
Public-key	B-Application
cryptography	I-Application
is	O
a	O
cryptographic	O
technique	O
that	O
enables	O
entities	O
to	O
securely	O
communicate	O
on	O
an	O
insecure	O
public	O
network	O
,	O
and	O
reliably	O
verify	O
the	O
identity	O
of	O
an	O
entity	O
via	O
digital	O
signatures	O
.	O
</s>
<s>
A	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
(	O
PKI	O
)	O
is	O
a	O
system	O
for	O
the	O
creation	O
,	O
storage	O
,	O
and	O
distribution	O
of	O
digital	O
certificates	O
which	O
are	O
used	O
to	O
verify	O
that	O
a	O
particular	O
public	B-Application
key	I-Application
belongs	O
to	O
a	O
certain	O
entity	O
.	O
</s>
<s>
The	O
PKI	O
creates	O
digital	O
certificates	O
which	O
map	O
public	B-Application
keys	I-Application
to	O
entities	O
,	O
securely	O
stores	O
these	O
certificates	O
in	O
a	O
central	O
repository	O
and	O
revokes	O
them	O
if	O
needed	O
.	O
</s>
<s>
A	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
that	O
stores	O
,	O
issues	O
and	O
signs	O
the	O
digital	O
certificates	O
;	O
</s>
<s>
A	O
certificate	B-Protocol
policy	I-Protocol
stating	O
the	O
PKI	O
's	O
requirements	O
concerning	O
its	O
procedures	O
.	O
</s>
<s>
Broadly	O
speaking	O
,	O
there	O
have	O
traditionally	O
been	O
three	O
approaches	O
to	O
getting	O
this	O
trust	O
:	O
certificate	B-Architecture
authorities	I-Architecture
(	O
CAs	O
)	O
,	O
web	O
of	O
trust	O
(	O
WoT	O
)	O
,	O
and	O
simple	O
public-key	B-Architecture
infrastructure	I-Architecture
(	O
SPKI	O
)	O
.	O
</s>
<s>
The	O
primary	O
role	O
of	O
the	O
CA	O
is	O
to	O
digitally	O
sign	O
and	O
publish	O
the	O
public	B-Application
key	I-Application
bound	O
to	O
a	O
given	O
user	O
.	O
</s>
<s>
This	O
is	O
done	O
using	O
the	O
CA	O
's	O
own	O
private	B-Application
key	I-Application
,	O
so	O
that	O
trust	O
in	O
the	O
user	O
key	O
relies	O
on	O
one	O
's	O
trust	O
in	O
the	O
validity	O
of	O
the	O
CA	O
's	O
key	O
.	O
</s>
<s>
The	O
term	O
trusted	O
third	O
party	O
(	O
TTP	O
)	O
may	O
also	O
be	O
used	O
for	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
.	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>
Revocation	O
is	O
performed	O
by	O
the	O
issuing	O
certificate	B-Architecture
authority	I-Architecture
,	O
which	O
produces	O
a	O
cryptographically	O
authenticated	O
statement	O
of	O
revocation	O
.	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>
According	O
to	O
NetCraft	O
report	O
from	O
2015	O
,	O
the	O
industry	O
standard	O
for	O
monitoring	O
active	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	B-Protocol
)	O
certificates	O
,	O
states	O
that	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
Sectigo	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>
This	O
approach	O
involves	O
a	O
server	O
that	O
acts	O
as	O
an	O
offline	O
certificate	B-Architecture
authority	I-Architecture
within	O
a	O
single	O
sign-on	O
system	O
.	O
</s>
<s>
Starting	O
Sep	O
2020	O
,	O
TLS	B-Protocol
Certificate	O
Validity	O
reduced	O
to	O
13	O
Months	O
.	O
</s>
<s>
An	O
