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: "short paper towards more reliable bitcoin timestamps"
  Abstract: Bitcoin provides freshness properties by forming a blockchain where each block is associated with its timestamp and the previous block. Due to these properties, the Bitcoin protocol is being used as a decentralized, trusted, and secure timestamping service. Although Bitcoin participants which create new blocks cannot modify their order, they can manipulate timestamps almost undetected. This undermines the Bitcoin protocol as a reliable timestamping service. In particular, a newcomer that synchronizes the entire blockchain has a little guarantee about timestamps of all blocks. #R##N#In this paper, we present a simple yet powerful mechanism that increases the reliability of Bitcoin timestamps. Our protocol can provide evidence that a block was created within a certain time range. The protocol is efficient, backward compatible, and surprisingly, currently deployed SSL/TLS servers can act as reference time sources. The protocol has many applications and can be used for detecting various attacks against the Bitcoin protocol.

Connected through hyperedge HE-1 (induced by source paper "the security reference architecture for blockchains towards a standardized model for studying vulnerabilities threats and defenses"):
  with members:
    - "ekiden a platform for confidentiality preserving trustworthy and performant smart contracts"
    - "a survey on ethereum systems security vulnerabilities attacks and defenses"
    - "security and privacy on blockchain"
    - "deconstructing blockchains a comprehensive survey on consensus membership and structure"
    - "truffle tests for free replaying ethereum smart contracts for transparency"
    - "blockchain technologies for the internet of things research issues and challenges"
    - "exploring the attack surface of blockchain a systematic overview"
    - "strongchain transparent and collaborative proof of work consensus"

Connected through hyperedge HE-2 (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"
    - "the latest gossip on bft consensus"
    - "finding the greedy prodigal and suicidal contracts at scale"
    - "sabre protecting bitcoin against routing attacks"
    - "scilla a smart contract intermediate level language"

Connected through hyperedge HE-3 (induced by source paper "strongchain transparent and collaborative proof of work consensus"):
  with members:
    - "decentralization in bitcoin and ethereum networks"
    - "the impact of uncle rewards on selfish mining in ethereum"
    - "the gap game"
    - "enhancing bitcoin security and performance with strong consistency via collective signing"
    - "optimal selfish mining strategies in bitcoin"
    - "bitcoin ng a scalable blockchain protocol"
    - "on subversive miner strategies and block withholding attack in bitcoin digital currency"
    - "the miner s dilemma"

Connected through hyperedge HE-4 (induced by source paper "permissionless blockchains and secure logging"):
  with members:
    - "blockchain based tls notary service"
    - "pdfs practical data feed service for smart contracts"
    - "scaling nakamoto consensus to thousands of transactions per second"
    - "contour a practical system for binary transparency"
    - "efficient gossip protocols for verifying the consistency of certificate logs"

Connected through hyperedge HE-5 (induced by source paper "blockchain based tls notary service"):
  with members:
    - "pki safety net pkisn addressing the too big to be revoked problem of the tls ecosystem"
    - "keeping authorities honest or bust with decentralized witness cosigning"

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)