Cryptography Glossary

Intro to Computer Security — Stallings + professor lectures

Foundations

Symmetric cipher: Encryption and decryption use the same secret key. Both parties must share the key securely beforehand. Avoid: private-key encryption (ambiguous with RSA).
Asymmetric cipher: Uses a key pair — public key encrypts, private key decrypts (or signs). Solves key distribution but is slower.
Block cipher: Encrypts fixed-size blocks (DES: 64 bits).
Stream cipher: Encrypts one byte at a time using a keystream XORed with plaintext (RC4).
Hash function: One-way mapping from arbitrary message → fixed-length digest. Used for integrity, not secrecy.
Confidentiality / Integrity / Availability (CIA): Core security goals: secrecy, no unauthorized change, systems accessible when needed.
Authentication vs authorization: Authentication proves identity; authorization decides permitted actions.

McCumber Cube: Framework linking CIA, data states, and security measures (policy, education, technology).
Sec-SDLC: Security integrated into the system development life cycle.
Asset: Anything of value to protect (data, systems, hardware).
Threat: Any source of danger (hackers, malware, disasters).
Vulnerability: Weakness an attacker can exploit.

Feistel network: Block cipher structure: Lₙ=Rₙ₋₁, Rₙ=Lₙ₋₁⊕f(Rₙ₋₁,Kₙ).
S-box: Substitution table: 6 input bits → 4 output bits. Row = b₁b₆; column = b₂b₃b₄b₅.
Effective key length (DES): 56 bits — 64-bit key with every 8th parity bit dropped by PC-1.
PC-1 / PC-2: Permuted Choice tables: PC-1 drops parity and splits key into C₀/D₀; PC-2 selects 48 bits for subkey Kₙ.
Key schedule: Generates 16 round subkeys from the 56-bit master key via left shifts on C and D halves.

KSA (Key Scheduling Algorithm): Initializes and permutes the S-box using the key: j=(j+S[i]+T[i]) mod n, swap.
PRGA (Pseudo-Random Generation Algorithm): Generates keystream bytes; ciphertext = plaintext ⊕ keystream.

Diffie-Hellman: Agree on shared secret over insecure channel via Y=α^X mod q. Does not encrypt messages alone.
Primitive root (generator α): Element whose powers cycle through all residues mod q (for DH).
RSA: Public-key cipher: C=M^e mod n, M=C^d mod n. Security based on factoring difficulty.
Euler totient φ(n): For n=p×q (primes): φ(n)=(p−1)(q−1).
Extended Euclidean algorithm: Finds d such that e·d≡1 (mod φ) — builds RSA private key.
Successive squaring: Binary exponentiation: compute M^e mod n by repeated squaring and selective multiply.

SHA-1: Produces 160-bit hash. Processes 512-bit padded blocks through 80 compression rounds.
Padding (SHA-1): Append bit 1, then zeros until length≡448 (mod 512), then append original 64-bit message length.
Message schedule (W): 16 initial 32-bit words expanded to 80 words: Wₜ = ROTL¹(Wₜ₋₃⊕Wₜ₋₈⊕Wₜ₋₁₄⊕Wₜ₋₁₆) for t≥16.
Avalanche effect: One-bit input change → completely different hash output.