We are used to thinking of “hash functions” as the method of producing cryptographic signatures, but actually that’s not true and it wasn’t until I saw a documentary about the retail chain TJ Maxx to find out what hashing really was.

TJ Maxx hash their stock. That is, they split each style of clothes into their varying sizes and hash (or split) them across display clothes rails around the shop, meaning that if you go to a rail that matches your size, you should see all the styles available within that single rail.

So it is with cryptographic hashing, the hashing part simply describes the splitting of an input into equal length sections ready to have modular arithmetic applied to it.

Modular arithmetic allows a one way function, important in cryptography.

Think of a clock face, if you add 4 to 10 on a clock face, you a result of 1 remainder 2. You discard the 1 and keep the 2.

Using this method, you can produce a unique output by adding up the input, which cannot be reversed, but generates the same result from the same input every time, but a completely different output for a slightly different input.

This is the basis a cryptographic signature