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Cybersecurity · 26

Every formula in this course and the easiest ways to remember them, on one page.

1. Security foundations2. Cryptography and network security3. Application security4. Security operations and careers5. Security engineering, ethics and law

1Security foundationsBeginner

The CIA triad, threats and risk

Open lesson
R = L × I
where:
  • Rrisk score (1–25)
  • Llikelihood, 1 = very unlikely … 5 = almost certain
  • Iimpact, 1 = negligible … 5 = catastrophic

A simple qualitative model: many organisations use a 5×5 risk matrix.

Personal security: passwords, MFA and phishing

Open lesson
H = L · log₂A
where:
  • Hentropy, in bits
  • Lpassword length (number of characters)
  • Aalphabet size: lowercase 26, uppercase 26, digits 10, symbols 33

2Cryptography and network securityIntermediate

Hashing and salting: how passwords are stored

Open lesson
N ≈ √(2ⁿ)
where:
  • Nroughly how many hashes are needed to find a collision (the “birthday paradox”)
  • nhash length, in bits

This is the limit for an ideal hash; MD5's weak design lets researchers find collisions in seconds.

Encryption, TLS and certificates

Open lesson
K = Bᵃ mod p = Aᵇ mod p = gᵃᵇ mod p
where:
  • p, gpublic parameters: a large prime and a generator
  • a, bthe parties' secret numbers (never sent)
  • A, Bvalues sent openly: A = gᵃ mod p, B = gᵇ mod p
  • Kthe shared key both parties obtain

Network security: ports, firewalls, VPNs and Wi-Fi

Open lesson

3Application securityAdvanced

Web security: the OWASP Top 10

Open lesson

Secure coding

Open lesson

4Security operations and careersAdvanced

Incident response and digital forensics

Open lesson

Careers and certifications in cybersecurity

Open lesson

5Security engineering, ethics and lawUniversity

Threat modelling with STRIDE

Open lesson
SLE = AV × EF
where:
  • SLEsingle loss expectancy, in manat
  • AVasset value, in manat
  • EFexposure factor: the share lost in one event, from 0 to 1
ALE = SLE × ARO
where:
  • ALEannualised loss expectancy, manat per year
  • AROannualised rate of occurrence: 0.5 = once every two years, 2 = twice a year
NB = ALE₀ − ALE₁ − C
where:
  • NBannual net benefit of the control, manat per year
  • ALE₀, ALE₁annualised loss expectancy before and after the control
  • Cannual cost of the control, manat per year

If NB > 0, the control pays for itself.

Security architecture and zero trust

Open lesson
P = p₁ · p₂ · … · pₙ
where:
  • Pprobability that an attack gets through every layer
  • pᵢprobability that layer i misses the attack (from 0 to 1)
  • nnumber of layers

Derivation: if A and B are independent, P(A and B) = P(A) · P(B); by induction the rule extends to n layers.

Aseq = A₁ · A₂ · … · Aₙ
where:
  • Aseqavailability of the series system
  • Aᵢprobability that component i is up
Apar = 1 − (1 − A₁)(1 − A₂)…(1 − Aₙ)
where:
  • Aparavailability of the parallel system
  • 1 − Aᵢprobability that component i is down

Derivation: a parallel system is down only when every component is down; that event's probability is the product of the failure probabilities, and availability is its complement. Yearly downtime: D = (1 − A) · 8760 hours.

Ethics and law in cybersecurity

Open lesson
Fmax = max(20 000 000 €; 0,04 · T)
where:
  • Fmaxthe GDPR fine cap for the most serious infringements
  • Tthe company's worldwide annual turnover for the previous financial year, in euros

Whichever is higher applies. For other infringements: max(€10,000,000; 0.02 · T). This is a cap; the actual fine is set case by case.