If you receive some information, how do you know if it’s intact or has been changed?

You use a hash in a similar way to a checksum by including it along with the data. If you want to verify that the data is correct, recalculate the hash and compare. There are many ways to calculate a hash. This episode will describe two broad categories of hash functions, non-cryptographic hash functions and cryptographic hash functions.

You can create or use hash functions that avoid accidental collisions or sometimes don’t worry much at all about collisions. I was at a pharmacy recently and noticed behind the counter was space to hold prescriptions that were waiting to be picked up. Each prescription was placed in a paper bag with what looked like the first two letters of the person’s last name. That’s a hash function. Not very complicated, but still a hash function. Two letters gives you 26 times 26 for a total of 676 possible hash values. There’s many reasons this is not a very good hash function for general use including the fact that it’s not very random. This means that some hash values will be more common than others. And I don’t know anybody who’s last name begins with double Q’s.

But for the pharmacy, using the first two letters of a person’s last name works great. It’s a good hash function for their needs. There’s no need for anything more complicated or elaborate.
Other hash functions could be more complicated and make use of thousands or millions or even more possible hash values. These are all non-cryptographic hash functions because while they can help detect accidental errors, they won’t help protect your information against attacks done on purpose. A non-cryptographic hash function is designed to meet your application’s needs assuming you don’t have to worry about your hash values being used by an attacker. For that, you need a cryptographic hash function.

Listen to the whole episode for more information about well-known cryptographic hash functions MD5, SHA-1, SHA-2 (including SHA-256 and SHA-512), and SHA-3.