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Android app that stores your passwords on paper, encrypted.
You encrypt a password with your master password, the app turns it into a card with a QR code, and you print an A4 sheet of cut-out cards. To get a password back, you scan its QR and type the master password.
The point is that losing the paper costs you nothing: a card is worthless without the master password. Keep them in a drawer, split them up, or leave a copy at a friend's place.
The app stores nothing — no passwords, no master password, no history. The paper is the only storage.
- F-Droid — search for PaperCript in the F-Droid app (or Droid-ify, Neo Store…); it keeps the app updated.
- APK from the latest release. F-Droid builds it reproducibly and ships this same signed APK, so you can switch between both sources and keep updating.
Encrypt → label, password (typed or generated) and master password → Encrypt → Add to sheet and next. The master password is kept between entries so you can chain several in one go.
Sheet → pick columns and rows, see how the page will look, then Print or save as PDF. The Android print dialog handles both a real printer and a PDF. Then cut them out.
Decrypt → Scan the card QR → master password. The label travels inside the QR, so the master password is all you type.
⚠️ The pending card list only lives in memory while the app is open. Print before closing.
⚠️ The master password has no recovery. Forget it and every printed card turns into recycling, all at once.
You choose the layout (1–6 columns × 1–8 rows) and the app tells you the real size in millimetres of both the card and the QR. Fewer cards per sheet means a bigger QR and an easier scan. Below roughly 18 mm of QR it warns you, because that is where each module drops under 0.45 mm and phone cameras start to struggle.
Each card carries the label, the QR, and the code in small print. That printed text is the rescue path if a QR is ever damaged: you type it back in the app with a 32-key keypad that only offers valid characters.
[ version 1B ][ salt 4B ][ ciphertext NB ][ tag 5B ]
All of it in Crockford-style Base32, an alphabet without I, L, O or U so they cannot be confused with 1, 1, 0 and V when typing.
- Key derivation:
scrypt(master_password, domain + version + salt)→ 64 bytes, split into an encryption key and a MAC key. v3 uses N=65536 (64 MB), roughly a second per operation on a current phone. - Encryption: AES-256-CTR with a fixed IV. This is safe because the salt is random per encryption, so the key never repeats and no keystream is ever reused.
- Integrity: HMAC-SHA256 truncated to 40 bits, encrypt-then-MAC. It catches a mistyped character and a wrong master password at the same time — which is why a failure cannot tell you which of the two it was.
- The label is not part of the derivation. It is your own note: change it or misspell it without breaking the card.
- The version byte selects the parameters, so raising the cost later does not invalidate what is already printed.
The QR holds LBR1|code|label with error correction level Q (survives up to 25% damage),
because these cards get folded and handled.
⚠️ The domain string and parameters are frozen from the first card you print. The key is never stored: it is recomputed in full on every decryption. Changing any ingredient makes already-issued cards unreadable forever.
Already post-quantum, with no effort: everything here is symmetric (scrypt, AES-256, HMAC-SHA256). Shor's algorithm breaks RSA and elliptic curves, neither of which appears anywhere in this app; Grover only cuts AES-256 down to about 128 effective bits, far out of reach. The real weak link is the entropy of your master password — a 6 or 7 word random passphrase (77–90 bits) is what actually moves the needle.
What the app does:
- No network permission. The APK declares only
CAMERA, andINTERNETis explicitly stripped withtools:node="remove". Without it the kernel will not let the app open a socket — that is not a promise, it is an inability. - Fully open source. QR codes are read with ZXing; there are no Google Play Services or other proprietary libraries.
FLAG_SECURE: no screenshots, no recents thumbnail, no screen recording.- The clipboard is flagged as sensitive and cleared 45 s after copying or when you return to the app, whichever happens while it is in the foreground. There is also a button to clear it immediately.
- No backups (
allowBackup=false) and nothing written to disk. - Derived keys are zeroed in memory as soon as they are used.
- Privacy policy inside the app: a button at the bottom of the Encrypt tab shows it as text, in your language, so reading it needs no network. It is also in PRIVACY.
What it does not cover, and you should know:
- A malicious accessibility service can read what is on screen.
FLAG_SECUREdoes not stop it. This is the realest hole. - Your keyboard sees everything you type, including the master password. Inherent to Android.
- No app can clear the clipboard from the background. Since Android 10 the system denies
access to anything without focus (
Denying clipboard access ... application is not in focus), so a timer that fires while you are pasting elsewhere clears nothing. Hence the clear-on-return and the manual button. If you never come back to the app, the password stays there until the system cleans it up. - The code length reveals the password length.
None of this has been externally audited. It is standard cryptography assembled with care, but run the full cycle with a throwaway password before trusting it with a real one.
Eighteen languages, all 70 translatable strings complete in every one: English (default), Spanish, Catalan, Galician, Basque, French, German, Italian, Portuguese, Dutch, Polish, Russian, Ukrainian, Simplified Chinese, Japanese, Korean, Thai and Hindi.
The app follows the phone language and falls back to English for anything else. On
Android 13+ it declares localeConfig, so you can pin a language for PaperCript alone in
Settings → Apps → PaperCript → Language, without touching the system.
The translations have not been reviewed by native speakers. If something reads wrong, it is a text file — fixing it touches no code.
To add a language, copy res/values/strings.xml to res/values-xx/, translate it, and
add <locale android:name="xx" /> to res/xml/locales_config.xml. No Kotlin involved.
Then check that the new language lines up:
pwsh tools/check-locales.ps1
It verifies that no key is missing or extra and that the format specifiers (%1$d,
%2$s…) match the English ones. That last part matters more than it looks: a %1$s
translated as %1$d compiles happily and blows up while formatting, in front of the user.
For Chinese and Japanese, each string must sit on a single line. Android joins wrapped lines with a space, which in languages without word separators lands in the middle of a sentence.
./gradlew testDebugUnitTest assembleDebug
The APK lands in app/build/outputs/apk/debug/. For real use, build a signed release with
your own keystore.
Requires JDK 21 and Android SDK 36. No runtime network dependencies.
See CONTRIBUTING. To report a security problem privately, see SECURITY.
MIT © javimentallab. Third-party licences are in NOTICE.