<s>
In	O
cryptography	O
and	O
computer	O
security	O
,	O
self-signed	B-Protocol
certificates	I-Protocol
are	O
public	O
key	O
certificates	O
that	O
are	O
not	O
issued	O
by	O
a	O
certificate	B-Architecture
authority	I-Architecture
(	O
CA	O
)	O
.	O
</s>
<s>
These	O
self-signed	B-Protocol
certificates	I-Protocol
are	O
easy	O
to	O
make	O
and	O
do	O
not	O
cost	O
money	O
.	O
</s>
<s>
For	O
instance	O
,	O
if	O
a	O
website	O
owner	O
uses	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
to	O
provide	O
HTTPS	B-Protocol
services	O
,	O
people	O
who	O
visit	O
that	O
website	O
cannot	O
be	O
certain	O
that	O
they	O
are	O
connected	O
to	O
their	O
intended	O
destination	O
.	O
</s>
<s>
For	O
all	O
they	O
know	O
,	O
a	O
malicious	O
third-party	O
could	O
be	O
redirecting	O
the	O
connection	O
using	O
another	O
self-signed	B-Protocol
certificate	I-Protocol
bearing	O
the	O
same	O
holder	O
name	O
.	O
</s>
<s>
In	O
comparison	O
,	O
a	O
certificate	O
signed	O
by	O
a	O
trusted	O
CA	O
prevents	O
this	O
attack	O
because	O
the	O
user	O
's	O
web	B-Application
browser	I-Application
separately	O
validates	O
the	O
certificate	O
against	O
the	O
issuing	O
CA	O
.	O
</s>
<s>
Self-signed	B-Protocol
certificates	I-Protocol
can	O
be	O
created	O
for	O
free	O
,	O
using	O
a	O
wide	O
variety	O
of	O
tools	O
including	O
OpenSSL	B-Language
,	O
Java	O
's	O
keytool	O
,	O
Adobe	O
Reader	O
,	O
wolfSSL	B-Protocol
and	O
Apple	O
's	O
Keychain	O
.	O
</s>
<s>
Self-signed	B-Protocol
certificate	I-Protocol
transactions	O
usually	O
present	O
a	O
far	O
smaller	O
attack	O
surface	O
by	O
eliminating	O
both	O
the	O
complex	O
certificate	O
chain	O
validation	O
,	O
and	O
CA	O
revocation	O
checks	O
like	O
CRL	B-Protocol
and	O
OCSP	B-Protocol
.	O
</s>
<s>
In	O
a	O
CA-based	O
PKI	B-Architecture
system	O
,	O
parties	O
engaged	O
in	O
secure	O
communication	O
must	O
trust	O
a	O
CA	O
,	O
i.e.	O
</s>
<s>
place	O
the	O
CA	B-Architecture
certificates	I-Architecture
in	O
a	O
whitelist	O
of	O
trusted	O
certificates	O
.	O
</s>
<s>
Developers	O
of	O
web	B-Application
browsers	I-Application
may	O
use	O
procedures	O
specified	O
by	O
the	O
CA/Browser	O
Forum	O
to	O
whitelist	O
well-known	O
,	O
public	O
certificate	B-Architecture
authorities	I-Architecture
.	O
</s>
<s>
Individual	O
groups	O
and	O
companies	O
may	O
whitelist	O
additional	O
,	O
private	O
CA	B-Architecture
certificates	I-Architecture
.	O
</s>
<s>
The	O
trust	O
issues	O
of	O
an	O
entity	O
accepting	O
a	O
new	O
self-signed	B-Protocol
certificate	I-Protocol
are	O
similar	O
to	O
the	O
issues	O
of	O
an	O
entity	O
trusting	O
the	O
addition	O
of	O
a	O
new	O
CA	B-Architecture
certificate	I-Architecture
.	O
</s>
<s>
The	O
parties	O
in	O
a	O
self-signed	O
PKI	B-Architecture
must	O
establish	O
trust	O
with	O
each	O
other	O
(	O
using	O
procedures	O
outside	O
the	O
PKI	B-Architecture
)	O
,	O
and	O
confirm	O
the	O
accurate	O
transfer	O
of	O
public	O
keys	O
e.g.	O
</s>
<s>
compare	O
the	O
certificate	O
's	O
cryptographic	B-Algorithm
hash	I-Algorithm
out	O
of	O