alternative	O
approach	O
to	O
the	O
problem	O
of	O
public	O
authentication	O
of	O
public	B-Application
key	I-Application
information	O
is	O
the	O
web-of-trust	O
scheme	O
,	O
which	O
uses	O
self-signed	O
certificates	O
and	O
third-party	O
attestations	O
of	O
those	O
certificates	O
.	O
</s>
<s>
Examples	O
of	O
implementations	O
of	O
this	O
approach	O
are	O
PGP	B-Application
(	O
Pretty	B-Application
Good	I-Application
Privacy	I-Application
)	O
and	O
GnuPG	B-Application
(	O
an	O
implementation	O
of	O
OpenPGP	O
,	O
the	O
standardized	O
specification	O
of	O
PGP	B-Application
)	O
.	O
</s>
<s>
Because	O
PGP	B-Application
and	O
implementations	O
allow	O
the	O
use	O
of	O
e-mail	O
digital	O
signatures	O
for	O
self-publication	O
of	O
public	B-Application
key	I-Application
information	O
,	O
it	O
is	O
relatively	O
easy	O
to	O
implement	O
one	O
's	O
own	O
web	O
of	O
trust	O
.	O
</s>
<s>
One	O
of	O
the	O
benefits	O
of	O
the	O
web	O
of	O
trust	O
,	O
such	O
as	O
in	O
PGP	B-Application
,	O
is	O
that	O
it	O
can	O
interoperate	O
with	O
a	O
PKI	O
CA	O
fully	O
trusted	O
by	O
all	O
parties	O
in	O
a	O
domain	O
(	O
such	O
as	O
an	O
internal	O
CA	O
in	O
a	O
company	O
)	O
that	O
is	O
willing	O
to	O
guarantee	O
certificates	O
,	O
as	O
a	O
trusted	O
introducer	O
.	O
</s>
<s>
A	O
PKI	O
is	O
only	O
as	O
valuable	O
as	O
the	O
standards	O
and	O
practices	O
that	O
control	O
the	O
issuance	O
of	O
certificates	O
and	O
including	O
PGP	B-Application
or	O
a	O
personally	O
instituted	O
web	O
of	O
trust	O
could	O
significantly	O
degrade	O
the	O
trustworthiness	O
of	O
that	O
enterprise	O
's	O
or	O
domain	O
's	O
implementation	O
of	O
PKI	O
.	O
</s>
<s>
The	O
web	O
of	O
trust	O
concept	O
was	O
first	O
put	O
forth	O
by	O
PGP	B-Application
creator	O
Phil	O
Zimmermann	O
in	O
1992	O
in	O
the	O
manual	O
for	O
PGP	B-Application
version	O
2.0	O
:	O
</s>
<s>
Another	O
alternative	O
,	O
which	O
does	O
not	O
deal	O
with	O
public	O
authentication	O
of	O
public	B-Application
key	I-Application
information	O
,	O
is	O
the	O
simple	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
(	O
SPKI	O
)	O
that	O
grew	O
out	O
of	O
three	O
independent	O
efforts	O
to	O
overcome	O
the	O
complexities	O
of	O
X.509	O
and	O
PGP	B-Application
's	O
web	O
of	O
trust	O
.	O
</s>
<s>
;	O
a	O
good	O
example	O
of	O
this	O
is	O
an	O
air-gapped	B-Architecture
network	O
in	O
an	O
office	O
.	O
</s>
<s>
Decentralized	B-Protocol
identifiers	I-Protocol
(	O
DIDs	O
)	O
eliminates	O
dependence	O
on	O
centralized	O
registries	O
for	O
identifiers	O
as	O
well	O
as	O
centralized	O
certificate	B-Architecture
authorities	I-Architecture
for	O
key	O
management	O
,	O
which	O
is	O
the	O
standard	O
in	O
hierarchical	O
PKI	O
.	O
</s>
<s>
The	O
public	O
disclosure	O
of	O
both	O
secure	O
key	O
exchange	O
and	O
asymmetric	B-Application
key	I-Application
algorithms	I-Application
in	O
1976	O
by	O
Diffie	O
,	O
Hellman	O
,	O
Rivest	O
,	O
Shamir	O
,	O
and	O
Adleman	O
changed	O
secure	O
communications	O
entirely	O
