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{{short description|Proprietary Internet telephony network}}
The '''Skype protocol''' is a [[Proprietary protocol|proprietary]] Internet telephony network used by [[Software:Skype|Skype]]. The protocol's specifications have not been made publicly available by Skype and official applications using the protocol are closed-source.

The Skype network is not interoperable with most other [[Engineering:Voice over IP|Voice over IP]] (VoIP) networks without proper licensing from Skype. Numerous attempts to study or [[Reverse engineering|reverse-engineer]] the protocol have been undertaken to reveal the protocol, investigate security or allow unofficial clients.

On June 20, 2014, Microsoft announced the deprecation of the old Skype protocol. Within several months from this date, in order to continue using Skype services, Skype users had to update to Skype applications released in 2014, and users were not able to [[Login|log in]] to older Skype versions (clients).<ref>{{cite web|url=https://blogs.skype.com/news/2014/06/20/making-way-for-the-next-generation-of-skype-on-desktop/|title=Making Way for the Next Generation of Skype on Desktop|date=20 June 2014}}</ref><ref>{{cite web|url=http://www.computerworld.com/s/article/9249275/Update_Skype_to_retire_recent_editions_for_Windows_Mac_upgrades_mandatory|title=Update: Skype to retire recent editions for Windows, Mac; upgrades mandatory|first=Gregg|last=Keizer|date=21 June 2014 }}</ref> No announcement has been made on whether SmartTV and hardware phones with built-in Skype functionality will continue to work without interruptions. The new Skype protocol—[[Microsoft Notification Protocol#MSNP24|Microsoft Notification Protocol 24]]—promised better offline messaging and better messages synchronization across Skype devices. The deprecation became effective in the second week of August 2014.<ref>{{cite web|title=Skype stops working on older Android phones leaving Linux users in the dark|url=http://www.theinquirer.net/inquirer/news/2360604/skype-stops-working-on-older-android-phones-leaving-linux-users-in-the-dark|archive-url=https://web.archive.org/web/20140816183820/http://www.theinquirer.net/inquirer/news/2360604/skype-stops-working-on-older-android-phones-leaving-linux-users-in-the-dark|url-status=unfit|archive-date=August 16, 2014|publisher=CNET|access-date=2 October 2014|author=Chris Merrimen|date=15 August 2014}}</ref>

==Peer-to-peer architecture==
Skype was the first [[Peer-to-peer|peer-to-peer]] IP telephony network.<ref>{{cite arXiv |eprint=cs/0412017v1 |title= An analysis of the Skype peer-to-peer Internet telephony protocol |year= 2004 |author1= Salman A. Baset |author2= Henning Schulzrinne |page=11}}</ref> The network contains three types of entities: ''[[Supernode (networking)|supernodes]]'', ordinary nodes, and the login server. Each client maintains a ''host cache'' with the [[IP address]] and port numbers of reachable supernodes. The Skype user directory is decentralized and distributed among the supernodes in the network.  Supernodes are grouped into ''slots'' (9–10 supernodes), and slots are grouped into ''blocks'' (8 slots).{{cn|date=November 2022}}

