<s>
Simple	B-Algorithm
file	I-Algorithm
verification	I-Algorithm
(	O
SFV	O
)	O
is	O
a	O
file	O
format	O
for	O
storing	O
CRC32	O
checksums	B-Algorithm
of	O
files	O
to	O
verify	O
the	O
integrity	O
of	O
files	O
.	O
</s>
<s>
The	O
file	O
extension	O
is	O
usually	O
used	O
for	O
SFV	B-Algorithm
files	I-Algorithm
.	O
</s>
<s>
Files	O
can	O
become	O
corrupted	O
for	O
a	O
variety	O
of	O
reasons	O
,	O
including	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
and	O
software	B-Error_Name
bugs	I-Error_Name
.	O
</s>
<s>
SFV	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
CRC	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
usually	O
negligible	O
with	O
random	O
corruption	O
.	O
</s>
<s>
(	O
The	O
number	O
of	O
possible	O
checksums	B-Algorithm
is	O
limited	O
though	O
large	O
,	O
so	O
that	O
with	O
any	O
checksum	B-Algorithm
scheme	O
many	O
files	O
will	O
have	O
the	O
same	O
checksum	B-Algorithm
.	O
</s>
<s>
However	O
,	O
the	O
probability	O
of	O
a	O
corrupted	O
file	O
having	O
the	O
same	O
checksum	B-Algorithm
as	O
its	O
original	O
is	O
exceedingly	O
small	O
,	O
unless	O
deliberately	O
constructed	O
to	O
maintain	O
the	O
checksum	B-Algorithm
.	O
)	O
</s>
<s>
SFV	O
cannot	O
be	O
used	O
to	O
verify	O
the	O
authenticity	O
of	O
files	O
,	O
as	O
CRC32	O
is	O
not	O
a	O
collision	O
resistant	O
hash	B-Algorithm
function	I-Algorithm
;	O
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
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>
In	O
cryptography	O
,	O
this	O
attack	O
is	O
called	O
a	O
collision	B-Algorithm
attack	I-Algorithm
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	O
md5sum	B-Device
and	O
sha1sum	O
utilities	O
are	O
often	O
preferred	O
in	O
Unix	B-Application
operating	I-Application
systems	I-Application
,	O
which	O
use	O
the	O
MD5	B-Algorithm
and	O
SHA-1	B-Algorithm
cryptographic	B-Algorithm
hash	I-Algorithm
functions	I-Algorithm
respectively	O
.	O
</s>
<s>
Even	O
a	O
single-bit	O
error	O
causes	O
both	O
SFV	O
's	O
CRC	O
and	O
md5sum	B-Device
's	O
cryptographic	B-Algorithm
hash	I-Algorithm
to	O
fail	O
,	O
requiring	O
the	O
entire	O
file	O
to	O
be	O
re-fetched	O
.	O
</s>
<s>
The	O
Parchive	B-General_Concept
and	O
rsync	B-Application
utilities	O
are	O
often	O
preferred	O
for	O
verifying	O
that	O
a	O
file	O
has	O
not	O
been	O
accidentally	O
corrupted	O
in	O
transmission	O
,	O
since	O
they	O
can	O
correct	O
common	O
small	O
errors	O
with	O
a	O
much	O
shorter	O
download	O
.	O
</s>
<s>
Despite	O
the	O
weaknesses	O
of	O
the	O
SFV	O
format	O
,	O
it	O
is	O
popular	O
due	O
to	O
the	O
relatively	O
small	O
amount	O
of	O
time	O
taken	O
by	O
SFV	O
utilities	O
to	O
calculate	O
the	O
CRC32	O
checksums	B-Algorithm
when	O
compared	O
to	O
the	O
time	O
taken	O
to	O
calculate	O
cryptographic	B-Algorithm
hashes	I-Algorithm
such	O
as	O
MD5	B-Algorithm
or	O
SHA-1	B-Algorithm
.	O
</s>
<s>
SFV	O
uses	O
a	O
plain	O
text	O
file	O
containing	O
one	O
line	O
for	O
each	O
file	O
and	O
its	O
checksum	B-Algorithm
in	O
the	O
format	O
FILENAME	O
<	O
whitespaces>CHECKSUM	O
.	O
</s>
<s>
Any	O
line	O
starting	O
with	O
a	O
semicolon	O
'	O
;	O
'	O
is	O
considered	O
to	O
be	O
a	O
comment	O
and	O
is	O
ignored	O
for	O
the	O
purposes	O
of	O
file	B-Error_Name
verification	I-Error_Name
.	O
</s>
<s>
The	O
delimiter	O
between	O
the	O
filename	O
and	O
checksum	B-Algorithm
is	O
always	O
one	O
or	O
several	O
spaces	O
;	O
tabs	O
are	O
never	O
used	O
.	O
</s>
<s>
A	O
sample	O
SFV	B-Algorithm
file	I-Algorithm
is	O
:	O
</s>
<s>
An	O
example	O
of	O
an	O
open-source	B-License
cross-platform	B-Operating_System
command-line	B-Application
utility	O
that	O
outputs	O
crc32	O
checksums	B-Algorithm
is	O
7-Zip	B-Language
.	O
</s>
<s>
Many	O
Linux	O
distributions	O
include	O
a	O
simple	O
command-line	B-Application
tool	I-Application
cksfv	O
to	O
verify	O
the	O
checksums	B-Algorithm
.	O
</s>
