How is this different from a Caesar cipher?

A Caesar cipher shifts only the 26 letters and wraps within the alphabet. This cipher shifts the raw byte value of every character across all 256 possibilities, so a letter can become punctuation, a control byte or part of a multi-byte character.

Why is the output shown as hex?

After shifting, the bytes often no longer form printable or valid UTF-8 text. Hex shows every byte unambiguously, so you can copy and later decrypt it exactly. When the shifted bytes do form valid UTF-8, the text version is shown too.

What does mod 256 mean here?

Each byte holds a value from 0 to 255. Adding the shift can exceed 255, so the value wraps around: 250 plus 10 becomes 4, for example. Decrypting subtracts the same way, so the original byte is always recovered.

Can I decrypt something I pasted as hex?

Yes. Switch the input format to hex and paste the bytes (spaces optional). The tool converts them back to bytes, subtracts the shift and shows the recovered text or hex.

Is an ASCII shift secure?

No. With only 256 possible shifts, an attacker can try them all instantly, and shifting every byte by the same amount leaves frequency patterns intact. Use it for obfuscation, encoding puzzles and learning, not real protection.

ASCII Shift Cipher Encoder & Decoder

The ASCII shift cipher is the byte-level generalisation of the classic Caesar cipher. Instead of rotating only the 26 letters of the alphabet, it adds a constant N to the numeric value of every byte, wrapping around modulo 256. That means it works on letters, digits, punctuation, whitespace and even the individual bytes of multi-byte UTF-8 characters.

How it works

The text is first encoded to UTF-8 bytes. Each byte b is then transformed:

encrypt: b' = (b + N) mod 256
decrypt: b' = (b - N) mod 256

Because the byte range is 0–255, adding N can push a value past 255, in which case it wraps around — 250 + 10 becomes 4. Decryption subtracts the same N and the modular wrap exactly undoes the encryption, recovering every original byte.

Since a shifted byte frequently is not a printable character (or not a valid start of a UTF-8 sequence), the tool shows the result as hex bytes by default, and additionally as text whenever the shifted bytes happen to form valid UTF-8.

Example

Shifting Gera Tools by N = 13 gives the bytes:

54 72 7f 6e 2d 61 7c 7c 79 80

Notice that space (0x20) became 0x2d (a hyphen) and the final s (0x73) wrapped to 0x80, which is not printable on its own. Decrypting these bytes with N = 13 subtracts 13 from each and returns Gera Tools.

Notes

Any whole number works as the shift; it is reduced modulo 256, so a shift of 256 is the same as 0 and 269 is the same as 13.
Paste hex back into the tool in decrypt mode to recover the original text exactly, even when the encrypted bytes were not printable.
This is an obfuscation/encoding toy, not encryption — there are only 256 keys and the byte statistics survive the shift.