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The Skype protocol is a proprietary Internet telephony network based on peer-to-peer architecture, used by 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 VoIP networks without proper licensing from Skype. Digium, the main sponsor of Asterisk PBX released a driver licensed by Skype dubbed 'Skype for Asterisk' to interface as a client to the Skype network, however this still remains closed source.[1] Numerous attempts to study and/or reverse engineer the protocol have been undertaken to reveal the protocol, investigate security or to allow unofficial clients.
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Skype was the first peer-to-peer IP telephony network,[2] requiring minimal centralized infrastructure. The Skype user directory is decentralized and distributed among the clients, or nodes, in the network.
The network contains three types of entities: supernodes, ordinary nodes, and the login server. Each client maintains a host cache with the IP address and port numbers of reachable supernodes.
Any client with good bandwidth, no restriction due to firewall or NAT, and adequate processing power can become a supernode. This puts an extra burden on those who connect to the Internet without NAT, as Skype may use their computers and Internet connections as third party for UDP hole punching (to directly connect two clients both behind NAT) or to completely relay other users' calls. Skype does not choose to supply server power with associated bandwidth required to provide the relay service for every client who needs it, instead it uses the resource of skype clients. [3]
Supernodes relay communications on behalf of two other clients, both of which are behind firewalls or "one to many" Network address translation. The reason that relaying is required is that without relaying clients with firewall or NAT difficulties , the two clients would be unable to make or receive calls from other . Skype tries to get the two ends to negotiate the connection details directly, but what can happen is that the sum of problems at both ends can mean that two cannot establish direct conversation.
The problems with firewalls and NAT can be
Supernodes are grouped into slots (9-10 supernodes), and slots are grouped into blocks (8 slots).
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 AES.[4]
The Skype client's 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.[5] Parts of the client use Internet Direct (Indy), an open source socket communication library.
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 classifiers are two approaches that were published in 2007.[6]
Abbreviations that are used:
The main functions of a Skype client are:
A Skype client authenticates the user with the login server, advertises its presence to other peers, determines the type of NAT and firewall it is behind and discovers nodes that have public IP addresses.
To connect to the Skype network, the host cache must contain a valid entry. A TCP connection must be established (i.e. to a supernode) otherwise the login will fail.
1. start 2. send UDP packet(s) to HC 3. if no response within 5 seconds then 4. attempt TCP connection with HC 5. if not connected then 6. attempt TCP connection with HC on port 80 (HTTP) 7. if not connected then 8. attempt TCP connection with HC on port 443 (HTTPS) 9. if not connected then 10. attempts++ 11. if attempts==5 then 12. fail 13. else 14. wait 6 seconds 15. goto step 2 16. Success
After a Skype client is connected it must authenticate the username and password with the Skype login server. There are many different Skype login servers using different ports. An obfuscated list of servers is hardcoded in the Skype executable.
Skype servers are:
Skype-SW connects randomly to 1-8.
On each login session, Skype generates a session key from 192 random bits. The session key is encrypted with the hard-coded login server's 1536-bit RSA key to form an encrypted session key. Skype also generates a 1024-bit private/public RSA key pair. An MD5 hash of a concatenation of the user name, constant string ("\nSkyper\n") and password is used as a shared secret with the login server. The plain session key is hashed into a 256-bit AES key that is used to encrypt the session's public RSA key and the shared secret. The encrypted session key and the AES encrypted value are sent to the login server.
On the login server side, the plain session key is obtained by decrypting the encrypted session key using the login server's private RSA key. The plain session key is then used to decrypt the session's public RSA key and the shared secret. If the shared secret match, the login server will sign the user's public RSA key with its private key. The signed data is dispatched to the super nodes.
Upon searching for a buddy, a super node will return the buddy's public key signed by Skype. The SC will authenticate the buddy and agree on a session key by using the mentioned RSA key.
UDP packets:
IP UDP Skype SoF Skype Crypted Data01
The Start of Frame (SoF) consists of:
The RC4 encryption algorithm is used to obfuscate the payload of datagrams.
The XOR of these two 32-bit values is transformed to a 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) are xored with the RC4 stream. Then, the cipher is reinitialized to encrypt the rest of the TCP stream.[7]
TCP packets:
TCP Skype Init TCP packet
The Skype Init TCP packet contains
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
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
Packets can be compressed. The algorithm is a variation of arithmetic compression that uses reals instead of bits.
Reverse engineering of the Skype protocol by inspecting/disassembling binaries is prohibited by the terms and conditions of Skype's license agreement. However there are legal precedents when the reverse-engineering is aimed at interoperability of file formats and protocols.[8][9][10] In the United States, the Digital Millennium Copyright Act grants a safe harbor to reverse engineer software for the purposes of interoperability with other software.[11][12] In addition, many countries specifically permit a program to be copied for the purposes of reverse engineering.[13]
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