Caesar Cipher & ROT13 Encoder/Decoder

Which characters get shifted?

Only the 26 unaccented Latin letters, A to Z and a to z, and case is kept: with shift 3, “Zulu” becomes “Cxox”. Digits, spaces, punctuation, accented letters such as é, and non-Latin scripts pass through unchanged. That makes the cipher easy to read for English text but means numbers in a message stay in the clear.

What shift values does the Shift box accept?

Whole numbers. The value wraps around the 26-letter alphabet, so 29 acts like 3 and 26 acts like 0, which leaves the text unchanged. A negative number shifts backwards: −3 is the same as 23, so “Zebra” encodes to “Wbyox”. Clearing the box counts as a shift of 0, which leaves the text unchanged. The Shift box is ignored in ROT13 and brute-force modes.

How does decode work?

Decoding with shift n applies a shift of 26 − n, which moves each letter n places back. Text encoded with shift 3 therefore decodes with shift 3: “Dwwdfn dw gdzq, Cxox 42!” returns “Attack at dawn, Zulu 42!”. If you do not know the shift, use brute-force mode and look for the readable line.

What is ROT13 and why is it special?

ROT13 is the Caesar cipher with shift 13. Because 13 is half of 26, running it twice returns the original text, so there is no separate decode step. “Hello, World” becomes “Uryyb, Jbeyq”, and putting that back through ROT13 returns the starting text.

What does brute-force mode show?

Shifts 1 to 25 applied to your input, one line each. It does not rank them or pick a winner, so you must find the readable line yourself. For “Khoor” the line at shift 23 reads “Hello”. Shift 0 is omitted because it is the original text. Note that brute force applies shift n forward, so shift 23 forwards is the same as decoding shift 3.

Does the tool do frequency analysis?

No. It lists all 25 candidate shifts but does not count letters or score the results. The mapping table shown for short messages lists the first 20 or fewer letters and their substitutes, which is not a frequency count. For longer ciphertext you will need to compare the 25 lines by eye.

Is the Caesar cipher secure?

No. There are only 25 useful shifts, and every occurrence of a letter is replaced by the same letter, so the cipher can be broken within seconds by hand or by the brute-force mode here. Use it for puzzles, teaching and light obfuscation only, never for secrets.