On Linux if you need to verify the file you just downloaded is untouched you run either sha1sum or md5sum on the file to get the checksum and verify it against the published checksum. There are some utilities available for Windows, but today I'm going to point the spotlight at a Microsoft offering, File Checksum Integrity Verifier or fciv for short. You can grab the download from Microsoft's site.
There are a few options available when you run the command, such as -md5 to get the md5 checksum of a file or -sha1 to get the sha1 checksum of a file. But you can also use the -both flag to get both checksums of a given file. You can even create you own database of checksums to verify the files later.
To generate the checksums and store them in the database (xml file) run:
>fciv -both -xml fcdatabase.xml filetocheck.txt
The command above will obviously store both checksums and on a large file could take a while to run, as it needs to run twice on the file, once for the sha1 sum and once for the md5 sum. You of course can use one or the other check sum algorithm if you prefer (I prefer sha1 for the time being).
To later verify a file against the database you run the command shown below:
>fciv -sha1 -xml fcdatabase.xml filetocheck.txt
Now if you make changes to filetocheck.txt and run the command below you will see that verifying the file against the database it has changed. If this where an executable it may either be corrupt or compromised. If it was the project you've been working on for weeks, now is a good time to restore from backup, if the change wasn't expected of course.
>fciv -v -xml fcdatabase.xml filetocheck.txt
You'll see something like:
Hash is : 48a5967227b85c6805f3210832a155da
It should be : 04518689efadfdf2393b533dc9c7c8b5
My only real complaint is that fciv doesn't show any sort of progress while processing a large directory. It would be nice to see a file names flying by the screen or something. All in all though this is a tool worth checking out if you run Windows and don't have another tool already in your arsenal.
I would like to know if any of you have a tool in your arsenal already for this purpose and what it is / where I can find it. Sadly I don't recall any of the tools I've used in the past, but this one may become my tool of choice for this job.