<s>
A5/1	B-Algorithm
is	O
a	O
stream	B-Algorithm
cipher	I-Algorithm
used	O
to	O
provide	O
over-the-air	O
communication	O
privacy	O
in	O
the	O
GSM	O
cellular	O
telephone	O
standard	O
.	O
</s>
<s>
It	O
is	O
one	O
of	O
several	O
implementations	O
of	O
the	O
A5	B-Algorithm
security	O
protocol	O
.	O
</s>
<s>
A5/1	B-Algorithm
is	O
used	O
in	O
Europe	O
and	O
the	O
United	O
States	O
.	O
</s>
<s>
A5/2	B-Algorithm
was	O
a	O
deliberate	O
weakening	O
of	O
the	O
algorithm	O
for	O
certain	O
export	O
regions	O
.	O
</s>
<s>
A5/1	B-Algorithm
was	O
developed	O
in	O
1987	O
,	O
when	O
GSM	O
was	O
not	O
yet	O
considered	O
for	O
use	O
outside	O
Europe	O
,	O
and	O
A5/2	B-Algorithm
was	O
developed	O
in	O
1989	O
.	O
</s>
<s>
In	O
2000	O
,	O
around	O
130	O
million	O
GSM	O
customers	O
relied	O
on	O
A5/1	B-Algorithm
to	O
protect	O
the	O
confidentiality	O
of	O
their	O
voice	O
communications	O
.	O
</s>
<s>
Security	O
researcher	O
Ross	O
Anderson	O
reported	O
in	O
1994	O
that	O
"	O
there	O
was	O
a	O
terrific	O
row	O
between	O
the	O
NATO	O
signal	B-General_Concept
intelligence	I-General_Concept
agencies	I-General_Concept
in	O
the	O
mid-1980s	O
over	O
whether	O
GSM	O
encryption	O
should	O
be	O
strong	O
or	O
not	O
.	O
</s>
<s>
A5/1	B-Algorithm
is	O
used	O
to	O
produce	O
for	O
each	O
burst	O
a	O
114	O
bit	O
sequence	O
of	O
keystream	B-Algorithm
which	O
is	O
XORed	O
with	O
the	O
114	O
bits	O
prior	O
to	O
modulation	O
.	O
</s>
<s>
A5/1	B-Algorithm
is	O
initialised	O
using	O
a	O
64-bit	O
key	O
together	O
with	O
a	O
publicly	O
known	O
22-bit	O
frame	O
number	O
.	O
</s>
<s>
When	O
operating	O
in	O
GPRS	O
/	O
EDGE	O
mode	O
,	O
higher	O
bandwidth	O
radio	O
modulation	O
allows	O
for	O
larger	O
348	O
bits	O
frames	O
,	O
and	O
A5/3	B-Algorithm
is	O
then	O
used	O
in	O
a	O
stream	B-Algorithm
cipher	I-Algorithm
mode	O
to	O
maintain	O
confidentiality	O
.	O
</s>
<s>
A5/1	B-Algorithm
is	O
based	O
around	O
a	O
combination	O
of	O
three	O
linear-feedback	B-Architecture
shift	I-Architecture
registers	I-Architecture
(	O
LFSRs	B-Architecture
)	O
with	O
irregular	O
clocking	O
.	O
</s>
<s>
After	O
this	O
is	O
completed	O
,	O
the	O
cipher	O
is	O
ready	O
to	O
produce	O
two	O
114	O
bit	O
sequences	O
of	O
output	O
keystream	B-Algorithm
,	O
first	O
114	O
for	O
downlink	O
,	O
last	O
114	O
for	O
uplink	O
.	O
</s>
<s>
A	O
number	O
of	O
attacks	O
on	O
A5/1	B-Algorithm
have	O
been	O
published	O
,	O
and	O
the	O
American	O
National	O
Security	O
Agency	O
is	O
able	O
to	O
routinely	O
decrypt	O
A5/1	B-Algorithm
messages	O
according	O
to	O
released	O
internal	O
documents	O
.	O
</s>
<s>
In	O
2006	O
Elad	O
Barkan	O
,	O
Eli	O
Biham	O
and	O
Nathan	O
Keller	O
demonstrated	O
attacks	O
against	O
A5/1	B-Algorithm
,	O
A5/3	B-Algorithm
,	O
or	O
even	O
GPRS	O
that	O
allow	O
attackers	O
to	O
tap	O
GSM	O
mobile	O
phone	O
conversations	O
and	O
decrypt	O
them	O
either	O
in	O
real-time	O
,	O
or	O
at	O
any	O
later	O
time	O
.	O
</s>
<s>
According	O
to	O
professor	O
Jan	O
Arild	O
Audestad	O
,	O
at	O
the	O
standardization	O
process	O
which	O
started	O
in	O
1982	O
,	O
A5/1	B-Algorithm
was	O
originally	O
proposed	O
to	O
have	O
a	O
key	O
length	O
of	O
128	O
bits	O
.	O
</s>
<s>
The	O
first	O
attack	O
on	O
the	O
A5/1	B-Algorithm
was	O
proposed	O
by	O
Ross	O
Anderson	O
in	O
1994	O
.	O
</s>
<s>
In	O
2000	O
,	O
Alex	O
Biryukov	O
,	O
Adi	O
Shamir	O
and	O
David	O
Wagner	O
showed	O
that	O
A5/1	B-Algorithm
can	O
be	O
cryptanalysed	O
in	O
real	O
time	O
using	O
a	O
time-memory	O
tradeoff	O
attack	O
,	O
based	O
on	O
earlier	O
work	O
by	O
Jovan	O
Golic	O
.	O
</s>
<s>
The	O
same	O
year	O
,	O
Eli	O
Biham	O
