Key
1.15 - 1.20.4
Lock your Chest to protect your items from other players and Creepers. Shift right click on the Chest to lock or unlock it. You can only unlock the Chest if you originally locked it. It doesn't matter which Key you use.
Even if something destroys the Chest or a Hopper is placed under the Chest, your items can't be stolen. However, be aware that items you put a locked Chest with a Hopper will drop out when you unlock it.
Crafting
The Crafting Table will show a Gold Nugget as the result. This will be replaced by the Key after crafting.
[1.16+] You can repair your Key, if you drop a Gold Ingot with the Key on an Anvil. To get more durability you can also enchant it with Unbreaking.
Observer Addon
Use this add-on to automatically unlock the locked Chest if you are in a radius of 2 blocks.
Settings
Show messages in the chat instead of above the hotbar.
/scoreboard players set #message chestlock 1
Get the Admin Key.
/function chest_lock:craft/admin_key
[1.16+] Disable tool damage when locking a Chest.
/scoreboard players set #damage chestlock 1
If you get a Green Shulker Box when you lock or unlock a chest you can use this two commands to fix it. Afterwards the Key won't get damaged and all items which are putted into the Chest, while it is looked will be lost if you unlock it.
/scoreboard players set #compatible chestlock 1
/scoreboard players set #damage chestlock 1
To disable the setting again you need to execute the same command except of the 1 replace with a 0.