Caesar Cipher Encoder & Decoder
Encode and decode text with the Caesar cipher online. Choose any shift 1–25. Free Caesar cipher encoder/decoder, no signup required.
Three steps to get started
Choose encode or decode
Select "Encode" to encrypt plaintext or "Decode" to decrypt a Caesar-encoded message.
Set the shift
Use the shift slider to choose a value from 1 to 25. Shift 13 is ROT13, the most common variant.
Paste and copy
Paste your text into the input and copy the result. Use the same shift value for encoding and decoding.
The Caesar cipher: history and how it works
Named after Julius Caesar who used it for military communications, the Caesar cipher is the foundation of classical cryptography. Despite being trivially breakable today, it introduced the core concept of substitution: replacing each character with another based on a fixed rule.
The algorithm works by treating the alphabet as a circular ring. With a shift of 3, the mapping becomes:
Plain: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Cipher: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
To decrypt, simply shift in the opposite direction by the same amount — or equivalently, shift forward by 26 − n.
Modern uses
- Puzzles and escape rooms — Caesar ciphers are a staple of puzzle hunts
- ROT13 on forums — shift 13 hides spoilers on Reddit and old Usenet posts
- Cryptography education — the simplest cipher to understand and implement
- CTF challenges — beginner capture-the-flag competitions frequently include Caesar-encoded flags
All processing is done locally in your browser. No text is ever transmitted to a server.