<s>
Py	B-Algorithm
is	O
a	O
stream	B-Algorithm
cipher	I-Algorithm
submitted	O
to	O
eSTREAM	O
by	O
Eli	O
Biham	O
and	O
Jennifer	O
Seberry	O
.	O
</s>
<s>
It	O
has	O
a	O
structure	O
a	O
little	O
like	O
RC4	B-Algorithm
,	O
but	O
adds	O
an	O
array	O
of	O
260	O
32-bit	O
words	O
which	O
are	O
indexed	O
using	O
a	O
permutation	O
of	O
bytes	O
,	O
and	O
produces	O
64	O
bits	O
in	O
each	O
round	O
.	O
</s>
<s>
The	O
authors	O
assert	O
that	O
the	O
name	O
be	O
pronounced	O
"	O
Roo	O
"	O
,	O
a	O
reference	O
to	O
the	O
cipher	O
's	O
Australian	O
origin	O
,	O
by	O
reading	O
the	O
letters	O
"	O
Py	B-Algorithm
"	O
as	O
Cyrillic	O
( Ру	O
)	O
rather	O
than	O
Latin	O
characters	O
.	O
</s>
<s>
This	O
somewhat	O
perverse	O
pronunciation	O
is	O
understood	O
to	O
be	O
their	O
answer	O
,	O
in	O
jest	O
,	O
to	O
the	O
difficult-to-pronounce	O
name	O
Rijndael	B-Algorithm
for	O
the	O
cipher	O
which	O
was	O
adopted	O
as	O
the	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
.	O
</s>
<s>
The	O
original	O
April	O
2005	O
proposal	O
included	O
the	O
cipher	O
Py	B-Algorithm
,	O
and	O
a	O
simplified	O
version	O
Py6	O
.	O
</s>
<s>
In	O
June	O
2006	O
,	O
the	O
authors	O
described	O
Pypy	O
(	O
even	O
more	O
confusingly	O
,	O
half-Cyrillic	O
Pyру	O
and	O
thus	O
pronounced	O
"	O
Pyroo	O
"	O
)	O
as	O
an	O
optional	O
stronger	O
variant	O
.	O
</s>
<s>
This	O
omits	O
one	O
of	O
the	O
output	O
words	O
from	O
each	O
iteration	O
of	O
Py	B-Algorithm
,	O
and	O
thus	O
operates	O
at	O
slightly	O
over	O
half	O
the	O
speed	O
of	O
Py	B-Algorithm
.	O
</s>
<s>
At	O
,	O
Gautham	O
Sekar	O
,	O
Souradyuti	O
Paul	O
and	O
Bart	O
Preneel	O
proposed	O
two	O
new	O
ciphers	O
RCR-32	O
and	O
RCR-64	O
based	O
on	O
the	O
design	O
principles	O
of	O
Pypy	O
and	O
Py	B-Algorithm
,	O
respectively	O
.	O
</s>
<s>
These	O
replace	O
a	O
variable	O
rotate	O
in	O
Py	B-Algorithm
with	O
a	O
fixed	O
rotate	O
,	O
eliminating	O
an	O
attack	O
and	O
speeding	O
up	O
the	O
cipher	O
slightly	O
.	O
</s>
<s>
,	O
the	O
best	O
cryptanalytic	O
attack	O
on	O
Py	B-Algorithm
(	O
by	O
Hongjun	O
Wu	O
and	O
Bart	O
Preneel	O
)	O
can	O
under	O
some	O
circumstances	O
(	O
e.g.	O
</s>
<s>
where	O
the	O
IV	O
is	O
much	O
longer	O
than	O
the	O
key	O
)	O
recover	O
the	O
key	O
given	O
partial	O
keystreams	B-Algorithm
for	O
224	O
chosen	O
IVs	O
.	O
</s>
<s>
In	O
a	O
more	O
difficult	O
scenario	O
from	O
the	O
point	O
of	O
view	O
of	O
attacker	O
,	O
given	O
only	O
known	O
plaintext	O
(	O
rather	O
than	O
chosen	O
plaintext	O
)	O
,	O
there	O
is	O
also	O
a	O
distinguishing	O
attack	O
on	O
the	O
keystream	B-Algorithm
(	O
by	O
Paul	O
Crowley	O
)	O
which	O
requires	O
around	O
272	O
bytes	O
of	O
output	O
and	O
comparable	O
time	O
.	O
</s>
<s>
There	O
is	O
a	O
still	O
a	O
debate	O
whether	O
these	O
attacks	O
constitute	O
an	O
academic	O
break	O
of	O
Py	B-Algorithm
.	O
</s>
<s>
When	O
the	O
attackers	O
claim	O
that	O
the	O
above	O
attacks	O
can	O
be	O
built	O
with	O
workload	O
less	O
than	O
the	O
exhaustive	O
search	O
under	O
the	O
design	O
specifications	O
of	O
Py	B-Algorithm
and	O
therefore	O
,	O
it	O
is	O
clearly	O
a	O
theoretical	O
break	O
of	O
the	O
cipher	O
,	O
the	O
designers	O
rule	O
out	O
the	O
attacks	O
because	O
Py	B-Algorithm
's	O
security	O
bounds	O
limit	O
any	O
attacker	O
to	O
a	O
total	O
of	O
264	O
bytes	O
of	O
output	O
across	O
all	O
keystreams	B-Algorithm
everywhere	O
.	O
</s>
<s>
Py	B-Algorithm
was	O
selected	O
as	O
Phase	O
2	O
Focus	O
Candidate	O
for	O
Profile	O
1	O
(	O
software	O
)	O
by	O
the	O
eSTREAM	O
project	O
but	O
did	O
not	O
advance	O
to	O
Phase	O
3	O
due	O
to	O
the	O
Wu	O
and	O
Preneel	O
chosen	O
IV	O
attack	O
.	O
</s>
<s>
In	O
January	O
2007	O
,	O
three	O
new	O
ciphers	O
namely	O
TPy	O
,	O
TPypy	O
and	O
TPy6	O
have	O
been	O
proposed	O
by	O
the	O
designers	O
of	O
Py	B-Algorithm
to	O
eliminate	O
the	O
above	O
attacks	O
.	O
</s>
<s>
The	O
best	O
attack	O
so	O
far	O
on	O
the	O
TPypy	O
,	O
which	O
is	O
conjectured	O
to	O
be	O
the	O
strongest	O
of	O
the	O
Py-family	O
of	O
ciphers	O
,	O
is	O
by	O
Sekar	O
et	O
al	O
.	O
</s>
<s>
Py	B-Algorithm
is	O
based	O
on	O
the	O
idea	O
of	O
"	O
sliding	O
arrays	O
"	O
:	O
arrays	O
are	O
indexed	O
relative	O
to	O
a	O
start	O
pointer	O
,	O
which	O
is	O
advanced	O
by	O
one	O
word	O
each	O
round	O
.	O
</s>
<s>
Where	O
modulo	O
indexing	O
is	O
available	O
(	O
hardware	O
,	O
and	O
many	O
digital	B-Architecture
signal	I-Architecture
processors	I-Architecture
)	O
,	O
these	O
can	O
be	O
implemented	O
as	O
circular	B-Data_Structure
buffers	I-Data_Structure
.	O
</s>
<s>
When	O
byte	O
output	O
is	O
required	O
,	O
Py	B-Algorithm
specifies	O
that	O
the	O
output	O
words	O
are	O
converted	O
little-endian	O
.	O
</s>
