Monoalphabetic Substitution Cipher

A monoalphabetic substitution cipher replaces each plaintext character with exactly one ciphertext character.

• The mapping remains fixed for the entire message.
• The cipher alphabet is a permutation of the plaintext alphabet.
• The Caesar cipher is a special case.

Example mapping: A → Q, B → M, C → L, …

Properties

• Preserves letter frequency distributions
• Vulnerable to frequency analysis
• Can be marginally strengthened using nulls or homophones

Caesar Cipher

The Caesar cipher is the simplest substitution cipher.

• Each letter is shifted by a fixed offset.
• Traditionally uses a shift of 3.

Example (shift = 3):

PLAINTEXT: ATTACK
CIPHERTEXT: DWWDFN

Properties

• Monoalphabetic substitution
• Key space of size 25
• Trivially broken by brute force or frequency analysis

Keyword Cipher

A keyword cipher constructs the substitution alphabet using a keyword.

Construction:

1. Choose a keyword
2. Remove repeated letters
3. Append remaining alphabet characters

Example:

Keyword: CIPHER
Substitution alphabet: C I P H E R A B D F G ...

Notes

• Stronger than Caesar
• Still preserves frequency
• Vulnerable to frequency analysis

Nulls

Nulls are meaningless characters inserted into the ciphertext.

• Known to the intended recipient
• Obscure patterns and frequencies
• Historically common in hand ciphers

Example:

CIPHERTEXT: XQDAZWWXDFNQZ
(remove X, Z, Q → DWWDFN)

Note:

Nulls increase confusion but do not provide cryptographic security.