SYSTEM/docs/prd/requirements.md

2. Requirements

Functional

1. FR1: System Initialization: The system must provide an interface for the first user ("Initiator") to create the "Община" and generate their initial cryptographic key pair.
2. FR2: Citizen Onboarding: The Initiator must be able to generate a unique, single-use invitation link to onboard new "Citizens". The onboarding process must include key pair generation and profile creation in the "Registry".
3. FR3: Optional Key Vault: During onboarding, a new "Citizen" must have the option to create a login/password account to store their private key in the server-side "Vault".
4. FR4: Immutable Ledger: Every state-changing action within the system (e.g., creating a Cell, joining, confirming payment) must be recorded as a new, cryptographically signed transaction in "The Main Ledger". The system must not allow direct modification or deletion of past transactions.
5. FR5: Cell Creation: Any "Citizen" must be able to create a financial "Cell" for a "Common Good", specifying the goal and the required items/costs ("Executor" roles).
6. FR6: Cell Participation: "Citizens" must be able to join a "Cell" as either an "Executor" (committing to purchase an item) or a "Consumer" (sharing the cost of the "Common Good").
7. FR7: Real-Time Calculation: The system must automatically and transparently calculate and display the financial obligations between all "Consumers" and "Executors" of a "Cell" as participants join or leave.
8. FR8: Payment Confirmation: An "Executor" must have the ability to sign a transaction confirming the receipt of payment from a specific "Consumer" for a specific "Cell".
9. FR9: Social Key Recovery: A "Citizen" who uses the "Vault" and has forgotten their password must be able to initiate a recovery process that requires cryptographic confirmation from a quorum of other "Citizens" (1 confirmation if <=4 members; 2 confirmations if >=5 members).

Non-Functional

1. NFR1: Performance on Embedded Hardware: The entire system (backend and database) must be optimized to run efficiently on a Raspberry Pi (Model 4 or newer recommended).
2. NFR2: High-Trust Environment Security: While designed for a trusted, private network, the application must include baseline web security practices (e.g., CSRF protection, secure headers). Direct database access should not be exposed.
3. NFR3: Cryptographic Integrity: All transactions must be signed using the Ed25519 algorithm or a similarly secure and lightweight standard. The integrity of the ledger's chain (verified by linking transaction hashes) must be maintained.
4. NFR4: Multi-Currency Support: The system must be able to handle calculations and record transactions in multiple fiat currencies (e.g., UAH, PLN, USD), with the currency specified at the "Cell" or "Plan" level.
5. NFR5: Internationalization (i18n) Readiness: The frontend architecture must be built to support multiple languages, even if the MVP only includes one language pack.
6. NFR6: Usability: The user interface for common, simple actions (like creating a quick "Cell") must be low-friction and require a minimal number of steps to encourage daily use.

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