<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
fingerprinting	B-Algorithm
algorithm	I-Algorithm
is	O
a	O
procedure	O
that	O
maps	B-Algorithm
an	O
arbitrarily	O
large	O
data	B-General_Concept
item	O
(	O
such	O
as	O
a	O
computer	B-Operating_System
file	I-Operating_System
)	O
to	O
a	O
much	O
shorter	O
bit	O
string	O
,	O
its	O
fingerprint	B-Algorithm
,	O
that	O
uniquely	O
identifies	O
the	O
original	O
data	B-General_Concept
for	O
all	O
practical	O
purposes	O
just	O
as	O
human	O
fingerprints	B-Algorithm
uniquely	O
identify	O
people	O
for	O
practical	O
purposes	O
.	O
</s>
<s>
This	O
fingerprint	B-Algorithm
may	O
be	O
used	O
for	O
data	B-General_Concept
deduplication	I-General_Concept
purposes	O
.	O
</s>
<s>
This	O
is	O
also	O
referred	O
to	O
as	O
file	B-Operating_System
fingerprinting	O
,	O
data	B-General_Concept
fingerprinting	O
,	O
or	O
structured	O
data	B-General_Concept
fingerprinting	O
.	O
</s>
<s>
Fingerprints	B-Algorithm
are	O
typically	O
used	O
to	O
avoid	O
the	O
comparison	O
and	O
transmission	O
of	O
bulky	O
data	B-General_Concept
.	O
</s>
<s>
For	O
instance	O
,	O
a	O
web	B-Application
browser	I-Application
or	O
proxy	B-Application
server	I-Application
can	O
efficiently	O
check	O
whether	O
a	O
remote	O
file	B-Operating_System
has	O
been	O
modified	O
,	O
by	O
fetching	O
only	O
its	O
fingerprint	B-Algorithm
and	O
comparing	O
it	O
with	O
that	O
of	O
the	O
previously	O
fetched	O
copy	O
.	O
</s>
<s>
Fingerprint	B-Algorithm
functions	O
may	O
be	O
seen	O
as	O
high-performance	O
hash	B-Algorithm
functions	I-Algorithm
used	O
to	O
uniquely	O
identify	O
substantial	O
blocks	O
of	O
data	B-General_Concept
where	O
cryptographic	B-Algorithm
hash	I-Algorithm
functions	I-Algorithm
may	O
be	O
unnecessary	O
.	O
</s>
<s>
Special	O
algorithms	O
exist	O
for	O
audio	B-Algorithm
fingerprinting	I-Algorithm
and	O
video	B-Algorithm
fingerprinting	I-Algorithm
.	O
</s>
<s>
To	O
serve	O
its	O
intended	O
purposes	O
,	O
a	O
fingerprinting	B-Algorithm
algorithm	I-Algorithm
must	O
be	O
able	O
to	O
capture	O
the	O
identity	O
of	O
a	O
file	B-Operating_System
with	O
virtual	O
certainty	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
the	O
probability	O
of	O
a	O
collision	B-Algorithm
—	O
two	O
files	O
yielding	O
the	O
same	O
fingerprint	B-Algorithm
—	O
must	O
be	O
negligible	O
,	O
compared	O
to	O
the	O
probability	O
of	O
other	O
unavoidable	O
causes	O
of	O
fatal	O
errors	O
(	O
such	O
as	O
the	O
system	O
being	O
destroyed	O
by	O
war	O
or	O
by	O
a	O
meteorite	O
)	O
:	O
say	O
,	O
10−20	O
or	O
less	O
.	O
</s>
<s>
This	O
requirement	O
is	O
somewhat	O
similar	O
to	O
that	O
of	O
a	O
checksum	B-Algorithm
function	O
,	O
but	O
is	O
much	O
more	O
stringent	O
.	O
</s>
<s>
To	O
detect	O
accidental	O
data	B-General_Concept
corruption	O
or	O
transmission	O
errors	O
,	O
it	O
is	O
sufficient	O
that	O
the	O
checksums	B-Algorithm
of	O
the	O
original	O
file	B-Operating_System
and	O
any	O
corrupted	O
version	O
will	O
differ	O
with	O
near	O
certainty	O
,	O
given	O
some	O
statistical	O
model	O
for	O
the	O
errors	O
.	O
</s>
<s>
In	O
typical	O
situations	O
,	O
this	O
goal	O
is	O
easily	O
achieved	O
with	O
16	O
-	O
or	O
32-bit	O
checksums	B-Algorithm
.	O
</s>
<s>
In	O
contrast	O
,	O
file	B-Operating_System
fingerprints	B-Algorithm
need	O
to	O
be	O
at	O
least	O
64-bit	B-Device
long	O
to	O
guarantee	O
virtual	O
uniqueness	O
