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: "an analysis of bitcoin op_return metadata"
  Abstract: The Bitcoin protocol allows to save arbitrary data on the blockchain through a special instruction of the scripting language, called OP_RETURN. A growing number of protocols exploit this feature to extend the range of applications of the Bitcoin blockchain beyond transfer of currency. A point of debate in the Bitcoin community is whether loading data through OP_RETURN can negatively affect the performance of the Bitcoin network with respect to its primary goal. This paper is an empirical study of the usage of OP_RETURN over the years. We identify several protocols based on OP_RETURN, which we classify by their application domain. We measure the evolution in time of the usage of each protocol, the distribution of OP_RETURN transactions by application domain, and their space consumption.

Connected through hyperedge HE-1 (induced by source paper "sharvot secret share based voting on the blockchain"):
  with members:
    - "distributed protocols at the rescue for trustworthy online voting"
    - "research on anonymization and de anonymization in the bitcoin system"
    - "deanonymisation of clients in bitcoin p2p network"

Connected through hyperedge HE-2 (induced by source paper "the evolution of embedding metadata in blockchain transactions"):
  with members:
    - "sustainable blockchain enabled services smart contracts"
    - "automated dynamic analysis of ransomware benefits limitations and use for detection"

Connected through hyperedge HE-3 (induced by source paper "a state space modeling framework for engineering blockchain enabled economic systems"):
  with members:
    - "cryptocurrencies without proof of work"

Connected through hyperedge HE-4 (induced by source paper "contour a practical system for binary transparency"):
  with members:
    - "certificate transparency with privacy"
    - "certificate transparency with privacy"
    - "enhancing bitcoin security and performance with strong consistency via collective signing"
    - "keeping authorities honest or bust with decentralized witness cosigning"
    - "efficient gossip protocols for verifying the consistency of certificate logs"
    - "majority is not enough bitcoin mining is vulnerable"

Connected through hyperedge HE-5 (induced by source paper "an empirical analysis of smart contracts platforms applications and design patterns"):
  with members:
    - "dissecting ponzi schemes on ethereum identification analysis and impact"
    - "new kids on the block an analysis of modern blockchains"
    - "smart contract templates foundations design landscape and research directions"
    - "note on fair coin toss via 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)