<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
is	O
a	O
non-profit	O
certificate	B-Architecture
authority	I-Architecture
run	O
by	O
Internet	O
Security	O
Research	O
Group	O
(	O
ISRG	O
)	O
that	O
provides	O
X.509	O
certificates	O
for	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
encryption	O
at	O
no	O
charge	O
.	O
</s>
<s>
It	O
is	O
the	O
world	O
's	O
largest	O
certificate	B-Architecture
authority	I-Architecture
,	O
used	O
by	O
more	O
than	O
300	O
million	O
websites	O
,	O
with	O
the	O
goal	O
of	O
all	O
websites	O
being	O
secure	O
and	O
using	O
HTTPS	B-Protocol
.	O
</s>
<s>
Major	O
sponsors	O
include	O
the	O
Electronic	O
Frontier	O
Foundation	O
(	O
EFF	O
)	O
,	O
the	B-Operating_System
Mozilla	I-Operating_System
Foundation	I-Operating_System
,	O
OVH	B-General_Concept
,	O
Cisco	O
Systems	O
,	O
Facebook	B-Application
,	O
Google	B-Application
Chrome	I-Application
,	O
Internet	O
Society	O
,	O
AWS	B-Application
,	O
NGINX	B-Operating_System
,	O
and	O
Bill	O
and	O
Melinda	O
Gates	O
Foundation	O
.	O
</s>
<s>
Other	O
partners	O
include	O
the	O
certificate	B-Architecture
authority	I-Architecture
IdenTrust	O
,	O
the	O
University	O
of	O
Michigan	O
(	O
U-M	O
)	O
,	O
and	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
.	O
</s>
<s>
The	O
mission	O
for	O
the	O
organization	O
is	O
to	O
create	O
a	O
more	O
secure	O
and	O
privacy-respecting	O
World-Wide	O
Web	O
by	O
promoting	O
the	O
widespread	O
adoption	O
of	O
HTTPS	B-Protocol
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
certificates	O
are	O
valid	O
for	O
90	O
days	O
,	O
during	O
which	O
renewal	O
can	O
take	O
place	O
at	O
any	O
time	O
.	O
</s>
<s>
By	O
eliminating	O
payment	O
,	O
web	O
server	O
configuration	O
,	O
validation	O
email	O
management	O
and	O
certificate	O
renewal	O
tasks	O
,	O
it	O
is	O
meant	O
to	O
significantly	O
lower	O
the	O
complexity	O
of	O
setting	O
up	O
and	O
maintaining	O
TLS	B-Protocol
encryption	I-Protocol
.	O
</s>
<s>
On	O
a	O
Linux	B-Application
web	O
server	O
,	O
execution	O
of	O
only	O
two	O
commands	O
is	O
sufficient	O
to	O
set	O
up	O
HTTPS	B-Protocol
encryption	O
and	O
acquire	O
and	O
install	O
certificates	O
.	O
</s>
<s>
To	O
that	O
end	O
,	O
a	O
software	O
package	O
was	O
included	O
into	O
the	O
official	O
Debian	O
and	O
Ubuntu	B-Operating_System
software	B-Application
repositories	I-Application
.	O
</s>
<s>
Current	O
initiatives	O
of	O
major	O
browser	O
developers	O
such	O
as	O
Mozilla	B-Operating_System
and	O
Google	B-Application
to	O
deprecate	B-General_Concept
unencrypted	O
HTTP	B-Protocol
are	O
counting	O
on	O
the	O
availability	O
of	O
Let	B-Application
's	I-Application
Encrypt	I-Application
.	O
</s>
<s>
The	O
service	O
only	O
issues	O
domain-validated	B-Protocol
certificates	I-Protocol
,	O
since	O
they	O
can	O
be	O
fully	O
automated	O
.	O
</s>
<s>
Organization	O
Validation	O
and	O
Extended	B-Application
Validation	I-Application
