<s>
Skype	B-Application
is	O
a	O
Voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
system	O
developed	O
by	O
Skype	B-Application
Technologies	O
S.A.	O
</s>
<s>
Skype	B-Application
users	O
can	O
search	O
for	O
other	O
users	O
and	O
send	O
them	O
messages	O
.	O
</s>
<s>
Skype	B-Application
reports	O
that	O
it	O
uses	O
256	O
bit	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
/	O
Rijnadel	O
encryption	O
to	O
communicate	O
between	O
Skype	B-Application
clients	O
;	O
although	O
when	O
calling	O
a	O
telephone	O
or	O
mobile	O
,	O
the	O
part	O
of	O
the	O
call	O
over	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
is	O
not	O
encrypted	O
.	O
</s>
<s>
User	O
public	B-Application
keys	I-Application
are	O
certified	O
by	O
the	O
Skype	B-Application
server	O
at	O
login	O
with	O
1536-bit	O
or	O
2048-bit	O
RSA	B-Architecture
certificates	I-Architecture
.	O
</s>
<s>
Skype	B-Application
's	O
encryption	O
is	O
inherent	O
in	O
the	O
Skype	B-Protocol
Protocol	I-Protocol
and	O
is	O
transparent	O
to	O
callers	O
.	O
</s>
<s>
Some	O
private	O
conversations	O
through	O
Skype	B-Application
such	O
as	O
audio	O
calls	O
,	O
text	O
messages	O
,	O
and	O
file	O
sending	O
(	O
image	O
,	O
audio	O
,	O
or	O
video	O
)	O
can	O
make	O
use	O
of	O
end-to-end	O
encryption	O
,	O
but	O
it	O
may	O
have	O
to	O
be	O
manually	O
turned	O
on	O
.	O
</s>
<s>
Messages	O
transmitted	O
between	O
Skype	B-Application
users	O
(	O
with	O
no	O
PSTN	O
users	O
included	O
)	O
are	O
encrypted	O
from	O
caller	O
to	O
caller	O
.	O
</s>
<s>
No	O
intermediate	O
node	O
(	O
router	B-Protocol
)	O
has	O
access	O
to	O
the	O
meaning	O
of	O
these	O
messages	O
.	O
</s>
<s>
This	O
claim	O
was	O
undermined	O
in	O
May	O
2013	O
by	O
evidence	O
that	O
Microsoft	O
(	O
owner	O
of	O
Skype	B-Application
)	O
has	O
pinged	O
unique	O
URLs	O
embedded	O
in	O
a	O
Skype	B-Application
conversation	O
;	O
this	O
could	O
only	O
happen	O
if	O
Microsoft	O
has	O
access	O
to	O
the	O
unencrypted	O
form	O
of	O
these	O
messages	O
.	O
</s>
<s>
Skype	B-Application
holds	O
registration	O
information	O
both	O
on	O
the	O
caller	O
's	O
computer	O
and	O
on	O
a	O
Skype	B-Application
server	O
.	O
</s>
<s>
Skype	B-Application
uses	O
this	O
information	O
to	O
authenticate	O
call	O
recipients	O
and	O
assure	O
that	O
callers	O
seeking	O
authentication	O
access	O
a	O
Skype	B-Application
server	O
rather	O
than	O
an	O
impostor	O
.	O
</s>
<s>
Skype	B-Application
says	O
that	O
it	O
uses	O
public-key	B-Application
encryption	I-Application
as	O
defined	O
by	O
RSA	B-Architecture
to	O
accomplish	O
this	O
.	O
</s>
<s>
The	O
Skype	B-Application
server	O
has	O
a	O
private	B-Application
key	I-Application
and	O
distributes	O
that	O
