Caesar Cipher & ROT13
Shift letters to encode or decode text — adjustable shift 0–25, ROT13 shortcut, and brute-force all 26 shifts.
Shift / ROT
Input
Output · shift 13
Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
Brute Force (all 26 shifts)
Click Show to see all 26 possible decryptions at once.
Letter Frequency (input)
English frequency: ETAOINSHRDLCUMWFGYPBVKJXQZ
About this tool
A Caesar cipher tool with ROT13 support. Encrypt or decrypt any text by shifting letters a chosen number of positions (0–25). Use the brute-force panel to view all 26 decryptions at once for cracking unknown shifts. Includes letter frequency analysis to aid cryptanalysis.
How to use it
Quick steps to get the most out of this utility.
- 1
Type or paste your text
Enter the plaintext to encrypt or the ciphertext to decrypt.
- 2
Set the shift
Drag the slider or type a number 0–25. Click ROT13 for shift 13.
- 3
Choose encrypt or decrypt
Decrypting reverses the shift direction automatically.
- 4
Brute-force unknown ciphers
Click Show under Brute Force to see all 26 possible decryptions and spot the readable one.
Caesar cipher history and uses today
Julius Caesar used a shift-of-3 cipher for military communications around 58 BC. While trivially broken by modern standards, the Caesar cipher is the foundation for understanding substitution ciphers, frequency analysis, and the basics of cryptography.
ROT13 (shift 13) lives on in internet culture: Reddit and forums use it to hide spoilers, puzzle answers, and punchlines. Its self-inverse property — applying it twice gives back the original — makes it convenient for toggling visibility without needing a separate decode step.
Frequently asked questions
What is a Caesar cipher?+
A Caesar cipher is a substitution cipher where each letter in the plaintext is shifted a fixed number of positions down the alphabet. Named after Julius Caesar, who reportedly used a shift of 3. It is one of the simplest encryption techniques.
What is ROT13?+
ROT13 is a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, applying ROT13 twice returns the original text — making encoding and decoding the same operation. It is used to hide spoilers and puzzle answers online.
How secure is the Caesar cipher?+
Not secure at all for real use. With only 25 possible shifts, a brute-force attack can try all possibilities in seconds. For real encryption, use AES or another modern cipher. The Caesar cipher is used for education and fun only.
What is frequency analysis?+
Frequency analysis exploits the fact that letters appear at predictable rates in natural language (E is most common in English). By comparing the frequency of letters in ciphertext to known language frequencies, you can often determine the shift without knowing it.
Does the cipher affect numbers and symbols?+
No. The Caesar cipher only shifts letters (A–Z, a–z). Numbers, spaces, and punctuation are passed through unchanged.
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