Certificates	I-Application
both	O
require	O
human	O
validation	O
of	O
any	O
registrants	O
,	O
and	O
are	O
therefore	O
not	O
offered	O
by	O
Let	B-Application
's	I-Application
Encrypt	I-Application
.	O
</s>
<s>
Support	O
of	O
ACME	B-Protocol
v2	O
and	O
wildcard	O
certificates	O
was	O
added	O
in	O
March	O
2018	O
.	O
</s>
<s>
The	O
domain	B-Protocol
validation	I-Protocol
(	O
DV	O
)	O
utilized	O
by	O
Let	B-Application
's	I-Application
Encrypt	I-Application
dates	O
back	O
to	O
2002	O
and	O
was	O
at	O
first	O
controversial	O
when	O
introduced	O
by	O
GeoTrust	O
before	O
becoming	O
a	O
widely	O
accepted	O
method	O
for	O
the	O
issuance	O
of	O
SSL	O
certificates	O
.	O
</s>
<s>
For	O
that	O
purpose	O
it	O
regularly	O
publishes	O
transparency	O
reports	O
,	O
publicly	O
logs	O
all	O
ACME	B-Protocol
transactions	O
(	O
e.g.	O
</s>
<s>
by	O
using	O
Certificate	B-Protocol
Transparency	I-Protocol
)	O
,	O
and	O
uses	O
open	O
standards	O
and	O
free	B-Application
software	I-Application
as	O
much	O
as	O
possible	O
.	O
</s>
<s>
The	O
Let	B-Application
's	I-Application
Encrypt	I-Application
project	O
was	O
started	O
in	O
2012	O
by	O
two	O
Mozilla	B-Operating_System
employees	O
,	O
Josh	O
Aas	O
and	O
Eric	O
Rescorla	O
,	O
together	O
with	O
Peter	O
Eckersley	O
at	O
the	O
Electronic	O
Frontier	O
Foundation	O
and	O
J	O
.	O
Alex	O
Halderman	O
at	O
the	O
University	O
of	O
Michigan	O
.	O
</s>
<s>
Internet	O
Security	O
Research	O
Group	O
,	O
the	O
company	O
behind	O
Let	B-Application
's	I-Application
Encrypt	I-Application
,	O
was	O
incorporated	O
in	O
May	O
2013	O
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
was	O
announced	O
publicly	O
on	O
November	O
18	O
,	O
2014	O
.	O
</s>
<s>
On	O
January	O
28	O
,	O
2015	O
,	O
the	O
ACME	B-Protocol
protocol	I-Protocol
was	O
officially	O
submitted	O
to	O
the	O
IETF	O
for	O
standardization	O
.	O
</s>
<s>
On	O
April	O
9	O
,	O
2015	O
,	O
the	O
ISRG	O
and	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
declared	O
their	O
collaboration	O
.	O
</s>
<s>
The	O
root	O
and	O
intermediate	B-Architecture
certificates	I-Architecture
were	O
generated	O
in	O
the	O
beginning	O
of	O
June	O
.	O
</s>
<s>
On	O
September	O
14	O
,	O
2015	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
issued	O
its	O
first	O
certificate	O
,	O
which	O
was	O
for	O
the	O
domain	O
.	O
</s>
<s>
On	O
the	O
same	O
day	O
,	O
ISRG	O
submitted	O
its	O
root	O
program	O
applications	O
to	O
Mozilla	B-Operating_System
,	O
Microsoft	O
,	O
Google	B-Application
and	O
Apple	O
.	O
</s>
<s>
On	O
October	O
19	O
,	O
2015	O
,	O
the	O
intermediate	B-Architecture
certificates	I-Architecture
became	O
cross-signed	O
by	O
IdenTrust	O
,	O
causing	O
all	O
certificates	O
issued	O
by	O
Let	B-Application
's	I-Application
Encrypt	I-Application
to	O
be	O
trusted	O
by	O
all	O
major	O
browsers	O
.	O
</s>
<s>
On	O
November	O
12	O
,	O
2015	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