band	O
.	O
</s>
<s>
There	O
are	O
many	O
subtle	O
differences	O
between	O
CA	O
signed	O
and	O
self-signed	B-Protocol
certificates	I-Protocol
,	O
especially	O
in	O
the	O
amount	O
of	O
trust	O
that	O
can	O
be	O
placed	O
in	O
the	O
security	O
assertions	O
of	O
the	O
certificate	O
.	O
</s>
<s>
With	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
by	O
contrast	O
,	O
trust	O
of	O
the	O
values	O
in	O
the	O
certificate	O
are	O
more	O
complicated	O
because	O
the	O
entity	O
possesses	O
the	O
signing	O
key	O
,	O
and	O
can	O
always	O
generate	O
a	O
new	O
certificate	O
with	O
different	O
values	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
validity	O
dates	O
of	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
might	O
not	O
be	O
trusted	O
because	O
the	O
entity	O
could	O
always	O
create	O
and	O
sign	O
a	O
new	O
certificate	O
that	O
contained	O
a	O
valid	O
date	O
range	O
.	O
</s>
<s>
The	O
values	O
in	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
can	O
only	O
be	O
trusted	O
when	O
the	O
values	O
were	O
verified	O
out-of-band	O
during	O
the	O
acceptance	O
of	O
the	O
certificate	O
,	O
and	O
there	O
is	O
a	O
method	O
to	O
verify	O
the	O
self-signed	B-Protocol
certificate	I-Protocol
has	O
not	O
changed	O
after	O
it	O
was	O
trusted	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
procedure	O
of	O
trusting	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
includes	O
a	O
manual	O
verification	O
of	O
validity	O
dates	O
,	O
and	O
a	O
hash	B-Algorithm
of	O
the	O
certificate	O
is	O
incorporated	O
into	O
the	O
white	O
list	O
.	O
</s>
<s>
When	O
the	O
certificate	O
is	O
presented	O
for	O
an	O
entity	O
to	O
validate	O
,	O
they	O
first	O
verify	O
the	O
hash	B-Algorithm
of	O
the	O
certificate	O
matches	O
the	O
reference	O
hash	B-Algorithm
in	O
the	O
white-list	O
,	O
and	O
if	O
they	O
match	O
(	O
indicating	O
the	O
self-signed	B-Protocol
certificate	I-Protocol
is	O
the	O
same	O
as	O
the	O
one	O
that	O
was	O
formally	O
trusted	O
)	O
then	O
the	O
certificate	O
's	O
validity	O
dates	O
can	O
be	O
trusted	O
.	O
</s>
<s>
Special	O
treatment	O
of	O
X.509	O
certificate	O
fields	O
for	O
self-signed	B-Protocol
certificate	I-Protocol
can	O
be	O
found	O
in	O
RFC	O
3280	O
.	O
</s>
<s>
Revocation	O
of	O
self-signed	B-Protocol
certificates	I-Protocol
differs	O
from	O
CA-signed	O
certificates	O
.	O
</s>
<s>
By	O
nature	O
,	O
no	O
entity	O
(	O
CA	O
or	O
others	O
)	O
can	O
revoke	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
.	O
</s>
<s>
Self-signed	B-Protocol
certificates	I-Protocol
have	O
limited	O
uses	O
,	O
e.g.	O
</s>
<s>
For	O
example	O
,	O
the	O
Encrypting	O
File	O
System	O
on	O
Microsoft	O
Windows	O
issues	O
a	O
self-signed	B-Protocol
certificate	I-Protocol
on	O
behalf	O
of	O
a	O
user	O
account	O
to	O
transparently	O
encrypt	O
and	O
decrypt	O
files	O
on	O
the	O
fly	O
.	O
</s>
<s>
Another	O
example	O
is	O
root	B-Protocol
certificate	I-Protocol
,	O
which	O
is	O
a	O
form	O
of	O
self-signed	B-Protocol
certificate	I-Protocol
.	O
</s>