.	O
</s>
<s>
Commercial	O
reasons	O
alone	O
(	O
e.g.	O
,	O
e-commerce	O
,	O
online	O
access	O
to	O
proprietary	O
databases	O
from	O
web	B-Application
browsers	I-Application
)	O
were	O
sufficient	O
.	O
</s>
<s>
Taher	O
Elgamal	O
and	O
others	O
at	O
Netscape	O
developed	O
the	O
SSL	B-Protocol
protocol	O
( 	O
 '	O
https	B-Protocol
 '	O
in	O
Web	O
URLs	O
)	O
;	O
it	O
included	O
key	O
establishment	O
,	O
server	O
authentication	O
(	O
prior	O
to	O
v3	O
,	O
one-way	O
only	O
)	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
PKIs	O
of	O
one	O
type	O
or	O
another	O
,	O
and	O
from	O
any	O
of	O
several	O
vendors	O
,	O
have	O
many	O
uses	O
,	O
including	O
providing	O
public	B-Application
keys	I-Application
and	O
bindings	O
to	O
user	O
identities	O
which	O
are	O
used	O
for	O
:	O
</s>
<s>
Encryption	O
and/or	O
sender	O
authentication	O
of	O
e-mail	O
messages	O
(	O
e.g.	O
,	O
using	O
OpenPGP	O
or	O
S/MIME	B-Protocol
)	O
;	O
</s>
<s>
Encryption	O
and/or	O
authentication	O
of	O
documents	O
(	O
e.g.	O
,	O
the	O
XML	B-Protocol
Signature	O
or	O
XML	B-Protocol
Encryption	O
standards	O
if	O
documents	O
are	O
encoded	O
as	O
XML	B-Protocol
)	O
;	O
</s>
<s>
Authentication	O
of	O
users	O
to	O
applications	O
(	O
e.g.	O
,	O
smart	O
card	O
logon	O
,	O
client	O
authentication	O
with	O
SSL/TLS	B-Protocol
)	O
.	O
</s>
<s>
There	O
's	O
experimental	O
usage	O
for	O
digitally	O
signed	O
HTTP	B-Protocol
authentication	O
in	O
the	O
Enigform	O
and	O
mod_openpgp	O
projects	O
;	O
</s>
<s>
Bootstrapping	B-Application
secure	O
communication	O
protocols	O
,	O
such	O
as	O
Internet	B-Protocol
key	I-Protocol
exchange	I-Protocol
(	O
IKE	O
)	O
and	O
SSL/TLS	B-Protocol
.	O
</s>
<s>
In	O
both	O
of	O
these	O
,	O
initial	O
set-up	O
of	O
a	O
secure	O
channel	O
(	O
a	O
"	O
security	B-Protocol
association	I-Protocol
"	O
)	O
uses	O
asymmetric	B-Application
key	I-Application
—	O
i.e.	O
,	O
public	B-Application
key	I-Application
—	O
methods	O
,	O
whereas	O
actual	O
communication	O
uses	O
faster	O
symmetric	B-Algorithm
key	I-Algorithm
—	O
i.e.	O
,	O
secret	O
key	O
—	O
methods	O
;	O
</s>
<s>
Internet	B-Operating_System
of	I-Operating_System
things	I-Operating_System
requires	O
secure	O
communication	O
between	O
mutually	O
trusted	O
devices	O
.	O
</s>
<s>
A	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
enables	O
devices	O
to	O
obtain	O
and	O
renew	O
X.509	O
certificates	O
which	O
are	O
used	O
to	O
establish	O
trust	O
between	O
devices	O
and	O
encrypt	O
communications	O
using	O
TLS	B-Protocol
.	O
</s>
<s>
OpenSSL	B-Language
is	O
the	O
simplest	O
form	O
of	O
CA	O
and	O
tool	O
for	O
PKI	O
.	O
</s>
<s>
It	O
is	O
a	O
toolkit	O
,	O
developed	O
in	O
C	O
,	O
that	O
is	O
included	O
in	O
all	O
major	O
Linux	B-Application
distributions	O
,	O
and	O
can	O
be	O
used	O
both	O
to	O
build	O
your	O
own	O
(	O
simple	O
)	O
CA	O
and	O
to	O
PKI-enable	O
applications	O
.	O
</s>
<s>
EJBCA	B-Language
is	O
a	O
full-featured	O
,	O
enterprise-grade	O
,	O
CA	O
