The short version
Scissor Zip never receives your files. When you open a tool, your browser downloads the website, including the archive engine. When you choose files, the browser reads them from your device and hands them to that engine, which runs in a background thread of the same tab. The finished archive, or the files you extract, are saved straight back to your device.
The engine
Archives are read and written by libarchive, the library behind the tar command on macOS, FreeBSD and Windows, compiled to WebAssembly so it runs in the browser at nearly native speed. zlib, bzip2, xz, Zstandard and LZ4 handle compression, and Mbed TLS handles the AES-256 encryption of password-protected zips.
Check it yourself
Open your browser's developer tools, go to the Network tab and use any tool. You will see the page load, and then nothing more while your archive is made or opened: no upload, no request carrying your files. You can also turn off your connection after the page loads; every tool keeps working.
Memory, not storage
Files are held in the tab's memory while you work and are gone when you close it. Nothing is written to a server, a database or your browser's storage. Because everything happens in memory, very large archives (over about 2 GB) may be more than some devices can hold.
What we do receive
Anonymous page counts and events (which tool was used, how long it took, whether it worked), short error reports without file names or contents, and whatever you choose to send through the feedback, bug report and contact forms. They go to our own customer care service on this domain, without cookies. Contact us with any question.
Open-source components
Scissor Zip is built with these open-source libraries, used under their licenses: libarchive (BSD 2-Clause, © Tim Kientzle and contributors), zlib (zlib license, © Jean-loup Gailly and Mark Adler), bzip2 (BSD-style license, © Julian Seward), XZ Utils / liblzma (BSD Zero Clause, © Lasse Collin and contributors), Zstandard (BSD license, © Meta Platforms, Inc. and affiliates), LZ4 (BSD 2-Clause, © Yann Collet) and Mbed TLS (Apache License 2.0, © The Mbed TLS Contributors). Their licenses permit use in closed-source software; their copyright notices are reproduced here as they require. Thank you to their authors.