<s>
Simon	B-Algorithm
is	O
a	O
family	O
of	O
lightweight	O
block	O
ciphers	O
publicly	O
released	O
by	O
the	O
National	O
Security	O
Agency	O
(	O
NSA	O
)	O
in	O
June	O
2013	O
.	O
</s>
<s>
Simon	B-Algorithm
has	O
been	O
optimized	O
for	O
performance	O
in	O
hardware	O
implementations	O
,	O
while	O
its	O
sister	O
algorithm	O
,	O
Speck	B-Algorithm
,	O
has	O
been	O
optimized	O
for	O
software	O
implementations	O
.	O
</s>
<s>
The	O
NSA	O
began	O
working	O
on	O
the	O
Simon	B-Algorithm
and	O
Speck	B-Algorithm
ciphers	O
in	O
2011	O
.	O
</s>
<s>
The	O
agency	O
anticipated	O
some	O
agencies	O
in	O
the	O
US	O
federal	O
government	O
would	O
need	O
a	O
cipher	O
that	O
would	O
operate	O
well	O
on	O
a	O
diverse	O
collection	O
of	O
Internet	B-Operating_System
of	I-Operating_System
Things	I-Operating_System
devices	O
while	O
maintaining	O
an	O
acceptable	O
level	O
of	O
security	O
.	O
</s>
<s>
The	O
Simon	B-Algorithm
block	O
cipher	O
is	O
a	O
balanced	B-Algorithm
Feistel	I-Algorithm
cipher	I-Algorithm
with	O
an	O
n-bit	O
word	O
,	O
and	O
therefore	O
the	O
block	O
length	O
is	O
2n	O
.	O
</s>
<s>
The	O
key	O
length	O
is	O
a	O
multiple	O
of	O
n	O
by	O
2	O
,	O
3	O
,	O
or	O
4	O
,	O
which	O
is	O
the	O
value	O
m	O
.	O
Therefore	O
,	O
a	O
Simon	B-Algorithm
cipher	O
implementation	O
is	O
denoted	O
as	O
Simon2n/nm	O
.	O
</s>
<s>
The	O
block	O
component	O
of	O
the	O
cipher	O
is	O
uniform	O
between	O
the	O
Simon	B-Algorithm
implementations	O
;	O
however	O
,	O
the	O
key	O
generation	O
logic	O
is	O
dependent	O
on	O
the	O
implementation	O
of	O
2	O
,	O
3	O
or	O
4	O
keys	O
.	O
</s>
<s>
Simon	B-Algorithm
supports	O
the	O
following	O
combinations	O
of	O
block	O
sizes	O
,	O
key	O
sizes	O
and	O
number	O
of	O
rounds	O
:	O
</s>
<s>
Let	O
notate	O
a	O
left	O
circular	B-Algorithm
shift	I-Algorithm
by	O
bits	O
.	O
</s>
<s>
The	O
key	B-Algorithm
schedule	I-Algorithm
structure	O
may	O
or	O
may	O
not	O
be	O
balanced	O
.	O
</s>
<s>
The	O
constant	O
sequence	O
,	O
,	O
is	O
created	O
by	O
a	O
Linear	B-Architecture
Feedback	I-Architecture
Shift	I-Architecture
Register	I-Architecture
(	O
LFSR	B-Architecture
)	O
.	O
</s>
<s>
The	O
LFSR	B-Architecture
is	O
created	O
by	O
a	O
5-bit	O
field	O
.	O
</s>
<s>
The	O
constant	O
bit	O
operates	O
on	O
a	O
key	O
block	O
once	O
per	O
round	O
on	O
the	O
lowest	O
bit	O
in	O
order	O
to	O
add	O
non-key-dependent	O
entropy	O
to	O
the	O
key	B-Algorithm
schedule	I-Algorithm
.	O
</s>
<s>
The	O
LFSR	B-Architecture
has	O
different	O
logic	O
for	O
each	O
sequence	O
;	O
however	O
,	O
the	O
initial	O
condition	O
is	O
the	O
same	O
for	O
encryption	O
.	O
</s>
<s>
The	O
initial	O
condition	O
of	O
the	O
LFSR	B-Architecture
for	O
decryption	O
varies	O
on	O
the	O
round	O
.	O
</s>
<s>
The	O
designers	O
claim	O
that	O
Simon	B-Algorithm
,	O
though	O
a	O
"	O
