<s>
X.500	B-Protocol
is	O
a	O
series	O
of	O
computer	O
networking	O
standards	O
covering	O
electronic	O
directory	O
services	O
.	O
</s>
<s>
The	O
X.500	B-Protocol
series	O
was	O
developed	O
by	O
the	O
Telecommunication	O
Standardization	O
Sector	O
of	O
the	O
International	O
Telecommunication	O
Union	O
(	O
ITU-T	O
)	O
.	O
</s>
<s>
X.500	B-Protocol
was	O
first	O
approved	O
in	O
1988	O
.	O
</s>
<s>
The	O
directory	O
services	O
were	O
developed	O
to	O
support	O
requirements	O
of	O
X.400	B-Protocol
electronic	O
mail	O
exchange	O
and	O
name	O
lookup	O
.	O
</s>
<s>
ISO/IEC	B-Protocol
9594	I-Protocol
is	O
the	O
corresponding	O
ISO/IEC	O
identification	O
.	O
</s>
<s>
The	O
protocols	O
defined	O
by	O
X.500	B-Protocol
include	O
:	O
</s>
<s>
Because	O
these	O
protocols	O
used	O
the	O
OSI	O
networking	O
stack	O
,	O
a	O
number	O
of	O
alternatives	O
to	O
DAP	O
were	O
developed	O
to	O
allow	O
Internet	O
clients	O
to	O
access	O
the	O
X.500	B-Protocol
Directory	I-Protocol
using	O
the	O
TCP/IP	B-Protocol
networking	O
stack	O
.	O
</s>
<s>
The	O
most	O
well-known	O
alternative	O
to	O
DAP	O
is	O
Lightweight	B-Protocol
Directory	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
LDAP	B-Protocol
)	O
.	O
</s>
<s>
While	O
DAP	O
and	O
the	O
other	O
X.500	B-Protocol
protocols	O
can	O
now	O
use	O
the	O
TCP/IP	B-Protocol
networking	O
stack	O
,	O
LDAP	B-Protocol
remains	O
a	O
popular	O
directory	O
access	O
protocol	O
.	O
</s>
<s>
The	O
X.500	B-Protocol
protocols	O
traditionally	O
use	O
the	O
OSI	O
networking	O
stack	O
.	O
</s>
<s>
However	O
,	O
the	O
Lightweight	B-Protocol
Directory	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
LDAP	B-Protocol
)	O
uses	O
TCP/IP	B-Protocol
for	O
transport	O
.	O
</s>
<s>
In	O
later	O
versions	O
of	O
the	O
ITU	O
Recommendation	O
X.519	O
,	O
the	O
Internet	O
Directly-Mapped	O
(	O
IDM	O
)	O
protocols	O
were	O
introduced	O
to	O
allow	O
X.500	B-Protocol
protocol	O
data	O
units	O
(	O
PDUs	O
)	O
to	O
be	O
transported	O
over	O
the	O
TCP/IP	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
The	O
primary	O
concept	O
of	O
X.500	B-Protocol
is	O
that	O
there	O
is	O
a	O
single	O
Directory	O
Information	O
Tree	O
(	O
DIT	O
)	O
,	O
a	O
hierarchical	O
organization	O
of	O
entries	O
which	O
are	O
distributed	O
across	O
one	O
or	O
more	O
servers	O
,	O
called	O
Directory	B-Application
System	I-Application
Agents	I-Application
(	O
DSA	O
)	O
.	O
</s>
<s>
Each	O
entry	O
has	O
a	O
unique	O
Distinguished	B-Protocol
Name	I-Protocol
,	O
formed	O
by	O
combining	O
its	O
Relative	B-Protocol
Distinguished	I-Protocol
Name	I-Protocol
(	O
RDN	O
)	O
,	O
one	O
or	O
more	O
attributes	O
of	O
the	O
entry	O
itself	O
,	O
and	O
the	O
RDNs	O
of	O
each	O
of	O
the	O
superior	O
entries	O
up	O
to	O
the	O
root	O
of	O
the	O
DIT	O
.	O
</s>
<s>
As	O
LDAP	B-Protocol
implements	O
a	O
very	O
similar	O
data	O
model	O
to	O
that	O
of	O
X.500	B-Protocol
,	O
there	O
is	O
further	O
description	O
of	O
the	O
data	O
model	O
