<s>
Skein	B-Algorithm
is	O
a	O
cryptographic	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
and	O
one	O
of	O
five	O
finalists	O
in	O
the	O
NIST	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
competition	I-Algorithm
.	O
</s>
<s>
Entered	O
as	O
a	O
candidate	O
to	O
become	O
the	O
SHA-3	B-Algorithm
standard	O
,	O
the	O
successor	O
of	O
SHA-1	B-Algorithm
and	O
SHA-2	B-Algorithm
,	O
it	O
ultimately	O
lost	O
to	O
NIST	O
hash	B-Error_Name
candidate	O
Keccak	B-Algorithm
.	O
</s>
<s>
The	O
name	O
Skein	B-Algorithm
refers	O
to	O
how	O
the	O
Skein	B-Algorithm
function	O
intertwines	O
the	O
input	O
,	O
similar	O
to	O
a	O
skein	B-Algorithm
of	O
yarn	O
.	O
</s>
<s>
Skein	B-Algorithm
was	O
created	O
by	O
Bruce	O
Schneier	O
,	O
Niels	O
Ferguson	O
,	O
Stefan	O
Lucks	O
,	O
Doug	O
Whiting	O
,	O
Mihir	O
Bellare	O
,	O
Tadayoshi	O
Kohno	O
,	O
Jon	O
Callas	O
and	O
Jesse	O
Walker	O
.	O
</s>
<s>
Skein	B-Algorithm
is	O
based	O
on	O
the	O
Threefish	B-Algorithm
tweakable	O
block	O
cipher	O
compressed	O
using	O
Unique	O
Block	O
Iteration	O
(	O
UBI	O
)	O
chaining	O
mode	O
,	O
a	O
variant	O
of	O
the	O
Matyas	O
–	O
Meyer	O
–	O
Oseas	O
hash	B-Error_Name
mode	O
,	O
while	O
leveraging	O
an	O
optional	O
low-overhead	O
argument-system	O
for	O
flexibility	O
.	O
</s>
<s>
Skein	B-Algorithm
's	O
algorithm	O
and	O
a	O
reference	O
implementation	O
was	O
given	O
to	O
public	O
domain	O
.	O
</s>
<s>
Skein	B-Algorithm
supports	O
internal	O
state	O
sizes	O
of	O
256	O
,	O
512	O
and	O
1024	O
bits	O
,	O
and	O
arbitrary	O
output	O
sizes	O
.	O
</s>
<s>
The	O
authors	O
claim	O
6.1	O
cycles	O
per	O
byte	O
for	O
any	O
output	O
size	O
on	O
an	O
Intel	B-Device
Core	I-Device
2	I-Device
Duo	O
in	O
64-bit	O
mode	O
.	O
</s>
<s>
The	O
core	O
of	O
Threefish	B-Algorithm
is	O
based	O
on	O
a	O
MIX	O
function	O
that	O
transforms	O
2	O
64-bit	O
words	O
using	O
a	O
single	O
addition	O
,	O
rotation	O
by	O
a	O
constant	O
and	O
XOR	O
.	O
</s>
<s>
Threefish	B-Algorithm
's	O
nonlinearity	O
comes	O
entirely	O
from	O
the	O
combination	O
of	O
addition	O
operations	O
and	O
exclusive-ORs	O
;	O
it	O
does	O
not	O
use	O
S-boxes	B-Algorithm
.	O
</s>
<s>
The	O
function	O
is	O
optimized	O
for	O
64-bit	O
processors	O
,	O
and	O
the	O
Skein	B-Algorithm
paper	O
defines	O
optional	O
features	O
such	O
as	O
randomized	O
hashing	B-Error_Name
,	O
parallelizable	O
tree	B-Algorithm
hashing	I-Algorithm
,	O
a	O
stream	B-Algorithm
cipher	I-Algorithm
,	O
personalization	O
,	O
and	O
a	O
key	B-Algorithm
derivation	I-Algorithm
function	I-Algorithm
.	O
</s>
<s>
In	O
October	O
2010	O
,	O
an	O
attack	O
that	O
combines	O
rotational	B-General_Concept
cryptanalysis	I-General_Concept
with	O
the	O
rebound	O
attack	O
was	O
published	O
.	O
</s>
<s>
The	O
attack	O
finds	O
rotational	O
collisions	O
for	O
53	O
of	O
72	O
rounds	O
in	O
Threefish-256	O
,	O
and	O
57	O
of	O
72	O
rounds	O
in	O
Threefish-512	O
.	O
</s>
<s>
It	O
also	O
affects	O
the	O
Skein	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
.	O
</s>
<s>
The	O
Skein	B-Algorithm
team	O
tweaked	O
the	O
key	B-Algorithm
schedule	I-Algorithm
constant	O
for	O
round	O
3	O
of	O
the	O
NIST	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
competition	I-Algorithm
,	O
to	O
make	O
this	O
attack	O
less	O
effective	O
,	O
even	O
though	O
they	O
believe	O
the	O
hash	B-Error_Name
would	O
still	O
be	O
secure	O
without	O
these	O
tweaks	O
.	O
</s>
<s>
Hash	B-Error_Name
values	I-Error_Name
of	O
empty	O
string	O
.	O
</s>
<s>
Even	O
a	O
small	O
change	O
in	O
the	O
message	O
will	O
(	O
with	O
overwhelming	O
probability	O
)	O
result	O
in	O
a	O
mostly	O
different	O
hash	B-Error_Name
,	O
due	O
to	O
the	O
avalanche	O
effect	O
.	O
</s>