in	O
large	O
file	B-Operating_System
systems	O
(	O
see	O
birthday	O
attack	O
)	O
.	O
</s>
<s>
A	O
good	O
fingerprinting	B-Algorithm
algorithm	I-Algorithm
must	O
ensure	O
that	O
such	O
"	O
natural	O
"	O
processes	O
generate	O
distinct	O
fingerprints	B-Algorithm
,	O
with	O
the	O
desired	O
level	O
of	O
certainty	O
.	O
</s>
<s>
Computer	B-Operating_System
files	I-Operating_System
are	O
often	O
combined	O
in	O
various	O
ways	O
,	O
such	O
as	O
concatenation	O
(	O
as	O
in	O
archive	B-Application
files	I-Application
)	O
or	O
symbolic	O
inclusion	O
(	O
as	O
with	O
the	O
C	B-Language
preprocessor	I-Language
's	O
directive	O
)	O
.	O
</s>
<s>
Some	O
fingerprinting	B-Algorithm
algorithms	I-Algorithm
allow	O
the	O
fingerprint	B-Algorithm
of	O
a	O
composite	O
file	B-Operating_System
to	O
be	O
computed	O
from	O
the	O
fingerprints	B-Algorithm
of	O
its	O
constituent	O
parts	O
.	O
</s>
<s>
Rabin	B-Algorithm
's	I-Algorithm
fingerprinting	I-Algorithm
algorithm	I-Algorithm
is	O
the	O
prototype	O
of	O
the	O
class	O
.	O
</s>
<s>
It	O
is	O
fast	O
and	O
easy	O
to	O
implement	O
,	O
allows	O
compounding	O
,	O
and	O
comes	O
with	O
a	O
mathematically	O
precise	O
analysis	O
of	O
the	O
probability	O
of	O
collision	B-Algorithm
.	O
</s>
<s>
Namely	O
,	O
the	O
probability	O
of	O
two	O
strings	O
r	O
and	O
s	O
yielding	O
the	O
same	O
w-bit	O
fingerprint	B-Algorithm
does	O
not	O
exceed	O
max( |r|	O
,	O
|s|	O
)	O
/2w	O
-1	O
,	O
where	O
|r|	O
denotes	O
the	O
length	O
of	O
r	O
in	O
bits	O
.	O
</s>
<s>
An	O
adversarial	O
agent	O
can	O
easily	O
discover	O
the	O
key	O
and	O
use	O
it	O
to	O
modify	O
files	O
without	O
changing	O
their	O
fingerprint	B-Algorithm
.	O
</s>
<s>
Mainstream	O
cryptographic	O
grade	O
hash	B-Algorithm
functions	I-Algorithm
generally	O
can	O
serve	O
as	O
high-quality	O
fingerprint	B-Algorithm
functions	O
,	O
are	O
subject	O
to	O
intense	O
scrutiny	O
from	O
cryptanalysts	O
,	O
and	O
have	O
the	O
advantage	O
that	O
they	O
are	O
believed	O
to	O
be	O
safe	O
against	O
malicious	O
attacks	O
.	O
</s>
<s>
A	O
drawback	O
of	O
cryptographic	B-Algorithm
hash	I-Algorithm
algorithms	O
such	O
as	O
MD5	B-Algorithm
and	O
SHA	O
is	O
that	O
they	O
take	O
considerably	O
longer	O
to	O
execute	O
than	O
Rabin	O
's	O
fingerprint	B-Algorithm
algorithm	O
.	O
</s>
<s>
They	O
also	O
lack	O
proven	O
guarantees	O
on	O
the	O
collision	B-Algorithm
probability	O
.	O
</s>
<s>
Some	O
of	O
these	O
algorithms	O
,	O
notably	O
MD5	B-Algorithm
,	O
are	O
no	O
longer	O
recommended	O
for	O
secure	O
fingerprinting	O
.	O
</s>
<s>
They	O
are	O
still	O
useful	O
for	O
error	O
checking	O
,	O
where	O
purposeful	O
data	B-General_Concept
tampering	O
is	O
n't	O
a	O
primary	O
concern	O
.	O
</s>
<s>
NIST	O
distributes	O
a	O
software	O
reference	O
library	O
,	O
the	O
American	O
National	O
Software	O
Reference	O
Library	O
,	O
that	O
uses	O
cryptographic	B-Algorithm
hash	I-Algorithm
functions	I-Algorithm
to	O
fingerprint	B-Algorithm
files	O
and	O
map	O
them	O
to	O
software	O
products	O
.	O
</s>
<s>
The	O
HashKeeper	O
database	O
,	O
maintained	O
by	O
the	O
National	O
Drug	O
Intelligence	O
Center	O
,	O
is	O
a	O
repository	O
of	O
fingerprints	B-Algorithm
of	O
"	O
known	O
to	O
be	O
good	O
"	O
and	O
"	O
known	O
to	O
be	O
bad	O
"	O
computer	B-Operating_System
files	I-Operating_System
,	O
for	O
use	O
in	O
law	O
enforcement	O
applications	O
(	O
e.g.	O
</s>
