An Overview of Secure Shell

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An Overview of Secure Shell
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Secure Shell

• A replacement for telnet, rlogin, etc.
• Developed in 1995 by Tatu Ylönen from Espoo,
  Finland
• Commercialized in 1998 by SSH Communications
  Security
• OpenSSH was created by the OpenBSD team as an
  open alternative to SSH
• http//www.openssh.org

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Open SSH Features

• Open Source Project
• Free Licensing
• Strong Encryption (3DES, Blowfish, AES, Arcfour)
• X11 Forwarding (encrypt X Window System traffic)
• Port Forwarding (encrypted channels for legacy
  protocols)

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Open SSH Features

• Strong Authentication
• Agent Forwarding (Single-Sign-On)
• Interoperability (Compliance with SSH 1.3, 1.5,
  and 2.0 protocol Standards)
• SFTP client and server support in both SSH1 and
  SSH2 protocols.
• Kerberos and AFS Ticket Passing
• Data Compression

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SSH services/functionality

• Services
• Confidentiality
• Integrity
• Authentication
• Functionality
• Secure command shell (remote execution of
  commands)
• Secure file transfer
• Data tunneling for TCP/IP applications (TCP port
  forwarding)

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Major SSH components

• SSH Transport Layer Protocol
• provides server authentication, confidentiality,
  and integrity services
• it may provide compression too
• runs on top of any reliable transport layer
  (e.g., TCP)
• SSH User Authentication Protocol
• provides client-side user authentication
• runs on top of the SSH Transport Layer Protocol

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Major SSH components (Cont)

• SSH Connection Protocol
• multiplexes the secure tunnel provided by the SSH
  Transport Layer and User Authentication Protocols
  into several logical channels
• these logical channels can be used for a wide
  range of purposes
• secure interactive shell sessions
• TCP port forwarding
• carrying X11 connections

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SSH security features

• strong algorithms
• uses well established strong algorithms for
  encryption, integrity, key exchange, and public
  key management
• large key size
• requires encryption to be used with at least 128
  bit keys
• supports larger keys too

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SSH security features (Cont)

• algorithm negotiation
• encryption, integrity, key exchange, and public
  key algorithms are negotiated
• it is easy to switch to some other algorithm
  without modifying the base protocol

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Supported Algorithms/Encryption

• 3des
• blowfish
• twofish256 / twofish192 / twofish128
• aes256 / aes192 / aes128
• serpent256 / serpent192 / serpent128
• arcfour (RC4)
• idea / cast128

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SSH TLP Overview
client
server
TCP connection setup
SSH version string exchange
SSH key exchange (includes algorithm negotiation)
SSH Transport Layer Protocol
SSH data exchange
termination of the TCP connection
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SFTP (secure file transport protocol)

• runs over a secure channel
• Server and client are inter-authenticated
• The channel will support encryption and integrity
• Run in request-response mode
• Independent from other SSH protocols
• may run on top of other secure protocols, e.g.,
  TLS
• Current use with the SSH Connection Protocol
• Run as a subsystem of the connection, name is
  sftp
• After a channel is opened, activate sftp server
  by starting a shell or command.

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Transport layer protocol

• Client initiates connection to the server
• Identification string exchange
• Algorithms exchange
• Key exchanges include host key sent to client
• Diffie-Hellman key exchange
• Client selects a random session key

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User Authentication Protocol

• the protocol assumes that the underlying
  transport protocol provides integrity and
  confidentiality (e.g., SSH Transport Layer
  Protocol)
• the protocol has access to the session ID
• the server should have a timeout for
  authentication and disconnect if the
  authentication has not been accepted within the
  timeout period
• recommended value is 10 minutes

SSH User Authentication Protocol
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User Authentication Protocol

• the server should limit the number of failed
  authentication attempts a client may perform in a
  single session
• recommended value is 20 attempts
• three authentication methods are supported
• publickey
• password
• hostbased

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User authentication overview
client
server
SSH_MSG_USERAUTH_REQUEST
SSH_MSG_USERAUTH_FAILURE (further authentication
needed)
SSH_MSG_USERAUTH_REQUEST
SSH User Authentication Protocol
SSH_MSG_USERAUTH_FAILURE (further authentication
needed)

SSH_MSG_USERAUTH_REQUEST
SSH_MSG_USERAUTH_SUCCESS
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SSH attacks

• Man-in-the-middle
• How does a client verify the host key?
• Check local database
• If exist, the server is authenticated, connection
  is established
• If not exist, currently let user determine
• (because public certificates are not used widely)
• Build connection with saving the server host key
• Build connection without saving.
• Therefore there exists a possibility of
  man-in-the-middle attack.
• Denial of service
• Attacker sets up many connections without user
  authentication.

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Tunneling

• A mechanism to encapsulate a message/packet
  within another message/packet and transfer
  through a network without touching the internal
  message/packet.
• Analogy to English Channel (railway tunneling)
• Typical scenario for Tunneling
• Two same networks connected by a different
  network
• Multicast backbone
• Mobile host moves out of its home area
• IPSec tunneling mode
• SSH port forwarding

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• Application streams are tunneled over Secure
  Shell by forwarding individual TCP ports.
• When a local port is forwarded the ssh client
  listens on the specified port on the local host.
• The ssh server opens a TCP connection to the
  remote host where the server application is
  actually running.
• localhost refers to the application client's host
  
• remotehost refers to the application server's
  host
• Privileged ports can only be forwarded by root

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Local Port Forwarding
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How to make it happen

• On host-1 type the following
• ssh l 2525localhost25 host-2
• You will then receive a login prompt from host-2
• As long as this connection is up all traffic
  hitting host-1 on port 2525 will be forwarded to
  host-2 via secure shell and port 22 to port
  localhost port 25 on host-2

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Class Exercise

• Team up into pairs and exchange IP addresses
• Create accounts for each other if the do not
  exist and password protect them
• Verify you have smtp (sendmail) running on each
  server. If not let me know.
• Verify you have the telnet client installed on
  each server.
• Ready?

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Class Exercise

• We will call the destination server host
• At the shell prompt type
• ssh L 2525localhost25 ltusergt_at_lthostgt
• Where lthostgt is the IP address of your partners
  server
• Now test the connection. On your server type
• telnet localhost 25

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Another Example

• Using the remote -R switch
• ssh R 8080there.com80 there.com
• Anyone connecting to www.here.com on port 8080
  will be forwarded down the ssh tunnel to the