Previously any client with good bandwidth, no restrictions due to firewall or network address translation (NAT), and adequate processing power could become a supernode. This placed an extra burden on those who connected to the Internet without NAT, as Skype used their computers and Internet connections as third parties for [[UDP hole punching]] (to directly connect two clients both behind NAT) or to completely relay other users' calls. In 2012, Microsoft altered the design of the network, and brought all supernodes under their control as hosted servers in data centres.<ref>{{cite news|last=Branscombe|first=Mary|title=Forget the conspiracy theories: Skype's supernodes belong in the cloud|url=http://www.zdnet.com/forget-the-conspiracy-theories-skypes-supernodes-belong-in-the-cloud-7000001720/|access-date=17 June 2013|newspaper=500 words into the future (ZDNet)|date=27 July 2012}}</ref>  Microsoft at the time defended the move, saying they "believe this approach has immediate performance, scalability and availability benefits for the hundreds of millions of users that make up the Skype community."<ref>{{cite news|title=Skype replaces P2P supernodes with Linux boxes hosted by Microsoft (updated)|url=https://arstechnica.com/business/2012/05/skype-replaces-p2p-supernodes-with-linux-boxes-hosted-by-microsoft/|access-date=17 June 2013|newspaper=Ars Technica|date=2 May 2012}}</ref> At the time there was some concern regarding the privacy implications of the change,<ref>{{cite news|last=Kosner|first=Anthony|title=Will Microsoft's Changes To The Architecture Of Skype Make It Easier To Snoop?|url=https://www.forbes.com/sites/anthonykosner/2012/07/18/did-microsoft-change-the-architecture-of-skype-to-make-it-easier-to-snoop/|access-date=17 June 2013|newspaper=Forbes blog|date=18 July 2012}}</ref> which appear to have been proven true with the revelation of the PRISM surveillance program in June 2013.<ref>{{cite news|last=Gallagher|first=Ryan|title=Newly Revealed PRISM Snooping Makes Verizon Surveillance Look Like Kids' Stuff|url=http://www.slate.com/blogs/future_tense/2013/06/06/nsa_prism_surveillance_private_data_from_google_microsoft_skype_apple_yahoo.html|access-date=17 June 2013|newspaper=Slate blog|date=6 June 2013}}</ref><ref>{{cite news|last=Greenwald|first=Glenn|title=NSA Prism program taps in to user data of Apple, Google and others|url=https://www.theguardian.com/world/2013/jun/06/us-tech-giants-nsa-data|access-date=17 June 2013|newspaper=The Guardian|date=7 June 2013}}</ref>

Skype does not support the use of the [[IPv6]] protocol,<ref>{{cite web|url=https://twitter.com/SkypeTalks/status/264057558262747136|title=Skype does not support IPv6.|publisher=Skype official Twitter account|date=1 Nov 2012|access-date=4 Nov 2012}}</ref> which would greatly reduce the complexity associated with the aforementioned IPv4 communication structure.

Supernodes relay communications on behalf of two other  clients, both of which are behind firewalls or "one-to-many" network address translation.
Without relaying by the supernodes, two clients with firewall or NAT difficulties would be unable to make or receive calls from one another. Skype tries to get the two ends to negotiate the connection details directly, but sometimes the sum of problems at both ends can prevent direct conversation being established.

The problems with firewalls and NAT can be:
*  The external port numbers or IP address are not derivable, because NAT rewrites them
*  The firewall and NAT in use prevents the session being received
*  UDP is not usable due to NAT issues, such as timeout
*  Firewalls block many ports
*  TCP through many-to-one NAT is always "outward only" by default – adding port-forwarding  settings to the NAT router can allow reception of TCP sessions

==Protocol==
Signaling is encrypted using [[RC4]]; however, the method only obfuscates the traffic as the key can be recovered from the packet. Voice data is encrypted with [[Advanced Encryption Standard|AES]].<ref>[http://www.ossir.org/windows/supports/2005/2005-11-07/EADS-CCR_Fabrice_Skype.pdf Introduction Skype analysis Enforcing anti-Skype policies], Skype uncovered Security study of Skype, Desclaux Fabrice, 7/11/2005, EADS CCR/STI/C</ref>

The Skype client's [[Application programming interface|application programming interface]] (API) opens the network to software developers. The Skype API allows other programs to use the Skype network to get "white pages" information and manage calls.

The Skype code is closed source, and the protocol is not standardized.<ref>{{cite web |url=http://support.skype.com/en_US/faq/FA153/Which-protocols-does-Skype-use |title=Which protocols does Skype use? |website=Help |publisher=Skype |archive-url=https://web.archive.org/web/20090303112252/http://support.skype.com/en_US/faq/FA153/Which-protocols-does-Skype-use |archive-date=March 3, 2009 }}</ref> Parts of the client use Internet Direct (Indy), an open source socket communication library.{{Citation needed|date=December 2008}}

