Skype Protocol

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Skype is a proprietary peer-to-peer Internet telephony (VoIP) network. The protocol has not been made publicly available and all applications using the protocol are closed-source.

Contents

[edit] Protocol

A Skype network is a peer-to-peer network with three main entities: supernodes, ordinary nodes and the login server. It is an overlay network: each client builds and refreshes a list of reachable nodes known as the host cache. The host cache contains IP address and port numbers of supernodes. Communication is encrypted using RC4.

Supernodes relay communications to other clients behind a firewall. Any skype client can become a supernode if it has good bandwidth, no firewall, and adequate processing power. Supernodes are grouped into slots (9-10 supernodes). Slots are grouped into blocks (8 slots).

[edit] Preliminaries

Abbreviations that are used:

  • SN: Skype network
  • SC: Skype client
  • HC: host cache

[edit] Skype client

The main functions of a Skype client are:

  • login
  • user search
  • start and end calls
  • media transfer
  • presence messages

[edit] Login

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 reponse 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=attempts+1
11.          if attempts==5 then 
12.            fail
13.          else
14.            wait 6 seconds
15.            return to step 1
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.

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 generate a 1024 private/public RSA key pair. A 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 it's 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.


[edit] UDP

UDP packets:

IP
UDP
Skype SoF
Skype Crypted Data

The Start of Frame (SoF) consists of:

  1. frame ID number (2 bytes)
  2. payload type (1 byte)
    • obfuscated payload
    • Ack/NAck packet
    • payload forwarding packet
    • payload resending packet
    • other


[edit] Obfuscation Layer

The RC4 encryption algorithm is used to obfuscate the payload of datagrams.

  1. The CRC32 of public source and destination IP, Skype's packet ID are taken
  2. Skype obfuscation layer's initialization vector (IV).

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.


[edit] 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

[edit] 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

[edit] 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

[edit] Packet compression

Packets can be compressed. The algorithm is a variation of arithmetic compression that uses reals instead of bits.

[edit] See also

  • Repository of articles on Skype analysis [1].

[edit] References

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