key	O
's	O
public	O
counterpart	O
with	O
every	O
copy	O
of	O
the	O
software	O
.	O
</s>
<s>
Skype	B-Application
locally	O
generates	O
public	O
and	O
private	B-Application
keys	I-Application
.	O
</s>
<s>
The	O
private	B-Application
key	I-Application
and	O
a	O
password	O
hash	O
are	O
stored	O
on	O
the	O
user	O
's	O
computer	O
.	O
</s>
<s>
Then	O
a	O
256-bit	O
AES-encrypted	O
session	O
is	O
established	O
with	O
the	O
Skype	B-Application
server	O
.	O
</s>
<s>
The	O
Skype	B-Application
server	O
verifies	O
that	O
the	O
selected	O
username	O
is	O
unique	O
and	O
follows	O
Skype	B-Application
's	O
naming	O
rules	O
.	O
</s>
<s>
For	O
each	O
call	O
,	O
Skype	B-Application
creates	O
a	O
session	O
with	O
a	O
256-bit	O
session	O
key	O
.	O
</s>
<s>
Skype	B-Application
securely	O
transmits	O
the	O
session	O
key	O
to	O
the	O
call	O
recipient	O
as	O
part	O
of	O
connecting	O
a	O
call	O
.	O
</s>
<s>
All	O
traffic	O
in	O
a	O
session	O
is	O
encrypted	O
using	O
the	O
AES	B-Algorithm
algorithm	O
running	O
in	O
Integer	O
Counter	O
Mode	O
(	O
ICM	O
)	O
.	O
</s>
<s>
Skype	B-Application
encrypts	O
the	O
current	O
counter	O
and	O
salt	O
with	O
the	O
session	O
key	O
using	O
the	O
256	O
bit	O
AES	B-Algorithm
algorithm	O
.	O
</s>
<s>
This	O
algorithm	O
returns	O
the	O
keystream	O
,	O
then	O
XORed	B-Application
with	O
the	O
message	O
content	O
.	O
</s>
<s>
Skype	B-Application
sessions	O
contain	O
multiple	O
streams	O
.	O
</s>
<s>
Skype	B-Application
uses	O
random	O
numbers	O
for	O
several	O
cryptographic	O
purposes	O
.	O
</s>
<s>
Purposes	O
include	O
protection	O
against	O
playback	O
attacks	O
,	O
creation	O
of	O
RSA	B-Architecture
key	B-Application
pairs	I-Application
,	O
and	O
creation	O
of	O
AES	O
key-halves	O
for	O
content	O
encryption	O
.	O
</s>
<s>
The	O
security	O
of	O
a	O
Skype	B-Application
peer-to-peer	O
session	O
depends	O
significantly	O
on	O
the	O
quality	O
of	O
the	O
random	O
numbers	O
generated	O
by	O
both	O
ends	O
of	O
the	O
Skype	B-Application
session	O
.	O
</s>
<s>
Skype	B-Application
uses	O
standard	O
cryptographic	O
primitives	O
to	O
achieve	O
its	O
security	O
goals	O
.	O
</s>
<s>
The	O
cryptographic	O
primitives	O
used	O
in	O
Skype	B-Application
are	O
the	O
AES	B-Algorithm
block	O
cipher	O
,	O
the	O
RSA	B-Architecture
public-key	B-Application
cryptosystem	I-Application
,	O
the	O
ISO	O
9796-2	O
signature	O
padding	O
scheme	O
,	O
the	O
SHA-1	B-Algorithm
hash	I-Algorithm
function	O
,	O
and	O
the	O
RC4	B-Algorithm
stream	O
cipher	O
.	O
</s>
<s>
Key-agreement	O
is	O
achieved	O
using	O
a	O
proprietary	B-Application
,	O
symmetric	O
protocol	O
.	O
</s>
<s>
The	O
challenge	O
response	O
is	O
to	O
customize	O
the	O
challenge	O
in	O
a	O
