<s>
Delegated	B-Protocol
credential	I-Protocol
is	O
a	O
short-lived	O
TLS	B-Protocol
certificate	O
used	O
to	O
improve	O
security	O
by	O
faster	O
recovery	O
from	O
private	B-Application
key	I-Application
leakage	O
,	O
without	O
increasing	O
the	O
latency	B-Protocol
of	O
the	O
TLS	B-Protocol
handshake	O
.	O
</s>
<s>
It	O
is	O
currently	O
an	O
IETF	O
Internet	O
Draft	O
,	O
and	O
has	O
been	O
in	O
use	O
by	O
Cloudflare	B-Application
and	O
Facebook	B-Application
,	O
with	O
browser	O
support	O
by	O
Firefox	B-Application
.	O
</s>
<s>
Modern	O
websites	O
and	O
other	O
services	O
use	O
Content	B-Operating_System
Delivery	I-Operating_System
Networks	I-Operating_System
(	O
CDNs	O
)	O
,	O
which	O
are	O
servers	O
potentially	O
distributed	O
all	O
over	O
the	O
world	O
,	O
in	O
order	O
to	O
respond	O
to	O
a	O
user	O
's	O
request	O
as	O
fast	O
as	O
possible	O
,	O
alongside	O
other	O
services	O
that	O
CDNs	O
provide	O
such	O
as	O
DDoS	O
mitigation	O
.	O
</s>
<s>
However	O
,	O
in	O
order	O
to	O
establish	O
a	O
secure	B-Protocol
connection	O
,	O
the	O
server	O
is	O
required	O
to	O
prove	O
possession	O
of	O
a	O
private	B-Application
key	I-Application
associated	O
with	O
a	O
certificate	O
,	O
which	O
serves	O
as	O
a	O
chain	O
of	O
trust	O
linking	O
the	O
public	B-Application
key	I-Application
and	O
a	O
trusted	O
party	O
.	O
</s>
<s>
The	O
trusted	O
party	O
is	O
normally	O
a	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
.	O
</s>
<s>
Knowledge	O
of	O
a	O
private	B-Application
key	I-Application
associated	O
to	O
a	O
valid	O
certificate	O
is	O
devastating	O
for	O
the	O
site	O
's	O
security	O
,	O
as	O
it	O
allows	O
Man-in-the-middle	O
attacks	O
,	O
in	O
which	O
a	O
malicious	O
entity	O
can	O
impersonate	O
to	O
a	O
user	O
as	O
a	O
legitimate	O
server	O
.	O
</s>
<s>
Therefore	O
,	O
these	O
private	B-Application
keys	I-Application
should	O
be	O
kept	O
secure	B-Protocol
,	O
preferably	O
not	O
distributed	O
over	O
every	O
server	O
in	O
the	O
CDN	O
.	O
</s>
<s>
Specifically	O
,	O
if	O
a	O
private	B-Application
key	I-Application
is	O
compromised	O
,	O
the	O
corresponding	O
certificate	O
should	O
optimally	O
be	O
revoked	B-Protocol
,	O
such	O
that	O
browsers	O
will	O
no	O
longer	O
support	O
this	O
certificate	O
.	O
</s>
<s>
Certificate	B-Protocol
revocation	I-Protocol
has	O
two	O
main	O
drawbacks	O
.	O
</s>
<s>
A	O
delegated	B-Protocol
credential	I-Protocol
is	O
a	O
short-lived	O
key	O
(	O
from	O
a	O
few	O
hours	O
to	O
a	O
few	O
days	O
)	O
that	O
the	O
certificate	O
's	O
owner	O
delegates	O
to	O
the	O
server	O
for	O
use	O
in	O
TLS	B-Protocol
.	O
</s>
<s>
It	O
is	O
in	O
fact	O
a	O
signature	B-General_Concept
:	O
the	O
certificate	O
's	O
owner	O
uses	O
the	O
certificate	O
's	O
private	B-Application
key	I-Application
to	O
sign	O
a	O
delegated	O
public	B-Application
key	I-Application
,	O
and	O
an	O
expiration	O
time	O
.	O
</s>
<s>
Given	O
this	O
delegated	B-Protocol
credential	I-Protocol
,	O
a	O
browser	O
can	O
(	O
if	O
it	O
supports	O
it	O
)	O
verify	O
the	O
server	O
's	O
authenticity	O
by	O
verifying	O
the	O
delegated	O
certificate	O
and	O
then	O
verify	O
the	O
certificate	O
itself	O
.	O
</s>
<s>
Delegated	B-Protocol
credentials	I-Protocol
are	O
short-lived	O
,	O
so	O
private	B-Application
keys	I-Application
can	O
be	O
changed	O
frequently	O
.	O
</s>
<s>
Therefore	O
,	O
even	O
if	O
a	O
particular	O
delegated	B-Protocol
credential	I-Protocol
is	O
compromised	O
without	O
certificate	O
owner	O
knowing	O
it	O
,	O
that	O
compromised	O
credential	O
could	O
only	O
be	O
abused	O
for	O
a	O
very	O
short	O
time	O
.	O
</s>
<s>
The	O
certificate	O
owner	O
does	O
not	O
need	O
to	O
expose	O
its	O
private	B-Application
key	I-Application
to	O
the	O
CDN	O
servers	O
,	O
but	O
rather	O
periodically	O
supply	O
them	O
with	O
delegated	B-Protocol
credentials	I-Protocol
.	O
</s>
<s>
Therefore	O
,	O
certificate	O
owner	O
can	O
maintain	O
exclusive	O
control	O
over	O
its	O
private	B-Application
key	I-Application
(	O
and	O
even	O
store	O
it	O
in	O
HSM	O
)	O
while	O
using	O
third-party	O
CDN	O
servers	O
.	O
</s>
<s>
The	O
delegated	B-Protocol
credential	I-Protocol
has	O
its	O
own	O
key	O
,	O
so	O
it	O
can	O
experiment	O
with	O
new	O
algorithms	O
such	O
as	O
Ed25519	O
.	O
</s>
<s>
This	O
can	O
facilitate	O
cryptographic	B-Algorithm
agililty	I-Algorithm
.	O
</s>
