Explanation of CRC files
Most people active on internet have encountered CRC files before,
but this page is for those who don't know what I'm talking about.
The explanation is a bit rude and crude but it is essentially right.
What is CRC?
CRC stands for Cyclic
Redundancy Check.
It is a code that is based on a given set of data (in our case:
a file). The combination of a CRC code and a file enables users
to check validity of a file.
This means that based on a given file a code can be generated
that at a later time can be used to check if the original file has
been changed/corrupted. This is useful to see if files transferred
over internet have been damaged during the transfer. Note that if
a file got infected by a virus during transfer the CRC test
will fail!
How is CRC used?
When someone creates some file(s) that person can generate a listing
of CRC codes for those files (see below). Somone who obtains a copy
of those files and a copy of that list can now run a check to see
if the codes in the list match the codes of the files, which then
tells the user with some certainty that the files are OK or not.
CRC file formats
The most commonly used CRC formats on the internet are CRC32 and
MD5. These two checksum formats and three filetypes are briefly
discussed below with links to programs used for creation and checking
of the files.
| CRC32 |
32 bit checksum. So there are:
different codes possible for a file. This
makes the chance of 2 different files with identical CRC32
pretty small.
|
| MD5 |
Officially this is not a checksum, but a Message Digest.
The idea is the same. It is a 128bit code very commonly used
on non-Windows operating systems. 128 bits means:
possible codes for a given file (that is 36 zeros!). I'm
pretty sure that there are not even that many different files
on the planet so the chance of finding 2 different files with
identical MD5 code is next to impossible. MD5 is really for
the paranoid amoung us.
|
| SFV |
Contains CRC32 code only. Use QSFV
or Dircheck
for creation and checking.
|
| CSV |
Is a Comma Separated datafile that is commonly linked to
Excel. If it contains CRC codes it also contains filesizes
making it a bit more secure than SFV. CSV files can also contain
directory information. For this reason CSV is commonly used
with collection program like The!Checker/PicCheck
or Hunter.
To simply check the CRC's use Dircheck
|
| MD5 |
Contains MD5 hash code only. Main problem with these files
is that it contains different entries for ASCII files as for
binary files. Use MD5Summer
if you prefer a windows user interface or MD5sum
for a fast and easy check.
|
| PAR |
Par is quite a new file format compared to the others. It
is not just a checksum file but it contains parity information
that can be used to restore missing files in a set of files.
Comicsort only uses the MD5 data that is contained in Par
files to test the files in a specific set and to analyse file
sets.
Use smartpar
to make full use of PAR files.
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