announced	O
that	O
general	O
availability	O
would	O
be	O
pushed	O
back	O
and	O
that	O
the	O
first	O
public	O
beta	O
would	O
commence	O
on	O
December	O
3	O
,	O
2015	O
.	O
</s>
<s>
On	O
March	O
3	O
,	O
2020	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
announced	O
that	O
it	O
would	O
have	O
to	O
revoke	B-Protocol
over	O
3	O
million	O
certificates	O
on	O
March	O
4	O
,	O
due	O
to	O
a	O
flaw	O
in	O
its	O
Certificate	B-Architecture
Authority	I-Architecture
software	I-Architecture
.	O
</s>
<s>
Through	O
working	O
with	O
software	O
vendors	O
and	O
contacting	O
site	O
operators	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
was	O
able	O
to	O
get	O
1.7	O
million	O
of	O
the	O
affected	O
certificates	O
renewed	O
before	O
the	O
deadline	O
.	O
</s>
<s>
They	O
ultimately	O
decided	O
not	O
to	O
revoke	B-Protocol
the	O
remaining	O
affected	O
certificates	O
,	O
as	O
the	O
security	O
risk	O
was	O
low	O
and	O
the	O
certificates	O
were	O
to	O
expire	O
within	O
the	O
next	O
90	O
days	O
.	O
</s>
<s>
In	O
March	O
2020	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
was	O
awarded	O
the	B-Operating_System
Free	I-Operating_System
Software	I-Operating_System
Foundation	I-Operating_System
's	O
annual	O
Award	O
for	O
Projects	O
of	O
Social	O
Benefit	O
.	O
</s>
<s>
On	O
February	O
27	O
,	O
2020	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
announced	O
having	O
issued	O
a	O
billion	O
certificates	O
.	O
</s>
<s>
As	O
of	O
September	O
2022	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
reports	O
having	O
issued	O
234	O
million	O
active	O
(	O
unexpired	O
)	O
certificates	O
.	O
</s>
<s>
In	O
June	O
2015	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
announced	O
the	O
generation	O
of	O
their	O
first	O
RSA	B-Architecture
root	O
certificate	O
,	O
ISRG	O
Root	O
X1	O
.	O
</s>
<s>
The	O
root	O
certificate	O
was	O
used	O
to	O
sign	O
two	O
intermediate	B-Architecture
certificates	I-Architecture
,	O
which	O
are	O
also	O
cross-signed	O
by	O
the	O
certificate	B-Architecture
authority	I-Architecture
IdenTrust	O
.	O
</s>
<s>
One	O
of	O
the	O
intermediate	B-Architecture
certificates	I-Architecture
is	O
used	O
to	O
sign	O
issued	O
certificates	O
,	O
while	O
the	O
other	O
is	O
kept	O
offline	O
as	O
a	O
backup	O
in	O
case	O
of	O
problems	O
with	O
the	O
first	O
intermediate	O
certificate	O
.	O
</s>
<s>
Because	O
the	O
IdenTrust	O
certificate	O
was	O
already	O
widely	O
trusted	O
by	O
major	O
web	O
browsers	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
certificates	O
can	O
normally	O
be	O
validated	O
and	O
accepted	O
by	O
relying	O
parties	O
even	O
before	O
browser	O
vendors	O
include	O
the	O
ISRG	O
root	O
certificate	O
as	O
a	O
trust	B-General_Concept
anchor	I-General_Concept
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
developers	O
planned	O
to	O
generate	O
an	O
ECDSA	O
root	O
key	O
back	O
in	O
2015	O
,	O
