<s>
Kerberos	B-Application
(	O
)	O
is	O
a	O
computer-network	B-Architecture
authentication	B-Protocol
protocol	I-Protocol
that	O
works	O
on	O
the	O
basis	O
of	O
tickets	O
to	O
allow	O
nodes	B-Protocol
communicating	O
over	O
a	O
non-secure	O
network	O
to	O
prove	O
their	O
identity	O
to	O
one	O
another	O
in	O
a	O
secure	O
manner	O
.	O
</s>
<s>
Its	O
designers	O
aimed	O
it	O
primarily	O
at	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
,	O
and	O
it	O
provides	O
mutual	O
authentication	O
—	O
both	O
the	O
user	O
and	O
the	O
server	O
verify	O
each	O
other	O
's	O
identity	O
.	O
</s>
<s>
Kerberos	B-Protocol
protocol	I-Protocol
messages	O
are	O
protected	O
against	O
eavesdropping	O
and	O
replay	O
attacks	O
.	O
</s>
<s>
Kerberos	B-Application
builds	O
on	O
symmetric-key	B-Algorithm
cryptography	I-Algorithm
and	O
requires	O
a	O
trusted	O
third	O
party	O
,	O
and	O
optionally	O
may	O
use	O
public-key	B-Application
cryptography	I-Application
during	O
certain	O
phases	O
of	O
authentication	O
.	O
</s>
<s>
Kerberos	B-Application
uses	O
UDP	B-Protocol
port	I-Protocol
88	O
by	O
default	O
.	O
</s>
<s>
The	O
protocol	B-Protocol
was	O
named	O
after	O
the	O
character	O
Kerberos	B-Application
(	O
or	O
Cerberus	B-Application
)	O
from	O
Greek	O
mythology	O
,	O
the	O
ferocious	O
three-headed	O
guard	O
dog	O
of	O
Hades	B-Language
.	O
</s>
<s>
Massachusetts	O
Institute	O
of	O
Technology	O
(	O
MIT	O
)	O
developed	O
Kerberos	B-Application
in	O
1988	O
to	O
protect	O
network	O
services	O
provided	O
by	O
Project	B-Operating_System
Athena	I-Operating_System
.	O
</s>
<s>
The	O
protocol	B-Protocol
is	O
based	O
on	O
the	O
earlier	O
Needham	B-Protocol
–	I-Protocol
Schroeder	I-Protocol
symmetric-key	I-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Several	O
versions	O
of	O
the	O
protocol	B-Protocol
exist	O
;	O
versions	O
1	O
–	O
3	O
occurred	O
only	O
internally	O
at	O
MIT	O
.	O
</s>
<s>
Kerberos	B-Application
version	O
4	O
was	O
primarily	O
designed	O
by	O
Steve	O
Miller	O
and	O
Clifford	O
Neuman	O
.	O
</s>
<s>
Published	O
in	O
the	O
late	O
1980s	O
,	O
version	O
4	O
was	O
also	O
targeted	O
at	O
Project	B-Operating_System
Athena	I-Operating_System
.	O
</s>
<s>
Authorities	O
in	O
the	O
United	O
States	O
classified	O
Kerberos	B-Application
as	O
"	O
Auxiliary	O
Military	O
Equipment	O
"	O
on	O
the	O
US	O
Munitions	O
List	O
and	O
banned	O
its	O
export	O
because	O
it	O
used	O
the	O
Data	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
DES	B-Algorithm
)	O
encryption	O
algorithm	O
(	O
with	O
56-bit	O
keys	O
)	O
.	O
</s>
<s>
The	O
Swedish	O
implementation	O
was	O
based	O
on	O
a	O
limited	O
version	O
called	O
eBones	B-Protocol
.	O
</s>
<s>
eBones	B-Protocol
was	O
based	O
on	O
the	O
exported	O
MIT	O
Bones	O
release	O
(	O
stripped	O
of	O
both	O
the	O
encryption	O
functions	O
and	O
the	O
calls	O
to	O
them	O
)	O
based	O
on	O
version	O
Kerberos4	O
patch-level9	O
.	O
</s>
<s>
In	O
2005	O
,	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
Kerberos	B-Application
working	O
group	O
updated	O
specifications	O
.	O
</s>
<s>
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
Encryption	O
for	O
Kerberos	B-Protocol
5	I-Protocol
(	O
RFC3962	O
)	O
.	O
</s>
<s>
A	O
new	O
edition	O
of	O
the	O
Kerberos	B-Application
V5	O
specification	O
"	O
The	O
