<s>
The	O
Skype	B-Protocol
protocol	I-Protocol
is	O
a	O
proprietary	B-Application
Internet	B-Application
telephony	I-Application
network	O
used	O
by	O
Skype	B-Application
.	O
</s>
<s>
The	O
protocol	O
's	O
specifications	O
have	O
not	O
been	O
made	O
publicly	O
available	O
by	O
Skype	B-Application
and	O
official	O
applications	O
using	O
the	O
protocol	O
are	O
closed-source	B-Application
.	O
</s>
<s>
The	O
Skype	B-Application
network	O
is	O
not	O
interoperable	O
with	O
most	O
other	O
Voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
networks	O
without	O
proper	O
licensing	O
from	O
Skype	B-Application
.	O
</s>
<s>
On	O
June	O
20	O
,	O
2014	O
,	O
Microsoft	O
announced	O
the	O
deprecation	O
of	O
the	O
old	O
Skype	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Within	O
several	O
months	O
from	O
this	O
date	O
,	O
in	O
order	O
to	O
continue	O
using	O
Skype	B-Application
services	O
,	O
Skype	B-Application
users	O
had	O
to	O
update	O
to	O
Skype	B-Application
applications	O
released	O
in	O
2014	O
,	O
and	O
users	O
were	O
not	O
able	O
to	O
log	B-General_Concept
in	I-General_Concept
to	O
older	O
Skype	B-Application
versions	O
(	O
clients	O
)	O
.	O
</s>
<s>
No	O
announcement	O
has	O
been	O
made	O
on	O
whether	O
SmartTV	O
and	O
hardware	O
phones	O
with	O
built-in	O
Skype	B-Application
functionality	O
will	O
continue	O
to	O
work	O
without	O
interruptions	O
.	O
</s>
<s>
The	O
new	O
Skype	B-Protocol
protocol	I-Protocol
—	O
Microsoft	O
Notification	O
Protocol	O
24	O
—	O
promised	O
better	O
offline	O
messaging	O
and	O
better	O
messages	O
synchronization	O
across	O
Skype	B-Application
devices	O
.	O
</s>
<s>
Skype	B-Application
was	O
the	O
first	O
peer-to-peer	O
IP	B-Application
telephony	I-Application
network	O
.	O
</s>
<s>
The	O
network	O
contains	O
three	O
types	O
of	O
entities	O
:	O
supernodes	B-Protocol
,	O
ordinary	O
nodes	O
,	O
and	O
the	O
login	B-General_Concept
server	O
.	O
</s>
<s>
Each	O
client	O
maintains	O
a	O
host	O
cache	O
with	O
the	O
IP	B-Protocol
address	I-Protocol
and	O
port	O
numbers	O
of	O
reachable	O
supernodes	B-Protocol
.	O
</s>
<s>
The	O
Skype	B-Application
user	O
directory	O
is	O
decentralized	O
and	O
distributed	O
among	O
the	O
supernodes	B-Protocol
in	O
the	O
network	O
.	O
</s>
<s>
Supernodes	B-Protocol
are	O
grouped	O
into	O
slots	O
(	O
9	O
–	O
10	O
supernodes	B-Protocol
)	O
,	O
and	O
slots	O
are	O
grouped	O
into	O
blocks	O
(	O
8	O
slots	O
)	O
.	O
</s>
<s>
Previously	O
any	O
client	O
with	O
good	O
bandwidth	O
,	O
no	O
restrictions	O
due	O
to	O
firewall	B-Application
or	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
,	O
and	O
adequate	O
processing	O
power	O
could	O
become	O
a	O
supernode	B-Protocol
.	O
</s>
<s>
This	O
placed	O
an	O
extra	O
burden	O
on	O
those	O
who	O
connected	O
to	O
the	O
Internet	O
without	O
NAT	O
,	O
as	O
Skype	B-Application
used	O
their	O
computers	O
and	O
Internet	O
connections	O
as	O
third	O
parties	O
for	O
UDP	O
hole	O
punching	O
(	O
to	O
directly	O
connect	O
two	O
clients	O
both	O
behind	O
NAT	O
)	O
or	O
to	O
completely	O
relay	O
other	O
users	O
 '	O
calls	O
.	O
</s>
<s>
In	O
2012	O
,	O
Microsoft	O
altered	O
the	O
design	O
of	O
the	O
network	O
,	O
and	O
brought	O
all	O
supernodes	B-Protocol
under	O
their	O
control	O
as	O
hosted	O
servers	O
in	O
data	O
centres	O
.	O
</s>
<s>
Microsoft	O
at	O
the	O
time	O
defended	O
the	O
move	O
,	O
saying	O
they	O
"	O
believe	O
this	O
approach	O
has	O
immediate	O
performance	O
,	O
scalability	O
and	O
availability	O
benefits	O
for	O
the	O
hundreds	O
of	O
millions	O
of	O
users	O
that	O
make	O
up	O
the	O
Skype	B-Application
community.	O
"	O
</s>
<s>
Skype	B-Application
does	O
not	O
support	O
the	O
use	O
of	O
the	O
IPv6	B-Protocol
protocol	O
,	O
which	O
would	O
greatly	O
reduce	O
the	O
complexity	O
associated	O
with	O
the	O
aforementioned	O
IPv4	O
