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Skype protocol

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The Skype protocol is a proprietary network used for Internet telephony. Its specifications are not publicly available, and all official applications based on the protocol are closed-source. It lacks interoperability with most Voice over IP (VoIP) networks, so it requires licensing from Skype for any integration.

Many attempts to reverse-engineer the protocol have been made to study its security features or to enable unofficial clients. On June 20, 2014, Microsoft announced that the old Skype protocol would be deprecated. Users had to upgrade to the 2014 version of Skype to continue accessing services, and older clients could no longer log in.[1][2] As of the second week of August 2014, the new protocol, Microsoft Notification Protocol 24, was implemented to improve offline messaging and message synchronization across devices.[3]

Peer-to-peer architecture

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Skype pioneered peer-to-peer (P2P) technology for IP telephony.[4] Its architecture includes supernodes, ordinary nodes, and a login server. Each client maintains a cache of reachable supernodes, while user directory data is distributed across these supernodes, organized into slots and blocks.[citation needed]

Initially, any client with sufficient bandwidth and processing power could become a supernode. This setup posed challenges for users behind firewalls or Network Address Translation (NAT) because their connections could be used to facilitate calls between other clients. In 2012, Microsoft transitioned control of supernodes to its data centers to enhance performance and scalability,[5][6] raising privacy concerns[7] that were later highlighted by the PRISM surveillance revelations in 2013.[8][9]

Skype does not support IPv6,[10] which could simplify its communication infrastructure.

Communication challenges

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Supernodes relay communications for clients that are behind firewalls or NAT, enabling calls that would otherwise be impossible. However, issues may arise, such as:

  • Non-derivable external port numbers or IP addresses due to NAT
  • Firewalls blocking incoming sessions
  • UDP issues like timeouts
  • Port restrictions

Protocol details

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Signaling in Skype is encrypted using RC4, but this method is considered weak because the encryption key can be recovered from the traffic. Voice data is protected with AES encryption.[11] The Skype API allows developers to access the network for user information and call management.

The code remains closed-source,[12] and parts of the client utilize an open-source socket communication library called Internet Direct (Indy).[citation needed]

In July 2012, a researcher revealed insights gained from reverse-engineering the Skype client.[13]

Protocol detection

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Various networking and security firms claim to have methods for detecting Skype's protocol. While their specific methods are proprietary, some published techniques include Pearson's chi-squared test and stochastic characterization using Naive Bayes classifiers.[14]

Obfuscation layer

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Skype employs RC4 to obfuscate the payload of data packets. The initialization vector (IV) is derived from a combination of the public source and destination IPs and a packet ID, transformed into an RC4 key.

Notably, the misuse of RC4 can occur on TCP streams, where the first 14 bytes of a stream are XOR-ed with the RC4 stream, impacting data security.[15]

Packet structure and compression

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Most Skype traffic is encrypted, with commands and their parameters organized in an object list that can be compressed using a variant of arithmetic compression.

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The terms of Skype's license agreement prohibit reverse engineering. However, EU law allows for reverse engineering for interoperability purposes,[16] and the U.S. Digital Millennium Copyright Act provides similar protections.[17][18][19][20][21] Certain countries also permit copying for reverse engineering.[22]

Notes

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  1. ^ "Making Way for the Next Generation of Skype on Desktop". 20 June 2014.
  2. ^ Keizer, Gregg (21 June 2014). "Update: Skype to retire recent editions for Windows, Mac; upgrades mandatory".
  3. ^ Chris Merrimen (15 August 2014). "Skype stops working on older Android phones leaving Linux users in the dark". CNET. Archived from the original on August 16, 2014. Retrieved 2 October 2014.{{cite web}}: CS1 maint: unfit URL (link)
  4. ^ Salman A. Baset; Henning Schulzrinne (2004). "An analysis of the Skype peer-to-peer Internet telephony protocol". p. 11. arXiv:cs/0412017v1.
  5. ^ Branscombe, Mary (27 July 2012). "Forget the conspiracy theories: Skype's supernodes belong in the cloud". 500 words into the future (ZDNet). Retrieved 17 June 2013.
  6. ^ "Skype replaces P2P supernodes with Linux boxes hosted by Microsoft (updated)". Ars Technica. 2 May 2012. Retrieved 17 June 2013.
  7. ^ Kosner, Anthony (18 July 2012). "Will Microsoft's Changes To The Architecture Of Skype Make It Easier To Snoop?". Forbes blog. Retrieved 17 June 2013.
  8. ^ Gallagher, Ryan (6 June 2013). "Newly Revealed PRISM Snooping Makes Verizon Surveillance Look Like Kids' Stuff". Slate blog. Retrieved 17 June 2013.
  9. ^ Greenwald, Glenn (7 June 2013). "NSA Prism program taps in to user data of Apple, Google and others". The Guardian. Retrieved 17 June 2013.
  10. ^ "Skype does not support IPv6". Skype official Twitter account. 1 Nov 2012. Retrieved 4 Nov 2012.
  11. ^ Introduction Skype analysis Enforcing anti-Skype policies, Skype uncovered Security study of Skype, Desclaux Fabrice, 7/11/2005, EADS CCR/STI/C
  12. ^ "Which protocols does Skype use?". Help. Skype. Archived from the original on March 3, 2009.
  13. ^ "Posts under Skype Reverse Category". oKLabs.
    "Skype Reverse Engineering: The (long) journey ;)." oKLabs.
  14. ^ Dario Bonfiglio; et al. "Revealing Skype Traffic: When Randomness Plays with You". ACM SIGCOMM Computer Communication Review. 37 (SIGCOMM 2007): 37–48. Archived from the original on 2011-04-30.
  15. ^ Fabrice Desclaux; Kostya Kortchinsky (2006-06-17). "Vanilla Skype part 2" (PDF). RECON2006.
  16. ^ Directive 2009/24/EC of the European Parliament and of the Council of 23 April 2009 on the legal protection of computer programs
  17. ^ 17 U.S.C. Sec. 1201(f).
  18. ^ WIPO Copyright and Performances and Phonograms Treaties Implementation Act
  19. ^ Sega vs Accolade, 1992
  20. ^ Sony vs Connectix, 2000
  21. ^ Pamela Samuelson & Suzanne Scotchmer (May 2002). "The Law and Economics of Reverse Engineering" (PDF). Yale Law Journal. 111 (7): 1575–1663. doi:10.2307/797533. JSTOR 797533. Archived from the original (PDF) on 2011-07-16. Retrieved 2015-03-17.
  22. ^ In the French "intellectual property" law set, there is an exception that allows any software user to reverse engineer it. See code de la propriété intellectuelle (in French). This law is the national implementation of a piece of EU legislation: Council Directive 91/250/EEC, since then repealed by Directive 2009/24/EC of the European Parliament and of the Council of 23 April 2009 on the legal protection of computer programs which also has a very similar provision allowing reverse engineering/decompilation for the purposes of development and testing of independent but inter-operating programs).

References

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