How to make a USB linux live system + Public partition + Private encrypted partition

Step 1

Take a USB key with no important data on it.

Plug it in. And format it in FAT32.

Download a Linux live distribution, for example :
https://www.linuxmint.com/edition.php?id=246

Verify the authenticity of your ISO image:
https://linuxmint.com/verify.php

Download UnetBootin:
https://unetbootin.github.io/linux_download.html

Use UnetBootin to write the iso of the linux mint to your usb key (/dev/sdb1).

Now reboot on your Live system.

Step 2

Open gparted, to edit the partitions.

Resize the first and only partition to 5g.

Make a second one of half the rest in FAT32 also.

Make a third one with the rest unformatted.

Now open "Disks" (the disks utility in linux mint), select the third partition, and choose to format it into LUKS+ext4.

now type your passphrase.

type it again.

Done !

You now have a USB key to boot on a fresh clean Linux, with a Public partition for sharing files accross devices and an encrypted partition for your secrets.
Do not connect to the internet with it and you should have a clean
live system.
Do not type your passphrase to decrypt your Private Encrypted Partition on another system than the live USB key system.