How does MD5Sum algorithm work?

Question

Md5sum is used to check integrity of iso image of say ubuntu.How does it work?

Answer 1

MD5Sum is a file checksum generating tool using MD5 as the hashing algorithm. It provides the user with a reasonable assurance that the file was untampered with.

In order to do this, the user will need to (via shell integration or manually) process their own MD5 hash and compare it to the hash provided by the uploader.

An MD5Sum hash won't assure you that the uploader who provided the checksum didn't fiddle with the file, to do that you should trust the source of the MD5 checksum (ie. get the checksum from Ubuntu instead of the download site providing the image).