proprietary	B-Application
way	O
and	O
returned	O
it	O
signed	O
with	O
the	O
responder	O
's	O
private	B-Application
key	I-Application
.	O
</s>
<s>
Because	O
an	O
Identity	O
Certificate	O
contains	O
a	O
public	B-Application
key	I-Application
,	O
each	O
end	O
can	O
then	O
confirm	O
signatures	O
created	O
by	O
the	O
other	O
peer	O
.	O
</s>
<s>
Chinese	O
,	O
Russian	O
and	O
United	O
States	O
law	O
enforcement	O
agencies	O
have	O
the	O
ability	O
to	O
eavesdrop	O
on	O
Skype	B-Application
conversations	O
and	O
to	O
have	O
access	O
to	O
Skype	B-Application
users	O
 '	O
geographic	O
locations	O
.	O
</s>
<s>
This	O
ability	O
was	O
deliberately	O
added	O
by	O
Microsoft	O
for	O
law	O
enforcement	O
agencies	O
around	O
the	O
world	O
after	O
they	O
purchased	O
Skype	B-Application
in	O
2011	O
.	O
</s>
<s>
This	O
is	O
implemented	O
by	O
switching	O
the	O
Skype	B-Application
client	O
for	O
a	O
particular	O
user	O
account	O
from	O
the	O
client-side	O
encryption	O
to	O
the	O
server-side	O
encryption	O
,	O
allowing	O
dissemination	O
of	O
an	O
unencrypted	O
data	O
stream	O
.	O
</s>
<s>
While	O
Skype	B-Application
encrypts	O
users	O
 '	O
sessions	O
,	O
other	O
traffic	O
,	O
including	O
call	O
initiation	O
,	O
can	O
be	O
monitored	O
by	O
unauthorized	O
parties	O
.	O
</s>
<s>
The	O
other	O
side	O
of	O
security	O
is	O
whether	O
Skype	B-Application
imposes	O
risk	O
on	O
its	O
users	O
 '	O
computers	O
and	O
networks	O
.	O
</s>
<s>
Those	O
flaws	O
made	O
it	O
possible	O
for	O
hackers	O
to	O
run	O
hostile	O
code	O
on	O
computers	O
running	O
vulnerable	O
versions	O
of	O
Skype	B-Application
.	O
</s>
<s>
The	O
first	O
security	O
bug	O
affected	O
only	O
Microsoft	B-Application
Windows	I-Application
computers	O
.	O
</s>
<s>
It	O
allowed	O
the	O
attacker	O
to	O
use	O
a	O
buffer	B-General_Concept
overflow	I-General_Concept
to	O
crash	O
the	O
system	O
or	O
to	O
force	O
it	O
to	O
execute	O
arbitrary	O
code	O
.	O
</s>
<s>
The	O
attacker	O
could	O
provide	O
a	O
malformed	O
URL	O
using	O
the	O
Skype	B-Application
URI	B-Protocol
format	O
,	O
and	O
lure	O
the	O
user	O
to	O
request	O
it	O
to	O
execute	O
the	O
attack	O
.	O
</s>
<s>
The	O
second	O
security	O
bug	O
affected	O
all	O
platforms	O
;	O
it	O
used	O
a	O
heap-based	O
buffer	B-General_Concept
overflow	I-General_Concept
to	O
make	O
the	O
system	O
vulnerable	O
.	O
</s>
<s>
Skype	B-Application
code	O
is	O
proprietary	B-Application
and	O
closed	B-Application
source	I-Application
,	O
and	O
it	O
is	O
not	O
planned	O
to	O
become	O
open-source	B-Application
software	I-Application
,	O
according	O
to	O
Niklas	O
Zennström	O
,	O
co-founder	O
of	O
Skype	B-Application
,	O
who	O
responded	O
in	O
2004	O
to	O
questions	O
on	O
the	O
