Time-Bucketed Balance Records: Bounded-Storage Ephemeral Tokens for Resource-Constrained Systems

Fungible tokens with time-to-live (TTL) semantics require tracking individual expiration times for each deposited unit. A naive implementation creates a new balance record per deposit, leading to unbounded storage growth and vulnerability to denial-of-service attacks. We present time-bucketed balance records, a data structure that bounds storage to O(k) records per account while guaranteeing that tokens never expire before their configured TTL. Our approach discretizes time into k buckets, coalescing deposits within the same bucket to limit unique expiration timestamps. We prove three key properties: (1) storage is bounded by k+1 records regardless of deposit frequency, (2) actual expiration time is always at least the configured TTL, and (3) adversaries cannot increase a victim's operation cost beyond O(k)[amortized] worst case. We provide a reference implementation in Solidity with measured gas costs demonstrating practical efficiency.