Short answer: “Decryption failed” during a backup restore means Mihon could not read your .tachibk file as a valid backup. The three usual causes are an outdated Mihon version trying to read a newer backup, a damaged or incompletely copied backup file, and a backup created by a different app family (a fork) than the one restoring it. Update Mihon to the latest 0.21.x, re-copy the file, and if needed re-export the backup from the original app.
Fix 1 — Update Mihon first
Backup formats evolve with the app. Before anything else, update to the current release (v0.21.2, from the official GitHub — see our Mihon download page with checksums) and try the restore again. This one step resolves most cases where the backup was created on a newer version than the app trying to open it. Note the direction of the deadline: 0.21.x is the last major line that restores Tachiyomi .tachibk files directly, so the restoring app being too old is the common failure, not too new.
Fix 2 — Check the file itself
- Re-copy the backup from its original location; a transfer interrupted halfway leaves a file that looks right but fails to decrypt.
- Do not rename the file extension or open/edit the file in another app — any change to its contents breaks it.
- If you keep dated copies (you should — see backup habits), try the previous copy: if the older one restores, the newest file is the damaged one.
Fix 3 — Restore in the app that made the backup
Backups made by forks (TachiyomiSY, TachiyomiAZ, Aniyomi and others) are usually compatible with Mihon — but not always, especially across big version jumps. If a fork’s backup refuses to restore in Mihon, restore it in that fork first, confirm the library is intact, then export a fresh backup from the fork’s current version and restore that in Mihon.
Fix 4 — Re-export from the old app
If the old app still runs, this is the most reliable path of all: open Tachiyomi (or your fork), create a brand-new backup with every category ticked, copy it over untouched, and restore. Our migration guide walks the full sequence, including adding extension repositories before restoring so entries link correctly.
What not to do
- Do not uninstall the old app or clear its data until a restore has succeeded and you have opened a few chapters in Mihon — your only copy of the library may be inside it.
- Do not download “backup fixer” tools or upload your
.tachibkto websites that promise to repair it. The file contains your library and tracking data; treat it like a password file. - Do not factory-reset or change phones mid-troubleshooting.
Still stuck? If the app itself hangs while processing the restore, see stuck on the updating screen. Related: all troubleshooting guides.