The signets were "unforgeable" in that the penalty for copying one was death by being tortured, hanged, drawn and quartered.
They were "unforgeable" because commissioning a jeweler to make an intricate carving on a ring by hand was very expensive in the first place. Finding one who could precisely replicate an existing carving, down to the mistakes of its original author and any wear and tear that would be visible on the wax seal, was almost impossible and even more expensive.
loading story #50197543
Indeed. In both this scenario and our modern one, we rely on the difficulty of "key material" reproduction being greater than the difficulty of its creation.
That would be retroactive security...
However, Kings likely benefitted from the option of a scapegoat, they could always blame someone for forging their unforgeable seal and have them killed, rather than admit that the seal was true.
If we add in attainder and escheat, the King had a pretty strong hand.