This is old news to most people, but do not use MIFARE Classic cards, they are not secure.
What was not clear to me is quite how insecure. Reading up on it a little, it sounds like several issues. Something to do with the random number generator being poor, and a nested attack. These apparently allow access very quickly, and there is even a standard command mfoc to crack such cases. By standard command I mean literally apt install mfoc, seriously.
Now, it seems, that modern cards are not quite as vulnerable, but can be cracked with a brute force attack in some way. This really shows why you don't make your own crypto, ever!
As I am doing some work with MIFARE Classic for a client (yes, I said it was insecure), I thought it worth seeing what that means in practice, in 2020. I found a site covering how to set up a hardnested attack, here.
I set up on a Pi, and realised that did not have much oomph. So I tried my very old linux box I normally use, not bad. But I decided to try my iMac. This meant a few brew commands instead of apt commands, but getting the hardnested mfoc compiled was simple.
It was a tad harder to run as the iMac grabs the NFC reader and I had to edit the Info.plist file in recovery mode to remove the ACR122U reader I am using, and compile libnfc with ./configure --with-drivers=acr122_pcsc
However, it then ran. I loaded a card where I had put a key it could not read.
It took nearly 10 minutes to get the key, and after several tests I found that was about the longest it takes, with it often finding in under 3 minutes. It will work through all of the unknown keys, and get you a dump of the card and all the keys.
This means you can read the card, and make a copy, and change it in any way including changing any "value" settings it has. Basically, it is, at that point, no more secure than a memory stick.
So please, do not use MIFARE Classic as if they are secure!
Of course, our card printing stuff at A&A will encode MIFARE Classic for you, but then it also does other insecure systems like mag stripe, and QR codes. MIFARE Classic is fine if you treat it just as a machine readable data store, much like mag stripe or a QR code - it is just not secure.
When it comes to more secure cards like DESFire, we don't encode them but we record which card has what card ID so you can encode them yourself later. This saves us having anyone's keys.
(P.S. I use MIFARE DESFire EV cards with AES for my door entry stuff)