What is the difference between a Caesar cipher and ROT13?

ROT13 is simply a Caesar cipher with a fixed shift of 13. This tool lets you pick any shift from 0 to 25, so a shift of 13 reproduces ROT13 exactly.

How do I decrypt a Caesar cipher message?

Switch to Decrypt and enter the same shift that was used to encrypt. The tool rotates each letter back by that amount to recover the original text.

What does a shift of 3 do?

A shift of 3 — the cipher Julius Caesar himself reportedly used — turns A into D, B into E, and so on. The word HELLO becomes KHOOR.

Does the Caesar cipher change numbers, spaces or punctuation?

No. Only letters A–Z and a–z are shifted. Digits, spaces, punctuation and symbols pass through unchanged, which preserves the layout of your text.

How do I break a Caesar cipher without the key?

Because there are only 25 possible non-zero shifts, you can brute-force it by trying each shift until readable text appears, or use letter-frequency analysis to find the most likely shift.

Is the Caesar cipher secure?

No. With only 25 keys it is trivially broken and offers no real protection. It is useful for puzzles, teaching and lightweight obfuscation, not for protecting sensitive data.

Why does the result wrap from Z back to A?

The alphabet is treated as a 26-letter ring, so shifting past Z wraps to A (modulo 26). This keeps every output a valid letter.

Is my message uploaded anywhere?

No. The cipher runs entirely in your browser with JavaScript, so your message never leaves your device.

Caesar Cipher — Gera Tools

The Caesar cipher is one of the oldest known encryption methods, named after Julius Caesar, who used a shift of 3 to protect military messages. It is a substitution cipher: every letter is replaced by another letter a fixed number of places further along the alphabet. This tool lets you encrypt or decrypt text with any shift from 0 to 25, instantly and entirely in your browser. It is ideal for puzzles, escape-room clues, classroom demonstrations and casual obfuscation.

How it works

Each letter is mapped onto the 26-letter alphabet as a number (A = 0 … Z = 25). The cipher adds the shift to that number, then wraps around using modulo 26:

encrypted = (letter index + shift) mod 26

Decryption subtracts the shift instead of adding it. The tool normalises the shift with ((shift % 26) + 26) % 26 so negative or large values still behave correctly, and it preserves letter case — uppercase stays uppercase, lowercase stays lowercase. Anything that is not a letter (digits, spaces, punctuation) is left exactly as typed.

Example

Encrypt HELLO with a shift of 3:

Letter	Index	+ shift 3	mod 26	Result
H	7	10	10	K
E	4	7	7	H
L	11	14	14	O
L	11	14	14	O
O	14	17	17	R

The output is KHOOR. To recover the original, set the mode to Decrypt with the same shift of 3, and KHOOR becomes HELLO again.

Everything runs locally in your browser — your text is never uploaded.