in	O
the	O
article	O
on	O
LDAP	B-Protocol
.	O
</s>
<s>
They	O
are	O
one	O
of	O
the	O
most	O
widely	O
deployed	O
white	B-Application
pages	I-Application
schema	I-Application
.	O
</s>
<s>
X.509	O
,	O
the	O
portion	O
of	O
the	O
standard	O
providing	O
for	O
an	O
authentication	O
framework	O
,	O
is	O
now	O
also	O
widely	O
used	O
outside	O
of	O
the	O
X.500	B-Protocol
directory	I-Protocol
protocols	O
.	O
</s>
<s>
It	O
specifies	O
a	O
standard	O
format	O
for	O
public-key	B-Application
certificates	O
.	O
</s>
<s>
The	O
current	O
use	O
of	O
X.509v3	O
certificates	O
outside	O
the	O
Directory	O
structure	O
loaded	O
directly	O
into	O
web	B-Application
browsers	I-Application
was	O
necessary	O
for	O
e-commerce	O
to	O
develop	O
by	O
allowing	O
for	O
secure	O
web	O
based	O
(	O
SSL/TLS	O
)	O
communications	O
which	O
did	O
not	O
require	O
the	O
X.500	B-Protocol
directory	I-Protocol
as	O
a	O
source	O
of	O
digital	O
certificates	O
as	O
originally	O
conceived	O
in	O
X.500	B-Protocol
(	O
1988	O
)	O
.	O
</s>
<s>
One	O
should	O
contrast	O
the	O
role	O
of	O
X.500	B-Protocol
and	O
X.509	O
to	O
understand	O
their	O
relationship	O
in	O
that	O
X.509	O
was	O
designed	O
to	O
be	O
the	O
secure	O
access	O
method	O
for	O
updating	O
X.500	B-Protocol
before	O
the	O
WWW	O
,	O
but	O
when	O
web	B-Application
browsers	I-Application
became	O
popular	O
there	O
needed	O
to	O
be	O
a	O
simple	O
method	O
of	O
encrypting	O
connections	O
on	O
the	O
transport	O
layer	O
to	O
web	O
sites	O
.	O
</s>
<s>
The	O
WWW	O
e-commerce	O
implementation	O
of	O
X.509v3	O
bypassed	O
but	O
did	O
not	O
replace	O
the	O
original	O
ISO	O
standard	O
authentication	O
mechanism	O
of	O
binding	O
distinguished	B-Protocol
names	I-Protocol
in	O
the	O
X.500	B-Protocol
Directory	I-Protocol
.	O
</s>
<s>
Should	O
a	O
problem	O
arise	O
,	O
such	O
as	O
what	O
occurred	O
with	O
DigiNotar	O
,	O
browser	B-Application
security	O
experts	O
can	O
issue	O
an	O
update	O
to	O
mark	O
a	O
certificate	O
authority	O
as	O
untrusted	O
,	O
but	O
this	O
is	O
a	O
serious	O
removal	O
effectively	O
of	O
that	O
CA	O
from	O
"	O
internet	O
trust	O
"	O
.	O
</s>
<s>
X.500	B-Protocol
offers	O
a	O
way	O
to	O
view	O
which	O
organization	O
claims	O
a	O
specific	O
root	O
certificate	O
,	O
outside	O
of	O
that	O
provided	O
bundle	O
.	O
</s>
<s>
The	O
contrast	O
of	O
this	O
browser	B-Application
bundled	O
approach	O
is	O
that	O
in	O
X.500	B-Protocol
or	O
LDAP	B-Protocol
the	O
attribute	O
"	O
caCertificate	O
"	O
can	O
be	O
"	O
bound	O
"	O
to	O
a	O
directory	O
entry	O
and	O
checked	O
in	O
addition	O
to	O
the	O
default	O
pre-loaded	O
bundle	O
of	O
certificates	O
of	O
which	O
end	O
users	O
typically	O
have	O
never	O
noticed	O
unless	O
an	O
SSL	O
warning	O
message	O
has	O
appeared	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
web	O
site	O
using	O
SSL	O
,	O
typically	O
the	O
DNS	O
site	O
name	O
"	O
www.foobar.com	O
"	O
is	O
verified	O
in	O
a	O
browser	B-Application
by	O
the	O
software	O
