<s>
File	B-Error_Name
verification	I-Error_Name
is	O
the	O
process	O
of	O
using	O
an	O
algorithm	O
for	O
verifying	O
the	O
integrity	O
of	O
a	O
computer	B-Operating_System
file	I-Operating_System
,	O
usually	O
by	O
checksum	B-Algorithm
.	O
</s>
<s>
A	O
more	O
popular	O
approach	O
is	O
to	O
generate	O
a	O
hash	B-Algorithm
of	O
the	O
copied	O
file	O
and	O
comparing	O
that	O
to	O
the	O
hash	B-Algorithm
of	O
the	O
original	O
file	O
.	O
</s>
<s>
A	O
file	O
can	O
become	O
corrupted	O
by	O
a	O
variety	O
of	O
ways	O
:	O
faulty	O
storage	B-General_Concept
media	I-General_Concept
,	O
errors	O
in	O
transmission	O
,	O
write	O
errors	O
during	O
copying	O
or	O
moving	O
,	O
software	B-Error_Name
bugs	I-Error_Name
,	O
and	O
so	O
on	O
.	O
</s>
<s>
Hash-based	O
verification	O
ensures	O
that	O
a	O
file	O
has	O
not	O
been	O
corrupted	O
by	O
comparing	O
the	O
file	O
's	O
hash	B-Error_Name
value	I-Error_Name
to	O
a	O
previously	O
calculated	O
value	O
.	O
</s>
<s>
Due	O
to	O
the	O
nature	O
of	O
hash	B-Algorithm
functions	I-Algorithm
,	O
hash	B-Algorithm
collisions	I-Algorithm
may	O
result	O
in	O
false	O
positives	O
,	O
but	O
the	O
likelihood	O
of	O
collisions	O
is	O
often	O
negligible	O
with	O
random	O
corruption	O
.	O
</s>
<s>
To	O
verify	O
the	O
authenticity	O
,	O
a	O
classical	O
hash	B-Algorithm
function	I-Algorithm
is	O
not	O
enough	O
as	O
they	O
are	O
not	O
designed	O
to	O
be	O
collision	O
resistant	O
;	O
it	O
is	O
computationally	O
trivial	O
for	O
an	O
attacker	O
to	O
cause	O
deliberate	O
hash	B-Algorithm
collisions	I-Algorithm
,	O
meaning	O
that	O
a	O
malicious	O
change	O
in	O
the	O
file	O
is	O
not	O
detected	O
by	O
a	O
hash	B-Algorithm
comparison	O
.	O
</s>
<s>
For	O
this	O
purpose	O
,	O
cryptographic	B-Algorithm
hash	I-Algorithm
functions	I-Algorithm
are	O
employed	O
often	O
.	O
</s>
<s>
As	O
long	O
as	O
the	O
hash	B-Error_Name
sums	I-Error_Name
cannot	O
be	O
tampered	O
with	O
for	O
example	O
,	O
if	O
they	O
are	O
communicated	O
over	O
a	O
secure	O
channel	O
the	O
files	O
can	O
be	O
presumed	O
to	O
be	O
intact	O
.	O
</s>
<s>
A	O
checksum	B-Algorithm
file	O
is	O
a	O
small	O
file	O
that	O
contains	O
the	O
checksums	B-Algorithm
of	O
other	O
files	O
.	O
</s>
<s>
There	O
are	O
a	O
few	O
well-known	O
checksum	B-Algorithm
file	O
formats	O
.	O
</s>
<s>
Several	O
utilities	O
,	O
such	O
as	O
md5deep	B-Language
,	O
can	O
use	O
such	O
checksum	B-Algorithm
files	O
to	O
automatically	O
verify	O
an	O
entire	O
directory	O
of	O
files	O
in	O
one	O
operation	O
.	O
</s>
<s>
The	O
particular	O
hash	B-Error_Name
algorithm	I-Error_Name
used	O
is	O
often	O
indicated	O
by	O
the	O
file	O
extension	O
of	O
the	O
checksum	B-Algorithm
file	O
.	O
</s>
<s>
The	O
"	O
.sha1	O
"	O
file	O
extension	O
indicates	O
a	O
checksum	B-Algorithm
file	O
containing	O
160-bit	O
SHA-1	B-Algorithm
hashes	I-Algorithm
in	O
sha1sum	O
format	O
.	O
</s>
<s>
The	O
"	O
.md5	B-Algorithm
"	O
file	O
extension	O
,	O
or	O
a	O
file	O
named	O
"	O
MD5SUMS	B-Device
"	O
,	O
indicates	O
a	O
checksum	B-Algorithm
file	O
containing	O
128-bit	O
MD5	B-Algorithm
hashes	I-Algorithm
in	O
md5sum	B-Device
format	O
.	O
</s>
<s>
The	O
"	O
.sfv	B-Algorithm
"	O
file	O
extension	O
indicates	O
a	O
checksum	B-Algorithm
file	O
containing	O
32-bit	O
CRC32	O
checksums	B-Algorithm
in	O
simple	B-Algorithm
file	I-Algorithm
verification	I-Algorithm
format	O
.	O
</s>
<s>
The	O
"	O
crc.list	O
"	O
file	O
indicates	O
a	O
checksum	B-Algorithm
file	O
containing	O
32-bit	O
CRC	O
checksums	B-Algorithm
in	O
brik	O
format	O
.	O
</s>
<s>
As	O
of	O
2012	O
,	O
best	O
practice	O
recommendations	O
is	O
to	O
use	O
SHA-2	B-Algorithm
or	O
SHA-3	B-Algorithm
to	O
generate	O
new	O
file	O
integrity	O
digests	O
;	O
</s>
<s>
and	O
to	O
accept	O
MD5	B-Algorithm
and	O
SHA1	B-Algorithm
digests	O
for	O
backward	O
compatibility	O
if	O
stronger	O
digests	O
are	O
not	O
available	O
.	O
</s>
<s>
The	O
theoretically	O
weaker	O
SHA1	B-Algorithm
,	O
the	O
weaker	O
MD5	B-Algorithm
,	O
or	O
much	O
weaker	O
CRC	O
were	O
previously	O
commonly	O
used	O
for	O
file	O
integrity	O
checks	O
.	O
</s>
<s>
even	O
if	O
the	O
hash	B-Error_Name
sum	I-Error_Name
file	O
is	O
not	O
tampered	O
with	O
,	O
it	O
is	O
computationally	O
trivial	O
for	O
an	O
attacker	O
to	O
replace	O
a	O
file	O
with	O
the	O
same	O
CRC	O
digest	O
as	O
the	O
original	O
file	O
,	O
meaning	O
that	O
a	O
malicious	O
change	O
in	O
the	O
file	O
is	O
not	O
detected	O
by	O
a	O
CRC	O
comparison	O
.	O
</s>