and	O
Orr	O
Dunkelman	O
also	O
published	O
an	O
attack	O
on	O
A5/1	B-Algorithm
with	O
a	O
total	O
work	O
complexity	O
of	O
239.91	O
A5/1	B-Algorithm
clockings	O
given	O
220.8	O
bits	O
of	O
known	O
plaintext	O
.	O
</s>
<s>
Ekdahl	O
and	O
Johansson	O
published	O
an	O
attack	O
on	O
the	O
initialisation	O
procedure	O
which	O
breaks	O
A5/1	B-Algorithm
in	O
a	O
few	O
minutes	O
using	O
two	O
to	O
five	O
minutes	O
of	O
conversation	O
plaintext	O
.	O
</s>
<s>
GSM	O
phones	O
can	O
be	O
convinced	O
to	O
use	O
the	O
much	O
weaker	O
A5/2	B-Algorithm
cipher	O
briefly	O
.	O
</s>
<s>
A5/2	B-Algorithm
can	O
be	O
broken	O
easily	O
,	O
and	O
the	O
phone	O
uses	O
the	O
same	O
key	O
as	O
for	O
the	O
stronger	O
A5/1	B-Algorithm
algorithm	O
.	O
</s>
<s>
A	O
second	O
attack	O
on	O
A5/1	B-Algorithm
is	O
outlined	O
,	O
a	O
ciphertext-only	O
time-memory	O
tradeoff	O
attack	O
which	O
requires	O
a	O
large	O
amount	O
of	O
precomputation	O
.	O
</s>
<s>
In	O
2006	O
,	O
Elad	O
Barkan	O
,	O
Eli	O
Biham	O
,	O
Nathan	O
Keller	O
published	O
the	O
full	O
version	O
of	O
their	O
2003	O
paper	O
,	O
with	O
attacks	O
against	O
A5/X	O
сiphers	O
.	O
</s>
<s>
In	O
2007	O
Universities	O
of	O
Bochum	O
and	O
Kiel	O
started	O
a	O
research	O
project	O
to	O
create	O
a	O
massively	O
parallel	O
FPGA-based	O
cryptographic	O
accelerator	O
.	O
</s>
<s>
COPACOBANA	O
was	O
the	O
first	O
commercially	O
available	O
solution	O
using	O
fast	O
time-memory	O
trade-off	O
techniques	O
that	O
could	O
be	O
used	O
to	O
attack	O
the	O
popular	O
A5/1	B-Algorithm
and	O
A5/2	B-Algorithm
algorithms	O
,	O
used	O
in	O
GSM	O
voice	O
encryption	O
,	O
as	O
well	O
as	O
the	O
Data	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
DES	B-Algorithm
)	O
.	O
</s>
<s>
In	O
2008	O
,	O
the	O
group	O
The	O
Hackers	O
Choice	O
launched	O
a	O
project	O
to	O
develop	O
a	O
practical	O
attack	O
on	O
A5/1	B-Algorithm
.	O
</s>
<s>
Together	O
with	O
the	O
scanning	O
capabilities	O
developed	O
as	O
part	O
of	O
the	O
sister	O
project	O
,	O
the	O
group	O
expected	O
to	O
be	O
able	O
to	O
record	O
any	O
GSM	O
call	O
or	O
SMS	O
encrypted	O
with	O
A5/1	B-Algorithm
,	O
and	O
within	O
about	O
3	O
–	O
5	O
minutes	O
derive	O
the	O
encryption	O
key	O
and	O
hence	O
listen	O
to	O
the	O
call	O
and	O
read	O
the	O
SMS	O
in	O
clear	O
.	O
</s>
<s>
It	O
created	O
the	O
look-up	O
tables	O
using	O
Nvidia	O
GPGPUs	B-Architecture
via	O
a	O
peer-to-peer	O
distributed	B-Architecture
computing	I-Architecture
architecture	I-Architecture
.	O
</s>
<s>
In	O
December	O
2009	O
,	O
the	O
A5/1	B-Algorithm
Cracking	O
Project	O
attack	O
tables	O
for	O
A5/1	B-Algorithm
were	O
announced	O
by	O
Chris	O
Paget	O
and	O
Karsten	O
Nohl	O
.	O
</s>
<s>
The	O
tables	O
use	O
a	O
combination	O
of	O
compression	O
techniques	O
,	O
including	O
rainbow	B-Algorithm
tables	I-Algorithm
and	O
distinguished	O
point	O
chains	O
.	O
</s>
<s>
These	O
tables	O
constituted	O
only	O
parts	O
of	O
the	O
1.7	O
TB	O
completed	O
table	O
and	O
had	O
been	O
computed	O
during	O
three	O
months	O
using	O
40	O
distributed	O
CUDA	B-Architecture
nodes	O
and	O
then	O
published	O
over	O
BitTorrent	B-Protocol
.	O
</s>
<s>
More	O
recently	O
the	O
project	O
has	O
announced	O
a	O
switch	O
to	O
faster	O
ATI	O
Evergreen	O
code	O
,	O
together	O
with	O
a	O
change	O
in	O
the	O
format	O
of	O
the	O
tables	O
and	O
Frank	O
A	O
.	O
Stevenson	O
announced	O
breaks	O
of	O
A5/1	B-Algorithm
using	O
the	O
ATI	O
generated	O
tables	O
.	O
</s>
<s>
Documents	O
leaked	O
by	O
Edward	O
Snowden	O
in	O
2013	O
state	O
that	O
the	O
NSA	O
"	O
can	O
process	O
encrypted	O
A5/1	B-Algorithm
"	O
.	O
</s>
