Free Online Caesar Cipher Encoder and Decoder Encrypt and decrypt text using the Caesar shift cipher.

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Caesar Cipher

Encrypt and decrypt text using the Caesar shift cipher.

Enter text

Type or paste your text (letters, numbers, punctuation).

Choose shift

Pick a shift value from 1 to 25.

View output

Encrypted or decrypted text appears instantly.

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What Is Caesar Cipher?

The oldest known encryption attributed to Julius Caesar (shift 3). Simple substitution shifting each letter. Preserves case. Only 25 shifts — trivially breakable. ROT13 (shift 13) is self-inverse. Pure JavaScript.

Why Use Caesar Cipher?

Fast, accurate processing
100% client-side — data never leaves your browser
Free online tool with no signup required
Mobile-friendly with 44px touch targets

Common Use Cases

Learning substitution ciphers

Introduce classic encryption concepts.

Puzzles and code games

Solve or create simple cipher challenges.

Quick obfuscation

Obfuscate text for fun or casual use.

Educational demos

Show how shift ciphers work in cryptography.

Technical Guide

E(x) = (x + s) mod 26. D(x) = (x + 26 - s) mod 26. Uppercase (base 65) and lowercase (base 97). Non-alphabetic via regex. 25 keys. ROT13 self-inverse: 13+13=0 mod 26.

Tips & Best Practices

1
Letters shift; numbers and punctuation stay unchanged.
2
Same shift for encode and decode (reverse direction).
3
ROT13 (shift 13) encodes and decodes with the same action.
4
Use Copy button for quick output.
5
"hello" with shift 3 becomes "khoor".

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Frequently Asked Questions

Q How does the Caesar cipher work?

Each letter shifts by a fixed number (1–25). A with shift 3 becomes D.

Q What is ROT13?

Shift 13 is self-inverse: encoding and decoding use the same action.

Q Can it handle numbers?

Yes. Numbers and punctuation pass through unchanged.

Q Is it case-sensitive?

Yes. Uppercase and lowercase letters are shifted separately.

Q How many possible shifts?

25. Shift 0 and 26 are the same as no shift.

About This Tool

Caesar Cipher is a free online tool by FreeToolkit.ai. All processing happens directly in your browser — your data never leaves your device. No registration or installation required.