<s>
Certificate	B-Protocol
Transparency	I-Protocol
(	O
CT	O
)	O
is	O
an	O
Internet	B-Application
security	I-Application
standard	O
for	O
monitoring	O
and	O
auditing	O
the	O
issuance	O
of	O
digital	O
certificates	O
.	O
</s>
<s>
The	O
standard	O
creates	O
a	O
system	O
of	O
public	O
logs	B-Application
that	O
seek	O
to	O
eventually	O
record	O
all	O
certificates	O
issued	O
by	O
publicly	O
trusted	O
certificate	B-Architecture
authorities	I-Architecture
,	O
allowing	O
efficient	O
identification	O
of	O
mistakenly	O
or	O
maliciously	O
issued	O
certificates	O
.	O
</s>
<s>
Version	O
2.0	O
of	O
the	O
Certificate	B-Protocol
Transparency	I-Protocol
mechanism	O
,	O
the	O
latest	O
,	O
is	O
described	O
in	O
the	O
experimental	O
,	O
which	O
obsoletes	O
the	O
earlier	O
version	O
1.0	O
described	O
in	O
.	O
</s>
<s>
The	O
certificate	B-Protocol
transparency	I-Protocol
system	O
consists	O
of	O
a	O
system	O
of	O
append-only	B-General_Concept
certificate	O
logs	B-Application
.	O
</s>
<s>
Logs	B-Application
are	O
operated	O
by	O
many	O
parties	O
,	O
including	O
browser	B-Application
vendors	O
and	O
certificate	B-Architecture
authorities	I-Architecture
.	O
</s>
<s>
Certificates	O
that	O
support	O
certificate	B-Protocol
transparency	I-Protocol
must	O
include	O
one	O
or	O
more	O
signed	O
certificate	O
timestamps	O
(	O
SCTs	O
)	O
,	O
which	O
is	O
a	O
promise	O
from	O
a	O
log	O
operator	O
to	O
include	O
the	O
certificate	O
in	O
their	O
log	O
within	O
a	O
maximum	O
merge	O
delay	O
(	O
MMD	O
)	O
.	O
</s>
<s>
Each	O
entry	O
in	O
a	O
log	O
references	O
the	O
hash	O
of	O
a	O
previous	O
one	O
,	O
forming	O
a	O
Merkle	B-Algorithm
tree	I-Algorithm
.	O
</s>
<s>
The	O
signed	O
tree	O
head	O
(	O
STH	O
)	O
references	O
the	O
current	O
root	O
of	O
the	O
Merkle	B-Algorithm
tree	I-Algorithm
.	O
</s>
<s>
Some	O
browsers	B-Application
require	O
TLS	O
certificates	O
to	O
have	O
proof	O
of	O
being	O
logged	O
with	O
certificate	B-Protocol
transparency	I-Protocol
,	O
either	O
through	O
SCTs	O
embedded	O
into	O
the	O
certificate	O
,	O
an	O
extension	O
during	O
the	O
TLS	O
handshake	O
,	O
or	O
through	O
OCSP	B-Protocol
:	O
</s>
<s>
Due	O
to	O
the	O
large	O
quantities	O
of	O
certificates	O
issued	O
with	O
the	O
Web	O
PKI	B-Architecture
,	O
certificate	B-Protocol
transparency	I-Protocol
logs	B-Application
can	O
grow	O
to	O
contain	O
many	O
certificates	O
.	O
</s>
<s>
This	O
large	O
quantity	O
of	O
certificates	O
can	O
cause	O
strain	O
on	O
logs	B-Application
.	O
</s>
<s>
Temporal	O
sharding	O
is	O
a	O
method	O
to	O
reduce	O
the	O
strain	O
on	O
logs	B-Application
by	O
sharding	O
a	O
log	O
into	O
multiple	O
logs	B-Application
,	O
and	O
having	O
each	O
shard	O
only	O
accept	O
precertificates/certificates	O
with	O
an	O
expiration	O
date	O
in	O
a	O
particular	O
time	O
period	O
(	O
usually	O
a	O
