Domain 3: Security Architecture and Engineering
Secure design principles
- Threat modelling early; attack surface minimisation.
- Least privilege, defence in depth (layered controls), secure defaults, fail securely (deny on failure).
- Separation of duties, keep it simple (complexity is the enemy).
- Zero trust – never trust, always verify; micro-segmentation; implicit deny.
- Shared responsibility in cloud: provider = of the cloud, customer = in the cloud.
- Privacy by design, secure access service edge patterns.
Security models (memorise the direction of the rules)
| Model | Protects | Rules |
|---|---|---|
| Bell-LaPadula | Confidentiality | no read UP (simple), no write DOWN (star property *) |
| Biba | Integrity | no read DOWN, no write UP (inverse of BLP) |
| Clark-Wilson | Integrity (commercial) | access triple subject–program(TP)–object(CDI); well-formed transactions |
| Brewer-Nash | Conflict of interest | Chinese Wall: access changes dynamically with user history |
| Graham-Denning / HRU / Take-Grant | creation/transfer of rights | formal models of protection-state change |
Evaluation standards
Common Criteria (ISO 15408): PP (protection profile = customer needs) → ST (security target = vendor claims) evaluated against a ToE (target of evaluation) producing an EAL1–EAL7 rating (higher = more assurance, not more security). Historical: TCSEC (Orange Book, US), ITSEC (EU).
Symmetric cryptography
One shared key; fast; key distribution is the hard part; n people need n(n−1)/2 keys.
- AES (128/192/256-bit, blocks of 128) — the standard.
- DES (56-bit) and 3DES (112-bit effective) — legacware only, know them as broken/retired.
- Blowfish/Twofish, RC4 (broken stream cipher), ChaCha20 (modern stream).
- Modes: ECB (never - pattern leakage), CBC, CTR, GCM (authenticated encryption).
Asymmetric + hashing + signatures
Asymmetric: keypair; slow; solves distribution and gives signatures. RSA (factoring), Diffie-Hellman (key agreement, no auth by itself, vulnerable to MITM without authentication), ECC (smaller keys, mobile/IoT), ElGamal.
Hashing: one-way, fixed output, collision-resistant. MD5/SHA-1 dead; use SHA-256/384/512 or SHA-3. HMAC = hash + secret key (integrity + authenticity).
Digital signature = hash of message encrypted with the SENDER's private key → gives integrity + authentication + non-repudiation. Confidentiality is added by encrypting with the receiver's public key.
PKI and key management
CA issues certificates, RA verifies identity, certificate revocation via CRL or OCSP (OCSP stapling = server brings its own proof). X.509 v3 certificate. Lifecycle: enrolment → issuance → validation → revocation/expiry → renewal.
Kerckhoffs's principle: security rests in the key, not the algorithm secrecy. Key rules: separation of duties in key management, split knowledge (no single person knows a whole key), dual control, M-of-N recovery, key escrow concerns.
Cryptanalysis attacks
- Brute force – try everything; longer keys resist.
- Known plaintext / chosen plaintext / chosen ciphertext – attacker has or crafts matching pairs.
- Meet-in-the-middle – why double-DES is useless (use 3DES/EDE).
- Birthday attack – hash collisions in ~2^(n/2).
- Side-channel – timing/power/EM leakage; fault injection.
- Pass the hash / replay – protocols, see D5/D4.
Hardware roots of trust
TPM (chip on motherboard: key storage, measured boot, disk-lock binding), HSM (dedicated appliance for CA/database keys), secure enclave/TEE. TCB = everything enforces policy; security kernel implements the reference monitor (must be small, tamper-proof, always invoked).
Memory protections: DEP/NX bits, ASLR, stack canaries — buffer-overflow mitigations (more in D8).
Systems and architectures
- Cloud IaaS/PaaS/SaaS responsibility split; container/microservice segmentation; serverless exposure.
- ICS/SCADA – availability-first, legacy protocols, air-gap/DMZ zones, Purdue model.
- IoT/embedded – cheap HW, no patching, fog/edge computing.
- High availability: redundancy, RAID (0 stripe, 1 mirror, 5 parity, 6 double parity, 10 mirror+stripe), clustering, failover, N+1 / 2N power.
- Covert channels: storage and timing; counter: traffic padding, audit.
Memory hooks
- Bell-LaPadula = confidentiality (secrets): ‘no read up, no write down’.
- Biba = integrity: exactly the inverse: ‘no read down, no write up’.
- Clark-Wilson = commercial integrity via programs (TP + CDI + UDI + IVP).
- Brewer-Nash = Chinese Wall (conflicts of interest).
- Signature = private key; confidentiality = recipient's public key.
- EAL = assurance level, not ‘more secure’.
Exam tips
- Translate model questions to the goal first (confidentiality vs integrity) — two answers usually die right there.
- ‘Shared key distribution between strangers’ → asymmetric; bulk data → symmetric (hybrid).
- DH alone has no authentication — pair with certificates or it's MITM-able.
- GCM appears as the safe default mode (encryption + integrity).
- TPM binds disk encryption keys to the machine state.