lightweight	O
"	O
cipher	O
,	O
is	O
designed	O
to	O
have	O
the	O
full	O
security	O
possible	O
for	O
each	O
block	O
and	O
key	O
size	O
,	O
against	O
standard	O
chosen-plaintext	O
(	O
CPA	O
)	O
and	O
chosen-ciphertext	O
(	O
CCA	O
)	O
attacks	O
.	O
</s>
<s>
No	O
effort	O
was	O
made	O
to	O
resist	O
attacks	O
in	O
the	O
known-key	B-General_Concept
distinguishing	I-General_Concept
attack	I-General_Concept
model	O
,	O
nor	O
did	O
the	O
designers	O
evaluate	O
Simon	B-Algorithm
for	O
use	O
as	O
a	O
hash	B-Error_Name
function	I-Error_Name
.	O
</s>
<s>
As	O
of	O
2018	O
,	O
no	O
successful	O
attack	O
on	O
full-round	O
Simon	B-Algorithm
of	O
any	O
variant	O
is	O
known	O
.	O
</s>
<s>
Due	O
to	O
interest	O
in	O
Simon	B-Algorithm
and	O
Speck	B-Algorithm
,	O
about	O
70	O
cryptanalysis	O
papers	O
have	O
been	O
published	O
on	O
them	O
.	O
</s>
<s>
The	O
best	O
published	O
attacks	O
on	O
Simon	B-Algorithm
in	O
the	O
standard	O
attack	O
model	O
(	O
CPA/CCA	O
with	O
unknown	O
key	O
)	O
are	O
differential	O
cryptanalysis	O
attacks	O
;	O
these	O
make	O
it	O
through	O
about	O
70	O
–	O
75%	O
of	O
the	O
rounds	O
of	O
most	O
variants	O
,	O
though	O
these	O
best	O
attacks	O
are	O
only	O
marginally	O
faster	O
than	O
brute-force	O
.	O
</s>
<s>
The	O
design	O
team	O
states	O
that	O
while	O
designing	O
Simon	B-Algorithm
,	O
they	O
found	O
differential	O
attacks	O
to	O
be	O
the	O
limiting	O
attacks	O
,	O
i.e.	O
</s>
<s>
the	O
type	O
of	O
attack	O
that	O
makes	O
it	O
through	O
the	O
most	O
rounds	O
;	O
they	O
then	O
set	O
the	O
number	O
of	O
rounds	O
to	O
leave	O
a	O
security	O
margin	O
similar	O
to	O
AES-128	B-Algorithm
'	O
s	O
at	O
approximately	O
30%	O
.	O
</s>
<s>
Simon	B-Algorithm
has	O
been	O
criticized	O
for	O
having	O
too	O
small	O
a	O
security	O
margin	O
,	O
i.e.	O
</s>
<s>
Simon	B-Algorithm
's	O
design	O
team	O
counters	O
that	O
there	O
is	O
a	O
real-world	O
cost	O
to	O
unnecessarily	O
large	O
security	O
margins	O
,	O
especially	O
on	O
lightweight	O
devices	O
,	O
that	O
cryptanalysis	O
during	O
the	O
design	O
phase	O
allowed	O
the	O
number	O
of	O
rounds	O
to	O
be	O
set	O
appropriately	O
,	O
and	O
that	O
they	O
targeted	O
AES	B-Algorithm
's	O
security	O
margin	O
.	O
</s>
<s>
Simon	B-Algorithm
includes	O
a	O
round	O
counter	O
in	O
the	O
key	B-Algorithm
schedule	I-Algorithm
.	O
</s>
<s>
The	O
designers	O
state	O
this	O
was	O
included	O
to	O
block	O
slide	O
and	O
rotational	B-General_Concept
cryptanalysis	I-General_Concept
attacks	O
.	O
</s>
<s>
Still	O
,	O
rotational-XOR	O
cryptanalysis	O
has	O
been	O
used	O
to	O
find	O
distinguishers	O
against	O
reduced-round	O
versions	O
of	O
related	O
ciphers	O
like	O
Speck	B-Algorithm
.	O
</s>
<s>
Though	O
the	O
authors	O
do	O
n't	O
describe	O
standard	O
key-recovery	O
attacks	O
based	O
on	O
their	O
distinguishers	O
,	O
