Caesar Cipher

Encode and decode messages with the classic shift cipher

Shift Configuration

3
Plaintext:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
↓
Ciphertext:
DEFGHIJKLMNOPQRSTUVWXYZABC

Your Message

Encrypted Message

Decrypted Message

Frequency Analysis

Letter frequency in the current text (useful for breaking unknown ciphers)

Brute Force Decryption

Try all possible shifts to break the cipher without knowing the key

Visual Alphabet Wheel

See how letters shift around the alphabet

OUTER:
Plaintext
INNER:
Ciphertext
(Shift: 3)

Help

How Does the Caesar Cipher Work?

The Caesar Cipher shifts each letter in the alphabet by a fixed number of positions.

Example with shift 3:

  • A → D
  • B → E
  • C → F
  • ...
  • X → A (wraps around)
  • Y → B
  • Z → C

Message: "HELLO" → "KHOOR" (with shift 3)

Formula: New_Position = (Old_Position + Shift) mod 26

What is ROT13?

ROT13 is a Caesar cipher with shift 13.

Special property: ROT13 is self-inverse!

  • Encrypt: ROT13("HELLO") = "URYYB"
  • Decrypt: ROT13("URYYB") = "HELLO"
  • Same operation for encryption and decryption!

Why it works: 13 is half of 26, so shifting twice returns to the original position.

Uses: Hiding spoilers, obscuring (not securing) text, puzzle games

Not for security: Trivially breakable!

Why is the Caesar Cipher Weak?

The Caesar Cipher is extremely weak and should never be used for real security:

1. Limited key space:

  • Only 25 possible shifts (shift 0 and 26 are same as plaintext)
  • Can try all possibilities in seconds

2. Frequency analysis:

  • Letter frequencies remain unchanged
  • Most common letter in ciphertext likely represents E
  • Can break cipher by analyzing letter patterns

3. Pattern preservation:

  • Word lengths unchanged
  • Double letters preserved (HELLO → KHOOR, LL → OO)
  • Common words recognizable by pattern
What is Frequency Analysis?

Frequency analysis is a technique to break substitution ciphers by analyzing letter frequencies.

English letter frequencies (most common):

  1. E (~13%)
  2. T (~9%)
  3. A (~8%)
  4. O, I, N (~7-8%)
  5. S, H, R (~6-7%)

How to break Caesar cipher:

  1. Count letter frequencies in ciphertext
  2. Most common letter likely represents E
  3. Calculate shift needed to map that letter to E
  4. Decrypt entire message with that shift
  5. Verify result is readable English

Limitations: Needs longer text (~100+ characters) for accurate results

How Does Brute Force Work?

Brute force means trying all possible solutions.

For Caesar cipher:

  • Try all 25 possible shifts (1-25)
  • Decrypt the message with each shift
  • Look for readable English text
  • One of them will be the correct plaintext!

Why it works: Only 25 possibilities to check - very fast!

Detection: The tool highlights likely plaintexts by checking for common English words.

Modern ciphers: Brute force doesn't work on AES-256 (2^256 possibilities would take longer than age of universe)

Historical Uses

Julius Caesar (50 BCE):

  • Used shift 3 to communicate with generals
  • Security came from enemies not knowing the system
  • Most people couldn't read Latin anyway

ROT13 (1980s-90s):

  • Used on Usenet to hide spoilers and offensive content
  • Never intended for security, just obscuration
  • Still used in some contexts (geocaching hints)

Educational:

  • Teaches basic cryptography concepts
  • Demonstrates substitution ciphers
  • Shows importance of key space
  • Illustrates frequency analysis
When Should I Use This?

✅ Appropriate uses:

  • Learning cryptography concepts
  • Puzzle games and challenges
  • Non-secret text obfuscation
  • Educational demonstrations
  • Historical reenactment

❌ Never use for:

  • Password storage
  • Sensitive communications
  • Financial data
  • Personal information
  • Anything requiring real security

Modern alternatives: Use AES-256, RSA-2048, or other modern encryption algorithms for actual security needs.