but	O
then	O
pushed	O
back	O
the	O
plan	O
to	O
early	O
2016	O
,	O
then	O
to	O
2019	O
,	O
and	O
finally	O
to	O
2020	O
.	O
</s>
<s>
On	O
September	O
3	O
,	O
2020	O
,	O
Let	B-Application
’s	I-Application
Encrypt	I-Application
issued	O
six	O
new	O
certificates	O
:	O
one	O
new	O
ECDSA	O
root	O
named	O
"	O
ISRG	O
Root	O
X2	O
"	O
,	O
four	O
intermediates	O
,	O
and	O
one	O
cross-sign	O
.	O
</s>
<s>
The	O
new	O
ISRG	O
Root	O
X2	O
is	O
cross-signed	O
with	O
ISRG	O
Root	O
X1	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
's	O
own	O
root	O
certificate	O
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
did	O
not	O
issue	O
an	O
OCSP	O
responder	O
for	O
the	O
new	O
intermediate	B-Architecture
certificates	I-Architecture
and	O
instead	O
plans	O
to	O
rely	O
solely	O
on	O
certificate	B-Protocol
revocation	I-Protocol
lists	I-Protocol
(	O
CRLs	O
)	O
to	O
recall	O
compromised	O
certificates	O
and	O
short	O
validity	O
periods	O
to	O
reduce	O
danger	O
of	O
certificate	O
compromise	O
.	O
</s>
<s>
The	O
challenge	O
–	O
response	O
protocol	O
used	O
to	O
automate	O
enrolling	O
with	O
the	O
certificate	B-Architecture
authority	I-Architecture
is	O
called	O
Automated	B-Protocol
Certificate	I-Protocol
Management	I-Protocol
Environment	I-Protocol
(	O
ACME	B-Protocol
)	O
.	O
</s>
<s>
It	O
can	O
query	O
either	O
Web	O
servers	O
or	O
DNS	B-Protocol
servers	O
controlled	O
by	O
the	O
domain	O
covered	O
by	O
the	O
certificate	O
to	O
be	O
issued	O
.	O
</s>
<s>
Based	O
on	O
whether	O
the	O
resulting	O
responses	O
match	O
the	O
expectations	O
,	O
control	O
of	O
the	O
enrollee	O
over	O
the	O
domain	O
is	O
assured	O
(	O
domain	B-Protocol
validation	I-Protocol
)	O
.	O
</s>
<s>
The	O
ACME	B-Protocol
client	O
software	O
can	O
set	O
up	O
a	O
dedicated	O
TLS	O
server	O
that	O
gets	O
queried	O
by	O
the	O
ACME	B-Protocol
certificate	B-Architecture
authority	I-Architecture
server	O
with	O
requests	O
using	O
Server	B-Protocol
Name	I-Protocol
Indication	I-Protocol
(	O
Domain	B-Protocol
Validation	I-Protocol
using	O
Server	B-Protocol
Name	I-Protocol
Indication	I-Protocol
,	O
DVSNI	O
)	O
,	O
or	O
it	O
can	O
use	O
hooks	O
to	O
publish	O
responses	O
to	O
existing	O
Web	O
and	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
Checking	O
DNS	B-Protocol
entries	O
are	O
provisioned	O
is	O
done	O
from	O
multiple	O
geographically	O
diverse	O
locations	O
to	O
make	O
DNS	B-Protocol
spoofing	O
attacks	O
harder	O
to	O
do	O
.	O
</s>
<s>
ACME	B-Protocol
interactions	O
are	O
based	O
on	O
exchanging	O
JSON	B-General_Concept
documents	O
over	O
HTTPS	B-Protocol
connections	O
.	O
</s>
<s>
A	O
draft	O
specification	O
is	O
available	O
on	O
GitHub	B-Application
,	O
and	O
a	O
version	O
has	O
been	O
submitted	O
to	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
as	O
a	O
proposal	O
for	O
an	O
Internet	O
standard	O
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
implemented	O
its	O
