Cryptography and Key Management
Core ideas
Cryptography provides confidentiality (encryption), integrity (hashing/MACs), authenticity, and non-repudiation (digital signatures). Know symmetric (AES-fast, shared key) vs. asymmetric (RSA/ECC-key exchange, signatures). Hashing (SHA-2/SHA-3) is one-way; HMAC adds a key for integrity/authenticity; digital signatures use the sender's private key.
Concepts to master
Block vs. stream ciphers, modes (GCM provides confidentiality + integrity), IV/nonce handling. PKI: CAs, certificates, chains of trust, revocation (CRL/OCSP). Key lifecycle: generation, distribution, storage, rotation, destruction. Perfect forward secrecy, hybrid encryption (asymmetric to exchange a symmetric session key), and TLS handshake basics. Post-quantum awareness.
Traps
Encryption does not provide integrity by itself (use AEAD/GCM or a MAC). Hashing is not encryption-hashes can't be reversed. Signing uses the private key; verification uses the public key. Never reuse an IV/nonce with the same key in stream/AEAD modes. MD5/SHA-1 are deprecated.
Worked example
Alice wants to send Bob a confidential, tamper-evident message. She encrypts with a symmetric session key using AES-GCM (confidentiality + integrity) and protects the session key by encrypting it with Bob's public key. Bob decrypts the session key with his private key.
Rule of thumb: symmetric for bulk data, asymmetric for key exchange/signatures, hashing/MAC for integrity.