Caesar Cipher Decoder & ROT13 Tool
Free online Caesar cipher decoder and ROT13 encoder. Convert text with any shift value, encrypt messages, or decode ciphertext instantly. Works directly in your browser — secure, private, and no signup required.
ROT13 & Caesar Cipher
Free online Caesar cipher decoder and ROT13 tool. Instantly encrypt or decrypt text with any shift value. Educational cryptography tool with instant results.
ROT13 is most common (shifts by 13 places)
Caesar Cipher Information
The Caesar cipher is one of the simplest and oldest encryption techniques, named after Julius Caesar who used it to protect his military communications. With over 18,100 monthly searches for "caesar cipher", it remains a popular tool for both educational and practical applications.
How to decode a Caesar cipher:
Each letter in the ciphertext is shifted backward by the same number of positions used for encryption. Our free online Caesar cipher decoder lets you decode any message by selecting the correct shift value (or trying all 25 possible shifts if unknown).
What is ROT13?
ROT13 is a special case of the Caesar cipher with a shift of exactly 13 places. It's the most popular cipher tool with approximately 2,000 monthly searches. Since the English alphabet has 26 letters, applying ROT13 twice returns the original text, making it perfect for hiding spoilers or puzzle solutions.
Example with ROT13:
Plain: HELLO WORLD
Cipher: URYYB JBEYQ
Example with ROT3:
Plain: ATTACK AT DAWN
Cipher: DWWDFN DW GDZQ
Key Features
Professional-grade tool designed for speed, security, and simplicity
Flexible Shift Values
Apply any numeric shift to encode or decode text, including ROT13 presets.
Support for Alphabets
Operate on ASCII letters with options to preserve case and non-letter characters.
Client-side & Instant
All transformations run in your browser with no external calls.
How the Caesar Cipher Tool Works
Shift each alphabetic character by the chosen offset; non-letter characters can be left unchanged.
Enter text
Paste or type the text to encode/decode.
Choose shift
Set the numeric shift value (e.g., 13 for ROT13).
Apply
View encoded/decoded output instantly and copy to clipboard.
Common Uses
Puzzles & Learning
Solve and create Caesar cipher puzzles for education and fun.
Legacy Systems
Quickly encode test strings for legacy systems using simple ciphers.
Technical Information
Processing Method
The Caesar cipher is one of the earliest recorded substitution ciphers, named after Julius Caesar who reportedly used a shift of 3 to protect military correspondence. With 18,000+ monthly searches for "caesar cipher" it remains a staple for teaching classical cryptography. Decoding is simply the reverse shift: each ciphertext letter is moved backward by the same offset used during encryption. If the shift is unknown, brute force by trying all 25 non‑zero shifts reveals the plaintext almost immediately. Examples: ROT13: HELLO WORLD → URYYB JBEYQ. ROT3 (classic Caesar): ATTACK AT DAWN → DWWDFN DW GDZQ.
Requirements
ROT variants overview: ROT1 shifts letters forward by 1 (HELLO → IFMMP). ROT3 is the historical Caesar shift. ROT5 and ROT7 are just different offsets (SHIFT → XMNKY with ROT5; CODE → JTKL with ROT7). ROT13 is special/involutive: applying it twice returns original text. ROT25 is equivalent to shifting backward by 1 (HELLO → GDKKN). All transformations ignore non‑alphabetic characters unless explicitly handled, and case can be preserved. Because there are only 26 possible shifts, every variant is trivial to enumerate.
Compatibility
Crack difficulty: Extremely low. A brute‑force loop over shifts (0–25) is O(26·n) ≈ O(n) in practice, and frequency analysis (looking for E/T/A patterns) further accelerates recognition. ROT13 offers no security—its involutive property makes it reversible with the same operation. ROT25 is just ROT‑1; none provide resistance against modern cryptanalysis.
Performance
Performance: Pure string/index arithmetic. Even brute forcing every shift on multi‑paragraph text completes instantly in the browser (linear time, negligible memory). Suitable strictly for obfuscation or educational demonstrations—not for real confidentiality.
Security & Privacy
Data Handling
No data is transmitted; everything runs locally.
Privacy Protection
Suitable for non-sensitive obfuscation only; not cryptographically secure.
Security Recommendations
Do not use classical ciphers to protect sensitive information; use modern encryption instead.
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