substitution ciphers

Summary

Encryption and decryption

Key space and size

• the substitution table is the key

number of ways to arrange objects without replacement

Concept

Substitution table, over an alphabet

substitution must be bijective

Known plaintext attack

• not secure
• given a sufficiently long ciphertext, the full substitution table can be found

Ciphertext only attack

• frequency analysis
• examining how frequently characters appear in the ciphertext and mapping them to their frequency in English
• quipquip and dcode.fr frequency analysis
  

Application

Ceasar/shift cipher

ROT3 on Cyberchef, shift cipher’s only have possible keys