<s>
An	O
authentication	B-Protocol
protocol	I-Protocol
is	O
a	O
type	O
of	O
computer	O
communications	O
protocol	O
or	O
cryptographic	B-Protocol
protocol	I-Protocol
specifically	O
designed	O
for	O
transfer	O
of	O
authentication	O
data	O
between	O
two	O
entities	O
.	O
</s>
<s>
The	O
task	O
of	O
the	O
authentication	B-Protocol
protocol	I-Protocol
is	O
to	O
specify	O
the	O
exact	O
series	O
of	O
steps	O
needed	O
for	O
execution	O
of	O
the	O
authentication	O
.	O
</s>
<s>
An	O
illustration	O
of	O
password-based	O
authentication	O
using	O
simple	O
authentication	B-Protocol
protocol	I-Protocol
:	O
</s>
<s>
This	O
is	O
an	O
example	O
of	O
a	O
very	O
basic	B-Protocol
authentication	I-Protocol
protocol	O
vulnerable	O
to	O
many	O
threats	O
such	O
as	O
eavesdropping	O
,	O
replay	O
attack	O
,	O
man-in-the-middle	O
attacks	O
,	O
dictionary	O
attacks	O
or	O
brute-force	O
attacks	O
.	O
</s>
<s>
Most	O
authentication	B-Protocol
protocols	I-Protocol
are	O
more	O
complicated	O
in	O
order	O
to	O
be	O
resilient	O
against	O
these	O
attacks	O
.	O
</s>
<s>
Protocols	O
are	O
used	O
mainly	O
by	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
(	O
PPP	O
)	O
servers	O
to	O
validate	O
the	O
identity	O
of	O
remote	O
clients	O
before	O
granting	O
them	O
access	O
to	O
server	O
data	O
.	O
</s>
<s>
Password	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
is	O
one	O
of	O
the	O
oldest	O
authentication	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Although	O
widely	O
supported	O
,	O
it	O
is	O
specified	O
that	O
if	O
an	O
implementation	O
offers	O
a	O
stronger	O
authentication	O
method	O
,	O
that	O
method	O
must	O
be	O
offered	O
before	O
PAP	B-Protocol
.	O
</s>
<s>
the	O
same	O
client	O
alternately	O
using	O
both	O
PAP	B-Protocol
and	O
CHAP	O
)	O
is	O
also	O
not	O
expected	O
,	O
as	O
the	O
CHAP	O
authentication	O
would	O
be	O
compromised	O
by	O
PAP	B-Protocol
sending	O
the	O
password	O
in	O
plain-text	O
.	O
</s>
<s>
Later	O
version	O
XTACACS	B-Protocol
(	O
Extended	O
TACACS	B-Protocol
)	O
added	O
authorization	O
and	O
accounting	O
.	O
</s>
<s>
Both	O
of	O
these	O
protocols	O
were	O
later	O
replaced	O
by	O
TACACS+	B-Protocol
.	O
</s>
<s>
TACACS+	B-Protocol
separates	O
the	O
AAA	O
components	O
thus	O
they	O
can	O
be	O
segregated	O
and	O
handled	O
on	O
separate	O
servers	O
(	O
It	O
can	O
even	O
use	O
another	O
protocol	O
for	O
e.g.	O
</s>
<s>
It	O
uses	O
TCP	B-Protocol
(	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
)	O
for	O
transport	O
and	O
encrypts	O
the	O
whole	O
packet	O
.	O
</s>
<s>
TACACS+	B-Protocol
is	O
Cisco	O
proprietary	O
.	O
</s>
<s>
Remote	B-Protocol
Authentication	I-Protocol
Dial-In	I-Protocol
User	I-Protocol
Service	I-Protocol
(	O
RADIUS	B-Protocol
)	O
is	O
a	O
full	O
AAA	O
protocol	O
commonly	O
used	O
by	O
ISP	O
.	O
</s>
<s>
Credentials	O
are	O
mostly	O
username-password	O
combination	O
based	O
,	O
it	O
uses	O
NAS	O
and	O
UDP	B-Protocol
protocol	I-Protocol
for	O
transport	O
.	O
</s>
<s>
Diameter	O
(	O
protocol	O
)	O
evolved	O
from	O
RADIUS	B-Protocol
and	O
involves	O
many	O
improvements	O
such	O
as	O
usage	O
of	O
more	O
reliable	O
TCP	B-Protocol
or	O
SCTP	O
transport	O
protocol	O
and	O
higher	O
security	O
thanks	O
to	O
TLS	B-Protocol
.	O
</s>
<s>
Kerberos	B-Protocol
is	O
a	O
centralized	O
network	O
authentication	O
system	O
developed	O
at	O
MIT	O
and	O
available	O
as	O
a	O
free	O
implementation	O
from	O
MIT	O
but	O
also	O
in	O
many	O
commercial	O
products	O
.	O
</s>
<s>
It	O
is	O
the	O
default	O
authentication	O
method	O
in	O
Windows	B-Application
2000	I-Application
and	O
later	O
.	O
</s>
<s>
The	O
authentication	O
process	O
itself	O
is	O
much	O
more	O
complicated	O
than	O
in	O
the	O
previous	O
protocols	O
-	O
Kerberos	B-Protocol
uses	O
symmetric	B-Algorithm
key	I-Algorithm
cryptography	I-Algorithm
,	O
requires	O
a	O
trusted	O
third	O
party	O
and	O
can	O
use	O
public-key	B-Application
cryptography	I-Application
during	O
certain	O
phases	O
of	O
authentication	O
if	O
need	O
be	O
.	O
</s>
