human: Given a node-centered hypergraph: <hypergraph>, where nodes represent papers and hyperedges represent sets of papers co-cited by a source paper.

Details of the hypergraph:
Center node: "when energy trading meets blockchain in electrical power system the state of the art"
  Abstract: With the rapid growth of renewable energy resources, the energy trading began to shift from centralized to distributed manner. Blockchain, as a distributed public ledger technology, has been widely adopted to design new energy trading schemes. However, there are many challenging issues for blockchain-based energy trading, i.e., low efficiency, high transaction cost, security & privacy issues. To tackle with the above challenges, many solutions have been proposed. In this survey, the blockchain-based energy trading in electrical power system is thoroughly investigated. Firstly, the challenges in blockchain-based energy trading are identified. Then, the existing energy trading schemes are studied and classified into three categories based on their main focus: energy transaction, consensus mechanism, and system optimization. And each category is presented in detail. Although existing schemes can meet the specific energy trading requirements, there are still many unsolved problems. Finally, the discussion and future directions are given.

Connected through hyperedge HE-1 (induced by source paper "smart contract development from the perspective of developers topics and issues discussed on social media"):
  with members:
    - "a survey on ethereum systems security vulnerabilities attacks and defenses"
    - "slither a static analysis framework for smart contracts"
    - "what are the actual flaws in important smart contracts and how can we find them"
    - "verisolid correct by design smart contracts for ethereum"
    - "manticore a user friendly symbolic execution framework for binaries and smart contracts"
    - "tool demonstration fsolidm for designing secure ethereum smart contracts"
    - "finding the greedy prodigal and suicidal contracts at scale"
    - "empirical vulnerability analysis of automated smart contracts security testing on blockchains"

Connected through hyperedge HE-2 (induced by source paper "cyberphysical blockchain enabled peer to peer energy trading"):
  with members:
    - "energy crowdsourcing and peer to peer energy trading in blockchain enabled smart grids"

Connected through hyperedge HE-3 (induced by source paper "peer to peer blockchain based energy trading"):
  with members:
    - "energy crowdsourcing and peer to peer energy trading in blockchain enabled smart grids"

Connected through hyperedge HE-4 (induced by source paper "smart contract development in practice trends issues and discussions on stack overflow"):
  with members:
    - "a survey on ethereum systems security vulnerabilities attacks and defenses"
    - "slither a static analysis framework for smart contracts"
    - "transax a blockchain based decentralized forward trading energy exchange for transactive microgrids"
    - "what are the actual flaws in important smart contracts and how can we find them"
    - "verisolid correct by design smart contracts for ethereum"
    - "manticore a user friendly symbolic execution framework for binaries and smart contracts"
    - "tool demonstration fsolidm for designing secure ethereum smart contracts"
    - "finding the greedy prodigal and suicidal contracts at scale"

Connected through hyperedge HE-5 (induced by source paper "blockchain for future smart grid a comprehensive survey"):
  with members:
    - "exploring the attack surface of blockchain a systematic overview"
    - "an efficient blockchain based hierarchical authentication mechanism for energy trading in v2g environment"
    - "energy crowdsourcing and peer to peer energy trading in blockchain enabled smart grids"
    - "blockchain based grid operation services for transactive energy systems"
    - "blockchain technologies for smart energy systems fundamentals challenges and solutions"
    - "key management systems for smart grid advanced metering infrastructure a survey"
    - "a survey on the security of blockchain systems"
    - "a survey on security and privacy issues of bitcoin"

Question: Please tell me which class the center node belongs to. The 40 classes are: cs.NA(Numerical Analysis), cs.MM(Multimedia), cs.LO(Logic in Computer Science), cs.CY(Computers and Society), cs.CR(Cryptography and Security), cs.DC(Distributed, Parallel, and Cluster Computing), cs.HC(Human-Computer Interaction), cs.CE(Computational Engineering, Finance, and Science), cs.NI(Networking and Internet Architecture), cs.CC(Computational Complexity), cs.AI(Artificial Intelligence), cs.MA(Multiagent Systems), cs.GL(General Literature), cs.NE(Neural and Evolutionary Computing), cs.SC(Symbolic Computation), cs.AR(Hardware Architecture), cs.CV(Computer Vision and Pattern Recognition), cs.GR(Graphics), cs.ET(Emerging Technologies), cs.SY(Systems and Control), cs.CG(Computational Geometry), cs.OH(Other Computer Science), cs.PL(Programming Languages), cs.SE(Software Engineering), cs.LG(Machine Learning), cs.SD(Sound), cs.SI(Social and Information Networks), cs.RO(Robotics), cs.IT(Information Theory), cs.PF(Performance), cs.CL(Computation and Language), cs.IR(Information Retrieval), cs.MS(Mathematical Software), cs.FL(Formal Languages and Automata Theory), cs.DS(Data Structures and Algorithms), cs.OS(Operating Systems), cs.GT(Computer Science and Game Theory), cs.DB(Databases), cs.DL(Digital Libraries), cs.DM(Discrete Mathematics). Directly output the class name.

gpt: cs.CR(Cryptography and Security)