- Caesar cipher shifts letters by a fixed integer key value (modulo 26).
- Monoalphabetic substitution maps each alphabet letter to a fixed alternative letter.
- Frequency analysis easily breaks monoalphabetic ciphers because letter frequencies in language are preserved.
### Caesar & Monoalphabetic Substitution
The **Caesar Cipher** shifts every letter in the plaintext by $k$ positions modulo 26:
$$E(x) = (x + k) \pmod{26}$$
#### Frequency Analysis Attack
In English, the letter **E** appears with a frequency of ~12.7%, followed by **T** (~9.1%) and **A** (~8.2%). Because substitution ciphers maintain a 1-to-1 mapping, letter frequency distributions remain unchanged, allowing cryptanalysts to break simple substitution ciphers effortlessly.