decayfmt: an open-source file format that corrupts itself every time you open it

alex_verem · x · 2026-09-01

Developer Arav Panwar released decayfmt, an open-source Rust tool that makes files decay on access: encode an image or text file, and every open damages the file on disk before showing it — no undo, no recovery from the file alone.

Decay speed lives in the filename: photo.idcy1 loses 9.5% of pixel data per open; photo.idcy10 scrambles 63% on the first open. Renaming changes how hard the next open hits.
Corruption comes from OS randomness, so there's no replayable seed to rebuild the original.
Image transparency is never touched (decay appears as color noise, not holes); text only swaps printable characters, so sentences rot into something that still looks like language.
The author calls it "a social contract enforced by math" — it isn't encryption or secure deletion, and anyone with a backup or hex editor can beat it.

In an era where digital files copy for free and AI can regenerate almost anything, a tool that makes reading cost part of the file is a deliberately backwards design.

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decayfmt: an open-source file format that corrupts itself every time you open it

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