Introduction to Cryptography and Encryption
Introduction to Cryptography and Encryption
This domain tests the vocabulary and security goals that every later domain depends on: what a cryptosystem is, which primitive delivers which property, and why classical ciphers survive as teaching models rather than usable tools.
Core rules and concepts
- Cryptosystem components. An algorithm, a key space, plaintext, ciphertext, and the encryption/decryption transforms. Kerckhoffs's principle: the system must stay secure when everything except the key is public - secrecy of the algorithm is never the control.
- Goal mapping. Confidentiality → encryption. Integrity → hash or MAC. Authenticity and non-repudiation → digital signature. Knowing which primitive answers which question is the most heavily examined idea in this domain.
- Symmetric vs asymmetric. One shared key (fast, bulk data) versus a public/private pair (slow, solves key distribution).
- Block vs stream. Block ciphers process fixed chunks with padding and modes; stream ciphers XOR a keystream and demand a never-reused nonce.
- Shannon's properties. Confusion obscures the relationship between key and ciphertext; diffusion spreads one plaintext bit across many ciphertext bits, producing the avalanche effect.
- Steganography hides the existence of a message; cryptography hides its meaning. They are different controls and can be layered.
Worked example
One-time pad over a 26-letter alphabet. Plaintext C A T = 2, 0, 19. Key = 5, 17, 3. Ciphertext = (2+5) mod 26 = 7 (H), (0+17) mod 26 = 17 (R), (19+3) mod 26 = 22 (W), giving "HRW". Decryption subtracts the same key. The pad is provably unbreakable only when the key is truly random, as long as the message, used once, and kept secret. Reuse two pads and C1 ⊕ C2 = P1 ⊕ P2, which leaks structure immediately.
Traps that cost marks
- Calling Base64 or hex encoding "encryption" - it is reversible encoding with no key.
- Accepting a one-time pad generated by a PRNG as information-theoretically secure.
- Swapping confusion and diffusion in a definition question.
- Assuming an unbreakable algorithm compensates for a weak key or a broken protocol.