Skype	B-Protocol
security	I-Protocol
model	O
saying	O
"	O
We	O
could	O
do	O
it	O
but	O
only	O
if	O
we	O
re-engineered	O
the	O
way	O
it	O
works	O
and	O
we	O
do	O
n't	O
have	O
the	O
time	O
right	O
now	O
"	O
.	O
</s>
<s>
On	O
13	O
November	O
2012	O
a	O
Russian	O
user	O
published	O
a	O
flaw	O
in	O
Skype	B-Protocol
security	I-Protocol
which	O
allowed	O
any	O
non-professional	O
attacker	O
to	O
take	O
over	O
a	O
Skype	B-Application
account	O
knowing	O
only	O
the	O
victim	O
's	O
email	O
in	O
seven	O
simple	O
steps	O
.	O
</s>
<s>
The	O
fact	O
that	O
Skype	B-Application
records	O
data	O
about	O
calls	O
(	O
but	O
not	O
the	O
message	O
contents	O
)	O
in	O
a	O
"	O
History	O
"	O
file	O
saved	O
on	O
the	O
user	O
's	O
computer	O
.	O
</s>
<s>
Skype	B-Application
can	O
consume	O
other	O
users	O
 '	O
bandwidth	O
.	O
</s>
<s>
There	O
are	O
some	O
20,000	O
supernodes	B-Protocol
out	O
of	O
many	O
millions	O
of	O
users	O
logged	O
on	O
.	O
</s>
<s>
Skype	B-Application
Guide	O
for	O
network	O
administrators	O
claims	O
that	O
supernodes	B-Protocol
carry	O
only	O
control	O
traffic	O
up	O
to	O
10	O
kB/s	O
and	O
relays	O
may	O
carry	O
other	O
user	O
data	O
traffic	O
up	O
to	O
15	O
kB/s	O
(	O
for	O
one	O
audio	O
conference	O
call	O
)	O
.	O
</s>
<s>
Skype	B-Application
's	O
file-transfer	O
function	O
does	O
not	O
integrate	O
with	O
any	O
antivirus	B-Application
products	I-Application
,	O
although	O
Skype	B-Application
claims	O
to	O
have	O
tested	O
its	O
product	O
against	O
antivirus	B-Application
"	O
Shield	O
"	O
products	O
.	O
</s>
<s>
Skype	B-Application
does	O
not	O
document	O
all	O
communication	O
activities	O
.	O
</s>
<s>
(	O
The	O
combination	O
of	O
an	O
invited	O
and	O
a	O
reverse-engineered	O
study	O
taken	O
together	O
suggest	O
Skype	B-Application
is	O
not	O
doing	O
anything	O
hostile	O
)	O
.	O
</s>
<s>
Skype	B-Application
can	O
be	O
easily	O
blocked	O
by	O
firewalls	B-Application
.	O
</s>
<s>
Skype	B-Application
consumes	O
network	O
bandwidth	O
,	O
even	O
when	O
idle	O
(	O
even	O
for	O
non-supernodes	O
,	O
e.g.	O
,	O
for	O
NAT	B-Protocol
traversal	I-Protocol
)	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
there	O
were	O
only	O
3	O
Skype	B-Application
users	O
in	O
the	O
world	O
and	O
2	O
were	O
communicating	O
,	O
the	O
3rd	O
computer	O
would	O
be	O
taxed	O
to	O
support	O
the	O
application	O
,	O
even	O
if	O
not	O
using	O
Skype	B-Application
at	O
the	O
time	O
.	O
</s>
<s>
The	O
large	O
number	O
of	O
Skype	B-Application
computers	O
means	O
that	O
this	O
activity	O
is	O
diffuse	O
,	O
it	O
can	O
lead	O
to	O
performance	O
issues	O
on	O
standby	O
Skype	B-Application
users	O
,	O
and	O
presents	O
a	O
conduit	O
for	O
security	O
breaches	O
.	O
</s>