own	O
draft	O
of	O
the	O
ACME	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
This	O
led	O
to	O
a	O
"	O
proposed	O
standard	O
"	O
(	O
RFC8555	B-Protocol
)	O
in	O
May	O
2019	O
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
implemented	O
the	O
new	O
version	O
and	O
started	O
pushing	O
existing	O
clients	O
into	O
upgrades	O
.	O
</s>
<s>
The	O
nudging	O
was	O
implemented	O
with	O
intermittent	O
down-times	O
of	O
the	O
ACMEv1	O
API	B-Application
.	O
</s>
<s>
Since	O
June	O
2020	O
,	O
ACMEv1	O
stopped	O
accepting	O
new	O
domain	B-Protocol
validations	I-Protocol
.	O
</s>
<s>
The	O
ACMEv1	O
API	B-Application
was	O
turned	O
off	O
completely	O
on	O
June	O
1	O
,	O
2021	O
.	O
</s>
<s>
The	O
certificate	B-Architecture
authority	I-Architecture
consists	O
of	O
a	O
piece	O
of	O
software	O
called	O
Boulder	O
,	O
written	O
in	O
Go	B-Application
,	O
that	O
implements	O
the	O
server	O
side	O
of	O
the	O
ACME	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
It	O
is	O
published	O
as	O
free	B-Application
software	I-Application
with	O
source	O
code	O
under	O
the	O
terms	O
of	O
version	O
2	O
of	O
the	O
Mozilla	B-License
Public	I-License
License	I-License
(	O
MPL	O
)	O
.	O
</s>
<s>
It	O
provides	O
a	O
RESTful	B-Protocol
API	I-Protocol
that	O
can	O
be	O
accessed	O
over	O
a	O
TLS-encrypted	O
channel	O
.	O
</s>
<s>
An	O
Apache-licensed	B-Application
Python	B-Language
certificate	O
management	O
program	O
called	O
certbot	O
(	O
formerly	O
letsencrypt	B-Application
)	O
gets	O
installed	O
on	O
the	O
client	O
side	O
(	O
the	O
Web	O
server	O
of	O
an	O
enrollee	O
)	O
.	O
</s>
<s>
This	O
is	O
used	O
to	O
order	O
the	O
certificate	O
,	O
to	O
conduct	O
the	O
domain	B-Protocol
validation	I-Protocol
process	O
,	O
to	O
install	O
the	O
certificate	O
,	O
to	O
configure	O
the	O
HTTPS	B-Protocol
encryption	O
in	O
the	O
HTTP	B-Protocol
server	O
,	O
and	O
later	O
to	O
regularly	O
renew	O
the	O
certificate	O
.	O
</s>
<s>
Additional	O
options	O
like	O
OCSP	B-Protocol
stapling	I-Protocol
or	O
HTTP	B-Protocol
Strict	I-Protocol
Transport	I-Protocol
Security	I-Protocol
(	O
HSTS	O
)	O
can	O
also	O
be	O
enabled	O
.	O
</s>
<s>
Automatic	O
setup	O
initially	O
only	O
works	O
with	O
Apache	B-Application
and	O
nginx	B-Operating_System
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
issues	O
certificates	O
valid	O
for	O
90	O
days	O
.	O
</s>
<s>
Initially	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
developed	O
its	O
own	O
ACME	B-Protocol
client	O
Certbot	O
as	O
an	O
official	O
implementation	O
.	O
</s>
<s>
This	O
has	O
been	O
transferred	O
to	O
Electronic	O
Frontier	O
Foundation	O
and	O
its	O
name	O
"	O
letsencrypt	B-Application
"	O
has	O
been	O
changed	O
to	O
"	O
certbot	O
"	O
.	O
</s>
<s>
There	O
is	O
a	O
large	O
selection	O
of	O
ACME	B-Protocol
clients	O
and	O
projects	O
for	O
a	O
number	O
of	O
environments	O
developed	O
by	O
the	O
community	O
.	O
</s>
