<s>
In	O
cryptography	O
,	O
Corrected	B-Algorithm
Block	I-Algorithm
TEA	I-Algorithm
(	O
often	O
referred	O
to	O
as	O
XXTEA	B-Algorithm
)	O
is	O
a	O
block	O
cipher	B-Application
designed	O
to	O
correct	O
weaknesses	O
in	O
the	O
original	O
Block	B-Algorithm
TEA	I-Algorithm
.	O
</s>
<s>
XXTEA	B-Algorithm
is	O
vulnerable	O
to	O
a	O
chosen-plaintext	O
attack	O
requiring	O
259	O
queries	O
and	O
negligible	O
work	O
.	O
</s>
<s>
The	O
cipher	B-Application
's	O
designers	O
were	O
Roger	O
Needham	O
and	O
David	O
Wheeler	O
of	O
the	O
Cambridge	O
Computer	O
Laboratory	O
,	O
and	O
the	O
algorithm	O
was	O
presented	O
in	O
an	O
unpublished	O
technical	O
report	O
in	O
October	O
1998	O
(	O
Wheeler	O
and	O
Needham	O
,	O
1998	O
)	O
.	O
</s>
<s>
Formally	O
speaking	O
,	O
XXTEA	B-Algorithm
is	O
a	O
consistent	O
incomplete	O
source-heavy	O
heterogeneous	O
UFN	O
(	O
unbalanced	B-Algorithm
Feistel	I-Algorithm
network	I-Algorithm
)	O
block	O
cipher	B-Application
.	O
</s>
<s>
XXTEA	B-Algorithm
operates	O
on	O
variable-length	O
blocks	O
that	O
are	O
some	O
arbitrary	O
multiple	O
of	O
32	O
bits	O
in	O
size	O
(	O
minimum	O
64	O
bits	O
)	O
.	O
</s>
<s>
The	O
original	O
Block	B-Algorithm
TEA	I-Algorithm
applies	O
the	O
XTEA	B-Algorithm
round	O
function	O
to	O
each	O
word	O
in	O
the	O
block	O
and	O
combines	O
it	O
additively	O
with	O
its	O
leftmost	O
neighbour	O
.	O
</s>
<s>
Slow	O
diffusion	O
rate	O
of	O
the	O
decryption	O
process	O
was	O
immediately	O
exploited	O
to	O
break	O
the	O
cipher	B-Application
.	O
</s>
<s>
Corrected	B-Algorithm
Block	I-Algorithm
TEA	I-Algorithm
uses	O
a	O
more	O
involved	O
round	O
function	O
which	O
makes	O
use	O
of	O
both	O
immediate	O
neighbours	O
in	O
processing	O
each	O
word	O
in	O
the	O
block	O
.	O
</s>
<s>
XXTEA	B-Algorithm
is	O
likely	O
to	O
be	O
more	O
efficient	O
than	O
XTEA	B-Algorithm
for	O
longer	O
messages	O
.	O
</s>
<s>
Needham	O
&	O
Wheeler	O
make	O
the	O
following	O
comments	O
on	O
the	O
use	O
of	O
Block	B-Algorithm
TEA	I-Algorithm
:	O
</s>
<s>
If	O
it	O
is	O
not	O
acceptable	O
to	O
have	O
very	O
long	O
messages	O
,	O
they	O
can	O
be	O
broken	O
into	O
chunks	O
say	O
of	O
60	O
words	O
and	O
chained	B-Algorithm
analogously	O
to	O
the	O
methods	O
used	O
for	O
DES	B-Algorithm
.	O
</s>
<s>
However	O
,	O
due	O
to	O
the	O
incomplete	O
nature	O
of	O
the	O
round	O
function	O
,	O
two	O
large	O
ciphertexts	O
of	O
53	O
or	O
more	O
32-bit	O
words	O
identical	O
in	O
all	O
but	O
12	O
words	O
can	O
be	O
found	O
by	O
a	O
simple	O
brute-force	O
collision	O
search	O
requiring	O
296−N	O
memory	O
,	O
2N	O
time	O
and	O
2N+296−N	O
chosen	O
plaintexts	O
,	O
in	O
other	O
words	O
with	O
a	O
total	O
time*memory	O
complexity	O
of	O
296	O
,	O
which	O
is	O
actually	O
2wordsize*	O
fullcycles/2	O
for	O
any	O
such	O
cipher	B-Application
.	O
</s>
<s>
It	O
is	O
currently	O
unknown	O
if	O
such	O
partial	O
collisions	O
pose	O
any	O
threat	O
to	O
the	O
security	O
of	O
the	O
cipher	B-Application
.	O
</s>
<s>
The	O
unusually	O
small	O
size	O
of	O
the	O
XXTEA	B-Algorithm
algorithm	O
would	O
make	O
it	O
a	O
viable	O
option	O
in	O
situations	O
where	O
there	O
are	O
extreme	O
constraints	O
e.g.	O
</s>
<s>
legacy	O
hardware	O
systems	O
(	O
perhaps	O
embedded	O
)	O
where	O
the	O
amount	O
of	O
available	O
RAM	B-Architecture
is	O
minimal	O
,	O
or	O
alternatively	O
single-board	B-Device
computers	I-Device
such	O
as	O
the	O
Raspberry	B-Operating_System
Pi	I-Operating_System
,	O
Banana	B-Application
Pi	I-Application
or	O
Arduino	O
.	O
</s>
<s>
An	O
attack	O
published	O
in	O
2010	O
by	O
E	O
.	O
Yarrkov	O
presents	O
a	O
chosen-plaintext	O
attack	O
against	O
full-round	O
XXTEA	B-Algorithm
with	O
wide	O
block	O
,	O
requiring	O
259	O
queries	O
for	O
a	O
block	O
size	O
of	O
212	O
bytes	O
or	O
more	O
,	O
and	O
negligible	O
work	O
.	O
</s>
<s>
To	O
cipher	B-Application
"	O
212	O
bytes	O
or	O
more	O
"	O
algorithm	O
performs	O
just	O
6	O
rounds	O
,	O
and	O
carefully	O
chosen	O
bit	O
patterns	O
allows	O
to	O
detect	O
and	O
analyze	O
avalanche	O
effect	O
.	O
</s>
<s>
The	O
original	O
formulation	O
of	O
the	O
Corrected	B-Algorithm
Block	I-Algorithm
TEA	I-Algorithm
algorithm	O
,	O
published	O
by	O
David	O
Wheeler	O
and	O
Roger	O
Needham	O
,	O
is	O
as	O
follows	O
:	O
</s>
<s>
Note	O
that	O
the	O
initialization	O
of	O
z	O
is	O
Undefined	B-Language
behavior	I-Language
for	O
n	O
<	O
1	O
which	O
may	O
cause	O
a	O
segmentation	B-Error_Name
fault	I-Error_Name
or	O
other	O
unwanted	O
behavior	O
–	O
it	O
would	O
be	O
better	O
placed	O
inside	O
the	O
'	O
Coding	O
Part	O
 '	O
block	O
.	O
</s>
