Cryptography and Secure Data Protection in Web Applications
Core concepts
Know the primitives and when each applies: symmetric encryption (AES-GCM, ChaCha20-Poly1305) for data at rest and bulk transport; asymmetric (RSA, ECDSA, Ed25519) for key exchange and signatures; hashing (SHA-256/384) for integrity; HMAC for authenticity with a shared key; and password hashing (bcrypt, scrypt, Argon2id) which is deliberately slow. Understand authenticated encryption (AEAD) and why encrypt-then-MAC or GCM tags prevent tampering. Know TLS 1.2/1.3 basics: certificate validation, HSTS, forward secrecy via ephemeral key exchange, and that TLS protects data in transit only.
Key and secret management
Keys belong in a KMS/HSM or secrets manager, never in source or client bundles. Rotate keys, separate environments, and never reuse an IV/nonce with the same key in GCM or CTR modes. Understand data classification and encryption at rest for PII, plus tokenization and masking for display. Know that hashing is one-way and not encryption, and that base64 is encoding, not protection.
Traps
Questions treat MD5/SHA-1 as acceptable for passwords or signatures; assume encryption provides integrity without AEAD; propose ECB mode; or believe HTTPS makes client-side secrets safe. Another trap: confusing signing (authenticity, non-repudiation) with encryption (confidentiality).
Rule of thumb
Use vetted libraries and AEAD; if you must choose between rolling your own crypto and changing the design, change the design.
Worked example
Encrypting a stored card number requires AES-GCM with a unique nonce per record, a key held in a KMS, and a key-rotation plan. Hashing would be wrong because the value must be recoverable for authorized use.