📘 What makes cryptography trustworthy?

What you’ll learn

1. Foundations: Goals, Threat Models, and Kerckhoffs's PrincipleDefine the core security goals cryptography provides and explain why a system's security must rest on the secrecy of the key, not the algorithm.Cryptography is the science of protecting information against an adversary, and modern practice frames it around four goals: confidentiality, integrity, authentication, and non-repudiation. A central design rule, Kerckhoffs's principle, holds that a system should remain secure even if everything about it except the key is public; Claude Shannon restated this as 'the enemy knows the system.' Security is meaningful only relative to a defined threat model and an adversary's assumed capabilities, and 'security through obscurity' is not a substitute for sound, openly reviewed design.
2. Symmetric Encryption: AES and Modes of OperationExplain how symmetric block ciphers like AES work, why a raw block cipher needs a mode of operation, and how nonces and IVs are used safely.Symmetric encryption uses a single shared secret key for both encryption and decryption, and the Advanced Encryption Standard (AES) is the dominant block cipher, standardized by NIST in FIPS 197 with 128, 192, or 256-bit keys and a fixed 128-bit block. Because a block cipher only transforms one fixed-size block, a mode of operation is needed to encrypt arbitrary-length data; ECB leaks structure and must be avoided, while modes like CTR and CBC use an IV or nonce to randomize output. The key management challenge is that both parties must already share the secret key, which motivates the public-key techniques covered later.
3. Hash Functions and Message AuthenticationDescribe the security properties of cryptographic hash functions and explain how HMAC uses a hash plus a key to authenticate messages.A cryptographic hash function maps arbitrary input to a fixed-length digest and must be one-way (preimage resistant), second-preimage resistant, and collision resistant. NIST standardizes the SHA-2 family in FIPS 180-4 and the sponge-based SHA-3 family in FIPS 202, both offering 224, 256, 384, and 512-bit digests. Hashing alone provides integrity only against accidental change, so authenticating a message against an active attacker requires a keyed construction such as HMAC, and passwords must be protected with slow, salted password-hashing functions rather than plain fast hashes.
4. Public-Key Cryptography: Key Exchange and Digital SignaturesExplain how asymmetric cryptography solves key distribution and enables digital signatures using RSA, Diffie-Hellman, and elliptic-curve methods.Public-key (asymmetric) cryptography gives each party a mathematically linked key pair, a public key that can be shared freely and a private key kept secret, which solves the key-distribution problem of symmetric ciphers. Diffie-Hellman lets two parties derive a shared secret over a public channel based on the hardness of the discrete logarithm problem, while RSA, based on the difficulty of factoring large integers, supports both encryption and signatures. Digital signatures use the private key to sign and the public key to verify, providing authentication, integrity, and non-repudiation, and elliptic-curve cryptography achieves equivalent security with much smaller keys.
5. Protocols, Pitfalls, and the Post-Quantum HorizonConnect cryptographic primitives into real protocols like TLS, identify common implementation pitfalls, and explain why post-quantum standards now exist.Cryptographic primitives only deliver security when composed correctly: TLS combines authenticated key exchange, certificates with a public-key infrastructure, and authenticated encryption to secure the web. Authenticated encryption with associated data (AEAD), such as AES-GCM standardized in NIST SP 800-38D, binds confidentiality and integrity together and avoids the fragile mistakes that arise when hand-composing separate encryption and MAC steps. Many real failures come from misuse rather than broken math, including nonce reuse, weak randomness, padding oracles, and side channels, and looming quantum computers led NIST to publish post-quantum standards in 2024 (FIPS 203, 204, and 205).
6. Capstone: Build a Mini Authenticated-Messaging ArtifactApply the course concepts by designing and assembling a small authenticated, confidential message scheme using only standard, vetted primitives.In this capstone you build a mini artifact, an authenticated secure-message scheme, by composing the primitives from this course in the correct order. You will establish a shared key with an authenticated key exchange, protect each message with an AEAD cipher using unique nonces, derive keys properly, and add a signature for non-repudiation, then validate your design against a checklist of common pitfalls. The deliverable is a concrete protocol specification and a reflection on its threat model, demonstrating that secure systems come from correctly combining standard building blocks rather than inventing new math.

Questions this course answers

Grounded in trusted sources

• Niels Ferguson, Bruce Schneier, and Tadayoshi Kohno, Cryptography Engineering — threat models and protocol hygiene
• NIST FIPS 197, Advanced Encryption Standard (AES), https://csrc.nist.gov/publications/detail/fips/197/final
• NIST FIPS 180-4, Secure Hash Standard, https://csrc.nist.gov/publications/detail/fips/180/4/final
• Auguste Kerckhoffs, “La cryptographie militaire” — Kerckhoffs’s principle (historical)
• RFC 8446, The Transport Layer Security (TLS) Protocol Version 1.3 — modern key exchange context, https://www.rfc-editor.org/rfc/rfc8446

Every Wunder lesson is built from real, reputable sources — never invented.

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