implementation	O
developed	O
in	O
Java	B-Language
.	O
</s>
<s>
XiPKI	O
,	O
CA	O
and	O
OCSP	B-Protocol
responder	O
.	O
</s>
<s>
With	O
SHA-3	B-Algorithm
support	O
,	O
implemented	O
in	O
Java	B-Language
.	O
</s>
<s>
XCA	O
uses	O
OpenSSL	B-Language
for	O
the	O
underlying	O
PKI	O
operations	O
.	O
</s>
<s>
CFSSL	O
open	O
source	O
toolkit	O
developed	O
by	O
CloudFlare	B-Application
for	O
signing	O
,	O
verifying	O
,	O
and	O
bundling	O
TLS	B-Protocol
certificates	O
.	O
</s>
<s>
Vault	O
tool	O
for	O
securely	O
managing	O
secrets	O
(	O
TLS	B-Protocol
certificates	O
included	O
)	O
developed	O
by	O
HashiCorp	O
.	O
</s>
<s>
Boulder	O
,	O
an	O
ACME-based	O
CA	O
written	O
in	O
Go	B-Application
.	O
</s>
<s>
Boulder	O
is	O
the	O
software	O
that	O
runs	O
Let	B-Application
's	I-Application
Encrypt	I-Application
.	O
</s>
<s>
Some	O
argue	O
that	O
purchasing	O
certificates	O
for	O
securing	O
websites	O
by	O
SSL/TLS	B-Protocol
and	O
securing	O
software	O
by	O
code	O
signing	O
is	O
a	O
costly	O
venture	O
for	O
small	O
businesses	O
.	O
</s>
<s>
However	O
,	O
the	O
emergence	O
of	O
free	O
alternatives	O
,	O
such	O
as	O
Let	B-Application
's	I-Application
Encrypt	I-Application
,	O
has	O
changed	O
this	O
.	O
</s>
<s>
HTTP/2	B-Protocol
,	O
the	O
latest	O
version	O
of	O
HTTP	B-Protocol
protocol	I-Protocol
,	O
allows	O
unsecured	O
connections	O
in	O
theory	O
;	O
in	O
practice	O
,	O
major	O
browser	B-Application
companies	O
have	O
made	O
it	O
clear	O
that	O
they	O
would	O
support	O
this	O
protocol	O
only	O
over	O
a	O
PKI	O
secured	O
TLS	B-Protocol
connection	O
.	O
</s>
<s>
Web	B-Application
browser	I-Application
implementation	O
of	O
HTTP/2	B-Protocol
including	O
Chrome	B-Application
,	O
Firefox	B-Application
,	O
Opera	B-Application
,	O
and	O
Edge	B-Application
supports	O
HTTP/2	B-Protocol
only	O
over	O
TLS	B-Protocol
by	O
using	O
the	O
ALPN	B-Protocol
extension	O
of	O
the	O
TLS	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
This	O
would	O
mean	O
that	O
,	O
to	O
get	O
the	O
speed	O
benefits	O
of	O
HTTP/2	B-Protocol
,	O
website	O
owners	O
would	O
be	O
forced	O
to	O
purchase	O
SSL/TLS	B-Protocol
certificates	O
controlled	O
by	O
corporations	O
.	O
</s>
<s>
Currently	O
the	O
majority	O
of	O
web	B-Application
browsers	I-Application
are	O
shipped	O
with	O
pre-installed	O
intermediate	O
certificates	O
issued	O
and	O
signed	O
by	O
a	O
certificate	B-Architecture
authority	I-Architecture
,	O
by	O
public	B-Application
keys	I-Application
certified	O
by	O
so-called	O
root	B-Protocol
certificates	I-Protocol
.	O
</s>
<s>
This	O
means	O
browsers	B-Application
need	O
to	O
carry	O
a	O
large	O
number	O
of	O
different	O
certificate	B-Architecture
providers	I-Architecture
,	O
increasing	O
the	O
risk	O
of	O
a	O
key	O
compromise	O
.	O
</s>
<s>
Browsers	B-Application
have	O
to	O
issue	O
a	O
security	O
patch	O
to	O
revoke	O
intermediary	O
certificates	O
issued	O
by	O
a	O
compromised	O
root	B-Protocol
certificate	I-Protocol
authority	I-Protocol
.	O
</s>