Kerberos	B-Application
Network	O
Authentication	O
Service	O
(	O
V5	O
)	O
"	O
(	O
RFC4120	O
)	O
.	O
</s>
<s>
This	O
version	O
obsoletes	O
RFC1510	O
,	O
clarifies	O
aspects	O
of	O
the	O
protocol	B-Protocol
and	O
intended	O
use	O
in	O
a	O
more	O
detailed	O
and	O
clearer	O
explanation	O
.	O
</s>
<s>
A	O
new	O
edition	O
of	O
the	O
Generic	O
Security	O
Services	O
Application	O
Program	O
Interface	O
(	O
GSS-API	O
)	O
specification	O
"	O
The	O
Kerberos	B-Application
Version5	O
Generic	O
Security	O
Service	O
Application	O
Program	O
Interface	O
(	O
GSS-API	O
)	O
Mechanism	O
:	O
Version2	O
"	O
(	O
RFC4121	O
)	O
.	O
</s>
<s>
MIT	O
makes	O
an	O
implementation	O
of	O
Kerberos	B-Application
freely	O
available	O
,	O
under	O
copyright	O
permissions	O
similar	O
to	O
those	O
used	O
for	O
BSD	B-Operating_System
.	O
</s>
<s>
In	O
2007	O
,	O
MIT	O
formed	O
the	O
Kerberos	B-Application
Consortium	O
to	O
foster	O
continued	O
development	O
.	O
</s>
<s>
Founding	O
sponsors	O
include	O
vendors	O
such	O
as	O
Oracle	B-Application
,	O
Apple	O
Inc.	O
,	O
Google	B-Application
,	O
Microsoft	O
,	O
Centrify	O
Corporation	O
and	O
TeamF1	O
Inc.	O
,	O
and	O
academic	O
institutions	O
such	O
as	O
the	O
Royal	O
Institute	O
of	O
Technology	O
in	O
Sweden	O
,	O
Stanford	O
University	O
,	O
MIT	O
,	O
and	O
vendors	O
such	O
as	O
CyberSafe	O
offering	O
commercially	O
supported	O
versions	O
.	O
</s>
<s>
Windows	B-Application
2000	I-Application
and	O
later	O
versions	O
use	O
Kerberos	B-Application
as	O
their	O
default	O
authentication	O
method	O
.	O
</s>
<s>
Some	O
Microsoft	O
additions	O
to	O
the	O
Kerberos	B-Application
suite	O
of	O
protocols	O
are	O
documented	O
in	O
RFC	O
3244	O
"	O
Microsoft	B-Application
Windows	I-Application
2000	I-Application
Kerberos	B-Application
Change	O
Password	O
and	O
Set	O
Password	O
Protocols	O
"	O
.	O
</s>
<s>
RFC	O
4757	O
documents	O
Microsoft	O
's	O
use	O
of	O
the	O
RC4	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
While	O
Microsoft	O
uses	O
and	O
extends	O
the	O
Kerberos	B-Protocol
protocol	I-Protocol
,	O
it	O
does	O
not	O
use	O
the	O
MIT	O
software	O
.	O
</s>
<s>
Kerberos	B-Application
is	O
used	O
as	O
the	O
preferred	O
authentication	O
method	O
:	O
in	O
general	O
,	O
joining	O
a	O
client	O
to	O
a	O
Windows	O
domain	O
means	O
enabling	O
Kerberos	B-Application
as	O
the	O
default	O
protocol	B-Protocol
for	O
authentications	O
from	O
that	O
client	O
to	O
services	O
in	O
the	O
Windows	O
domain	O
and	O
all	O
domains	O
with	O
trust	O
relationships	O
to	O
that	O
domain	O
.	O
</s>
<s>
Internet	O
web	O
applications	O
can	O
enforce	O
Kerberos	B-Application
as	O
an	O
authentication	O
method	O
for	O
domain-joined	O
clients	O
by	O
using	O
APIs	O
provided	O
under	O
SSPI	B-Protocol
.	O
</s>
<s>
Microsoft	O
Windows	O
and	O
Windows	O
Server	O
include	O
,	O
a	O
command-line	B-Application
utility	O
that	O
can	O
be	O
used	O
to	O
read	O
,	O
modify	O
,	O
or	O
delete	O
the	O
Service	B-Protocol
Principal	I-Protocol
Names	I-Protocol
(	O
SPN	O
)	O
for	O
an	O
Active	B-Protocol
Directory	I-Protocol
service	I-Protocol
account	O
.	O
</s>
<s>
Many	O
Unix-like	O
operating	O
systems	O
,	O
including	O
FreeBSD	B-Operating_System
,	O
OpenBSD	B-Operating_System
,	O
Apple	O
's	O
macOS	B-Application
,	O
Red	O
Hat	O
Enterprise	O
Linux	O
,	O
