Introduction and History of Cryptography
This domain tests the vocabulary that the rest of the exam assumes, plus the classical ciphers and the principles that explain why modern designs look the way they do. Expect questions that distinguish cryptology, cryptography and cryptanalysis, identify a cipher from its mechanics, and test Kerckhoffs's principle and the one-time pad.
Core definitions
- Cryptology is the umbrella field: cryptography (defensive design) plus cryptanalysis (offensive analysis).
- Cryptography provides confidentiality, integrity, authentication and non-repudiation. Authentication proves origin; non-repudiation prevents later denial and normally requires a signature or trusted audit trail.
- Kerckhoffs's principle: a system should remain secure when everything except the key is public. Shannon restated it as “the enemy knows the system.” Security through obscurity violates this.
- Substitution replaces symbols; transposition rearranges them. A code replaces words or phrases; a cipher works on characters or bits. Steganography hides the existence of a message and is not encryption.
Historical ciphers
The Caesar cipher is a monoalphabetic shift. Vigenère uses a repeating keyword to create a polyalphabetic stream. Playfair encrypts digraphs. The one-time pad (Vernam) XORs plaintext with a truly random key that is as long as the message and used only once; it is the only cipher with perfect secrecy. Enigma used rotors and a plugboard but was still breakable because of operational mistakes, cribs and the Bombe.
Worked example
Encrypt CAT with a Caesar shift of +3. Map A=0: C(2)+3=F(5), A(0)+3=D(3), T(19)+3=W(22) modulo 26, giving FDW. Decryption subtracts 3. The shift changes letters but preserves the pattern of repeated letters and the overall frequency distribution, which is why a short ciphertext can be broken by frequency analysis.
Traps that cost marks
- Calling steganography “encryption”; it provides concealment, not cryptographic protection.
- Forgetting that a one-time pad reused twice leaks the XOR of the two plaintexts.
- Claiming transposition changes letter frequencies; it does not.
- Treating authentication as equivalent to non-repudiation.
- Describing a cipher as safe because its algorithm is secret.