On July 8, 2012, a researcher from Benin, Ouanilo Medegan, released articles and proof of concept code, results of his reverse engineering the Skype client.<ref>{{cite web |url=http://www.oklabs.net/category/skype-reverse/ |title=Posts under Skype Reverse Category |website=oKLabs }}<br />{{cite web |url=http://www.oklabs.net/skype-reverse-engineering-the-long-journey/ |title=Skype Reverse Engineering: The (long) journey ;).. |website=oKLabs }}</ref>

===Protocol detection===

Many networking and security companies claim to detect and control Skype's protocol for enterprise and carrier applications. While the specific detection methods used by these companies are often proprietary, [[Pearson's chi-squared test]] and stochastic characterization with [[Naive Bayes classifier]]s are two approaches that were published in 2007.<ref>{{cite journal |url=https://www.dpacket.org/articles/revealing-skype-traffic-when-randomness-plays-you |author=Dario Bonfiglio |display-authors=0 |title=Revealing Skype Traffic: When Randomness Plays with You |journal=ACM SIGCOMM Computer Communication Review |volume=37 |issue=SIGCOMM 2007 |pages=37–48 |url-status=dead |archive-url=https://web.archive.org/web/20110430074127/https://www.dpacket.org/articles/revealing-skype-traffic-when-randomness-plays-you |archive-date=2011-04-30 }}</ref>

==Obfuscation layer==
The [[RC4]] encryption algorithm is used to obfuscate the payload of datagrams.
# The [[Cyclic redundancy check|CRC32]] of public source and destination IP, Skype's packet ID are taken
# Skype obfuscation layer's [[Initialization vector|initialization vector]] (IV).
The XOR of these two 32-bit values is transformed to an 80-byte RC4 key using an unknown key engine.

A notable misuse of RC4 in Skype can be found on TCP streams (UDP is unaffected). The first 14 bytes (10 of which are known to the user, since they consist of a hash of the username and password) are XOR-ed with the RC4 stream. Then, the cipher is reinitialized to encrypt the rest of the TCP stream.<ref>{{cite conference |author1=Fabrice Desclaux |author2=Kostya Kortchinsky |date=2006-06-17 |title=Vanilla Skype part 2 |book-title=RECON2006 |url=http://www.recon.cx/en/f/vskype-part2.pdf }}</ref>

==TCP==
TCP packets:
 TCP
 Skype Init TCP packet

The Skype Init TCP packet contains
* the seed (4 bytes)
* init_str string 00 01 00 00 01 00 00 00 01/03

==Low-level datagrams==
Almost all traffic is ciphered. Each command has its parameters appended in an object list. The object list can be compressed.
                                  / Object List     ... -|
         Enc      -> Cmd -> Encod
                      ^           \ Compressed List ... -|
         Frag         |                                  |
                      |------------------<---------------|
         Ack

         NAck

         Forward  -> Forwarded..Message

===Object lists===
An object can be a number, string, an IP:port, or even another object list.  Each object has an ID.  This ID identifies which command parameter the object is.

  Object:
          Number
          IP:Port
          List of numbers
          String
          RSA key

  Object List
          List Size (n)
          Object 1
          .
          .
          Object n

===Packet compression===
Packets can be compressed. The algorithm is a variation of [[Arithmetic coding|arithmetic compression]] that uses reals instead of bits.

==Legal issues==

[[Reverse engineering#Legality|Reverse engineering]] of the Skype protocol by inspecting/disassembling binaries is prohibited by the terms and conditions of Skype's license agreement. However European Union law allows reverse-engineering a computer program without getting a permission from an author for inter-operability purposes.<ref>[http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:111:0016:01:EN:HTML Directive 2009/24/EC of the European Parliament and of the Council of 23 April 2009 on the legal protection of computer programs]</ref> In the United States, the [[Digital Millennium Copyright Act]] provides protections for reverse engineering software for the purposes of interoperability with other software.<ref>17 U.S.C. Sec. 1201(f).</ref><ref>WIPO Copyright and Performances and Phonograms Treaties Implementation Act</ref> There are also legal precedents in the United States when the reverse-engineering is aimed at interoperability of file formats and protocols.<ref>Sega vs Accolade, 1992</ref><ref>Sony vs Connectix, 2000</ref><ref>{{cite journal |author=Pamela Samuelson |author2=Suzanne Scotchmer |name-list-style=amp |title=The Law and Economics of Reverse Engineering |volume=111 |journal=Yale Law Journal |pages=1575–1663 |date=May 2002 |issue=7 |doi=10.2307/797533 |jstor=797533 |url=http://www.yalelawjournal.org/pdf/111-7/SamuelsonFINAL.pdf |access-date=2015-03-17 |url-status=dead |archive-url=https://web.archive.org/web/20110716095222/http://www.yalelawjournal.org/pdf/111-7/SamuelsonFINAL.pdf |archive-date=2011-07-16 }}</ref>

