Substitution Cipher

Cryptography · Axiom Academy

Understanding monoalphabetic substitution ciphers and why a large key space doesn't guarantee security In a simple substitution cipher , each letter of the alphabet is replaced by exactly one other letter. This creates a one-to-one mapping between plaintext and ciphertext alphabets. The animation shows how individual letters are transformed. Notice that the same plaintext letter always produces the same ciphertext letter. The key is a permutation of the 26-letter alphabet. It defines the complete mapping from plaintext to ciphertext. With this key, the word "HELLO" becomes "ITSSG": How many possible substitution keys exist? Since each key is a permutation of 26 letters, the total number of possible keys is: This equals approximately 4 × 10 26 possible keys - an astronomically large number! 4. Why Size Doesn't Equal Security Despite the enormous key space, substitution ciphers are easily broken using frequency analysis. The key weakness: the cipher preserves the statistical properties of the language . Other exploitable patterns include: Letter frequency: T, A, O, I, N are common; Q, Z, X are rare Digraphs: Common two-letter combinations like TH, HE, AN Trigraphs: Three-letter patterns like THE, AND, ING Word patterns: Single-letter words (A, I), common short words 5. Monoalphabetic vs Polyalphabetic Simple substitution ciphers are called monoalphabetic because they use a single, fixed alphabet substitution throughout the entire message.

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