using	O
libraries	O
that	O
would	O
check	O
to	O
see	O
if	O
the	O
certificate	O
was	O
signed	O
by	O
one	O
of	O
the	O
trusted	O
root	O
certificates	O
given	O
to	O
the	O
user	O
.	O
</s>
<s>
To	O
contrast	O
this	O
with	O
X.500	B-Protocol
,	O
the	O
certificate	O
is	O
one	O
attribute	O
of	O
many	O
for	O
an	O
entry	O
,	O
in	O
which	O
the	O
entry	O
could	O
contain	O
anything	O
allowable	O
by	O
the	O
specific	O
Directory	O
schema	O
.	O
</s>
<s>
Thus	O
X.500	B-Protocol
does	O
store	O
the	O
digital	O
certificate	O
,	O
but	O
it	O
is	O
one	O
of	O
many	O
attributes	O
that	O
could	O
potentially	O
verify	O
the	O
organization	O
,	O
such	O
as	O
physical	O
address	O
,	O
a	O
contact	O
telephone	O
number	O
and	O
an	O
email	O
contact	O
.	O
</s>
<s>
CA	O
Certs	O
or	O
certificate	O
authority	O
certs	O
are	O
loaded	O
into	O
the	O
browser	B-Application
automatically	O
(	O
in	O
the	O
case	O
of	O
Microsoft	O
's	O
update	O
mechanism	O
)	O
,	O
or	O
in	O
new	O
version	O
updates	O
of	O
browsers	B-Application
,	O
and	O
the	O
user	O
is	O
given	O
further	O
choices	O
to	O
import	O
,	O
delete	O
,	O
or	O
develop	O
an	O
individual	O
trust	O
relationship	O
with	O
the	O
loaded	O
Certificate	O
Authorities	O
and	O
determine	O
how	O
the	O
browser	B-Application
will	O
behave	O
if	O
OCSP	O
revocation	O
servers	O
are	O
unreachable	O
.	O
</s>
<s>
Thus	O
the	O
browser	B-Application
can	O
verify	O
the	O
SSL	O
cert	O
of	O
the	O
website	O
by	O
means	O
of	O
the	O
loaded	O
group	O
of	O
accepted	O
certificates	O
or	O
the	O
root	O
certificates	O
can	O
be	O
looked	O
up	O
in	O
an	O
X.500	B-Protocol
or	O
LDAP	B-Protocol
Directory	O
(	O
or	O
via	O
HTTP/S	O
)	O
and	O
imported	O
into	O
the	O
list	O
of	O
trusted	O
certificate	O
authorities	O
.	O
</s>
<s>
The	O
"	O
bound	O
"	O
distinguished	B-Protocol
name	I-Protocol
is	O
located	O
in	O
the	O
subject	O
fields	O
of	O
the	O
certificate	O
which	O
matches	O
the	O
Directory	O
entry	O
.	O
</s>
<s>
An	O
end	O
user	O
who	O
relies	O
on	O
the	O
authenticity	O
of	O
a	O
certificate	O
being	O
presented	O
to	O
a	O
browser	B-Application
or	O
email	O
has	O
no	O
simple	O
way	O
to	O
compare	O
a	O
forged	O
certificate	O
presented	O
(	O
perhaps	O
which	O
triggers	O
a	O
browser	B-Application
warning	O
)	O
with	O
a	O
valid	O
certificate	O
,	O
without	O
also	O
being	O
given	O
the	O
opportunity	O
to	O
validate	O
the	O
DN	O
or	O
Distinguished	B-Protocol
Name	I-Protocol
which	O
was	O
designed	O
to	O
be	O
looked	O
up	O
in	O
an	O
X.500	B-Protocol
DIT	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
X.500	B-Protocol
software	O
that	O
runs	O
the	O
Federal	O
Bridge	O
has	O
cross	O
certificates	O
that	O
enable	O
trust	O
between	O
certificate	O
authorities	O
.	O
</s>
<s>
If	O
a	O
X.509v3	O
certificate	O
is	O
bound	O
to	O
a	O
valid	O
organization	O
's	O
distinguished	B-Protocol
name	I-Protocol
within	O
the	O
Directory	O
,	O
then	O
a	O
simple	O
check	O
can	O
be	O
made	O
in	O
regards	O
to	O
the	O
authenticity	O
of	O
