Security Architecture and Engineering
Models
Bell-LaPadula protects confidentiality: no read up, no write down. Biba protects integrity and mirrors it: no read down, no write up. Clark-Wilson enforces integrity through well-formed transactions and separation of duty. Brewer-Nash (Chinese Wall) prevents conflicts of interest. Questions swap the simple (read) and star (write) properties deliberately, so name the property before answering.
The Trusted Computing Base is all hardware, software, and firmware enforcing policy; the reference monitor mediates every access and must be tamper-proof and verifiable; the security kernel implements it. Rings 0–3 separate privilege, with the hypervisor occupying a more privileged position than the guest OS.
Cryptography
Symmetric (AES) is fast but key distribution is hard; asymmetric (RSA, ECC) solves distribution and enables signatures but is slow; hashing (SHA-256/3) provides integrity, and HMAC adds keying. Digital signatures give authentication, integrity, and nonrepudiation - not confidentiality. In PKI, know root and intermediate CAs, CRL versus OCSP, and pinning. Traps: confusing encoding with encryption, and key direction - encrypt with the recipient's public key, sign with your own private key.
Physical
CPTED, mantraps, and fire classes matter: Class A water, B foam/CO2, C inert gas. Never use water suppression in an energized electrical room, and confirm interlocks. Fail-safe versus fail-secure door selection is driven by life safety.
Worked example
To stop a DBA from altering audit records, combine separation of duty with the TCB: ship logs to an append-only system administered by someone else. The technical control must match the stated threat.