What is a checksum?

A checksum of any data such as a string, file, image or cd-rom is a large hexadecimal number. Checksums serve as a finger print of the data concerned: All forms of data contain unique fingerprints.

MD5 is a standard method to compute checksums. RFC 1321

String	MD5 checksum
Hello World	b10a8db164e0754105b7a99be72e3fe5
hello world	5eb63bbbe01eeed093cb22bb8f5acdc3
helloworld	fc5e038d38a57032085441e7fe7010b0

Try it for yourself

 

There are two properties that make MD5 checksums very useful:

If two file are different, then their MD5 checksums will also be different; even if they only differ by just 1 bit.
Instead of comparing two files, you can compare checksums. A file producing an MD5 checksum, is impossible to create with the same checksum.
It is impossible to change a file, and keep the checksum the same.
If you have copies of a file, you can check their validity by comparing checksums using the checksum of the original file.

There are many files with the same checksum,
but you will never find two different files containing the same MD5 checksum. In practical situations, a collision never occurs.




Computing a checksum

Most Unix/Linux systems have a command line utility to compute an MD5 checksum. gnu coreutils, in the textutils subset.

For Microsoft platforms, you can use Bitser




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