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.

What is the ASCII Shift Cipher Encoder & Decoder?

Free ASCII byte-shift cipher tool — adds a constant N modulo 256 to every byte of your text, covering all 256 values not just the 26 letters. Encrypt and decrypt with hex output in your browser, no upload. It runs free in your browser on Gera Tools, with nothing uploaded.

ASCII Shift Cipher Encoder & Decoder

Name: ASCII Shift Cipher Encoder & Decoder
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

How this differs from a Caesar cipher

A traditional Caesar cipher:

Operates only on the 26 letters A–Z (and sometimes a–z).
Leaves digits, punctuation, and spaces unchanged.
Wraps at 26 (the alphabet size).

The ASCII byte shift:

Operates on every byte in the UTF-8 encoding of the text.
Letters, digits, punctuation, whitespace, and control characters are all shifted.
Wraps at 256 (the byte range).

The consequence is that an ASCII shift with N = 13 on a purely alphabetic string is not the same as ROT13 — ROT13 keeps non-letter characters in place, while a byte shift also moves them. On strings containing only uppercase letters, though, a byte shift of 13 and a Caesar shift of 13 both shift those letters by the same amount.

Practical uses

Encoding puzzles and educational exercises. The cipher is easy to explain, illustrate, and manually verify, making it useful in cryptography education.
Obfuscating human-readable text in config files. A byte shift makes a value not immediately readable at a glance without offering any real security. Use it only for low-stakes obfuscation where the goal is not to prevent a determined reader.
Exploring byte-level encoding. Seeing which bytes produce printable output after shifting gives hands-on intuition for how character encodings work.

Security note

There are only 256 possible shifts (N = 0 through 255), so brute-force decryption takes at most 256 attempts — effectively instant for any software. The cipher also preserves the statistical byte distribution of the original text, which makes frequency analysis straightforward. Do not use this for any real data protection. For that, use a modern authenticated encryption scheme.

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 and learning tool, not encryption. All processing runs locally in your browser.