Saakshi means witness. It is a web app (PWA) that looks and works like an ordinary calculator. Behind it, a woman can start a silent SOS that records audio, video and location in encrypted 5-second pieces, fingerprints each piece on her phone, and seals the fingerprints on Polygon, so the evidence survives even if the phone is taken and anyone can check it hasn’t been changed.
Built by team Appvengers for the SHAKTI Hackathon (Women Safety / Blockchain track, “Dharma & Trust”).
Thirteen seconds of Saakshi
What works today
The disguise and the SOS
- Calculator disguise. A fully working calculator. The home-screen name and icon say “Calculator”, and so do the page title, browser storage names and service worker cache.
- Her PIN and
= opens Saakshi. Any other number just calculates.
- Silent SOS. Three taps on the calculator display. The screen goes plum, then black after 8 seconds. A wake lock keeps recording alive.
- Encrypted 5-second pieces.
MediaRecorder slices video and audio every 5 seconds, plus location fixes. Each piece is fingerprinted (SHA-256 of the plaintext), encrypted on the phone (AES-GCM), written to an IndexedDB outbox and uploaded at once. Uploads retry when the signal returns.
- Self-healing capture. If the phone takes the camera away mid-SOS (a call, another app), recording restarts within about a second as a new segment, falling back to microphone only.
- Duress PIN. Typed while she is being forced to stop, it looks as if the SOS stopped, but recording carries on and the session is flagged.
- Ten seconds to undo. Undo erases every trace of an accidental SOS, on the phone and on the server.
- Vault. After her PIN she can play recordings back (decrypted on the phone), check their fingerprints, and delete one recording or everything.
Sealed and checkable
What she shows the police. Every recording has a proof screen with a QR code that opens its public result. One tap prints the certificate the Bharatiya Sakshya Adhiniyam asks for with electronic evidence, filled in from the seal records; she fills the blanks and signs it.
- Sealed on Polygon. Each fingerprint is salted and gathered into a Merkle tree, and only the root goes on-chain, through
EvidenceAnchor on Polygon Amoy (about 24,000 gas a root). A relayer pays, so she never needs a wallet. A database job seals every minute, even after her phone is gone.
- Public Verify page. Anyone can drop a downloaded recording on it. It is hashed in the browser, split into its pieces, and every Merkle proof and transaction is checked on-chain from the browser. A recording’s proof screen shows a QR code that opens its result. Without a file of your own, the page checks a sealed sample recording, then the same file with one byte changed.
- Checking without us. After a file checks out, the Verify page saves its proof receipt. The offline checker (
web/public/verify-offline.html, served at /verify-offline.html) is one self-contained page: given the file and its receipt, it re-hashes the pieces, climbs each Merkle proof and reads each transaction from public Polygon servers. It never talks to Saakshi, and the contract address is fixed in the page, so a forged receipt fails.
- Section 63 certificate. One tap prints the certificate the Bharatiya Sakshya Adhiniyam requires for electronic evidence: fingerprint, times, block and the verify QR code.
- Court packet. A complaint from Sahayak prints as one document: the complaint, the Section 63 certificate with a hash report for every sealed item it lists, and where to get free legal help.
Beyond the night itself
- Journal and photo vault. Dated entries and photos of injuries or messages, encrypted and sealed like recordings.
- Guardians on Telegram. Trusted people link through the Saakshi Guardian bot. When she starts an SOS they get her live location after the 10-second undo, at once if the duress PIN is used, and a warning if her phone goes silent.
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