communication	O
structure	O
.	O
</s>
<s>
Supernodes	B-Protocol
relay	O
communications	O
on	O
behalf	O
of	O
two	O
other	O
clients	O
,	O
both	O
of	O
which	O
are	O
behind	O
firewalls	B-Application
or	O
"	O
one-to-many	O
"	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
.	O
</s>
<s>
Without	O
relaying	O
by	O
the	O
supernodes	B-Protocol
,	O
two	O
clients	O
with	O
firewall	B-Application
or	O
NAT	O
difficulties	O
would	O
be	O
unable	O
to	O
make	O
or	O
receive	O
calls	O
from	O
one	O
another	O
.	O
</s>
<s>
Skype	B-Application
tries	O
to	O
get	O
the	O
two	O
ends	O
to	O
negotiate	O
the	O
connection	O
details	O
directly	O
,	O
but	O
sometimes	O
the	O
sum	O
of	O
problems	O
at	O
both	O
ends	O
can	O
prevent	O
direct	O
conversation	O
being	O
established	O
.	O
</s>
<s>
The	O
problems	O
with	O
firewalls	B-Application
and	O
NAT	O
can	O
be	O
:	O
</s>
<s>
Signaling	O
is	O
encrypted	O
using	O
RC4	B-Algorithm
;	O
however	O
,	O
the	O
method	O
only	O
obfuscates	O
the	O
traffic	O
as	O
the	O
key	O
can	O
be	O
recovered	O
from	O
the	O
packet	O
.	O
</s>
<s>
Voice	O
data	O
is	O
encrypted	O
with	O
AES	B-Algorithm
.	O
</s>
<s>
The	O
Skype	B-Application
client	O
's	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
opens	O
the	O
network	O
to	O
software	O
developers	O
.	O
</s>
<s>
The	O
Skype	B-Application
API	B-Application
allows	O
other	O
programs	O
to	O
use	O
the	O
Skype	B-Application
network	O
to	O
get	O
"	O
white	O
pages	O
"	O
information	O
and	O
manage	O
calls	O
.	O
</s>
<s>
The	O
Skype	B-Application
code	O
is	O
closed	B-Application
source	I-Application
,	O
and	O
the	O
protocol	O
is	O
not	O
standardized	O
.	O
</s>
<s>
Parts	O
of	O
the	O
client	O
use	O
Internet	B-Language
Direct	I-Language
(	O
Indy	O
)	O
,	O
an	O
open	O
source	O
socket	O
communication	O
library	O
.	O
</s>
<s>
On	O
July	O
8	O
,	O
2012	O
,	O
a	O
researcher	O
from	O
Benin	O
,	O
Ouanilo	O
Medegan	O
,	O
released	O
articles	O
and	O
proof	O
of	O
concept	O
code	O
,	O
results	O
of	O
his	O
reverse	O
engineering	O
the	O
Skype	B-Application
client	O
.	O
</s>
<s>
Many	O
networking	O
and	O
security	O
companies	O
claim	O
to	O
detect	O
and	O
control	O
Skype	B-Application
's	O
protocol	O
for	O
enterprise	O
and	O
carrier	O
applications	O
.	O
</s>
<s>
While	O
the	O
specific	O
detection	O
methods	O
used	O
by	O
these	O
companies	O
are	O
often	O
proprietary	B-Application
,	O
Pearson	B-General_Concept
's	I-General_Concept
chi-squared	I-General_Concept
test	I-General_Concept
and	O
stochastic	O
characterization	O
with	O
Naive	B-General_Concept
Bayes	I-General_Concept
classifiers	I-General_Concept
are	O
two	O
approaches	O
that	O
were	O
published	O
in	O
2007	O
.	O
</s>
<s>
The	O
RC4	B-Algorithm
encryption	O
algorithm	O
is	O
used	O
to	O
obfuscate	O
the	O
payload	O
of	O
datagrams	O
.	O
</s>
<s>
Skype	B-Application
obfuscation	O
layer	O
's	O
initialization	B-Algorithm
vector	I-Algorithm
(	O
IV	O
)	O
.	O
</s>
<s>
The	O
XOR	O
of	O
these	O
two	O
32-bit	O
values	O
is	O
transformed	O
to	O
an	O
80-byte	O
RC4	B-Algorithm
key	O
using	O
an	O
unknown	O
key	O
engine	O
.	O
</s>
<s>
A	O
notable	O
misuse	O
of	O
RC4	B-Algorithm
in	O
Skype	B-Application
can	O
be	O
found	O
on	O
TCP	O
streams	O
(	O
UDP	O
is	O
unaffected	O
)	O
.	O
</s>
<s>
The	O
first	O
14	O
bytes	O
(	O
10	O
of	O
which	O
are	O
known	O
to	O
the	O
user	O
,	O
since	O
they	O
consist	O
of	O
a	O
hash	O
of	O
the	O
username	O
and	O
password	O
)	O
are	O
XOR-ed	O
with	O
the	O
RC4	B-Algorithm
stream	O
.	O
</s>
<s>
The	O
algorithm	O
is	O
a	O
variation	O
of	O
arithmetic	B-Algorithm
compression	I-Algorithm
that	O
uses	O
reals	O
instead	O
of	O
bits	O
.	O
</s>
<s>
Reverse	O
engineering	O
of	O
the	O
Skype	B-Protocol
protocol	I-Protocol
by	O
inspecting/disassembling	O
binaries	O
is	O
prohibited	O
by	O
the	O
terms	O
and	O
conditions	O
of	O
Skype	B-Application
's	O
license	O
agreement	O
.	O
</s>