the	O
certificate	O
by	O
a	O
comparison	O
with	O
what	O
is	O
presented	O
to	O
the	O
browser	B-Application
with	O
what	O
is	O
present	O
in	O
the	O
Directory	O
.	O
</s>
<s>
If	O
the	O
cert	O
is	O
likely	O
to	O
be	O
trusted	O
and	O
is	O
failing	O
because	O
the	O
domain	O
name	O
is	O
a	O
slight	O
mismatch	O
,	O
it	O
will	O
then	O
initially	O
fail	O
in	O
the	O
browser	B-Application
,	O
but	O
then	O
be	O
subjected	O
to	O
the	O
notary	O
trust	O
,	O
which	O
can	O
then	O
bypass	O
the	O
browser	B-Application
warning	O
.	O
</s>
<s>
This	O
would	O
have	O
normally	O
triggered	O
a	O
browser	B-Application
warning	O
,	O
but	O
would	O
not	O
if	O
the	O
MITM	O
certificate	O
was	O
issued	O
by	O
a	O
valid	O
certificate	O
authority	O
already	O
trusted	O
by	O
a	O
browser	B-Application
or	O
other	O
software	O
.	O
</s>
<s>
This	O
necessitated	O
an	O
emergency	O
patch	O
to	O
major	O
browsers	B-Application
.	O
</s>
<s>
Some	O
projects	O
designed	O
to	O
exchange	O
PHI	O
,	O
protected	O
Health	O
Information	O
(	O
which	O
is	O
considered	O
to	O
be	O
highly	O
HIPAA	O
sensitive	O
)	O
may	O
obtain	O
X.509v3	O
certs	O
via	O
a	O
CERT	O
DNS	O
resource	O
record	O
,	O
or	O
via	O
LDAP	B-Protocol
to	O
a	O
X.500[2008]	O
Directory	O
.	O
</s>
<s>
The	O
name	O
space	O
associated	O
with	O
X.500	B-Protocol
has	O
traditionally	O
been	O
thought	O
to	O
start	O
with	O
a	O
national	O
naming	O
authority	O
,	O
which	O
mirrors	O
the	O
ISO/ITU	O
approach	O
to	O
global	O
systems	O
with	O
national	O
representation	O
.	O
</s>
<s>
Thus	O
different	O
countries	O
will	O
create	O
their	O
own	O
unique	O
X.500	B-Protocol
services	O
.	O
</s>
<s>
The	O
U.S.	O
X.500	B-Protocol
was	O
privatized	O
in	O
1998	O
,	O
when	O
the	O
U.S.	O
Government	O
no	O
longer	O
offered	O
X.500	B-Protocol
or	O
DNS	O
registration	O
outside	O
of	O
known	O
government	O
agencies	O
.	O
</s>
<s>
The	O
X.500	B-Protocol
pilot	O
project	O
has	O
been	O
in	O
development	O
in	O
the	O
commercial	O
space	O
,	O
and	O
the	O
technology	O
continues	O
to	O
be	O
present	O
in	O
major	O
installations	O
of	O
millions	O
of	O
users	O
within	O
corporate	O
data	O
centers	O
,	O
and	O
within	O
the	O
U.S.	O
Government	O
for	O
credentialing	O
.	O
</s>
<s>
The	O
authors	O
of	O
RFC	O
2693	O
(	O
concerning	O
SPKI	O
)	O
note	O
that	O
"	O
The	O
original	O
X.500	B-Protocol
plan	O
is	O
unlikely	O
ever	O
to	O
come	O
to	O
fruition	O
.	O
</s>
<s>
Collections	O
of	O
directory	O
entries	O
...	O
are	O
considered	O
valuable	O
or	O
even	O
confidential	O
by	O
those	O
owning	O
the	O
lists	O
and	O
are	O
not	O
likely	O
to	O
be	O
released	O
to	O
the	O
world	O
in	O
the	O
form	O
of	O
an	O
X.500	B-Protocol
directory	I-Protocol
sub-tree.	O
"	O
</s>
<s>
and	O
that	O
"	O
The	O
X.500	B-Protocol
idea	O
of	O
a	O
distinguished	B-Protocol
name	I-Protocol
(	O
a	O
single	O
,	O
globally	O
unique	O
name	O
that	O
everyone	O
could	O
use	O
when	O
referring	O
to	O
an	O
entity	O
)	O
is	O
also	O
not	O
likely	O
to	O
occur.	O
"	O
</s>
