HSM
A box whose entire business model is that the key never leaves it, not even for you.
A Hardware Security Module generates, stores and uses cryptographic keys inside tamper-resistant hardware. The application sends data in and gets a signature or a decryption back, but the key never leaves the box in plaintext. Attempts to open the casing, change the temperature or measure power consumption are meant to trigger erasure of the keys.
Levels are certified under FIPS 140, where a higher level means stronger physical protection. The modules are mandatory for certificate authorities, for card payments under the PCI rules, and for the DNSSEC root key. Cloud providers sell them as a service. The cost is not only the price — key recovery requires ceremonies with several people and split secrets, because keys that genuinely cannot be copied cannot be casually regenerated either.