<s>
Skype	B-Application
prior	O
to	O
version	O
3.0.0.216	O
created	O
a	O
file	O
called	O
1.com	O
in	O
the	O
temp	O
directory	O
which	O
was	O
capable	O
of	O
reading	O
all	O
BIOS	O
data	O
from	O
a	O
PC	O
.	O
</s>
<s>
According	O
to	O
Skype	B-Application
this	O
was	O
used	O
to	O
identify	O
computers	O
and	O
provide	O
DRM	O
protection	O
for	O
plug-ins	O
.	O
</s>
<s>
The	O
URI	B-Protocol
handler	O
that	O
checks	O
URLs	O
for	O
verification	O
of	O
certain	O
file	O
extensions	O
and	O
file	O
formats	O
uses	O
case	O
sensitive	O
comparison	O
techniques	O
and	O
does	O
n't	O
check	O
all	O
potential	O
file	O
formats	O
.	O
</s>
<s>
While	O
Skype	B-Application
does	O
encrypt	O
most	O
of	O
its	O
communications	O
,	O
unencrypted	O
packets	O
containing	O
advertisements	O
are	O
pulled	O
from	O
several	O
places	O
,	O
exposing	O
a	O
cross-site	O
scripting	O
vulnerability	O
.	O
</s>
<s>
The	O
privacy	O
of	O
Skype	B-Application
traffic	O
may	O
have	O
limits	O
.	O
</s>
<s>
Although	O
Skype	B-Application
encrypts	O
communication	O
between	O
users	O
,	O
a	O
Skype	B-Application
spokesman	O
did	O
not	O
deny	O
the	O
company	O
's	O
ability	O
to	O
intercept	O
communication	O
.	O
</s>
<s>
On	O
the	O
question	O
of	O
whether	O
Skype	B-Application
could	O
listen	O
in	O
on	O
their	O
users	O
 '	O
communication	O
,	O
Kurt	O
Sauer	O
,	O
head	O
of	O
the	O
security	O
division	O
of	O
Skype	B-Application
,	O
replied	O
evasively	O
:	O
"	O
We	O
provide	O
a	O
secure	O
means	O
of	O
communication	O
.	O
</s>
<s>
This	O
suggests	O
that	O
Skype	B-Application
has	O
the	O
capacity	O
to	O
eavesdrop	O
on	O
connections	O
.	O
</s>
<s>
One	O
of	O
Skype	B-Application
's	O
minority	O
owners	O
,	O
eBay	B-Application
,	O
has	O
divulged	O
user	O
information	O
to	O
the	O
U.S.	O
government	O
.	O
</s>
<s>
Security	O
researchers	O
Biondi	O
and	O
Desclaux	O
have	O
speculated	O
that	O
Skype	B-Application
may	O
have	O
a	O
back	O
door	O
,	O
since	O
Skype	B-Application
sends	O
traffic	O
even	O
when	O
it	O
is	O
turned	O
off	O
and	O
because	O
Skype	B-Application
has	O
taken	O
extreme	O
measures	O
to	O
obfuscate	O
their	O
traffic	O
and	O
functioning	O
of	O
their	O
program	O
.	O
</s>
<s>
Several	O
media	O
sources	O
have	O
reported	O
that	O
at	O
a	O
meeting	O
about	O
the	O
"	O
Lawful	O
interception	O
of	O
IP	O
based	O
services	O
"	O
held	O
on	O
25	O
June	O
2008	O
,	O
high-ranking	O
but	O
unnamed	O
officials	O
at	O
the	O
Austrian	O
interior	O
ministry	O
said	O
that	O
they	O
could	O
listen	O
in	O
on	O
Skype	B-Application
conversations	O
without	O
problems	O
.	O
</s>
<s>
Austrian	O
public	O
broadcasting	O
service	O
ORF	O
,	O
citing	O
minutes	O
from	O
the	O
meeting	O
,	O
have	O
reported	O
that	O
"	O
