What is a checksum and why do you need it?

Posted on July 9, 2008 at 5:38 am

A checksum is a form of mechanism to ensure that the file we downloaded is properly downloaded. Checksum acts like a “fingerprint” of a file so if a file gets corrupted this fingerprint will change and thus letting us know that this is not the same file.

Any serious file providers provide a mechanism to have a checksum on their downloadable files. Symantec for example has the MD5, a form of checksum, on their download page:

The long string on the right part of the page is the MD5 checksum. An MD5 checksum is now probably the most used checksum algorithm implemented on the Net. To learn more about MD5, checkout this cool article from Wikipedia: http://en.wikipedia.org/wiki/Md5sum

So how do we use the MD5 checksum? Simple, after you downloaded a file compare it’s MD5 checksum information to the one posted on the download page. This is assuming that the page provides the checksum information, if not, you are out of luck. You can try a free tool from Microsoft called File Checksum Integrity Verifier, FCIV, (URL: http://support.microsoft.com/kb/841290). FCIV works like charm but my only complaint about this tool is that it is a command line utility with no graphical interface. FCIV is perfect for folks who want direct to the point command line functionality. You can also try another command line utility called MD5SUMS by Jem Berkes http://www.pc-tools.net/win32/md5sums/

For more casual users who wants something with a GUI, The MD5Sums is a simple and easy to use tool: http://www.md5summer.org/download.html

What you will get there is an executable that you don’t have to install. After unzipping the file, run it and open the folder where the file you want to check is located. In my case, the file is in the Downloads folder:

Click the Create sums button to generate the MD5 hash value then choose the file you want to check:

Click OK to generate the checksum, depending on the size of the file it could take from a few seconds to a few minutes for larger files. It will ask that you save the generated file on

The final result is the MD5 has value of the file I downloaded:

This MD5 value could then be compared with the one posted on the Symantec Website:

After a gruesome visual string comparison, I am now confident that what I downloaded is not corrupted since the two MD5 hash values are equal.

If the downloaded file’s MD5 value is not equal to the one posted on the page, you could try to re-download the whole thing. Re-downloading usually fixes the problem especially on a slow connection like a dial-up.

You could also use a downloader tool like Getright that could generate the checksum value. Poor Internet connection usually causes data or file corruption but with the knowledge of checksums and a good downloading software like Getright you can easily get over this limitation.