calendar	O
year	O
)	O
.	O
</s>
<s>
Cloudflare	B-Application
's	O
Nimbus	O
series	O
of	O
logs	B-Application
was	O
the	O
first	O
to	O
use	O
temporal	O
sharding	O
.	O
</s>
<s>
One	O
of	O
the	O
problems	O
with	O
digital	O
certificate	O
management	O
is	O
that	O
fraudulent	O
certificates	O
take	O
a	O
long	O
time	O
to	O
be	O
spotted	O
,	O
reported	O
and	O
revoked	B-Protocol
.	O
</s>
<s>
An	O
issued	O
certificate	O
not	O
logged	O
using	O
Certificate	B-Protocol
Transparency	I-Protocol
may	O
never	O
be	O
spotted	O
at	O
all	O
.	O
</s>
<s>
Certificate	B-Protocol
Transparency	I-Protocol
makes	O
it	O
possible	O
for	O
the	O
domain	O
owner	O
(	O
and	O
anyone	O
interested	O
)	O
to	O
get	O
in	O
knowledge	O
of	O
any	O
certificate	O
issued	O
for	O
a	O
domain	O
.	O
</s>
<s>
Certificate	B-Protocol
Transparency	I-Protocol
depends	O
on	O
verifiable	O
Certificate	B-Protocol
Transparency	I-Protocol
logs	B-Application
.	O
</s>
<s>
A	O
log	O
appends	O
new	O
certificates	O
to	O
an	O
ever-growing	O
Merkle	B-Algorithm
hash	I-Algorithm
tree	I-Algorithm
.	O
</s>
<s>
Verify	O
that	O
each	O
submitted	O
certificate	O
or	O
precertificate	O
has	O
a	O
valid	O
signature	O
chain	O
leading	O
back	O
to	O
a	O
trusted	O
root	O
certificate	B-Architecture
authority	I-Architecture
certificate	O
.	O
</s>
<s>
Monitors	O
check	O
logs	B-Application
to	O
make	O
sure	O
they	O
are	O
behaving	O
correctly	O
.	O
</s>
<s>
An	O
inconsistency	O
is	O
used	O
to	O
prove	O
that	O
a	O
log	O
has	O
not	O
behaved	O
correctly	O
,	O
and	O
the	O
signatures	O
on	O
the	O
log	O
's	O
data	O
structure	O
(	O
the	O
Merkle	B-Algorithm
tree	I-Algorithm
)	O
prevent	O
the	O
log	O
from	O
denying	O
that	O
misbehavior	O
.	O
</s>
<s>
Certificate	B-Protocol
Transparency	I-Protocol
auditors	O
use	O
partial	O
information	O
about	O
a	O
log	O
to	O
verify	O
the	O
log	O
against	O
other	O
partial	O
information	O
they	O
have	O
.	O
</s>
<s>
Apple	O
and	O
Google	B-Application
have	O
separate	O
log	O
programs	O
with	O
distinct	O
policies	O
and	O
lists	O
of	O
trusted	O
logs	B-Application
.	O
</s>
<s>
Certificate	B-Protocol
Transparency	I-Protocol
logs	B-Application
maintain	O
their	O
own	O
root	O
stores	O
and	O
only	O
accept	O
certificates	O
that	O
chain	O
back	O
to	O
the	O
trusted	O
roots	O
.	O
</s>
<s>
A	O
number	O
of	O
misbehaving	O
logs	B-Application
have	O
been	O
publishing	O
inconsistent	O
root	O
stores	O
in	O
the	O
past	O
.	O
</s>
<s>
In	O
2011	O
,	O
a	O
reseller	O
of	O
the	O
certificate	B-Architecture
authority	I-Architecture
Comodo	O
was	O
attacked	O
and	O
the	O
certificate	B-Architecture
authority	I-Architecture
DigiNotar	O
was	O
compromised	O
,	O
demonstrating	O
existing	O
flaws	O
in	O
the	O
certificate	B-Architecture
authority	I-Architecture
ecosystem	O
and	O
prompting	O
work	O
on	O
various	O
mechanisms	O
to	O
prevent	O
