<s>
UUHash	B-Algorithm
is	O
a	O
hash	B-Error_Name
algorithm	I-Error_Name
employed	O
by	O
clients	O
on	O
the	O
FastTrack	B-Protocol
network	O
.	O
</s>
<s>
It	O
is	O
employed	O
for	O
its	O
ability	O
to	O
hash	B-Error_Name
very	O
large	O
files	O
in	O
a	O
very	O
short	O
period	O
of	O
time	O
,	O
even	O
on	O
older	O
computers	O
.	O
</s>
<s>
This	O
weakness	O
makes	O
it	O
trivial	O
to	O
create	O
a	O
hash	B-Error_Name
collision	O
,	O
allowing	O
large	O
sections	O
to	O
be	O
completely	O
altered	O
without	O
altering	O
the	O
checksum	B-Algorithm
.	O
</s>
<s>
This	O
method	O
is	O
used	O
by	O
Kazaa	B-Protocol
.	O
</s>
<s>
The	O
weakness	O
of	O
UUHash	B-Algorithm
is	O
exploited	O
by	O
anti-p2p	O
agencies	O
to	O
corrupt	O
downloads	O
.	O
</s>
<s>
The	O
UUHash	B-Algorithm
is	O
a	O
160-bit	O
string	O
that	O
is	O
usually	O
Base64-encoded	O
for	O
presentation	O
.	O
</s>
<s>
It	O
is	O
a	O
concatenation	O
of	O
an	O
MD5	B-Algorithm
hash	I-Algorithm
and	O
a	O
CRC32	O
sum	O
of	O
selected	O
chunks	O
of	O
the	O
file	O
.	O
</s>
<s>
The	O
first	O
307,200	O
bytes	O
(	O
300	O
Kibibyte	O
,	O
one	O
"	O
chunk	O
size	O
"	O
)	O
of	O
the	O
file	O
are	O
MD5-hashed	O
(	O
less	O
if	O
file	O
is	O
shorter	O
)	O
.	O
</s>
<s>
The	O
160-bit	O
UUHash	B-Algorithm
is	O
now	O
the	O
concatenation	O
of	O
the	O
128-bit	O
MD5	B-Algorithm
hash	I-Algorithm
and	O
the	O
final	O
32-bit	O
smallhash	O
value	O
.	O
</s>
<s>
Given	O
are	O
hashes	B-Error_Name
(	O
base64	B-Protocol
and	O
hex	O
)	O
for	O
strings	O
of	O
various	O
lengths	O
containing	O
only	O
0x00	O
or	O
0xFF	O
bytes	O
:	O
</s>
<s>
Notice	O
that	O
all	O
strings	O
that	O
have	O
a	O
complete	O
MD5	B-Algorithm
chunk	O
have	O
the	O
same	O
128-bit	O
prefix	O
.	O
</s>
<s>
For	O
files	O
up	O
to	O
300	O
KiB	O
,	O
the	O
file	O
length	O
can	O
be	O
extracted	O
from	O
the	O
last	O
four	O
bytes	O
of	O
the	O
hash	B-Error_Name
;	O
smallhash	O
is	O
~	O
0	O
.	O
</s>
<s>
The	O
name	O
UUHash	B-Algorithm
derives	O
from	O
the	O
utility	O
which	O
creates	O
URIs	B-Protocol
referencing	O
files	O
on	O
Kazaa	B-Protocol
.	O
</s>
<s>
These	O
URIs	B-Protocol
are	O
of	O
the	O
form	O
:	O
</s>
<s>
Not	O
considering	O
the	O
fact	O
that	O
this	O
URI	B-Protocol
format	O
is	O
not	O
RFC	O
compliant	O
,	O
UUHash	B-Algorithm
refers	O
to	O
the	O
Base64-encoding	O
of	O
the	O
hash	B-Error_Name
and	O
not	O
the	O
hash	B-Error_Name
itself	O
.	O
</s>
