system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why was a King's Seal considered such a reliable way of making sure it was the King's own letter?

It seems like it would be fairly easy to replicate a stamp, or steal one. Going to sleep, so I won't respond for a while.
gpt
Security measures never prevent something, they only make something difficult. Hopefully difficult enough that it discourages criminals from doing whatever the security measure is meant to discourage, or stealing what it is meant to protect. 

Seals were made by skilled craftsmen so there are limited numbers of people able to replicate them. These craftsmen are generally well compensated and part of court in order to secure their loyalty as well. 

They also only had one or a few seals made at a time and they were either rings or other jewelry worn by the king at all times, or they were locked in a box that was then locked in a room. You also have guards to contend with everywhere the king would go.