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: "a first look at the usability of bitcoin key management"
  Abstract: Bitcoin users are directly or indirectly forced to deal with public key cryptography, which has a number of security and usability challenges that differ from the password-based authentication underlying most online banking services. Users must ensure that keys are simultaneously accessible, resistant to digital theft and resilient to loss. In this paper, we contribute an evaluation framework for comparing Bitcoin key management approaches, and conduct a broad usability evaluation of six representative Bitcoin clients. We find that Bitcoin shares many of the fundamental challenges of key management known from other domains, but that Bitcoin may present a unique opportunity to rethink key management for end users.

Connected through hyperedge HE-1 (induced by source paper "a security reference architecture for blockchains"):
  with members:
    - "ekiden a platform for confidentiality preserving trustworthy and performant smart contracts"
    - "strongchain transparent and collaborative proof of work consensus"
    - "insight into insiders and it a survey of insider threat taxonomies analysis modeling and countermeasures"
    - "dfinity technology overview series consensus system"
    - "short paper towards more reliable bitcoin timestamps"
    - "the latest gossip on bft consensus"
    - "finding the greedy prodigal and suicidal contracts at scale"
    - "sabre protecting bitcoin against routing attacks"

Connected through hyperedge HE-2 (induced by source paper "sok blockchain technology and its potential use cases"):
  with members:
    - "decentralization in bitcoin and ethereum networks"
    - "hunting the ethereum smart contract color inspired inspection of potential attacks"
    - "a survey on security and privacy issues of bitcoin"
    - "a new look at the refund mechanism in the bitcoin payment protocol"
    - "consensus in the age of blockchains"
    - "when the cookie meets the blockchain privacy risks of web payments via cryptocurrencies"
    - "solida a blockchain protocol based on reconfigurable byzantine consensus"
    - "bitcoin ng a scalable blockchain protocol"

Connected through hyperedge HE-3 (induced by source paper "smartotps an air gapped 2 factor authentication for smart contract wallets"):
  with members:
    - "ekiden a platform for confidentiality preserving trustworthy and performant smart contracts"
    - "an air gapped 2 factor authentication for smart contract wallets"
    - "rethinking blockchain security position paper"

Connected through hyperedge HE-4 (induced by source paper "cerberus a blockchain based accreditation and degree verification system"):
  with members:
    - "blockbench a framework for analyzing private blockchains"
    - "untangling blockchain a data processing view of blockchain systems"
    - "towards an ontology driven blockchain design for supply chain provenance"

Connected through hyperedge HE-5 (induced by source paper "a survey on security and privacy issues of bitcoin"):
  with members:
    - "concurrency and privacy with payment channel networks"
    - "dandelion redesigning the bitcoin network for anonymity"
    - "consensus in the age of blockchains"
    - "when the cookie meets the blockchain privacy risks of web payments via cryptocurrencies"
    - "be selfish and avoid dilemmas fork after withholding faw attacks on bitcoin"
    - "socially optimal mining pools"
    - "the balance attack against proof of work blockchains the r3 testbed as an example"
    - "hijacking bitcoin routing attacks on cryptocurrencies"

Connected through hyperedge HE-6 (induced by source paper "an air gapped 2 factor authentication for smart contract wallets"):
  with members:
    - "ekiden a platform for confidentiality preserving trustworthy and performant smart contracts"
    - "rethinking blockchain security position paper"

Connected through hyperedge HE-7 (induced by source paper "untangling blockchain a data processing view of blockchain systems"):
  with members:
    - "ustore a distributed storage with rich semantics"
    - "enhancing bitcoin security and performance with strong consistency via collective signing"
    - "bitcoin ng a scalable blockchain protocol"
    - "keeping authorities honest or bust with decentralized witness cosigning"
    - "majority is not enough bitcoin mining is vulnerable"

Connected through hyperedge HE-8 (induced by source paper "of two minds multiple addresses and one history characterizing opinions knowledge and perceptions of bitcoin across groups"):
  with members:
    - "operational distributed regulation for bitcoin"
    - "nowcasting the bitcoin market with twitter signals"
    - "the miner s dilemma"
    - "practical aspects of the bitcoin system"
    - "majority is not enough bitcoin mining is vulnerable"
    - "on bitcoin and red balloons"

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)