🛡️ Last Mile — Zero-Knowledge Family Continuity Vault

Kalvium Hackathon MVP
Team: Mayank Sharma (Team Lead) & Shyam Sharma
Live Deployment: https://bjp-tau.vercel.app

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💡 What is Last Mile?

Last Mile is a zero-knowledge emergency continuity vault that helps individuals securely organize life-critical financial records, insurance policies, and legal directives.

If the vault owner is active, the vault remains completely sealed. If an unexpected tragedy or prolonged inactivity occurs, trusted family nominees can collectively recover and decrypt the vault instructions using 2-of-3 Shamir Secret Sharing, with zero reliance on centralized custodians or cloud master keys.

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🏛️ Core Security Invariants

                                  +------------------------------------+
                                  |    Client Browser (Zero-Knowledge) |
                                  +------------------------------------+
                                    |              |                 |
                   AES-256-GCM      |              | scrypt KDF      | Shamir (2-of-3)
               (Encrypted Payload)  |              | (N=131072)      | Polynomial Split
                                    v              v                 v
                       +-------------------+  +----------+   +-------------------+
                       | Ciphertext + AAD  |  | Wrapped  |   | 3 Recovery Cards  |
                       | Nonce & Timestamp |  | DEK (KEK)|   | Held Out-of-Band  |
                       +-------------------+  +----------+   +-------------------+
                                    |              |                   |
                         (HTTPS)    |              |                   | (Given to
                                    v              v                   |  Nominees)
                       +---------------------------------+             v
                       |      Supabase PostgreSQL        |     +-------------------+
                       |   Zero Plaintext / Zero DEK     |     | Nominee 1: Card 1 |
                       |   Tamper-Evident SHA-256 Audit  |     | Nominee 2: Card 2 |
                       +---------------------------------+     | Nominee 3: Card 3 |
                                                               +-------------------+

1. True Zero-Knowledge Encryption:
   The server and database never receive, see, or store the master passphrase, the raw 256-bit Data Encryption Key (DEK), or unencrypted asset payload. All encryption and key derivations occur client-side via the Web Crypto API.
2. 2-of-3 Threshold Cryptography (Shamir Secret Sharing):
   The DEK is split into 3 mathematical shares over $\text{GF}(2^8)$. Any single nominee holding 1 card possesses zero bytes of recoverable key material. Any 2 distinct nominees can combine their physical or digital recovery cards in their browser to reconstruct the exact 256-bit DEK.
3. Server-Authoritative Inactivity State Machine:
   Vault states strictly progress through server-enforced lifecycle phases:
   ACTIVE → CHECKIN_DUE → GRACE_PERIOD → RECOVERY_COOLING_OFF → READY_FOR_RECOVERY.
4. Owner Cooling-Off Abort Safety Window:
   If an emergency recovery request is initiated, the owner is alerted and has a mandatory cooling-off window to cancel the attempt with a single click, instantly restoring active vault security.
5. Tamper-Evident Cryptographic Audit Trail:
   Every state change, check-in, nominee decision, and payload update is hashed canonically using RFC 8785 JSON canonicalization and appended to a SHA-256 hash chain, guaranteeing verifiable auditability.

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🚀 Live Demo Walkthrough

Try the live production application at bjp-tau.vercel.app:

Step	Page	Description
1. Create Vault	/vault/new	Enter assets, choose 3 nominees, and generate 3 cryptographic Recovery Cards client-side.
2. Owner Check-In	/dashboard/checkin	Verify your presence with a cryptographic check-in heartbeat to extend the vault deadline by 30 days.
3. Audit Trail	/dashboard/activity	View the tamper-evident SHA-256 Merkle chain with live verification.
4. Nominee Recovery	/recovery	Paste any 2 of the 3 Recovery Cards to reconstruct the DEK and decrypt the vault locally in memory.

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