their	O
best	O
distinguishers	O
on	O
Simon32	O
and	O
Simon48	O
in	O
the	O
known-key	B-General_Concept
distinguishing	I-General_Concept
attack	I-General_Concept
model	O
for	O
certain	O
weak	O
key	O
classes	O
make	O
it	O
through	O
slightly	O
more	O
rounds	O
than	O
the	O
best	O
differential	O
distinguishers	O
.	O
</s>
<s>
The	O
designers	O
also	O
state	O
that	O
Simon	B-Algorithm
was	O
not	O
designed	O
to	O
resist	O
known-key	B-General_Concept
distinguishing	I-General_Concept
attacks	I-General_Concept
(	O
which	O
do	O
not	O
directly	O
compromise	O
the	O
confidentiality	O
of	O
ciphers	O
)	O
.	O
</s>
<s>
Simon	B-Algorithm
's	O
designers	O
have	O
been	O
criticized	O
for	O
not	O
providing	O
more	O
details	O
on	O
NSA	O
cryptanalysis	O
of	O
the	O
ciphers	O
.	O
</s>
<s>
The	O
NSA	O
has	O
approved	O
Simon128/256	O
and	O
Speck128/256	O
for	O
use	O
in	O
U.S.	O
National	O
Security	O
Systems	O
,	O
though	O
AES-256	B-Algorithm
is	O
still	O
recommended	O
for	O
non-constrained	O
applications	O
.	O
</s>
<s>
Initial	O
attempts	O
to	O
standardise	O
Simon	B-Algorithm
and	O
Speck	B-Algorithm
failed	O
to	O
meet	O
International	O
Organization	O
for	O
Standardization	O
super-majority	O
required	O
by	O
the	O
process	O
and	O
the	O
ciphers	O
were	O
not	O
adopted	O
.	O
</s>
<s>
Expert	O
delegates	O
to	O
the	O
ISO	O
from	O
several	O
countries	O
including	O
Germany	O
,	O
Japan	O
and	O
Israel	O
opposed	O
the	O
efforts	O
by	O
the	O
NSA	O
to	O
standardise	O
the	O
Simon	B-Algorithm
and	O
Speck	B-Algorithm
ciphers	O
,	O
citing	O
concerns	O
that	O
the	O
NSA	O
is	O
pushing	O
for	O
their	O
standardisation	O
with	O
knowledge	O
of	O
exploitable	O
weaknesses	O
in	O
the	O
ciphers	O
.	O
</s>
<s>
In	O
response	O
to	O
concerns	O
,	O
the	O
NSA	O
stated	O
that	O
more	O
than	O
70	O
security	O
analysis	O
papers	O
from	O
some	O
of	O
the	O
world	O
's	O
leading	O
cryptographers	O
support	O
NSA	O
's	O
conclusion	O
that	O
the	O
algorithms	O
are	O
secure	O
and	O
NSA	O
affirmed	O
that	O
it	O
is	O
not	O
aware	O
of	O
any	O
cryptanalytic	O
techniques	O
that	O
would	O
allow	O
them	O
or	O
anyone	O
else	O
to	O
exploit	O
Simon	B-Algorithm
or	O
Speck	B-Algorithm
.	O
</s>
<s>
After	O
initial	O
attempts	O
to	O
standardise	O
the	O
ciphers	O
failed	O
,	O
the	O
ISO	O
standardised	O
Simon	B-Algorithm
and	O
Speck	B-Algorithm
in	O
other	O
working	O
groups	O
.	O
</s>
<s>
As	O
of	O
October	O
2018	O
,	O
the	O
Simon	B-Algorithm
and	O
Speck	B-Algorithm
ciphers	O
have	O
been	O
standardized	O
by	O
ISO	O
as	O
a	O
part	O
of	O
the	O
RFID	O
air	O
interface	O
standard	O
,	O
International	O
Standard	O
ISO/29167	O
-21	O
(	O
for	O
Simon	B-Algorithm
)	O
and	O
International	O
Standard	O
ISO/29167	O
-22	O
(	O
for	O
Speck	B-Algorithm
)	O
,	O
making	O
them	O
available	O
for	O
use	O
by	O
commercial	O
entities	O
.	O
</s>
