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Skype
1
Overview
 P2P architecture
 Closed and proprietary design
 Encryption of data
 Obfuscation of protocol
 Knowledge of protocol is due to analysis,
not to standards or code
 Different client platforms, e.g. mobile
devices
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Features
 Voice
Client to client
 Skypein and Skypeout connection with
Telephone networks

 Video
 Conferencing
 Chat

Usual features: e.g. status, profiles etc.
 File transfer
3
History
 Developed by the same Estonians that
developed Kazaaa: Ahti Heinla, Priit
Kasesalu and Jaan Tallinn
 2003 - First version
 2005 – Purchased by eBay
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Network Architecture
 Skype Login server
 Only centralized node
 Authenticates users
 Super node
 Assists in calls
 Location of peers
 NAT traversal
 Must not be behind firewall or NAT
 Host node
 Skypein servers
 Skypeout servers
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Registration
 Register a username with Skype web
server
 No real identity check
 Install Skype client
 Client software contains hard-coded,
obfuscated list of login servers
 Obfuscation?
Various tricks to hide what is actually there
 Example: list of super nodes is encrypted and
decryption only occurs when the software is
running

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Host Cache
 Each client has a host cache
 Stores a list of super nodes and login servers
 Similar in function to Chord finger table or
Gnutella neighborhood
 List of super nodes is updated during regular
Skype operation
 Upon login, client will attempt to contact one of
the hosts in the host cache
 In Windows XP, appears in C:\Documents and
Settings\user\Application
Data\Skype\shared.xml
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Ports
 Each client listens on several ports
 At installation a random port P is chosen
 The client listens on P in UDP
 The client listens on P in TCP
 TCP 80 (HTTP)
 TCP 443 (HTTPS)
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Buddy List
 List of users which client “knows”
 Can open a session with buddy
 Aware of buddy presence
 In Windows XP appears in C:\Documents and
Settings\user\Application Data\Skype\Skype
name\config.xml
 Buddy list is also stored on a central Skype server
 Skype stores 4 bytes per contact (this is some
function of both the contact and the current user,
i.e. two users with the same buddy have a
different 4-byte identifier)
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Login
 Functions
 Connect to Skype network
 Authenticate Skype client
 Determine if client is behind NAT and discover
real address and port
 Update software version (if necessary)
 Advertise presence
 Login proceeds by connecting to:
 Host in host cache (if exists)
 One of hard-coded servers
 Repeat many times
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It just works
 Skype users are a cross section of society

Many are not knowledgeable about technology
 Skype login may be hindered by
 NAT
 Firewall rules: blocking UDP, blocking certain IP
addresses etc.
 The Skype login process needs to work in
the background and overcome obstacles
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Connect to Skype Network
 Choose a host from Host Cache (one of at most
200 possible choices)


Attempt to establish contact on host’s UDP port
If unsuccessful try next host in cache
 If unsuccessful for 20 hosts
 Assume that there is a firewall blocking Skype
 Repeat the above with TCP ports
 If unsuccessful, attempt to establish contact with
7 bootstrap nodes


Connect to UDP 33033 and if unsuccessful
Connect to TCP 33033
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Skype Authentication
 Function: network authenticates Skype client
 If client passes authentication, it can connect to




other clients
If client doesn’t pass authentication, it can’t
connect
The authenticator is a central node – one of the
Skype login servers
The nodes that require data on the authentication
are other nodes
How can the other nodes know of the
authentication result?
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(Failed) Attempts
Login server contacts all nodes and
notifies them of authentication result
2. Login server gives Skype client a “cookie”,
a unique random quantity that identifies it
as legitimate
3. Skype client routes all its connections
through login server
4. Login server generates a key for
symmetric authentication and sends it to
Skype client who uses it for every
authentication
1.
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Overview of Skype Authentication
 The last idea is not bad except we need a
public key instead of a symmetric key
 How does a node know that a public key is
authorized by the login server?

A signature by the login server!
 Two stages
 The client authenticates to the login server
using a <username, password> pair
 The login server returns a signed public key
15
Authentication Process
 Skype has a list of trusted RSA public keys
 Client and login server have a shared key
 Hashed password
 Client generates a random session key
 RSA with 1024 bit modulus
 Client encrypts the shared secret with:
 Symmetric encryption (AES 256 bits)
 The key is a hash of the session key
 Client uses one of the trusted RSA keys to
encrypt a concatenation of


Session key
Random string
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Authentication process (cont.)
 Client sends to login server
Encrypted session key
 Encrypted shared key

 Login server decrypts both keys
 Login server verifies that shared key is
correct
 If shared key is correct
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Authentication (client – login
server)
18
Global Index
 Many Skype operations are based on a Skype
client finding data



What is my buddy’s IP address (to place a call)
What is the status of a contact
Find a user based on profile information
 Relevant data must be published to network, e.g.
 Client status
 How does this work?
 Skype needs the same functionality that Chord
has
 It is called Global Index (by Joltid corporation),
but details are not public
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Global Index (What is known?)
 Super nodes have hierarchy
 9-10 super nodes to a slot
 8 slots per block
 In 2006 there were 2050 slots (so about 20000 super
nodes)
 Each super node knows almost any other super
node
 Data is replicated (unlike Chord)
 Finding an object requires limited multicast
(request data from 4 super nodes, then 8 super
nodes, then 16 etc.)
 Skype claims that any user that has been logged
on in the last 72 hours can be found
20
Global Index (cont.)
 Failed search for users goes through
central login servers
 Possibly, these servers store all users who
have logged on in the past 72 hours.
21
Skype Super Node Map (2006)
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Voice call
 The call can be placed between two buddies
 Two stages
 Signaling
 Media
 Several cases
Both peers have public addresses
 One behind NAT
 Both behind NAT

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Two Public IP addresses
 Signaling
Direct TCP connection between initiator and
responder
 Includes negotiation on CODEC, agreement on
encryption key etc.
 Includes extended services e.g. conferences

 Media
 Direct UDP connection between them
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Addresses behind NAT
 One address behind NAT
Signaling through super node
 Media directly between the peers

 Two addresses behind NAT
 Both signaling and media are relayed through
one or more super nodes
25
Encryption
 RC-4 to obfuscate packet headers
 AES 256-bit key to encrypt data (media,
IM etc.)
 RC-4 key is computed based on publicly
available data such as the IP address (but
what happens with NAT?)
 AES key is transferred using the client’s
RSA session key
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