In addition, some countries specifically permit a program to be copied for the purposes of reverse engineering.<ref>In the French "intellectual property" law set, there is an exception that allows any software user to reverse engineer it. See [http://legifrance.gouv.fr/affichCodeArticle.do?cidTexte=LEGITEXT000006069414&idArticle=LEGIARTI000006278920&dateTexte=20080329&categorieLien=cid ''code de la propriété intellectuelle''] {{in lang|fr}}. This law is the national implementation of a piece of EU legislation: [http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31991L0250:EN:NOT Council Directive 91/250/EEC], since then repealed by [http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32009L0024:EN:NOT 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).</ref>

==Notes==
{{Reflist|30em}}

==References==
{{Refbegin}}
* {{cite arXiv |eprint=cs/0412017v1 |title= An analysis of the Skype peer-to-peer Internet telephony protocol |year= 2004 |author1= Salman A. Baset |author2= Henning Schulzrinne |name-list-style=amp }}
* {{Cite web|author1=P. Biondi  |author2=F. Desclaux  |name-list-style=amp | title = Silver Needle in the Skype | url = https://www.blackhat.com/presentations/bh-europe-06/bh-eu-06-biondi/bh-eu-06-biondi-up.pdf | date = March 3, 2006}}
* {{Cite web|author1=F. Desclaux  |author2=K. Kortchinsky  |name-list-style=amp | title = Vanilla Skype - part 1 | url = http://www.recon.cx/en/f/vskype-part1.pdf| date = June 6, 2006}}
* {{Cite web|author1=F. Desclaux  |author2=K. Kortchinsky  |name-list-style=amp | title = Vanilla Skype - part 2 | url = http://www.recon.cx/en/f/vskype-part2.pdf| date = June 17, 2006}}
* {{Cite web|author1=L. De Cicco |author2=S. Mascolo |author3=V. Palmisano | title = An Experimental Investigation of the Congestion Control Used by Skype VoIP. | work = WWIC 07 | publisher = Springer | date = May 2007 | url = http://c3lab.poliba.it/images/d/d2/Skype_wwic07.pdf}}
* {{Cite web|author1=L. De Cicco |author2=S. Mascolo |author3=V. Palmisano | title = A Mathematical Model of the Skype VoIP Congestion Control Algorithm. | work = Proc. of IEEE Conference on Decision and Control 2008 | date = December 9–11, 2008 | url = http://c3lab.poliba.it/images/2/22/Skype_voip_model.pdf }}
* {{Cite web|author1=Dario Bonfiglio |author2=Marco Melia |author3=Michela Meo |author4=Dario Rossi |author5=Paolo Tofanelli | title = Revealing Skype Traffic: When Randomness Plays With You | publisher = ACM SIGCOMM Computer Communication Review | date = August 27–31, 2007 | url = http://www.sigcomm.org/node/2623 }}
{{Refend}}

==External links==
* [http://kirils.org/skype/ Website containing articles and tools related to Skype protocol and behaviour analysis ]
* [http://www1.cs.columbia.edu/~salman/skype/ Repository of articles on Skype analysis]
* [https://translate.google.com.ua/translate?sl=ru&tl=en&js=y&prev=_t&hl=ru&ie=UTF-8&u=https%3A%2F%2Fmarakew.github.io%2Fskype_research.html&edit-text= Skype Architecture Inside]

{{Instant messaging}}

{{DEFAULTSORT:Skype Protocol}}
[[Category:VoIP protocols]]
[[Category:Instant messaging protocols]]

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{{Sourceattribution|Skype protocol}}