A Spring Boot backend that demonstrates offline UPI payments routed through a Bluetooth-style mesh network
• Built a Spring Boot backend for a mesh-routed offline payment demo, exposing REST APIs for packet ingestion, settlement, account tracking, and mesh simulation.
• Designed the backend flow to process encrypted payment packets through a clean pipeline: hash verification, deduplication, decryption, freshness validation, and transactional settlement.
• Implemented idempotent, exactly-once settlement using ConcurrentHashMap.putIfAbsent as an atomic claim gate, keyed on the SHA-256 hash of the ciphertext (not the packet ID, which relay nodes can forge), verified under concurrent load with a 3-thread integration test. Secured packets end-to-end with hybrid RSA-OAEP + AES-256-GCM encryption, making payloads opaque to relay devices and rejecting any tampering before settlement.
• Engineered Spring Data JPA and transactional service logic to manage account balances and ledger entries reliably, with optimistic locking as an added safety layer.
• Added a 5-device virtual mesh simulator and interactive dashboard to demonstrate TTL-decremented gossip rounds, bridge uploads, replay rejection, and end-to-end backend behavior.
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