New file format corrupts itself irreversibly with each open based on filename parameter

Decayfmt is a Rust crate that encodes files into a format where each open operation permanently corrupts the file. The corruption rate is determined by a numeric parameter embedded in the filename, such as `.idcy3` for images or `.tdcy7` for text. This parameter defines how aggressively the file degrades with each access, ranging from gradual decay to near-instant destruction. The design enforces a one-way transformation: once opened, the file is irrevocably altered, and no recovery is possible without an external backup or manual intervention.

The corruption mechanism is probabilistic, with the likelihood of byte corruption derived from the instability parameter. For example, a parameter of 1 corrupts roughly 9.5% of eligible bytes per open, while a parameter of 10 corrupts about 63%. The randomness is sourced from cryptographically secure entropy, ensuring that each corruption event is unique and unpredictable. This prevents replay attacks or deterministic recovery attempts, though the format itself is not cryptographically secure and can be defeated with a hex editor or backup.

The format is explicitly not intended for secure deletion, DRM, or encryption. Instead, it serves as a social contract: the file’s decay is transparent and irreversible, but only if no backups exist. For engineers, this presents a novel way to enforce ephemerality at the file system level, though its practical applications are limited. The implementation is lightweight, with no runtime dependencies, and can be installed via Cargo or pre-built binaries. However, its utility is constrained to scenarios where data loss is an intentional feature, not a bug.

Text and image files decay differently in presentation but identically in mechanism. Text corruption swaps in printable characters, producing readable but nonsensical output, while images degrade into visual noise. The decay rate can be observed in real time by repeatedly opening the same file, with higher instability parameters causing rapid degradation. This makes the format a useful tool for visualizing entropy accumulation, though its primary value lies in its conceptual demonstration of irreversible data transformation rather than practical storage.