I would imagine the firmware signing key is stored on a Hardware Security Module (HSM). Devices like that are tamper resistant (for example, they might be configured to delete the key if someone is messing with the device).<p>HSMs generally support signing operations, so it&#x27;s likely the key doesn&#x27;t leave the device at all. They might be using a configuration with shared secrets (smart card + PIN, etc.) where at least <i>n</i> shares need to be present to operate the HSM. Those secrets are probably held by a small number of high-level Apple employees.<p>It&#x27;s probably similar to the DNSSEC Root Signing Ceremony[1]. Well, hopefully slightly less insane.<p>[1]: <a href="https:&#x2F;&#x2F;www.cloudflare.com&#x2F;dnssec&#x2F;root-signing-ceremony&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.cloudflare.com&#x2F;dnssec&#x2F;root-signing-ceremony&#x2F;</a>