<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
hash	B-Algorithm
list	I-Algorithm
is	O
typically	O
a	O
list	O
of	O
hashes	B-Algorithm
of	O
the	O
data	O
blocks	O
in	O
a	O
file	O
or	O
set	O
of	O
files	O
.	O
</s>
<s>
Lists	O
of	O
hashes	B-Algorithm
are	O
used	O
for	O
many	O
different	O
purposes	O
,	O
such	O
as	O
fast	O
table	O
lookup	O
(	O
hash	B-Algorithm
tables	I-Algorithm
)	O
and	O
distributed	O
databases	O
(	O
distributed	B-Operating_System
hash	I-Operating_System
tables	I-Operating_System
)	O
.	O
</s>
<s>
A	O
hash	B-Algorithm
list	I-Algorithm
is	O
an	O
extension	O
of	O
the	O
concept	O
of	O
hashing	O
an	O
item	O
(	O
for	O
instance	O
,	O
a	O
file	O
)	O
.	O
</s>
<s>
A	O
hash	B-Algorithm
list	I-Algorithm
is	O
a	O
subtree	B-Application
of	O
a	O
Merkle	B-Algorithm
tree	I-Algorithm
.	O
</s>
<s>
Often	O
,	O
an	O
additional	O
hash	B-Algorithm
of	O
the	O
hash	B-Algorithm
list	I-Algorithm
itself	O
(	O
a	O
top	O
hash	B-Algorithm
,	O
also	O
called	O
root	O
hash	B-Algorithm
or	O
master	O
hash	B-Algorithm
)	O
is	O
used	O
.	O
</s>
<s>
Before	O
downloading	O
a	O
file	O
on	O
a	O
p2p	O
network	O
,	O
in	O
most	O
cases	O
the	O
top	O
hash	B-Algorithm
is	O
acquired	O
from	O
a	O
trusted	O
source	O
,	O
for	O
instance	O
a	O
friend	O
or	O
a	O
web	O
site	O
that	O
is	O
known	O
to	O
have	O
good	O
recommendations	O
of	O
files	O
to	O
download	O
.	O
</s>
<s>
When	O
the	O
top	O
hash	B-Algorithm
is	O
available	O
,	O
the	O
hash	B-Algorithm
list	I-Algorithm
can	O
be	O
received	O
from	O
any	O
non-trusted	O
source	O
,	O
like	O
any	O
peer	O
in	O
the	O
p2p	O
network	O
.	O
</s>
<s>
Then	O
the	O
received	O
hash	B-Algorithm
list	I-Algorithm
is	O
checked	O
against	O
the	O
trusted	O
top	O
hash	B-Algorithm
,	O
and	O
if	O
the	O
hash	B-Algorithm
list	I-Algorithm
is	O
damaged	O
or	O
fake	O
,	O
another	O
hash	B-Algorithm
list	I-Algorithm
from	O
another	O
source	O
will	O
be	O
tried	O
until	O
the	O
program	O
finds	O
one	O
that	O
matches	O
the	O
top	O
hash	B-Algorithm
.	O
</s>
<s>
In	O
some	O
systems	O
(	O
for	O
example	O
,	O
BitTorrent	B-Protocol
)	O
,	O
instead	O
of	O
a	O
top	O
hash	B-Algorithm
the	O
whole	O
hash	B-Algorithm
list	I-Algorithm
is	O
available	O
on	O
a	O
web	O
site	O
in	O
a	O
small	O
file	O
.	O
</s>
<s>
Such	O
a	O
"	O
torrent	B-Protocol
file	O
"	O
contains	O
a	O
description	O
,	O
file	O
names	O
,	O
a	O
hash	B-Algorithm
list	I-Algorithm
and	O
some	O
additional	O
data	O
.	O
</s>
<s>
Hash	B-Algorithm
lists	I-Algorithm
can	O
be	O
used	O
to	O
protect	O
any	O
kind	O
of	O
data	O
stored	O
,	O
handled	O
and	O
transferred	O
in	O
and	O
between	O
computers	O
.	O
</s>
<s>
An	O
important	O
use	O
of	O
hash	B-Algorithm
lists	I-Algorithm
is	O
to	O
make	O
sure	O
that	O
data	O
blocks	O
received	O
from	O
other	O
peers	O
in	O
a	O
peer-to-peer	O
network	O
are	O
received	O
undamaged	O
and	O
unaltered	O
,	O
and	O
to	O
check	O
that	O
the	O
other	O
peers	O
do	O
not	O
"	O
lie	O
"	O
and	O
send	O
fake	O
blocks	O
.	O
</s>
<s>
Usually	O
a	O
cryptographic	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
such	O
as	O
SHA-256	B-Algorithm
is	O
used	O
for	O
the	O
hashing	O
.	O
</s>
<s>
If	O
the	O
hash	B-Algorithm
list	I-Algorithm
only	O
needs	O
to	O
protect	O
against	O
unintentional	O
damage	O
unsecured	O
checksums	B-Algorithm
such	O
as	O
CRCs	O
can	O
be	O
used	O
.	O
</s>
<s>
Hash	B-Algorithm
lists	I-Algorithm
are	O
better	O
than	O
a	O
simple	O
hash	B-Algorithm
of	O
the	O
entire	O
file	O
since	O
,	O
in	O
the	O
case	O
of	O
a	O
data	O
block	O
being	O
damaged	O
,	O
this	O
is	O
noticed	O
,	O
and	O
only	O
the	O
damaged	O
block	O
needs	O
to	O
be	O
redownloaded	O
.	O
</s>
<s>
With	O
only	O
a	O
hash	B-Algorithm
of	O
the	O
file	O
,	O
many	O
undamaged	O
blocks	O
would	O
have	O
to	O
be	O
redownloaded	O
,	O
and	O
the	O
file	O
reconstructed	O
and	O
tested	O
until	O
the	O
correct	O
hash	B-Algorithm
of	O
the	O
entire	O
file	O
is	O
obtained	O
.	O
</s>
<s>
Hash	B-Algorithm
lists	I-Algorithm
also	O
protect	O
against	O
nodes	O
that	O
try	O
to	O
sabotage	O
by	O
sending	O
fake	O
blocks	O
,	O
since	O
in	O
such	O
a	O
case	O
the	O
damaged	O
block	O
can	O
be	O
acquired	O
from	O
some	O
other	O
source	O
.	O
</s>