Oracle	B-Application
's	O
Solaris	B-Application
,	O
IBM	O
's	O
AIX	B-Application
,	O
HP-UX	B-Application
and	O
others	O
,	O
include	O
software	O
for	O
Kerberos	B-Application
authentication	O
of	O
users	O
or	O
services	O
.	O
</s>
<s>
A	O
variety	O
of	O
non-Unix	O
like	O
operating	O
systems	O
such	O
as	O
z/OS	B-Application
,	O
IBM	B-Application
i	I-Application
and	O
OpenVMS	B-Operating_System
also	O
feature	O
Kerberos	B-Application
support	O
.	O
</s>
<s>
Embedded	O
implementation	O
of	O
the	O
Kerberos	B-Application
V	O
authentication	B-Protocol
protocol	I-Protocol
for	O
client	O
agents	O
and	O
network	O
services	O
running	O
on	O
embedded	O
platforms	O
is	O
also	O
available	O
from	O
companies	O
.	O
</s>
<s>
When	O
the	O
client	O
needs	O
to	O
communicate	O
with	O
a	O
service	O
on	O
another	O
node	O
(	O
a	O
"	O
principal	O
"	O
,	O
in	O
Kerberos	B-Application
parlance	O
)	O
,	O
the	O
client	O
sends	O
the	O
TGT	O
to	O
the	O
TGS	O
,	O
which	O
usually	O
shares	O
the	O
same	O
host	O
as	O
the	O
KDC	O
.	O
</s>
<s>
The	O
service	O
must	O
have	O
already	O
been	O
registered	O
with	O
the	O
TGS	O
with	O
a	O
Service	B-Protocol
Principal	I-Protocol
Name	I-Protocol
(	O
SPN	O
)	O
.	O
</s>
<s>
The	O
protocol	B-Protocol
is	O
described	O
in	O
detail	O
below	O
.	O
</s>
<s>
Other	O
credential	O
mechanisms	O
like	O
pkinit	O
(	O
RFC	O
4556	O
)	O
allow	O
for	O
the	O
use	O
of	O
public	B-Application
keys	I-Application
in	O
place	O
of	O
a	O
password	O
.	O
</s>
<s>
The	O
client	O
transforms	O
the	O
password	O
into	O
the	O
key	O
of	O
a	O
symmetric	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
This	O
either	O
uses	O
the	O
built-in	O
key	B-Algorithm
scheduling	I-Algorithm
,	O
or	O
a	O
one-way	B-Algorithm
hash	I-Algorithm
,	O
depending	O
on	O
the	O
cipher-suite	B-Protocol
used	O
.	O
</s>
<s>
The	O
server	O
receives	O
the	O
username	O
and	O
symmetric	B-Algorithm
cipher	I-Algorithm
and	O
compares	O
it	O
with	O
the	O
data	O
from	O
database	O
.	O
</s>
<s>
If	O
it	O
is	O
,	O
the	O
AS	O
generates	O
the	O
secret	O
key	O
by	O
hashing	O
the	O
password	O
of	O
the	O
user	O
found	O
at	O
the	O
database	O
(	O
e.g.	O
,	O
Active	B-Protocol
Directory	I-Protocol
in	O
Windows	O
Server	O
)	O
and	O
sends	O
back	O
the	O
following	O
two	O
messages	O
to	O
the	O
client	O
:	O
</s>
<s>
Message	O
B	O
:	O
Ticket-Granting-Ticket	O
(	O
TGT	O
,	O
which	O
includes	O
the	O
client	O
ID	O
,	O
client	O
network	B-Protocol
address	I-Protocol
,	O
ticket	O
validity	O
period	O
,	O
and	O
the	O
Client/TGS	O
Session	O
Key	O
)	O
encrypted	O
using	O
the	O
secret	O
key	O
of	O
the	O
TGS	O
.	O
</s>
<s>
Message	O
C	B-Language
:	O
Composed	O
of	O
the	O
message	O
B	O
(	O
the	O
encrypted	O
TGT	O
using	O
the	O
TGS	O
secret	O
key	O
)	O
and	O
the	O
ID	O
of	O
the	O
requested	O
service	O
.	O
</s>
<s>
Upon	O
receiving	O
messages	O
C	B-Language
and	O
D	O
,	O
the	O
TGS	O
retrieves	O
message	O
B	O
out	O
of	O
message	O
C	B-Language
.	O
It	O
decrypts	O
message	O
B	O
using	O
the	O
TGS	O
secret	O
key	O
.	O
</s>
<s>
Message	O
E	O
:	O
Client-to-server	O
ticket	O
(	O
which	O
includes	O
the	O
client	O
ID	O
,	O
client	O
network	B-Protocol
address	I-Protocol
,	O
validity	O
period	O
,	O
and	O
Client/Server	B-Operating_System
Session	O
Key	O
)	O
encrypted	O
using	O
the	O
service	O
's	O
secret	O
key	O
.	O
</s>
<s>
