The Caesar Cipher / ROT13 tool applies a letter-shift substitution to whatever you paste, live, in the browser. The page holds a text area whose placeholder says Enter text..., a row of three preset buttons labeled ROT13, ROT5 and Custom, a row of two direction buttons labeled Encrypt and Decrypt, a shift slider marked from 1 to 25 with a Shift Amount label, a read-only output text area of the same size, a copy button next to it, and a shift table below. Whatever you type appears in the output on the same keystroke; there is no run button and nothing to press. At the bottom a small report-an-issue link rounds out the page. Nothing is sent anywhere: the shifting, the table, the copy, all of it runs on the device.
What the tool takes and what it gives
The inputs are four: the text itself, the preset, the direction, and the shift amount. The preset decides which alphabet moves; the direction decides which way the letters move; the shift decides how many places. With ROT13 or Custom the Caesar shift applies, and with ROT5 the digit shift applies instead. Because the output is computed live, there is no moment when a result waits to be produced: clear the text area and the output clears with it, and paste a sentence and the shifted version is already sitting in the read-only box. The copy button writes that box to the clipboard and shows a Copied state for two seconds; it only acts when there is something to copy.
How the Caesar shift works
The shift moves each letter of the Latin alphabet forward by the chosen amount and wraps around at the end: with a shift of 3, a becomes d, b becomes e, and x, y, z wrap to a, b, c. Uppercase and lowercase move in their own lanes and keep their case, so under ROT13 a becomes n and Z becomes M. Every character that is not a Latin letter passes through exactly as typed: digits, punctuation, spaces, tabs, line breaks, and all non-Latin characters, accented letters and CJK text included, are untouched. The classic test: with the ROT13 preset the sentence Hello, World! becomes Uryyb, Jbeyq!, the comma, the capitals, and the exclamation all riding along unmoved.
ROT13 and the decrypt direction
ROT13 is the Caesar shift of exactly 13, which is half the alphabet, so applying it twice returns the original text; the same operation encodes and decodes. The other presets use the direction buttons. Encrypt moves letters forward by the shift, and Decrypt moves them back, which the tool implements as a forward shift of 26 minus the shift, so decrypting with a shift of 3 is a forward move of 23. Concretely: encrypt abc with a shift of 3 to get def, then decrypt def with a shift of 3 and the table lands back on abc. The extremes behave as the math says: a shift of 25 is the same as a shift of minus 1, so encrypting A with 25 gives Z, and decrypting with a shift of 1 is a forward move of 25. Every shift from 1 to 25 round-trips exactly, which is what makes the decrypt direction reliable.
ROT5 and the preset buttons
The ROT5 preset swaps the alphabet for the ten digits: each digit is moved forward by 5 and wraps at 9, so 12345 becomes 67890, 9 wraps to 4, and 0 wraps to 5, while every letter stays where it is; the mixed text A1a comes back as A6a, and 19 becomes 64. Two UI details follow from the preset. The direction row hides itself under ROT5, because a digit shift has the same shape on both sides and there is no direction to choose, and the slider value is ignored for the output, though the table below shows the ten digit pairs instead of the twenty-six letters. Pressing the ROT13 or the Custom preset resets the slider back to 13; pressing ROT5 leaves whatever shift you had alone, so switching from ROT5 back to Custom returns to a 13-shift Caesar, not to the last custom value you dialed.
The slider and the shift table
The slider runs from 1 to 25 with those two values labeled at the ends. Zero is deliberately out of reach, since a shift of zero is the identity and the identity is not a cipher; the smallest change you can make is one place. The shift table under the output renders the current mapping live: twenty-six rows of uppercase letter pairs, A to N under ROT13, recomputed for any custom shift, and recomputed again the moment you flip to Decrypt, so the table always shows the mapping the output is actually using right now. Under ROT5 the same panel shows the ten digit pairs, 0 to 5 through 9 to 4, in the same pair format. The table is a reference aid, not an input: nothing you do in it changes the text.
What happens with empty input
An empty text area produces an empty output, under both presets, and the output box is always on the page, empty or full, so there is no placeholder result and no stale value lingering after you clear the input. The copy button sits idle until the output holds something; pressing it with an empty box does nothing, and the Copied label appears for two seconds after a successful copy and then returns to Copy. There is no stored history: reload the page and the tool starts from the empty input, shift 13, the Encrypt direction, and the ROT13 preset, the same state every visit.
Where the text goes, and what it is not
Nowhere off the device. The text area is read by the page, the substitution runs in the browser, and no network request is made at any point; the shifted text lives in the page state only, and the copy button is the only way it leaves the machine, to your clipboard, when you ask. And the honest note: the Caesar cipher is a toy, not a security tool. There are only 25 possible shifts, each one reversible in a single pass, so it can hide a party hint or a puzzle answer from casual eyes and nothing more; a password or a secret never belongs in it. When real confidentiality is the job, the AES Encrypt/Decrypt tool does symmetric encryption properly, the Base64 Encoder/Decoder shows how encoding is a different animal from encryption, the Password Generator makes strong passwords and keys to protect real secrets with, and for the message-integrity side of the family, see the HMAC guide.