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: "covert channels in sip for voip signalling"
  Abstract: In this paper, we evaluate available steganographic techniques for SIP (Session Initiation Protocol) that can be used for creating covert channels during signaling phase of VoIP (Voice over IP) call. Apart from characterizing existing steganographic methods we provide new insights by introducing new techniques. We also estimate amount of data that can be transferred in signalling messages for typical IP telephony call.

Connected through hyperedge HE-1 (induced by source paper "fundamental limits of covert packet insertion"):
  with members:
    - "covert communication over a k user multiple access channel"
    - "fundamental limits of invisible flow fingerprinting"
    - "fundamental limits of covert bit insertion in packets"
    - "privacy preserving mhealth data release with pattern consistency"
    - "covert capacity of non coherent rayleigh fading channels"
    - "towards provably invisible network flow fingerprints"
    - "covert wireless communication with artificial noise generation"
    - "covert communication over noisy channels a resolvability perspective"

Connected through hyperedge HE-2 (induced by source paper "voip steganography and its detection a survey"):
  with members:
    - "influence of speech codecs selection on transcoding steganography"
    - "steganalysis of transcoding steganography"
    - "using transcoding for hidden communication in ip telephony"
    - "lost audio packets steganography the first practical evaluation"
    - "what are suspicious voip delays"
    - "steganography of voip streams"
    - "lack a voip steganographic method"
    - "new security and control protocol for voip based on steganography and digital watermarking"

Connected through hyperedge HE-3 (induced by source paper "using transcoding for hidden communication in ip telephony"):
  with members:
    - "lost audio packets steganography the first practical evaluation"
    - "steganography of voip streams"
    - "lack a voip steganographic method"
    - "new security and control protocol for voip based on steganography and digital watermarking"

Connected through hyperedge HE-4 (induced by source paper "lost audio packets steganography the first practical evaluation"):
  with members:
    - "what are suspicious voip delays"
    - "steganography of voip streams"
    - "lack a voip steganographic method"
    - "new security and control protocol for voip based on steganography and digital watermarking"

Connected through hyperedge HE-5 (induced by source paper "steganography of voip streams"):
  with members:
    - "new security and control protocol for voip based on steganography and digital watermarking"

Connected through hyperedge HE-6 (induced by source paper "analysis of a procedure for inserting steganographic data into voip calls"):
  with members:
    - "steganography of voip streams"

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.MM(Multimedia)