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Symmetric cryptography, substitution, Caesar

Short answer

In symmetric cryptography, the same key encrypts and decrypts data (C=E(K,P), P=D(K,C)). It is fast but suffers from key distribution problems. Mono-alphabetic substitution replaces each letter with a fixed mapping (key space 26!), breakable by frequency analysis. Caesar cipher is a special case, shifting letters by k (mod 26), with just 25 keys.

Explain symmetric cryptography. Also explain mono-alphabetic substitution and Caesar cipher.

Symmetric cryptography

In symmetric (secret-key) cryptography the same key is used to encrypt and to decrypt: C = E(K, P) and P = D(K, C). The sender and the receiver must share the key beforehand over a secure channel. Examples: DES, 3DES, AES, Blowfish.

 P --[Encrypt, K]--> C --(channel)--> [Decrypt, K]--> P
  • Advantages: fast, suitable for large data.
  • Disadvantages: key distribution problem; n users need n(n−1)/2 keys; no non-repudiation.
  • Two kinds: block ciphers (fixed size blocks, e.g. 128 bits in AES) and stream ciphers (bit by bit).
  • Classical techniques: substitution and transposition.

Mono-alphabetic substitution cipher

Each letter of the plaintext alphabet is replaced by another letter from a fixed one-to-one mapping (the key is a permutation of the 26 letters).

Plain : a b c d e f g h i j k l m n o p q r s t u v w x y z
Cipher: Q W E R T Y U I O P A S D F G H J K L Z X C V B N M

"attack" → "QZZQEA".

  • Key space is 26! ≈ 4 × 10²⁶, too large for brute force.
  • Weakness: letter frequencies of the language remain (e, t, a most common in English), so frequency analysis easily recovers the key.

Caesar cipher

A special mono-alphabetic cipher where each letter is shifted by a fixed k positions: C = (P + k) mod 26. With k = 3: A → D, B → E; "attack" → "DWWDFN". Only 25 keys, so it is broken at once by trying all shifts.

✓ Answer checked by Claude Opus 5.5 (high effort)

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