Depending on criticity, use (frequency and effective use), consider: * HSM https://en.wikipedia.org/wiki/Hardware_security_module https://en.wikipedia.org/wiki

Depending on criticity, use (frequency and effective use), consider: * HSM https://en.wikipedia.org/wiki/Hardware_security_module https://en.wikipedia.org/wiki/Hardware_security_module * Handwritten base64 sheets of paper in different vaults * a USB token (Yubikey, or whatnot)