the	O
Austrian	O
police	O
are	O
able	O
to	O
listen	O
in	O
on	O
Skype	B-Application
connections	O
"	O
.	O
</s>
<s>
Skype	B-Application
declined	O
to	O
comment	O
on	O
the	O
reports	O
.	O
</s>
<s>
The	O
Skype	B-Application
client	O
for	O
Linux	O
has	O
been	O
observed	O
accessing	O
the	O
Firefox	O
profile	O
folder	O
during	O
execution	O
.	O
</s>
<s>
Access	O
to	O
this	O
file	O
was	O
confirmed	O
by	O
tracing	O
system	O
calls	O
made	O
by	O
the	O
Skype	B-Application
binary	O
during	O
execution	O
.	O
</s>
<s>
The	O
Skype	B-Application
client	O
for	O
Mac	O
has	O
been	O
observed	O
accessing	O
protected	O
information	O
in	O
the	O
system	O
Address	O
Book	O
even	O
when	O
integration	O
with	O
the	O
Address	O
Book	O
(	O
on	O
by	O
default	O
)	O
is	O
disabled	O
in	O
the	O
Skype	B-Application
preferences	O
.	O
</s>
<s>
Users	O
may	O
see	O
a	O
warning	O
about	O
Skype.app	O
attempting	O
to	O
access	O
protected	O
information	O
in	O
the	O
address	O
book	O
under	O
certain	O
conditions	O
,	O
e.g.	O
</s>
<s>
launching	O
Skype	B-Application
while	O
syncing	O
with	O
a	O
mobile	O
device	O
.	O
</s>
<s>
Skype	B-Application
has	O
no	O
legitimate	O
reason	O
to	O
access	O
the	O
Address	O
Book	O
if	O
the	O
integration	O
is	O
not	O
enabled	O
.	O
</s>
<s>
Further	O
,	O
the	O
extent	O
of	O
the	O
integration	O
is	O
to	O
add	O
all	O
cards	O
from	O
the	O
Address	O
Book	O
to	O
the	O
list	O
of	O
Skype	B-Application
contacts	O
along	O
with	O
their	O
phone	O
numbers	O
,	O
which	O
can	O
be	O
accomplished	O
without	O
accessing	O
any	O
protected	O
information	O
(	O
neither	O
the	O
name	O
nor	O
numbers	O
on	O
cards	O
are	O
protected	O
)	O
and	O
thus	O
the	O
attempt	O
to	O
access	O
information	O
beyond	O
the	O
scope	O
of	O
the	O
integration	O
,	O
regardless	O
of	O
whether	O
or	O
not	O
that	O
integration	O
is	O
enabled	O
,	O
raises	O
deeper	O
questions	O
as	O
to	O
possible	O
spying	O
on	O
users	O
.	O
</s>
<s>
The	O
United	O
States	O
Federal	O
Communications	O
Commission	O
(	O
FCC	O
)	O
has	O
interpreted	O
the	O
Communications	O
Assistance	O
for	O
Law	O
Enforcement	O
Act	O
(	O
CALEA	O
)	O
as	O
requiring	O
digital	B-Application
phone	I-Application
networks	O
to	O
allow	O
wiretapping	O
if	O
authorized	O
by	O
an	O
FBI	O
warrant	O
,	O
in	O
the	O
same	O
way	O
as	O
other	O
phone	O
services	O
.	O
</s>
<s>
In	O
February	O
2009	O
Skype	B-Application
said	O
that	O
,	O
not	O
being	O
a	O
telephone	O
company	O
owning	O
phone	O
lines	O
,	O
it	O
was	O
exempt	O
from	O
CALEA	O
and	O
similar	O
laws	O
which	O
regulate	O
US	O
phone	O
companies	O
.	O
</s>
<s>
It	O
is	O
also	O
not	O
clear	O
whether	O
wiretapping	O
of	O
Skype	B-Application
communications	O
is	O
technically	O
possible	O
.	O
</s>