Message	O
F	O
:	O
Client/Server	B-Operating_System
Session	O
Key	O
encrypted	O
with	O
the	O
Client/TGS	O
Session	O
Key	O
.	O
</s>
<s>
Message	O
G	O
:	O
A	O
new	O
Authenticator	O
,	O
which	O
includes	O
the	O
client	O
ID	O
,	O
timestamp	O
and	O
is	O
encrypted	O
using	O
Client/Server	B-Operating_System
Session	O
Key	O
.	O
</s>
<s>
The	O
SS	O
decrypts	O
the	O
ticket	O
(	O
message	O
E	O
)	O
using	O
its	O
own	O
secret	O
key	O
to	O
retrieve	O
the	O
Client/Server	B-Operating_System
Session	O
Key	O
.	O
</s>
<s>
Message	O
H	O
:	O
The	O
timestamp	O
found	O
in	O
client	O
's	O
Authenticator	O
(	O
plus	O
1	O
in	O
version	O
4	O
,	O
but	O
not	O
necessary	O
in	O
version	O
5	O
)	O
,	O
encrypted	O
using	O
the	O
Client/Server	B-Operating_System
Session	O
Key	O
.	O
</s>
<s>
The	O
client	O
decrypts	O
the	O
confirmation	O
(	O
message	O
H	O
)	O
using	O
the	O
Client/Server	B-Operating_System
Session	O
Key	O
and	O
checks	O
whether	O
the	O
timestamp	O
is	O
correct	O
.	O
</s>
<s>
Kerberos	B-Application
has	O
strict	O
time	O
requirements	O
,	O
which	O
means	O
that	O
the	O
clocks	O
of	O
the	O
involved	O
hosts	O
must	O
be	O
synchronized	O
within	O
configured	O
limits	O
.	O
</s>
<s>
The	O
tickets	O
have	O
a	O
time	O
availability	O
period	O
,	O
and	O
if	O
the	O
host	O
clock	O
is	O
not	O
synchronized	O
with	O
the	O
Kerberos	B-Application
server	O
clock	O
,	O
the	O
authentication	O
will	O
fail	O
.	O
</s>
<s>
In	O
practice	O
,	O
Network	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
daemons	O
are	O
usually	O
used	O
to	O
keep	O
the	O
host	O
clocks	O
synchronized	O
.	O
</s>
<s>
The	O
administration	O
protocol	B-Protocol
is	O
not	O
standardized	O
and	O
differs	O
between	O
server	O
implementations	O
.	O
</s>
<s>
In	O
case	O
of	O
symmetric	B-Algorithm
cryptography	I-Algorithm
adoption	O
(	O
Kerberos	B-Application
can	O
work	O
using	O
symmetric	O
or	O
asymmetric	O
(	O
public-key	B-Application
)	O
cryptography	O
)	O
,	O
since	O
all	O
authentications	O
are	O
controlled	O
by	O
a	O
centralized	O
key	O
distribution	O
center	O
(	O
KDC	O
)	O
,	O
compromise	O
of	O
this	O
authentication	O
infrastructure	O
will	O
allow	O
an	O
attacker	O
to	O
impersonate	O
any	O
user	O
.	O
</s>
<s>
Each	O
network	O
service	O
that	O
requires	O
a	O
different	O
host	O
name	O
will	O
need	O
its	O
own	O
set	O
of	O
Kerberos	B-Application
keys	O
.	O
</s>
<s>
Kerberos	B-Application
requires	O
user	O
accounts	O
and	O
services	O
to	O
have	O
a	O
trusted	O
relationship	O
to	O
the	O
Kerberos	B-Application
token	O
server	O
.	O
</s>
<s>
The	O
Data	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
DES	B-Algorithm
)	O
cipher	O
can	O
be	O
used	O
in	O
combination	O
with	O
Kerberos	B-Application
,	O
but	O
is	O
no	O
longer	O
an	O
Internet	O
standard	O
because	O
it	O
is	O
weak	O
.	O
</s>
<s>
Security	O
vulnerabilities	O
exist	O
in	O
many	O
legacy	O
products	O
that	O
implement	O
Kerberos	B-Application
because	O
they	O
have	O
not	O
been	O
updated	O
to	O
use	O
newer	O
ciphers	O
like	O
AES	B-Algorithm
instead	O
of	O
DES	B-Algorithm
.	O
</s>
<s>
In	O
November	O
2014	O
,	O
Microsoft	O
released	O
a	O
patch	O
(	O
MS14-068	O
)	O
to	O
rectify	O
an	O
exploitable	O
vulnerability	O
in	O
Windows	O
implementation	O
of	O
the	O
Kerberos	B-Application
Key	O
Distribution	O
Center	O
(	O
KDC	O
)	O
.	O
</s>