or	O
monitor	O
unauthorized	O
certificate	O
issuance	O
.	O
</s>
<s>
Google	B-Application
employees	O
Ben	O
Laurie	O
,	O
Adam	O
Langley	O
and	O
Emilia	O
Kasper	O
began	O
work	O
on	O
an	O
open	B-Application
source	I-Application
framework	B-Architecture
for	O
detecting	O
mis-issued	O
certificates	O
the	O
same	O
year	O
.	O
</s>
<s>
In	O
March	O
2013	O
,	O
Google	B-Application
launched	O
its	O
first	O
certificate	B-Protocol
transparency	I-Protocol
log	O
.	O
</s>
<s>
In	O
September	O
2013	O
,	O
DigiCert	O
became	O
the	O
first	O
certificate	B-Architecture
authority	I-Architecture
to	O
implement	O
Certificate	B-Protocol
Transparency	I-Protocol
.	O
</s>
<s>
In	O
2015	O
,	O
Google	B-Application
Chrome	I-Application
began	O
requiring	O
Certificate	B-Protocol
Transparency	I-Protocol
for	O
newly	O
issued	O
Extended	B-Application
Validation	I-Application
Certificates	I-Application
.	O
</s>
<s>
It	O
began	O
requiring	O
Certificate	B-Protocol
Transparency	I-Protocol
for	O
all	O
certificates	O
newly	O
issued	O
by	O
Symantec	O
from	O
June	O
1	O
,	O
2016	O
,	O
after	O
they	O
were	O
found	O
to	O
have	O
issued	O
187	O
certificates	O
without	O
the	O
domain	O
owners	O
 '	O
knowledge	O
.	O
</s>
<s>
On	O
March	O
23	O
,	O
2018	O
,	O
Cloudflare	B-Application
announced	O
its	O
own	O
CT	O
log	O
named	O
Nimbus	O
.	O
</s>
<s>
In	O
March	O
2018	O
,	O
"	O
Certificate	B-Protocol
Transparency	I-Protocol
Version	O
2.0	O
"	O
draft	O
was	O
published	O
.	O
</s>
<s>
In	O
May	O
,	O
2019	O
,	O
certificate	B-Architecture
authority	I-Architecture
Let	B-Application
's	I-Application
Encrypt	I-Application
launched	O
its	O
own	O
CT	O
log	O
called	O
Oak	O
.	O
</s>
<s>
Since	O
February	O
2020	O
,	O
it	O
is	O
included	O
in	O
approved	O
log	O
lists	O
and	O
is	O
usable	O
by	O
all	O
publicly-trusted	O
certificate	B-Architecture
authorities	I-Architecture
.	O
</s>
<s>
In	O
December	O
2021	O
,	O
IETF	O
RFC	O
9162	O
“	O
Certificate	B-Protocol
Transparency	I-Protocol
Version	O
2.0	O
”	O
was	O
published	O
.	O
</s>
<s>
In	O
February	O
2022	O
,	O
Google	B-Application
published	O
an	O
update	O
to	O
their	O
CT	O
policy	O
,	O
which	O
removes	O
the	O
requirement	O
for	O
certificates	O
to	O
include	O
a	O
SCT	O
from	O
their	O
own	O
CT	O
log	O
service	O
,	O
matching	O
all	O
the	O
requirements	O
for	O
certificates	O
to	O
those	O
previously	O
published	O
by	O
Apple	O
.	O
</s>
<s>
In	O
Certificate	B-Protocol
Transparency	I-Protocol
Version	O
1.0	O
,	O
a	O
log	O
must	O
use	O
NIST	O
P-256	O
curve	O
,	O
or	O
RSA	B-Architecture
signatures	O
(	O
using	O
a	O
key	O
of	O
at	O
least	O
2048	O
bits	O
)	O
.	O
</s>
<s>
In	O
the	O
updated	O
Certificate	B-Protocol
Transparency	I-Protocol
Version	O
2.0	O
standard	O
,	O
the	O
allowed	O
algorithms	O
are	O
NIST	O
P-256	O
,	O
Deterministic	O
ECDSA	O
,	O
or	O
Ed25519	O
.	O
</s>
