What is a Caesar Cipher?

A Caesar cipher is one of the oldest and simplest forms of encryption. It is a 'substitution cipher' where each letter in the plaintext is 'shifted' a certain number of places down the alphabet. For example, with a shift of 3, 'A' becomes 'D', 'B' becomes 'E', and so on.

ROT13 ('Rotate by 13') is a specific, well-known example of a Caesar cipher. It uses a fixed shift of 13 places. The advantage of ROT13 is that it's reversible: applying ROT13 twice gets you back to the original text (since 13 is half of 26). It's famously used to hide spoilers or puzzle hints online.

How do you decrypt a Caesar Cipher?

You decrypt it by applying the *opposite* shift. If the message was encrypted with a shift of 3 (A -> D), you decrypt it with a shift of -3 (or +23) to get back to the original (D -> A). If you don't know the shift, you can simply try all 25 possible shifts until one produces readable text.

Caesar Cipher Tool (Shift Cipher)

Encrypt/Decrypt text by shifting letters (e.g., ROT13).

Plain Text / Cipher Text

Mode

Shift (1-25)

Output

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What is the Caesar Cipher (or Shift Cipher)?

The Caesar cipher, named after Julius Caesar, is one of the simplest and most widely known encryption techniques in history. It's a type of **substitution cipher** where each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet.

This fixed number is the "key" or **"shift"**.

Example (Shift of 3):

Plaintext: A B C D E
Ciphertext: D E F G H

To decrypt, you simply shift the letters back by the same amount.

Is the Caesar Cipher Secure?
**No. It is completely insecure.** Because there are only 25 possible shifts (a shift of 26 brings you back to the start), an attacker can easily "brute-force" the message by simply trying all 25 keys. The correct plaintext will quickly become obvious.

Key Use Cases (Educational / Puzzles):

Educational Tool: It is the perfect "first cipher" to learn when studying cryptography, as it simply demonstrates the concept of a key and a substitution.
ROT13: A special case of the Caesar cipher with a **shift of 13**. Its main property is that it's its own inverse: encrypting and decrypting use the *same* key (13). It is famously used on internet forums to hide puzzle answers, movie spoilers, or punchlines.

For any real-world security, you must use a modern, secure algorithm like **AES-256**.

Caesar Cipher Examples

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Caesar Cipher Key Concepts & Warnings

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NOT FOR SECURITY

This cannot be repeated enough. **Do not use this to protect any sensitive data.** The Caesar cipher is a "toy" cipher and can be broken instantly by a human, and even faster by a computer. Its only purpose is for education or simple puzzles (like ROT13).

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ROT13: The Reversible Cipher

The most famous "shift" is 13, known as **ROT13**. Because 13 is half of the 26 letters in the alphabet, applying the shift twice (13 + 13 = 26) brings you right back to the original letter. This means the same "encrypt" function is used to "decrypt." (e.g., A → N, and N → A).

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Vulnerable to Frequency Analysis

Beyond brute-force, all simple substitution ciphers are weak against **frequency analysis**. In English, the letter 'E' is the most common. An attacker just has to count the letters in the ciphertext. If 'H' is the most common, the shift was likely 3 (E → H). This is how they are broken.