The blockchain offered a distributed way to provide security guarantees for financial transactions that avoid the single-point failure drawback of centralized approaches. However, this ability comes with the cost of storing a large (distributed) blockchain state and introducing additional computation and communication overheads to all participants. All these drawbacks raise a challenging scalability problem for resource-constrained devices in the blockchain network. Most scaling solutions typically require resource-constrained devices to rely on peers with a higher computational and storage capability. Such scaling solutions, however, expose the data of the resource-constrained devices to risks of compromise of the more powerful peers they rely on (e.g., accidental, or malicious through a break-in, insider misbehavior, malware infestation). This potential for leakage raises a privacy concern for these constrained clients, in addition to other scaling-related concerns.
In this talk, we propose two cryptographic constructions that enable resource-constrained devices to securely and efficiently participate in the blockchain network. First, we investigate the Bitcoin Simplified Payment Verification (SPV) client, a widely adopted solution to resolve the storage problem for constrained devices. However, the current SPV solutions raise privacy concerns for the SPV clients when they rely on potentially malicious nodes. To address those concerns, we present T^3, a trusted hardware-secured Bitcoin full client that supports efficient oblivious search/update for Bitcoin SPV clients without endangering the clients’ privacy. Second, to address the computational overhead of the gossip protocol used in all popular blockchain protocols, we propose a cryptographic primitive called the Flexible Signature. In a flexible signature scheme, the verification algorithm quantifies the validity of a signature based on the computational effort performed by the verifier. This primitive allows the resource-constrained devices to prevent the adversary from flooding spam transactions to the blockchain network with minimal computational overheads.